Hybrid metal-based carbon nanotubes: Novel platform for multifunctional applications

Progress in Materials Science 69 (2015) 183–212

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Hybrid metal-based carbon nanotubes: Novel platform for multifunctional applications Caterina Soldano ⇑ ETC s.r.l., Via P. Gobetti 101, 40129 Bologna, Italy

a r t i c l e

i n f o

Article history: Received 24 August 2014 Accepted 12 November 2014 Available online 27 November 2014 Keywords: One-dimensional hybrid carbon nanostructure Metal-filled carbon nanotube Metal-decorated carbon nanotube (Ferro)magnetic carbon nanotube Multifunctional applications In-situ synthesis Ex-situ synthesis

a b s t r a c t Combining objects with diverse properties has often the advantage of giving rise to novel functionalities. In this scenario, metal-filled and decorated carbon nanotubes (m-CNTs) represent a class of hybrid carbon-based nanostructured materials with enormous interest for application in several fields, ranging from nanoelectronics and spintronics to nanomedicine and magnetic data recording. The present review will provide the reader with an overview of state-of-the-art hybrid architectures based on m-CNT systems, methods currently employed for their fabrication, the set of their unique properties and how they can be applied toward novel devices with multifunctional properties for a broad range of applications. Ó 2014 Elsevier Ltd. All rights reserved.

Contents 1. 2.

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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis of metal-filled and decorated-carbon nanotubes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. In-situ synthesis (combined growth and filling) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Ex-situ filling (post-growth filling) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Properties and applications of metal-filled and decorated carbon nanotubes . . . . . . . . . . . . . . . . . . . . 3.1. Electrical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Optical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Magnetic properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

⇑ Tel.: +39 051 6398332; fax: +39 051 6398540. E-mail addresses: [email protected], [email protected] http://dx.doi.org/10.1016/j.pmatsci.2014.11.001 0079-6425/Ó 2014 Elsevier Ltd. All rights reserved.

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3.4. Medical properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5. Sensing properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Nomenclature AAO Anodic Aluminum Oxide ALD atomic layer deposition AFM Atomic Force Microscopy CA contrast agent CNTs carbon nanotubes CVD chemical vapor deposition DFT density functional theory DNA Deoxyribonucleic Acid ESFDA electrostatic force directed assembly FET Field-Effect Transistor HAMR heat-assisted magnetic recording HBT high-bias treatment IV current–voltage MFM magnetic force microscopy MFP mean-free path MRI magnetic resonance imaging MRFM magnetic resonance force microscopy MWNT multi-wall carbon nanotube MPCVD microwave plasma chemical vapor deposition NEMS NanoElectroMechanical System NC nanocrystal NP nanoparticle SEM scanning electron microscopy SERS surface-enhanced Raman scattering SPR surface plasmon resonance STEM–BF Scanning Transmission Electron Microscopy–Bright Field STEM–HAADF Scanning Transmission Electron Microscopy–High Angle Anular Dark Field STM scanning tunneling microscopy SWNT single-wall carbon nanotube TEM Transmission Electron Microscopy

1. Introduction One approach toward building nanoscale hybrid heterostructures is to use one component as a scaffold and build the second component in or around it. Carbon nanotubes (CNTs) form a robust, low-cost, exotic platform for the fabrication of a number of different kinds of nanoscale architectures. Since their discovery in the early 90s [1], carbon nanotubes have attracted constantly and increasingly interest in the scientific community both from a technological and a fundamental point of view. In particular, carbon nanotubes present novel properties, which render them outstanding potential candidates for a broad variety of applications ranging from optics to electronics [2,3]. Carbon nanotubes

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are made of either one (single wall, SWNT) or multiple (multi wall, MWNT) rolled-up graphene sheets [4,5]. They can be either metallic or semiconducting, and their intrinsic electronic properties are determined by the chiral structure of the atomic assembly and the one-dimensional quantum confinement. Depending on the mean-free path (MFP) of the electrons, ballistic transport [6] as well as signatures of diffusive quantum transport [7] has been reported. Because of their remarkable electrical, thermal, mechanical and optical properties, and extremely high aspect and surface-to-volume ratios, carbon nanotubes find applications, for example, in FETs [8], NEMS [9], flat-panel displays [10], sensors [11,12], switches [13], supercapacitors [14], batteries [15], targeted drug delivery systems [16], transparent conductive electrodes [17], flash memories [18], spintronics [19], filters [20], energy storage [21], nanomedicine [22–24] and interconnects for the sub-22 nm technology node [25–27]. Several aspects render carbon nanotubes ideal candidates for the development of a platform for novel and innovative nano- and micro-scale devices and architectures. Nanotubes can be synthesized straightforwardly, show striking transport properties at low temperatures and are extremely robust and easy to handle. Further, carbon nanotubes are chemically very stable and their stability is not affected if tubes are either opened and/or filled [28]. Then, the possibility of using nanotubes as building blocks for various carbon-based heterostructures opens up to a broad range of possible research and original applications. Carbon nanotubes can be filled with metals, semiconductors, salts, organic materials, therapeutics, either during the synthesis process (in-situ filling) [29,30] or through opening and subsequent filling of the nanotube (ex-situ filling) [31,32]. In this scenario, one area of research that has been growing very rapidly is the design, development and characterization of metal hybrid carbon-based nanostructures, generally consisting of nanoscale metal objects (individual atoms or particles, clusters, nanowires, atomic chains, etc.) and carbon nanotubes [33]. Throughout this review, this novel class of metal hybrid carbon nanotubes will be referred as m-CNTs. Combining carbon nanotubes with metallic nanostructures leads to new functionalities in terms of electronic, optical and mechanical properties [34–37]. In this class of hybrid materials, the outer carbon wall represents an interesting shell-type building block onto or into which a number of different species can be selectively placed (i.e. DNA [38], fullerene [39], atomic chain [40]). In particular, the geometric nature of the tube further provides an important built-in barrier against oxidation or interaction with the external environment, ensuring long-term stability of the inner core. Depending on the kind of specific applications, filled- and decorated-carbon nanotubes with tailored and tuned properties are thus required. Partially or fully-filled carbon nanotubes with for example magnetic materials present both small size and enhanced magnetic coercitivity, opening the way to applications in magnetic data storage technology. For example, in the field of biomedicine, a partial filling of the inner volume of the tube is often preferable over a complete filling since the nanostructure functionality is to behave channel-like, and where the filling represents only one functional component of the entire structure. On the other hand, if a high-aspect ratio of the filling is required (i.e. magnetic probes in magnetic force microscopy, MFM) a continuous filling is indeed in need and it is of fundamental importance to lead to a geometrically extended magnetic dipole. From a fundamental point of view, m-CNTs further represent an interesting platform to study fundamental physical phenomenon such as weak localization in the presence of adatoms [41], Kondo effect in case of magnetic impurity [42], as well as enhanced spin–orbit coupling [43]. In fact, it has been shown that in the limit of small external magnetic fields, the charge transport of a CNT filled with magnetic (cobalt) clusters is extremely sensitive to the magnetic state of the cluster inside the tube [44]. Similarly, it has been proposed that those hybrid structures can be tailored for applications in fields such as photo-thermal therapy [45] as well as heat-assisted magnetic recording (HAMR) [46– 48], for which each individual cluster (and potentially atom) can be regarded as an individual magnetic bit carrying information. The possibility of selectively tuning the properties in these hybrid CNT-based nanostructures holds immense potential for a broad range of applications in numerous fields. This review will provide in the first section a comprehensive overview of current techniques and methodologies used to fabricate hybrid metal-based carbon nanotubes. Then, in the second section, different properties of this class of one-dimensional hybrid carbon nanostructures will be presented and particular attention will be devoted to the potential fields of applications for each class of specific

