Non-symmetric hybrids of noble metal-semiconductor: Interplay of nanoparticles and nanostructures in formation dynamics and plasmonic applications

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Review

Non-symmetric hybrids of noble metal-semiconductor: Interplay of nanoparticles and nanostructures in formation dynamics and plasmonic applications☆ Yinghui Suna, Xianzhong Yangb, Haofei Zhaoa, Rongming Wanga,

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a Beijing Key Laboratory for Magneto-Photoelectrical Composite and Interface Science, School of Mathematics and Physics, University of Science and Technology Beijing, Beijing 100083, China b Beijing National Laboratory for Condensed Matter Physics, Beijing Key Laboratory for Nanomaterials and Nanodevices, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China

A R T I C L E I N F O

A BS T RAC T

Keywords: Noble metal Semiconductor Nanoparticle Nanostructure Hybrid

Noble metal-semiconductor hybrids have been employed as fundamental structures in modern technologies. In these hybrids, their cooperative multiple functions attract much attention in recent years because of the interplay of nanoparticles and nanostructures. In this review, we summarize the interplay of nanoparticles and nanostructures in specific kinds of noble metal-semiconductor hybrids, termed as non-symmetric hybrids of noble metal-semiconductor. It particularly refers to metal nanoparticles (or semiconducting quantum dots) at 1dimensinal (1D) and 2-dimensional (2D) semiconductor (or metal) nanostructures, in contrast to the core/shell and heterodimer nanostructures. First, we discuss the formation dynamics, especially in chemical growth and assembly as well as physical coating and deposition, of non-symmetric noble metal-semiconductor hybrids with nanoparticles on nanostructures. Second, we introduce the plasmon-related applications of these hybrids in heterogeneous catalysis, optoelectronic or photovoltaic devices, all-optical devices, and surface detection or modulation. This review not only provides a comprehensive understanding of the formation mechanisms of the non-symmetric metal-semiconductor hybrid nanostructures, but also may inspire new ideas of novel functional devices and applications based on these systems.

1. Introduction Noble metal was discovered thousands of years ago and used as currency for a long time. Semiconductor, however, is a much ‘younger’ material as its discovery dates back to only two hundred years ago. Noble metal and semiconductor were originally distinguished with their different electrical transport behaviors. Their optical, mechanical, and thermal properties were further found to be drastically different. With the building up of condensed matter theory, these differences are well understood from their electronic band structures. Noble metal typically has no band gap and possesses a high density of free electrons, leading to high electrical and thermal conductivities as well as a wide range of absorption and a strong reflection of light. At a noble metaldielectric interface exist surface plasmons (SPs), which originate from collective electron oscillations and deliver propagating surface excitations and localized SP resonance (LSPR) through coupling of the electromagnetic fields to the oscillations of the electron plasma in the

conductor [1]. SPR is the main phenomenon giving rise to surfaceenhanced Raman scattering (SERS) [2,3] and tip-enhanced Raman scattering (TERS) [4], which is greatly applicable in sensitive detection [5–7] and photothermal therapy [8–10]. Semiconductor, in contrast, has a band gap in its band structure. The conductivity of a semiconductor is dominated by electrons in the conduction band (n-type) or holes in the valence band (p-type). Only the light with a higher energy than the band gap could be absorbed by an intrinsic semiconductor. Electron-hole pairs can be generated by optical excitation and could further form excitons due to the strong Coulomb interaction. Radiative recombination of excitons would induce photoluminescence in a semiconductor with, in particular, a direct band gap [11]. Both noble metal and semiconductor have become the key materials in the fields of electronics, optics, catalysis, and energy storage. Noble metal-semiconductor hybrids have also been employed as fundamental structures in modern technologies. For example, the building of logic circuits and solar cells usually requests semiconduc-

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Peer review under responsibility of Chinese Materials Research Society. Corresponding author. E-mail address: [email protected] (R. Wang).

http://dx.doi.org/10.1016/j.pnsc.2017.03.006 Received 12 December 2016; Received in revised form 2 March 2017; Accepted 9 March 2017 1002-0071/ © 2017 Chinese Materials Research Society. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).

