Porous AAO template-assisted rational synthesis of large-scale 1D hybrid and hierarchically branched nanoarchitectures

Progress in Materials Science 95 (2018) 243–285

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Progress in Materials Science journal homepage: www.elsevier.com/locate/pmatsci

Porous AAO template-assisted rational synthesis of large-scale 1D hybrid and hierarchically branched nanoarchitectures Qiaoling Xu a, Guowen Meng a,b,⇑, Fangming Han a a Key Laboratory of Materials Physics and Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, PR China b University of Science and Technology of China, Hefei 230026, PR China

a r t i c l e

i n f o

Article history: Received 5 September 2016 Received in revised form 6 February 2018 Accepted 8 February 2018

Keywords: Hybrid nanostructures Branched nanostructures Anodic aluminum oxide Template-assisted synthesis

a b s t r a c t One-dimensional (1D) hybrid and hierarchically branched nanoarchitectures have been the focus of intensive research due to their fascinating size-, shape-, and material-dependent properties, with potential applications in catalysis, biomolecular separation, solar cells, nanoelectronics, nanophotonics, and nanodevices. Up to now, various approaches have been developed for the building of these complex nanoarchitectures, such as chemical vapor deposition, solution-based synthesis, thermal evaporation route, and various combinatorial multi-step approaches. However, the methods mentioned above exhibited limited control over the nanoarchitectural geometry, density, sizes, and locations of the branches. As the nanochannel diameters, lengths, and morphologies inside porous anodic aluminum oxide (AAO) membranes can be tuned by rationally adjusting the anodizing conditions of high-purity Al foils, porous AAO template-assisted synthesis has been considered as a general approach to the large-scale fabrication of 1D hybrid and hierarchically branched nanoarchitectures with controlled geometry, composition of different components and complexity. This review highlights the advances in the porous AAO template-assisted rational synthesis of 1D hybrid and hierarchically branched nanoarchitectures, including axial hetero-nanostructures, coaxial nanocables, hierarchically branched 1D homonanostructures, and hierarchically branched 1D hetero-nanostructures. It also discusses the fascinating applications of these complex nanoarchitectures in various fields. Ó 2018 Elsevier Ltd. All rights reserved.

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Axial hetero-nanostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Segmented hybrid nanowires . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1. Electrochemical deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2. Combination of ECD and other methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.3. Galvanic deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Segmented nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Segmented NW/NT hetero-nanostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1. Electrochemical deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

⇑ Corresponding author at: Institute of Solid State Physics, Chinese Academy of Sciences, P.O. Box 1129, Hefei, Anhui 230031, PR China. E-mail address: [email protected] (G. Meng). https://doi.org/10.1016/j.pmatsci.2018.02.004 0079-6425/Ó 2018 Elsevier Ltd. All rights reserved.

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2.3.2. Combination of ECD and CVD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Coaxial nanocables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Coaxial nanocables with solid cores. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1. Electrochemical deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2. Combination of ECD and other methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.3. Other methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Coaxial nanocables with hollow cores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Coaxial nanocables with hybrid nanostructures as inner cores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1. Segmented hybrid NWs as inner cores of the coaxial nanocables. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2. Segmented NW/NT heterostructures as inner cores of the coaxial nanocables . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.3. Sub-nanocables as inner cores of the coaxial nanocables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hierarchically branched 1D homo-nanostructures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Branched NWs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Branched NTs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Multi-generation branched NWs and NTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hierarchically branched 1D hetero-nanostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Y-branched two-segment NW/NT hetero-nanostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Y-branched three-segment NT/NW/NT hetero-nanostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.1. With two end NT segments consisting of the same material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.2. With two end NT segments consisting of different materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and future outlook. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1. Introduction One-dimensional (1D) hybrid and hierarchically branched nanoarchitectures have gained tremendous attention in the last two decades due to their fascinating size-, shape-, and material-dependent properties [1–8]. Their interesting electronic, optical, and magnetic properties have led to a wide range of applications in barcodes [3], catalysis [8], biomolecular separation [9], field-emitter [10], magnetic manipulation [11], solar cells [12], nanoelectronics, nanophotonics, nanodevices, and nanosystems [2,6,13–16]. Rational synthesis of these multi-component hybrid complex nanoarchitectures is crucial for the investigation of their properties and potential applications in various fields in the future. Coaxial nanocables are one type of the 1D hybrid nanoarchitectures, and various approaches have been developed for the nanocables. For example, chemical vapor deposition (CVD) was used for the construction of various coaxial nanocables, such as solid ZnS-nanowire (NW)@SiO2-nanotube (NT) (The symbol ‘‘@” denotes the interface between the NW-core and the NTsheath of the nanocable) [17] and Sn-NW@amorphous carbon-NT (CNT) nanocables [18]. The CVD-grown coaxial nanocables have applications in many fileds. For instance, the AuSi-NW@b-Ga2O3-NT nanocables could be employed as wide range hightemperature nanothermometers [19]. The carbon-NW@Si-NT nanocables could be used as high power and long life lithium battery electrodes [20]. The Ge-NW@Si-NT, Si-NW@Ge-NT, and Si-NW@Ge-NT@Si-NT nanocables cold be served as field-effect transistors [21]. Meanwhile, a low-temperature solution-based cation exchange reaction was developed for CdS-NW@Cu2S-NT nanocables for solar cell application [12,22]. Thermal evaporation route was employed to synthesize In/Si-NW@SiO2-NT nanocables [23] and Si-NW@CdSSe-NT nanocables [24]. In addition, combinatorial methods were developed for the fabrication of coaxial nanocables. For example, Meng et al. [25] firstly synthesized b-SiC-NW@SiO2-NT nanocables by a combinatorial process of carbothermal reduction of silica xerogels containing nanosized carbon particles to get b-SiC-NW and thermal evaporation-condensation of silica to achieve the SiO2-NT nanocable-sheath. Later on, Ga/ZnS-NW@SiO2-NT nanocables were synthesized by combining thermal evaporation route with thermochemical reaction [26]. Moreover, a combination of CVD and a low-temperature supercritical-solution route was employed for constructing hollow CNT@Al2O3-NT nanocables [27]. However, these approaches exhibited limited control over the materials, diameter, and core/shell ratio of the coaxial nanocables. Homo-material branched nanostructures, with their trunks and branches consisting of the same materials, are simple 1D hierarchically branched nanoarchitectures. Diverse synthetic approaches have been developed for the homo-material branched nanostructures. For example, solution-based synthesis was used to grow branched Pd-nanostructures [8] and branched CdTe-nanocrystals with a controlled diameter of identical arms [28]. Thermal evaporation route was employed for the synthesis of multi-angularly branched ZnS-nanostructures with needle-shaped tips [10]. Vapor-liquid-solid (VLS) growth process was also developed for the fabrication of hyper-branched PbS-NWs and PbSe-NWs [7], twistingly branched PbS-NWs [29] and PbSe-NWs [30], and Si-nanotrees [31]. However, these approaches exhibited limited control over the density and sizes of the branches in the 1D branched nanostructures that are ultimately central to the rational design of building

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blocks for future nanodevices and nanosystems [32]. Furthermore, multi-step nanocluster-catalyzed VLS process was developed for the synthesis of hierarchically branched nanoarchitectures, such as branched GaP-NWs [33], as well as branched SiNWs and GaN-NWs [5]. In this multi-step synthetic approach, the resultant branched NWs were formed by depositing metal nanoparticles or nanoclusters (as a catalyst) on the trunks or the former branches to seed the growth of the next generation branches. Thus, the posterior seeding steps will deposit catalytic nanoparticles on not only the previously formed branches but also even the primary NW stem or trunk, thus losing control over the density and locations of the growing NW-branches. As for the complex 1D hierarchically branched hetero-nanoarchitectures, combinatorial multi-step synthetic approaches to constructing different materials in each step were developed. For example, branched hetero-nanostructures consisting of different semiconducting materials, such as SnO2-backbone/ZnO-branches (The slash ‘‘/” denotes the interface between the backbone or stem or trunk and the branches) [34,35], CdSe/ZnSe [36], and ZnS/CdS [37] were fabricated via the multi-step synthetic approaches, such as a combination of VLS and hydrothermal processes [34], and a combination of sequential seeding strategy and solution-liquid-solid growth process [36]. The multi-step synthetic approaches could also be used to build semiconducting and metallic branches onto the NW stem or trunk. For example, to grow semiconducting and metallic branches on Si-NW or Si-NW@SiO2-NT nanocable backbones, the following steps were carried out [38]. Firstly, Si-NW backbones were synthesized by a nanocluster-catalyzed CVD method, and the Si-NW@SiO2-NT nanocables were achieved by partial surface oxidation of the Si-NWs. Then, branched NWs were grown on the backbones by using an aqueous solution-based method for metallic branches and vapor-phase approaches for semiconducting branches. However, these multi-step approaches exhibited limited control over the density and locations of the branches in the hierarchically branched hetero-nanostructures. In addition, localized joining methods were also exploited for the construction of branched and hybrid nanoarchitectures. For example, high-temperature electron beam welding was developed for the synthesis of single-walled CNTs with junctions of various geometries, such as X-shape, Y-shape, and T-shape [39,40]. Laser-welding nano-assembly technique was also employed for constructing W18O49-nanotip/W-microtip assembly [41]. A nanoscale electrical welding technique was developed for assembling various individual nano-objects into complex nanoarchitectures [42]. However, these joining or welding methods are costly and time-consuming, and only one junction could be constructed each time. Porous anodic aluminum oxide (AAO) template-assisted approach can overcome the drawbacks of the techniques mentioned above. Porous AAO membranes are often fabricated by electrochemical anodization of high-purity Al foils in a proper acidic electrolyte. Under optimal anodization condition, the resultant AAO membranes contain a large number of selforganized cylindrical channels arranged in a hexagonal array. These nanochannels have a uniform diameter, and the channel density is as high as 1011 channels/cm2. Therefore, porous AAO membranes can be used as templates for large-scale synthesis of 1D nanostructures with homogeneous structures. Furthermore, the channel shapes and sizes inside the AAO membranes can be rationally tuned by simply adjusting the anodizing conditions of the high-purity Al foils, especially the applied anodization voltage or current. Thus, by using the porous AAO membranes with tailored pores as templates, 1D hierarchically branched nanoarchitectures can be rationally achieved.

Fig. 1. The general procedure for ECD of segmented hybrid NWs in porous AAO membranes.

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After Masuda and co-workers created ordered porous AAO membranes with straight nanochannels in 1995 [43], the AAO membranes were widely used for constructing linear 1D nanostructures, including single-component NWs and NTs, as well as multi-component axial hetero-nanostructures and radial hetero-nanostructures of coaxial nanocables. Then, Li and coworkers created porous AAO membranes with Y-branched nanochannels and grew CNTs by CVD inside the Y-branched nanochannels to achieve Y-branched CNTs (denoted as Y-CNTs) [44]. Later, Meng et al. [45] created porous AAO membranes with nanochannels of various complex shapes, such as multiple-branched shapes and multi-generation branched shapes. By using the AAO membranes with complex-shaped nanochannels as templates, they built CNTs and Ni-NWs with the corresponding complex shapes [45]. Meng and co-workers [46] also demonstrated the morphological and component tuning of the two-segment 1D NW/NT (The slash ‘‘/” denotes the interface between the NW and NT segments) heteronanostructures and the three-segment NT/NW/NT hybrid nanostructures with branched topologies. In this review, we summarize the large-scale synthesis of 1D hybrid and hierarchically branched nanoarchitectures via the porous AAO template-assisted techniques, and also introduce various fascinating applications of these complex nanostructures or their assemblies. We organize the review based on the types of 1D complex nanostructures or the classification of the fabrication methods. The details are as follows: we begin with the synthesis of one of the multi-component 1D hybrid nanostructures, i.e., axial hetero-nanostructures (including segmented hybrid NWs, segmented NTs, and segmented NW/NT hetero-nanostructures), by using straight-nanochannel AAO membranes as templates (Section 2). Afterwards, straightnanochannel AAO template-assisted synthesis of another multi-component 1D hybrid nanostructure, i.e., coaxial nanocables, is introduced (Section 3). After that, the discussion will focus on the synthesis of 1D hierarchically branched nanoarchitectures, including hierarchically branched 1D homo-nanostructures (Section 4) and hierarchically branched 1D hetero-nanostructures (Section 5), by using porous AAO membranes with the pre-designed complex-shaped nanochannels as templates. Finally, we conclude with perspectives for the 1D hybrid nanostructures and hierarchically branched nanoarchitectures. 2. Axial hetero-nanostructures Axial hetero-nanostructures, with multiple chemical components along their axis, have great potentials in biomolecular separation [9], bioassay [3], catalysis [47], nanoelectronics [48], and nanophotonics [13,32]. In this section, we will introduce the building of three types of axial hetero-nanostructures, i.e., segmented hybrid NWs, segmented NTs, and segmented NW/ NT (The slash ‘‘/” denotes the interface between the NW and NT segments) hetero-nanostructures, by using straightnanochannel AAO membranes as templates via diverse nanochannel-confined deposition techniques, including electrochemical deposition (ECD), combinations of ECD and other methods, and galvanic deposition. 2.1. Segmented hybrid nanowires 2.1.1. Electrochemical deposition ECD is most commonly used for the synthesis of segmented hybrid NWs in the straight-nanochannel AAO membranes. It offers marked advantages over other deposition techniques, such as simple, inexpensive, high yield, and all the NWs in the same batch with similar geometric parameters. Moreover, it doesn’t require any special equipment, high temperatures, or low-vacuum pressures. The general procedure for ECD of segmented hybrid NWs in the straight-nanochannel AAO membranes is outlined in Fig. 1, as also discussed in previous review articles [49,50]. First, a thin metal (Ag or Cu) layer is deposited onto one face of the AAO membrane to act as a working electrode for ECD of desired materials. Next, short segments of sacrificial metal (Ag or Ni) are electrodeposited into the bottom of the nanochannels to prevent a ‘‘puddling” effect, which causes one end of the NWs to have a deformed mushroom shape [49]. Then, ECD of different segmented hybrid NWs of desired materials is performed sequentially. After that, the sputtered metal layer electrode and the sacrificial metal segments are dissolved by chemical etching. Finally, segmented hybrid NWs are released after the removal of the AAO membrane. The diameters of the resultant segmented hybrid NWs are determined by the pore diameters of the AAO membranes used, while the length of each segment can be well controlled by monitoring the charge passed through during the corresponding ECD process. It should be noticed that a large variety of materials, such as metals, metal alloys, semimetals, conducting polymers, and semiconductors, can be electrodeposited inside the nanochannels of the AAO membranes as components of the segmented hybrid NWs. Segmented metallic NWs can be conveniently prepared by sequentially electrodepositing pre-desired components in the nanochannels of the AAO membranes via changing the electrolyte and accordingly varying the potential during the ECD process. The resultant segmented metallic NWs have applications in different fields. For example, Natan and co-workers [3] constructed segmented NWs of Au/Ag, Au/Ag/Au, and Au/Ag/Ni/Pd/Pt by ECD in the straight nanochannels of the AAO membranes. They found that each segment of the hybrid NWs can be identified by the pattern of differential optical reflectivity of adjacent segments. This enables the use of the segmented hybrid NWs as supports for bioassays, such as DNA and protein bioassays. Later, Meyer and co-workers reported the selective surface functionalization of Au-NW and Ni-NW segments in two-segment Au/Ni NWs with thiols and carboxylic acids, respectively [51]. They further demonstrated that the surface-functionalized two-segment Au/Ni NWs could selectively absorb proteins on the Ni-NW segments functionalized with palmitic acid but not on the Au-NW segments [52]. Three-segment metallic NWs can be applied in protein separation.

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Fig. 2. (a) Schematic of the procedure for the separation of His-tagged proteins and biotin-tagged proteins from nontagged proteins. (b) Optical microscopy image and SEM image (the inset) of the three-segment Au/Ni/Au NWs. (c) Fluorescence microscopy image of the three-segment Au/Ni/Au NWs after modification of the Ni-NW and Au-NW segments with fluorescein-tagged poly-His and atto 590-tagged biotin, respectively. Reprinted with permission from Ref. [53], copyright 2006, American Chemical Society.