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hybrid materials (i.e. SWNTs vs. MWNTs, individual tubes vs. tubes in bulk assembly/array, different filling materials, and so on). 2. Synthesis of metal-filled and decorated-carbon nanotubes Numerous techniques are currently available to fill and decorate carbon nanotubes, the choice of which largely depends on the type of carbon nanotubes (SWNTs or MWNTs, individual tubes or bundles/arrays) and the filling material and its physical properties. Liquid, solid or vapor phase methods can be used to fill carbon nanotube inner volumes. Therefore, solubility, melting and boiling points, and decomposition temperature are crucial parameters that need to be taken into account during the synthesis process. In this review, two main approaches toward the synthesis of these hybrid carbon-based nanostructures will be presented: (i) in-situ, where the growth of carbon nanotubes and the filling procedure are achieved within the same process and (ii) ex-situ, or post-growth in which the filling or decoration is performed in a separate step following the synthesis of carbon nanotubes. These approaches are schematically summarized and presented in Fig. 1. Both methodologies will require at least some fundamental knowledge on the growth of pristine (empty) carbon nanotubes. However, it is beyond the scope of the present review to present a detailed description of the various synthesis processes and growth mechanisms; for further details, the reader is referred to more in depth and specific literature (arc-discharge [49], laser ablation [50], chemical vapor deposition (CVD) [51]). 2.1. In-situ synthesis (combined growth and filling) In-situ synthesis of m-CNT can mainly be pursued by two different growth methods: chemical vapor deposition in the presence of a catalyst and arc-discharge [52,53]. In the latter, the synthesis process requires a catalyst (often a metal) to control the reaction kinetics (i.e. decomposition of the precursor, diameter of tubes, etc.) and a hydrocarbon precursor compound, representing the carbon source for the formation of the CNT walls (for pristine tubes, often an additional hydrocarbon source is used). The entire process is largely dependent on specific experimental conditions for which, the catalyst needed to initiate the growth of the carbons walls, is wedged and embedded inside the tubes.

Fig. 1. Techniques and processes currently available for the fabrication of m-CNTs (see manuscript for details).

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Clearly, this approach is fundamentally limited to materials that are catalysts for carbon (transition metals such as nickel, cobalt and iron), although some efforts to encapsulate other materials have been also reported [54]. Use of organometallic compounds containing those metals are suitable for the synthesis of (Fe, Co, Ni)-filled CNTs; the ferromagnetic nature of those catalysts thus also allows an easy route toward the synthesis of ferromagnetic cores/nanowires encapsulated in CNTs [55,56], the fabrication of which would not otherwise be straightforward. The low solubility of those metallocenes (ferrocene, cobaltocene and nickelocene) leads to the formation of metastable carbides, making those catalysts quite efficient. Filling with alloys (i.e. Fe–Co) is also possible by mixing for example ferrocene and cobaltocene powders [57] or by dissolving in toluene both compounds prior to carbon nanotube growth [58]. In the case of nanotubes grown by arc-discharge, the standard growth procedure is slightly modified so that the graphite anode can also host the filling material powder; this can be achieved both by mixing it directly with the graphite powder or filling a hole in the graphite target itself. Then the growth process is run similarly to synthesis of pristine carbon nanotubes, resulting in carbon nanotubes (partially) filled with the desired element. One drawback of this technique is that, due to the elevated temperature gradients formed inside the chamber, efficiency and control of the filling process is hard to achieve, as compared for example to CVD methods. The presence of sulfur in the chamber favors a more effective filling in arc-plasma processes [59,60]. However, arc-discharge methods are not generally suitable for the synthesis of ferromagnetic nanotube (besides few exceptions [61]) because those metals tend to form C-metal solid droplets that are encapsulated as carbides within the tubes or that escape the plasma zone to subsequently initiate the growth of new empty tubes. In the case of m-CNTs, it is certainly interesting to investigate the process leading to combined growth, for which not only the formation of the carbon shell but also the realization of the in-situ filling has to be explained, the latter posing an additional constraint on the entire synthesis process. Zhang and coworkers [62] have proposed an open-tip growth, as shown in Fig. 2a along with the existence of two different growth rates. This consists in a first stage in which the catalyst detaches from the substrate with the hydrocarbon initiating the formation of the wall (slow open-tip growth phase) followed by the catalyst falling on top of the already initiated and growing tube becoming thus a new site for nucleation and growth (fast growth phase of the carbon walls). This two-step growth mechanism leads to the final inclusion of metal catalyst inside the inner volume of the tube. At this point, it is then essential to note that the assumption of open-tip growth is a necessary condition for the filling procedure to take place, while allowing the continuation of the wall growth process. In this regard, Lee and coworkers have suggested that tip closure can be prevented by the so-called scooter-motion mechanism, where a moving metal cluster around the open tip prevents the closure of the CNT cap [63]. On the other hand, it has also been suggested that the in-situ growth of filled metal nanotubes can be determined by an open-tip base mechanism [64] as depicted in Fig. 2b, where the filling results from the building-up of iron nanoclusters that fall on the open tip and, if the diameter allows, diffuse inside the inner volume of the tube itself. A combined approach [65] based on the two previously described mechanisms has been recently proposed (see Fig. 2c). In fact, the initial stage is characterized by basegrowth, followed by tip-growth based on the assumption that catalyst particles are floating: this can suitably describes the synthesis of metal-filled carbon nanotubes. This ‘‘mixed’’ approach also provides some insight into the spatial distribution of metal particles and wires inside the CNT as well as the occurrence of unexpected features such as kinks and branches, often observed in these cases [66,67]. Zhang et al. [68] has proposed a modified microwave plasma-assisted chemical vapor deposition (MPCVD) method, where an additional copper source (in the form of a rod) is introduced into the chamber during the growth process. Varying the length of the exposed copper electrodes allows for a good control of the concentration of the copper clusters in the plasma; once the plasma is ignited, the copper atomic clusters are sputtered out from the exposed copper electrodes. The amount of copper clusters depends on the plasma density and the exposed surfaces of the Cu electrode, thus leading to a good control of the concentration of Cu clusters. However, it is difficult to directly determine quantitatively, and thus controlling, the cluster density within the plasma during the growth process.

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Fig. 2. (a) Base and (b) tip-based growth. (c) Combined approach suggesting a possible combination of both base- and tipgrowth mechanisms.

2.2. Ex-situ filling (post-growth filling) As previously mentioned, any post-growth technique to fill at any extent carbon nanotubes strictly requires the existence of open tips; thus, unless the as-grown tubes are already open, the opening tip procedures is needed before filling can take place. The de-capping of the tubes can be achieved mainly in two ways: thermally in oxidizing environment (air or oxygen) and chemically, by using acids capable of oxidizing poly-aromatic carbon [69,70]. Those methods are quite efficient in opening SWNTs as well as MWNTs, however in most cases the experimental conditions to which carbon nanotubes are subjected are very harsh and present some disadvantages. In the case of the liquid route, the oxidation process leaves behind residues that tend to cover the tube, limiting further treatments (i.e. filling) and structural and morphological characterization. Thus, oxidation in the gas phase is preferable most of the time, due also to the simplicity of the process itself: thermal treatment at temperature of approximately 400 °C in air conditions within a furnace, where convection leads to motion of the oxidizing gas and creates openings in the tubes. In both cases (liquid and gas phase) and for all kinds of nanotubes, due to the extreme strength of the carbon–carbon bonds in the graphene lattice forming the walls, the process of tip opening often is initiated at defect sites (in fact, closed tips often present pentagons and heptagons accounting for the cap curvature). For the filling and decorating procedures, various methodologies are available, depending mostly on materials and types of tubes. Wet chemistry approaches have been proposed for the fabrication

