Applications of electron microscopy to the characterization of semiconductor nanowires

We review our current progress on semiconductor nanowires of β-Ga₂O₃, Si and GaN. These nanowires were grown using both vapor–solid (VS) and vapor–liquid–solid (VLS) mechanisms. Using transmission electron microscopy (TEM) we studied their morphological, compositional and structural characteristics. Here we survey the general morphologies, growth directions and a variety of defect structures found in our samples. We also outline a method to determine the nanowire growth direction using TEM, and present an overview of device fabrication and assembly methods developed using these nanowires. PACS 61.14.-x; 81.07.-b; 61.14.Lj; 81.05.-t     

Vapor–liquid–solid growth and narrow-band ultraviolet photoluminescence of well-aligned GeO2 nanowire arrays with controllable aspect ratios

GeO₂ nanowire arrays have been fabricated by thermal evaporation of Ge powder in air via a vapor–liquid–solid mechanism with Au as a catalyst. The GeO₂ nanowires are single crystalline with a hexagonal structure and have a controllable aspect ratio in the range of 23–167. The controllable synthesis of GeO₂ nanowire arrays with different aspect ratios was achieved by adjusting the heating temperature and time. Photoluminescence spectra of the GeO₂ nanowire arrays were measured at room temperature, and two ultraviolet emission peaks at 347 and 364 nm are observed for the first time, which indicates that the GeO₂ nanowire arrays may have potential applications in nanoscale photonic and electronic devices. Moreover, this vapor–liquid–solid growth process may be employed for synthesis of the highly oriented nanowire arrays of other oxides, and provides opportunities for both fundamental research and technological applications.     

Growth peculiarities during vapor–liquid–solid growth of silicon nanowhiskers by electron-beam evaporation

One-dimensional (1D) silicon (Si) nanostructures were grown by electron-beam evaporation catalyzed by gold nanoparticles on silicon substrates following the vapor–liquid–solid growth mechanism. We report three strikingly different growth morphologies of the 1D Si nanostructures and discuss their formation. The morphology of the silicon nanostructures strongly depends on gold layer thickness, annealing temperature before deposition and growth temperature during the deposition. The formation of nanoscale silicon features such as nanobelts, nanowires and nanowhiskers was observed. The nanoscale silicon features were characterized by transmission and scanning electron microscopy using imaging, diffraction and energy-dispersive X-ray spectroscopy, atomic force microscopy and UV micro-Raman spectroscopy. PACS 68.37.Lp; 68.70.+w; 78.30.-j; 81.15.Jj     

Vapor–liquid–solid growth and Sb doping of Ge nanowires from a liquid Au-Sb-Ge ternary alloy

Single-crystalline Sb-doped Ge nanowires (NWs) with excellent structural properties and uniform composition have been synthesized with high yield by vapor–liquid–solid (VLS) growth by low-temperature thermal evaporation from a mixture of Ge and Sb powders. During deposition, both the Ge and the Sb dopant became incorporated in the VLS seed nanoparticle. In situ annealing experiments during transmission electron microscopy establish that a liquid ternary Au-Sb-Ge alloy constitutes the active phase of the VLS seed drop at high temperatures, which governs the growth of the one-dimensional Ge NW and its doping by Sb.     

Competitive growth of In2O3 nanorods with rectangular cross sections

In₂O₃ nanorods with rectangular cross sections have been successfully synthesized using Au as a catalyst through chemical vapor deposition methods. The synthesized nanorods possessed larger size than that of the catalyst particle. The growth process was discussed through detailed theory analysis and experimental validation, and a competitive growth model between axial growth under the vapor–liquid–solid (VLS) mechanism and lateral growth controlled by the vapor–solid (VS) mechanism was proposed to explain the formation of a rectangular cross section and the size change of the nanorods. The research regarding controlled growth under the two mechanisms, viz. VLS and VS, was beneficial for exploration into the controlled growth of complicated functional nanomaterials. Furthermore, the photoluminescence property was also studied. PACS  61.46.-W     

ZnO-coated zinc antimonate nanocables

ZnO-coated zinc antimonate (ZnSb₂O₄) nanocables have been fabricated via a simple one-step thermal evaporation of layered powders of Sb₂O₃ and ZnO on a brass substrate. The samples were characterized using X-ray diffraction, scanning electron microscopy and high resolution transmission electron microscopy with a high-angle annular dark-field attachment. The nanocables are several tens of micrometers in length; the ZnSb₂O₄ core is ∼30 nm in diameter and the thickness of the ZnO sheath is ∼5 nm. Both the ZnSb₂O₄ core and the ZnO sheath are single crystalline. The controlled growth mechanism of ZnO-coated ZnSb₂O₄ nanocables was considered to be the combination of oxide-assisted evaporation and vapor–solid processes. PACS 61.46.+w; 81.07.-b     

