Nucleation and growth of Si nanowires by laser ablation and thermal evaporation of Si powder sources mixed with SiO 2 have been investigated by means of transmission electron microscopy. At the initial nucleation stage, Si oxide vapor condensed on the substrate and formed Si nanoparticles ͑the nuclei of nanowires͒. Each Si nanowire nucleus consisted of a polycrystalline Si core containing a high density of defects and a Si oxide shell. A growth mechanism was proposed based on the microstructure and different morphologies of the Si nanowires observed. ͓S0163-1829͑98͒51348-3͔

Semiconductor wires with nanometer widths have attracted much attention in recent years for their potential applications in mesoscopic research and nanodevices. Since the 1960s, Si whiskers grown from the vapor-liquid-solid (VLS) reaction have been extensively studied. In the VLS reaction, Au particles are generally used as the mediating solvent on a Si substrate since Au and Si form a molten alloy at a relatively low temperature. Si in the vapor phase diffuses into the liquid-alloy droplet and bonds to the solid Si at the liquid-solid interface, which results in the growth of Si whiskers. The diameter of the whisker is determined by the diameter of the liquid-alloy droplet at its tip. Si whiskers generally grow along ⟨111⟩ directions epitaxially on Si(111) substrates in the form of single crystals by the VLS reaction.In different materials systems, however, a variety of whisker forms can be obtained. For example, GaP whiskers display rotational twins around their ⟨111⟩ growth axes, while GaAs whiskers grow in the form of the wurtzite structure.Thus far, the synthesis of one-dimensional nanostructured materials on a large scale remains a challenge. In recent years, many efforts have been made to synthesize Si nanowires by employing different methods such as photolithography and etching techniques and scanning tunneling microscopy. One method of particular interest is a recently developed laser ablation of metal-containing semiconductor targets, by which bulk quantities of semiconductor nanowires can be readily obtained. Our recent studies show that oxides play a dominant role in the nucleation and growth of high-quality semiconductor nanowires in bulk quantities by laser ablation, thermal evaporation, or chemical vapor deposition. A new growth mechanism called oxide-assisted nanowire growth has therefore been established. The ability to synthesize large quantities of high-purity (no contamination), ultra-long (in millimeters), and uniform-sized semiconductor nanowires (a few nanometers to tens of nanometers in diameter) from this new technique offers exciting possibilities in fundamental and applied research.

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We report a new membrane system showing dynamic controllable multiphase transport and separation under steady-state pressure.

Si nanowires with uniform size have been synthesized by laser ablation of highly pure Si powder targets mixed with SiO2. A bulk quantity of Si nanowires was successfully obtained by mixing 30%–70% of SiO2 into the Si powder target. SiO2 played a crucial role in enhancing the formation and growth of the Si nanowires. The morphology and microstructure of the Si nanowire tips have been systematically characterized by means of high-resolution transmission electron microscopy. No evidence of metal was found at the tips. The results suggest that Si oxide is more important than metal in catalyzing the formation of Si nanowires.

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Silicon nanowires (SiNWs) were synthesized using laser ablation. A continuous SiNW film was prepared by grinding the pieces of sponge-like SiNWs to powder, then dispersing and sticking the powder onto a Si wafer. The field emission characteristics of the SiNW film were studied based on current–voltage measurements and the Fowler–Nordheim equation. The electron field emission increased with decreasing diameter of SiNWs. A hydrogen plasma treatment of the SiNW film aimed at reducing the oxide overlayer improved the emission uniformity of the film.

ter of the nanopillars was quite noticeable and would be useful in fabricating nanostructures with feature sizes smaller than those of the original master. The aspect ratio was as high as 5 for the nanostructures produced using SPS, and might be increased further with structures of smaller feature sizes. The black spots next to the pillars in the figures are dimples formed by pressure buildup.In summary, we have presented the observation of several intriguing nanostructures, such as mushroom-like nanopillars, vertical nanopillars, and nanospheres, using capillary lithography with a UV-curable, polyurethane acrylate mold. It has been shown that air permeation during capillary rise plays an important role in pattern replication, which has not been previously observed in any type of nanofabrication involving PDMS molds. Depending on the film thickness of the polymer solution or the wetting conditions at the time of contact, nanopillars or nanospheres were observed for two different step heights of the mold used in the experiment. Furthermore, the step height could be adjusted to obtain well-defined vertical nanopillars with diameters less than that of the step height. This simple method would be potentially useful in fabricating unique nanostructures without resorting to other complicated, multistep methods. ExperimentalFabrication of UV-Curable Mold: The UV-curable mold material consisted of a functionalized prepolymer with acrylate groups for crosslinking, a monomeric modulator, a photoinitiator, and a radiation-curable releasing agent for surface activity. Details on the synthesis and characterization of the polymer have been published elsewhere [26]. The UV-curable mold used in the experiment was a thin sheet with a thickness ranging from 0.3 to 1 mm (see Fig. 1d).Polymers: We used a PEG-based random copolymer, poly(3-trimethoxysilyl)propyl methacrylate-r-polyethylene glycol methyl ether) (poly(TMSMA-r-PEGMA)) that has potential for use in biological applications. This polymer contains surface-reactive trimethoxysilyl groups as part of its backbone, which allows for the formation of multivalent bonds onto oxide surfaces, as well as multiple PEG chains. Detailed information on the synthesis and characterization of the polymer has been published elsewhere [28]. For comparison, we also used poly(sodium 4-styrenesulfonate) with a molecular weight of 200 000 (30 wt.-% in water, Aldrich). The polymer solution was diluted prior to use.Capillary Lithography: A few drops of the polymer solution, of varying concentration (1±10 wt.-%), were placed on a silicon substrate and thin films were obtained by spin coating (Model CB 15, Headaway Research, Inc.) at 1000 rpm for 10 s. To make conformal contact, the polyurethane acrylate molds were carefully placed onto the surface and then the samples were stored overnight at room temperature to allow for evaporation of the solvent. The molds were peeled off using a sharp tweezer after complete evaporation of the solvent.Scanning Electron Microscopy (SEM): Images were taken using a h...