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of filled carbon nanotubes. Zhang et al. [62] have used SWNTs produced by laser ablation technique; once opened and purified by acid treatment, carbon nanotubes are transferred into a beaker containing an over-saturated solution of iron chloride (FeCl3) and sonicated for few minutes. Long stirring times (48 h) and careful washing in hydrogen chloride (HCl), followed by multiple centrifugation steps in distilled water, were crucial to obtain a solution of tubes filled with FeCl3. Heating at 600 K in air decomposes iron chloride in iron and chlorine [71], thus leaving carbon nanotubes filled with iron. Moreover, Yin and coworkers [72] have used shortened and opened MWNTs (using a mixture of concentrated sulfuric and nitric acid, followed by sonication, filtration and thorough rinsing) onto which super-paramagnetic nanoparticles were strongly attached through the decomposition of ferrocene at 425 °C for 2 h, thus obtaining iron filled-MWNTs [73]. Filling of carbon nanotubes can also be achieved from the gas-phase: the vapor of the filling material is introduced in a sealed evacuated flask, heated up at the temperature (or slightly higher) of vaporization or sublimation of the filling material. In conditions of high partial pressure, the vapor starts to condense and under the capillarity force tends to diffuse toward the inner volume of the tube when the system is cooled down. The gas phase approach represents an easy route for the fabrication of m-CNTs, holding the potential for a high-filling rate, high-purity and homogeneity of the filling material. It can be applied to SWNTs and MWNTs, although available literature works on the latter are rather limited [74–76]. Some restrictions in terms of the filling materials are inherent to experimental conditions such as (i) no vaporization and sublimation temperature greater than 1000– 1200 °C to reduce the sealing effect of the nanotube openings (graphitization effect becomes significant at and above these temperatures) and any reaction with the carbon, thus leading to an efficient filling process and (ii) in the case of compounds, the process is limited to materials that vaporize/sublime without decomposing. Although these aspects have to be considered, the gas phase approach has enormous potentials for implementation of the filling procedure in large-scale production of hybrid carbon nanotubes. Filling by the liquid phase is extremely dependent on the physical interaction between the filling liquid and the hosting tube, typically governed via the Young–Laplace law for capillary wetting. If using a solvent, wetting does not represent a major limitation since the surface tensions of most frequently used solvents are smaller than 80 mN/m. On the other hand, viscosity certainly represents another parameter to be considered when this synthetic route is used. In addition, for materials that are strong oxidants (i.e. CrO3), a two-stages process has been proposed [77], in which first, the raw material is filled inside the CNT by capillary action [78], then the filling in converted to the desired oxide by a specific annealing procedures. In case of cobalt nanoparticles-filled CNTs, the filling can be easily achieved by dispersing CNTs in a previously prepared suspension containing oleic acid along with a cobalt stearate complex, and then bringing the entire mixture to thermal decomposition (318 °C) [79]. Fig. 3 shows transmission electron microscope images at different magnification of individual carbon nanotubes filled with 50 nm diameter cobalt particles, indicated by the darker (brighter) regions in A, B and C (D), depending on the detection technique used. These experimental conditions lead to a high level filling of tubes (about 90%) as well as a high filling density of 60 wt% (within each individual tube). Further, nanoparticles and metal agglomerates can be grown and/or deposited directly onto the CNT surface using covalent linking through organic fragments [80–82]. Different treatments have been proposed in order to ease and favor the subsequent attachment of metal nanostructures onto the tubes. Oxygen plasma treatment of carbon nanotubes has the overall effect of breaking the carbon–carbon bonds, creating numerous defects sites on the surface of the tube itself, which then act as preferred sites for atoms to attach and forming clusters [83,84]. It has been shown in fact, that oxygen plasma treatment in a cold, low-pressure plasma reactor favors the decoration with gold nanoparticles [85] by thermal evaporation [86], as shown in Fig. 4. Further, investigations on different types of plasma [87] have revealed the key-role played by the oxygen in favoring adhesion and enhancing the amount of NPs decorating the tube surface. Claessens et al. has used atmospheric argon plasma as a pre-treatment and then sprayed a colloidal solution (gold, platinum and ruthenium nanoparticles) on the tubes, reaching a smooth and neat surface coverage. This approach is very simple, robust and efficient and has the advantage of achieving both the surface activation and the deposition process in a single step. Potentially it can also be adjusted and tuned for large-scale production of m-CNTs. Other techniques to

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Fig. 3. (A and B) Representative TEM micrographs of cobalt nanoparticles with different filling density within the inner volume of the carbon nanotube. (C) High-resolution STEM-BF and (D) STEM–HAADF of the cobalt oxide NPs encapsulated inside the CNT channel in B showing the faceted and highly porous nature of the particles. [Reprinted with permission from Chemistry of Materials 24, 1549–1551. Copyright 2012 American Chemical Society.]

introduce defects within the atomic structure of a carbon nanotube include treatments in acid environment [88], electron and ion irradiation [89–91], laser irradiation [92], and more. Creating preferred sites for nucleation can be also achieved by microwave irradiation, which has the effect of introducing different terminal groups (carboxyl, carbonyl, hydroxyl and allyl) on the surface of the tube, which then becomes the preferred nucleation centers for the reduction of the metal ions present in the solution [93]. With this method, a good uniformity in terms of the coverage of the MWNT wall can be achieved. Decoration of pristine MWNTs can be obtained by dispersing them in a cobalt-nitrate solution (0.5–1.0 wt%), then sonicating in an ultrasound bath and finally stirring to facilitate the dispersion of the nanotubes. Repeating this process multiple times leads to MWNTs filled with cobalt nanoparticles. If the particles need to be oxidized, the solution can be dehydrated and annealed in air at 300 °C [94]. This approach is very easy and can potentially be applied successfully for different types of NPs (both metallic and metal oxides). Metal deposition techniques such as e-beam evaporation (Pd, Pt, Sn, Rh) [12,95], sputtering (Pd, Ti, Au, Pt) [96–99] and thermal evaporation, can be used to decorate carbon nanotubes in various forms

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Fig. 4. Transmission electron microscope (TEM) micrographs of different Au coatings on (left) pristine and (right) oxygenplasma-treated MWNTs. [Reprinted with permission from Nanotechnology 20, 375501. Copyright 2009. Institute of Physics.]

Fig. 5. (Top) SEM images of the high-density SWNT arrays on quartz (a) before and (b–c) after gold decoration via gold-seeds approach by repeating both the gold seed deposition and the seeded growth process several times. Palladium seeds are used in (c). Insets are higher magnification images showing the gold nanoparticle decoration. (Bottom) AFM topographical images of corresponding images on top row. [Reprinted with permission from Journal of American Chemical Society 131(40), 14310–6. Copyright 2009. American Chemical Society.]

(from individual tubes to arrays and bundles, as well as manufactured devices already containing the tube(s)). Silver nanocrystals (NCs), first synthesized by physical vapor deposition using a mini-arc plasma reactor [100] can be directly deposited onto the tubes (on electrical pads) using an electrostatic force directed assembly (ESFDA) approach [101]. Electro-less approaches have also been also proposed. Chu et al. have shown the possibility of decorating single wall carbon nanotubes with gold clusters through a two-stage process based on a combination of seed deposition and seeded growth [102]. Previously grown SWNTs on a substrate were first immersed in a solution of gold chloride for few minutes to deposit gold seeds and then, the entire sample was immersed into a seeded growth

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Fig. 6. (a) Schematic depicting the attachment of silver nanoparticles (orange dots) onto an individual carbon nanotube under an electric field. (b) SEM micrograph of a Ag NPs–SWCNT-based hybrid two-terminal electrical device. (c) Zoomed image of the selected portion indicated by white dotted square region in (b) showing in more details the decoration of the tube. [Reprinted with permission from Journal of American Chemical Society 133(11), 4005–9. Copyright 2011. American Chemical Society.]