For example, Mirkin et al. used three-segment Au/Ni/Au NWs for efficiently separating His-tagged proteins from non-his tagged proteins [9]. They also found that the three-segment Au/Ni/Au NWs could be employed for the separation of a mixture of three proteins [53]. His-tagged proteins bind to the Ni-NW segment, and biotin-tagged proteins bind to the functionalized Au-NW segments, allowing the separation of His-tagged and biotin-tagged proteins from a mixed solution (Fig. 2a). The Au-NW and Ni-NW segments can be clearly seen in the scanning electron microscopy (SEM) and optical microscopy images (Fig. 2b). Fluorescence microscopy images show that the red dye-labeled biotin binds to the functionalized AuNW segments, while the green dye-labeled polyhistidine binds to the Ni-NW segments (Fig. 2c), providing evidence for their proposed protein separation mechanism. Segmented metallic NWs can also be self-assembled, and the surface functionality is crucial for their two-dimensional self-assembly because specific attractive and repulsive interactions between the selectively functionalized segments can

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Fig. 3. (a) Alignment of an individual Ni/Bi/Ni NW on Ni lines. (b) An optical image of an individual Ni/Au/Ni NW between solder-plated Ni electrodes. (c) Magnetic alignment of the Ni/Au/Ni NWs. (d–f) Optical images of magnetically aligned Ni/Au/Ni NWs at (d) 45°, (e) 90°, and (f) 135° with respect to the ferromagnetic lines. Reprinted with permission from Ref. [58], copyright 2005, American Chemical Society.

direct the assembly of the segmented hybrid NWs [54]. For example, Searson and co-workers [55] demonstrated directed end-to-end assembly of three-segment Au/Pt/Au NWs using the biotin/avidin linkage. Biotin-terminated thiol and butane isocyanide were bound to the Au-NW and Pt-NW segments, respectively. Then, avidin is bound to the biotin groups on the Au-NW segments in aliquots of bifunctionalized NWs. The coupling between the three-segment NWs with avidinterminated and biotin-terminated Au-NW segments results in directed end-to-end assembly. Using a similar approach, they further demonstrated the directed end-to-end assembly of three-segment Au/Ni/Au NWs [56]. As for the kinetics of the assembly process, it was found that the directed end-to-end assembly of segmented metallic NWs is similar to the polycondensation of linear polymers. It was also demonstrated that multi-segment Au/Ni NWs could self-assemble into highly stable three-dimensional microstructures, guided by magnetic interactions between Ni-NW segments [57]. Hangarter and coworkers [58] demonstrated assembling, positioning, and spatial manipulating of three-segment Ni/Au/Ni NWs and Ni/Bi/ Ni NWs on ferromagnetic contacts. By applying an external magnetic field, magnetic alignment of these segmented hybrid NWs could be achieved (Fig. 3a–c). The directionality of the segmented hybrid NWs can be controlled by the magnetic field, with angles from 45° to 135° with respect to the electrodes being achieved (Fig. 3d–f). In addition, researchers also used the AAO-nanochannel-confined ECD for constructing a large variety of segmented metal/metal (alloy) NWs, such as Pt/Ni NWs [59], Pt/Ru NWs, and Pt/RuNi NWs [47] with high catalytic activity in room-temperature electrooxidation of methanol under acidic conditions, Au/Ag NWs with multiple surface plasmon resonances [60], Ni/CuSn/Ni NWs that could be magnetically manipulated [11], as well as NiFe/Cu/Co NW [61] and FeMn/NiFe/Co/Cu/Co/NiFe/Cu NW [62] spin-valve structures. Segmented metallic NWs achieved via AAO-nanochannel-confined ECD can also be utilized to create structures with controllable gaps between the desired metal segments. These structures could be achieved by using a synthetic strategy termed on-wire lithography that was pioneered by Mirkin and co-workers [63]. In the so-called on-wire lithography process (Fig. 4a), multi-segment Au/Ni or Au/Ag NWs were first deposited in the AAO-nanochannels via ECD. After dissolving the AAO membrane, the hybrid NW aqueous suspension was deposited on a solid substrate. Then a thin support layer (SiO2 layer or Au/Ti bi-layer) was deposited on the NWs-coated substrate by CVD or thermal evaporation. Selective chemical etching of the sacrificial segments (Ni or Ag) eventually yielded free-standing Au-NWs with controllable gaps (Fig. 4b and c). The gap size could be tuned by controlling the length of the sacrificial metal segments, and in this case, can be limited to as small as 5 nm or even down to approximately 1 nm [64]. The on-wire lithography method allowed the fabrication of 1D wire structures with precise control over the compositions, morphologies, locations, and sizes of the blocks, as well as the locations and sizes of the gaps between the NW-blocks. The

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Fig. 4. (a) Schematic of the procedure for the on-wire lithography. (b) SEM image of the Au-NW with gaps of different sizes. (c) SEM image of the Au-NW with a gap of 5 nm. Reprinted with permission from Ref. [63], copyright 2005, The American Association for the Advancement of Science.

blocks were tunable to be wire and disk of different materials, including Au, Pt, Ag, and Ni [65–68]. These structures could be used to create a wide variety of functional architectures, such as molecular transport junctions [65,69], field effect transistors [70], electrical nanotraps [71], catalytic nanorotors [72], novel energy conversion materials [67], encoded materials [73], and probes for bio-diagnostics assays [73–75]. Additionally, Mirkin and co-workers also used the on-wire lithography method to fabricate free-standing bimetallic nanorings and nanoring arrays [76]. They found that the nanoring dimers exhibited greater surface-enhanced Raman scattering effect than the analogous nanodisk dimers, showing potentials as labels for biological and chemical detection. Components in the segmented hybrid NWs can also be other materials rather than metals, such as conducting polymers. To deposit conducting polymer in the AAO-nanochannels, a positive potential should be applied to an organic electrolyte solution containing appropriate monomers in the ECD process. Mirkin and co-workers [77] found that segmented metal/conducting polymer NWs could behave as mesoscopic amphiphiles and self-organized into mesoscopic architectures including bundles, tubes, and sheets. For example, segmented Au/polypyrrole (Ppy) NWs were fabricated by ECD of Au into the AAO membrane, followed by electrochemical polymerization of pyrrole. The resultant segmented Au/Ppy NWs exhibited amphiphilic characteristics that originated from hydrophilic Au segments and hydrophobic Ppy segments. As a result, threedimensional microscopic tubular architectures (Fig. 5a–c) could be formed by adjusting the lengths of the Au and Ppy segments in the two-segment Au/Ppy NWs, while three-segment Au/Ppy/Au NWs could gather together to form planar sheetlike assemblies (Fig. 5d). The two-segment Au/Ppy NWs could also self-assemble into three-dimensional superstructures with various well-defined shapes by pre-designing an appropriate lithographic pattern and controlling the length ratio of the Au/Ppy segment [78]. They further investigated the electrical properties of the three-segment Au/Ppy/Au NWs [79]. It was found that the PPy segment dictated the electrical properties of the three-segment NW, while the Au segments acted as electrical lead to the microscopic circuits (Fig. 6a and b). They also demonstrated that the microelectrode device constructed from a single four-segment Au/Ppy/Cd/Au NW exhibited diode-like behavior at room temperature (Fig. 6c). In addition, Mallouk and co-workers [80] fabricated three-segment Au/Ppy/Au NWs with the Ppy-NW segments modified with proteins for developing nanoscale biosensors and assemblies. Segmented metal/conducting polymer NWs can also be used to produce field-effect transistors. For example, Chung and co-workers [81] synthesized three-segment Co/Ppy/Co NWs via AAO-nanochannel-confined ECD. Then they used the segmented Co/Ppy/Co NWs to construct field-effect transistors with output and transfer characteristics being as good as or even better than those of the Ppy-film field-effect transistors.

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Fig. 5. (a–c) SEM images of the assemblies of the two-segment Au/Ppy NWs with a block-length ratio of (a) 1:4, (b) 3:2, (c) 4:1. (d) Optical image of a planar assembly of the three-segment Au/Ppy/Au NWs and its side-view image (the inset). Reprinted with permission from Ref. [77], copyright 2004, The American Association for the Advancement of Science.

Fig. 6. (a) Current-voltage (I-V) curves for the Au segments (1–2, 3–4) within a single three-segment Au/Ppy/Au NW at room temperature and an optical microscope image of a single three-segment NW on the microelectrodes (the inset). (b) Temperature-dependent I-V curves for the PPy segment (2–3) and a plot of log r(T) vs. 1/T (upper-left inset. r: conductivity, T: temperature). (E) I-V curves for a single four-segment Au-Ppy-Cd-Au NW at room temperature. Reprinted with permission from Ref. [79], copyright 2004, American Chemical Society.

Except for metals and conducting polymers, semiconductor compounds can also be built as components in segmented hybrid NWs. For example, Skinner and co-workers [82] constructed three-segment Au/CdSe/Au NWs via AAOnanochannel-confined sequential ECD, and demonstrated the selective surface functionalization of the three-segmented NWs with self-assembled monolayers of (3-mercaptopropyl)-trimethoxysilane. Wang et al. [83] reported that surfacefunctionalized segmented semiconductor/metal NWs can be used for the detection of DNA molecules. They constructed three-segment CdTe/Au/CdTe NWs (Fig. 7a and b) via the AAO-nanochannel-confined sequential ECD and found that the segmented NWs exhibited a p-type behavior. After surface functionalization of the Au-NW segment with thiol-ended single strand DNA (ssDNA) fragments, the three-segment CdTe/Au/CdTe NWs were used to fabricate sensors with a field-effect transistor configuration (Fig. 7c). These sensors could be used for ultra-sensitive detection of specific ssDNA sequences at a very low concentration based on the modulation of the NW conductance (Fig. 7d–g). In addition, researchers also employed

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Fig. 7. (a) SEM image of the three-segment CdTe/Au/CdTe NWs. The inset is energy dispersive X-ray spectroscopy (EDS) spectrum taken from the CdTe-NW segment. (b) Transmission electron microscopy (TEM) image of the interconnection between the CdTe-NW and Au-NW segments. (c) Schematic of the modified three-segment CdTe/Au/CdTe-NW field-effect transistor device for the detection of DNA. (d) I-V curves for a three-segment CdTe/Au/CdTe-NW device before and after immersion in water. (e and f) Conductance responds from two different three-segment CdTe/Au/CdTe-NW devices for ssDNA(II). (g) I-V curves for a three-segment CdTe/Au/CdTe-NW device in the solutions without DNA molecules, with mismatched ssDNA(III), and with matched ssDNA (II). Reprinted with permission from Ref. [83], copyright 2008, American Chemical Society.

the AAO-nanochannel-confined sequential ECD to prepare a large variety of segmented hybrid NWs with semiconductors as components, such as Ag/Cu2O NWs with their room temperature photoluminescence spectrum exhibiting UV emission band at 383 nm [84], Au/TiO2/Au NWs with nonlinear I-V response for individual NW [85], Sb/Bi NWs exhibiting a resistive switching behavior [86], as well as Au/CdSe/Au and Ni/CdSe/Ni NWs that are resistive in dark but showing pronounced visible light photoconductivity [87].

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Fig. 8. (a) Side-view SEM image of the two-segment CdS/Ppy NWs embedded inside the porous AAO membrane. (b) SEM image of a single CdS/Ppy NW. (c) Element mapping of a single CdS/PpY NW. (d) I-V curves for a single CdS/PpY NW under light illumination with different intensities at room temperature. The inset is an SEM image and EDS line analysis of the measured nanodevice. Reprinted with permission from Ref. [88], copyright 2008, American Chemical Society.

Furthermore, segmented semiconductor/conducting polymer NWs can also be constructed via sequential ECD in porous AAO membranes. For example, Guo and co-workers [88] fabricated two-segment CdS/Ppy NWs in the straight nanochannels of the AAO membranes by sequential ECD of CdS and Ppy, as shown in Fig. 8a–c. The individual CdS/Ppy NW can act as a diode with a strong photodependent rectifying effect, and the conducting property of a single segmented CdS/Ppy NW can be adjusted by tuning the intensity of the incident light (Fig. 8d). In comparison, pulsed ECD is a more convenient approach to segmented hybrid NWs in porous AAO membranes. For this purpose, a single electrolyte bath containing multiple metal ions is often used. Taking two-component electrolyte bath as an example, the cathodic potential is alternately pulsed between values above and below the reduction potential of the less noble metal [89]. As the potential is held at a less negative potential, the more noble metal is deposited exclusively. However, when the more negative potential is pulsed to deposit the less noble metal, the more noble metal is often co-deposited. Thus the concentration of the more noble metal ions must be kept at a value lower enough than that of the less noble metal ions, to ensure that the deposition of the more noble metal during the pulsing of more negative potential is limited by ion diffusion [49,50,89]. By using pulsed ECD, multi-layered metallic NWs could be created in the nanochannels of the AAO membranes. For example, Ohgai et al. [90] used pulsed ECD to construct multi-layered Co/Cu NWs. They further demonstrated that the 100-bilayered Co/Cu NWs could reach 20% giant magnetoresistance and tri-layered Co/Cu/Co NWs exhibited typical resistance switching of spin-valves. In addition, multi-layered Ni/Cu NWs with enhanced coercivity [91], multi-layered Co/Pt NWs with tunable ferromagnetism [92], and multi-layered Fe/Pt NWs with their ordered phase formed at a low annealing temperature [93], were also constructed by using pulsed ECD in the AAO-nanochannels. Recently, Liu and co-workers [94] employed the AAO-nanochannel-confined pulsed ECD to synthesize multi-layered metallic NWs and further construct inorganic nanopeapods. As an example, Pt@CoAl2O4 inorganic nanopeapods were obtained by a combination of pulsed ECD of

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Fig. 9. (a) SEM image of the multi-segmented Bi2Te3/Te NWs. (b) TEM image of the multi-segmented NWs. (c) TEM image of the two segments of an individual multi-segmented NW, (d) High-resolution TEM (HRTEM) image of each segment. (e) Schematic of the formation process. Reprinted with permission from Ref. [99], copyright 2007, American Chemical Society.

multi-layered Co/Pt NWs in porous AAO membrane and a subsequent solid-state reaction between Co and Al2O3 at a high temperature. The AAO-nanochannel-confined pulsed ECD was also extended to fabricate superlattice Sb/Bi NWs [95] and Bi/BiSb NWs [96] with promising applications in thermoelectrics. 2.1.2. Combination of ECD and other methods Segmented hybrid NWs can also be created by combining ECD with other growth methods, including CVD, annealing reaction, and electroless deposition. Metals, metal alloys, metal oxides, semimetals, and semiconductors can be constructed as components in the segmented hybrid NWs. Combining ECD with CVD, Luo and co-workers built two-segment Ag/Si NWs [97,98] and Pt6Si5/Si NWs [98] in the straight-nanochannel AAO membranes. The two-segment Ag/Si NWs were achieved by first ECD of Ag-NWs in partial AAO-nanochannels and subsequent CVD-growth of Si-NWs in the remaining AAO-nanochannels on the tips of the AgNWs. For the two-segment Pt6Si5/Si NWs, Pt-NWs were first electrodeposited in partial AAO-nanochannels, followed by a CVD process in which the Pt-Si alloy NWs were formed, and then catalyzed the growth of the Si-NWs in the remaining nanochannels on their tips. By a combination of ECD and an annealing reaction, Wang and co-workers [99] fabricated multi-segmented Bi2Te3/Te NWs (Fig. 9a–d) in the straight-nanochannel AAO membranes, by first ECD of Bi-Te alloy NWs in the AAO-nanochannels and subsequent annealing of the as-deposited NW arrays. The formation of the multi-segmented Bi2Te3/Te NWs can be attributed to the classical precipitation in alloy system consisting of three stages: nucleation, growth, and coarsening process (Fig. 9e). Using this combined method, Perego et al. [100] recently constructed three-segment Au/NiO/Au NWs in the straight-nanochannel AAO membranes, via the following procedure with three steps. First, three-segment Au/Ni/Au NWs were electrodeposited in the AAO-nanochannels. Then, mechanical polishing was performed on the sample to make the tips of the Au segments exposed to the template surface. Finally, in situ annealing of the Au/Ni/Au NWs embedded in the AAO membrane was applied, during which the Ni-NW segments were transformed to NiO-NW segments, therefore threesegment Au/NiO/Au NWs were formed. This approach allows one to obtain an entire array of metal-oxide-metal NWs whose electrical properties can be individually accessed. By a combination of ECD and electroless deposition, our group constructed two-segment Bi/CdS NWs in the straightnanochannel AAO membranes [101]. In a typical process, the Bi-NWs were first electrodeposited in half channels of the AAO membrane with a thick Au layer sputtered on one planar surface side as an electrode. Then, the remaining empty AAO-nanochannels were activated by Sn2+ and Pd2+. The Pd2+ was deoxidized by Sn2+ via a reaction of Sn2+ + Pd2+ = Pd + Sn4+, leading to Pd nanoparticles attached on the channel walls that served as seeds for the subsequent heterogeneous nucleation and growth of the CdS-NWs. Next, the AAO membrane was immersed in a mixed solution of CH3CSNH2 and CdCl2,

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Fig. 10. (a) Schematic for the formation of the metallic NWs inside the nanochannels of the Au-coated, Al-surrounded AAO template. (b) The formation process of the metallic NWs with three stages. Reprinted with permission from Ref. [102], copyright 2009, American Chemical Society.

Fig. 11. (a) SEM image of a bundle of two-segment Au/Ni NWs. (b) TEM image near the junction area of the two-segment NW, along with the SAED patterns taken from the Au-NW (c) and Ni-NW (e) segments. (d) HRTEM image of the interface of Au and Ni. Reprinted with permission from Ref. [102], copyright 2009, American Chemical Society.

resulting in the formation of the CdS-NWs on the tips of the Bi-NWs in the remaining channels. Finally, two-segment Bi/CdS NWs with polycrystalline CdS-NW and single-crystalline Bi-NW segments were achieved. Measurements of resistancetemperature relationship reveal that the Bi/CdS NW with diameters about 30 nm show semiconductor/semiconductor behavior, i.e., both of the Bi-NW and CdS-NW segments display semiconducting behavior.

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2.1.3. Galvanic deposition Previously, we developed a generic nanochannel-confined galvanic deposition technique for mono-segmented and multisegmented metallic NWs of various metals (e.g., Au, Pt, Pd, Cu, Ni, and Co), by merely infiltrating aqueous solutions of metal chloride salts into the nanochannels of AAO membranes with ring-shaped Al foil surrounded on the circumjacent outside edge and an Au layer coated on the bottom surface side (ab. Au-coated, Al-surrounded AAO templates) [102]. Redox reactions of two galvanic cells (termed as top and bottom galvanic cells) are responsible for the formation of the metallic NWs, as shown in Fig. 10a. In the top galvanic cell, some locations on the surface of the Al foil serve as anodes, where Al is oxidized into Al3+ that subsequently enters into the solution, 3þ

Al ! Al

þ 3e

The electrons then transfer to some other locations on the Al foil surface (i.e., the cathodes) across the Al foil itself. As the redox potential of metallic ions (Mn+) in the solution is much higher than that of Al, the metallic ions gain electrons at the cathodes and are reduced to metal atoms (M0),

Mnþ þ ne ! M0 In the bottom galvanic cell, the surrounding Al foil acts as an anode, where Al is oxidized into Al3+ that subsequently enters into the solution; whereas the Au layer serves as a cathode, where metallic ions are reduced into metal atoms. The redox reactions on the electrodes are the same as those in the top galvanic cell. The formation of the metallic NWs follows three different representative steps: metal atom formation on both of the Al foil top surface and the bottom Au layer, crystal nucleus formation and NW growth from metal atoms coming from the bottom galvanic cell, and NW growth from metal atoms coming from both the top and the bottom galvanic cells (Fig. 10b). On the basis of the above formation mechanism, segmented metallic NWs with various metals (e.g., Au, Pt, Pd, Cu, Ni, and Co) can be constructed in the Au-coated, Al-surrounded AAO templates. For example, two-segment Au/Ni NWs were obtained by infiltrating aqueous solutions of HAuCl4 and NiCl2 into the same piece of Au-coated, Al-surrounded AAO template in sequence. Fig. 11a shows a side-view SEM image of the two-segment Au/Ni NWs, with the two segments having different contrasts. TEM image (Fig. 11b) clearly shows the morphology of an individual two-segment Au/Ni NW. Selected area electron diffraction (SAED) patterns (Fig. 11c and e) taken from the two segments reveal that the dark contrast segment is single-crystalline Au-NW, while the light contrast segment is polycrystalline Ni-NW. HRTEM image (Fig. 11d) taken on the junction interface of the Au-NW and Ni-NW segments reveals that Au (1 1 1) plane is nearly parallel to Ni (1 1 1) plane. Similarly, three-segment Au/Ni/Au NWs could be achieved by sequentially infiltrating HAuCl4, NiCl2, and HAuCl4 aqueous solutions into the same piece of Au-coated, Al-surrounded AAO template.

