Macroscopic-Scale Assembled Nanowire Thin ... - ACS Publications

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Macroscopic-Scale Assembled Nanowire Thin Films and Their Functionalities Jian-Wei Liu, Hai-Wei Liang, and Shu-Hong Yu* Division of Nanomaterials & Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemistry, National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230026, P. R. China. Corresponding Author Notes Biographies Acknowledgments List of Symbols and Abbreviations References

1. INTRODUCTION A nanowire is a structure with a diameter of the order of a nanometer. When the diameter puts the radial dimension of the nanowire at or below certain characteristic lengths, such as the Bohr radius, the wavelength of the light, phonon mean-free path, and others, quantum mechanical effects become important.1,2 With a large surface-to-volume ratio and twodimensional confinement, nanowires show unique optical, magnetic, and electronic properties.3−14 Moreover, the large aspect ratio of nanowires, as an ideal energy transport material, can direct the conduction of quantum particles such as electrons, phonons, and photons, improving their technological application. As nanotechnology developed, significant advancement has been made in the last two decades; a wide range of low-dimensional nanostructured materials with good qualities can now be produced in large quantities.3−5,14−26 Research attention has now shifted to creating and constructing functional, well-defined ordered superstructures or complex architectures engineered for desired properties.27−30 Macroscopic-scale nanofabrication using wirelike nanostructures has become one of the most active research areas of materials science.31−34 A diverse range of properties are accessible through the numerous types of nanowires that can be produced by synthetic efforts. Integrating these nanowires in wafer scale as key components will require the development of appropriate methods to assemble these materials. This level of control also provides an advanced understanding of the phenomena of aggregation, which has long been a subject of research on small-sized nanowire fabrication. These techniques complement many different approaches to nanofabrication. The self-assembly paradigm in chemistry, physics, and biology has matured scientifically over the past two decades to a point of sophistication such that one can begin to exploit its numerous attributes in nanofabrication. Self-assembly (SA) in nanotechnology refers to the process by which spontaneous organization of pre-existing nanomaterials into the organized, ordered, or functional desired systems occurs. The reason for the self-assembly is the direct specific interactions and/or

CONTENTS 1. Introduction 2. Thin Films Composed of Assembled Ordered Nanowire Arrays 2.1. Interface-Induced Nanowire Assembly 2.1.1. Assembly of Nanowires by the Langmuir−Blodgett Technique 2.1.2. Evaporation-Induced Assembly of Nanowires 2.1.3. Other Nanowire Assemblies at Complex Interfaces 2.1.4. Nanowire Assemblies by Mechanical Force 2.1.5. Nanowire Assemblies Induced by External Nanostructures 2.2. Assembly of 1D Nanostructures by External Fields 2.2.1. Magnetic Field-Assisted Assembly of 1D Nanostructures 2.2.2. Electric Field- or Dielectrophoresis-Assisted Assembly of 1D Nanostructures 2.3. Nanowire Assemblies by Microfluidic Flow 2.4. Nanowire Assemblies by Bubble-Blowing Process 2.5. Nanowire Assemblies by Electrospinning 3. Thin Films Composed of Disordered Nanowires 3.1. Nanowire Films by Layer-by-Layer Assembly 3.2. Nanowire Films by Spin-Coating 3.3. Nanowire Films by Vacuum Filtration or Solvent Evaporation 4. Applications of Nanowire Thin Films 4.1. Nanowire Thin Films for Nanodevice Fabrication 4.2. Nanowire Membrane for Fuel Cells 4.3. Nanowire Thin Films for Separation and Environmental Application 5. Concluding Remarks Author Information © XXXX American Chemical Society

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Received: September 17, 2011

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dx.doi.org/10.1021/cr200347w | Chem. Rev. XXXX, XXX, XXX−XXX

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Scheme 1. Schematic Illustration of Macroscopic-Scale Nanowire Thin-Film Assembling Methods That Make Disordered 1-D Nanowires Ordered

indirect interactions, through their environment.35,36 An understanding of these systems provides insights into how the unique electrical, optical, magnetic, and spectroscopic properties of the nanowire assemblies can be exploited for potential applications. Characterized by a minimum in the system’s free energy, self-assembly, which is essential for organization of disordered structures into ordered structures, either through direct or indirect interactions, is typically associated with thermodynamic equilibrium. The fact that nanowires and nanowire assemblies exhibit unique electrical, optical, magnetic, and catalytic properties is not only because of the dramatic increase in surface area/volume ratio but also because of the emergence of collective and nanoscale properties as a result of the interwire arrangement or assembly. The ability to create nanowire networks, arrays, and composites, however, depends on our ability to fully understand and control the assembly process of these materials. Nanowires with high aspect ratio often curl and twist, hampering their self-assembly into ordered nanostructures.37,38 At the beginning of the century, many comprehensive and impressive reviews were published on topics related to nanowire, mainly one-dimensional nanowire synthesis and characterization.1,3,4,39,40 With the research attention shift toward nanowire integration, the field is undergoing a significant expansion and is becoming one of the most active research areas within the nanoscience community. However, few reviews have charted this research development. Recently, the group of Yang studied and reviewed nanocrystal and nanowire assembly by the Langmuir−Blodgett (LB) method,41 and as a versatile method, the LB assembly method for organic functional nanowires was also reviewed by Ariga and coworkers.42 Meanwhile, on the basis of their own excellent work, the group of Lieber reviewed nanowire integration and

application.43 For soft organic functional nanowires only, controlled growth and assembly were also reviewed by the groups of Ariga and Hu.38,44 From the viewpoint of interaction forces, the Gates group published a short review about nanowire assembly.45 Herein, we intend to overview the developments in the field of emerging macroscopic-scale nanowire thin films that are composed of integrated nanowires or their random components, including assembly principles and strategies for design, fabrication, and applications. This review also discusses how to engineer “bottom-up” assembly processes to direct nanowires assembled into various device-based architectures and achieve ordering on macroscale, which can affect their functional properties. It emphasizes the latest advancements, and examples specific to well-aligned nanowire systems will be highlighted. Overall, the objectives of this review are (i) to review the methods and principles used for generating ordered nanowire films from disordered; (ii) to review the methods and principles used for generating nanowire films with random placement; and (iii) to sketch some of the interesting applications and research directions involving both ordered and disordered nanowire films.

2. THIN FILMS COMPOSED OF ASSEMBLED ORDERED NANOWIRE ARRAYS Although 1D nanowires are of great interest for applications, their disordered structures seem to be problematic for use in device fabrication (for example, microelectronics and photonics) that often requires well-aligned and highly ordered architectures.43,46−51 Rational assembly strategies are needed not only to build complex structures with novel collective properties but also to pattern nanoscale building blocks for device fabrication at a practical scale. Device fabrication using B


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Figure 1. (a) Illustration of LB nanowire assembly process. Reproduced with permission from ref 67. Copyright 2003 Nature. (b) SEM image of the aligned Ge NW film. Inset: photo of a Ge nanowire suspension in chloroform. Reproduced with permission from ref 64. Copyright 2005 American Chemical Society. (c) Typical SEM image of VO2 nanowire LB films deposited at surface pressures of 39.5 mN/m. Reproduced with permission from ref 65. Copyright 2009 American Chemical Society. (d) Scanning electron microscopy image of the silver nanowire monolayer deposited on a silicon wafer. Reproduced with permission from ref 63. Copyright 2003 American Chemical Society. (e) TEM image of large-scale assembly of PbS nanowires at surface pressures of 23 mN/m and (f) TEM image of thicker PbS nanowires after coalescence at surface pressures of 25 mN/m for 90 min. (e and f) Reproduced with permission from ref 66. Copyright 2007 American Chemical Society.

wire-based film is sometimes pursued out of necessity. Achieving the substantial potential of nanowires (NWs) and nanotubes (NTs) in these and other areas of nanotechnology will require the controlled and predictable assembly of wellordered structures. Thin films composed of organized onedimensional nanostructures with well-controlled location, orientation, and spacing across multiple length scales could enable next-generation high-performance electronic, optoelectronic, and electromechanical systems.52 Assembly of complex nanowire structures requires not only manipulation of individual wires but also the controlled connection of one wire to another. For low-cost applications of 1D nanomaterials, it is necessary to develop suitable assembling processes for forming films of nanostructures with good packing order. A grand challenge is assembling and positioning the nanowires in desired locations to construct complex, higher-order functional structures. There have now been some reviews published that relate to the nanowire assembly.41,44,45,53,54 However, it is still challenging to manipulate either an array of nanowires or individual nanowires in a controllable manner. The lack of this ability is currently hindering the realization of integrated devices and circuits over macroscopic length scales. In general, there are two strategies for nanowire assembly, one is the “topdown” method,55,56 mainly based on photolithography, and the other is the “bottom-up” process, typically self-assembly methods. Compared with previous techniques, self-assembly approaches have lower processing costs and higher production efficiency, offering more flexibility in selecting functional materials and fabrication processes. However, major efforts are still needed to further develop such approaches. Here, we provide an overview of “bottom-up” assembly of onedimensional (1D) nanostructure with different methods and mechanisms (Scheme 1). This research is of interest to physicists, chemists, and material scientists alike, especially in light of efforts to find “green” methods for the assembly of 1D

nanostructures. Integrating these nanowires as key components will require the development of appropriate methods to assemble these materials. These techniques can complement many different approaches to nanofabrication. 2.1. Interface-Induced Nanowire Assembly

2.1.1. Assembly of Nanowires by the Langmuir− Blodgett Technique. The Langmuir−Blodgett (LB) technique was traditionally used to fabricate amphiphilic molecule monolayers onto a water surface. Now, the LB technique has been shown to be a high-throughput, low-cost, easily integrated method to assemble nanosized building blocks to fabricate both closely packed nano superstructures and well-defined patterns with low density.41,42,57−62 Figure 1a illustrates the LB assembly process for nanowire orientation. Nanomaterials were first dissolved in an immiscible volatile solvent and spread onto a water-supported surface using a microsyringe. After solvent evaporation, the sample was compressed slowly while the surface pressure was monitored and a Langmuir thin film consisting of a nanostructure monolayer can be obtained.41 On the basis of the application of an appropriate level of compression, the initial preparation of the monolayer at the interface can be followed by dynamic manipulation via compression or expansion during the LB assembly process, and as a result, suitable packing within the monolayer is achieved. The monolayers can be repeatedly deposited in a layer-by-layer fashion onto a solid substrate through verticaldipping (Langmuir−Blodgett) or horizontal-lifting (Langmuir− Schaefer) techniques, which provides a means for preparation of a certain thickness of the film device and offers the additional possibility for fundamental scientific study of interactions among assembled layers. The Yang group initiated a research program in the area of nanoparticle and nanowire assembly by LB techniques and published an impressive review recently.41,61,63 Subsequently, this versatile method has attracted C


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Figure 2. (a and b) TEM and high-resolution TEM (HRTEM) images of the monolayer assembly of Te nanowires. (c) SAXRD pattern measured on the monolayer of aligned Te nanowires. (d−f) Crossed Te nanowire layers can be formed by uniformly transferring a second layer of aligned parallel Te nanowires perpendicular to the first layer and turning the angle between two monolayers of nanowires. The insets of (d−f) show the fast Fourier transform of the TEM images and the simulation plot for the assembled mesostructures of the nanowires. Reproduced with permission from ref 58. Copyright 2010 American Chemical Society.