of HAuCl4 and hydroxylamine hydrochloride for an hour. This approach can offer a moderate control over the coverage of the tube and its surface, as shown by SEM and AFM images in Fig. 5. Metallic nanostructures can also be embedded in/onto CNTs by electro-chemical approaches exploiting the application of a constant bias between two electrodes (working and counter), which causes a chemical reaction to take place at the interface of an electronic conductor (the electrode composed of a metal or a semiconductor) and an ionic conductor (the electrolyte). This approach can be used both on individual tubes as well as on bulk systems such as bundles and arrays. Sahoo and coworkers [103] have shown a very simple and cheap method to decorate SWNTs previously grown by CVD on Si/SiO2 substrates. The carbon nanotube is electrically contacted and it represents the cathode, while an additional silver electrode is used as the anode, as shown in Fig. 6. Using deionized water as electrolyte, the bias applied between the two terminals induces a release of Ag ions from the anode and forcing them to move them toward the cathode (tubes), where the reduction process takes place. Silver ions then tend to accumulate on the tube, thus forming clusters on its surface. This procedure, and hence the coverage of the tube itself, can be tailored by tuning the electrode size, the electrode separation and the applied bias between the electrodes, thus achieving a precise control of the amount of silver coating. This approach is also successful in the case of large bundles of SWNTs [104], where in fact the surface decoration in aligned arrays can be obtained by immersion of an array of already grown SWNTs in a chloroplatinic acid solution and then applying a negative potential. Thus, the platinum ions having positive charges are nucleated selectively on the surface of the contact pads and the SWNT architectures. Decoration of pre-contacted nanotubes by different metals (depending on the electrode materials) can additionally be achieved by applying in vacuum a rapid controlled cyclic current–voltage (IV) sweep with increasing biases (also known as high-bias treatment or HBT). This generates enough Joule heating [105,106] across the tube to anneal defects and decorate the outer surface of the nanotube with metal nanoclusters migrating away from the contacts [107]. Fig. 7 shows the overall change in

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Fig. 7. Scanning electron microscope image of a pristine two-terminal device (a) before and (b) after the HBT process. Images on the right represent zoomed areas on the tube/metal interface as indicated by the white dashed box, respectively. The HBT process produces surface decoration of the outer surface of the tube with platinum clusters coming from the electrodes, as shown in (c) and (d). Diameter of clusters is on average less than 10 nm. [Reprinted with permission from Rensselaer Polytechnic Institute, C. Soldano PhD Dissertation, Copyright 2007.]

the morphology of the nanotube on the surface, where the tube presents platinum clusters with average diameter less than 10–20 nm decorating the entire surface. For MWNTs grown in anodized alumina (AAO) template and still in the matrix [44], the fabrication of MWNT-based electrodes to be used as a cathode (MWNTs were coated with metal on one side and masked on the other side to leave unexposed the part of the template to fill) has been successfully proposed. The MWNT array is then immersed in the electrolyte, where the tube and the deposited metal film form an array of extended electrodes, with only the inner volume of each tube exposed to the electrolyte. Upon application of a bias between the MWNT electrode and reference electrode, metal ions are driven toward the MWNT, hence filling the tube. One advantage of using carbon nanotubes grown in template is the fact that the filling is selective, in the sense that the insertion is only limited to the inner volume of each tube. Fig. 8 shows AAO-grown MWNTs filled with various materials (including alloy) and where the filling itself has diverse shapes. Compared to electro-less [108] and sol–gel methods [109], electro-deposition in nanopore templates provides the possibility of controlling the composition of the filling material and the metal used, by simply modifying the electrolyte composition. Electrochemical deposition in an AAO template can also lead to interesting hierarchical one-dimensional structures composed of segments of MWNTs and metallic nanowire structures, as shown in Fig. 9. Thus, varying the deposition parameters (applied bias, electrolytes composition, etc.) it is possible to fabricate multi-segmented

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Fig. 8. Scanning electron microscope (SEM) images of multi wall carbon nanotubes (grown in alumina template) with various types of ferromagnetic filling materials by electro-deposition technique. In particular, tubes are filled with (a–b) nickel, (c) cobalt–nickel alloy and (d) cobalt. [Reprinted with permission from Rensselaer Polytechnic Institute, C. Soldano PhD Dissertation, Copyright 2007.]

Fig. 9. Scanning electron microscope (SEM) images of arrays of (a) and (b) CNT–Au hybrid multi-segmented structures and individual (c) Au–CNT hybrid and (d) CNT–Au–CNT. (e) Transmission electron microscope (TEM) image showing Au–CNT junction. [Reprinted with permission from Applied Physics Letters 89, 243122. Copyright 2006. American Institute of Physics.]

structures based on carbon and metal with desired structures and materials. These nanostructures have the advantages of having all the shells connected to the adjacent metal segment, thus providing an electrical contact to all the carbon shells. Atomic layer deposition (ALD) can be used to fill carbon nanotubes grown in template [110]. Consisting in the repeated deposition of thin layers (usually a

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monolayer or less) from two different vapor-phase reactants, ALD has the advantage of an accurate control of the growth rate and a conformal coating on 3D structures. Daub et al. have reported the fabrication of magnetic nanotubes in templates by ALD, based on three subsequent and combined steps: alternate exposure to metal–organic precursor, water, and hydrogen [111]. 3. Properties and applications of metal-filled and decorated carbon nanotubes Combining carbon nanotubes with nanoscale metal structures often leads to enhanced and/or novel functionalities which make hybrid carbon nanotubes potential candidates for a variety of applications and fields. In the next few paragraphs, various properties characterizing m-CNTs will be presented and described, along with their possible and potential applications in many fields of interest for both the scientific and technological community. 3.1. Electrical properties Ab-initio density functional theory (DFT) calculations have shown that in the case of single wall carbon nanotube the addition of platinum atoms on the outer surface of the tube gives rise to new electronic bands near the Fermi level, directly affecting the band gap, which defines the semiconducting or metallic nature of the nanotubes, as shown in Fig. 10 [104]. Adding more Pt atoms causes more bands to appear with further enhancement of available states near the Fermi level; in terms of band gap, DEg rapidly vanishes when reaching n (number of Pt atoms) = 3 for semiconducting nanotubes ([8,0] and [10,0]), remaining unchanged for further additions. In the case of the metallic tube ([9,0]), the gap remains approximately zero for almost all values of n. The presence of these additional bands near the Fermi level also contributes to the conductance of the tube (bottom panel in Fig. 10),

Fig. 10. (a) Band gap closing in semiconducting [8,0] and [10,0] SWNTs due to Pt nanocluster decoration. The gaps close within 3-Pt atoms coverage and then remain close to zero. The metallic [9,0] nanotube remains metallic after 3-Pt coverage. (b) Effect of Pt decoration on the zero-bias Landauer conductance at T = 300 K in a semilog plot. The conductance of the semiconducting nanotubes [8,0] and [10,0] increases by several orders of magnitude and approach G = 4e2/h within 3-Pt decoration and fluctuate around this value, while the metallic tube remains metallic without any significant drop in conductance. [Reprinted with permission from ACS Nano 3(9), 2018. Copyright 2009. American Chemical Society.]