Fig. 12. (a) SEM image of multi-segmented Au/Ni NTs embedded in an AAO membrane. (b and c) SEM images of the multi-segmented NTs released from the AAO membrane. Reprinted with permission from Ref. [103], copyright 2005, Wiley-VCH.

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Fig. 13. (a) Schematic of the fabrication process of the Bi-NW/Cu-NT hetero-nanostructures. (b) SEM image of the hetero-nanostructures and an enlarged image (the inset). (c) TEM image of the hetero-nanostructures and SAED patterns taken from the marked areas (the insets). (d and e) I–V curves of the BiNW/Cu-NT (d) and Cu-NW/Bi-NT (e) hetero-nanostructures at various temperatures. Reprinted with permission from Ref. [104], copyright 2007, The Royal Society of Chemistry.

Fig. 14. Schematic of the fabrication procedure for the (a) two-segment metal-NW/CNT hetero-nanostructures and (b) three-segment metal-NW/CNT hetero-nanostructures in porous straight-nanochannel AAO membranes. Reprinted with permission from Ref. [110], copyright 2010, American Chemical Society.

2.2. Segmented nanotubes Segmented NTs can also be constructed by using the straight-nanochannel AAO template-assisted approach. For example, Lee et al. [103] fabricated multi-segmented metallic NTs based on preferential ECD of metal along the channel wall surfaces of the AAO membranes decorated with metallic nanoparticles. By modifying the well-established sensitization-preactivation process applied to the AAO membranes prior to the electroless deposition, Ag nanoparticles were immobilized on the inner surfaces of the AAO-channel walls by spontaneous reduction of Ag+ by Sn2+. Sn2+ was first deposited on the channel walls by immersing the empty porous AAO membrane in an aqueous solution of SnCl2. After being dried, the resultant AAO membrane was soaked in an aqueous solution of AgNO3. This cycle repeated about six times, leading to the uniform deposition of Ag nanoparticles on the surfaces of the AAO-channel walls via a reaction of 2Ag+ + Sn2+ ? 2Ag + Sn4+. Subsequent ECD of metals in the Ag-nanoparticle-decorated AAO-nanochannels leads to the formation of metallic NTs. Multi-segmented Au/Ni NTs could be built by periodically altering the electrolyte during ECD process inside the nanoparticle-decorated

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AAO-nanochannels. Fig. 12 shows the morphology of the resultant multi-segmented Au/Ni NTs embedded in the straight nanochannels of the AAO membrane (Fig. 12a) and those released from the membrane (Fig. 12b and c), clearly showing the stacking configuration of the multi-segmented metallic NTs. 2.3. Segmented NW/NT hetero-nanostructures 2.3.1. Electrochemical deposition ECD could be used for constructing not only segmented hybrid NWs but also segmented NW/NT hetero-nanostructures in the straight-nanochannel AAO membranes. For example, our group fabricated metal-semimetal NW/NT heteronanostructures by the AAO-nanochannel-confined ECD [104]. We built Bi-NW/Cu-NT hetero-nanostructures by first ECD of Cu-NTs in partial channels of the AAO membranes with one side coated with a mesh-like Au layer to leave the pores open, and then ECD of Bi-NWs in the remaining straight nanochannels of the AAO membrane (Fig. 13a). The resultant Bi-NW/CuNT hetero-nanostructures exhibit obvious interfaces between the Bi-NW segment and the Cu-NT segment (Fig. 13b). The junction between Cu-NT and Bi-NW can also be observed in a TEM image (Fig. 13c). The SAED patterns (the insets in Fig. 13c) indicate that Cu-NT is polycrystalline and Bi-NW is single-crystalline. Using a modified process applied to the Bi-NW/Cu-NT hetero-nanostructures, Cu-NW/Bi-NT hetero-nanostructures could also be constructed by changing the ECD sequence to Bi-NTs first and then Cu-NWs. We further found that the Bi-NW/Cu-NT hetero-nanostructures exhibited metal/metal behavior, while the Cu-NW/Bi-NT hetero-nanostructures showed metal/semiconductor behavior (Fig. 13d and e). 2.3.2. Combination of ECD and CVD Segmented NW/NT hetero-nanostructures can also be synthesized by combining ECD with CVD in the porous straightnanochannel AAO membranes. Metals and semiconductor compounds can be used as components in the NW segments, while the NT segments can be composed of carbon and semiconductors. Segmented metal-NW/NT hetero-nanostructures are often constructed by the combination of ECD and CVD in the porous straight-nanochannel AAO membranes. A general procedure for the fabrication of two-segment and three-segment metalNW/NT hetero-nanostructures is shown in Fig. 14. Before the ECD process, a thin Ag layer was coated on one face of the straight-nanochannel AAO membrane to serve as a working electrode. The two-segment metal-NW/NT heteronanostructures can be obtained by first ECD of metal NWs in the bottom half nanochannels and subsequent CVD growth of the NTs in the top remaining channels. Modify the procedure for the two-segment hetero-nanostructures mentioned above, three-segment metal-NW/NT hetero-nanostructures can also be achieved. First, sacrificial metal (Cu or Co) NWs were

Fig. 15. (a and b) SEM images of the two-segment Au-NW/CNT hetero-nanostructures. (c and d) SEM images of a single two-segment Au-NW/CNT heteronanostructure (c) and a single three-segment CNT/Au-NW/CNT hetero-nanostructure (d). Reprinted with permission from Ref. [106], copyright 2006, American Institute of Physics.

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Fig. 16. (a) (i) The schematic formation process of bifunctional assemblies from two-segment Au-NW/CNT hetero-nanostructures. (ii–iv) optical images of self-assembled golden spheres (ii), a larger golden droplet formed by merging the smaller spheres using ultrasound (iii), and wrinkled golden sphere removed from the solution and dried (iv). (b) Schematic (i) and an optical image (ii) of a large black sphere formed by the reverse assembly of the twosegment Au-NW/CNT hetero-nanostructures. Reprinted with permission from Ref. [107], copyright 2008, American Chemical Society.

electrodeposited in part of the straight AAO-nanochannels, followed by ECD of the desired metal NWs on their tops in the channels. After chemical etching of the Ag layer and the sacrificial metal NWs, NTs were grown in both of the two end remaining empty channels of the AAO membrane via CVD. At the earliest, Luo et al. synthesized two-segment Ni-NW/ CNT [97,98] and Ag-NW/a-CNT [98] hetero-nanostructures using this approach, and further found that the contacts between Ag-NW and a-CNT of every heterojunction were ohmic. Subsequently, they constructed three-segment Ni-NW/multi-walled CNT (MWCNT)/a-CNT hetero-nanostructures and found that the Ni-NW/MWCNT and MWCNT/a-CNT heterojunctions possessed distinct electrical properties, showing characteristics of nearly ideal Schottky contacts [105]. Ajayan and coworkers [106] used this AAO template-assisted combinatorial method for constructing two-segment and three-segment metal-NW/CNT hetero-nanostructures. The morphologies of the resultant two-segment Au-NW/CNT heteronanostructures (Fig. 15a–c) and three-segment CNT/Au-NW/CNT hetero-nanostructures (Fig. 15d) can be clearly observed. The interfaces between Au-NWs and CNTs are obvious. They further demonstrated that the two-segment Au-NW/CNT hetero-nanostructures could sense their environment in a mixture of two immiscible solutions, e.g., water and dichloromethane (DCM) [107]. When DCM drops were added to water containing dispersed Au-NW/CNT hetero-nanostructures, the hetero-nanostructures oriented themselves at the hydrophobic/hydrophilic interface between the two liquids, to form assemblies with Au segments pointing outward and CNT segments facing inward. It should be noted that the smaller assemblies could also merge to form a larger one under ultrasound (Fig. 16a). When water was introduced into the DCM, assemblies with reverse structures of CNT segments facing outward and Au segments pointing inward were formed (Fig. 16b). They also demonstrated that these properly designed assemblies could be manipulated by using a magnetic field and optical irradiation. The two-segment Au-NW/CNT hetero-nanostructures could also be employed as electrodes for fabricating high power supercapacitors (Fig. 17a–e) [108]. Compared to CNT electrodes, the Au-NW/CNT electrodes exhibited large improvement in power density, being attributed to the low contact resistance arising from the well-adhered interface between the Au-NW and CNT junctions. Ajayan et al. also demonstrated that two-segment Au-NW/(CNT@MnO2-NT) heteronanostructures could serve as efficient electrodes for supercapacitors [109]. The two-segment Au-NW/(CNT@MnO2-NT) hetero-nanostructures were fabricated by infiltrating MnO2-NTs on the tops of the Au-NWs embedded in the half channels of the AAO membrane before CVD-growth of the CNTs. The hetero-nanostructure electrodes showed further improvement in specific capacitance, energy, and power densities of a supercapacitor. Other materials rather than carbon could also be used as component of the NT segment in the segmented NW/NT hetero-nanostructures. For example, we fabricated two-segment Au-NW/Si-NT hetero-nanostructures and three-segment Si-NT/Au-NW/Si-NT hetero-nanostructures by the combination of ECD and CVD in the porous straight-nanochannel AAO membranes [110].

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Fig. 17. (a) Schematic of the supercapacitor device with Au-NW/CNT electrodes. (b and c) Cyclic voltammograms measured at different scan rates. (d) The galvanostatic charge-discharge behavior for supercapacitor using Au-NW/CNT electrodes. (e) Nyquist plots for the Au-NW/CNT and CNT electrodes. The inset shows an enlarged scale. Reprinted with permission from Ref. [108], copyright 2008, The Royal Society of Chemistry.

Except for metallic NWs, semiconductor compound NWs could also be built in the segmented NW/NT heteronanostructures by using the combination of ECD and CVD in the straight-nanochannel AAO membranes. For example, our group synthesized two-segment CdS-NW/CNT hetero-nanostructures and three-segment CNT/CdS-NW/CNT heteronanostructures by using this combinatorial method [111]. For the two-segment CdS-NW/CNT hetero-nanostructures, a similar approach to that for the metal-NW/NT hetero-nanostructures was applied, namely by first ECD of CdS-NWs in partial straight nanochannels of the AAO membrane and then growing CNTs in the remaining empty channels via CVD. By modifying the above approach, three-segment CNT/CdS-NW/CNT hetero-nanostructures could also be obtained by selectively etching part of the deposited CdS-NWs before CVD-growth of the CNTs. We further found that the CdS-NW/CNT hetero-nanostructures showed Ohmic-like behavior [111].

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Fig. 18. (a) SEM image of the MnO2-NW@PEDOT-NT nanocables. (b) TEM image of a single nanocable. (c and d) EDS mapping of S and Mn from the boxed area in (b). (e) The specific capacitance of MnO2-NWs (closed blue square), PEDOT-NWs (open purple dots), MnO2 thin film (open green square), and MnO2NW@PEDOT-NT nanocables (closed red dots) at difference charge/discharge current densities. Reprinted with permission from Ref. [117], copyright 2008, American Chemical Society.

Fig. 19. (a) Schematic for the synthesis of Bi-NW@Cu-NT and Cu-NW@Bi-NT coaxial nanocables. (b) HRTEM image of Bi-NW@Cu-NT nanocable and the SAED pattern (the inset). (c and d) TEM images of the middle (c) and end (d) segments of one nanocable. (e-g) Cu-NW@Bi-NT nanocables with the same outer diameter but different shell thicknesses and core diameters. Reprinted with permission from Ref. [119], copyright 2008, American Institute of Physics.

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3. Coaxial nanocables Coaxial nanocables, consisting of core@shell heterojunctions in the radial direction, have potential applications in transistors [15,112], solar cells [113,114], and logical gates [114]. In this section, we focus on the synthesis of coaxial nanocables with cores of different structures including solid NWs, hollow NTs, and hybrid nanostructures of diverse materials in the straight-nanochannel AAO membranes, via various deposition techniques, such as ECD, the combination of ECD and other methods, etc. 3.1. Coaxial nanocables with solid cores 3.1.1. Electrochemical deposition One-step and two-step ECD can be used for the synthesis of solid-core nanocables in the straight-nanochannel AAO membranes. In a one-step ECD process, solid-core nanocables are often formed in the porous AAO membranes by using one electrolyte. In a two-step ECD process, the core and shell materials are often electrodeposited separately in porous AAO membranes by using different electrolyte. A large variety of materials that are amenable to ECD, such as metals, metal oxides, semimetals, semiconductors, and polymers, can be used as components in the solid cores and the shells of the coaxial nanocables. One-step ECD can be employed for the fabrication of coaxial nanocables with metallic shells in the straight-nanochannel AAO membranes since some materials will react with the AAO-channel wall to form metallic shells. For example, Wang and co-workers [115] fabricated Cu-NW@Ni-NT nanocables by co-depositing Ni and Cu atoms in the straight AAO-nanochannels. During the co-electrodeposition process, Ni ions were adsorbed on the AAO-channel walls by a chemical complexation through hydroxyl groups. Using a similar approach, Bi-NW@Ni-NT nanocables could also be synthesized by Jia and coworkers [116]. Solid-core nanocables with shells of other materials, such as polymers and semiconductors, could also be constructed via one-step ECD in the straight-nanochannel AAO membranes. For example, Liu et al. [117] used the one-step ECD for constructing MnO2-NW@poly(3,4-ethylenedioxythiophene)(PEDOT)-NT coaxial nanocables in the straight-nanochannel AAO membranes. Fig. 18a–d shows the morphology and composition analysis of the coaxial nanocables. The structural parameters of the coaxial nanocables, such as the shell thickness and the cable length, can be easily controlled by varying the applied potential. The MnO2-NW@PEDOT-NT nanocables exhibited high specific capacitances at a high current density (Fig. 18e), and could serve as promising electrochemical energy storage materials. Zhu and co-workers [118] used the one-step ECD for constructing Ni-NW@TiO2-NT nanocables in the straight-nanochannel AAO membranes, where the TiO2-NT shell and the Ni-NW core were formed in the nanochannels simultaneously. It was referred that the hydrolysis of the TiF4 was responsible for the formation of TiO2-NTs on the inner wall surface of the nanochannels and the deoxidization of the Ni ions led to the formation of the Ni-NW core from the bottom of the nanochannels. In the meantime, two-step ECD was also developed for the synthesis of the solid-core nanocables in the straightnanochannel AAO membranes. For example, our group used the two-step ECD for constructing Bi-NW@Cu-NT and CuNW@Bi-NT nanocables in the straight-nanochannel AAO membranes [119]. The fabrication process is shown in Fig. 19a. Firstly, the nanocable shells were electrodeposited on the channel walls of the AAO membrane coated with a mesh-like thin Au layer covering the top-view surface of pore walls but still leaving the pores open. Then, the cavities of the shells were filled by a second ECD process to obtain the solid core of the nanocables. The resultant Bi-NW@Cu-NT nanocable can be obviously seen in Fig. 19c and d. HRTEM image together with the SAED pattern (Fig. 19b) reveals that the preferred growth direction of the Bi-NW core is along [0 0 3] orientation. Fig. 19e-g are SEM images of the Cu-NW@Bi-NT nanocables with the same outer diameter but different diameters of the inner cores. The diameter of the cable core and the thickness of the cable shell can be tailored by tuning the thickness of the sputtered Au layer. Usually, thinner sputtered Au layer leads to nanocables with thinner shells and thicker cores. Using a similar approach, Li et al. [120] fabricated Cu-NW@Ni-NT nanocables with a high remanence ratio. 3.1.2. Combination of ECD and other methods Coaxial nanocables with solid cores can also be constructed by combining ECD with other methods, such as chemical etching, sol-gel technique, and CVD, in the straight-nanochannel AAO membranes. The solid cores of these nanocables can be composed of metals, metal alloys, semimetals, semiconductors, and conducting polymers that are amenable to ECD, while the shell materials can be metal oxides, semiconductors, and polymers. Coaxial nanocables with insulating shells (e.g., SiO2 and Al2O3) are ideal components in nanoelectronics because the shells can act as a protective layer, gate oxide, and dielectric [15,121,122]. A combination of ECD and chemical etching was often used to construct Al2O3-sheathed coaxial nanocables. The Al2O3-sheathed nanocables were often constructed by first ECD of NW cores in the straight nanochannels of the AAO membranes and then chemical etching part of the AAO membranes. Using this approach, Fe(or Cu)-NW@Al2O3-NT [123] and InSb-NW@Al2O3-NT [124] nanocables were achieved. For a generic approach to the Al2O3-sheathed coaxial nanocables, we recently developed a unique two-layer-pore AAO membrane and constructed Al2O3-sheathed nanocables with solid cores of a large variety of materials, such as metals (e.g., Cu or Zn), semimetals (e.g., Bi), semiconductor compounds (e.g., CdS or ZnO), and conducting polymers (e.g., Ppy) [125]. The unique

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Fig. 20. (a) Schematic fabrication procedure of the two-layer-pore AAO membrane. (b-d) Side-view (b), top-view (c), and bottom-view (d) SEM images of the two-layer-pore AAO membrane. Reprinted with permission from Ref. [125], copyright 2011, Wiley-VCH.

two-layer-pore AAO membranes, consisting of parallel two-end-through pores (denoted as through-pores) and top-throughbottom-capped pores (denoted as capped-pores), with each through-pore surrounded by six capped pores in a hexagonal arrangement after removing the remaining Al foil and the bottom barrier layer, were obtained by increasing the anodizing pffiffiffi voltage by a factor of 3 in the final anodization stage of the Al foils (Fig. 20a). The bottom cross-section, the upper layer, and the lower layer of the two-layer-pore AAO membrane can be observed in Fig. 20b, c, and d, respectively. We take the Cu-NW@Al2O3-NT nanocables as an example to introduce the synthetic procedure (Fig. 21a). Firstly, a metal layer was sputtered onto the bottom planar surface side (with only the through-pore opening) of the two-layer-pore AAO membrane as the working electrode. Then, solid Cu-NWs were electrodeposited inside the through-pores. After chemical etching from the capped-pores, Cu-NW@Al2O3-NT nanocables with both hexahedral (Fig. 21b and c) and columnar (Fig. 21d and e) sheaths were formed. Usually, shorter etching duration leads to the hexahedral Al2O3-sheaths with a larger thickness, while longer etching duration results in columnar Al2O3-sheaths with a smaller thickness, and much longer etching duration leads to much thinner Al2O3-sheaths (Fig. 21f). Using this method, ZnO-NW@Al2O3-NT nanocables could also be constructed, where the ZnO-NW cores were first achieved by using a combined method reported previously [126], i.e., electrodepositing pure metal Zn-NW cores inside the through-pores of the two-layer-pore AAO membranes and then converting the Zn-NWs into ZnO-NWs by simple oxidation. The sol-gel technique can also be incorporated with ECD to create coaxial nanocables with solid cores in the straightnanochannel AAO membranes. The solid-core nanocables were often synthesized by first depositing NT-shells in the straight AAO-nanochannels via a sol-gel technique and then ECD of NW-cores within the cavities of the NTs. Using this combined method, solid-core nanocables with oxide shells, such as Co-NW@ZrO2-NT [127], Co-NW@TiO2-NT [128], and Au-NW@SiO2-NT [129], were constructed. It should be noted that polymers can also be built as shells of the solid-core nanocables. For example, by first ECD of polymer NT-shells in the straight nanochannels of the AAO membranes and then depositing metal oxide NW-cores into the as-prepared NTs via a sol-gel technique, ZnO-NW@polyaniline (PANI)-NT nanocables were constructed by Zheng and co-workers [130]. The photoluminescence (PL) spectrum of the ZnO-NW@PANI-NT nanocables embedded in the AAO membrane exhibited about 100 times PL enhancement when compared with that of the ZnO-NWs embedded in the AAO membrane. This could be ascribed to the existence of the PANI-NTs.