explosive attention worldwide. Chemically functionalized, uniform Ge nanowires are soluble in organic solvents and can be readily assembled into a close-packed monolayer by LB processes (Figure 1b).64 In addition, ordered nanowire films, such as VO2 nanowires (Figure 1c), Ag nanowires (Figure 1d), and PbS nanowires (Figure 1e and f), have been prepared.63,65,66 Though the technique has proven to be a versatile and general tool for assembly of 1D nanosized building blocks, there are still some limitations inherent in this fabrication method. For example, for hydrophilic nanosized building blocks, the surface of the nanomaterials must be functionalized with hydrophobic ligands for the LB experiment, which greatly restricts their future application.41,42 Recently, our group has shown that well-defined periodic mesostructures of hydrophilic flexible Te nanowires with aspect ratios of at least 104 can be produced by the LB technique without any extra hydrophobic pretreatment or functionalization.58 To avoid the hydrophobization, before adding to the water−air interface, hydrophilic ultrathin Te nanowires were first dissolved in N,Ndimethylformamide (DMF) and then added to a mixture of DMF and chloroform. The effect of the nanowire assembly is shown by transmission electron microscopy (TEM) images of Figure 2a and b and small-angle X-ray diffraction pattern (SAXRD) measurement of Figure 2c. Referring to the intuitive periodic information shown by TEM images, the corresponding SAXRD data shows that the period of the Te nanowire monolayer is 7.16 nm. Packing the arrayed nanowire monolayers makes it possible to construct nanomesh-like mesostructures or more complex multilayered structures composed of ultrathin nanowires on a planar substrate by

delicately controlling the cross angles (Figure 2d−f). Besides the Te nanowires, this improved LB technique approach has been found to be a general method for the self-assembly of other flexible, ultralong one-dimensional nanomaterials such as Ag2Te nanowires and Pt nanotubes.58 2.1.2. Evaporation-Induced Assembly of Nanowires. Although the dynamics of evaporation are rather complex and involve many challenging problems in soft matter physics,68−71 assembly induced by evaporation is very common.72−76 When a spilled drop of nanomaterials dispersed in a volatile solution dries on a solid surface or a liquid−air interface, the material that settles out of this solution will adopt random orientations and finally leaves dense, well-organized assemblies because of both material−material and material−substrate interactions.77 The resulting morphologies and the degree of the ordering achieved vary significantly and depend on not only the nanomaterial’s size distribution but also the dewetting of the solvent during the assembly process. The nanomaterial is initially dispersed over the entire drop of the solution, and then the concentration of nanostructures is gradually increased as a result of solvent evaporation. The resulting outward flow can carry virtually all of the dispersed material to the edge. During drying, liquid evaporating from the edge is replenished by liquid from the interior and the contact line of the droplet undergoes stick−slip motion, that is, small movements interspersed with large and rapid displacements as a result of competition between the pinning and capillary forces, leaving a small amount of nanostructures distributed randomly on the substrate.77 When the contact line is pinned (fixed), the solvent evaporating from the edge is replenished by solvent from the interior. Because the outward solvent flow carries D


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Figure 3. (a) Illustration of sphere-on-flat evaporation setup. (b) Representation of as-prepared concentric rings of organic nanowires. (c and d) SEM images of the concentric rings of DMQA nanowires formed during solvent evaporation, with different magnification. Reproduced with permission from ref 85. Copyright 2011 Wiley. (e−g) TEM images of binary assemblies containing Au nanoparticles (e) and Au nanorods (f) at constant nanowire concentration. (g) SEM images of binary assemblies of Au nanoparticles induced by flexible aligned Au nanowires. Reproduced with permission from ref 87. Copyright 2010 Wiley. (h) Stepwise schematic showing the contact line moving process on a vertical substrate induced by solvent evaporation. In the initial stage (1), the contact angle between solvent and substrate is θ0 and the meniscus height is a0. (2) As evaporation time elapses, contact angle decreases and meniscus height increases because the contact line sticks on the substrate. (3) When the meniscus stretches to its maximum, the contact line cannot stick but slips to a new position. Thus, the contact angle and meniscus height return to their initial values, respectively. Reproduced with permission from ref 72. Copyright 2008 Wiley.

evaporation could be well-controlled inside a fume cupboard at room temperature. After the solution was dried on the substrate, it left a number of ordered rings of nanowires that aligned and formed concentric rings on both surfaces, as shown schematically in Figure 3b. The confined geometry consisted of a glass slide and a spherical lens made of fused silica, and a drop of N,N′-dimethylquinacridone (DMQA), which was selected as a nonvolatile solute, was dropped into the gap between slide and lens. Then, a steady upstream of nitrogen gas was blown nearby the setup to help remove the solvent. After the solution was dried on the substrate, it left a number of ordered rings of nanowires that aligned and formed concentric rings on both surfaces.85 The scanning electron microscope (SEM) images in Figure 3c and d show the concentric rings of DMQA nanowires formed during solvent evaporation. The density, length, and periodicity of the nanowire arrays can be tuned by controlling the evaporation rate. Concentric rings of remarkable regularity formed over a large scale of several hundred micrometers. Each ring consists of a number of DMQA nanowires.85 Liz-Marzán’s group in Spain reviewed the nanoparticle self-assembly process that can be directed, enhanced, or controlled by either changing the energy or entropy landscapes, using templates or applying external fields.86 Recently, their group reported binary selfassembly of gold nanowires with nanoparticles and nanorods induced by solvent evaporation, as shown in Figure 3e−g. Long nanowires can drive the oriented assembly of particles and rods into extended ordered arrays. Moreover, nanowires can tune

dispersed nanostructures to the edge, the concentration of nanostructures at the edge is higher than that at the center. When the nanostructure concentration at the edge increases up to a certain point, liquid-crystalline phases of nanostructures start to form out of the isotropic dispersions. The resultant liquid-crystalline phases precipitate to the substrate at the edge, which helps to pin the contact line.72 As more nanostructures are carried to the edge by the outward solvent flow, the liquidcrystalline phases grow larger. When the contact angle decreases to a critical value, owing to the continuous solvent evaporation, the contact line starts to recede until the solvent is completely evaporated.72 Here we take some recently reported thin nanowire organizations by evaporation as examples.78,79 Yan and co-workers reported the synthesis of thin nanowires with a uniform diameter of ∼1.8 nm and length up to several micrometers.80 The ultrathin nanowires could spontaneously assemble into three-dimensional (3D) superstructures via a parallel arrangement after evaporation of a mixture composed of cyclohexane and ethanol.80 Evaporation-induced nanowire assembly can be controlled by using a confined geometry to obtain a number of ordered nanostructures.81−84 Zhang et al. have reported a facile approach to the fabrication of large-scale regular concentric arrays of aligned organic nanowires by simply allowing solvent evaporation in a confined geometry.85 Figure 3a shows the illustration of sphere-on-flat evaporation setup during the whole evaporation process, and the direction and rate of solvent E


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Figure 4. (a) SEM image of the disordered Ag nanowires. (b) SEM image of the assembly nanowires. The insets of a and b are optical photographs of Ag nanowires dispersed in distilled water and the assembled nanowires transferred to a silicon wafer. (c) Cross section of the three-phase interface. (d) Schematic illustration of the movement of the Ag nanowires in the first step with the evaporation of the oil phase. (e) Schematic representation of the stages of film formation of the Ag nanowires at the three-phase interface. (f−i) Corresponding experimental optical photographs of the steps shown in e. Reproduced with permission from ref 100. Copyright 2010 Wiley.

could be removed with tweezers.90 Films containing plasticizers could be cut to a desired shape using a razor blade, whereas films from pure protein fibrils were more brittle and could be broken to size with the use of tweezers.90 Solvent loss due to evaporation in a drying drop can drive capillary flows and solute migration.72,91−93 The substrate was treated to make the surface hydrophilic, so that the solvent meets the surface at a nonzero contact angle and the contact line is pinned due to surface roughness (Figure 3h). The contact line is pinned to its initial position, as is commonly the case.91 The physical phenomena of contact line pinning and solvent evaporation caused the solutes (nanosized building blocks) in the droplet to flow toward the contact line (i.e., capillary flow).91 Owing to the anisotropic nature of the interactions, the tips of the nanowires would be directed along the flow of solution. This explains the self-assembly of rods that are predominantly aligned along the dewetting direction. As a result, the combined effect of the capillary flow and strong interaction leads to the formation of ordered nanowire arrays.72 On the other hand, contact line pinning and dewetting create

distances between nanoparticles and nanorods, thereby altering the overall optical response of the film.87 Choi and co-workers reported a nanowire self-assembly process during the evaporation of a colloid droplet of nanowires on nanoengineered superhydrophobic surfaces. The self-assembly of nanowires is achieved by the interactions between nanowires and the superhydrophobic surface engineered with sharp-tip latching nanostructures of micropillars.88 Xu and co-workers introduced a facial solvent-evaporation method of radialoriented anthracene nanowires and their self-assembled concentric ring arrays.89 Welland and co-workers reported a scalable self-assembly approach induced by evaporation to making free-standing films from amyloid protein fibrils.90 The films were well-ordered and highly rigid, with a Young’s modulus of up to 5−7 GPa, which is comparable to the highest values for proteinaceous materials found in nature. Briefly, films were fabricated by transferring 1 mL of the nanowires containing hydrogel onto a flat polytetrafluoroethylene film; the solvent and the volatile acid were left to evaporate for 24 h, and the resulting protein films F


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silicon wafer. Because the Ag nanowires were well-aligned with high ordering, the optical photograph showed a similar luster of the film as a silver mirror. Using this facile approach, Ag nanowires with a high aspect ratio were close-packed and aligned parallel to each other. The self-assembly procedure was conducted when an appropriate amount of an aqueous dispersion of Ag nanowires was dropped onto the surface of chloroform to form the water−oil−air three-phase interface, which is necessary for the ultralong Ag nanowire aggregation and self-assembly. The mechanism of three-phase interface assembly nanowires was also discussed.100 The formation of Ag nanowire arrays through this three-phase-interface approach is illustrated by Figure 4c−e. Figure 4f−i shows the corresponding experimental optical photographs of the steps shown in Figure 4e. To start with, the three-phase interface of oil− water−air was formed by dropping an appropriate amount of aqueous solution of the Ag nanowires onto the surface of chloroform.100 First, the Ag nanowires transferred step-by-step from the water−oil interface to the water−air interface through the oil−water−air interface line due to the chloroform evaporation. Second, the Ag nanowires at the water−air interface self-assembled, initially at the contact line of the wall of the beaker and the water phase, which can be denoted as the water−air−substrate interface. At last, the film of the Ag nanowires at the water−air−substrate interface grew into a continuous, well-aligned nanowire film. Thus, the three-phase interface is thought to play a key role in the process, and could drive the movement and self-assembly of the Ag nanowires.100 In addition, evaporation of the oil phase, capillary force, and poly(vinyl pyrrolidone) (PVP) molecules are the main factors that promote the self-assembly of ultralong Ag nanowires. Ag nanowires that were prepared by the polyol reduction of AgNO3 in the presence of PVP had very good dispersion in water due to the protecting layer of PVP on their surface. PVP also provides a stronger interaction among the adjacent Ag nanowires, which may improve the nanowires ability to bind together and align parallel to each other. The longer the PVPcapped Ag nanowires were washed, the less ordered the selfassembled film of the Ag nanowires became.100 Recently, Yu and co-workers reported a water−oil-interface assembly strategy to fabricate large-area self-assembled nanofilms composed of various nanobuilding blocks at room temperature, including nanoparticles, nanocubes, nanowires, and nanosheets.101 A family of liquid−liquid interfacial systems could be selected as platforms for fabrication of nanofilms to deepen the impression of assembly and aggregation at nanoscales. Both planar and uneven substrates could be selected to transfer nanofilms, and the thickness of the nanofilms could be controlled by layer-by-layer dipping. Interestingly, nanowires and nanoparticles can be wellcoassembled at liquid−liquid interfaces.101 The as-prepared coassembled Ag NW and nanoparticle (NP) films show high SERS intensity, and the SERS performance will be described in the latter part of this review. 2.1.4. Nanowire Assemblies by Mechanical Force. Controlled and uniform assembly of nanowire materials with high order and large area presents one of the significant bottleneck challenges facing the integration of nanowires for electronic applications. A simple and effective method for the assembly of highly aligned nanowires on stretchable substrates is called mechanical force-induced assembly at the interface. Here we take the contact-printing method,102,103 the knocking-

