One-Dimensional Nanostructures

In this chapter, we have examined several of our recent results on InAs nanowires that have implications to the vapor—liquid—solid (VLS) growth mechanism as well as the newly proposed Si-assisted growth mechanism. In summary, the study on the effect of oxygen during the nanowire growth showed the inhibiting effect of oxygen on the VLS growth mechanism. The results on the observation of size-dependent liquidus depression, more importantly, do not seem applicable on the results of Ti-catalyzed grown Si nanowires, but bring into question the validity of the vapor—solid—solid (VSS) growth mechanism in the Au-catalyzed grown GaAs and InAs nanowires. In the newly proposed nanowire growth mechanism, namely the Si-assisted mechanism using SiO _x , a growth model is proposed based on the phase separation of SiO at higher temperature, which forms a stable SiO₂ and reactive, nanometer-sized Si clusters. It is suggested that these clusters consequently serve as the nucleating/catalyst sites for the growth of InAs nanowires with the growth mechanism different from VLS, VSS, and OA.

SiC with unique properties, such as wide band gap, excellent thermal conductivity, chemical inertness, high electron mobility, and biocompatibility, promises well for applications in microelectronics and optoelectronics, as well as nanocomposites. The chapter reviews the recent progress on one-dimensional SiC nanostructures in both experimental and theoretical level, including synthesis methods and some properties (field emission, optical, electronic transport, mechanical, photocatalyst, and hydrogen storage) of SiC nanowires. Importantly, some novel results on SiC nanowires were elucidated clearly in our laboratory. Personal remarks end with some views on development and application of one-dimensional SiC nanostructures.

I summarize our results of the nanowire fabrication of various Si-based materials on substrate surfaces. By heating source materials in an evacuated silica tube, we have grown various types of wire-like one-dimensional structures such as Si nanochains, SiGe core/shell needles, and SiC fractal nanowires. Furthermore, Cu silicide nanochains and Fe silicide/Si heterostructured nanowires have also been fabricated using nanowire templates. We discuss their structure, formation mechanism, and properties based on transmission electron microscopy (TEM) observations and related analytical methods.

The formation of large-scale arrays of individually seeded, electrically addressable Si nanowires with controlled dimension, placement, and orientation is demonstrated. E-beam evaporated gold nanoparticles were used for nanowire synthesis by the vapor–liquid–solid growth mechanism. By controlling the lithography and metal deposition conditions, nanowire arrays with narrow size distributions have been achieved. Low-energy postgrowth ion-beam treatment has been utilized to control the orientation of Si nanowires. This process also leads to the attachment of nanowires on the substrate surface. Fabrication of planar devices with robust metal contact formation becomes feasible. Our method, combining bottom-up and top-down approaches, can enable efficient and economical integration of nanowires into device architectures for various applications.

Low-dimensional nanomaterials are a new class of advanced materials that have been receiving a lot of research interest in the last decade because of their superior physical and chemical properties. Nanowires (NWs) have been demonstrated to exhibit superior electrical [1], optical [2,3], mechanical [4], piezoelectric [5], and field emission [6] properties, and can be used as fundamental building blocks for nanoscale science and technology, ranging from chemical and biological sensors, field effect transistors to logic circuits.

Field effect transistors (FETs) based on NWs have been demonstrated as good candidates for ultrasensitive, miniaturized molecule sensors [7, 8]. Because of the high surface-to-volume ratio of these one dimensional nanostructures, their electronic characteristics may be sensitive enough to a very small amount of charge transfer such that single molecule detection becomes possible [7]. Nanosensors based on Si nanowires (SiNWs) are a promising candidate for label-free, direct, real-time electrical detection of the event of biomolecule binding, because of their several appealing properties including the following: (i) The electrical properties and sensitivity of SiNWs can be tuned by controlling NW diameter and the dopant type and concentration. (ii) The modification of silicon oxide surface, required for the preparation of interfaces selective for binding various analytes of interest, is well established.

The vapor–liquid–solid (VLS) growth mechanism, studied in detail in the 1960s and 1970s by Wagner et al. [9], is an ideal growth technique in the gas phase to produce NWs with high crystalline quality, required for sensing applications. Superior performance based on individual SiNW devices has been demonstrated [7, 10]. However, most of the existing studies based on nanostructures assembled using a bottom-up approach are limited by complex integration. Devices have been constructed around single, or several dispersed SiNWs. For practical applications, efficient and precise manipulation and placement of NWs at desired locations needs to be established to allow the integration with microfluidics and CMOS circuits. Methods that can enable reliable contact formation are essential in order for the intrinsic properties of the nanostructures to be realized.