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Fig. 21. (a) The fabrication process of the Cu-NW@Al2O3-NT nanocables. (b and c) Top-view SEM image (b) and TEM image (c) of the nanocables with hexahedral sheaths, the inset in (c) is the SAED pattern. (d and e) Top-view SEM image (d) and TEM image (e) of the nanocables with columnar sheaths. (f) A typical nanocable obtained by etching at 30 °C for 2 h. Reprinted with permission from Ref. [125], copyright 2011, Wiley-VCH.

A combination of CVD and ECD can also be used for the synthesis of coaxial nanocables with solid cores in the straightnanochannel AAO membranes. In this case, the fabrication starts with CVD-growth of NT-shells in the straight nanochannels of the AAO membrane and subsequent ECD of NW-cores in the cavities of the as-prepared NTs. For example, Bao and coworkers [131] synthesized Co-NW@CNT nanocables and found that the nanocables exhibited enhanced coercivity when compared with bulk Co. Lately, Zhang et al. [132] fabricated Ppy-NW@ZnS-NT nanocables and found that the ZnS-NT showed semiconducting behavior and Ppy-NW exhibited ohmic behavior. Moreover, the I-V characteristics of a single Ppy-NW@ZnSNT nanocable exhibited a rectification behavior due to the electron transfer between Ppy and ZnS (Fig. 22). In addition, researchers also employed the combination of ECD and other methods for solid-core nanocables in the straight-nanochannel AAO membranes. For example, a combination of ECD and heat treatment was used to construct BiNW@Bi2O3-NT nanocables [133]. Bi-NW@SiO2-NT nanocables could be synthesized by a combination of ECD and atomic layer deposition (ALD) [122]. The Bi-NW@SiO2-NT nanocables with the core diameters of 30 nm and 50 nm are semiconducting and those with the core diameters of 200 nm exhibit semimetallic behavior. By combining ECD with high-temperature chemical reduction, FeCo-NW@CoPt-NT nanocables could be fabricated [134]. Further experiments reveal that the hysteresis loop of the FeCo-NW@CoPt-NT nanocable array shows two apparent kinks, due to partial or even complete decoupling of the hard-magnetic phase from the soft-magnetic phase. In addition, Co-NW@BaTiO3-NT nanocables could also be synthesized by a combination of ECD and template wetting [135]. It was further demonstrated that the nanocables exhibited room temperature ferromagnetism and ferroelectricity.

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Fig. 22. (a) Schematic of a top-contact nanoscale device. (b) Optical image and (c) SEM image of a single Ppy-NW@ZnS-NT nanocable with top-contact electrodes. (d) I-V curves of a single Ppy-NW, ZnS-NT, and Ppy-NW@ZnS-NT nanocable. Reprinted with permission from Ref. [132], copyright 2011, American Chemical Society.

3.1.3. Other methods Other methods, such as supercritical fluid inclusion process, sol-gel technique, and a combination of galvanic deposition technique and oxidation can also be employed for the synthesis of the solid-core nanocables inside the porous AAO membranes. Supercritical fluid inclusion process (using the supercritical fluid as an inclusion medium) was first developed by Holmes and coworker to construct Si-NWs in mesoporous silica [136]. The high diffusivity and low viscosity of the supercritical fluid lead to rapid diffusion of reactant precursor into the pores of the mesoporous silica, where nucleation and growth of the SiNWs readily occur. Subsequently, they used this technique to fabricate Co-NWs, Cu-NWs, and Fe3O4-NWs in the mesoporous silica [137]. After that, the supercritical fluid inclusion process was also employed for constructing solid-core nanocables in the straight-nanochannel AAO membranes. The synthesis of the nanocables followed a two-step process, in which the NT-shells were first deposited on the channel walls of the AAO membrane and subsequently filled with the core material to form coaxial nanocables. Using the approach, they constructed Ge-NW@Co-NT nanocables and found that the nanocables were ferromagnetic with near room temperature Curie temperature [138]. Later, they fabricated Co-NW@Fe3O4-NT and Fe3O4-NW@Co-NT nanocables [139]. Fig. 23a and b show the morphology of the Co-NW@Fe3O4-NT nanocable and the Fe3O4-NW@Co-NT nanocable, respectively. They found that these nanocables exhibited ferromagnetic and ferrimagnetic properties at room temperature with the saturation magnetization up to 20% (Fig. 23c and d), and demonstrated that the magnetic properties of the nanocables were mainly determined by the cores. The solid-core nanocables could also be constructed by a sol-gel technique in the straight-nanochannel AAO membranes. Using this technique, Zhao and co-workers [140] fabricated SnO2-NW@a-CNT nanocables in the straight-nanochannel AAO membranes. The a-CNTs were in situ formed on the as-prepared SnO2-NWs during the sol-gel process, due to the pyrolysis of citric acid under anoxic conditions in the nanochannels of the porous AAO membranes. It was also demonstrated that the SnO2-NW@a-CNT nanocables could be used as anodes in lithium ion batteries with much-improved cycling performance. In addition, our group used a combination of galvanic deposition technique and oxidation for the synthesis of AuNiNW@NiO-NT coaxial nanocables in the straight-nanochannel AAO membranes [141]. Using a galvanic deposition technique similar to that for the fabrication of the two-segment Au/Ni NWs [102], AuNi-NW@Ni-NT nanocables were first deposited in the nanochannels of the AAO membrane by using a mixed aqueous solution of NiCl2 and HAuCl4. Subsequent immersion of

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Fig. 23. (a and b) TEM image of a single Co-NW@Fe3O4-NT nanocable (a) and Fe3O4-NW@Co-NT nanocable (b). (c and d) Magnetization curves for the CoNW@Fe3O4-NT nanocables and the Fe3O4-NW@Co-NT nanocables measured at 300 K (c) and 1.8 K (d). Reprinted with permission from Ref. [139], copyright 2006, Wiley-VCH.

the AuNi-NW@Ni-NT nanocable-embedded AAO membrane in an aqueous NaOH solution led to the oxidation of the Nisheaths to form AuNi-NW@NiO-NT coaxial nanocables.

3.2. Coaxial nanocables with hollow cores Coaxial nanocables can also be built with hollow cores rather than the usual solid cores in the straight-nanochannel AAO membranes by using various approaches, such as ECD, combinatorial methods, ALD, and wet chemical approach. The core materials can be metals, metal oxides, semiconductors, and conducting polymers. The AAO-nanochannel-confined sequential ECD can be used for constructing the hollow-core coaxial nanocables. For example, Park et al. [142] built Ppy-NT@Ni-NT nanocables by first ECD of Ppy-NTs in the straight nanochannels of the AAO membrane and subsequent ECD of Ni-NTs outside the Ppy-NTs. It was demonstrated that the Ni-shells of the nanocables had an anisotropic ferromagnetic nature with the maximum of coercivity and remanent-saturation magnetization when a magnetic field was applied along the parallel direction of the NTs. Liu and co-workers [143] fabricated RuO2-NT@PEDOTNT nanocables (Fig. 24a) via the AAO-nanochannel-confined sequential ECD. The nanocables were achieved by first ECD of PEDOT-NTs in the straight nanochannels of the AAO membrane and subsequent ECD of RuO2-NTs inside the inner walls of the PEDOT-NTs. The RuO2-NT@PEDOT-NT nanocables exhibited high specific capacitance and fast charging/discharging capability (Fig. 24b and c), with potential application as promising electrode materials for high-energy and high-power supercapacitors. The authors demonstrated that the addition of RuO2 to the PEDOT-NTs dramatically enhanced the energy density of the PEDOT-NTs and the high surface area of the nanocable allowed the realization of its high specific capacitance. Hollow-core coaxial nanocables can also be constructed in the straight-nanochannel AAO membranes by combinatorial methods, such as a combination of ECD and CVD, a combination of ECD and chemical etching, and a combination of CVD and chemical etching. At the earliest, Bao and co-workers [144] synthesized Ni-NT@CNT nanocables in the straight-nanochannel AAO membranes via a combination of ECD and CVD. The nanocables were fabricated first by ECD of Ni-NTs in the straight nanochannels of the AAO membrane and then CVD-growth of CNTs outside the as-prepared Ni-NTs. It was found that the electrodeposited Ni retained its tubular structure, but the diameter of the Ni-NTs became smaller after CVD-growth of the CNTs. Based on this phenomenon, the authors proposed a possible formation mechanism for the CNTs. Later on, our

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Fig. 24. (a) SEM image of the RuO2-NT@PEDOT-NT nanocables after removing the AAO membrane. (b) Cyclic voltammograms of the PEDOT-NTs nanocables (- - -) and the RuO2-NT@PEDOT-NT nanocables (—). (c) A plot of energy density versus power density (Ragone plot) for the RuO2-NT@PEDOT-NT nanocables obtained at different charge/discharge current densities. Reprinted with permission from Ref. [143], copyright 2010, PCCP Owner Societies.

group demonstrated the use of a combination of ECD with chemical etching for the fabrication of Ni-NT@Al2O3-NT nanocables, by using our unique two-layer-pore AAO membranes (as shown in Fig. 20) [125]. It should be noted that for ECD of NiNTs as the Al2O3-sheathed nanocable cores, a mesh-like thin Au layer has to be sputtered onto the bottom planar surface side of the AAO membrane. By using the unique two-layer-pore AAO membranes, our group also demonstrated the synthesis of CNT@Al2O3-NT and Si-NT@Al2O3-NT nanocables by first CVD-growth of CNTs (or Si-NTs) within the through-pores of the unique two-layer-pore AAO membranes and then chemical etching from the empty capped-pores [125]. Sequential ALD could also be used for constructing hollow-core coaxial nanocables in the straight-nanochannel AAO membranes. For example, Liu and co-workers [145] deposited V2O5-NT@Ru-NT nanocables in the nanochannels of the AAO membranes via sequential ALD, to serve as the anode and cathode of a battery. This battery is composed of an array of nanobatteries connected in parallel. Each of the nanobatteries is composed of an anode, a cathode, and a liquid electrolyte

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Fig. 25. (a) The schematic fabrication process of the NiSn/PANI-NW@PEO-NT nanocables. (b) TEM image of the NiSn-NW@PEO-NT nanocable segment. (c) Schematic of the nanocable. (d) TEM image of the PANI-NW@PEO-NT nanocable segment. (e) Voltage versus time graphs for the galvanostatic measurements conducted on the nanocable energy storage device. (f) Discharge capacity versus cycle number plot showing reversible capacities at three different current rates (0.03, 0.05, 0.07 mA/cm2) up to 30 cycles. Reprinted with permission from Ref. [149], copyright 2011, American Chemical Society.

region. Sequential ALD was also employed for the synthesis of TiN-NT@CNT nanocables in the straight-nanochannel AAO membranes, which could serve as negative electrode materials for aqueous based supercapacitors [146]. In addition, a wet chemical approach could also be used for fabricating CdS-NT@TiO2-NT nanocables in the straight-nanochannel AAO membranes [147]. 3.3. Coaxial nanocables with hybrid nanostructures as inner cores 3.3.1. Segmented hybrid NWs as inner cores of the coaxial nanocables Coaxial nanocables with segmented hybrid NWs as inner cores can be constructed via the combination of ECD with other techniques, such as sol-gel technique, chemical etching, and drop coating, in the straight-nanochannel AAO membranes. Kovtyukhova and co-workers [148] combined sequential ECD with a sol-gel technique to fabricate Au/CdS/Au-NW@SiO2-NT nanocables in the straight-nanochannel AAO membranes, via first surface sol-gel deposition of SiO2-NTs on the straight channel walls of the AAO membrane and then sequential ECD of each component of the segmented NWs inside the SiO2-NTs. It was demonstrated that the Au/CdS/Au-NW@SiO2-NT nanocable-based thin-film transistors could operate at drain voltages lower than 1 V and showed better ON/OFF current ratio, threshold voltage, and subthreshold slope than the chemically similar planar thin-film transistors. By using the unique two-layer-pore AAO membranes mentioned above, our group constructed Au/CdS-NW@Al2O3-NT and Au/Ge-NW@Al2O3-NT nanocables via the combination of ECD and other techniques [125]. The fabrication of theses nanocables is similar to that for the Cu-NW@Al2O3-NT nanocables (as shown in Fig. 21a), i.e., deposition of segmented NW cores inside the through-pores of the two-layer-pore AAO membranes and then chemical etching of partial walls of the AAO membranes. The Au/CdS-NW@Al2O3-NT nanocables were obtained by sequential ECD of Au/CdS-NWs inside the through-pores of the two-layer-pore AAO membranes and then chemical etching of partial AAO walls. The Au/GeNW@Al2O3-NT nanocables were fabricated by ECD of Au-NWs inside the through-pores of the two-layer-pore AAO membranes, CVD-growth of Ge-NWs at the end of the Au-NWs, and then chemical etching of partial AAO walls. Coaxial nanocables with segmented hybrid NWs as inner cores can also be built by combining ECD with drop coating. For example, Ajayan and co-workers [149] synthesized NiSn/PANI-NW@PEO-NT nanocables in the straight-nanochannel AAO membranes as energy storage device. The nanocable energy storage device was built by first ECD of NiSn-NWs in the straight nanochannels of the AAO membrane, chemical widening of the pores, and then drop coating of the separator (polyethylene oxide or PEO) and PANI (Fig. 25a–d). This device showed good charge/discharge characteristics with a discharge capacity of 3 lAh/cm2 at a current rate of 0.03 mA/cm2 (Fig. 25e and f).

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Fig. 26. (a) The schematic fabrication process of the Cu-NW/CNT@Al2O3-NT nanocables. (b) TEM image of a single nanocable and SAED pattern taken from the region marked with a white circle (the inset). (c and d) HRTEM images of the nanocable taken from the two red rectangular areas marked in (b). Reprinted with permission from Ref. [125], copyright 2011, Wiley-VCH.

3.3.2. Segmented NW/NT heterostructures as inner cores of the coaxial nanocables The inner cores of the coaxial nanocables can consist of not only segmented hybrid NWs, but also segmented NW/NT heterostructures. For example, Our group built two-segment Cu-NW/CNT heterostructures as inner cores of the Al2O3-sheathed nanocables by using the two-layer-pore AAO membranes mentioned above [125]. The fabrication process is shown in Fig. 26a. Firstly, Cu-NWs are electrodeposited inside the bottom part of the through-pores of the two-layer-pore AAO membrane with a metal layer coated on its bottom side. Then, CNTs are deposited on the tips of Cu-NWs in the remaining empty channels by CVD to form Cu-NW/CNT heterostructures. After chemically etching the metal layer and the walls between the capped-pores, Cu-NW/CNT@Al2O3-NT nanocables are formed. TEM image of a typical nanocable (Fig. 26b) shows that the inner core consists of well-connected solid NW and hollow NT. SAED pattern taken from the NW segment (the inset in Fig. 26b) reveals the single-crystalline nature of the Cu-NW. HRTEM image of the segment with CNT as core (Fig. 26c) indicates that the multi-walled CNT well adheres to the Al2O3 sheath, and the HRTEM image taken from the nanocable where the Cu-NW and the CNT meet (Fig. 26d) reveals that the Cu-NW and the CNT are well connected. Any materials, which are amenable to ECD and stable during the growth of CNTs, can be built as NW-segment in the segmented NW/NT heterostructures as inner cores of the Al2O3-sheathed nanocables.

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Fig. 27. SEM images of the TiN-NT@Al2O3-NT@TiN-NT nanocable capacitors. (a) Bottom of the nanocables, showing the AAO barrier layer and three layers of the TiN bottom electrode (BE), Al2O3, and the TiN top electrode (TE). (b) Pore openings at the top showing a similar trilayer structure. Reprinted with permission from Ref. [156], copyright 2009, Nature Publishing Group.