the competition between the pinning and depinning forces. During evaporation of solvent, the contact angle decreases (θi < θ0) and the meniscus interface area increases (ai > a0). These changes cause an increase in depinning force, without altering the pinning force. When the depinning force reaches a value greater than the pinning force, the contact line becomes unstable and hops to a new position, and thus a new stripe of nanowires develops.72 The deposited nanowires enhance the surface roughness and increase the pinning force. Higher nanowire density also makes the pinning force greater, and then the meniscus can be stretched longer before the contact line slips. Thus, at a higher evaporation rate, intervals between nanowire arrays are also expected to be larger.72 2.1.3. Other Nanowire Assemblies at Complex Interfaces. Interfaces provide a unique heterodielectric environment where phases having totally different dielectric constants are in contact. Interfacial assembly has been shown to offer a significant platform for the organization of nanosized building block films in recent years, based on interfacialordering effects.94 There are mainly two methods of forming an ordered film at interfaces: exchanging the ligands capped on nanowires before mixing the suspension with another solvent to form an interface,95 or changing the interaction forces between the nanowires by adding inorganic salts, organic solvent, or ultrasonic waves after forming the interface.96 Choi et al. reported on a self-assembly method in which well-defined superhydrophobic structures on a template surface can configure three-phase (liquid−solid−gas) contact lines at the structure's tips and direct the site-specific self-assembly of nanowires when the colloidal droplet of nanowires recedes in evaporation.97 Zhang, Lee, and their co-workers have developed a simple one-step process for forming and assembling single-crystal organic nanowires into aligned films at the organic solvent/water interface.98 Solvent evaporation plays a key role in the assembly process. During dichloromethane (DCM) evaporation, organic nanowires would be trapped at the DCM/water interface driven by the compression force. As more DCM evaporated, the density of the nanowires would increase and the DCM/water interface would continuously shrink. Upon complete solvent evaporation, a compact monolayer of well-aligned nanowires was left on the water surface. As a result, disordered nanowires were compressed into an ordered array. These films can be transferred directly onto any desired substrate or stacked layer-by-layer to form a multilayer film for device applications.98 The Elemans group reported a spontaneous assembly and formation of periodic patterns of exceptionally long (up to 1 mm) columnar stacks of porphyrin dye molecules at a solid/ liquid interface, with highly defined spatial and parallel ordering.99 First, 3-mL droplets of chloroform solution of porphyrin dye with a concentration of 4.8 × 10−6 M were dropcasted on mica. After evaporation, very large domains (up to ∼3 mm2) containing a highly ordered pattern of equidistant, nearly parallel, wirelike architectures were observed. By combined self-assembly and dewetting, the orientation process takes place simultaneously when a drop-casted solution of the porphyrin molecules is evaporated on a surface.99 Recently, we introduced a novel and powerful three-phaseinterface assembly approach to produce free-standing ordered Ag nanowire thin films.100 Figure 4a and b shows the SEM images of disordered Ag nanowires before assembly and ordered Ag nanowires after assembly. The insets of Figure 4b show an optical photograph of the nanowires transferred onto a G


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Figure 5. Schematic illustration showing the process of NW assembly. (a) Contact printing of NWs. (b) Knocking-down assembly method. (c) Strain-release NW assembly method. (a−c) Reproduced with permission from refs 103−105, respectively. Copyright 2008, 2010, and 2011 American Chemical Society. (d) Illustration of the transfer printing NW assembly method. Reproduced with permission from ref 109. Copyright 2007 Nature. H


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Figure 6. Nanowire alignment with GO on a Si substrate. (a) Scheme showing the experimental setup: a piece of cleaned Si wafer is immersed into the well-dispersed nanowire/GO solution. Upon solution evaporation, an aligned nanowire pattern is deposited on the substrate with the orientation parallel to the horizontal liquid−substrate contact line. (b and c) Typical SEM images of nanowire alignment pattern. Reproduced with permission from ref 110. Copyright 2009 American Chemical Society. (d) General scheme of Au nanowire self-assembly onto CNTs. Reproduced with permission from ref 113. Copyright 2011 American Chemical Society. (e and f) AFM images of highly aligned collagen fibrils on muscovite mica. [KCl] = 200 mM. The concentration of collagen and incubation time on mica are (e) 10 μg/mL collagen and 1 min of incubation and (f) 50 μg/mL and 108 min of incubation. Inset: 2 μm scan. Reproduced with permission from ref 115. Copyright 2011 American Chemical Society.

down method,104 and the strain-release assembly105 for examples. The contact-printing method introduced here involves directional sliding of a growth substrate, consisting of a dense “lawn” of NWs, on top of a receiver substrate coated with a lithographically patterned resist.102,103 During the process, NWs are, in effect, combed by the sliding step and eventually detached from the donor substrate as they are anchored by the van der Waals interactions with the surface of the receiver substrate, resulting in the direct transfer of aligned NWs to the receiver chip. Lieber et al. used the contact-printing method, which was developed to directly transfer regular arrays of semiconductor NWs from donor to patterned receiver substrates, to assemble both single and highly dense parallel arrays of NWs on substrates and in large scale, as shown in Figure 5a.102 Weak interactions between the chemically unmodified NWs and surface chemical modification of the receiver substrate play an important role in the assembly process. The knocking-down nanowire assembly method involves directional “knock-down” of a growth substrate, consisting of a preprogrammed ordered dense “lawn” of nanowire arrays, for the controlled in-place planarization of the vertical nanowire elements. One of the initial works reported by Patolsky and coworkers was shown in Figure 5b with the schematic illustration of the knock-down process.104 First, a typical vertical nanowire array consisting of silicon nanowire elements, 4 μm long and 80 nm in diameter, was formed by the simple top-down sculpting of an appropriate silicon-on-insulator substrate. Then followed manual rolling of the elastomer-based roller, made of poly(dimethylsiloxane) (PDMS), Teflon, or other elastomers with different rigidity and surface properties, over the nanowire-

array substrate. Compared with the contact-printing method, nanowire elements are directly grown on the final device substrate, which is advantageous over transferring the nanowires from a “donor” substrate.104 Lei et al. reported a two-step knock-down method for successful fabrication of large-area well-aligned arrays of ZnO nanowires with large area and high density along their c-axis on a flexible substrate.106 First, a modified chemical vapor deposition (CVD) process is initially used for synthesizing vertical ZnO NW arrays perpendicular to the donor substrate surface. Then, the manual contact-printing method was introduced to knock the nanowires down and transfer them to a receiver substrate with well patterns.106 As an elective method for assembly of highly aligned NWs, the strain-release assembly method involves first transferring NWs to a strained stretchable substrate and then releasing the strained substrate.105 As a result, the NWs aligned in the transverse direction and the area coverage of the NWs on the substrate increased. This method can be applied to any NWs synthesized by different methods or processed in different conditions deposited on a stretchable film and can be repeated multiple times to increase the alignment and density of the NWs. The elasticity of the substrate and the static friction between the NWs and the substrate play key roles in this assembly method. Ag and Si NWs were selected as two model materials by Zhu and co-workers to demonstrate the general applicability of the proposed alignment method, which is illustrated in Figure 5c, showing the process of NW assembly.105 First, Ag or Si NWs were transferred to the PDMS-strained substrate based on two different strategies, contact-line deposition and contact printing. After the growth substrate was removed, the PDMS was coated with a roughly aligned film of Ag or Si NWs. When the PDMS substrate was I


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however, these methods are identical to those previously developed and currently used for carbon nanotubes.

released, the alignment and density of the NWs increased. Moreover, subsequent transfers of the NWs can be performed to further increase the alignment and density. The handspinning technique was introduced, a novel spinning method, to prepare uniaxially aligned nanofibers.107 The easy processing is the most crucial advantage in this hand-spinning method. Moreover, it was found that the handspun fiber diameter and its surface morphologies dramatically depended on PAD and solvent system. Cao and co-workers reported a scale selfassembly method to regulate suspended single-walled carbon nanotubes (SWNTs) across regular TiO2 gel islands.108 First, random SWNTs were coated on the top of a small and uniformly dispersed TiO2 nanoparticle supernatant. After the TiO2 gel was completely dried and chapped into microscale islands, random SWNTs could be straightened into aligned arrays and suspended across the TiO2 island gaps. The transfer printing nanowire assembly method involves transferring the prefabricated ordered nanowire topographical features of a mold into a flat polymeric film over large areas.109 First, a hard mold that contains topographical features is pressed into a polymeric material cast on a substrate, generating well-defined patterns in polymeric materials, and repeating the process can transfer the pattern to other substrates (Figure 5d). The merit of this technique is independent of nanowires and is suitable for a wide range of polymeric materials, including conjugated polymers. The shortcoming of this method is that it requires a prefabricated pattern.109 2.1.5. Nanowire Assemblies Induced by External Nanostructures. Appropriate and selective materials can promote the ordering of some nanowire at the interface. The interaction between the nanowires and assistant materials induces the alignment of the nanowires. Wu and Li studied coassembly behavior, i.e., using graphene oxide (GO) nanosheets to make disordered Na0.44MnO2 nanowires ordered (Figure 6a−c).110 With the addition of GO nanosheets to the nanowire aqueous suspension, GO nanosheets can absorb onto the nanowire surface because of the interactions between GO and nanowires, mainly hydrogen bonding and ion−dipole interactions; modify the surface properties of nanowires; and lead to their enrichment at the solution surface.110 The adsorption of GO also increases the negative surface charge density of the nanowires and thus further stabilizes the colloidal solution. When a critical concentration is reached, the alignment of nanowires occurs according to Onsager’s theory.111 Figure 6d shows the report of Yang and co-workers that gold nanowires112 with the diameter of 1.6 nm and ultrahigh aspect ratios (L/d > 500) self-assembled along the axes of multiwall carbon nanotubes.113 Strong van der Waals and hydrophobic interactions induced self-assembly of oleylamine-coated Au nanowires along sidewalls of carbon nanotubes (CNTs).113 Adsorption and lattice direction can induce the wires into ordered films (Figure 6e and f). Fibrillar collagen assembles on mica with highly ordered arrays via the initial adsorption to mica, surface diffusion, nucleation, and growth into a 2dimensional network captured by atomic force microscopy (AFM). The mechanism of the assembly process could be that the mica lattice determines the growth direction of fibrils during the nucleation step, while potassium ions affect surface adsorption and diffusion of collagen molecules by neutralizing the mica surface.114,115 It should be pointed out that interface indeed provides an economical and effective platform for assembly of nanowires;

