Semiconductor nanowires (NWs) have been studied extensively for over two decades for their novel electronic, photonic, thermal, electrochemical and mechanical properties. This comprehensive review article summarizes major advances in the synthesis, characterization, and application of these materials in the past decade. Developments in the understanding of the fundamental principles of "bottom-up" growth mechanisms are presented, with an emphasis on rational control of the morphology, stoichiometry, and crystal structure of the materials. This is followed by a discussion of the application of nanowires in i) electronic, ii) sensor, iii) photonic, iv) thermoelectric, v) photovoltaic, vi) photoelectrochemical, vii) battery, viii) mechanical, and ix) biological applications. Throughout the discussion, a detailed explanation of the unique properties associated with the one-dimensional nanowire geometry will be presented, and the benefits of these properties for the various applications will be highlighted. The review concludes with a brief perspective on future research directions, and remaining barriers which must be overcome for the successful commercial application of these technologies.

/pdf/25th-anniversary-article-semiconductor-nanowires-synthesis-4ccslz11ke.pdf

25th Anniversary Article: Semiconductor Nanowires – Synthesis, Characterization, and Applications

1D metal-oxide nanostructures have attracted much attention because metal oxides are the most fascinating functional materials. The 1D morphologies can easily enhance the unique properties of the metal-oxide nanostructures, which make them suitable for a wide variety of applications, including gas sensors, electrochromic devices, light-emitting diodes, field emitters, supercapacitors, nanoelectronics, and nanogenerators. Therefore, much effort has been made to synthesize and characterize 1D metal-oxide nanostructures in the forms of nanorods, nanowires, nanotubes, nanobelts, etc. Various physical and chemical deposition techniques and growth mechanisms are exploited and developed to control the morphology, identical shape, uniform size, perfect crystalline structure, defects, and homogenous stoichiometry of the 1D metal-oxide nanostructures. Here a comprehensive review of recent developments in novel synthesis, exceptional characteristics, and prominent applications of one-dimensional nanostructures of tungsten oxides, molybdenum oxides, tantalum oxides, vanadium oxides, niobium oxides, titanium oxides, nickel oxides, zinc oxides, bismuth oxides, and tin oxides is provided.

One-Dimensional Metal-Oxide Nanostructures: Recent Developments in Synthesis, Characterization, and Applications

Harnessing the potential of single crystal inorganic nanowires for practical advanced nanoscale applications requires not only reproducible synthesis of highly regular one-dimensional (1D) nanowire arrays directly on device platforms but also elegant device integration which retains structural integrity of the nanowires while significantly reducing or eliminating complex critical processing steps. Here we demonstrate a unique, direct, and bottom-up integration of a semiconductor 1D nanowire, using zinc oxide (ZnO) as an example, to obtain a vertical surround-gate field-effect transistor (VSG-FET). The vertical device structure and bottom-up integration reduce process complexity, compared to conventional top-down approaches. More significantly, scaling of the vertical channel length is lithographically independent and decoupled from the device packing density. A bottom electrical contact to the nanowire is uniquely provided by a heavily doped underlying lattice-match substrate. Based on the nanowire-integrated platform, both n- and p-channel VSG-FETs are fabricated. The vertical device architecture has the potential for use in tera-level ultrahigh-density nanoscale memory and logic devices.

Single Crystal Nanowire Vertical Surround-Gate Field-Effect Transistor

Quasi-one dimensional metal oxide semiconductors: Preparation, characterization and application as chemical sensors

We report the fabrication of vertically aligned NiO nanowalls on nickel foils using a plasma assisted oxidation method. Electrochemical properties of as-synthesized NiO nanowalls were evaluated by galvanostatic cycling and cyclic voltammetery. The results show a capacity of ∼638 (mA h)/g (at 1.25C rate), with excellent capacity retention of up to 85 cycles, when cycled in the range, 0.005−3.0 V vs Li. The superior electrochemical performance of NiO nanowalls in comparison to the previously reported results on nanosized NiO particles can be attributed to its large surface area and shorter diffusion length for mass and charge transport. A possible reaction mechanism is discussed. We also report that electron field emission studies show that the verticllay aligned NiO nanowalls are efficient field emitters with a turn-on field of 7.4 V/µm and a maximum current density of ∼160 µA/cm2 can be achieved.

Fabrication of NiO Nanowall Electrodes for High Performance Lithium Ion Battery

Interfacing semiconducting nanostructures with conducting or insulating substrates to attain a three-dimensional (3D) integrated platform is highly desirable for advanced nanoscale electronics and optoelectronics applications ( [1][1] ). As such, the assembly and synthesis of these nanostructures, which demonstrate multiple dimensionality, using a bottom-up approach would be useful. Taking ZnO as an example, we show here that high crystallinity 1D and 2D nanostructures can be epitaxially and vertically grown into 3D architectures on both conducting and insulating single crystalline substrates.

Our objective was to grow vertically well-aligned 2D and 1D semiconducting nanostructures for nanoelectronics and nanosensor applications. Using a carbothermal reduction process and gold (Au)-catalyzed epitaxial growth ( [2][2] – [4][3] ), we have been able to reproducibly grow hierachically well-ordered and vertically aligned 2D and 1D ZnO nanostructures on electrically conducting, highly oriented pyrolytic graphite (HOPG) and on insulating (1120) sapphire substrates. We achieve these by specifically controlling either the spacing between the catalyst spots or the thickness of the catalyst thin film.