3.3.3. Sub-nanocables as inner cores of the coaxial nanocables The inner cores of the coaxial nanocables could also be composed of sub-nanocables with solid cores of NWs. For example, by using the two-layer-pore AAO membranes mentioned above, our group built Al2O3-sheathed nanocables with Cu2O-NW@CNT sub-nanocables as inner cores via a combination of CVD, ECD, and chemical etching [125]. The fabrication procedure of the Cu2O-NW@CNT@Al2O3-NT nanocables is shown as follows. Firstly, CNTs are deposited via a CVD process inside the through-pores of the two-layer-pore AAO membrane with its bottom side coated with a metal layer. Then, Cu2O-NWs are electrodeposited inside the CNTs embedded in the two-layer-pore AAO membrane. After chemically etching the metal layer and the pore walls between the capped-pores, Al2O3-sheathed nanocables with Cu2O-NW@CNT sub-nanocables as inner cores are obtained. In addition, via a combination of ECD with a wet chemical method, more complicated coaxial nanocables of Au-NW@multilayered poly(styrene sulfonate) (PSS)/poly(allylamine hydrochloride) (PAH) film-NT [150] and Au-NW@TiO2/W12O41-NT@PANI-NT/CNT [151] can be constructed by using the straight-nanochannel AAO membranes. It was also demonstrated that the complicated Au-NW@TiO2/W12O41-NT@PANI-NT/CNT nanocables exhibited the characteristics of a built-in p-n junction at the TiO2/PANI interface. Sub-nanocables with hollow cores of NTs can also be built as inner cores of the coaxial nanocables. For example, via a combination of ECD and electrochemical oxidation, Chen and co-workers [152] fabricated Co-NT@NiO-NT@Ni-NT nanocables in the straight-nanochannel AAO membranes. The fabrication followed a three-step procedure. Firstly, Ni-NTs were electrodeposited in the straight nanochannels of the AAO membrane. Then, electrochemical oxidation on the Ni-NTs was carried out to form NiO-NTs on the inner walls of the Ni-NTs. Finally, Co-NTs were electrodeposited inside the as-prepared NiO-NTs. Further investigations on the magnetization reversal mechanisms revealed a transition from curling to transverse in Co-NT@NiO-NT@Ni-NT nanocable arrays at large angles. Besides, the saturated magnetization of the Co-NT@NiO-NT@Ni-NT nanocables sharply increased at low temperature, being attributed to the superparamagnetic nanoparticles on the nanocables. Coaxial nanocables with hollow-core sub-nanocables as inner cores can also be constructed via alternately depositing inner-core/intermediate-shell/outer-shell materials inside the nanochannels of the AAO membranes. For example, by CVD, Shelimov and co-workers [153] grew CNT@BN-NT@CNT nanocables in the porous AAO membrane as a capacitor. For a 50-mm-thick AAO membrane, the specific capacitance of the capacitor was up to 2.5 mF/cm2. Recently, Zhao and co-workers [154] fabricated CNT@Si-NT@CNT nanocables in the straight-nanochannel AAO membranes via CVD. The coaxial nanocables embedded in the porous AAO membrane could serve as a high-capacity and long-life anode for lithium-ion battery. The anode materials have a high first Coulombic efficiency of 90% and high specific capacities (4000 mAh g1 for Si and more than 600 mAh g1 for the whole anode). Significantly, a high area specific capacity of 6 mAh cm2 was also obtained. Except for CVD, ALD can also be used for constructing coaxial nanocables with hollow-core sub-nanocables as inner cores in the straight-nanochannel AAO membranes. Using this approach, metal@insulator@metal nanocables could be constructed by depositing successive ALD layers of metal, insulator, and metal with atomic layer thickness control inside the nanochannels of the AAO membranes. For example, Lee and co-workers demonstrated that the ALD-grown TiN-NT@Al2O3-NT@TiN-NT nanocables embedded in the porous AAO membranes (Fig. 27) could serve as capacitors for energy storage [155,156]. The capacitors have a capacitance per unit planar area of 10 mF/cm2 for 1-mm-thick AAO membrane and 100 mF/cm2 for 10mm-thick AAO membrane. Later on, they used ALD for constructing AZO (Al-Zn oxide)-NT@Al2O3-NT@AZO-NT nanocable capacitors with improved electrical performance [157]. The AZO-NT@Al2O3-NT@AZO-NT nanocable capacitors were fabricated by using the porous AAO membranes with tailored nanotopography produced via an electrochemical nanoengineering technique. These capacitors have energy densities of 1.5 Wh/kg, about two times of that for the TiN-NT@Al2O3-NT@TiN-NT nanocable capacitors.

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Fig. 28. (a) SEM image of the Y-branched Pt-NWs released from the AAO membrane. (b) SEM image of some broken Y-branched Pt-NWs. (c–f) Comparison of the electrocatalytic activity of Y-branched-Pt /C, Pt-NW/C, and Pt/C. (c and d) Transient current density curves of formic acid (at 0.24 V) and ethanol oxidation (at 0.04 V), the inset in (d) shows an enlarged view. (e and f) Potential-dependent steady-state current density for both formic acid and ethanol oxidation reactions. Reprinted with permission from Ref. [162], copyright 2008, American Chemical Society.

4. Hierarchically branched 1D homo-nanostructures Hierarchically branched 1D homo-nanostructures have potential applications in drug delivery [158], nanoelectronics [159,160], and nanoelectromechanical systems [161]. As the morphologies of the nanochannels inside the porous AAO membranes can be tuned by adjusting the anodizing conditions of the Al foils, porous AAO membranes with nanochannels of more complex shapes could be constructed. Thus, 1D hierarchically branched homo-nanostructures can be built by using the porous AAO membranes with pre-designed hierarchically branched nanochannels as templates. In this section, we will first highlight how to get the porous AAO membranes with hierarchically branched nanochannels, and then how to build hierarchically branched NWs and NTs of different materials by using the porous AAO membranes with nanochannels of the corresponding shapes as templates.

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Fig. 29. (a) Schematic of the growth process of the Y-branched Ge-NWs in the Y-branched nanochannels of the AAO membranes. (b) TEM image of a typical Y-branched Ge-NW. (c) TEM image of a Y-branched Ge-NW with its two branches having different lengths. Reprinted with permission from Ref. [170], copyright 2011, American Chemical Society.

4.1. Branched NWs On the basis of our previous work [45], AAO membranes with nanochannels having a predetermined number of branches pffiffiffi can be obtained by merely reducing the applied anodizing voltage by a factor of 1/ n (n is the number of the branches) during the anodization process of the Al foils. Using the branched-nanochannel AAO membranes as templates, branched NWs of various materials, such as metals, semimetals, alloys, semiconductors, and polymers, could be achieved via various techniques including ECD, CVD, galvanic deposition, electroless deposition, and combinatorial methods. ECD is a very convenient technique for constructing branched NWs in the branched-nanochannel AAO membranes. For example, Mahima and co-workers [162] used this method for constructing Y-branched Pt-NWs in the Y-branchednanochannel AAO membranes. Fig. 28a and b show the morphology of the Y-branched Pt-NWs released from the AAO membrane. They further demonstrated that the Y-branched Pt-NWs showed unique electrocatalytic activity for formic acid and ethanol oxidation (Fig. 28c-f), demonstrating potential application as a catalyst of the fuel cell. In addition, our and other groups also employed the branched-AAO-nanochannel-confined ECD for the synthesis of branched NWs of various materials, such as Cu [163], Au [164,165], Ni [165,166], Fe [167], Ag [168], Bi [169], and Ppy [164], and investigated their thermal [163], electrical [169], and magnetic [166] properties. Except for ECD, other techniques, such as galvanic deposition, CVD, electroless deposition, and combinatorial methods, can also be used for constructing branched NWs in the branched-nanochannel AAO membranes. For example, the galvanic deposition technique (as shown in Fig. 10) that we used for constructing metallic NWs can also be used to build branched metallic NWs, such as Y-branched Au-NWs. The Y-branched Au-NWs were fabricated by infiltrating HAuCl4 aqueous solutions into the Au-coated, Al-surrounded AAO membranes with Y-branched nanochannels [102]. Later on, we fabricated Y-branched Ge-NWs in the Y-branched-nanochannel AAO membranes via a low-temperature CVD process catalyzed by metal nanorods at the channel bottom [170]. As shown in Fig. 29a, the growth of the Y-branched Ge-NWs follows five steps: the formation of liquid Au-Ge eutectic alloy nanorod at the channel bottom, nucleation of Ge nanocrystals at the solid-liquid interface of the catalyst nanorod, the growth of the NWs inside the stem channel, split of the liquid catalyst nanorod into two nanorods inside the nanochannel-branches of the AAO membrane, and the growth of Ge-NW branches inside the branched nanochannels of the AAO membrane. A typical resultant Y-branched Ge-NW released from the AAO membrane can be observed in a TEM image (Fig. 29b), with the nanorod catalysts still on the tips of the two branches. Fig. 29c shows

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Fig. 30. (a) Top-view SEM image of the CNTs aligned in the porous AAO membrane after ion-milling of amorphous carbon on the surface. Top inset shows stem part of the Y-branched CNTs. Bottom left inset is close-up of the region between the stem and branch portions still embedded in the membrane. Bottom right inset is a close-up view of the top of the CNT in its hexagonal cell. (b) TEM image of an individual Y-branched CNT. Reprinted with permission from Ref. [44], copyright 1999, Nature Publishing Group. (c and d) I-V curve for an individual Y-branched CNT (c) and their arrays (d). The inset in (c) shows the bias configuration used. The inset in (d) shows results obtained for straight CNT arrays. Reprinted with permission from Ref. [159], copyright 2000, American Physical Society.

a Y-branched Ge-NW with two branches of different lengths. It can be seen that the longer the catalyst nanorod is, the shorter the branch is. Similarly, by using AAO membranes with three-branched nanochannels as templates, threebranched Ge-NWs can also be achieved via CVD. Electroless deposition can also be employed to construct three-branched NWs [171]. For three-branched Ni-P alloy NWs, Pd nanoparticles were first attached on the channel walls of the threebranched-nanochannel AAO membrane to serve as catalysts in the following chemical reaction, by using a wellestablished sensitization-preactivation process that was used for the fabrication of the CdS segments in the two-segment Bi/CdS NWs [101]. Then, Ni-P alloy NWs were deposited in the branched nanochannels of the AAO membrane by immersing the Pd nanoparticle-modified AAO membrane into an aqueous solution containing nickel salt and hypophosphite. In addition, Jo et al. [172] used a combination of infiltration and polymerization process for the fabrication of multi-branched poly(pentafluorophenyl acrylate)-NWs in the multi-branched-nanochannel AAO membranes. They further demonstrated that the chemical modification of the multi-branched poly(pentafluorophenyl acrylate)-NWs with spiropyran amine could result in ultraviolet-responsive nanostructures.

4.2. Branched NTs Using the porous AAO membranes with branched nanochannels as templates, not only branched solid NWs but also branched hollow NTs could be constructed. The branched NTs consisting of various materials including metals, carbon, and semiconductors can be fabricated via ECD and CVD techniques in the porous AAO membranes with branched nanochannels. Our group used ECD to construct Y-branched metallic Cu-NTs in the Y-branched AAO membranes and studied their thermal properties [163]. It should be noted that the Au layer (as an electrode) coated on the planar surface of the stem-channel side of the Y-branched AAO membranes must be thin enough to leave the pores open and cover only the top-view surface of the pore walls. Otherwise, Cu-NWs rather than Cu-NTs could be formed. The thermal properties of the Y-branched Cu-NTs are similar to that of the Y-branched Cu-NWs, i.e., the thermal expansion of the Y-branched nanostructures is temperature-

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Fig. 31. (a) Schematic of two representative nanochannels with 3-generation upside-down Y-branching inside the AAO membrane. (b) SEM image of a bundle of the Ni-NWs with 3-generation Y-branched morphology. (c) TEM image of a two-generation Ni-NW broken from the 3-generation Y-branched NiNW (left) and a close-up view of the marked area (right). Reprinted with permission from Ref. [165], copyright 2010, Wiley-VCH.

dependent, and the thermal expansion coefficient of the stem with a larger diameter is more sensitive than that of the branches with a smaller diameter. CVD can be employed to construct branched CNTs in the branched-nanochannel AAO membranes. Li et al. [44] grew Ybranched CNTs in the Y-branched-nanochannel AAO membranes. As the Y-branched CNTs (Fig. 30a and b) totally replicated the morphology of the Y-branched nanochannels in the AAO membrane, the length and diameter of the stem and branches could be controlled. Experiments revealed that either individual Y-branched CNT or their arrays showed intrinsic nonlinear transport and reproducible rectifying behavior at room temperature (Fig. 30c and d), revealing their potential application in nanoelectronics [159]. Sui et al. [173] used the CVD technique for constructing multiple-branched CNTs in the AAO membranes with multiple-branched nanochannels. Meng et al. [45] systematically reported the CVD-growth of the multiplebranched (e.g., 2-, 3-, 4-, 8-, and 16-branched) CNTs in the porous AAO membranes with pre-designed number of branched nanochannels, and demonstrated that the branching number of the CNTs can be tuned by controlling the branching number of the nanochannels in the porous AAO membranes. Other materials that can be grown in tube-morphology inside the nanochannels of the AAO membranes can also be used to construct branched NTs. For example, our group fabricated Y-branched Ge-NTs in the Y-branched-nanochannel AAO membranes with the assistance of Ni nanorod catalysts at the bottom of the nanochannels, by using GeH4 as a precursor in a low-temperature CVD growth process [170]. Similarly, by using SiH4 rather than GeH4 as a precursor, three-branched Si-NTs could be fabricated via CVD inside the three-branched-nanochannel AAO membranes [165].

4.3. Multi-generation branched NWs and NTs pffiffiffi By reducing the anodizing voltage a pre-desired number of times, with each time by a factor of 1/ n during the anodization process of the Al foils, the pore-branching takes place the same number of times along the axial direction of the stem channels of the AAO membranes [45], to produce AAO membranes with tree-like multi-generation branched nanochannels. By using these AAO membranes as templates, multi-generation branched NWs could be constructed. For example, our group constructed AAO membranes with 3-generation Y-branched channels by sequentially reducing the applied anodizing voltage pffiffiffi for three times during the anodization process of the Al foils, with each time by a factor of 1/ 2 (Fig. 31a) [165]. Then NiNWs with 3-generation Y-branched morphology were built via ECD in the AAO membranes with 3-generation Y-branched channels. Fig. 31b is an SEM image of a bundle of the resultant 3-generation Y-branched Ni-NWs, showing three interfaces where each of the Y-branches takes place. The first two generations of the Y-branches of the Ni-NW and a close-up view of one generation can be seen in a TEM image (Fig. 31c). The diameters of the stem, the first-generation branches, and the

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Fig. 32. (a) Schematic of two representative nanochannels with 3-branch-2-subbranch tree-like channels inside the AAO membrane. (b) SEM image of a bundle of Si-NTs with 3-branch-2-subbranch morphology. (c and d) Close-up views taken from each interface of the Si-NTs in (b). (e) TEM image showing one of the Si-NT with three branches broken from the first generation of 3-branch-2-subbranch tree-like Si-NTs, the inset is the corresponding SAED pattern. (f) EDS spectrum of the Si-NTs. Reprinted with permission from Ref. [165], copyright 2010, Wiley-VCH.

second-generation branches are about 120 nm, 65 nm, and 40 nm, respectively. As desired, the diameters of the primary stem and the branches are proportional to the anodizing voltages for creating the corresponding nanochannels. The length of each generation of the branches can be controlled independently by tuning the corresponding anodization duration for each generation channel segment. Later on, using a similar approach, Xu et al. [167] synthesized 2-generation Y-branched and 3-generation Y-branched Fe-NWs. Multi-generation branched NTs can also be constructed by using the AAO membranes with multi-generation branched nanochannels as templates. For example, Meng et al. [45] synthesized 4-generation Y-branched and 3-branch-3subbranch tree-like CNTs via CVD, by using the AAO membranes with the corresponding-shaped nanochannels as templates. The 4-generation Y-branched channels of the AAO membranes are obtained by sequentially reducing the applied anodizing pffiffiffi voltage for four times during the anodization process of the Al foils, with each time by the same factor of 1/ 2. The 3-branch3-subbranch tree-like channels (the channel structures of a single stem dividing into three branches and each of those branches further dividing into three sub-branches) are achieved by sequentially reducing the anodizing voltages for two pffiffiffi times during the anodization process of an Al foil, with each time by the same factor of 1/ 3. Except for the CNTs, multigeneration branched Si-NTs can also be constructed. For example, our group synthesized Si-NTs with two-generation Y-branched morphology by using the AAO membranes with two-generation Y-branched nanochannels as templates via a

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Fig. 33. (a) The schematic fabrication procedure of the Y-Ni-NW/2CNTs hetero-nanostructures. (b) Left: SEM image of a bundle of the heteronanostructures released from the AAO membrane. Middle: close-up views of the Y-branchings (top) and Ni-NW/CNT junctions (bottom). Right: HRTEM (top) and TEM (bottom) images of a Ni-NW/CNT junction. Reprinted with permission from Ref. [46], copyright 2009, Wiley-VCH.

CVD process using silane as a precursor [165]. Using a similar technique, Si-NTs with more complex morphologies, e.g., 3branch-2-subbranch morphology, could also be created by using the AAO membranes with 3-branch-2-subbranch tree-like nanochannels as templates [165]. These multi-generation branched AAO membranes are obtained by reducing the applied pffiffiffi pffiffiffi anodizing voltage by factors of 1/ 3 first and then 1/ 2 during the anodization process of the Al foils (Fig. 32a). Fig. 32b shows a bundle of the Si-NTs with 3-branch-2-subbranch morphology after template removal and two interfaces of the branchings could be observed. Enlarged views taken from the interfaces of the branching (Fig. 32c and d) clearly display the first generation of three branches and the second generation of two branches of the Si-NTs. TEM image (Fig. 32e) shows that one of the NTs has three branches, corresponding to the first-generation three-branches of the 3-branch-2-subbranch tree-like Si-NTs. SAED pattern (inset of Fig. 32e) reveals that the NT is polycrystalline Si. EDS analysis (Fig. 32f) further confirms that the composition of the NT is Si. 5. Hierarchically branched 1D hetero-nanostructures The 1D hetero-nanostructures with hierarchically branched topology may have potential applications in nanophotonics [36], nanoelectronics [36,37], and nanodevices [37,38]. These complex structures can be constructed by using porous AAO membranes with pre-designed hierarchically branched nanochannels as templates via various combinatorial methods. For simplicity, in this section, we only focus on the fabrication of Y-branched two-segment NW/NT heterostructures and three-segment NT/NW/NT heterostructures by using the porous AAO membranes with Y-branched nanochannels as templates. 5.1. Y-branched two-segment NW/NT hetero-nanostructures By using the AAO membranes with Y-branched nanochannels as templates, different types of Y-branched NW/NT heteronanostructures can be constructed. For example, our group constructed Y-branched hetero-nanostructures of a Y-shaped Ni-NW connected with two parallel CNTs in the branches of the Y construct (denoted as Y-Ni-NW/2CNTs) [46], as shown

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Fig. 34. (a) The schematic fabrication procedure of the Y-CNT/2Ni-NWs hetero-nanostructures. (b) Left: SEM image of a bundle of the heteronanostructures after the removal of the AAO membrane. Middle: close-up views of the Y-branchings (top) and Ni-NW/CNT junctions (bottom). Right: TEM (top) and HRTEM (bottom) images of a Y-branched CNT. Reprinted with permission from Ref. [46], copyright 2009, Wiley-VCH.