2.2. Assembly of 1D Nanostructures by External Fields

To control or modulate the assembled structures, physical forces, such as electric or magnetic fields, are often employed.116 An extra field would affect the growth and directional aggregation of nanosized building blocks, resulting in the formation of self-assembly structures of magnetic or charged materials. It is, therefore, important to tune the interactions of the nanomaterials by an external field, and thus, attempts to control the clustering and transport properties have attracted significant interest. 2.2.1. Magnetic Field-Assisted Assembly of 1D Nanostructures. A magnetic field is a field of force produced by moving electric charges, by electric fields that vary in time, and by the “intrinsic” magnetic field of elementary particles associated with the spin of the particles. An interesting phenomenon is that magnetic fields can be used for navigation by migratory and homing animals as well as bacteria. Iron-laden cells were thought to provide these creatures with a legend to the earth’s magnetic roadmap.117 These ferromagnetic crystals are polarized like a bar magnet under geomagnetic induction, which implies that a magnetic field would affect the growth and directional aggregation of nanocrystallites, resulting in the formation of self-assembled structures of nanomaterials. 45,118−127 Chen and co-workers showed a method combining arc-discharge growth with magnetic-field-induced alignment to prepare well-aligned and closely packed singlewalled carbon nanotube (SWNT) films over a large area on various substrates, including flexible plastics.128 This approach involves electric arc-discharge growth of SWNTs, diffusion and alignment of the in situ grown SWNTs in a parallel magnetic field, and then deposition of these aligned SWNTs on substrates at the desired location and orientation. In detail, Figure 7a illustrates the fabrication process of well-aligned and

Figure 7. (a) Scheme of fabrication of well-aligned and closely packed SWNT film on an arbitrary substrate by electric arc discharge with the assistance of a magnetic field. (b) Photograph of patterned, aligned SWNT films on glass. (c) Photograph of the 3D structure of aligned SWNTs with different orientations. (d) SEM image of a two-layered deposited aligned SWNT film with a different orientation. Reproduced with permission from ref 128. Copyright 2010 Wiley.

closely packed SWNT films on a substrate. The film hierarchical structure, thickness, area, location, and orientation can all be controlled. Furthermore, the evaporation coating character can also enable a controllable deposition of aligned SWNT films with desired geometries by controlling the deposition parameters. Thus, by employing the commonly used mask technique in vacuum evaporation coating, various patterned structures with aligned SWNTs on a designated area can be fabricated directly.128 Because the orientation of an J


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aligned SWNT film is only determined by the relative direction between applied magnetic field and substrate, the alignment orientation of SWNTs can be easily changed by rotating the substrate. Therefore, multiple layers and a 3D structure in which each layer has different orientation and location can be fabricated simply by adjusting the orientation and position of the substrate/mask (Figure 7b−d).128 Fragouli et al. reported a simple technique for magnetic-field-induced formation, assembling, and positioning of magnetic nanowires in a polymer film. The dimensions localized at specific layers in the polymer matrix could be controlled by varying the duration of the applied magnetic field, in combination with the evaporation dynamics.129 Figure 8a shows the scheme of the fabrication

particular, 1D arrays of well-aligned magnetic nanowires with magnetically anisotropic behavior based on the nanoparticles in plastic films can be formed thick enough to be free-standing. 2.2.2. Electric Field- or Dielectrophoresis-Assisted Assembly of 1D Nanostructures. In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. Nanowires suspended in a dielectric medium can be positioned between two electrodes by introducing an electric field that exerts a force on other electrically charged objects.21,25,130−141 Dielectrophoresis (DEP) is a phenomenon in which a force is exerted on dielectric building blocks when they are subjected to an inhomogeneous electric field, usually alternating current (AC) electric fields, offering a method to attract nanowires onto predefined electrodes. It is analogous to the related phenomenon of electrophoresis, in which motion of suspensoid building blocks are produced by the action of an electrostatic field on the charged building blocks.142 An alternating electric field can lead to nanowire polarization due to charge separation at the surface of the nanowire. When nanowires are more polarizable than the dielectric medium, they will experience a dielectrophoresis force that produces net movement in the direction of increasing field strength, which occurs at the periphery of the electrode fingers. So, the electric field-assisted assembly method has been applied to align nanowires, normally by the dielectrophoresis forces that direct the nanowires toward regions of high field strength.136,143−150 DEP can be tailored to work with nanowires of various conductivities and materials and allows for integration upon arbitrary substrates including those that require low-temperature processing, such as flexible substrates. Because of its ability to position nanowires or nanotubes precisely on a substrate, dielectrophoretic assembly of nanowires has become increasingly popular.151−154 As we know, the DEP process depends on the dielectric medium, the nanowire density, the bias on-time, and the bias field strength.140 First, gap distances between the two electrodes should have comparable lengths to the length of the nanowires, so that the NWs bridge the gap after the alignment. When an electric field is applied across the two electrodes, the charge-neutral regions of the nanowires become polarized and are then subjected to the DEP forces. Second, the nanowires should be more conductive than the dielectric

Figure 8. (a) Scheme of the fabrication method. (b−e) Optical microscope images of various colloidal NP/polymer assemblies (NPbuilt NWs). (b) 1 wt % γ-Fe 2 O 3 in 99 wt % of poly(methylmethacrylate) (PMMA). (c) 1 wt % γ-Fe2O3 in 99 wt % of polystyrene (PS). (d) 2 wt % γ-Fe2O3 in 98 wt % of poly[33′(vinylcarbazole)] (PVK). (e) 1 wt % Fe2O3−TiO2 heterostructures, in 99 wt % of PEMMA. Reproduced with permission from ref 129. Copyright 2010 American Chemical Society.

method. The well-defined film is produced by exposing nanosized building blocks with a certain concentration to an external magnetic field. Figure 8b−e shows the optical microscope images of various colloidal NP/polymer assemblies. Besides the applied magnetic field, the polymer matrix also plays an important role in irreversible formation of welldetermined NWs at specific depths in the film, because it induces the aggregation process in the initial stage of formation and defines a viscosity gradient in the film volume.129 In

Figure 9. Nanowires are attracted toward regions of electric field or a typical DEP assembly. (a−c) Optical dark-field and DUV images of nanowires assembled onto electrodes on a 4-in. quartz substrate after the complete process. Reproduced with permission from ref 140. Copyright 2010 Nature. (d−i) Scanning electron microscopy (SEM) images for a typical DEP assembly with varying nanotube density: (d) ∼1 SWNT/μm; (e) 10 SWNT/ μm; (f) 20 SWNT/μm; and (g) 30 SWNT/μm. “S” and “D” in (d−g) mean source and drain electrodes. (h and i) Magnified images of (f) and (g) reproduced with permission from ref 143. Copyright 2011 American Chemical Society. K


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Figure 10. Schematic of fluidic channel structures for flow assembly. (a) Channel formed when the PDMS mold was brought in contact with a flat substrate. NW assembly was carried out by flowing an NW suspension inside the channel with a controlled flow rate for a set duration. (b) Parallel arrays of NWs are observed in the flow direction on the substrate when the PDMS mold is removed. (c) Multiple crossed NW arrays can be obtained by changing the flow direction sequentially in a layer-by-layer assembly process. Reproduced with permission from ref 50. Copyright 2001 Science.

are parallel to each other. However, at higher density, ∼90% are aligned within ±10° of the longitudinal axis. Seen from these images, by simply varying the concentration of the SWNT solution, the density of the SWNT in the channel can be varied from 1 to ∼30 SWNT/μm. The group of Myoung reported a DEP process that provided a high yield and a large degree of freedom-positioning nanowires on the substrate, as well as fixed the DEP medium as isopropyl alcohol (IPA) for Si NWs at a direct current (DC) bias of 10 V.157 Keating and co-workers have introduced a hybrid approach that uses electric-field forces to direct different populations of biofunctionalized nanowires to specific regions of the chip by providing accurate registry between each individual nanowire and the photolithographic features within that region.158 After synchronized sequential injecting of nanowires carrying different DNA sequences, a programmed, spatially confined electric-field profile directs nanowire assembly. The nanowire arrays are considered as highly promising materials for electronics because of their relatively high carrier mobility, reliable control on geometry, and electronic properties.159

medium, so the nanowires are attracted to the electrode edges where the electric-field gradient is the highest. As a result, the nanowires can be designed to be assembled and positioned accurately in complex devices. Freer et al. reported a promising method of nanowire assembly to achieve an unprecedented combination of precision and area of coverage in the selfassembly of boron-doped silicon nanowires.140 Figure 9a−c shows the optical dark-field and deep ultraviolet (DUV) images of nanowires assembled onto electrodes on a wafer-scale quartz substrate after the complete process. As the nanowire suspension flows through the channel, nanowires that are within a distance less than the characteristic decay length of the AC electric field originating from the patterned electrodes polarize. The polarized nanowires are therefore attracted to the electrodes by means of dielectrophoretic forces, the magnitude of which depend on voltage, frequency, material properties, and electrode geometry. The nanowire-assembly process depends on the relative balance of the dielectrophoretic, hydrodynamic, and electrostatic double-layer interactions between the nanowire and surface.140 Under the correct conditions, nanowire assembly is self-limiting. Thus, single nanowires can be assembled on each electrode with high probability by carefully controlling the hydrodynamic and dielectrophoretic forces.140 The area over which this single-nanowire assembly is predictable is limited only by the ability to maintain uniformity of the hydrodynamic and electrostatic forces. Just like nanowires, nanotubes also can be assembled in devices by the DEP process.153,155,156 As an example of this, Khondaker and co-workers have reported a single-walled carbon nanotube (SWNT) assembly process induced by DEP in a probe station under ambient conditions.143 To start with, a small drop of the SWNT solution was cast onto the chip containing the electrode arrays with an AC voltage of 5 Vp‑p with a frequency of 300 kHz using a function generator between the source and drain electrodes for 30 s. The induced dipole moment of the nanotubes interacting with the strong electric field causes the nanotubes to move in a translational motion along the electric field gradient and align in the direction of the electric field lines.143 Figure 9d−i shows SEM images for a typical DEP assembly when the concentration of the SWNT solution was varied by simply diluting the original solution using deionized (DI) water. Different concentrations of the SWNT solution, for example, 0.08, 0.34, 1.67, and 3.4 μg/mL, can result in an increase in the density of the SWNT from 1 to ∼30/μm, respectively, as shown in Figure 9d−g. A magnified view of these images is shown in Figure 9h and i. At low densities, almost all of the nanotubes are well-aligned and