When the Au film thickness is maintained above the threshold limit of 15 A, we obtained highly intricate quasi-3D nanostructures ([Fig. 1A][4] and inset), which are composed of an array of vertical 1D nanowires on top of a 2D network of intricate “nanowalls.” The scanning electron microscope (SEM) (top view) further reveals a random honeycomb-like pattern, resembling typical structures observed from anodization of thin-film aluminum. The nanowires are observed to grow from the “nodes” of the nanowalls. The average diameters of the nanowires and the thickness of the nanowalls are of the same order of magnitude (∼80 nm). The hexagonal facets (0001) of the nanowires ( [4][3] ) are clearly visible in the SEM images. This suggests that epitaxial growth of the nanowires occurs at the nodes of the nanowalls, which are themselves assembled epitaxially on the substrates and presumably also have their c -plane parallel to the substrates. The nanowalls have essentially vertical side wall profiles. By controlling the growth time, 2D nanowalls ([Fig. 1B][4]) could be obtained before onset of the 1D nanowires. When the catalyst thickness is kept below the threshold limit of 15 A, discrete individual free-standing 1D nanowires are obtained ([Fig. 1C][4]).

![ Fig. 1. ][5] Fig. 1. 
Zinc oxide nanowalls and nanowires. ( A ) SEM image of quasi-3D ZnO nanostructures grown on a sapphire using ∼40 to 50 A Au thin film as the catalyst. The inset shows a SEM perspective view. ( B ) 2D ZnO nanowalls on a sapphire with a height ∼5 μm. ( C ) An array of free-standing 1D ZnO nanowires on a HOPG substrate using ∼15 A Au ultrathin film as the catalyst. ( D ) Schematic illustration showing the growth mechanism of ZnO nanowalls and nanowires. The first four sketches are top views; the last, a perspective view.



Vertical growth of the ZnO nanostructures are observed as a result of the good epitaxial lattice match of the c plane of ZnO with the hexagonal basal plane of HOPG and (\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(11{\bar{2}}0\) \end{document}) plane of sapphire. Unlike the governing mechanism in the growth of carbon nanowalls ( [5][6] ), the observed growth phenomena appear to be a result of the interplay between the dynamic wetting behavior and thermal-enhanced surface diffusion of gold adatoms on the substrates. The limited wettability between the substrate and Au film facilitates the formation of Au network patterns at an elevated temperature, in contrast to totally isolated droplets that form due to a total incompatibility or a continuous film that forms due to a complete wetting.

Control experiments with different growth durations but without ZnO and carbon feed-stock have been performed and examined under an atomic force microscope (AFM). Consistency in the AFM granulometry results has suggested aggregation of the Au nanograins forming larger particulates with narrow grain boundaries within complex channeled Au networks that resemble the 2D nanowall patterns, as depicted in [Fig. 1D][4]. We believe that, with improved wetting to the substrate, ZnO epitaxial growth is initiated along the grain boundaries because these are the most thermodynamically active sites for saturation and precipitation of the Zn atoms (green arrows). As the ZnO nanowalls continue to grow via vapor-liquid-solid (VLS) mechanism, the surface energy at the nodes increases with resultant Au atoms diffusing and accumulating at the nodes (red arrows) for overall energy compensation. At a critical saturation point, 1D nanowires begin to grow from the nodes via the same mechanism. In the 1D individual nanowire growth, the Au nanoparticles are typically well-separated and the surface diffusion length may not be extended enough to allow a continuous network formation.

Though it is well known that nanowires are useful in many electronics and optoelectronics applications, the nanowalls and high surface-to-volume ratio nanostructures with multiple dimensionality may be useful in applications in energy storage or conversion and data storage and memory devices.

Supporting Online Material 

[www.sciencemag.org/cgi/content/full/300/5623/1249/DC1][7] 

Materials and Methods

Fig. S1

1. [↵][8] International Technology Roadmap for Semiconductors 2001 (Semiconductor Industry Association, San Jose, CA, 2001); available at 

2. [↵][9] H. T. Ng et al. , Appl. Phys. Lett. 82, 2023 (2003).
 [OpenUrl][10][CrossRef][11]

3. Materials and methods are available as supporting material on Science Online.
 

4. [↵][12] M. Huang et al ., Science 292, 1897 (2001).
 [OpenUrl][13][Abstract/FREE Full Text][14]

5. [↵][15] Y. Wu, P. Qiao, T. Chong, Z. Shen, Adv. Mater. 14, 64 (2002).
 [OpenUrl][16]

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https://science.sciencemag.org/content/sci/300/5623/1249.full.pdf

Growth of Epitaxial Nanowires at the Junctions of Nanowalls

We demonstrate seamless direct integration of a semiconductor nanowire grown using a bottom-up approach to obtain a vertical field-effect transistor (VFET). We first synthesize single crystalline semiconductor indium oxide (In2O3) nanowires projecting vertically and uniformly on a nonconducting optical sapphire substrate. Direct electrical contact to the nanowires is uniquely provided by a self-assembled underlying In2O3 buffer layer formed in-situ during the nanowire growth. A controlled time-resolved growth study reveals dynamic simultaneous nucleation and epitaxial growth events, driven by two competitive growth mechanisms. Based on the nanowire-integrated platform, a depletion mode n-channel VFET with an In2O3 nanowire constituting the active channel is fabricated. Our unique vertical device architecture could potentially lead to tera-level ultrahigh-density nanoscale electronic, and optoelectronic devices.

/pdf/direct-integration-of-metal-oxide-nanowire-in-vertical-field-gq84a23uwu.pdf

Michael K. Smith

Papers

Growth of Epitaxial Nanowires at the Junctions of Nanowalls

Direct integration of metal oxide nanowire in vertical field-effect transistor