Please cite this article as: Sun, Y., Progress in Natural Science: Materials International (2017), http://dx.doi.org/10.1016/j.pnsc.2017.03.006

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1D or 2D nanostructures in several typical noble metal-semiconductor hybrids.

tors as channels and noble metals as electrodes. The most known noble metal-semiconductor hybrid structure is Schottky junction, which is caused by the band alignment between the metal and the semiconductor. With the requirement of miniaturization of devices, nanostructured noble metal-semiconductor hybrids have become highly desirable. Many interface interactions, such as charge transfer, interface strain, and exciton-plasmon interaction, have been comprehensively studied in noble metal-semiconductor hybrid nanostructures. Through designing the device morphology and geometry, the functions of the devices based on noble metal-semiconductor hybrid nanostructures can be effectively modified [12–18]. Among various noble metal-semiconductor hybrid nanostructures, the core/shell and heterodimer nanostructures have been well studied as powerful bottom-up nanosystems to achieve fundamental interactions and functionality enhancements [19–22]. In particular, the precise control of the core/shell metal/semiconductor nanostructures, especially under large lattice mismatch, can maintain the monocrystalline of both metal and semiconductor [23], and facilitate the applications based on metal-semiconductor coupling [24,25]. On the other hand, the systems of nanoparticles at nanostructures, such as nanoparticles on nanowires, have attracted intense attention in recent years. In these systems, metal or semiconductor nanostructures provide a platform with a high surface-to-volume ratio, while semiconductor or metal nanoparticles could interact with the nanostructures to offer cooperative multiple functions. The variety of nanoparticles and nanostructures of metal or semiconductor has the hybrid systems deliver a huge amount of functions and a wide range of applications, including heterogeneous catalysis, optoelectronic devices, surfaceplasmon related devices, etc. In this review, we summarize the interplay of nanoparticles and nanostructures in specific kinds of noble metal-semiconductor hybrids, termed as non-symmetric hybrids of noble metal-semiconductor. It particularly refers to metal nanoparticles (or semiconducting quantum dots) at 1-dimensinal (1D) and 2-dimensional (2D) semiconductor (or metal) nanostructures. First, we discuss the formation dynamics, especially in chemical growth and assembly as well as physical coating and deposition, of non-symmetric noble metal-semiconductor hybrids with nanoparticles at nanostructures. Second, we introduce the plasmon-related applications of these hybrids in heterogeneous catalysis, optoelectronic or photovoltaic devices, all-optical devices, and surface detection or modulation. This review not only provides a comprehensive understanding of the formation mechanisms of the non-symmetric noble metal-semiconductor hybrid nanostructures, but also may inspire new ideas of novel functional devices based on these systems.

2.1. Chemical growth or assembly 2.1.1. Interface energy Interface energy dominates many formation processes of nanoparticles at nanostructures systems, especially in electrochemical deposition and reduction. The wettability derived from interface energy is the key factor to determine whether a metal or semiconductor become nanoparticles on another nanostructure. It has been proven both theoretically and experimentally that noble metals like Au and Ag prefer to form nanoparticles selectively on many types of nanostructures, such as cadmium chalcogenides [28–31], graphene [42,43], transition metal di-chalcogenides [39,40], etc. As promising photocatalysts, cadmium chalcogenides have been extensively investigated due to the facile alternation of bandgaps throughout the visible region by the precise control of their sizes. Their single-crystalline feature of nanostructures also provides an ideal platform to study the effect of interface energy on the formation of nanoparticles on semiconductor nanostructures. Different transition metals, such as Au, Pt, Co, were controllably coated along anisotropic nanorods of, in particular, CdS and CdSe [28–31]. The semiconductor nanorods can be decorated with metal nanoparticles (NPs) by reducing metal precursors with amines at room temperature [28–30]. Furthermore, it is possible to vary the shape and size of the individual components of hybrid nanostructure through proper choice of surface capping group, precursor ratio, temperature, and reaction duration [30,44]. Fig. 1 shows the transmission electron microscopy (TEM) images of metal-semiconductor heterostructure composed of CdS nanorods (NRs) with selective growth of Pt, Ni, and Co. Fig. 1B-D clearly show the preferential adsorption of Pt complex onto the rod tips [30]. The tips are more reactive because of the increased surface energy and also possibly because of imperfect passivation of the ligands on these faces. Once metal nucleates at the edge, it is preferential for additional metal to adhere and grow on that seed [28]. Banin et al. have demonstrated that a ripening process would turn a double-sided Au-tipped CdS nano-heterostructures (NHS) into a single-sided style upon the addition of more precursors of gold [29]. Fig. 2 shows the theoretical simulation of the process. When the gold concentration is below a threshold, a fluctuation in the gold density on the tips leads to growth on the two tips, with relative stabilization of gold on one tip (Fig. 2b). When the concentration is over the threshold, the further growth shows the consumption of the smaller tip (Fig. 2c). To understand the mechanism of ripening, the surface energy and the standard reduction electrode potential for metal NPs should be considered. Smaller gold NPs are more easily oxidized compared with the larger ones, and are relatively destabilized. This ripening process was achieved by the electron transfer in an electrochemical reaction along the NRs. In the small particle, Au must be oxidized to be released to solution and the electrons released are transferred through the rod to the large particle on which a gold ion is reduced leading to growth on that side.