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Fig. 11. (a–c) Differential conductance (dI/dV) measured at temperature T = 40 mK as a function of in-plane magnetic field l0H applied at an angle 25° with respect to the nanotube axis for different gate voltages. The red (blue) arrow in (a–c) indicates the up (down) magnetic field sweep direction. The strong dependence of both amplitude and sign of the jumps is observed in a large window of Vg in (d). (e) TEM micrograph showing an individual SWNT discretely filled with elongated cobalt nanoparticles. [Reprinted with permission from ACS Nano 5(3), 2348. Copyright 2011. American Chemical Society.]

with almost 4–5 orders of magnitude increase for n as low as 3, approaching the value G = 4e2/h, then remaining approximately constant for increasing n. On the other hand, the metallic tube remains metallic, as expected. These findings strongly suggest that uniform decoration of small (few nanometers) metallic clusters can potentially convert semiconducting SWNTs into metallic ones, thus representing a powerful tool to tune their electric properties. These results also explained well the experimental findings by the same group in the case of parallel nanotube arrays (of different widths) decorated with Pt clusters, for which improved conductance is found. Similarly, in the case of an individual AAO grown carbon nanotube decorated with Pt clusters by HBT treatment [112], the conductance is enhanced as a result of combined surface decoration with nanoclusters and defect annealing effect on an overall disordered carbon structure. A set of novel properties arises if the decorating/filling material is also magnetic, thus broadening functionalities and corresponding potential applications. Soldano and coworkers [44] have shown that the low-temperature transport properties of individual cobalt nanocluster filled-AAO multi wall carbon nanotubes are sensitive to the magnetic state of the embedded clusters, thus acting as the ultimate conducting sensor for ferromagnetic activity at the nanoscale. In fact, it has been predicted that the proximity of ferromagnetic adatoms on the walls of carbon nanotubes may give rise to non-collinear alignment of their magnetization, leading to non-Heisenberg-like behavior in the limit of low-dimensions [113]. Cleuziou et al. have shown a large sensitivity of single wall carbon nanotube transport properties to the magnetization reversal of filling clusters constituted by only few hundreds of atoms (Co), as shown in Fig. 11 [34]. In this case, cobalt nanoparticles retain their magnetic properties upon encapsulation in the tube, and they show enhanced surface magnetic anisotropy inducing a large magnetization perpendicularly to the tube axis. In addition, magneto-Coulomb effect leads to gate-modulated magnetoconductance in this hybrid m-SWNT. Further, Rossella et al. have recently demonstrated a magneto-size effect in the limit of very high pulsed magnetic field, which leads to overall negative magneto-conductance with G/G0 value of 0.6–0.7 of the maximum value of the applied field [114]. These works described so far, open the way to the design and development of novel prototype structures of spin-based devices such as spin valves [115,116], where instead of

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Fig. 12. (Top) Imaginary, e2 and (bottom) real, e1 part of the pseudodielectric function for MWNT and Co-MWNT from spectroscopic ellipsometry measurements. The segments in the lower panel refer to inter-band transitions. [Reprinted with permission from ACS Nano 4(11), 6573. Copyright 2010. American Chemical Society.]

having separate ferromagnetic electrodes, one can envision each cluster (accurately positioned at desired locations) as the site of the spin injection and/or detection. This can potentially lead to a very precise nanoscale control of the magnetic states of each ferromagnetic agglomerates, where thus spin densities can be formed at one location, propagate through a non-ferromagnetic medium such as the hosting tube and then detected by another ferromagnetic agglomerate. In particular, precise control over the spin can be obtained since the electron path from the injection to the detection sites can potentially be extremely short (few nms), thus making negligible any issue related to the diffusion of the electrons within a non-ferromagnetic medium. Engineering the materials, shapes and locations of the magnetic clusters might allow exploring the ultimate limits in spin control and eventually controlling spin injection devices. Single wall carbon nanotubes filled with magnetic nanoparticles (iron) can exhibit highperformance unipolar n-type semiconducting behavior at high-filling levels (defined as the ratio between the number of Fe and C atoms) with the onset of Coulomb blockade for low temperatures (<120 K) and p–n junction diode behavior at low-filling levels as a result of possible enhanced electron-donating capabilities [117]. Study of the local electronic density of states (DoS) of nanometer-size cobalt clusters on metallic single-wall nanotubes by means of scanning tunneling microscopy (STM) measurement at low temperatures has unraveled a narrow peak feature near the Fermi level, identified as a Kondo resonance [118,119]. In the case of ordered and vertically aligned MWNTs (grown in template), the dielectric response is significantly changed upon inclusion of nanometer-size cobalt nanoclusters, suggesting a marked modification of the energy band structure of the material. In particular, ellipsometry measurements reveal an enrichment of the energy band structure with evidence of wide gap semiconducting material, as shown in Fig. 12 [120]. Nanotubes grown in template further

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Fig. 13. Angular dependence of the reflectivity of (a) MWNTs, (b) Co-MWNTs, and (c) normalized reflectivity (DR/RMWNT) in the [6000 cm 1; 10,000 cm 1]. Angles and colors are labeled consistently in the three panels. [Reprinted with permission from ACS Nano 4(11), 6573. Copyright 2010. American Chemical Society.]

offer the possibility of easy and large-scale production of multi-segmented metal-CNT nanostructures [121] (both in bulk as well as single nanotube after release from the template); in this case, it is possible to obtain the junction and the electrical contact of all the shells of a CNT with a metallic side. The great advantage of this approach is the very good control and uniformity in length and diameter of the resulting nanostructures. This approach has enormous potential in interconnect applications [122], where all shells are electrically connected to the metal electrodes-metal nanowires (nanoscale contacts taking advantage of the all-walls-connected condition). In addition, decoration of horizontal mats of carbon nanotubes by small platinum clusters has been shown to be an efficient method to transform semiconducting nanotubes into metallic, leading to an overall enhanced conductivity of the entire parallel arrays. Interestingly these results can be obtained rather easily through postnanotube growth approaches, including the application of high-bias application procedure or through electro-deposition [26]. 3.2. Optical properties In the case of carbon nanotubes grown in template, filled with cobalt clusters and not yet released from the matrix itself, the filling procedure has the effect of enhancing the overall reflectivity, outcome that cannot be explained only in terms of geometry of the system and the simple cluster ‘‘physical presence‘‘. Thus, Rossella et al. invoke the plasmonic coupling between the metallic clusters and the carbon/alumina matrix to explain the large enhancement (almost two orders of magnitude) in reflectivity of the bulk hybrid nanostructure, as shown in Fig. 13 [112]. This effect is independent of the materials of the cluster (however the strength of this coupling depends on the nature of the materials). This suggests the possibility of exploring the potential of hybrid AAO-grown MWNT arrays as a platform to develop photonic band gap materials based on enhanced optical response, for which

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Fig. 14. Stokes and anti-Stokes Raman spectra (D- and G-lines shown) characteristic of cobalt filled multi-walled carbon nanotubes shown in the top panel, measured in different positions along the tubes (Co-MWNT and MWNT-as indicated by the arrows). The Stokes G-lines have been normalized to highlight the change in the ratio of anti-Stokes to Stokes signal. [Reprinted with permission from Advanced Materials 24(18), 2453–8. Copyright 2012. Wiley.]

appropriate pore size and filling materials could be tailored for selective applications. Plasmonic coupling is responsible for the large enhancement of the local temperature while studying an individual cobalt clusters-filled tube coming from the very same array. Fig. 14 shows how using a laser source, it is possible to significantly enhance the Stokes to anti-Stokes intensity ratio, which is directly related to the local temperature at the site of illumination (metal cluster within the tube) [45]. This temperature increase (approximately 100 K) further induces a thermal gradient along the tube, with a lower limit estimated to be approximately 100 K/lm. The locally-induced temperature enhancement, estimated by studying the Stokes to anti-Stokes intensity ratio in the Raman spectra, is due to the combination of (i) absorption by the tube wall of the light reflected by the cobalt surface and (ii) the heating of the cobalt cluster induced by enhanced optical absorption due to surface plasmons excited by the incident laser radiation. Thus, the clusters behave as heat radiators for the adjacent environment, suggesting that this effect can be explored for several cutting-edge innovative photothermal systems such as light-activated, thermal gradient-driven device and actuators, original building blocks for heat-assisted magnetic recording and novel platforms for heat-driven medical applications. Raman studies on ferrocene-single wall nanotubes [117] also suggest both (i) sensitivity of the RBM mode (radial breathing mode) to the presence of the filling material (even at low concentrations) and (ii) a diameter selective filling process [123]. In many cases, surface electrons of metal nanoparticles such as gold and silver can be resonant with the electrical field of incident light, resulting in strong surface plasmon resonance (SPR) with strong adsorption of light. This has the overall effect of intensifying the Raman scattering signal for nearby chemical or biological molecules. Thus, surface-enhanced Raman scattering (SERS) in m-CNTs has succeeded in detecting organic compounds even at single molecule level [124,125]. SERS of CNTs, which could be realized upon metal nanoparticle decoration of CNTs, can also provide more detailed