We begin this chapter with an overview of prior methods that have been used for nanostructure assembly and device fabrication, including top-down (Sect. 4.1.1) and bottom-up (Sect. 4.1.2) approaches. The advantage and disadvantage of various approaches are discussed.

A pathway to fabricating large-scale arrays of individually seeded, electrically addressable SiNWs with controlled dimension, placement, and orientation is described (Sect. 4.2). SiNWs with controlled dimensions and specific placement were produced by the conventional chemical vapor deposition via the VLS process. By controlling the lithography and metal deposition conditions, NW arrays with narrow size distributions have been achieved (Sect. 4.2.2). Low-energy postgrowth ionbeam treatment has been utilized to control the orientation of SiNWs (Sect. 4.2.3). This process also leads to the attachment of NWs on the substrate surface. Fabrication of planar devices with robust metal contact formation becomes feasible. Electrical and structural characterizations were used to assess the quality of NWs after device fabrications (Sect. 4.2.4). Our method, which combines bottom-up and top-down approaches, can enable efficient and economical integration of NWs into device architectures for various applications.

Single-crystalline wurtzite GaN nanotubes have been synthesized recently with proposed applications in nanoscale electronics, optoelectronics, and the biochemical sensing field. In this work, molecular dynamics methods with a Stillinger-Weber potential have been used to investigate the melting behavior, thermal conductivity, and mechanical properties of wurtzite-type single-crystalline GaN nanotubes. (1) The simulations show that the melting temperature of the GaN nanotubes increases with the thickness of the nanotubes to a saturation value, which is close to the melting temperature of a bulk GaN. The results reveal that the nanotubes begin to melt at the surface, and then the melting rapidly extends to the interior of the nanotubes as the temperature increases. (2) The thermal conductivity of nanotubes is smaller than that of the bulk GaN single crystal. The thermal conductivity is also found to decrease with temperature and increase with increasing wall thickness of the nanotubes. The change of phonon spectrum and surface inelastic scattering may account for the reduction of thermal conductivity in the nanotubes, while thermal softening and high-frequency phonon interactions at high temperatures may provide an explanation for its decrease with increasing temperature. (3) The simulation results show that at low temperatures, the nanotubes show brittle properties, whereas at high temperatures, they behave as ductile materials. The brittle to ductile transition temperature generally increases with increasing thickness of the nanotubes and strain rate. (4) The simulation temperature, tube length, and strain rate all can affect the buckling behavior of GaN nanotubes. The critical stress decreases with the increase of simulation temperature and tube length. The tube length dependence of buckling is compared with those from the analysis of equivalent continuum structures using Euler buckling theory.

Semiconductor nanowires are novel nanostructures full of promise for optical applications. Nanowires have subwavelength diameters and large aspect ratios, which combined with the high permittivity of semiconductors lead to a strong optical anisotropy. We review in this chapter this optical anisotropy, focusing on the polarization anisotropy of the photoluminescence of individual nanowires and the propagation of light through birefringent ensembles of aligned nanowires.

Recent developments in bottom-up nanofabrication techniques allow the growth of free-standing semiconductor nanowires with controlled composition, lateral dimensions of typically 10–100 nm, and lengths of several micrometers (see Fig. 6.1). The small lateral dimensions of nanowires enables to grow them heteroepitaxially onto different substrates [1–3] or even to design heterostructures with segments, shells, and/or quantum dots of different semiconductors in a single nanowire [4–8]. Nanowires are full of promise for monolithic integration of high-performance semiconductors with new functionality [8–11] into existing silicon technology [2, 3, 12]. These nanostructures will offer new possibilities as next generation of optical and optoelectronical components. Junctions in semiconductor nanowires and light emitting devices have been demonstrated [4, 13–17]. Although the quantum efficiency of these nano-LEDs is still low, fast progress is being made on the passivation of the nanowire surface and the increase of their efficiency [18, 19]. Also, optically and electrically driven nanowire lasing have been reported [9, 20, 21]. Nanowires have been proposed as polarization sensitive photodetectors [22, 23] and as a source for single photons [8, 24].

