H. Y. Peng

Synthesis of Large Areas of Highly Oriented, Very Long Silicon Nanowires**

Silicon is one of the most important electronic materials. Its nanoscale forms, such as nanocrystals, porous silicon, quantum wells, and nanowires, have stimulated great interest among scientists because of their peculiar physical properties, such as light emission, field emission, and quantum confinement effects. The progress made in the synthesis of silicon nanostructures and nanowires in recent years has attracted considerable attention. Today, large quantities of silicon nanowires can be produced by the laser ablation of metalor SiO2-containing silicon targets, [8] and a few properties, such as electric and thermal conductivity and optical properties, have also been studied. However, the experimental characterization and application of silicon nanowires, for example, the measurement of the elastic properties, the realization of efficient field emission of nanoscale silicon, and the fabrication of nanometer field effect transistors and planar displays, have been hampered so far because of the difficulty in growing oriented silicon nanowires. As a result, the production of highly oriented and very long silicon nanowires is a very important and challenging issue. In this communication, we report the successful synthesis of highly oriented, largescale, and very long silicon nanowires on flat silicon substrates by thermal evaporation of silicon monoxide (SiO). The growth mechanism and optical properties of the oriented silicon nanowires are also discussed. To the best of our knowledge, the synthesis of oriented silicon nanowires has not yet been reported. The equipment used for the present work is similar to that described previously. An alumina tube was mounted inside a tube furnace. The SiO powders (Gooodfellow, 99.95 %) were placed near the middle of the high-temperature zone of the furnace. The polished silicon (100) substrates about 5 mm in width and 50 mm in length were ultrasonically cleaned in acetone, ethanol, and deionized water for 20 min each, dipped in 20 % HF for 20 min, and finally rinsed in deionized water for 20 min before they were placed abreast at one end of the alumina tube. The tube had previously been evacuated to a base pressure of 10 torr by a mechanical pump before the starting materials were heated. The carrier gas of argon mixed with 5 % H2 admitted at the other end of the alumina tube flowed at 50 sccm (standard cubic centimeters per minute) at 400 torr. The temperature of the furnace was increased to 1300 C at 6 C/min and kept at this temperature for 7 h. The temperature of the silicon substrate surface where the oriented silicon nanowires grew was found to be approximately 930 C, which differed from that at the center due to the temperature gradient within the tube. The product was first directly examined by scanning electron microscopy (SEM, Philips XL 30 FEG). Microstructural characterization was carried out in a conventional Philips CM 20 transmission electron microscope (TEM) at 200 kV. The high-resolution transmission electron microscopy (HRTEM) study was performed in a Philips CM200 FEG transmission electron microscope, operated at 200 kV accelerating voltage at room temperature. The chemical compositions of the samples were determined by an energy dispersive X-ray (EDX) spectrometer attached to both the SEM and HRTEM instruments. Raman scattering spectra were measured with a Renishaw micro-Raman spectrometer at room temperature. Excitation was by means of the 514 nm line of an Ar laser, and the Raman signals were measured in a backscattering geometry with a spectral resolution of 1.0 cm. The deposited silicon nanowire product is light yellow in color. SEM images at different magnifications of a typical sample in Figures 1a, 1b, 1c, and 1d clearly show the large area of highly oriented nanowires on the surface of the silicon substrate. The low magnification SEM image (Fig. 1a) shows that the area of highly oriented silicon nanowires is about 2 mm ́ 3 mm and the lengths of individual nanowires are up to 1.5±2 mm. The thickness of the oriented nanowire product was about 10 lm, as estimated from the cross-sectional image (Fig. 1d) of the sample prepared by focused ion beam cutting. The highly oriented array of Si nanowires can also be observed from the cross-sectional image. The EDX results show that the nanowires are composed of silicon and oxygen. No metal was found in the sample. Such results are consistent with our previous theory that silicon nanowire growth is enhanced by silicon oxide instead of a metal particle catalyst. Because of local charging effects, the diameters observed from SEM images appear larger than the actual wire diameters. More information about the morphology of silicon nanowires is given by the following TEM characterization Small pieces of oriented silicon nanowire samples were peeled off from a silicon substrate and mounted on a folding grid for TEM and HRTEM observations. Figure 2 shows the typical morphology of silicon nanowires. As reported previously, these nanowires show a better orientation than those synthesized by laser ablation. Silicon nanowires as observed by TEM are quite clean, with very few particles attached to their surfaces, and are relatively homogeneous. Analysis of a number of nanowires shows that the diameters of these silicon nanowires vary from 18 to 46 nm, and the mean value is about 30 nm. The selected-area electron dif-

β-SiC nanorods synthesized by hot filament chemical vapor deposition

A one-step procedure has been developed to grow β-SiC nanorods from a solid carbon and silicon source on a Si substrate by hot filament chemical vapor deposition. This process is catalyzed by metallic particles which come from impurities in the solid source which is a plate made by pressing a mixture graphite and silicon powders at 150 °C. Hydrogen was introduced into the reaction chamber to react with the solid plate to produce hydrocarbon and hydrosilicon radicals which presumably reacted to form SiC nanorods. The nanorods consisted of a crystalline β-SiC core with an amorphous silicon oxide shell layer and grew along the [100] direction. The nanorods were 10–30 nm in diameter and less than 1 μm in length.