2.3. Nanowire Assemblies by Microfluidic Flow

The flow of a river can be used for delivering logs down to a distant sawmill since the 19th century. This old technique for transporting logs is the inspiration for a new method of assembling nanowires.67 One-dimensional (1D) nanowires can be strongly affected by the rheological behavior of suspensions, even at the very low loading, because of their large aspect ratios.160 Microfluidic flow can be used to align nanowires by combining fluidic alignment with surface-patterning techniques.8,50,161−169 First, a homogeneous, stable nanowire suspension is achieved, and when the suspensions pass through the microfluidic channels, nanowires are integrated.50 The nanowires are aligned along the flow direction driven by the shear force, and the density of assembled nanowires is controlled by the concentration of nanowires in the suspension and flow time. The degree of alignment can be controlled by the flow rate, because higher flow rates produce larger shear forces and hence lead to better alignment. The average NW surface coverage can be controlled by the flow duration where the average density is calculated by dividing the average number of NWs at any cross section of the channel by the width of the channel. Complex geometries such as crossed nanowire arrays can be formed through a layer-by-layer process. Lieber and coworkers reported an approach for the hierarchical assembly of one-dimensional nanostructures into well-defined functional networks using microfluidic flow.50 Figure 10a shows the fluidL


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Figure 11. Illustration of blown-bubble film (BBF) process. (a) NW/NT polymer suspension, (b) bubble expansion over a circular die, and (c) films transferred to crystalline wafers, plastics, curved surfaces, and open frames. Nitrogen gas at pressure P flows through the die and expands a bubble from the NW/NT−epoxy suspension (dark-blue color) on the top of the die while a stable vertical force, F, is applied by means of a wire-ring connected to a controlled speed motor. Black straight lines represent aligned NWs/NTs embedded in the bubble film. (d−g) Control of aligned NW density in BBFs. (d) Photograph of 0.01, 0.03, and 0.15 wt % (left to right) epoxy suspensions of Si NWs. (e−g) Dark-field optical images recorded from 0.01 (e), 0.03 (f), and 0.15 (g) wt % Si NW-BBFs, respectively. The scale bars in e, f, and g are 50, 20, and 10 mm, respectively. Reproduced with permission from ref 175. Copyright 2007 Nature.

Figure 12. (a) Laboratory setup for an electrospinning experiment with a perpendicular arrangement of the electrodes. (b) Photograph of the PVA nanofiber mat (with a PVA/Ag molar ratio of 530:3) by electrospinning for 1 h. (c) Typical SEM image of Ag/PVA nanofiber mat. (d−g) Typical TEM image of Ag/PVA nanofibers with a molar ratio of PVA/Ag ratio of (d) 530:1, (e) 530:2, (f) 530:3, and (g) 530:4. Insets are the photographs of the corresponding Ag/PVA nanofiber mats. Reproduced with permission from ref 181. Copyright 2009 American Chemical Society.

flow-induced assembly method of macroscopic arrangements of nanowires through rheological control.170 The alignment mainly involved an intense shear process, which was carried out on the rheometer. Generally, the applied shear rates were >100 s−1, and the shear time was >1 s. We found that higher shear rates usually led to better alignment effect. Moreover, longer shear time had no influence after the steady state had been reached. The aligned structures form under intense shear, which can be reversibly controlled by different shear rates. Recently, Dittrich et al. used microfluidic chips to trap in situ-formed bundles of nanowires in microsized cages and clamps, thereby enabling immobilization, positioning, and

assisted assembly arrays of nanowires. First, the nanowires were suspended in ethanol solution. Passing suspensions of the NWs through fluidic channel structures formed between a poly(dimethylsiloxane) (PDMS) mold and a flat substrate hierarchical assembly of 1D NWs are aligned in fluid flows with the separation and spatial location readily controlled (Figure 10b). Crossed NW arrays were also prepared by layerby-layer assembly with different flow directions for sequential steps (Figure 10c). Parallel and crossed arrays of NWs can be readily achieved with single (Figure 10b) and sequential crossed (Figure 10c) flows, respectively, for the assembly process as described below.50 Kim and co-workers reported a M


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Figure 13. (a and b) Electrospinning nanowire assembly process with the collector modified by gold electrode pattern. (a) Schematic illustration of a test pattern consisting of four gold electrodes patterned on a quartz substrate. (b) Optical micrograph of a mesh made of PVP nanofibers collected in the central region of the electrodes. Reproduced with permission from ref 201. Copyright 2004 Wiley. (c and d) Electrospinning nanowire assembly process with the collector modified by magnetic field. (c) Schematic setup for fabrication of aligned nanofibrous films. (d) SEM images of PVP fiber arrays fabricated using the method. Reproduced with permission from ref 195. Copyright 2010 Wiley. (e and f) Electrospinning nanowire assembly process with the collector modified by high-speed roller. (e) Schematic diagram of the setup for fabrication of aligned nanofibrous scaffolds by electrospinning with a rotating disk collector. (f) Optical micrograph of aligned organic nanofibrous scaffolds. Reproduced with permission from ref 192. Copyright 2004 Elsevier. (g and h) The electrospinning nanowire assembly process with the collector modified by high-speed insulated roller covered with parallel steel sticks. (g) Schematic diagram of the modified setup for the electrospinning process. (h). SEM image of fibers collected with a roller with parallel sticks and a baffle. Reproduced with permission from ref 184. Copyright 2007 Wiley.

cutting-out of desired lengths.171 Liu et al. presented a method of positioning and aligning both large-scale NW arrays and single lines of Ag NWs by hydrodynamic focusing that functions as “hydrotweezers”.172 The density, width, and position of the nanowire arrays can be tuned in the hydrodynamic focusing process by adjusting the flow duration and flow rates of the sheath flows and sample flow.

expands primarily in the vertical direction, with a continuous supply of NW suspension from the top surface of the die, the orientation of the NWs in the BBFs should always follow the upward (longitude) direction, which is consistent with optical images. Expansion along a defined direction is crucial to obtain consistent alignment of NWs over large areas and enables the overall orientation to be fixed in an absolute sense during transfer to a substrate, independent of high-resolution imaging.175

2.4. Nanowire Assemblies by Bubble-Blowing Process

Blown film extrusion is the most common method to make polymer films, especially for the manufacture of plastic films in large quantities, and involves extruding a molten polymer and inflating it to obtain a balloon, which can be collapsed and slit to form continuous flat films. The blown-bubble assembly strategy is a general and scalable assembly method that has been recently developed for uniformly aligned and controlleddensity nanowires and nanotubes films by controlling bubble expansion of homogeneous polymer suspensions containing 1D nanomaterials.173−175 The basic steps are illustrated in Figure 11. First, the functionalized nanowires were dispersed in a controlled concentration polymer to form a homogeneous, stable suspension (Figure 11a). Then expansion of the polymer suspension occurs, using a circular die to form a bubble at controlled pressure, P, and expansion rate, where stable vertical expansion is achieved using an external vertical force, F (Figure 11b). Figure 11c shows the transferral of the bubble film to substrates or open-frame structures.175 Excellent orientational alignment of Si NWs was observed for blown-bubble films (BBFs) with different NW densities prepared from 0.01−0.22 wt % Si NW−epoxy suspensions (Figure 11d−g). Qualitatively, the shear stress associated with the suspension passing through the circumferential edge of the die could align the high-aspect-ratio nanowires in a polymer fluid along the principal direction of strain.175 As the bubble

2.5. Nanowire Assemblies by Electrospinning

Electrospinning is a highly versatile method that utilizes high electrostatic forces to process solutions or melts, mainly of polymers, into continuous fibers with diameters ranging from a few micrometers to a few nanometers.176−181 Electrospinning mainly makes use of the electrostatic repulsions between surface charges to reduce the diameter of a viscoelastic jet or a glassy filament. Under the influence of a strong electrostatic field, assembly of electrospinning fibers along the axial direction occurs such that composites can be formed by imposing additional spatial confinement to the polymer chains. In a typical electrospinning experiment (Figure 12a), a polymer solution or melt is pumped through a thin nozzle with an inner diameter on the order of 100 μm. The nozzle simultaneously serves as an electrode, to which a high electric field of 100−500 kV m−1 is applied, and the distance to the counter electrode is 10−25 cm in laboratory systems. Our group has used poly(vinyl alcohol) (PVA) and high SERSactive Ag dimers or aligned aggregates, which are assembled within nanofibers with chainlike arrays, to synthesize a freestanding and flexible surface-enhanced Raman scattering (SERS) substrate via the electrospinning technique (Figure 12b).181 A typical SEM image of Ag/PVA nanofibers produced by electrospinning is shown in Figure 12c, which shows a threeN


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Figure 14. (a) Scheme of the LBL film deposition. Steps 1 and 3 represent the adsorption of polyanion and polycation; steps 2 and 4 are washing steps. Reproduced with permission from ref 204. Copyright 2008 American Chemical Society. (b) SEM image of (PDDA/NW)2, Te film consisting of two bilayers. (c) UV−vis absorbance spectra of LBL films with an increasing number of deposition cycles (PDDA/NW)15, Te film consisting of 15 bilayers. Reproduced with permission from ref 213. Copyright 2006 Wiley.

magnetic nanoparticles are mixed with polymers, aligned fibers can also be fabricated through external magnetic field.184−191 In anisotropic electrospinning, collector design is critical to derive oriented nanofibers. Meanwhile, electrospinning has been explored to align nanofibers using either rotating collector184,192−194 or electrostatic forces from specific counterelectrode collector.177,186,195−199 The Xia group reported a modified electrospinning process for generating uniaxially aligned nanofibers with various compositions.200,201 To obtain the parallel nanowire alignment, the conventional collector electrode was cut into two pieces, and these were separated then with a void gap. When the collector was modified by pairing gold electrodes patterned on the insulating substrate and alternately grounding (Figure 13a), a double-layered mesh of nanowires could be obtained (Figure 13b). Moreover, the electrode collectors were modified by the magnetic or electric field to align nanowires. Recently, Yang et al. reported a magnetic-field-assisted electrospinning process to fabricate aligned nanowires.195 Figure 13c shows the schematic setup of electrospinning with the collectors modified by magnetic field. The SEM image shows in Figure 13d uniaxially aligned PLGA fibers collected for 120 min. The magnetic field could generate an additional force on the jet and also increase the velocity of the jet reaching a substrate. The advantage of using an external magnetic field instead of parallel auxiliary electrodes to fabricate aligned nanofibers is that the alignment can be maintained for thick fibrous membranes.195 Besides parallel

dimensional network structure consisting of a large quantity of randomly deposited fibers, in which the individual fiber has a high aspect ratio and a smooth surface. TEM images in Figure 12d−g clearly showed that Ag aggregates were embedded in the PVA matrix and assembled to some ordered linear chainlike structures along the fiber axial direction and that the average diameter and length of Ag/PVA nanofibers were ∼170 nm and several millimeters, respectively. In contrast to the solution before electrospinning with a random dispersion of the aggregates, the Ag NP aggregates with chainlike arrays in the PVA matrix resulted after the electrospinning process.181 Recently, our group reported the fabrication of a free-standing, flexible, and stable Au nanorod (NR)/poly(vinyl alcohol) (PVA) nanofiber mat on a large scale by using electrospinning to direct self-assembly of AuNRs.182 The AuNRs were assembled within the PVA fibers and along the fiber axial direction. The distance between adjacent nanorods, including the side-to-side and end-to-end distance, can be turned by changing the concentration of the AuNRs in the PVA solutions. Moreover, Tracy and co-workers used electrospinning poly(ethylene oxide) (PEO) fiber films for macroscale fabrication of nanorods (GNRs) with long-range order.183 Thus, nanowires might be assembled with good orientation, which is a kind of forecast of what will happen in the future. To create aligned or patterned bundles of electrospinning fibers, electrodes either in parallel (arranged at a particular angle with a gap) or rotation collector have been used. When O