2. Formation dynamics In the last two decades, a huge amount of 1D and 2D semiconducting nanostructures have been studied, such as silicon/zinc oxide/ titanium dioxide nanowires, cadmium chalcogenides nanorods, graphene (a semi-metal; it is considered as either a metal or semiconductor in this review), and transition metal di-chalcogenides. Various methods have been employed to prepare nanostructured metal-semiconductor hybrids through electrochemical deposition [26,27], reduction [28–34], photochemistry [32,35], ligand-exchange assisted formation [36], and physical deposition [37–40]. The formation of these heterostructures fundamentally involves heterogeneous nucleation and growth, or chemical/physical assembly, of one component on the other. Interface energy and related wettability of the two components must be considered in the dynamic model of the growth process. In addition, atomic arrangements and inter-atomic interactions also play vital roles in the growth process. Many review articles have been published to describe these growth methods that lead to a variety of heterostructures of nanoparticles at 1D or 2D nanostructures [12–18,41]. Thus in the following, instead of describing formation methods in details, we will discuss the formation dynamics or mechanisms of nanoparticles on

2.1.2. Atomic arrangements Besides the surface energetics, atomic arrangements and interatomic interactions also play vital roles in the formation of heterostructures [41]. At the metal-oxide interface, the metal and the oxide usually possess different crystal symmetries. The distinct difference in the symmetry of the overgrown islands and the substrate makes them hard to match each other, such as in the case of Ru or Pt NPs on a TiSi2 nanonet [45,46]. Ru was grown on TiSi2 by atomic layer deposition (ALD). In the C49 structure of TiSi2, the b planes consist of alternate layers of a Ti-Si mixture and Si-only atoms. The calculation of the absorption energy and adhesion energy of Ru NPs on a, b, c planes of TiSi2 shows that the NPs prefer to grow on the b-plane. A strong mixing 2

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Fig. 1. Metal-semiconductor nano-heterostructures composed of nanorods with selective growth of Pt, Ni, and Co. (A) CdS nanorods; (B) Pt-CdS NRs; (C) PtNi-CdS NRs; (D) PtCo-CdS NRs. Reproduced with permission from [30,44]. Copyright: 2008, American Chemical Society; 2012, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

phase, nanoparticle/solution interface energies can be strongly influenced by adhesion of solvent and capping ligands. Ligand-initiated cation exchange has been demonstrated as an effective way to tailor core/shell and heterodimer hybrid nanocrystals under large lattice mismatches [23,24,47]. Similar to the core/shell and heterodimer hybrid nanocrystals, ligand binding is an important factor to affect the formation dynamics of non-symmetric hybrid nanostructures. In the study of Co-TiO2 nanorod heterostructures, Cozzoli et al. found that the ligand or surfactant composition and concentration are the most influential parameters in governing the growth mode of the heterostructures [48]. If the surfactant/TiO2 ratio was very low, the Co would nucleate randomly onto both the tips and the sidewalls of the nanorod.

at the interface between Ru38 and C49 TiSi2 is obvious (Fig. 3A), suggesting greater interaction between the Ru NP and the Si layer of the b planes in TiSi2. The calculation based on density functional theory (DFT) is consistent with the experimental observations by TEM. As shown in Fig. 3C and D, metallic Ru NPs were observed only siteselectively grown on the top and bottom surfaces of C49 TiSi2 (bplane). 2.1.3. Ligand binding In addition to the intrinsic facet-dependent reactivity, controlling of ligand exchange or reaction conditions also provide effective ways to achieve topological selectivity in heterostructures. Typically in colloidal

Fig. 2. Simulation of the growth dynamics of Au on the nanorod. (a) A random initial configuration of Au (time t=1 in Monte Carlo steps). (b) Au grows on both ends of the rod, leading to the formation of nano-dumbbells (t=260,000). (c) Formation of a nano-bell-tongue (t=33,500,000). The larger and more stable right tip continues to grow, while the left tip is nearly fully consumed. Reproduced with permission from [29]. Copyright: 2005, Nature Publishing Group.

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Fig. 3. DFT calculations show that Ru NPs prefer the b planes (A) over the c planes of C49 TiSi2 (B). It is consistent with experimental observations by TEM from the top (C), where b planes are parallel to the viewing direction. Inset: size distribution of Ru NPs. When viewed from the side (D), where b planes are perpendicular to the viewing direction, no Ru NPs are seen on the c or a plane. Reproduced with permission from [45]. Copyright: 2014, American Chemical Society.