Fig. 35. Schematic showing four types of Y-branched Au-NW/Si-NT hetero-nanostructures. The left column presents the building blocks. The ordinal number designates each of the architectures. Reprinted with permission from Ref. [110], copyright 2010, American Chemical Society.

schematically in Fig. 33a. A metal layer was coated onto the planar surface of the stem-channel side as a working electrode, and then Ni-NWs were electrodeposited in the nanochannels of the AAO membrane. The ECD duration was adjusted to fill the channels beyond the Y-junction level, leaving some empty space in the branched channels. Finally, CNTs were deposited

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Fig. 36. (a) The schematic fabrication procedure of the Au-NW/Y-Si-NT hetero-nanostructures. (b) side-view SEM image of a bundle of Au-NW/Y-Si-NT hetero-nanostructures released from the AAO membrane. (c and d) Close-up SEM images of the corresponding Au-NW/Si-NT junction area (c) and Ybranchings of Y-shaped Si-NTs (d). (e and f) TEM images of a typical Au-NW/Si-NT junction (e) and Y-shaped Si-NT (f) of the hetero-nanostructure. Reprinted with permission from Ref. [110], copyright 2010, American Chemical Society.

in the empty spaces in the branched channels by CVD. A bundle of the resultant Y-Ni-NW/2CNTs architectures released from the template is shown in an SEM image (left panel in Fig. 33b). Two interfaces of the Y-branchings and the Ni-NW/CNT junctions can be observed in the middle panel. Enlarged SEM images of the Ni-NW/CNT junction indicate that Ni-NWs and CNTs connected longitudinally in the branches have the same diameter, and the interface between the Ni-NW and the CNT is contiguous. TEM and HRTEM images taken on a typical Ni-NW/CNT junction (right panel in Fig. 33b) reveal that the interface is well-connected and the CNT is multi-walled. By modifying the fabrication process for the Y-Ni-NW/2CNTs heteronanostructures, different hetero-nanostructures of Ni-NW/Y-CNT (one straight Ni-NW longitudinally connects with the stem end of a Y-shaped CNT) could be constructed by using the following procedure: a shorter duration of ECD was performed to achieve Ni-NWs that do not reach the Y-junction region of the AAO-nanochannels, which is then filled from the opposite side with Y-branched CNTs in a CVD process. Similarly, Y-branched hetero-nanostructures of one Y-branched CNT connected with two parallel Ni-NWs in the branches of the Y construct (denoted as Y-CNT/2Ni-NWs), which mirror the ones in Fig. 33, can also be created by electrodepositing NiNWs in the half branched channel bottom and then growing CNTs by CVD in the remaining empty Y-channels (Fig. 34) [46]. The left panel in Fig. 34b shows an SEM image of a bundle of the resultant Y-CNT/2Ni-NWs hetero-nanostructures after template removal. Two interfaces of Y-branchings and Ni-NW/CNT junctions can be seen in the middle panel. In the right panel of Fig. 34b, it can be seen that the closely curved inner side of the Y-branched CNT is graphitic. Longer Ni-NWs can also be grown with this configuration to achieve NWs beyond the Y-junction level. Subsequent CVD to fill CNTs in the stems of the AAO membrane can result in CNT/Y-Ni-NW (one straight CNT longitudinally connects with the stem end of a Y-shaped NiNW) hetero-nanostructures. Except for CNTs as the segments of the Y-branched hetero-nanostructures, Si-NTs can also be built as the segments in the Y-branched hetero-nanostructures. For example, our group constructed Y-branched Au-NW/Si-NT hetero-nanostructures [110]. For this kind of hetero-nanostructures, four types of connections can be constructed, as shown schematically in Fig. 35. The fabrication of Y-branched hetero-nanostructures of a straight Au-NW longitudinally connected with the stem end of a Y-shaped Si-NT (denoted as Au-NW/Y-Si-NT, the 1st architecture in Fig. 35) is shown in Fig. 36a. The fabrication process involves coating a silver layer on the planar surface of the stem-channel side of the AAO membrane, ECD of short straight segments of Au-NWs into the bottom part of the stem channels, and AAO self-catalyzed and post-annealing growth

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Fig. 37. (a) The schematic fabrication procedure of the CNT/Y-Ni-NW/2CNTs hetero-nanostructures. (b) The second panel from the left shows an SEM image of a bundle of the hetero-nanostructures after the removal of the AAO membrane. The left panels show the Y-branchings of the Y-shaped Ni-NWs (top) and a TEM image of a single Y-shaped Ni-NW (bottom, inset: the SAED pattern). The third panel from the left shows close-up views of CNT/Ni-NW junctions in the stem (top) and the Ni-NW/CNT junctions in the branch (bottom). The right panels are TEM (top) and HRTEM (bottom) images of a CNT/Ni-NW junction in the stem. Reprinted with permission from Ref. [46], copyright 2009, Wiley-VCH.

of Si-NTs inside the remaining empty Y-shaped nanochannels. Fig. 36b shows an SEM image of a bundle of the resultant AuNW/Y-Si-NT hetero-nanostructures after template removal. Two sharp interfaces of the Au-NW/Si-NT junction (Fig. 36c) and the Y-branchings of the Si-NTs (Fig. 36d) can be seen. The morphologies of the Au-NW/Si-NT junction and the Y-shaped Si-NT can be further confirmed in the TEM images (Fig. 36e and f). Longer ECD will lead to Au-NWs growing beyond the Y-junction to form Y-shaped Au-NWs, leaving some channels near the end of branched side empty. After AAO self-catalyzed and postannealing growth of Si-NTs in the remaining empty channels, Y-branched hetero-nanostructures of Y-shaped Au-NW longitudinally connected with two straight parallel Si-NTs in the ends of two branches (denoted as Y-Au-NW/2Si-NTs, the 2nd architecture in Fig. 35) would be achieved. If the Au-NWs were electrodeposited from the planar surface of the Y-branched channel side of the AAO membrane, straight and Y-shaped Au-NWs would be grown in part section of the Y-shaped channels. Subsequently filling the remaining empty nanochannels with Si-NTs would result in Y-branched hetero-nanostructures of one Y-shaped Si-NT longitudinally connected with two parallel Au-NWs in the two branches of the Y construct (denoted as Y-Si-NT/2Au-NW, the 3rd architecture in Fig. 35) and one Si-NT longitudinally connected with the stem end of a Y-shaped Au-NW (denoted as Si-NT/Y-Au-NW, the 4th architecture in Fig. 35), respectively. 5.2. Y-branched three-segment NT/NW/NT hetero-nanostructures Using the Y-branched-nanochannel AAO membranes as templates, not only the Y-branched two-segment NT/NW heteronanostructures (as shown above) can be built, but also Y-branched three-segment NT/NW/NT hetero-nanostructures can be achieved by more complex combinatorial processes. 5.2.1. With two end NT segments consisting of the same material Y-branched three-segment NT/NW/NT hetero-nanostructures with the two end NT segments consisting of the same material can be constructed. For example, our group synthesized Y-branched NT/NW/NT hetero-nanostructures of CNT/YNi-NW/2CNTs (i.e., one CNT longitudinally connects with the stem end of a Y-shaped Ni-NW, and sequentially connects with another two parallel CNTs in the branches of the Y-shaped Ni-NWs) [46]. The combinatorial fabrication process of the CNT/YNi-NW/2CNTs hetero-nanostructures is shown schematically in Fig. 37a. First, a layer of metal electrode is sputtered onto the planar surface of the stem-channel side of the AAO membrane. Ni-NWs with Y junctions are subsequently electrodeposited, followed by chemical removal of the metal layer and a short part of the deposited Ni-NWs, leaving the Y junction of the NW intact and also providing empty space for CNT deposition in the stem-channels. Then CNTs are deposited inside the empty channels at both ends of the branches and the stems by CVD. Fig. 37b displays three junctions of CNT/Ni-NW in the stem, Y junction in the Y-shaped Ni-NW, and Ni-NW/CNT in the branches of the CNT/Y-Ni-NW/2CNTs hetero-nanostructures,

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Fig. 38. (a) The schematic fabrication procedure of the Si-NT/Y-Au-NW/2Si-NTs hetero-nanostructures. (b) Side-view SEM image of a bundle of Si-NT/Y-AuNW/2Si-NTs hetero-nanostructures released from the AAO membrane. (c and d) Close-up SEM images of Au-NW/Si-NT junction area (c) and Y-branchings (d) of Y-shaped Au-NWs. (e and f) TEM images of a typical Au-NW/Si-NT junction (e) and a Y-shaped Au-NW (f) of the hetero-nanostructure. Reprinted with permission from Ref. [110], copyright 2010, American Chemical Society.

respectively. TEM image together with SAED pattern (lower left) reveals single-crystalline nature of the Y-shaped Ni-NW, which resulted from the high-temperature heating during CNT growth. A typical TEM image of the CNT/Ni-NW junction located in the stem (top of the right panel) and the lattice-resolved image near the junction area (bottom of the right panel) indicate that the multi-walled CNT ends are close to the junction interface, revealing good adherence between the Ni-NW and the CNT. Modifications of the above approach for the CNT/Y-Ni-NW/2CNTs hetero-nanostructures, by positioning the Ni-NW segments away from the Y-junction area, will lead to two other types of Y-branched CNT/Ni-NW/CNT heteronanostructures, i.e., architectures with the Ni-NW segments in the middle of the stem (CNT/Ni-NW/Y-CNT heteronanostructures) and in the middle of the branches (Y-CNT/2Ni-NWs/2CNTs hetero-nanostructures). It should also be mentioned that the materials for the NW segments can be changed to other materials amenable to ECD, to be selectively etched, and stable under the growth conditions of the NTs. For example, we demonstrated the fabrication of Y-branched CNT/NW/ CNT hetero-nanostructures with metals of Cu and Au, semiconductor of CdS, magnetic alloys of CoPt and NiCo, and even segmented NWs of Ag/Ni as the NW segments [46]. Except for the CNTs, AAO-confined grown NTs of other materials can also be built as the two end segments of the Y-branched three-segment NT/NW/NT hetero-nanostructures. For example, our group constructed insulating SiO2-NTs and semiconducting Si-NTs as the two end NT segments in the Y-branched three-segment NT/NW/NT architectures to form branched hetero-nanostructures of SiO2-NT/NW/SiO2-NT [46] and Si-NT/NW/Si-NT [46,110]. It should be noted that even very stable noble metals (e.g., Au) can also be incorporated as the NW segments in the middle of the three-segment NT/NW/NT architectures. Fig. 38a shows the fabrication procedure of the Si-NT/Y-Au-NW/2Si-NTs (one Si-NT connects the stem end of a Y-shaped Au-NW, and then connects two parallel Si-NTs) hetero-nanostructures. First, a short straight sacrificial segment of Co-NWs and the pre-designed Y-shaped Au-NWs are sequentially electrodeposited beyond the Y-junction region of the Y-branched-nanochannel AAO membrane, still leaving a portion of branched channels near the planar surface of the template empty. Then, selective chemical etching of the Ag layer and sacrificial Co-NW segments leads to the Y-shaped Au-NWs to locate in the Y-junction region of the nanochannels. Subsequent filling of the two end remaining empty nanochannels with Si-NTs results in the Si-NT/Y-Au-NW/2Si-NTs hetero-nanostructures. SEM image of the resultant hetero-nanostructures released from the AAO membrane (Fig. 38b) shows three obvious interfaces. Enlarged SEM images show the Au-NW/Si-NT interface (Fig. 38c) and Y-branchings of the Y-shaped Au-NWs (Fig. 38d). TEM image of a typical Au-NW/Si-NT junction (Fig. 38e) reveals good adherence between the Au-NW segment and the Si-NT segment, and the Y-shaped topology of the Au-NW segment can be verified (Fig. 38f). Using the approaches similar to that for the

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Fig. 39. (a) The schematic fabrication procedure of the Y-Si-NT/2Au-NWs/2CNTs hetero-nanostructures. (b) The second panel from the left is an SEM image of a bundle of the structures after the removal of the AAO membrane. The left panel shows SEM image of Y-shaped Si-NTs (top) and TEM image of a Yshaped Si-NT (bottom; the inset is the SAED pattern). The third panel from the left shows the close-up views of the Si-NT/Au-NW (top) and Au-NW/CNT (bottom) junctions. The right panel shows TEM image of well-connected Si-NT/Au-NW junction (top) with the SAED pattern from the Au-NW (the inset) and HRTEM image of the Si-NT (bottom). Reprinted with permission from Ref. [46], copyright 2009, Wiley-VCH.

Si-NT/Y-Au-NW/2Si-NTs hetero-nanostructures, two other types of Y-branched Si-NT/Au-NW/Si-NT hetero-nanostructures could be constructed, by electrodepositing Au-NWs in the middle portion of either stem or branched channels of the Y-branched-nanochannel AAO membranes. If the Au-NWs were electrodeposited in the middle portion of the stem channels, Si-NT/Au-NW/Y-Si-NT (one straight Si-NT connects one straight Au-NW, and then connects the stem end of a Y-shaped Si-NT) hetero-nanostructures would be achieved. If the Au-NWs were electrodeposited in the middle portion of the branched channels, Y-Si-NT/2Au-NWs/2Si-NTs (one Y-shaped Si-NT with its two legs connected with two parallel Au-NWs, and further connected the two parallel Si-NTs) hetero-nanostructures would be obtained. 5.2.2. With two end NT segments consisting of different materials Using the Y-branched-nanochannel AAO membranes as templates, Y-branched three-segment NT/NW/NT heteronanostructures with the two end NT segments comprising two different materials can also be built. For example, our group constructed Y-branched three-segment NT/NW/NT hetero-nanostructures with the two end NT segments composed of Si-NTs and CNTs respectively [46]. Fig. 39a shows the fabrication procedure of Y-Si-NT/2Au-NWs/2CNTs (a Y-shaped Si-NT connects with two parallel Au-NWs and two CNTs in the branches in sequence) hetero-nanostructures. First, sacrificial segments of Co-NWs and the desired Au-NWs are electrodeposited in sequence from the small-diameter branched channel side of the Y-branched-nanochannel AAO membrane. Then, Y-shaped Si-NTs are grown inside the empty Y-shaped channels of the membrane by CVD. Next, the metal electrode and the sacrificial Co-NW segments are selectively etched away. Finally, CNTs are grown inside the bottom branched channels by CVD. The second panel from the left in Fig. 39b shows an SEM image of a bundle of the resultant three-segment Y-Si-NT/2Au-NWs/2CNTs heteronanostructures after template removal. Three parallel interfaces observed in the Y-branched hetero-nanostructures are Y branches of Y-shaped Si-NTs (top of the left panel), Si-NT/Au-NW junction (top of the third panels from the left), and Au-NW/CNT junction (bottom of the third panels from the left), respectively. TEM image (bottom of the left panel) verifies the formation of the Y-shaped Si-NT, and the SAED pattern (the inset in the bottom of the left panel) reveals the polycrystalline nature of the Si-NT. TEM image of a Si-NT/Au-NW junction (top of the right panel) indicates that the Si-NT and the Au-NW are well connected. The SAED pattern taken from the Au-NW (inset in the top of the right panel) reveals that the Au-NW is single-crystalline. HRTEM image (bottom of the right panel) further confirms the polycrystalline nature of the Si-NT.