Blue-light emission from amorphous SiOx nanoropes

High-yield synthesis of amorphous silicon oxide nanoropes (SiONRs) was achieved by using simple evaporation and oxidation of Si. Transmission electron microscopy observations show that the amorphous SiONRs have a length of up to several hundreds of micrometers and a diameter of 20 to 40 nm. Energy-dispersive X-ray analysis reveals that the SiONRs consist of Si and O elements in an atomic ratio of approximately 1:1.2. The formation process of the SiONRs is closely related to a vapor–solid method. The SiONRs emit blue light at energies of 2.53 and 3.0 eV. Received: 2 January 2002 / Accepted: 7 January 2002 / Published online: 20 March 2002     

Large-scale synthesis of SnO2 nanobelts

Tin dioxide (SnO₂) nanobelts have been successfully synthesized in bulk quantity by a simple and low-cost process based on the thermal evaporation of tin powders at 800 °C. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations reveal that the nanobelts are uniform, with lengths from several-hundred micrometers to a few millimeters, widths of 60 to 250 nm and thicknesses of 10 to 30 nm. X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX) and selected-area electron diffraction analysis (SAED) indicate that the nanobelts are tetragonal rutile structure of SnO₂. The SnO₂ nanobelts grow via a vapor–solid (VS) process. Received: 3 June 2002 / Accepted: 10 June 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +86-551/559-1434, E-mail: gwmeng@mail.issp.ac.cn     

Self-catalytic ZnSe nanorods on grains synthesized using thermal evaporation method

In this paper, ZnSe nanorods grown on in-situ synthesized ZnSe grains are reported. The ZnSe products are synthesized through thermal evaporation of elementary materials of Zn and Se powders in a horizontal resistance furnace. It is interesting to note that the ZnSe nanorods of nearly the same diameter and length are obtained, and they grow in the same direction on a facet of the ZnSe grain. The ZnSe grains are random in shape, with well-defined bounded facets. The ZnSe grains can be synthesized in various growth conditions while the ZnSe nanorods can be synthesized on the ZnSe grains with the fulfillment of the Zn enriched condition. The growth of ZnSe nanorods can be described by the self-catalytic vapor–liquid–solid (VLS) mechanism. PACS 73.21.-b; 78.55.Et; 61.10.Nz; 61.46.+w; 68.65.-k     

Synthesis and characterization of silica nanosprings by a low temperature chemical vapor deposition

Silica nanosprings were synthesized using a simple, low temperature, chemical vapor deposition method via a vapor–liquid–solid mechanism. Nanosprings with excellent uniformity and helicity in high and repeatable yields have been observed. The morphology and crystal structure of the nanosprings were characterized by scanning electron microscopy and transmission electron microscopy. The chemical composition of the nanosprings was determined using the energy-filtered transmission electron microscopic method. The as-grown nanomaterials were confirmed to be amorphous silica with irregularly shaped Au catalytic particles located at the tips. In addition, we propose a spontaneous spinning growth model to explain the formation of such helical nanostructures.     

The Influence of B<Subscript>2</Subscript>H<Subscript>6</Subscript> on the Growth of Silicon Nanowire

The un-doped and boron-doped silicon nanowires (SiNWs) were grown via vapor–liquid–solid (VLS) mechanism by low pressure chemical deposition (LPCVD). The diameters of un-doped and boron-doped SiNWs varied from 18.5 to 75.3 nm and 26.6 to 66.1 nm, respectively. The critical growth temperature of boron-doped SiNWs is 10°C lower than that of un-doped ones and the diameters of the boron-doped SiNWs is always larger than that of the un-doped ones under different growth temperatures. This is because that the introduction of diborane enhanced the dissociation of SiH₄ which determines the growth process of SiNW. A growth process of silicon nanowire is proposed to describe the influence of B₂H₆.     