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Figure 15. (a) AFM topography (top) and phase (bottom) images for a P3HT/PS (5:95) blend spincast from a CH2Cl2 solution: top surface (left), bottom surface (middle), and interface after selectively dissolving PS (right). The scale bar is 500 nm. (b) Schematic representation of the formation of a nanofibrillar network in the PS matrix. Reproduced with permission from ref 218. Copyright 2009 Wiley.

nanowire research along various directions.3,5,14,47,202,203 Several nanowire film-formation methods for the fabrication of membranes with random nanowire placement have been reviewed here, to which readers can refer.

electrodes, researchers have used high-speed rollers as the collector for anisotropic electrospinning. Aligned nanofibrous scaffolds could be fabricated by the electrospinning process using a rotating disk collector (200 mm in diameter) with a sharp edge (Figure 13e and f).192 Figure 13e shows the setup for fabrication of aligned nanofibrous scaffold by the electrospinning with a rotating disk collector. A linear rate of the rotation disk at the edge was set at 11 m/s. Optical micrographs of electrospinning nanofibers were shown in Figure 13f, indicating that the three-dimensional fibrous mesh consists of fibers with diameters ranging from 200 to 800 nm, and the thickness of the nanowire film was ca. 0.5 mm.192 The majority of the fibers were oriented along the longitudinal axis, which forms a unique aligned topography. Combining the two kinds of collector-modified methods, i.e., rotating collector and parallel electrodes, Gu et al. reported an anisotropic electrospinning process.184 Figure 13g and h shows the modified setup for the electrospinning process and an SEM image of the welldefined fibers. In most cases, nanotubes are just similar with nanowires and nanowire-assembly strategies are identical to those previously developed and currently used for carbon nanotubes. To be honest, the boundary of different nanowire assembly methods is blurry and many assembly strategies are the combination of more than two definitions, such as LB technique is a combination of mechanical force and interfacial effect. In most cases, we make the classification by methods. In some cases, contact printing, knocking-down method, and strainrelease methods are mainly force-related assembly strategies, so we make the definition mechanical-force method.

3.1. Nanowire Films by Layer-by-Layer Assembly

Layer-by-layer (LBL) assembly is a simple, versatile, and very inexpensive approach, based on the sequential adsorption of polyanions and polycations on the substrate, by which nanocomponents of different groups can be combined to coat both macroscopically flat and nonplanar (e.g., colloidal core− shell particles) surfaces.204−212 LBL can be used to combine a wide variety of species including nanoparticles (NPs), nanosheets, and nanowires (NWs) with polymers, thus merging the properties of each type of material. In general, for the LBL assembly process, the common positively charged polyelectrolytes used are poly(allylamine hydrochloride) (PAH), poly(diallyldimethylammonium chloride) (PDDA), and polyethyleneimine (PEI). Commonly used negatively charged polyelectrolytes are poly(acrylic acid) (PAA), poly(styrene sulfonate) (PSS), and poly(vinyl sulfate), which leads to the reversal of net charge on the substrate.204 Nanoparticles, nanosheets, and nanowires have been combined with the charged polymer. Then, the films are consequently rinsed with pure water; the aim of rinsing is the removal of loosely adsorbed polyelectrolytes. Figure 14a shows the scheme of the LBL-assembly process: steps 1 and 3 represent the adsorption of polyanion and polycation, steps 2 and 4 are washing steps, and steps 1−4 are repeated continuously until the desired numbers of “bilayers” are achieved.204 The group of Kotov fabricated Te nanowire films via the LBL technique by dipping the substrate alternatively into negatively charged NWs and positively charged polyelectrolyte, poly(diallyldimethylammonium chloride) shown in Figure 14b and c.204 SEM images show the Te NWs homogeneously distributed over the substrate’s surface (Figure 14b). Successful film formation was evidenced by a linear increase of the ultraviolet−visible spectroscopy (UV−vis) absorbance with an increasing number of deposition cycles (Figure 14c).

3. THIN FILMS COMPOSED OF DISORDERED NANOWIRES Nanowire films with random placement of wires have attracted explosive attention in nanoscience owing to their unique structures, interesting physical properties, and potential for novel applications. There are now many comprehensive review articles on the topic where readers can get a complete picture of P


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Figure 16. (a) Schematic illustrations showing the filtration process for preparing free-standing nanofibrous films. Reprinted with permission from ref 234. Copyright 2007 American Chemical Society. (b) AFM image of a SWNT film (color scale: black to bright yellow, 30 nm). Reprinted with permission from ref 241. Copyright 2004 Science. (c and d) SEM images of the double-walled carbon nanotube (DWNT) and cellulose nanofiber papers, respectively. Reprinted with permission from refs 219 and 236, respectively. Copyright 2005 Nature and 2009 Wiley. (e) SEM image of cadmium hydroxide−Au composite nanofiber ultrathin film. Reprinted with permission from ref 234. Copyright 2007 American Chemical Society. The insets in c−e show the photographs of corresponding film or paper materials.

3.2. Nanowire Films by Spin-Coating

and chemical engineering in modern industry. Solvent evaporation is a physical process in which the solvent molecules in a solution have enough heat energy to escape from the liquid, resulting in only solute molecules left. Vacuum filtration219,220 and solvent evaporation34,221−233 as traditional and efficient techniques for film formation can be used for forming nanowire films. The filtration method for nanowire film formation is similar to traditional paper manufacturing process, where cellulose microfibers are used. In a typical process,234 nanowires are first dispersed in a suitable solvent by vigorous magnetic stirring or ultrasonic treatment to form a homogeneous solution, which is then filtered on a microporous membrane filter at a pressure difference of 20−90 kPa. The filtration can generally be finished within several minutes. After that, the nanowire films are peeled off from the membrane filter directly or with the assistance of another solvent to generate free-standing structures (Figure 16a). Compared with other methods, the filtration strategy possesses several obvious advantages for assembling nanowires into macroscopic films. The film size only depends on the diameter of filtration funnels; thus, it is easy to prepare largescale films by choosing suitable funnels. The thickness of the nanowire film can be precisely controlled from tens of nanometers to hundreds of micrometers by adjusting the concentration and volume of nanowire dispersion for filtration. Furthermore, during the filtration process, the nanowires gradually deposit on the substrate and overlap and inter-

Spin-coating is a procedure that has been widely used in microfabrication to apply uniform thin films to flat substrates. A small amount of a solution, usually consisting of a volatile solvent, which simultaneously evaporates, is placed on the substrate, which is then rotated at high speed to spread the fluid by centrifugal force, and rotation is continued until the desired thickness of the film is achieved. So, the higher the angular speed of spinning and the lower the concentration of the solution and the solvent, the thinner is the film.214−216 Nanowire film can be fabricated by spin-coating.217,218 Cho and co-workers fabricated thin films on a silicon substrate with different poly(3-hexylthiophene) (P3HT) and polystyrene (PS) ratios, using spin-casting blend solutions with a total concentration of 0.5 vol %. AFM images of the top surface, the bottom surface, and the interface (after PS etching) of P3HT/ PS (10:90) clearly confirm this structure (Figure 15a).218 Figure 15b shows a schematic representation of the formation of embedded P3HT nanofibers in a PS matrix during the fabrication process. 3.3. Nanowire Films by Vacuum Filtration or Solvent Evaporation

Vacuum filtration is commonly used for the separation of solids from liquids flowing from the high-pressure side to the lowpressure side of the filter, leaving the solids behind. Vacuum filtration as a physical operation is very important in chemistry Q


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Figure 17. (a) Optical images of the flexible CNF membrane; the inset shows the optical image of the CNF solution used for casting the membrane. (b) Low and high (inset) magnification SEM images showing the surface morphology of the CNF membrane. Reproduced with permission from ref 245. Copyright 2010 Wiley. (c and d) SEM and TEM images of the free-standing Pt NW membrane, respectively. The inset in c shows the optical image of the Pt membrane. Reproduced with permission from ref 246. Copyright 2011 Wiley.

flexible polyester (PE) substrates using the dry-transfer approach, which could be used as transparent conductive electrodes for organic light-emitting diodes (OLEDs), organic photovoltaic devices, or other optoelectronic devices. Also using the filtration method, Endo et al. fabricated a paper-like material consisting of hexagonally packed bundles of clean, double-walled carbon nanotubes (DWNTs), which were prepared in high yields using chemical vapor deposition with a conditioning catalyst and two-step purification. This material was called “buckypaper”, and it was very flexible and tough enough to fold into an origami plane (Figure 16c).219 Recently, native cellulose nanofiber has become an attractive material for generating macroscopic functional materials, such as aerogels and membranes, because of its high aspect ratio, good mechanical properties, and biological and sustainable origin.243 Nogi and co-workers fabricated optically transparent paper-like materials from cellulose nanofibers using the filtration and subsequent polishing processes.236 In the first step, cellulose nanofibers with a uniform diameter of 15 nm were obtained from wood flour by chemical purification and physical grinding. After slow filtration of the nanofiber suspension on a hydrophilic polytetrafluoroethylene membrane filter, a wet sheet with a moisture content of 560 wt % was formed. Further drying, pressing, and polishing treatment resulted in a cellulose nanofiber transparent paper (Figure 16d). In contrast to conventional opaque cellulose paper, which consists of micrometer fibers, the nanofiber cellulose paper is highly transparent because the interstices between the nanofibers of this paper are small enough to avoid light scattering. Besides the above carbon nanotubes, some inorganic nanostructures can also be employed as 1-D structural units

penetrate with each other, resulting in mechanically stable and flexible macroscopic film materials. The main limitation of this method is its requirement of nanofibrous materials possessing very high aspect ratio. Thus, only a few 1D nanomaterials have been assembled into films using the vacuum filtration method up to now.219,235−240 In 2004, Rinzler et al. described a simple filtration process for the fabrication of ultrathin, transparent, and electrically conducting films consisting of pure SWNTs.241 The purified SWNTs were first dispersed in 1 wt % Triton X-100 solution by bath ultrasonication. Subsequently, the surfactant-based suspension of SWNTs was vacuum-filtered onto a mixed cellulose ester (MCE) filter membrane. After washing away the surfactant with purified water and dissolving the MCE filter membrane, the ultrathin SWNT film could be released into a suitable solvent or transferred to other desired substrates by multiple techniques. An AFM image of the SWNT film confirmed the nanofibrous network porous structure (Figure 16b). The films were highly transparent and could be fabricated as large as 10 cm in diameter (the inset in Figure 16b). For demonstrating the application potentials in optoelectronic device, the film has been used to construct an electric fieldactivated optical modulator, which constitutes an optical analogue of the nanotube-based field-effect transistor. In another similar work, Zhou and co-workers reported a transparent, conductive, and flexible carbon nanotube film using commercial SWNTs as starting materials.242 The sodium dodecyl sulfate (SDS)-stabilized SWNT suspension was first filtered through a porous alumina filtration membrane, forming a homogeneous gray layer. The supported SWNT films were then transferred from the alumina membrane to glass and R


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Figure 18. (a and b) SEM images of the two-block NW logic tile. Scale bar, 1 μm. (c) Circuit design implementing a one-bit full adder. /A, /B, and/ C denote the complementary inputs of A, B, and C, respectively. The left- and right-hand dashed boxes outline block 1 and block 2, respectively. (d) Voltage-transfer function for S (red) and Cout (blue) from input states (0, 0, 0) to (1, 1, 1).The dashed tangent lines show the maximal voltage gains of the outputs. Reproduced with permission from ref 47. Copyright 2011 Nature. (e−h) ordered nanowire pattern used for field-effect transistors (FETs) and gas sensors. (e) Scanning electron micrographs of the superlattice nanowire pattern (SNAP) nanowires on plastic. (f) Electrical characterization of nanowire thin-film transistors (TFTs) on plastic. (g) SEM image of an array of nanowire sensors. Inset: Digital photograph of the flexible sensor chip. Electrical response of a nanowire sensor to 20 ppm (red curve), 2 ppm (blue curve), 200 ppb (green curve), and 20 ppb (black curve) NO2 diluted in N2. Inset: An extended response of the sensor to 20 ppb NO2; the gas is introduced after 20 min of flowing N2. Reproduced with permission from ref 109. Copyright 2007 Nature.