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structural information of CNTs. In particular, SERS of many types of m-carbon nanotubes is characterized by an electromagnetic enhancement, the greatest when both the nanotube optical transition and the surface plasmon of the metal particle are in resonance with the excitation wavelength. Further, the nanoparticles morphology as well as size and their distribution can be directly correlated to the resulting optical properties and the enhancement of the Raman spectra. Due to the high aspect ratio, stability and strong electron–phonon coupling, SWNTs are also unique probing one-dimensional objects for SERS studies. Thus, tuning the enhancement of Raman scattering from m-CNT enables high sensitivity SERS measurements. Nonetheless, these hybrid nanostructures have also significant potential as a standard SERS substrate for the investigation of a variety of target molecules in air and in liquid phase. 3.3. Magnetic properties In recent years, nanotubes filled with magnetic materials such as iron, cobalt or nickel have increasingly gained interest due to their small size and enhanced magnetic coercitivity for potential applications in magnetic-driven applications. In fact, coercitivity represents a key parameter since it estimates the intensity of the applied magnetic field required to decrease the magnetization of a material to zero after it has reached saturation. One interesting aspect for m-CNTs, in particular when the metallic objects are magnetic and located within the nanotube itself, is the fact that the carbon shells provide an effective barrier against oxidation and consequently ensure long-term stability of the magnetic core, a condition that is highly desired when designing and developing long-lasting magnetic storage media applications. Magnetic activity and the state of the magnetic core (in the form of nanoparticles, agglomeration or nanowires) can be investigated using the properties of unfilled carbon nanotubes as a reference, to study if and how they change depending on the amplitude of the magnetic field and its direction [126]. A large amount of work has been done to investigate the effect of ferromagnetic impurities on the band structure of the hosting tube, and how its electronic and magnetic properties vary depending on the filling material, when spin is considered [127,128]. Ab-initio density functional theory calculations [129] reveal that the presence of an iron chain in a metallic tube with chirality (3,3) changes the electronic behavior inducing a metal-to-semiconductor transition. In fact, Fe atoms tend to form r bonds with four (or six atoms), while carbon forms sp3 hybrid-like orbitals; the Fe-filled SWNT becomes a semiconductor and the magnetic moment on the Fe atoms disappears. The result is different if iron is replaced by cobalt along the chain; in fact cobalt has seven d electrons, of which four electrons will bond with the p electron from the carbon atoms and other two will participate in the bonding with the neighbor cobalt atom, one each side; thus the remaining free electron is responsible for the conduction and for the absence of any gap in the band structure [40]. Furthermore, Fe nanoparticles-filled SWNTs exhibit a ferromagnetic characteristic at room temperature and superparamagnetic properties at low temperatures [117]. In this case, there is very little change due to the presence of the iron atoms that can be explained in terms of missing magnetocrystalline anisotropy of magnetic particles [130]; in fact, for very small dimensions of the nanoparticles, most likely a perfect crystalline structure is not formed due to very limited number of available Fe atoms. Fig. 15 shows the collective magnetic of an array of iron-filled tubes grown vertically on a substrate by CVD [131]. In the case of almost fully filled nanotubes (as shown by the TEM image in Fig. 15b), the value of the coercitivity for the magnetic field direction perpendicular to the substrate is about double (56 mT) as compared to the direction parallel to the substrate (25 mT). It is then possible to expect that if any residual magnetization is present, this will affect both the charge and spin transport in these hybrid structures. Single magnetic carbon nanotubes and groups of them can also be studied in terms of their spatial magnetic response with techniques such as magnetic force microscopy (MFM) [132,133]. Very much like AFM, MFM is a scanning probe technique able to map the spatial distribution of magnetic response through the magnetic interaction between the sample and the tip (both magnetic or magnetized). Fig. 16 shows (top) scanning electron microscope and (bottom) corresponding magnetic force microscopy images of a single carbon nanotube partially filled with iron nanowires segments along its axis. Bright (and yellowish) regions in panel (a) (and (b)) denote remnant magnetization of the iron cores along longitudinal nanowire axis suggesting that the magnetic anisotropy is dominated by

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Fig. 15. (a) Low-magnification SEM image of vertically aligned iron-filled tubes grown on oxidized Si. (b) TEM image of an individual iron-filled tube and (c) magnetization curve of Fe-filled array of tubes. [Reprinted with permission from Diamond and Related Materials 12, 790–3. Copyright 2003. Elsevier.]

Fig. 16. (Top) Scanning electron microscope mage and corresponding (bottom) magnetic force microscope phase image of four individual Fe nanowires segments inside a carbon nanotube. [Image courtesy by Andreas Winkler and Thomas Mühl, IFW Dresden.]

the shape. High aspect ratio iron nanowires can be thus regarded as formed by two well-distinguished magnetic monopoles located near the two wire ends. Carbon nanotubes coated with metal can themselves be used as the ultimate MFM tips. In fact, Deng et al. have shown that a carbon nanotube grown directly on the apex of an AFM tip and decorated with cobalt by electron beam of cobalt (two titanium layers in a sandwich structure are needed to prevent cobalt oxidation), is capable of resolving magnetic domains smaller than 20 nm [134]. This important result has surely opened the way to the use of these hybrid magnetic probes for the development of a new class of MFM tips that can be operated on high aspect ratio features and might possibly contribute to a more quantitative analysis of MFM experimental data [135]. In fact, this probe can in principle be modified in size and filling material to fulfill specific needs or to optimize the response in the magnetic measurements. Detailed study of magnetization reversal in hybrid nanoscale ferromagnetic carbon-based nanostructures has been motivated by the large interest both in the fundamental physics governing the switching process and in the numerous potential applications as logic and memory elements. Banerjee et al. have shown a highly precise magnetization reversal in an individual 25 nm diameter iron nanowire within a carbon nanotube by means of cantilever magnetometry, switching that can be driven and controlled by temperature [136]. Tuning of the magnetic properties thus in this class of nanotubes has great potential in fields such as force sensing applications (i.e. scanned probe nuclear and ferromagnetic resonance imaging [137,138]) where the ability to produce strongly localized and inhomogeneous magnetic fields at the nanoscale is of fundamental importance. Magnetic resonance force microscopy (MRFM), which measures very small magnetic force (in the range of attoNewton) between the magnetic scanning tip and the nuclear spins of the sample, can lead to imaging methods characterized by high spatial resolution as a result of the cantilever intense local magnetic field. Various techniques based on highly precise force-measurement at the nanoscale can lead to substantial