Fig. 4. (a) Schematic of the synthesis of Au/TiO2 hybrid nanostructures by the ligand-exchange method; (b) TEM image of the typical Au/TiO2 hybrid nanostructures; (c) HRTEM image shows the close contact and lattice correlation between the Au NP and the TiO2 nanocrystal. Reproduced with permission from [50]. Copyright: 2014, American Chemical Society.

2.1.4. Reaction condition-related selectivity The selectivity of chemical assembly can also be affected by the reaction conditions in liquid, such as temperature, pressure, and pH value. For example, Liang et al. synthesized different Au–AgCdSe hybrid nanostructures with controlled morphologies and spatial distributions [51]. The monodispersed Au NRs (Fig. 5a) are covered with a thin layer of Ag after the Step 1. The products have two different shapes under different pH values, as shown in Fig. 5b and c. By simply manipulating the pH value of the system, the AgCdSe NPs could selectively grow at one end, at both the ends, or on the side surface of an Au-nanorod, as shown in Fig. 5d-f. These three types of Au–AgCdSe hybrid NRs displayed distinct localized surface plasmon resonance and photoluminescence properties [51]. The synthesized semiconductor NPs could enhance the stability of metal nanostructures and tune their localized surface plasmon resonance frequencies.

At suitable surfactant/TiO2 ratios, the surfactants would preferentially bind to the sidewall of the nanorod, and in this case, the Co NPs would grow only onto the unprotected tips. If the surfactant/TiO2 ratio exceeds a threshold value, the Co cannot nucleate on the nanorods, but homogeneous nucleation in the solution takes place. Although they dealt with magnetic metal (not exactly noble metal) NPs, the ligand exchange has been successfully utilized by other groups to deposit Au NPs on bulk semiconductors [49] and TiO2 nanocrystals [50]. Ding et al. fabricated Au/TiO2 hybrid nanostructures by depositing presynthesized colloidal uniform Au NPs to TiO2 nanocrystals and then removing capping ligands on Au surface through a delicately designed ligand-exchange method and a mild annealing process [50]. Fig. 4 shows the schematic of their fabrication process (Fig. 4a) and the resulted Au NPs at anatase TiO2 crystals in TEM and high resolution transmission electron microscopy (HRTEM) (Fig. 4b and c). 4

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Fig. 5. Schematic illustration with TEM images shows the synthesis of three different types of Au–AgCdSe hybrid NRs under different reaction conditions. (a) TEM image of the Au NRs (15 nm×58 nm in width and length, respectively); (b) the I-shaped Au-Ag hybrid NRs obtained at pH=6.8; (c) the shuttle-shaped Au-Ag NRs obtained at pH=8.6; (d) the mike-like AuAgCdSe NRs; (e) dumbbell-like Au-AgCdSe NRs; (f) toothbrush-like Au-AgCdSe NRs. Reproduced with permission from [51]. Copyright: 2012, American Chemical Society.

developed to apply in various fields, such as catalysis, energy harvest, and tunable optics. In this review, we will focus on the plasmon-related applications based on surface plasmon resonance (SPR) modulation and exciton-plasmon interaction.

2.2. Physical coating or deposition In addition to chemical assembly, physical coating or deposition is also an efficient and mature method to construct nanoparticles on other nanostructures [45]. Similar dynamics, like surface energy, dominates the formation of heterostructures. Spin-coating is a facile approach to physically disperse solution-based nanoparticles on target nanostructures [52–54]. For example, Wei et al. spin coated a commercial solution to make quantum dots with a CdSeTe core and a ZnS shell on SiO2-coated Ag nanowires [52]. The density of coated nanoparticles can be well controlled by the solution concentration, the spinning speed, and the wettability between the solution and the target. Evaporation and sputtering are two typical approaches of physical deposition that can be commercially scaled up. They are all corresponding to vapor-solid procedure. The deposited materials can be metal or semiconductor, which could form nanoparticles on target nanostructures depending on the wettability of the materials with the substrate. It is well known that there are three types of growth modes (island, layer, and island-layer) for physically deposited thin films. The growth of noble metals, such as Au and Ag, follows the Volmer–Weber island growth mode on semiconducting materials. In this mode, atom clusters nucleate and grow on a semiconductor surface, forming isolated metal islands. Nucleation continues to occur as long as exposed area exists. The islands keep growing until a continuous and polycrystalline film forms. By controlling the quantity of deposited metals, the size and the shape of metal nanoparticles can be tuned in physical deposition. The physical deposition has been widely employed to deposit noble metal nanoparticles on carbon nanotubes, graphene, and transition metal di-chalcogenides [37–40]. Fig. 6 shows the morphologies of different metal-coated monolayer MoS2 measured by atomic force microscopy (AFM) [40]. Au and Ag apparently follow the Volmer–Weber island growth mode, while Pd appears to follow the layer growth mode. Compared with chemical methods, physical deposition is expected to be able to create cleaner interface between metal and semiconductor nanostructures.