17b). The fabricated CNFs membranes were very flexible and mechanically robust enough for filtration under a high applied pressure without any damage.245 On the basis of the feature that the CNFs could self-assemble into free-standing membranes, we designed a smart multistep templating route to prepare free-standing Pt nanowire membranes, which could be used as an electrocatalyst for the oxygen reduction reaction (ORR).246 Te@C nanocables were first synthesized through the HTC process using Te nanowires as templates.247 Then Pt@C nanocables were fabricated by the second templating process, in which the galvanic replacement reaction between Te and PtCl62− occurred.248 Similar to the formation of CNF membranes, Pt@C nanocables could also be assembled into free-standing membranes by the solventevaporation process. Finally, after calcination of the Pt@C membrane at 400 °C in air for 1 h, the free-standing Pt NW membrane (the inset in Figure 17c) was obtained. The SEM and TEM images show that the Pt membrane is composed of very fine wirelike nanostructures, which interconnect with each other to form a highly porous nanowire network structure (Figure 17c and d). This unique porous network structure facilitates electron transport and gas diffusion on the Pt NW membrane electrode, which is important for electrocatalysis applications.246

for constructing macroscopic film materials via the filtration method.235,237,239,244 For example, Ichinose and co-workers reported a simple and general route for the preparation of freestanding ultrathin films of nanofibrous composite materials (Figure 16e).234 This work started the positively charged cadmium hydroxide nanostrands of only 1.9 nm in diameter and micrometers in length, which were prepared by mixing dilute aqueous solutions of cadmium chloride and aminoethanol. Then negatively charged nanoparticles, proteins, and dye molecules were adsorbed onto the surface of the nanostrands by the electrostatic interaction to form composite nanostructures. The well-dispersed solution of the nanofibrous composites was filtered with a polycarbonate membrane filter. Uniform films with a thickness of tens to hundreds of nanometers were yielded on the filter. The nanofibrous ultrathin films could be readily peeled off from the filter by immersion in ethanol. The authors also showed the optical, biological, metallic, and magnetic properties of these freestanding films.234 The solvent-evaporation-induced self-assembly process is another simple and efficient route for assembling 1D nanostructures with high aspect ratio into free-standing film materials.34,221 Recently, our group fabricated a series of freestanding nanofibrous membranes by the solvent-evaporation method.245 All these membranes were based on highly uniform carbonaceous nanofibers (CNFs), which were synthesized by a template-directed hydrothermal carbonization (HTC) process.17 The as-prepared CNFs were dispersed in ethanol with vigorous magnetic stirring to form a brown wool-like homogeneous suspension (the inset in Figure 17a). After casting the suspension onto a Teflon substrate and drying at ambient temperature slowly, a brown paper-like material was obtained, which could be easily detached from the substrate without cracking (Figure 17a). The SEM observation revealed that the membranes consisted of abundant randomly oriented nanofibers that were very flexible and intertwisted with each other to form a network structure with high porosity (Figure

4. APPLICATIONS OF NANOWIRE THIN FILMS Evidence of the emerging application of nanowire thin films, either in the form of ordered or disordered states includes nanodevice fabrication, nanowire membranes for fuel cells, and nanowire thin films for separation and environmental application. 4.1. Nanowire Thin Films for Nanodevice Fabrication

When the diameter of the nanowire structures is close to or below the characteristic length scale of various solid-state phenomena, such as the exciton Bohr radius, the wavelength of light, the phonon mean free path, the critical size of magnetic domains, or the exciton diffusion length, many physical S


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Figure 19. Photoelectric properties of the Te nanowire nanodevices. (a−c) I−V curves and reversible switching photoconducting properties of a 20layer Te nanowire device. (a) I−V curves measured in the dark. (b) I−V curves measured under different light intensities: 0, 0.37, 3.04, 5.76 mW/ cm2, respectively. (b, inset) Linear characteristics between photocurrent and the power of illumination (0, 0.37, 3.04, 5.76 mW/cm2) at bias 1 V. (c) Reversible switching of a 20-layer Te nanowire device between low- and high-conductivity states when the light was turned on and off. The bias on the nanowire films is 1 V. (e and f) Reversible switching photoconducting properties of the Te nanowire films, which are composed of monolayers, three layers and five layers, respectively. All the Te nanowire films were assembled without tuning an angle, and all the nanowires have the same axial direction. Reproduced with permission from ref 58. Copyright 2010 American Chemical Society.

bly and the scaling of charge-trapping devices indicate that an area 103-fold smaller, ∼0.0017 mm2, is achievable. The twoblock programmable non-volatile nanowire transistor arrays (PNNTA) tile was initially programmed to function as a full adder, an important combinational circuit in the arithmetic logic unit in modern digital computers. Configuration of the one-bit full-adder logic circuit comprising two blocks with the output of block 1 (Figure 18c, left-hand box) fed into block 2.47 Duan et al. fabricated high-speed graphene transistors with a self-aligned nanowire gate to provide unprecedented transistor performance.30 First, the physical assembly of the nanowire gate preserves the high carrier mobility in graphene. In addition, the self-alignment process ensures that the edges of the source, drain, and gate electrodes are automatically and precisely positioned so that no overlapping or significant gaps exist between these electrodes, thus minimizing access resistance. Myoung et al. reported a programmable nanowire integration route for fabricating field-effect Si NW transistors that have uniform transfer characteristics on intentionally organized gate sites.157 On the basis of the number of aligned nanowires as the channel width of transistor, three groups of PDMS blocks that include nanowire bridges between electrodes can be classified. With programmable decaling on the predesigned gate sites by drawing the letter N, nine nanowire bridges in each group were individually converted into bottomgate field-effect Si NW transistors having uniform transfer characteristics with ∼80% fabrication yields. Heath and co-workers used highly ordered nanowire arrays on plastic substrates for FET and ultrasensitive flexible chemical sensor application.109 First, ordered arrays of doped silicon nanowires can be comprehensively transferred to flexible plastic substrates (section 2.1.4). Then, FETs fabricated from the transferred nanowires using standard microprocessing techniques yield large on/off ratios and low-power operation. On the basis of the array of nanowires, sensor arrays on bendable plastic substrate exhibit high sensitivities.

properties of semiconductors are significantly altered within the confines of the nanowire surfaces.249 Developing organized and interconnected well-defined arrays of functional nanowire building blocks is central to the efforts directed toward the ultimate applications of assembled nanowire systems. The components, sophisticated multiscale assembled nanowire structures, and their applications have been reviewed.53 There are many potential applications that would benefit from the directed assembly of nanowires. In the following section, we will describe a series of applications of integrated NW structures, such as electric applications, photon applications, and photoconductive applications. A broad range of functional devices and integrated nanoelectronic systems can be fabricated based on well-defined NW array structures. NW-based memory and logic devices can be fabricated during the synthesis of NW building blocks and their subsequent assembly.47−49,250−255 To exploit the unique properties, both the locations and the interconnections of nanowires are decided after fabrication. The Lieber group studied the crossed NWs and metal 1D and 2D NW arrays device composed of Si/a-Si NWs crossing six Ag NWs.255 Recently, their group described the design, fabrication, and use of programmable and scalable logic tiles for nanoprocessors that surmount these hurdles (Figure 18).47 First, a parallel array of Ge/Si nanowires was assembled by the method of contact printing, which was discussed in section 2.1.4; electron beam lithography (EBL) was used to fabricate the source and drain electrodes, atomic layer deposition was used to deposit the Al2O3−ZrO2−Al2O3 charge trapping structure, and then a second step of EBL was used to define input gate lines. A total of 496 programmable NW devices were laid out in two separate arrays with a total area of ∼960 μm2, where each device node consists of a single nanowire crossed by a gate line.47 The average area per node, ∼1.9 mm2, is relatively large in these proof-of-concept studies but does not represent a lower limit, as previous studies demonstrating close-packed nanowire assemT


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Photoconductivity is an optical and electrical phenomenon in which a material usually becomes more electrically conductive due to the absorption of electromagnetic radiation such as visible light, ultraviolet light, infrared light, or γ-radiation.256−258 A type of photoconductive nanodevice composed of 20-layer parallel Te nanowire monolayer assemblies on a Si/SiO2 (SiO2 layer with a thickness of 500 nm) substrate can be fabricated by LB techniques, as reported by our group.58 Photoelectric properties of well-defined Te nanowire nanodevice were investigated, and the current−voltage (I−V) curves, measured in the dark and under white light illumination of different light intensities, are shown in Figure 19a and b. Figure 19c shows a reversible switching of a Te nanodevice between low- and highconductivity states when the lamp was turned on and off. The properties of the photoconductive Te nanowires suggest that they are candidates for optoelectronic switches, with the dark insulating state as “OFF” and the light exposed conducting state as “ON”. In Figure 19d−f, we can see that even a monolayer device gives an obvious response to the white light.58 The I−V curves exhibit that the device was insulating in the dark and the electrical resistance decreased when the light intensity was increased from dark. Moreover, the current of the device increased linearly with increasing illumination power (inset of Figure 19b, at bias 1 V), indicating the extremely high sensitivity of the devices to light.58 Compared with the work by Kotov and Wang,213 the resistance of this nanowire device is 3 orders of magnitude smaller, though the diameter of the nanowires is smaller and the length is much longer. The reason for this is that the nanowires here are parallel arrayed and the film device contains no polyelectrolytes or polymer as used previously.25 Thus, it should be helpful to align functional nanowires for measuring their electrical and optical properties. When the number of layers is increased, from monolayer to five layers, the current of the device increased.58 Ag nanowires, because of their high electrical conductivity, have potential uses in many electronic devices.259−263 Novel optical properties can be generated by well-aligned Ag nanowires because of the large electromagnetic (EM) fields that were localized in the interstitial areas of adjacent nanowires, which have great application in the field of ultrasensitive, molecular specific sensing. Pendry and coworkers reported that metallic microstructure comprising a regular array of thin wires exhibits novel electromagnetic properties in the GHz region, analogous to those exhibited by a solid metal in the UV. Incident radiation can also excite surface plasmons trapped in the interstitials between the cylinders, which can generate EM coupling between the aligned silver half-cylinders.264 The free-standing ordered Ag nanowire thin film obtained by our group100 showed that the dependence of the UV−vis spectra on the polarization angle of the incident light demonstrates the existence of large EM fields in the films (Figure 20). The polarization angle of the incident light, i.e., the angle between the polarized electric field and the long axes of the nanowires, was recorded by rotating a half-wave plate in the laser path. When the polarization angle increased from 0° to 90°, the transmission intensity increased. Broadening in the 700−900 nm range was attributed to the coupling of electromagnetic waves among adjacent nanowires.100 Importantly, because of the existence of the large EM fields, a film of well-aligned Ag nanowires could serve as an excellent SERS substrate for molecular sensing with high sensitivity and selectivity. Recently, our group reported the capability of wellaligned Ag nanowire films for use in the detection of

Figure 20. (a) Schematic illustration of the polarization angle θ, which is defined as the angle between the polarization direction and the long nanowire axis. (b) UV−vis transmission spectra of the film of silver nanowires. All of the spectra were obtained at normal incidence with polarization angles (θ) in the range of 0°−90°. (c) UV−vis transmission spectra of the film made by the randomly dispersed Ag nanowires. The spectra are obtained at polarization angles (θ) of 0° (cycle) and 90° (square), respectively. Reproduced with permission from ref 100. Copyright 2010 Wiley.