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advancement in the development of probing methods able to directly map the 3D organization and composition different nanostructures, with significant impact on the imaging of biological species and the ability to target and locate specific proteins. 3.4. Medical properties Carbon nanotubes provide unique and unprecedented opportunities to meet a number of biological and biomedical applications including, but not limited to, protein and peptide transporters [139,140], drugs delivery [141], medical imaging [142–147] and cancer targeting/therapeutics [148,149]. mCNTs are expected to combine advantages from both carbon nanotubes and metal objects in one individual and unique structure, potentially of great interest for the medical community, especially for the advanced and broad use of nanomaterials in treatment and disease detection. So far a lot of effort has been focused toward the use of different types of nanoparticles: even though at initial stage, encouraging results have been obtained on tests on animals [150,151] and in some cases clinical trial stage has been reached [152]. Functionalized magnetic carbon nanotubes, biocompatible and hydrophilic in nature, have shown an elevated contrast effect in magnetic resonance imaging (MRI) [72]. Iron-filled tubes have been successfully detected by commercially available MRI systems currently in use in hospitals and medical facilities in targeting in vitro cancer cells, thus showing immense potentials in cancer detection. The possibility of functionalizing these magnetic carbon nanotubes with fluorescent molecules [153] and therapeutic agents [154–156], potentially open also the way to future development of novel tools and platform for both magnetic and optical-based diagnostics as well as for real-time monitoring of drug delivery processes. Krupskaya et al. have proposed the use of iron-containing carbon nanotubes for magnetic hyperthermia [157], medical practice in which the precise control of tissue temperature is exploited to locally address tumors (cancer cells are supposedly killed for temperature above 41– 42 °C) [158]. Currently, available approaches directly introduce heating elements (thermocouple or fiber-optical thermometers) nearby the tissue of interest [159]. Rossella et al. have shown that a metal (cobalt) nanocluster inside a multi wall carbon nanotube can be individually heated with a laser source producing a temperature increase of approximately 100 K (with respect to regions where there is no metal cluster) [45]. This preliminary study can potentially lead to non-invasive and contact-less local activation of heat to destroy cancer cells, with an elevated and precise control of the generated temperature. Immediate and direct implementation of this approach, however, faces rather challenging issues, including engineering a proper light source to thermally activate those clusters inside the tubes within a specific region of the human body. Alternatively, local temperature increase can also be achieved with the application of a DC or AC field to magnetic CNTs (in powder form as well as in solution), which produces a variation in the magnetization. Local heat production treatment is expected to have minor side effects for patients, definitely holding a lot of interest in the scientific and medical community. Moreover, engineering the filling and choosing the proper materials, can give rise to an easy way to develop specific treatments to selectively target different types of tumors and cancers. Hybrid carbon nanotubes including para- or superparamagnetic materials with free electron spins (i.e. Gd3+, Mn2+/3+, Fe2+/3+) have been shown to represent an ideal platform on which to develop contrast agents (CAs) for different imaging techniques, where enhanced magnetic interaction between electrons of contrast agent and 1H nucleus is required. In 2005, Sitharaman and coworkers have reported the first carbon nanotube-based contrast agent called gadonanotube [144], a nanoscale object comprising of an ultra-short SWNT and gadolinium clusters. Investigation of the magnetic properties has revealed that these hybrid nanostructures behave as linear superparamagnetic molecular magnets with enhanced MRI efficacies (almost two orders of magnitude) larger than any current Gd-based agent in standard clinical use [160]. As such, gadonanotubes with 2–5 nm embedded superparamagnetic Gd clusters, demonstrate potential as a radically novel approach for the development of highperformance MRI contrast agents, molecular imaging and other advanced applications. Besides their intrinsic magnetic properties, metal-filled and -decorated carbon nanotubes also offer an elevated ‘‘mass’’ contrast in specific imaging techniques based on atomic weight differences (12 for carbon and 50–200 for most metals), thus representing a novel object of interest for imaging techniques based on mass contrast (i.e. tomography [161]).

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Fig. 17. Sequential TEM images showing the induced motion of iron within the nanotube. A gold tip (on the left, positively biased) is in electrical contact with an iron filled carbon nanotube(on the right): (a) a current of electrons flows from the right to the left (at t = 0); (b) the iron core breaks up (at t = 2 min); (c) iron migrates in the same direction as the electron flows (at t = 3mins). [Reprinted with permission from Physical Review Letters 93(14):145901. Copyright 2004. American Physical Society.]

Additionally, mass transport within the inner volume of a carbon nanotube can be of interest due to the possibility to deliver very small amount of materials to very defined locations using for example the carbon nanotube as a ‘‘nano-syringe’’ [162] or generating nanodroplets or wire in a tube [163]. Svensson et al. have shown that metal-filled carbon nanotubes can successfully work as ‘‘nanocargo’’ to transport metal through the inner volume of the tube (and, if needed out of the tube itself) under the action of an electron beam, as shown in Fig. 17 [162]. The beam irradiation induces internal pressure build-up as result of melting and thermal expansion of the encapsulated materials and the shrinkage of the nanotube shells, which finally pushes out the molten metal and forms bubbles. A similar results obtained by applying electrical current at the ends of a Fe-filled CNT has also been reported, where the motion on Fe filling inside a MWNT in a reversible way, as a result of the Joule heating produced [164]. Although at very embryonic stage, these examples can represent the proof-of-principle for local delivery of molecules or objects (therapeutics) within the tube, thus the m-CNT behaving as a very tiny nano-syringe. Nonetheless, for any medical and biomedical applications, it is however of crucial importance to investigate the possible impact of carbon nanotubes (whether filled/decorated or empty) on the

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Fig. 18. Time response of the electrical resistance change for a chemiresistor based on (black) pristine MWNT films and (green) Pt- and (red) Au-modified MWNT films toward (top) NH3 and (bottom) NO2 gases at working temperature of 150 °C. [Reprinted with permission from Applied Physics Letters 90:173123. Copyright 2007. American Institue of Physics.]

human body system. For this we refer to more specific reviews and literature work on this topic and the references therein [165–167]. 3.5. Sensing properties Metal-filled and decorated-carbon nanotubes have the possibility for the metal (whether on the outer surface or in the inner volume) to play the active role for numerous and different types of detection. Due to high aspect ratio and the carbon nature, CNTs represent already an interesting platform on which to develop sensors, where the actuating mechanism is often based on electrical response coming from the interaction of the outer shell with foreign species. SWNTs are expected to be intrinsically more sensitive compared to multi-shell tubes [168], where the sensitivity is limited by the inter-shell interaction; however, in the limit of small applied biases, this effect is drastically reduced, thus leading to overall good performance in the case of MWNTs too. It is here important to note that an individual carbon nanotube (whether SWNT or MWNT) naturally already represents one of the smallest available sensing structures. Fig. 18 shows the increase sensitivity of functionalized (Pt and Au) MWNTs upon gas exposure as compared to pristine tubes, where the enhancement factor depends on the specific gas (6–8 times for NH3 and 2–3 times for NO2) as result of the combined effect of a more efficient charge injection and

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Fig. 19. (a–b) Tilted cross-sectional schematics with corresponding top-view scanning electron microscopy image of single wall carbon nanotube on porous alumina template decorated with Pd nanocubes coated with a thin layer of Au. (c) Amperometric sensing of H2O2 oxidation (0.5 V) in 20 mL of phosphate buffer solution using a three-electrodes potentiostat. The biosensor was tested by increasing the concentration of H2O2 by 10 mM for (green) pristine SWNTs, (blue) Pd and (red) (Au/Pd) nanocubes decorated SWNTs. (d) Regression analysis of experimental data shows a linearity between current increase and H2O2 concentration in both cases of tube decoration. [Reprinted with permission from ACS Nano 3(1):37–44. Copyright 2009. American Chemical Society.]

catalytically induced charge into functionalized MWNTs. Moreover, silver nanocrystals-decorated carbon nanotubes are characterized by enhanced sensitivity (almost 10%). Quite fast response (few seconds) and full recovery within few minutes upon exposure to ammonia (1%) have been reported [100]. In this case, a key role is played by the fully oxidized silver surface, where the ammonia adsorption is thus favored leading to a conductance variation. Similar results are found for the detection at room temperature of several other gases (NO2 and CO) on mats of MWNTs initially treated with oxygen plasma and further decorated with Au nanoparticles. In particular, the combined treatment allows for the NO2 absorption to take place through the surface nanoparticles as well as the oxygenateddefected sites. On the other hand, since CO does not bind to those defected sites, the presence of gold is crucial for the absorption to take place. These findings are also strongly supported by first principle quantum electron conductance calculations [85]. Simulations as well as experimental results show that gold decoration of single wall carbon nanotubes dramatically improves gas detection. First-principle calculations have shown that hybrid CNT–NC structures (with NC = Ni13 and Pt13) are sensitive to benzene adsorption through a large variation of the net magnetic moment [169]. In fact, the interaction between the p electrons (CNT) and d orbitals (nanocrystal) is very strong, thus affecting structural and electronic properties. Interestingly, for very small nanocrystals, the tube is affected by a net spin polarization and it is possible to observe spin filtering effect in specific energetic conditions (0.5 eV around EF).