3.1. Heterogeneous catalysis Photocatalysts for various chemical transformations induced by UV–vis light are almost exclusively semiconductors. In a photocatalyst process, a flux of photons is absorbed by a semiconductor, yielding charge carriers (electron (e−)–hole (h+) pairs) in the semiconductor. The charge carriers separate from each other and diffuse to catalytically active sites at the semiconductor/liquid interface where they drive chemical transformations. For an efficient semiconductor photocatalyst, long lived charge carriers, less charge trapping centers, proper energy level offsets, and stability against light are always desirable. Synthetic advancement makes it possible to design the desired NHSs with particular band alignment favoring expected charge separation. It has been reported that composite noble-metal/semiconductor photocatalysts achieved significantly higher rates in various photocatalytic reactions compared with their pure semiconductor counterparts. Photocatalytic conversion of solar energy into hydrogen fuel through water photolysis is a very attractive field considering the clean and renewable energy resources [13,55–58]. Au/CdS hybrid nanostructures with precisely controlled structural symmetry were reported to maintain the monocrystalline of CdS under large lattice mismatch during the growth process. Optimized heterodimer structured Au/CdS manifest a dramatic photoactivity enhancement of ∼73 times, compared to pure CdS quantum dots [24]. Banin and co-workers revealed that CdS NRs with even small size Pt cluster can generate hydrogen with an efficiency up to 3.9% [59]. It was discovered that noble metal NPs are capable of sensitizing wide-bandgap semiconductors, exhibiting much enhanced visible-light response [60–63]. Ingram et al. reported that the rate of oxygen evolution under broadband visible illumination ( > 400 nm) is significantly enhanced on composites of plasmonic Ag and nitrogendoped TiO2 (N-TiO2) compared with pure N-TiO2 [61]. Garcia and coworkers obtained similar conclusions in their studies, which showed that the rate of hydrogen-evolution half-reaction was higher on plasmonic Au/TiO2 composites than on TiO2 under broadband visible illumination (Fig. 7a) [55]. The enhanced catalytic rate in the literatures can be attributed to two mechanisms. The first is the charge transfer mechanism, which depends on the band alignment of plasmonic metal and

3. Plasmonic applications The nanoparticle-nanostructure hybrid structure introduces a unique system containing nanometric metal-semiconductor interfaces, which lead to multifunctional capabilities with enhanced properties and potential applications. The noble metal-semiconductor hybrids are 5

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Fig. 6. AFM images of monolayer MoS2 coated with 2 nm-thick (a) Pd, (b) Au, and (c) Ag film. The bottom schematic of each image shows the growth mode for the three metals. Reproduced with permission from [40]. Copyright: 2013, American Chemical Society.

Fig. 7. (a) Volume of hydrogen evolved (VH2) during the photocatalytic runs (λ > 400 nm) using catalysts with different gold loadings and methanol as sacrificial electrons donor: TiO2 (■), Au(0.25%)/TiO2 (•), Au(1.5%)/TiO2 (▲) and Au(2.2%)/TiO2 (▼) [55]. (b) Mechanism of SPR-induced charge transfer with approximate energy levels on the normal hydrogen electrode (NHE) scale. Dashed red lines refer to the water-splitting redox potentials. (i) Electrons near the metal Fermi level (Ef) are excited to surface plasmon (SP) states; (ii) the electrons transfer to a nearby semiconductor nanostructure; (iii) this activates electron-driven processes such as the hydrogen-evolution half-reaction. Reproduced with permission from [13]. Copyright: 2011, Macmillan Publishers Limited.

Fig. 8. (a) Schematic view of graphene-assisted, plasmon-driven reaction of the transformation of PATP-to-DMAB. (b) Scanning electron microscopy (SEM) image of the large-area, well-ordered, uniform-sized, graphene-coated Ag bowtie nanoantenna arrays (below the yellow dashed line); the yellow dashed line shows the graphene border. (c) SEM image of bare Ag bowtie nanoantenna arrays. (d) SEM images of the bare Ag bowtie nanoantenna arrays after oxidation and (e) chemically inert, graphene covered Ag nanoantenna arrays after a month. The Ag bowtie nanoantenna arrays are protected by the graphene monolayer. Reproduced with permission from [64]. Copyright: 2015, CIOMP.