Synthesis of zirconium dioxide nanotubes, nanowires, and nanocables by concentration dependent solution deposition

Nanowires and nanotubes of ZrO₂ were successfully synthesized in this study by concentration dependent solution deposition (CDSD) using Zr[O(CH₂)₃CH₃]₄ as the precursor and anodic aluminum oxide (AAO) as the substrate. Here, we synthesized one-dimensional zirconium dioxide with a simple and efficient method by sucking the precursor solution though the AAO channels. In these experiments, the key factor in the formation of nanowires and nanotubes is the concentration of the precursor, Zr[O(CH₂)₃CH₃]₄ butanol solution. Concentrated Zr[O(CH₂)₃CH₃]₄ solutions cause the formation of solid one-dimensional materials, whereas dilute precursor solutions form hollow nanotubes. The dimensions of the nanomaterials synthesized correspond to the scope of the AAO template are 200–450 nm in diameter and about 60 μm in length. Furthermore, the cable structures of these nanomaterials, wire-in-tube and tube-in-tube, were synthesized by alternately operating the processes. We are reporting an uncomplicated, fast and versatile method applicable to a wide range of materials, taking advantage of the sol–gel process to prepare a vast variety of nanocomposite full of potential applications, especially cable nanomaterials.     

Synthesis and photoluminescence of aligned SiO<Subscript>x</Subscript> nanowire arrays

Aligned SiO_x nanowire arrays standing on a Si substrate were successfully synthesized using a simple method by heating a single-crystalline Si slice covered with SiO₂ nanoparticles at 1000 °C in a flowing Ar atmosphere. The SiO_x nanowire arrays were characterized by scanning electron microscopy and transmission electron microscopy. The SiO_x nanowires become progressively thinner from bottom to top. The formation process of the SiO_x nanowire arrays is closely related to a vapor–solid mechanism. Room-temperature photoluminescence measurements under excitation at 260 nm showed that the SiO_x nanowire arrays had a strong blue–green emission at 500 nm (about 2.5 eV), which may be related to oxygen defects. Received: 29 April 2002 / Accepted: 30 April 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +86-551-559-1434, E-mail: gwmeng@mail.issp.ac.cn     

Long silicon nitride nanowires synthesized in a simple route

Long silicon nitride (Si₃N₄) nanowires with high purity were synthesized by heating mixtures of SiO₂ powders and short carbon fibers at 1430°C for 2 h in a flowing N₂ atmosphere. The nanowires had the length of 1–2 millimeters and the diameters of 70–300 nm, and were mainly composed of ⍺-Si₃N₄, growing along the [001] direction. The vapor–solid (VS) mechanism was employed to interpret the nanowires growth.     

Effect of surface roughness on thermal conductivity of VLS-grown rough Si<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> nanowires

The recent demonstration of thermal conductivity of rough electrolessly etched Si nanowire (Hochbaum et al., Nature, 451:163, 2008) attracted a lot of interest, because it could not be explained by the existing theory; thermal conductivity of rough Si nanowires falls below the boundary scattering of the thermal conductivity. However, nanoscale pores presented in the nanowires (Hochbaum et al., Nano Letters, 9:3550–3554, 2009) hinder one to be fully convinced that the surface roughness solely made a contribution to the significant reduction in thermal conductivity. In this study, we synthesized vapor–liquid–solid (VLS) grown rough Si_1−x Ge _x nanowire and measured and theoretically simulated thermal conductivity of the nanowire. The thermal conductivity of rough Si_0.96Ge_0.04 nanowire is an order of magnitude lower than that of bulk Si_0.96Ge_0.04 and around a factor of four times lower than that of smooth Si_0.96Ge_0.04 nanowire. This significant reduction could be explained by the fact that the surface roughness scatters medium-wavelength phonons, whereas the long-wavelength phonons are scattered by phonon boundary scattering, and the short-wavelength phonons are scattered by alloy scattering.     

Surfactant-free synthesis of novel copper oxide (CuO) nanowire–cobalt oxide (Co<Subscript>3</Subscript>O<Subscript>4</Subscript>) nanoparticle heterostructures and their morphological control

A simple and surfactant-free synthesis of novel heterostructures comprising of copper oxide (CuO) nanowires uniformly decorated with cobalt oxide (Co₃O₄) nanoparticles was demonstrated by combining thermal growth and wet-coating method. The heterostructures were synthesized by thermally decomposing cobalt salt (cobalt nitrate) into Co₃O₄ nanoparticles onto vapor–solid (VS)-grown CuO nanowires. X-ray diffraction (XRD) and high resolution transmission electron microscopy (TEM) confirmed the presence of CuO and Co₃O₄ phases as well as a narrow size distribution of Co₃O₄ nanoparticles (average diameter ~7.0 ± 1.5 nm) on CuO nanowires (average diameter of nanowire tips ~67.9 ± 18.6 nm). Unique interfacial lattice relationship was observed for (111) Co₃O₄ nanoparticles on (200) CuO nanowire surface resulting in hemispherical shape of the former. For the first time, further systematic studies were performed to understand the influence of various parameters (cobalt salt concentration and annealing temperature, atmosphere, and time) on the morphological evolution of Co₃O₄ nanoparticles on CuO nanowires. Interestingly, by varying these parameters, it was possible to grow Co₃O₄ in different shapes (spherical, triangular, rectangular, cubical, and hexagonal nanoparticles) and forms (shells and nanorods). It was observed that all these parameters play a critical role in influencing the surface migration, nucleation, and growth of Co₃O₄ nanoparticles on CuO nanowires and this assisted in understanding the involved growth mechanisms. Finally, UV–vis–NIR spectroscopy and band gap energies for these heterostructures were evaluated that showed higher photocatalytic degradation efficiency for Rhodamine B under low-power visible-light illumination.     