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6. Conclusions and future outlook In this review, we have summarized the porous AAO template-assisted rational synthesis of four types of 1D hybrid and hierarchically branched nanoarchitectures, i.e., axial hetero-nanostructures, coaxial nanocables, hierarchically branched 1D homo-nanostructures, and hierarchically branched 1D hetero-nanostructures, by deliberately controlling the geometric shape and size of the nanochannels inside the AAO membranes. A large variety of materials, such as metals, metal alloys, metal oxides, semimetals, conducting polymers, as well as elemental and compound semiconductors, can be used as components of the 1D complex nanoarchitectures. The fascinating applications of these 1D complex nanoarchitectures in various fields were also discussed. The 1D hybrid and hierarchically branched nanoarchitectures may have development prospect in the following aspects. For the hybrid nanostructures, future development relies on improving the diversity of the segment materials for axial hetero-nanostructures and the core materials for coaxial nanocables, to construct novel hybrid nanostructures with various interfaces that may have potential as building blocks for various nanodevices. Meanwhile, developing better characterization methods to fully understand the nature of the interfaces of the hybrid nanostructures is also necessary. For the hierarchically branched nanoarchitectures whose morphologies totally replicate those of the branched nanochannels of the AAO membranes, engineering the pore structures of the AAO membranes is crucial for constructing hierarchically branched nanoarchitectures with novel pre-designed shapes. For example, Jin et al. [174] recently reported the synthesis of AAO membranes with a novel pore structure, i.e., the AAO membranes with 3D interconnected pores, based on patterned aluminum surface. They further demonstrated the fabrication of a unique supporting AAO skeleton structure, by tuning the anodization and etching conditions. Control over the composition, structure, and interface of the 1D complex nanoarchitectures will affect their fundamental properties, such as electronic, optical, and magnetic properties. Thus, efforts should also be directed to developing tools to investigate the fundamental properties of the individual or collective ensembles of the 1D complex nanoarchitectures. The most important is to design simple and reproducible strategies for assembling and integrating the 1D complex nanoarchitectures into multifunctional nanodevices and nanosystems. Acknowledgments This work was supported by the National Natural Science Foundation of China (grant numbers 51632009, 51472245, 51628202, 21207134, 51372247), the CAS/SAFEA International Partnership Program for Creative Research Teams, and Key Research Program of Frontier Sciences, Chinese Academy of Sciences (grant number QYZDJ-SSW-SLH046). References [1] Deepak FL, Govindaraj A, Rao CNR. Synthetic strategies for Y-junction carbon nanotubes. Chem Phys Lett 2001;345:5–10. [2] Gothard N, Daraio C, Gaillard J, Zidan R, Jin S, Rao AM. Controlled growth of Y-junction nanotubes using Ti-doped vapor catalyst. Nano Lett 2004;4:213–7. [3] Nicewarner-Pena SR, Freeman RG, Reiss BD, He L, Pena DJ, Walton ID, et al. Submicrometer metallic barcodes. Science 2001;294:137–41. [4] Milliron DJ, Hughes SM, Cui Y, Manna L, Li JB, Wang LW, et al. Colloidal nanocrystal heterostructures with linear and branched topology. Nature 2004;430:190–5. [5] Wang D, Qian F, Yang C, Zhong Z, Lieber CM. Rational growth of branched and hyperbranched nanowire structures. Nano Lett 2004;4:871–4. [6] Bandaru PR, Daraio C, Jin S, Rao AM. Novel electrical switching behaviour and logic in carbon nanotube Y-junctions. Nat Mater 2005;4:663–6. [7] Bierman MJ, Lau YKA, Jin S. Hyperbranched PbS and PbSe nanowires and the effect of hydrogen gas on their synthesis. Nano Lett 2007;7:2907–12. [8] Watt J, Cheong S, Toney MF, Ingham B, Cookson J, Bishop PT, et al. Ultrafast growth of highly branched palladium nanostructures for catalysis. ACS Nano 2010;4:396–402. [9] Lee KB, Park S, Mirkin CA. Multicomponent magnetic nanorods for biomolecular separations. Angew Chem Int Ed 2004;43:3048–50. [10] Fang XS, Gautam UK, Bando Y, Dierre B, Sekiguchi T, Golberg D. Multiangular branched ZnS nanostructures with needle-shaped tips: Potential luminescent and field-emitter nanomaterial. J Phys Chem C 2008;112:4735–42. [11] Bentley AK, Trethewey JS, Ellis AB, Crone WC. Magnetic manipulation of copper-tin nanowires capped with nickel ends. Nano Lett 2004;4:487–90. [12] Tang JY, Huo ZY, Brittman S, Gao HW, Yang PD. Solution-processed core-shell nanowires for efficient photovoltaic cells. Nat Nanotech 2011;6:568–72. [13] Gudiksen MS, Lauhon LJ, Wang J, Smith DC, Lieber CM. Growth of nanowire superlattice structures for nanoscale photonics and electronics. Nature 2002;415:617–20. [14] Sun BQ, Marx E, Greenham NC. Photovoltaic devices using blends of branched CdSe nanoparticles and conjugated polymers. Nano Lett 2003;3:961–3. [15] Xiang J, Lu W, Hu YJ, Wu Y, Yan H, Lieber CM. Ge/Si nanowire heterostructures as high-performance field-effect transistors. Nature 2006;441:489–93. [16] Liang GC, Xiang J, Kharche N, Klimeck G, Lieber CM, Lundstrom M. Performance analysis of a Ge/Si core/shell nanowire field-effect transistor. Nano Lett 2007;7:642–6. [17] Moore D, Morber JR, Snyder RL, Wang ZL. Growth of ultralong ZnS/SiO2 core-shell nanowires by volume and surface diffusion VLS process. J Phys Chem C 2008;112:2895–903. [18] Tao XY, Dong LX, Zhang WK, Zhang XB, Chen JP, Huang H, et al. Controllable melting and flow of beta-Sn in flexible amorphous carbon nanotubes. Carbon 2009;47:3122–7. [19] Gong NW, Lu MY, Wang CY, Chen Y, Chen LJ. Au(Si)-filled beta-Ga(2)O(3) nanotubes as wide range high temperature nanothermometers. Appl Phys Lett 2008;92. [20] Cui LF, Yang Y, Hsu CM, Cui Y. Carbon-Silicon Core-Shell Nanowires as High Capacity Electrode for Lithium Ion Batteries. Nano Lett 2009;9:3370–4. [21] Lauhon LJ, Gudiksen MS, Wang CL, Lieber CM. Epitaxial core-shell and core-multishell nanowire heterostructures. Nature 2002;420:57–61. [22] Wong AB, Brittman S, Yu Y, Dasgupta NP, Yang PD. Core-Shell CdS-Cu2S Nanorod Array Solar Cells. Nano Lett 2015;15:4096–101. [23] Zhan JH, Bando Y, Hu JQ, Liu ZW, Yin LW, Golberg D. Fabrication of metal-semiconductor nanowire heterojunctions. Angew Chem Int Ed 2005;44:2140–4. [24] Pan AL, Yao LD, Qin Y, Yang Y, Kim DS, Yu RC, et al. Si-CdSSe Core/Shell Nanowires with Continuously Tunable Light Emission. Nano Lett 2008;8:3413–7.

282

Q. Xu et al. / Progress in Materials Science 95 (2018) 243–285

[25] Meng GW, Zhang LD, Mo CM, Zhang SY, Qin Y, Feng SP, et al. Preparation of beta-SiC nanorods with and without amorphous SiO2 wrapping layers. J Mater Res 1998;13:2533–8. [26] Hu JQ, Bando Y, Zhan JH, Golberg D. Fabrication of silica-shielded Ga-ZnS metal-semiconductor nanowire heterojunctions. Adv Mater 2005;17:1964–9. [27] Fu L, Liu YQ, Liu ZM, Han BX, Cao LC, Wei DC, et al. Carbon nanotubes coated with alumina as gate dielectrics of field-effect transistors. Adv Mater 2006;18:181–5. [28] Manna L, Milliron DJ, Meisel A, Scher EC, Alivisatos AP. Controlled growth of tetrapod-branched inorganic nanocrystals. Nat Mater 2003;2:382–5. [29] Bierman MJ, Lau YKA, Kvit AV, Schmitt AL, Jin S. Dislocation-driven nanowire growth and Eshelby twist. Science 2008;320:1060–3. [30] Zhu J, Peng HL, Marshall AF, Barnett DM, Nix WD, Cui Y. Formation of chiral branched nanowires by the Eshelby Twist. Nat Nanotech 2008;3:477–81. [31] Doerk GS, Radmilovic V, Maboudian R. Branching induced faceting of Si nanotrees. Appl Phys Lett 2010;96. [32] Wu Y, Fan R, Yang P. Block-by-Block growth of single-crystalline Si/SiGe superlattice nanowires. Nano Lett 2002;2:83–6. [33] Dick KA, Deppert K, Larsson MW, Martensson T, Seifert W, Wallenberg LR, et al. Synthesis of branched ’nanotrees’ by controlled seeding of multiple branching events. Nat Mater 2004;3:380–4. [34] Cheng CW, Liu B, Yang HY, Zhou WW, Sun L, Chen R, et al. Hierarchical assembly of ZnO nanostructures on SnO2 backbone nanowires: lowtemperature hydrothermal preparation and optical properties. ACS Nano 2009;3:3069–76. [35] Sun SH, Meng GW, Zhang GX, Zhang LD. Controlled growth and optical properties of one-dimensional ZnO nanostructures on SnO2 nanobelts. Cryst Growth Des 2007;7:1988–91. [36] Dong A, Tang R, Buhro WE. Solution-based growth and structural characterization of homo- and heterobranched semiconductor nanowires. J Am Chem Soc 2007;129:12254–62. [37] Jung Y, Ko D-K, Agarwal R. Synthesis and structural characterization of single-crystalline branched nanowire heterostructures. Nano Lett 2007;7:264–8. [38] Jiang X, Tian B, Xiang J, Qian F, Zheng G, Wang H, et al. Rational growth of branched nanowire heterostructures with synthetically encoded properties and function. Proc Natl Acad Sci USA 2011;108:12212–6. [39] Terrones M, Terrones H, Banhart F, Charlier JC, Ajayan PM. Coalescence of single-walled carbon nanotubes. Science 2000;288:1226–9. [40] Terrones M, Banhart F, Grobert N, Charlier JC, Terrones H, Ajayan PM. Molecular junctions by joining single-walled carbon nanotubes. Phys Rev Lett 2002;89:075505. [41] She JC, An S, Deng SZ, Chen J, Xiao ZM, Zhou J, et al. Laser welding of a single tungsten oxide nanotip on a handleable tungsten wire: a demonstration of laser-weld nanoassembly. Appl Phys Lett 2007;90:073103. [42] Peng Y, Cullis T, Inkson B. Bottom-up nanoconstruction by the welding of individual metallic nanoobjects using nanoscale solder. Nano Lett 2009;9:91–6. [43] Masuda H, Fukuda K. Ordered metal nanohole arrays made by a two-step replication of honeycomb structures of anodic alumina. Science 1995;268:1466–8. [44] Li J, Papadopoulos C, Xu J. Growing Y-junction carbon nanotubes. Nature 1999;402:253–4. [45] Meng G, Jung YJ, Cao A, Vajtai R, Ajayan PM. Controlled fabrication of hierarchically branched nanopores, nanotubes, and nanowires. Proc Natl Acad Sci USA 2005;102:7074–8. [46] Meng G, Han F, Zhao X, Chen B, Yang D, Liu J, et al. A general synthetic approach to interconnected nanowire/nanotube and nanotube/nanowire/nanotube heterojunctions with branched topology. Angew Chem Int Ed 2009;48:7166–70. [47] Liu F, Lee JY, Zhou WJ. Segmented Pt/Ru, Pt/Ni, and Pt/RuNi nanorods as model bifunctional catalysts for methanol oxidation. Small 2006;2:121–8. [48] Keating CD, Natan MJ. Striped metal nanowires as building blocks and optical tags. Adv Mater 2003;15:451–4. [49] Hurst SJ, Payne EK, Qin LD, Mirkin CA. Multisegmented one-dimensional nanorods prepared by hard-template synthetic methods. Angew Chem Int Ed 2006;45:2672–92. [50] Lee W, Park SJ. Porous anodic aluminum oxide: anodization and templated synthesis of functional nanostructures. Chem Rev 2014;114:7487–556. [51] Bauer LA, Reich DH, Meyer GJ. Selective functionalization of two-component magnetic nanowires. Langmuir 2003;19:7043–8. [52] Birenbaum NS, Lai BT, Chen CS, Reich DH, Meyer GJ. Selective noncovalent adsorption of protein to bifunctional metallic nanowire surfaces. Langmuir 2003;19:9580–2. [53] Oh BK, Park S, Millstone JE, Lee SW, Lee KB, Mirkin CA. Separation of tricomponent protein mixtures with triblock nanorods. J Am Chem Soc 2006;128:11825–9. [54] Martin BR, Dermody DJ, Reiss BD, Fang MM, Lyon LA, Natan MJ, et al. Orthogonal self-assembly on colloidal gold-platinum nanorods. Adv Mater 1999;11:1021–5. [55] Salem AK, Chen M, Hayden J, Leong KW, Searson PC. Directed assembly of multisegment Au/Pt/Au nanowires. Nano Lett 2004;4:1163–5. [56] Chen M, Guo L, Ravi R, Searson PC. Kinetics of receptor directed assembly of multisegment nanowires. J Phys Chem B. 2006;110:211–7. [57] Love JC, Urbach AR, Prentiss MG, Whitesides GM. Three-dimensional self-assembly of metallic rods with submicron diameters using magnetic interactions. J Am Chem Soc 2003;125:12696–7. [58] Hangarter CM, Myung NV. Magnetic alignment of nanowires. Chem Mater 2005;17:1320–4. [59] Liu F, Lee JY, Zhou WJ. Template preparation of multisegment PtNi nanorods as methanol electro-oxidation catalysts with adjustable bimetallic pair sites. J Phys Chem B 2004;108:17959–63. [60] Kim S, Kim SK, Park S. Bimetallic gold-silver nanorods produce multiple surface plasmon bands. J Am Chem Soc 2009;131:8380–1. [61] Wang HX, Wu YC, Wang M, Zhang YG, Li GH, Zhang LD. Fabrication and magnetotransport properties of ordered sub-100 nm pseudo-spin-valve element arrays. Nanotechnology 2006;17:1651–4. [62] Wang HX, Wu YC, Li QS, Wang M, Li GH, Zhang LD. Synthesis and characterization of FeMn-pinned spin valve arrays. Appl Phys Lett 2006;89:052107. [63] Qin LD, Park S, Huang L, Mirkin CA. On-wire lithography. Science 2005;309:113–5. [64] Osberg KD, Schmucker AL, Senesi AJ, Mirkin CA. One-dimensional nanorod arrays: independent control of composition, length, and interparticle spacing with nanometer precision. Nano Lett 2011;11:820–4. [65] Chen XD, Yeganeh S, Qin LD, Li SZ, Xue C, Braunschweig AB, et al. Chemical fabrication of heterometallic nanogaps for molecular transport junctions. Nano Lett 2009;9:3974–9. [66] Wei W, Li SZ, Millstone JE, Banholzer MJ, Chen XD, Xu XY, et al. Surprisingly long-range surface-enhanced raman scattering (SERS) on Au-Ni multisegmented nanowires. Angew Chem Int Ed 2009;48:4210–2. [67] Wei W, Li S, Qin L, Xue C, Millstone JE, Xu X, et al. Surface plasmon-mediated energy transfer in heterogap AuAg nanowires. Nano Lett 2008;8:3446–9. [68] Qin LD, Jang JW, Huang L, Mirkin CA. Sub-5-nm gaps prepared by on-wire lithography: correlating gap size with electrical transport. Small 2007;3:86–90. [69] Chen X, Jeon YM, Jang JW, Qin L, Huo F, Wei W, et al. On-wire lithography-generated molecule-based transport junctions: a new testbed for molecular electronics. J Am Chem Soc 2008;130:8166. [70] Liu SH, Tok JBH, Bao ZN. Nanowire lithography: fabricating controllable electrode gaps using Au-Ag-Au nanowires. Nano Lett 2005;5:1071–6. [71] Zheng GF, Qin LD, Mirkin CA. Spectroscopically enhancing electrical nanotraps. Angew Chem Int Ed 2008;47:1938–41. [72] Qin L, Banholzer MJ, Xu X, Huang L, Mirkin CA. Rational design and synthesis of catalytically driven nanorotors. J Am Chem Soc 2007;129:14870–1. [73] Qin L, Banholzer MJ, Millstone JE, Mirkin CA. Nanodisk codes. Nano Lett 2007;7:3849–53. [74] Banholzer MJ, Qin L, Millstone JE, Osberg KD, Mirkin CA. On-wire lithography: synthesis, encoding and biological applications. Nat Protocols 2009;4:838–48.