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Fig. 9. SEM images of (a) silicon nanocone array, which were fabricated by the maskless etching in the ICP system, (b) the floral-clustered graphene nanosheets, which were grown on the nanocone array, (c) the high-resolution image of petaliform graphene nanosheets on the nanocone tips, (d) Ag NPs attached to both sides of a graphene nanosheet, (e) SEM images of three different substrates (left side) corresponding to their SERS spectra (right side).Reproduced with permission from [66]. Copyright: 2016, Macmillan Publishers Limited, part of Springer Nature.

formation on the surface of semiconductor (nearby the plasmonic metal) increases because the rate of electron-hole formation is proportional to the local intensity of the electric field. These mechanisms also work in novel systems of metal/2D material hybrids [64,65]. Recently, Dai et al. reported that the number of graphene layers could control the plasmon-driven, surface-catalyzed reaction that converts para-aminothiophenol (PATP) to p, p-dimercaptoazobenzene (DMAB) on graphene-coated Ag bowtie nanoantenna

semiconductor. The plasmonic metal NPs and semiconductor are in direct contact with each other, allowing a rapid transfer of charge carriers. Owing to the band alignment of semiconductor and SPR excitation of noble metal, the plasmonic-metal/semiconductor systems allow for the transfer of only energetic electrons from the metal to the semiconductor, as shown in Fig. 7b [13]. The second is the radiative energy transfer from metal SPR to the semiconductor, which can also take place by enhanced local electric field. The rate of electron-hole 7

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arrays (ABNA) hybrids [64]. Fig. 8a is a schematic of the grapheneassisted, plasmon-driven reaction of the transformation of PATP-toDMAB. They found that after a month, bare ABNA were oxidized while the graphene-covered ABNA did not show any signs of changes (Fig. 8d vs. e). Then they systematically studied the plasmon-driven surfacecatalyzed reactions on this hybrid, and found that monolayer graphene could enhance the PATP-to-DMAB conversion, but bilayer graphene decreased the reaction. This is because monolayer graphene introduces a strong dipole that is enhanced by the local electric fields, resulting in an electron transition between PATP and graphene. A couple of PATP molecules then lose electrons to become DMAB molecules on the graphene surface. For the bilayer graphene, however, the electron transition is weak and hot electron transfer from Ag is difficult. Three-dimensional (3D) hybrids have been investigated in recent years because they have 3D space and ultrahigh surface area to accommodate NPs. Therefore, they are expected to have much more catalytic active sites than 1D or 2D hybrids. The reported Ag-3D graphene hybrids can significantly enhance the surface catalytic reactions. Zhao et al. successfully fabricated a 3D hierarchical hybrid of vertical flower-like graphene nanosheets (FGNSs) sandwiched by Ag NPs on a silicon substrate with nanocone arrays (Fig. 9a-d) [66]. They studied the surface catalytic reactions of 4-nitrobenzenethiol (4NBT) on 2D and 3D graphene-Ag NPs hybrids (Fig. 9e). Through comparing the intensity ratio of DMAB peak at 1432 cm−1 to the D peak of graphene at 1335 cm−1, they found that 3D structure of graphene/AgNPs is much more efficient for enhancing plasmon-driven catalytic reactions than 2D planar structure.

optoelectronic and photovoltaic devices [67,68]. The efficient light trapping originates from not only the increase of the cross-section of absorption induced by the surface plasmons of metal nanostructures, but also the elongation of optical path of the incident light caused by the subwavelength scattering of metal NPs. The plasmonic enhanced absorption has been utilized to improve the efficiency of photo-detection or solar energy conversion of semiconducting thin films [69–71]. Intensive interests in 2D materials with atomic thickness, such as graphene and MoS2, have inspired a new sight to a “film” and shown great promising properties in optoelectronic and photovoltaic devices [72–81]. Plasmonic enhanced 2D systems for optoelectronic and photovoltaic applications have been hot topics of scientific research in recent years [82,83]. In solar energy conversion, Yang et al. fabricated Au NPs-decorated MoS2 sheets as a hole transport layer to make use of plasmonics for organic solar cells [84]. It resulted in an enhanced efficiency of ~17%, compared with the system without Au NPs. In photo-detection, Miao et al. demonstrated a deposition of Au nanoparticles or nanoplates onto a few-layer MoS2-based photodetector (Fig. 10a and b) [85]. Due to the localized surface plasmon resonance, they obtained a two-fold increase in the photocurrent after sparsely depositing 4 nm-thick (nominal thickness) Au onto MoS2 (Fig. 10c and d). The photocurrent can be further increased with periodic Au nanoarrays, which is mainly attributed to the near-field oscillation and scattering effect of the periodic Au nanoarrays. These results show that the coupling of plasmonic metal NPs with atomically thin semiconductors would have great potential to enable high-performance devices in solar energy conversion and photo-detection.