Laser pulse heating: Cavity formation into steel, nickel and tantalum surfaces

The cavity formation during laser pulse heating of steel, nickel, and tantalum is examined and evaporation rate from the cavity surface is predicted. The mushy zones generated across the vapor–liquid and liquid–solid phases are modeled using the energy method. Temperature-dependent thermal properties are accommodated in the analysis and the laser pulse shape resembling the actual laser pulse is employed in the simulations. A numerical scheme using the control volume method is used to predict the cavity size, recession velocity of the vapor front, and temperature field in the laser irradiated region. It is found that cavity depth for steel is the largest, then follows nickel and tantalum. The recession velocity of the vapor front is high for steel due to the low evaporation temperature and latent heat of evaporation of steel.     

Li<Subscript>1−<Emphasis Type="BoldItalic">x</Emphasis></Subscript>Sm<Subscript>1+<Emphasis Type="BoldItalic">x</Emphasis></Subscript>SiO<Subscript>4</Subscript> as solid electrolyte for high temperature solid-state lithium batteries

Lithium lanthanoid silicates are projected as promising solid electrolytes for solid-state high-temperature lithium batteries. Synthesis of Li_1−x Sm_1+x SiO₄ (x = 0.2 to 0.6) was carried using sol–gel method, and these compounds were characterized by thermogravimetry differential thermal analysis, X-ray diffraction, Fourier transform infrared, and SEM. Impedance measurements were carried out at different temperatures, and conductivity at different temperatures was calculated. The effect of an increase of samarium content on the conductivity of the solid electrolyte was studied in this paper. It was found that less samarium content exhibits good conductivity at higher temperatures.     

Ferroelectric phase stabilization, phase transformations and?thermal properties in (1? x )PbZrO3– x Pb(Co1/3Nb2/3)O3 solid solution

The solid solution between the antiferroelectric PbZrO₃ (PZ) and relaxor ferroelectric Pb(Co_1/3Nb_2/3)O₃ (PCoN) was synthesized by the columbite method. The phase structure and thermal properties of (1−x)PZ–xPCoN, where x=0.0–0.3, were investigated. With these data, the ferroelectric phase diagram between PZ and PCoN has been established. The crystal structure data obtained from XRD indicates that the solid solution PZ–PCoN, where x=0.0–0.3, successively transforms from orthorhombic to rhombohedral symmetry with an increase in PCoN concentration. The AFE→FE phase transition was found in the compositions of 0.0≤x≤0.10. The AFE→FE phase transition shift to lower temperatures with higher compositions of x. The width of the temperature range of FE phase was increased with increasing amount of PCoN. It is apparent that the replacement of the Zr⁴⁺ ion by (Co_1/3Nb_2/3)⁴⁺ ions would decrease the driving force for antiparallel shift of Pb²⁺ ions, because they interrupt the translational symmetry. This interruption caused the appearance of a rhombohedral ferroelectric phase when the amount of PCoN was more than 10 mol%.     

ZnO nanorods grown on ZnSe particles by the chemical vapor deposition method

A novel structure of ZnO nanorods on microsized ZnSe particles has been prepared through a chemical vapor deposition technique using Zn and Se powders as the sources. The dimension of the nanorods can be controlled by adjusting the growth temperature, time and the Zn : Se ratio. Through the investigation of the effects of synthesis time and Zn : Se ratio on the formation of ZnO nanorods on ZnSe microparticles, it is proposed that the synthesis of the ZnO–ZnSe structures involves a two-stage formation. The growth of ZnO nanorods can be described by the vapor–solid mechanism. The photoluminescence of the ZnO–ZnSe structures has also been studied. PACS 73.21.-b; 78.55.Et; 61.10.Nz; 61.46.+w; 68.65.-k