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Depending on the metal ‘‘deposited’’ onto the CNT structures, it is thus possible to tune and deliver specific functionalities (i.e. sensitivity to specific gases or species) [170–172]. Decoration of carbon nanotubes with platinum nanospheres has led to a glucose sensitivity of 70 lA/mM/cm2, a low detection limit of 380 nM and a linear sensing response within a wide concentration range between 1 lM and 0.75 mM [173]. Those Pt-CNTs nanostructures and potentially similar hybrid nanomaterials thus represent a suitable platform for cell physiology research and still retain great potentials for blood glucose sensing, where there exists the need of an efficient detection in the physiological range 3.8–6.1 mM [174]. Very similar results have been obtained by the same research group using also gold/palladium nanocubes on networks of SWNTs, as shown in Fig. 19 [175]. In particular, the biocompatibility of the Au/Pd nanocubes additionally renders this class of sensors exceptional and promising candidates for a wide range of bio-functionalization schemes and biomarker detection strategies. Further, the possibility of controlling both size and density of the decorating objects [176], allows for a precise control of the different functionalities of the sensor itself. It is important to note that due to the fabrication process, this approach is in principle scalable for mass production, thus allowing future integration into commercial sensors and monitoring systems with accurate sensitivity and precise glucose sensing. Similarly, Tang and coworkers have developed an amperometric biosensor based on a platinum NP-modified carbon nanotube electrode, which presents excellent electrocatalytic activity combined with a peculiar three-dimensional geometry leading to a broad detection range, very fast response and enhanced sensitivity (91 mA M 1 cm 2) and stability (more than 70% intact after few weeks). In addition, those characteristics can be controlled by varying experimental conditions (pH, applied potential, electrode fabrication process [177]. Finally, even if beyond the scope of the present review, there is also great interest in the use of metal-hybrid carbon nanotubes as novel objects onto/into which additional active species (i.e. molecules, organic molecules and DNA) can perform the sensor active function. In some cases, the presence of the metal might potentially favor the attachment of chemical groups and molecules, as in the case of Ag-decorated SWNT-based device, where the variation of electrical resistance is used to monitor the effective attachment of DNA molecules onto the carbon nanotubes themselves [103]. Pt nanoparticledecorated multi wall nanotubes (in the form of a glassy carbon electrode (GCE) in combination with Nafion) have been demonstrated to achieve a limit for DNA detection of 1.0  10 11 mol l 1, as a result of the capability of carbon nanotubes to promote electron-transfer reactions and the high catalytic activities of platinum nanoparticles for chemical reactions [178]. Similar experimental results have been also obtained in the case of Au nanoparticle–MWNTs [179–181].

4. Conclusions The present review has provided a broad and detailed overview of a novel class of hybrid onedimensional carbon nanostructures comprising of a carbon nanotube (single wall as well as multi wall) and metallic objects in diverse forms. Combining those structures leads to the development of innovative functionalities or the enhancement of some of the existing properties. As shown, there are numerous methods to fabricate those m-CNTs, including in-situ and ex-situ methods, the choice of which mainly depends on the class of carbon nanotubes and the physical and chemical properties of the filling materials. Various applications in diverse fields have also been presented based on their properties, ranging from spintronics to gas sensing, to medicine, where for example m-CNTs have been tested as a contrast agent in imaging techniques as well as nanoscale vehicles to deliver therapeutics or localized heat. The diversity of the literature presented here clearly shows the large ongoing efforts in various fields (engineering, chemistry, physics, biology, materials science, etc.) toward the development of radically new material platforms on which to design and fabricate original architectures, targeted mainly at nanoscale specific needs in specific fields. Some of the works presented here are by now potentially ready for fast application as in the case of gadolinium–carbon nanotube complexes for advanced MRI contrast agents, where tests have already been performed using commercially available systems, thus establishing their real efficacy. With wellestablished potential, future and ongoing efforts will surely foresee the implementation and

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exploitation of those structures in commercial systems available within medical facilities. At the same, these very promising results will pave the way to the optimization of those structures, in terms of materials, shapes and architectures. On the other hand, ferromagnetic tubes, if a precise control of their magnetic response can be achieved, represent clearly interesting individual objects, with very small dimensions, with immense applications in data storage devices (i.e. HAMR) and spintronics, where each atom-nanoparticle-cluster can be easily envisioned as the active element carrying the magnetic information (state) and the hosting tube is the scaffold for such architectures. For example, adjacent clusters can be envisioned as the ferromagnetic leads in spin valve-type spintronic devices, where each cluster can play the role of the injected and collecting electrode. Analogous applications absolutely require a very precise control of the filling materials as well as the position and location of the magnetic object within or onto the tubes: this still requires great effort and further studies for future implementation. Carbon nanotubes grown in an alumina template represent for example a class of same lengthsame diameter tubes, where controlled filling by electrochemistry-based techniques can lead to the production of a large amount of carbon nanotubes (individual or bulk arrays) with the same set of properties along with a high level of reproducibility. In particular, step-like growth of carbon nanotubes and of metallic nanowire segments within the pores of the alumina matrix can potentially provide: (i) all electrically-connected carbon shells, (ii) precise length control of the carbon nanotubes as well as of the metal segments, (iii) numerous choices in terms of electrodeposited metal (or other materials). This represent a very promising solution for interconnect applications. This review has presented a summary on diverse m-CNT structures, however there is still a large and increasing amount of work dedicated to how to properly choose filling/decorating materials and engineer their shape and composition to perform specific functionalities. Although mainly focused on materials and the development of new architectures based on m-CNTs, this review targets some of the current issues and technological challenges in different scientific communities, and where novel functionalities arising from combining carbon nanotubes with metal nanoscale objects leads to radically novel and successful materials. Acknowledgements CS would like to acknowledge many of her past and current colleagues for their support: many colleagues at Rensselaer Polytechnic Institute and University of Pavia (Dr. Bellani and Dr. Rossella) for their shared interest and continuing effort in the study of hybrid carbon-based nanostructures. CS is very thankful in particular to Prof. S. Kar from Northeastern University for his numerous, lengthy and inspiring discussions during those years. CS acknowledges Dr. L. Ortolani for helping with the graphics of the manuscript and the current financial support from Italian MIUR funded project FIRB RBAP115AYN: ‘‘Oxides at the Nanoscale: Multifunctionality and Applications’’. References [1] Iijima S. Helical microtubules of graphitic carbon. Nature 1991;354:56–8. [2] Dresselhaus MS, Dresselhaus G, Avouris P. Carbon nanotubes: synthesis, structure, properties, and applications. Berlin: Springer-Verlag; 2001. [3] Terrones M. Science and technology of the twenty-first century: synthesis, properties and applications of carbon nanotubes. Annu Rev Mater Res 2003;33:419–501. [4] Novoselov KS, Geim AK, Morozov SV, Jiang D, Zhang Y, Dubonos SV, et al. Electric field effect in atomically thin carbon films. Science 2004;306:666–9. [5] Soldano C, Mahmood A, Dujardin E. Production, properties and potential of grapheme. Carbon 2010;48:2127–50. [6] Frank S, Poncharal P, Wang ZL, de Heer WA. Carbon nanotube quantum resistors. Science 1998;280:1744–6. [7] Bachtold A, Strunk C, Salvetat J-P, Bonard J-M, Forró L, Nussbaumer T, et al. Aharonov-Bohm oscillations in carbon nanotubes. Nature 1999;397:673–5. [8] Martel R, Schmidt T, Shea HR, Hertel T, Avouris P. Single- and multiwall carbon nanotube field-effect transistors. Appl Phys Lett 1998;73(17):2447–9. [9] Gayathri V, Geetha R. Carbon nanotube as NEMS sensor – effect of chirality and stone-wales defect intend. J Phys: Conf Series 2006;34:824–8. [10] Kwo JL, Yokoyama M, Wang WC, Chuang FY, Lin IN. Characteristics of flat panel display using carbon nanotubes as electron emitters. Diam Rel Mater 2000;9(3):1270–4.

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