Rhodamine 6G (R6G) molecules using spontaneous Raman instruments (Figure 21a).100 Figure 21b shows the SERS spectra of the R6G molecules (25 μL, 1 × 10−7 M) on the Ag nanowire films. When the polarization angle between the polarization direction and the long nanowire axis increased from 0° to 90°, the intensities of the corresponding SERS signals increased. To express this periodicity, we analyzed the typical Raman band (1362 cm−1) and plotted a diagram of the SERS intensities and the polarization angle. The solid line in Figure 21c is the best fit to a cos2 θ function.100 The group of Yang used the well-defined silver nanowires for a systematic study of the SERS signal with respect to polarization and U


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Figure 21. (a) Schematic illustration showing the spontaneous Raman scattering instruments used for the 180° back-scattered SERS experiments. (b) SERS spectra of adsorbed Rhodamine 6G (25 μL, 1 × 10−7 M) on a film taken at different polarization directions of incident light. (c) The line represents the best fit to a periodic cosine function; the square dots represent the relationship between the polarization angle and the SERS intensities of the peak at 1362 cm−1. Reproduced with permission from ref 100. Copyright 2010 Wiley. (d−f) Ordered Ag nanowire monolayers SERS spectra of adsorbed 1-hexadecanethiol. (d) SERS spectra of adsorbed 1-hexadecanethiol on Ag nanowire monolayer taken at different polarization directions of incident light. (e) Polar plot of SERS intensities for various low-frequency Raman bands with respect to polarization angle. The dark lines represent the best fit to a periodic cosine function. (f) Schematic illustration of the electromagnetic field generated between two adjacent nanowires upon incident irradiation, shown in a cross-sectional view. The surfaces on opposing sides of the junction have opposite polarization charges, leading to the highly dipolar nature of this geometry. Reproduced with permission from ref 265. Copyright 2005 American Chemical Society.

Figure 22. (a) Low and high (the inset) magnification SEM images of Ag NW and NP coassemblies. (b) SERS spectra of 2 mM 2-mercaptobenzoic acid (2-MBA) molecules collected on a set of Ag NW−NC films with different NW/NC ratios. From sample 1 to sample 5, the NW/NC ratios are ca. 1:0, 6:1, 1:3, 1:6, and 0:1, respectively. The acquisition time is 10 s. Reproduced with permission from ref 101. Copyright 2012 Wiley.

structural ordering.265 Figure 21d−f shows that the observed dependence of the extinction spectra is the result of the dependence of the extinction on longitudinal plasmons within the monolayers. In contrast, for films composed of randomly distributed Ag nanowires, the transmission intensities showed no change as the polarization angle increased from 0° to 90°. Therefore, the existence of the large EM fields in the film demonstrates the high order of the film of the Ag nanowires. Binary coassembly of nanobuilding blocks with different morphologies has attracted increasing attention owing to their unique structures and potential properties.87,101 Co-assembly of Ag NWs and Ag NCs with the same surfactant PVP was performed at the interface as shown in Figure 22a. The SEM images show the binary nature of the assemblies consisting of Ag NWs and Ag NCs. Ag NWs array almost in the same direction, and Ag NCs disperse regularly among Ag NWs. The as-prepared coassembled Ag NW and NC films show high

SERS intensity, and the SERS performance is strongly dependent on the ratio of the two kinds of nanobuilding blocks.101 From sample 1 to sample 5, the number of Ag NWs in the NWs/NCs assemblies decreases, and sample 1 and sample 5 are the films of pure Ag NWs and pure Ag NCs. Their corresponding SERS spectra are as shown in Figure 22b. The best SERS sensitivity in comparison to the other four substrates is shown in sample 2, indicating that introducing a tiny amount of NCs to NW films leads to a better SERS performance than pure Ag NW and Ag NC films.63,265 When a small amount of Ag NCs are coassembled with NWs, the NWs keep the orientation with NCs arrayed loosely in the same direction of NWs, so that this coassembly creates hot spots like NW−NC and NC−NC interstitials that possess much higher SERS intensity than aligned NWs.101 V


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nanofibrous films for filtration application from the dispersion of cadmium hydroxide nanostrands and anionic surfactants.274 This ultrathin composite nanofibrous film supported on an alumina membrane was stable enough to be used for ultrafiltration at pressure difference of 90 kPa. In addition, the film could reject 40 nm Au nanoparticles with an extremely high filtration rate of 14 000 L/(h m2 bar).274 Later, the same group developed another ultrathin nanofiber film by employing ultrathin β-MnOOH nanofibers as units, which could be produced in a large scale using Mn(NO3)2 and aminoethanol at room temperature. The membranes can separate 10 nm nanoparticles from water at a flux of 15 120 L/(h m2 bar), which was 2−3 times higher than that of commercial membranes with similar rejection properties.275 For achieving high selectivity and maintaining high flux simultaneously, nanofibrous membranes with unsymmetrical layered structures have been developed. Ke and co-workers designed a novel high-flux filtration medium, consisting of a three-tier composite structure, using ceramic nanofibers as the key component.276 The substrates of the hierarchical filtration medium consisted of alumina microparticles, which were first overlaid on a uniform layer of titanate fibers with diameter of 40−100 nm by the spin-coating method. The pore size of the two-tier filtration medium is in the range of 100−200 nm, much lower than that of original alumina substrates (∼10 μm). Further reduction in pore size to tens of nanometers could be realized by the spin-coating of another layer using thinner boehmite nanofibers of only 2−5 nm in diameter. The final resulting membranes could effectively filter out particles >60 nm at flow rates orders of magnitude greater than with conventional membranes. Following this work, Zhang et al. developed a similar two-layer structure using ultrafine (10 nm in diameter) and large (20−100 nm in diameter) TiO2 nanofibers for forming the functional layer and supporting layer of the filtration media, respectively.220 The filtration tests revealed that the cutoff size of the TiO2 nanofibers membranes was ∼20 nm. Interestingly, the membrane possessed multifunctional capabilities under UV irradiation, such as antifouling, antibacterial, concurrent separation, and photocatalytic oxidation. Although filtration out of particles below 100 nm has been successfully realized with nanofiber membranes as described above, it is difficult to regulate the cutoff size of these membranes because of poor control of the diameter of nanofibers. These filters are not suitable for filtration and separation of different size nanoparticles efficiently. On the other hand, the filtrate flux dramatically depends on the pore size of the filter; thus, membranes with various precise pore sizes are required for performing different practical filtration operation. Recently, Yu and co-workers developed a novel nanofibrous filtration media consisting of very uniform carbonaceous nanofibers (CNFs).245 Importantly, these CNF membranes had very narrow pore-size distributions and showed excellent size-selective rejection properties. A family of CNF membranes with varied, controllable, and precise cutoff sizes could be obtained by carefully regulating the diameters of the CNFs. For example, the membrane consisting of CNFs with a diameter of 50 nm could reject 25 nm Au particles and allowed 5 nm particles to pass through freely (Figure 24a). The membranes consisting of CNFs with larger diameters possessed larger cutoff sizes and also showed similar size-selective rejection properties. Large porosity and hydrophilic properties of the

4.2. Nanowire Membrane for Fuel Cells

The electrocatalyst durability of the oxygen reduction reaction (ORR) at the cathode of a proton-exchange membrane fuel cell (PEMFC) has been recognized as one of the most important issues that must be addressed before the commercialization of PEMFCs.266 One of the strategies for improving the durability of Pt-based ORR catalysts is the use of 1D Pt-nanostructural catalysts owing to their inherent structural characteristics.78,267 Recently, Yu and co-workers reported a new type of electrocatalyst, i.e., free-standing Pt nanowire (Pt NW) membrane, which was fabricated via a smart multistep templating process.268 This membrane catalyst system consists of long crystalline Pt nanowires that can improve catalytic activity owing to the preferential exposure of certain crystal facets and few surface defect sites in 1D nanostructures. In addition, the unique nanofibrous network structure facilitates the electron transport and gas diffusion on the Pt NW electrode. Therefore, the Pt NW membrane has 2.1 and 1.8 times higher specific activity than that of commercial Pt/C and Pt black for ORR, respectively (Figure 23a). More importantly,

Figure 23. (a) ORR curves of the Pt/C, Pt black, and PtNW membrane catalysts in O2-saturated 0.5 M H2SO4 aqueous solution at room temperature (sweep rate = 20 mV/s). The inset shows the specific activity for the three catalysts at 0.85 V versus RHE. (b) Loss of ECSA of Pt/C, Pt black, and PtNW membrane with the number of CV cycles. Reproduced with permission from ref 268. Copyright 2011 Wiley.

compared with the commercial catalysts, this unique freestanding Pt nanostructural catalyst exhibits remarkably high stability. After 3000 cyclic voltammetry cycles, the electrochemical surface areas (ECSAs) of Pt NW membrane only decreased by 18%, much lower than that of Pt/C (95%) and Pt black (61%) catalysts (Figure 23b).268 4.3. Nanowire Thin Films for Separation and Environmental Application

Fibrous membrane materials have been used extensively in water treatment as prefilters or to support the medium that performs the separation.269 Compared with conventional granular filtration media, the fibrous membranes possess several unique advantages, such as very large porosity, interconnected open pore structure, good flexibility, and high permeability.269−272 The current fibrous filtering media are mostly based on nonwoven polymer mats obtained by electrospinning technique.273 However, the diameters of electrospun fibers were generally in the range of micrometers or submicrometers, resulting in a large cutoff size of several micrometers. Thus, the use of nonwoven filter media was limited to prefilters and was not used further downstream as high-performance filters. It is expected that by reducing the fiber size into the nanometer range, a highly efficient membrane filter with nanoscale pores (

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