Bulk-quantity GaN nanowires synthesized from hot filament chemical vapor deposition

The bulk-quantity synthesis of single-crystal GaN nanowires has been achieved through a simple method of hot filament chemical vapor deposition without using a nanometer-sized catalyst. The microstructures and optical properties of GaN . nanowires have been studied by electron microscopy and photoluminescence PL measurements at room temperature. The GaN nanowires had diameters of 5-12 nm and lengths of a few micrometers, and were highly pure. They possessed a 4 hexagonal wurtzite structure and had a growth direction perpendicular to the 1101 plane. The PL spectra showed a broad emission peak centered at 420 nm. q 2000 Published by Elsevier Science B.V.

Thin β-SiC nanorods and their field emission properties

Abstract Beta-silicon carbide (β-SiC) nanorods (diameter, ca. 5–20 nm; length, 1 μm) have been grown on porous silicon substrates by chemical vapor deposition with an iron catalyst. The turn-on field of the grown β-SiC nanorods on a porous silicon substrate is 13–17 V/μm.

Diameter modification of silicon nanowires by ambient gas

Si nanowires (SINWs) with different diameters have been synthesized by laser ablation in different ambient gases. SINWs with the diameter distribution peaks at ∼13.2 and ∼9.5 nm have been obtained respectively in He and Ar (5% H2). SINWs produced in N2 had the smallest peak diameter at 6 nm, and are mixed in with some spherical particles with diameters ranging from ∼9 nm to several hundreds nm. Elements from the ambient gas were not detected in the SINWs. SINWs produced in Ar(5% H2) and N2 atmospheres exhibited photoluminescence and spectral blue-shift with diameter reduction, which are attributable to two-dimensional quantum confinement effects in crystalline nanowires.

Straight β-SiC nanorods synthesized by using C–Si–SiO2

Straight beta-silicon carbide nanorods have been grown on silicon wafers using hot filament chemical vapor deposition with iron particles as catalyst. A plate made of a C–Si–SiO2 powder mixture was used as carbon and silicon sources. Hydrogen, which was the only gas fed into the deposition system, acts both as a reactant and as a mass transporting medium. The diameter of the β-SiC nanorod ranged from 20 to 70 nm, while its length was approximately 1 μm. A growth mechanism of beta-silicon carbide nanorods was proposed. The field emission properties of the beta-silicon carbide nanorods grown on the silicon substrate are also reported.

Highly efficient and stable photoluminescence from silicon nanowires coated with SiC

Abstract A reaction of silicon nanowires (SiNW) with methane and hydrogen has been performed to produce a thin coating layer of cubic silicon carbide (β-SiC) using an ion beam deposition technique. High resolution transmission electron microscopy (HRTEM) showed that silicon oxide shells originally cladding the as-grown SiNW were removed and replaced by a thin layer of nano-sized crystals of β-SiC. This has led to stable photoluminescence (PL) observed from the SiC-coated SiNW with high efficiency almost tripled as compared with that before SiC coating.

Bulk-quantity Si nanosphere chains prepared from semi-infinite length Si nanowires

Bulk-quantity Si nanosphere chains have been fabricated. This is accomplished via the spheroidization of Si nanowires of semi-infinite lengths. The process has been extensively investigated by transmission electron microscopy. The nanosphere chains consisted of equally spaced Si crystalline nanospheres connected by Si-oxide bars. The transition from Si nanowires to Si nanosphere chains was determined by the annealing temperature, ambient pressure, initial Si nanowire diameters, and the oxide state of the outer layers of Si nanowires. The relationships between the geometry (size and spacing) of Si nanospheres, the initial state (diameter and oxide state) of Si nanowires, and the experimental conditions are discussed.

Smallest diameter carbon nanotubes

Mass-selected carbon ion beam deposition (MSIBD) was used to demonstrate that the diameter of a carbon nanotube could be as small as 0.4 nm, the theoretical limit predicted but never experimentally reached so far. The deposition was performed at an elevated temperature much lower than the high temperatures (800–1000 °C) needed for deposition of carbon nanotubes by conventional methods. High-resolution transmission electron microscopy showed that the combination of the stress induced by the ion impact and the C migration at the temperature applied formed graphitic sheets with their normal (c axis) parallel to the surface of the silicon substrate. Some sheets closed to form multiwall nanotubes. The smallest diameter of the innermost tube was found to be 0.4 nm. The novel use of MSIBD (a pure method, catalyst free, low deposition temperature, easily applied to large surfaces without surface pretreatment capable of pattern-writing) may significantly advance the carbon nanostructure technology.

Si nanowires synthesized by laser ablation of mixed SiC and SiO2 powders

Abstract By using a KrF excimer laser to ablate a target of SiC powder mixed with 10 wt.% SiO 2 powder at 1400°C, Si nanowires were deposited on the inner wall of a ceramic tube. Transmission electron microscopy shows that the nanowires are around 14 nm in diameter and co-exist with a small amount of nanoparticles. High-resolution transmission electron microscopy shows that the nanowires are crystalline Si nanowires and the nanoparticles are cubic SiC. The intergrowth of heterocrystal nanowires and nanoparticles verifies that the oxide-assisted growth model of Si nanowires is reasonable.