3.2. Optoelectronic and photovoltaic devices

3.3. All-optical devices

Light trapping based on surface plasmon resonance effect of metal nanostructures is a promising strategy to improve light harvesting for

Under the specific irradiation of light, there are localized electromagnetic modes in metal nanostructures, and relatively long-lived

Fig. 10. (a) Schematic of a MoS2 phototransistor coated with Au nanoparticles. (b) SEM images of Au nanoparticles with the nominal thicknesses ranging from 4 to 1 nm. Scale bar, 50 nm. (c) UV–vis absorption spectra of Au nanoparticles on glass. (d) Dependence of photocurrent on source-drain voltage of MoS2 phototransistors with/without Au nanoparticles. Reproduced with permission from [85]. Copyright: 2015, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

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excitations [90]. Wei et al. observed a reversed process in the coupled system of quantum dot and metallic nanowire, as illustrated in Fig. 12a [52]. When launching the surface plasmons at the discontinuity of Ag nanowires, the propagating SPs interact with quantum dots on the nanowire and excite the excitons whose radiative decay can be directly observed in the quantum dot emission image shown in Fig. 12b. The two images correspond primarily to the SP-exciton conversion process. The bright spots at the opposite ends of the launching spots (marked with green circles) are actually due to the exciton-SP-photon conversion.

excitations in semiconductor. The complementary optical properties of combining metal and semiconductor nanostructures can provide novel properties and phenomena based on exciton-plasmon interactions. The exciton-plasmon interactions lead to an effective increase of the absorption cross section of the semiconductor nanostructure in proximity of metal NPs. Therefore, the performance of photovoltaic systems can increase due to the thinner absorption layers that reduce losses compared with thicker layers [86,87]. Exciton-plasmon interactions can also give rise to modified emission properties, such as emission enhancement used to increase the sensitivity of optical sensing [88] and the change of emission wavelength of semiconductor nanostructures [39]. Moreover, the energy transfer between the excitonic and plasmonic system is very attractive, and has been exploited in surface plasmon polariton (SPP) propagation [52,89,90], surface plasmon amplification [91,92], and lasing [54,93,94]. Among the diverse applications based on exciton-plasmon interactions, we take the SPP propagation as an example in the system of semiconductor NPs on a metal nanowire. When semiconductor quantum dots or nanocrystals couple with metallic nanowires, the energy is guided, locally concentrated, and released by surface plasmon. Fig. 11a shows the schematic of a single CdSe quantum dot optically excited near a silver nanowire. The excited optical emitters have three channels for energy relaxation: photoluminescence (PL) into free space, dipoledipole interaction with the damped mirror dipole, excitation of surface plasmons in metal and plasmon-photon-conversion as the result of scattering at surface roughnesses and structural discontinuities such as the Ag-nanowire ends (high efficiency) [89,90]. Therefore, the emission from the quantum dot couples directly to surface plasmons in the nanowire causing the wire's ends to light up, as shown in Fig. 11b (indicated by blue circle). The transferred energy maintains the quantized character of the exciton source and can lead to single SP

3.4. Surface detection and modulation Surface plasmon resonance of metal nanostructures could be utilized to enhance optical signals coming from a surface that they contact. When the surface goes to the atomic limit, like 2D semiconductors, the interactions between metal NPs and 2D semiconducting materials can exhibit interesting physical phenomena that are of both fundamental interests and technological importance [39,40,95,96]. Sun et al. investigated the interactions between noble metal NPs and atomically thin 2D materials, such as MoS2 and WS2 [39]. The Ag NPs deposited on 2D layered materials can not only generate local mechanical strain, but also affect the Raman vibrational modes and PL emission [39]. The deposited metal can introduce new nonradiative recombination pathways that suppress PL emission, as shown in Fig. 13a. Compared with the Raman spectra of pristine MoS2, two new peaks red shifted from E12g and A1g modes, as well as an E1g peak, emerge in monolayer MoS2 covered by Ag NPs (Fig. 13b). Highly inhomogeneous strain spreads around the circular edge of the Ag NP contacting with MoS2, where the electric field is highly localized and enhanced as shown in Fig. 13c and d. Strongly enhanced local electric field from surface plasmon excitations enables the selective probing of the metal–MX2 boundaries through optical methods. Exciton-plasmon coupling also exists in the hybrids of metal NPs/ 2D semiconductors, and could be used to modulate the properties of the ultrathin 2D materials. Kang et al. deposited Au NPs onto MoS2 monolayer. Because the Schottky barrier between MoS2 monolayer and Au NPs is lower than 0.8 eV, hot electrons generated by plasmon

Fig. 11. (a) A coupled quantum dot can either spontaneously emit into free space or into the guided surface plasmons of the nanowire with respective rates Γrad, Γpl. (b) Left, channel I: nanowire image. Middle, channel II: image of quantum dots. The red circle denotes the position of the coupled quantum dot, and the same point is also denoted in the leftmost image. Right, channel III: the excitation laser was focused on the quantum dot (red circle). The largest bright spot corresponds to the quantum dot fluorescence, while two smaller spots correspond to surface plasmons scattered from the nanowire ends. The blue circle indicates the farthest end of the nanowire. Reproduced with permission from [90]. Copyright: 2007, Nature Publishing Group.