The encouraging perspectives for novel applications has lead to improved control over nanowire synthesis and materials composition [4, 5, 25–27]. However, little is known about how light is emitted by individual nanowires or how light is scattered by ensembles of these nanostructures. The large geometrical anisotropy of nanowires and the high refractive index of semiconductors give rise to a huge optical anisotropy, which has been reported as a strongly polarized photoluminescence of individual nanowires along their long axis [22, 28]. In this chapter we review the polarization anisotropy in the photoluminescence of individual nanowires. We also describe the propagation of light through ensembles of nanowires oriented perpendicularly to the surface of a substrate. The controlled growth and alignment of the nanowires leads to a medium with giant birefringence [29], i.e., a medium with a large difference in refractive indexes for different polarizations. The giant birefringence in ensembles of nanowires can be easily tuned by changing the semiconductor filling fraction and is not restricted to narrow frequency bands as in periodic structures [30]. Broadband and giant birefringence constitutes an elegant example of the extreme optical anisotropy of nanowires, which may lead to nanoscale polarization controlling media [31], the efficient generation of nonlinear signals [32], and the observation of novel surface electromagnetic modes on birefringent materials [33].

We investigate the plasmonic properties of silver nanowires. By comparing our computations with previously published experimental results, we propose a way to correct tabulated permittivities to obtain a better description of the dispersive properties of this kind of structure.

Electromagnetic resonances of metal nanowires lead to strong enhancement of the near field of the particle. Antenna-like resonances that give the biggest enhancement are explained theoretically. The preparation of high-quality wires is introduced. Spectroscopic results for resonance curves are shown and discussed with respect to field enhancement. Surface-enhanced Raman scattering and surfaceenhanced infrared absorption are introduced focusing on nanowire-assisted configurations, and examples of these enhanced spectroscopies for molecules on resonant nanowires are shown.

Through recent publications, as reviewed in this article, we have determined the effects of contact barrier change on the electrical transport properties of carbon nanotube field-effect transistors. To analyze the Fermi level alignment and the Schottky barrier at the contact, we used the first-principles electronic structure calculations of different types of metal electrodes with various bonding configurations. In parallel, we have used various experimental techniques to engineer the contact barrier: decorations of metal nanoparticles, the self-assembled monolayers of molecules, and protein nanoparticles. We investigated the changes in the electron transport properties of the nanotube transistors in relation to the adjustment of the contact barrier. Overall reviews of these studies are presented here, and a few potential applications are also suggested.

Moore’s Law in microelectronic technology will break down as the size of individual bits approaches the dimension of atoms; this has been called the end of the silicon road map. For this reason and also for enhancing the multifunctionality of devices, the spin degree of freedom of electron is being investigated for magnetoelectronics applications, i.e., spintronics. Spin-based devices are closely connected with the development of nanotechnology. In this chapter, recent developments of the low-dimensional nanomaterials for spintronics are reviewed. In the first section, the main concepts of spintronics including nanospintronics are briefly discussed. Experimental studies on transition-metal-doped nanowires and nanotubes are summarized in the second section. Extensive theoretical works in this field are reviewed in the third section. Finally, an outlook is given in the last section.

The electrically operated phase-change random access memory (PRAM) features faster write/read, improved endurance, and much simpler fabrication as compared with the traditional transistor-based nonvolatile semiconductor memories. Low-dimensional phase-change materials in nanoscale dimensions offer advantages over their bulk or thin-film counterparts in several aspects such as reduced programmable volume and reduced thermal energies in phase transition. These features contribute to low-power operation, excellent scalability, and fast write/erase time. In this chapter, we present a general bottom-up synthesis approach and systematic material analysis study of one-dimensional chalcogenide-based phase-change materials including germanium telluride (GeTe), and indium selenide (In₂Se₃) nanowires that are targeted for nonvolatile resistive switching data storage. The phase-change nanowires have been synthesized via thermal evaporation method under vaporliquid—solid (VLS) mechanism. The morphology, composition, and crystal structure of the synthesized nanowires were investigated by scanning electron microscopy, energy dispersive X-ray spectroscopy, and high-resolution transmission electron microscopy. The as-synthesized nanowires are structurally uniform with single crystalline structures. The one-dimensional phase-change chalcogenide nanowires exhibit significantly reduced melting points, low activation energy, and excellent morphology, making them promising nanomaterials for data storage devices with very low energy consumption and excellent scalability.

InAs/InAl(Ga)As quantum wires (QWRs) have been grown on InP (001) substrates by molecular beam epitaxy (MBE) technology. A modified S-K growth mode has been presented for the formation of InAs QWRs on InAl(Ga)As/InP (001) substrate, in which the effect of lateral composition modulation in InAlAs buffer layers plays an important role. Vertical anticorrelation of InAs quantum wire superlattices has been observed and attributed to the interplay of strain field distribution and alloy phase separation in InAlAs matrix around InAs QWRs. The structural and optical properties of InAs/InAlAs QWR superlattices have also been discussed.