Q. Xu et al. / Progress in Materials Science 95 (2018) 243–285

283

[75] Osberg KD, Rycenga M, Harris N, Schmucker AL, Langille MR, Schatz GC, et al. Dispersible gold nanorod dimers with sub-5 nm gaps as local amplifiers for surface-enhanced Raman scattering. Nano Lett 2012;12:3828–32. [76] Liusman C, Li S, Chen X, Wei W, Zhang H, Schatz GC, et al. Free-standing bimetallic nanorings and nanoring arrays made by on-wire lithography. ACS Nano 2010;4:7676–82. [77] Park S, Lim JH, Chung SW, Mirkin CA. Self-assembly of mesoscopic metal-polymer amphiphiles. Science 2004;303:348–51. [78] Ciszek JW, Huang L, Wang Y, Mirkin CA. Kinetically controlled, shape-directed assembly of nanorods. Small 2008;4:206–10. [79] Park S, Chung SW, Mirkin CA. Hybrid organic-inorganic, rod-shaped nanoresistors and diodes. J Am Chem Soc 2004;126:11772–3. [80] Hernandez RM, Richter L, Semancik S, Stranick S, Mallouk TE. Template fabrication of protein-functionalized gold-polypyrrole-gold segmented nanowires. Chem Mater 2004;16:3431–8. [81] Chung HJ, Jung HH, Cho YS, Lee S, Ha JH, Choi JH, et al. Cobalt-polypyrrole-cobalt nanowire field-effect transistors. Appl Phys Lett 2005;86:213113. [82] Skinner K, Dwyer C, Washburn S. Selective functionalization of arbitrary nanowires. Nano Lett 2006;6:2758–62. [83] Wang X, Ozkan CS. Multisegment nanowire sensors for the detection of DNA molecules. Nano Lett 2008;8:398–404. [84] Hou JW, Yang XC, Cui MM, Huang M, Wang QY. Synthesis and optical property of one-dimensional Ag-Cu2O heterojunctions. Mater Lett 2012;74:159–62. [85] Herderick ED, Tresback JS, Vasiliev AL, Padture NP. Template-directed synthesis, characterization and electrical properties of Au-TiO(2)-Au heterojunction nanowires. Nanotechnology 2007;18:155204. [86] Zhang Y, Li L, Li GH. Fabrication and anomalous transport properties of an Sb/Bi segment nanowire nanojunction array. Nanotechnology 2005;16:2096–9. [87] Pena DJ, Mbindyo JKN, Carado AJ, Mallouk TE, Keating CD, Razavi B, et al. Template growth of photoconductive metal-CdSe-metal nanowires. J Phys Chem B 2002;106:7458–62. [88] Guo YB, Tang QX, Liu HB, Zhang YJ, Li YL, Hu WP, et al. Light-controlled organic/inorganic P-N junction nanowires. J Am Chem Soc 2008;130:9198–9. [89] Yahalom J, Zadok O. Formation of composition-modulated alloys by electrodeposition. J Mater Sci 1987;22:499–503. [90] Ohgai T, Hoffer X, Gravier L, Wegrowe JE, Ansermet JP. Bridging the gap between template synthesis and microelectronics: spin-valves and multilayers in self-organized anodized aluminium nanopores. Nanotechnology 2003;14:978–82. [91] Guo YG, Wan LJ, Zhu CF, Yang DL, Chen DM, Bai CL. Ordered Ni-Cu nanowire array with enhanced coercivity. Chem Mater 2003;15:664–7. [92] Choi JR, Oh SJ, Ju H, Cheon J. Massive fabrication of free-standing one-dimensional Co/Pt nanostructures and modulation of ferromagnetism via a programmable barcode layer effect. Nano Lett 2005;5:2179–83. [93] Wang HX, Wu YC, Zhang LD, Hu XY. Fabrication and magnetic properties of Fe/Pt multilayered nanowires. Appl Phys Lett 2006;89:232508. [94] Liu LF, Lee W, Scholz R, Pippel E, Gosele U. Tailor-made inorganic nanopeapods: structural design of linear noble metal nanoparticle chains. Angew Chem Int Ed 2008;47:7004–8. [95] Xue FH, Fei GT, Wu B, Cui P, Zhang LD. Direct electrodeposition of highly dense Bi/Sb superlattice nanowire arrays. J Am Chem Soc 2005;127:15348–9. [96] Dou X, Li G, Lei H. Kinetic versus thermodynamic control over growth process of electrodeposited Bi/BiSb superlattice nanowires. Nano Lett 2008;8:1286–90. [97] Luo J, Zhang L, Zhang YJ, Zhu J. Controlled growth of one-dimensional metal-semiconductor and metal-carbon nanotube heterojunctions. Adv Mater 2002;14:1413–4. [98] Luo J, Zhu J. Arrays of one-dimensional metal/silicon and metal/carbon nanotube heterojunctions. Nanotechnology 2006;17:S262–70. [99] Wang W, Lu XL, Zhang T, Zhang GQ, Jiang WJ, Li XG. Bi2Te3/Te multiple heterostructure nanowire arrays formed by confined precipitation. J Am Chem Soc 2007;129:6702–3. [100] Perego D, Franz S, Bestetti M, Cattaneo L, Brivio S, Tallarida G, et al. Engineered fabrication of ordered arrays of Au-NiO-Au nanowires. Nanotechnology 2013;24:045302. [101] Yang D, Meng G, Han F, Zhang L. Two-segment CdS/Bi nanowire heterojunctions arrays and their electronic transport properties. Mater Lett 2008;62:3213–6. [102] Xu Q, Meng G, Wu X, Wei Q, Kong M, Zhu X. A generic approach to desired metallic nanowires inside native porous alumina template via redox reaction. Chem Mater 2009;21:2397–402. [103] Lee W, Scholz R, Nielsch K, GÖsele U. A template-based electrochemical method for the synthesis of multisegmented metallic nanotubes. Angew Chem Int Ed 2005;44:6050–4. [104] Yang D, Meng G, Zhang S, Hao Y, An X, Wei Q, et al. Electrochemical synthesis of metal and semimetal nanotube–nanowire heterojunctions and their electronic transport properties. Chem Commun 2007:1733–5. [105] Luo J, Xing YJ, Zhu J, Yu DP, Zhao YG, Zhang L, et al. Structure and electrical properties of Ni nanowire/multiwalled-carbon nanotube/amorphous carbon nanotube fleterojunctions. Adv Funct Mater 2006;16:1081–5. [106] Ou FS, Shaijumon MM, Ci L, Benicewicz D, Vajtai R, Ajayan PM. Multisegmented one-dimensional hybrid structures of carbon nanotubes and metal nanowires. Appl Phys Lett 2006;89:243122. [107] Ou FS, Shaijumon MM, Ajayan PM. Controlled manipulation of giant hybrid inorganic nanowire assemblies. Nano Lett 2008;8:1853–7. [108] Shaijumon MM, Ou FS, Ci LJ, Ajayan PM. Synthesis of hybrid nanowire arrays and their application as high power supercapacitor electrodes. Chem Commun 2008:2373–5. [109] Reddy ALM, Shaijumon MM, Gowda SR, Ajayan PM. Multisegmented Au-MnO2/carbon nanotube hybrid coaxial arrays for high-power supercapacitor applications. J Phys Chem C 2010;114:658–63. [110] Chen BS, Meng GW, Xu QL, Zhu XG, Kong MG, Chu ZQ, et al. Crystalline silicon nanotubes and their connections with gold nanowires in both linear and branched topologies. ACS Nano 2010;4:7105–12. [111] Han F, Meng G, Zhao X, Xu Q, Liu J, Chen B, et al. Building desired heterojunctions of semiconductor CdS nanowire and carbon nanotube via AAO template-based approach. Mater Lett 2009;63:2249–52. [112] Lauhon LJ, Gudiksen MS, Wang D, Lieber CM. Epitaxial core-shell and core-multishell nanowire heterostructures. Nature 2002;420:57–61. [113] Tian BZ, Zheng XL, Kempa TJ, Fang Y, Yu NF, Yu GH, et al. Coaxial silicon nanowires as solar cells and nanoelectronic power sources. Nature 2007;449. 885-U8. [114] Ku JR, Vidu R, Talroze R, Stroeve P. Fabrication of nanocables by electrochemical deposition inside metal nanotubes. J Am Chem Soc 2004;126:15022–3. [115] Wang Q, Wang G, Han X, Wang X, Hou JG. Controllable template synthesis of Ni/Cu nanocable and Ni nanotube arrays: a one-step coelectrodeposition and electrochemical etching method. J Phys Chem B 2005;109:23326–9. [116] Jia YY, Yang DC, Luo B, Liu SM, Tade MO, Zhi LJ. One-pot synthesis of Bi-Ni nanowire and nanocable arrays by coelectrodeposition approach. Nanoscale Res Lett 2012;7:1–6. [117] Liu R, Lee SB. MnO2/Poly(3,4-ethylenedioxythiophene) coaxial nanowires by one-step coelectrodeposition for electrochemical energy storage. J Am Chem Soc 2008;130:2942–3. [118] Zhu W, Wang G, Hong X, Shen X. One-step fabrication of Ni/TiO2 core/shell nanorod arrays in anodic aluminum oxide membranes. J Phys Chem C 2009;113:5450–4. [119] Yang D, Meng G, Xu Q, Zhao X, Liu J, Kong M, et al. A generic approach to nanocables via nanochannel-confined sequential electrodeposition. Appl Phys Lett 2008;92:083109. [120] Li XR, Wang YQ, Song GJ, Peng Z, Yu YM, She XL, et al. Fabrication and magnetic properties of Ni/Cu Shell/Core nanocable arrays. J Phys Chem C 2010;114:6914–6.

284

Q. Xu et al. / Progress in Materials Science 95 (2018) 243–285

[121] Ng HT, Han J, Yamada T, Nguyen P, Chen YP, Meyyappan M. Single crystal nanowire vertical surround-gate field-effect transistor. Nano Lett 2004;4:1247–52. [122] Lee J, Farhangfar S, Yang RB, Scholz R, Alexe M, Gosele U, et al. A novel approach for fabrication of bismuth-silicon dioxide core-shell structures by atomic layer deposition. J Mater Chem 2009;19:7050–4. [123] Huang GS, Xie Y, Wu XL, Yang LW, Shi Y, Siu GG, et al. Formation mechanism of individual alumina nanotubes wrapping metal (Cu and Fe) nanowires. J Cryst Growth 2006;289:295–8. [124] Yang YW, Li L, Huang XH, Ye M, Wu YC, Li GH. Fabrication of InSb-core/alumina-sheath nanocables. Mater Lett 2006;60:569–71. [125] Han FM, Meng GW, Xu QL, Zhu XG, Zhao XL, Chen BS, et al. Alumina-sheathed nanocables with cores consisting of various structures and materials. Angew Chem Int Ed 2011;50:2036–40. [126] Li Y, Meng GW, Zhang LD, Phillipp F. Ordered semiconductor ZnO nanowire arrays and their photoluminescence properties. Appl Phys Lett 2000;76:2011–3. [127] Bao JC, Tie CY, Xu Z, Ma Q, Hong JM, Sang H, et al. An array of concentric composite nanostructures of zirconia nanotubules/cobalt nanowires: preparation and magnetic properties. Adv Mater 2002;14:44–7. [128] Ye Z, Liu H, Schultz I, Wu W, Naugle DG, Lyuksyutov I. Template-based fabrication of nanowire-nanotube hybrid arrays. Nanotechnology 2008;19:325303. [129] Kovtyukhova NI, Mallouk TE, Mayer TS. Templated surface sol-gel synthesis of SiO2 nanotubes and SiO2-insulated metal nanowires. Adv Mater 2003;15:780–5. [130] Zheng ZX, Xi YY, Dong P, Huang HG, Zhou JZ, Wu LL, et al. The enhanced photoluminescence of zinc oxide and polyaniline coaxial nanowire arrays in anodic oxide aluminium membranes. Phys Chem Comm 2002;5:63–5. [131] Bao JC, Tie C, Xu Z, Suo ZY, Zhou QF, Hong JM. A facile method for creating an array of metal-filled carbon nanotubes. Adv Mater 2002;14:1483–6. [132] Zhang DZ, Luo LA, Liao Q, Wang H, Fu HB, Yao JN. Polypyrrole/ZnS core/shell coaxial nanowires prepared by anodic aluminum oxide template methods. J Phys Chem C 2011;115:2360–5. [133] Li L, Yang YW, Li GH, Zhang LD. Conversion of a Bi nanowire array to an array of Bi-Bi2O3 core-shell nanowires and Bi2O3 nanotubes. Small 2006;2:548–53. [134] Zhou D, Zhu MG, Zhou MG, Guo ZH, Li W. Preparation and magnetic properties of hard-magnetic (CoPt)/soft-magnetic (FeCo) composite nanocable array. J Appl Phys 2011;109:07B720. [135] Narayanan TN, Mandal BP, Tyagi AK, Kumarasiri A, Zhan XB, Hahm MG, et al. Hybrid multiferroic nanostructure with magnetic-dielectric coupling. Nano Lett 2012;12:3025–30. [136] Coleman NRB, Morris MA, Spalding TR, Holmes JD. The formation of dimensionally ordered silicon nanowires within mesoporous silica. J Am Chem Soc 2001;123:187–8. [137] Crowley TA, Ziegler KJ, Lyons DM, Erts D, Olin H, Morris MA, et al. Synthesis of metal and metal oxide nanowire and nanotube arrays within a mesoporous silica template. Chem Mater 2003;15:3518–22. [138] Crowley TA, Daly B, Morris MA, Erts D, Kazakova O, Boland JJ, et al. Probing the magnetic properties of cobalt-germanium nanocable arrays. J Mater Chem 2005;15:2408–13. [139] Daly B, Arnold DC, Kulkarni JS, Kazakova O, Shaw MT, Nikitenko S, et al. Synthesis and characterization of highly ordered cobalt–magnetite nanocable arrays. Small 2006;2:1299–307. [140] Zhao NH, Wang GJ, Huang Y, Wang B, Yao BD, Wu YP. Preparation of nanowire arrays of amorphous carbon nanotube-coated single crystal SnO2. Chem Mater 2008;20:2612–4. [141] Xu QL, Meng GW, Chen BS, Li XD, Zhu XG, Chu ZQ, et al. Synthesis of AuNi/NiO nanocables by porous AAO template assisted galvanic deposition and subsequent oxidation. Eur J Inorg Chem 2010:4309–13. [142] Park DH, Lee YB, Cho MY, Kim BH, Lee SH, Hong YK, et al. Fabrication and magnetic characteristics of hybrid double walled nanotube of ferromagnetic nickel encapsulated conducting polypyrrole. Appl Phys Lett 2007;90:093122. [143] Liu R, Duay J, Lane T, Lee SB. Synthesis and characterization of RuO2/poly(3,4-ethylenedioxythiophene) composite nanotubes for supercapacitors. Phys Chem Chem Phys 2010;12:4309–16. [144] Bao JC, Wang KY, Xu Z, Zhang H, Lu ZH. A novel nanostructure of nickel nanotubes encapsulated in carbon nanotubes. Chem Commun 2003:208–9. [145] Liu CY, Gillette EI, Chen XY, Pearse AJ, Kozen AC, Schroeder MA, et al. An all-in-one nanopore battery array. Nat Nanotech 2014;9:1031–9. [146] Grote F, Zhao HP, Lei Y. Self-supported carbon coated TiN nanotube arrays: innovative carbon coating leads to an improved cycling ability for supercapacitor applications. J Mater Chem A 2015;3:3465–70. [147] Hsu MC, Leu IC, Sun YM, Hon MH. Fabrication of CdS@TiO2 coaxial composite nanocables arrays by liquid-phase deposition. J Cryst Growth 2005;285:642–8. [148] Kovtyukhova NI, Kelley BK, Mallouk TE. Coaxially gated in-wire thin-film transistors made by template assembly. J Am Chem Soc 2004;126:12738–9. [149] Gowda SR, Reddy ALLM, Zhan XB, Ajayan PM. Building energy storage device on a single nanowire. Nano Lett 2011;11:3329–33. [150] Yu JS, Kim JY, Lee S, Mbindyo JKN, Martin BR, Mallouk TE. Template synthesis of polymer-insulated colloidal gold nanowires with reactive ends. Chem Commun 2000:2445–6. [151] Kovtyukhova NL, Mallouk TE. Nanowire p-n heterojunction diodes made by templated assembly of multilayer carbon-nanotube/polymer/ semiconductor-particle shells around metal nanowires. Adv Mater 2005;17:187–92. [152] Chen JY, Ahmad N, Shi DW, Zhou WP, Han XF. Synthesis and magnetic characterization of Co-NiO-Ni core-shell nanotube arrays. J Appl Phys 2011;110:073912. [153] Shelimov KB, Davydov DN, Moskovits M. Template-grown high-density nanocapacitor arrays. Appl Phys Lett 2000;77:1722–4. [154] Zhao CL, Li Q, Wan W, Li JM, Li JJ, Zhou HH, et al. Coaxial carbon-silicon-carbon nanotube arrays in porous anodic aluminum oxide templates as anodes for lithium ion batteries. J Mater Chem 2012;22:12193–7. [155] Banerjee P, Perez I, Henn-Lecordier L, Lee SB, Rubloff GW. ALD based metal-insulator-metal (MIM) nanocapacitors for energy storage. ECS Trans 2009;25:345–53. [156] Banerjee P, Perez I, Henn-Lecordier L, Lee SB, Rubloff GW. Nanotubular metal-insulator-metal capacitor arrays for energy storage. Nat Nanotech 2009;4:292–6. [157] Haspert LC, Lee SB, Rubloff GW. Nanoengineering strategies for metal-insulator-metal electrostatic nanocapacitors. ACS Nano 2012;6:3528–36. [158] Patri AK, Majoros IJ, Baker Jr JR. Dendritic polymer macromolecular carriers for drug delivery. Curr Opin Chem Biol 2002;6:466–71. [159] Papadopoulos C, Rakitin A, Li J, Vedeneev AS, Xu JM. Electronic transport in Y-junction carbon nanotubes. Phys Rev Lett 2000;85:3476–9. [160] Routkevitch D, Tager AA, Haruyama J, Almawlawi D, Moskovits M, Xu JM. Nonlithographic nano-wire arrays: fabrication, physics, and device applications. IEEE Trans Electron Devices 1996;43:1646–58. [161] Srivastava D, Menon M, Cho K. Computational nanotechnology with carbon nanotubes and fullerenes. Comput Sci Eng 2001;3:42–55. [162] Mahima S, Kannan R, Komath I, Aslam M, Pillai VK. Synthesis of platinum Y-junction nanostructures using hierarchically designed alumina templates and their enhanced electrocatalytic activity for fuel-cell applications. Chem Mater 2008;20:601–3. [163] Yang D, Meng G, Zhu C, Han F, Chen L. Synthesis and thermal expansion of copper nanotubes and nanowires with Y- and step-shaped topologies. Small 2010;6:381–5. [164] Xu Q, Meng G, Han F, Zhao X, Kong M, Zhu X. Controlled fabrication of gold and polypyrrole nanowires with straight and branched morphologies via porous alumina template-assisted approach. Mater Lett 2009;63:1431–4. [165] Chen B, Xu Q, Zhao X, Zhu X, Kong M, Meng G. Branched silicon nanotubes and metal nanowires via AAO-template-assistant approach. Adv Funct Mater 2010;20:3791–6.

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[166] Guo Q, Qin LR, Zhao JW, Hao YH, Yan ZK, Mu F, et al. Structural analysis and angle-dependent magnetic properties of Y-branched Ni nanowires. Physica E 2012;44:1988–91. [167] Xu LP, Yuan ZH, Zhang XG. Fabrication of multi-level branched metal nanowires by AAO template electrodeposition. Chin Sci Bull 2006;51:2055–8. [168] Zhang JP, Day CS, Carroll DL. Controlled growth of novel hyper-branched nanostructures in nanoporous alumina membrane. Chem Commun 2009:6937–9. [169] Tian Y, Meng G, Biswas UK, Ajayan PM, Sun S, Zhang L. Y-branched Bi nanowires with metal-semiconductor junction behavior. Appl Phys Lett 2004;85:967–9. [170] Li XD, Meng GW, Xu QL, Kong MG, Zhu XG, Chu ZQ, et al. Controlled synthesis of germanium nanowires and nanotubes with variable morphologies and sizes. Nano Lett 2011;11:1704–9. [171] Guo D, Fan L, Sang J, Liu Y, Huang S, Zou X. Fabrication of a regular tripod Ni–P nanorod array and an AAO template with regular branched nanopores using a current-controlled branching method. Nanotechnology 2007;18:405304. [172] Jo H, Haberkorn N, Pan JA, Vakili M, Nielsch K, Theato P. Fabrication of chemically tunable, hierarchically branched polymeric nanostructures by multi-branched anodic aluminum oxide templates. Langmuir 2016;32:6437–44. [173] Sui YC, Gonzalez-Leon JA, Bermudez A, Saniger JM. Synthesis of multi branched carbon nanotubes in porous anodic aluminum oxide template. Carbon 2001;39:1709–15. [174] Jin SY, Li Y, Li ZX, Hu X, Ling ZY, He XH, et al. Controllable fabrication and microstructure modulation of unique AAO structures based on patterned aluminum surface. J Electrochem Soc 2016;163:H1053–9. Qiaoling Xu is an associate professor of Institute of Solid State Physics (ISSP), Chinese Academy of Sciences (CAS). She received her PhD degree in Materials Physics and Chemistry from ISSP in 2009. Her main research interests focus on the synthesis of hierarchically branched- and hetero-nanostructures and their applications in nanodevices. Professor Guowen Meng is currently the Director of ISSP, CAS. He received his B. E. in 1984, M. E. in 1987, and PhD. in 1996 from Northwestern Polytechnic University, Xi’an, China. He worked as a post-doc at ISSP in 1996-1998, and later he was appointed as Full Professor in ISSP in 1999. Once he worked in Dept. of M.S. & E. at Rensselaer Polytechnic Institute (Troy, NY, USA) from Nov. 2002 to Dec. 2004. In 2005, he was selected in ‘‘100 Talent Program” of CAS, and in the same year, he earned the National Science Fund for Distinguished Young Scholars. His main research interests focus on rational design and building of hierarchically branched- and hetero-nanostructures for various prototype nanodevices and environmental applications of nanomaterials. He has authored and co-authored more than 200 peer-reviewed international journal papers. His entire publications have been cited for over 9970 times. The H-index of his publications is 50. He has received the 2nd class of the State Natural Science Award in China in 2006, the 1st class Natural Science Award (Anhui Province) in 2014 and 2003, the Science & Technology Award for Yong Scientists (Anhui Province) in 2006. Fangming Han is an Associate Professor of ISSP, CAS. He received his B. S. degree in semiconductor materials and devices from Lanzhou University in 1999, and PhD degree in condensed matter physics from ISSP in 2010. His main research interests focus on the synthesis, characterization and properties of the nanomaterials and nanostructures, especially on new type carbon nanomaterials and two-dimensional materials for electric energy storage.