Fig. 12. (a) Schematic illustration of the excitation process of excitons in quantum dot (QD) by launching the surface plasmons at one end of Ag nanowire. (b) QD emission images obtained by focusing a 710 nm laser on the wire ends marked with green circles. Reproduced with permission from [52]. Copyright: 2009, American Chemical Society.

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Fig. 13. (a) PL spectra of monolayer MoS2 before (black line) and after 1 nm Ag deposition (red line). Inset: AFM image of monolayer with Ag NPs. The scale bar is 100 nm. (b) Raman spectra of monolayer before (black dash line) and after Ag deposition (red line). Inset: Lorentzian fitting of the splitting Raman peaks. (c) Simulated distribution of the amplitude and the direction of local electric field upon an optical excitation of 488 nm wavelength. The arrows in the selected area indicate the electric field direction, and their lengths indicate the field strength. The diameter of the Ag NP is 10 nm. (d) Schematic diagrams of the distribution of local strain in monolayer. Reproduced with permission from [39]. Copyright: 2014, American Chemical Society.

devices, and surface detection or modulation. This review may inspire new ideas of novel functional devices based on SPR modulation and exciton-plasmon interaction. Although researchers have made intense efforts to rationally design and controllably fabricate noble metal-semiconductor hybrids, the clarification of the formation mechanisms in the atomic sale is still limited. Much work was focused on the pure metal or hybrid metal nanocrystals [41]. DFT calculation plays an important role in the explanation of the growth mechanism by atomically modeling the interface and structure-property relationships [41]. Furthermore, in experimental, TEM characterization with atomic resolution is an indispensable method to study nanostructures. The in situ TEM experiments can provide valuable information on growth or reaction mechanisms. Recently, the aberration-corrected scanning transmission electron microscopy (STEM) with high angle annular dark field (HAADF) imaging has become a powerful tool. Since HAADF signal is highly sensitive to the atomic number [97], even individual atoms could be directly observed as well as atom columns [98,99]. By analyzing the image contrast, the atomic species can be identified and the number of atoms along each column can be obtained [100– 102]. Based on the obtained column numbers, the 3D atom structures of Pt nanoparticles on the surface of NiPt hollow spheres before and after catalyzing CO oxidation were established to evidence the migration of Pt atoms during the catalysis [102]. With the advance of technology, dynamic phenomena in the synthesis and application of nanomaterials can also be studied in situ within TEM [103–105]. In addition, the high spatial and energy resolution characterization is also helpful for the deep understanding of the plasmon-related interaction in noble metal-semiconductor hybrids. For example, Wei et al. mapped the spatial distribution of the exciton-surface plasmon

excitation in Au NPs under an excitation wavelength of 488 nm can transfer into the surrounding MoS2 monolayer, thus enabling an effective hot electron doping. With the observation of Raman and PL spectroscopies, they found that MoS2 has a transient reversible 2H to 1T phase transition due to the hot electron doping [95]. On the other hand, Zhao et al. recently hybridized the MoS2 with different sizes of Ag NPs, and they found the exciton-plasmon coupling can be tuned in monolayer MoS2-Ag NPs hybrid systems by changing the sizes and shapes of the particles. In monolayer MoS2, significant PL enhancement was achieved by altering the localized SPR to the excitation and exciton energy. The coherent dipole–dipole coupling between excitons and localized surface plasmon induces a transparency dips in the hetero system [96]. These results highlight viable approaches for the active control and engineering of light–matter interactions in the hetero-systems of 2D semiconductors with plasmonic nanostructures. 4. Summary and outlook In this review, we summarize the interplay of nanoparticles and nanostructures in specific kinds of noble metal-semiconductor hybrids, termed as non-symmetric hybrids of noble metal-semiconductor. That contains metal nanoparticles (or semiconducting quantum dots) at 1D and 2D semiconductor (or metal) nanostructures. The preparation methods of metal-semiconductor hybrids include chemical growth and assembly, as well as physical coating and deposition. The formation dynamics was discussed in various processing, such as interface energy, atomic arrangements, ligand binding, and reaction condition. Among the multifunctional capabilities and diversified applications, we introduce the plasmon-related applications of these hybrids in heterogeneous catalysis, optoelectronic or photovoltaic devices, all-optical 10

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