Sulfide-assisted growth of silicon nano-wires by thermal evaporation of sulfur powders

Silicon nanowires (SiNWs) with a diameter of 20 nm were synthesized by the thermal evaporation of sulfur powders on silicon wafers. The source of the SiNWs came from the silicon substrates. It is considered that the generated SiS compound assisted the formation of SiNWs. Finally, the Raman shift of SiNWs was discussed.     

XPS study on the evaporation of gold submonolayers on carbon surfaces

We investigated the early nucleation stages of evaporated gold submonolayers on different carbon surfaces (pristine HOPG, argon-ion irradiated HOPG and amorphous carbon). Gold core-level and valence band spectra were measured by monochromatised X-ray photoelectron spectroscopy (MXPS). The Au 4f spectra for the lowest coverages (0.1 Å equivalent thickness) on irradiated HOPG and amorphous carbon surprisingly exhibited two well-separated doublets. We attribute this phenomenon to a bimodal particle size distribution caused by gold atom pinning at carbon defect sites. Deposition at elevated temperatures (on irradiated HOPG) opens a possibility to grow particles preferentially on defect sites. The influence of carbon surface defects on the cluster morphology was checked by SEM imaging. These results are interesting for future applications as they help to improve control over metal nanodots growth.     

The intrinsic lifetime of low-frequency zone-centre phonon modes in silicon nanowires and carbon nanotubes

Estimates of the intrinsic lifetime of low-frequency zone-centre phonon modes in silicon nanowires and carbon nanotubes have been presented from the application of Fermi’s golden rule formula based upon an elastic continuum model for cubic anharmonicity. In particular, results have been presented for the lowest non-zero mode in both nanostructures, and also the breathing mode in the nanotube. Except for the ultrathin nanowire, the lifetime increases with size and decreases with an increase in temperature. Typically, these modes have a lifetime of the order of nanoseconds, almost a thousand times larger than the lifetimes of optical phonon modes in the corresponding bulk materials. Also, at room temperature the lifetime of the lowest non-zero mode is nearly an order of magnitude larger in the (20,20) nanotube than in the nanowire of similar thickness (width 2.2 nm).     

Silicon nano-wires fabricated by thermal evaporation of silicon wafer

Thin silicon nano-wires (SiNWs) with a diameter of 10–20 nm were fabricated by a simple thermal evaporation of silicon wafer at 1523 K. The gold produced by an electrochemical method was covered on the wafer surface as catalyst. It was found that the SiNWs are amorphous and its Raman peak shifted down maybe due to the effect of laser heating and quantum confinement. Finally, a temperature gradient growth model is suggested to explain the growth direction of SiNWs.     

Oxide reduced silicon nanowires

Core crystalline silicon nanowires with a heavily reduced amorphous shell have been successfully synthesised using palladium as a metal catalyst. We present two approaches to reduce the oxidation of the nanowires during the thermal annealing growth. The ratios of the amorphous shell to crystalline core of the nanowires produced, from the two methods, are compared and show a remarkable drop (hence thinner oxide) compared to wires fabricated using currently available techniques. In addition, a focused ion beam was utilised to contact the oxide-reduced nanowires for transport measurements, without first removing the thin oxide shell. The oxygen-reduced core-shell silicon nanowires showed a very low electrical resistivity (4 × 10⁻¹ Ω cm). Our novel approach presents a new alternative to the production of low cost, high yield, highly conducting silicon nanowires offering a wide range of opportunities for semiconductor based technology.     

One-dimensional and quasi-one-dimensional ZnO nanostructures prepared by spray-pyrolysis-assisted thermal evaporation

One-dimensional (1D) and quasi-1D ZnO nanostructures have been fabricated by a kind of new spray-pyrolysis-assisted thermal evaporation method. Pure ZnO powder serves as an evaporation source. Thus-obtained products have been characterized by X-ray diffraction (XRD) analysis, scanning electron microscope (SEM) equipped with energy dispersive X-ray spectroscopy (EDS), transmission electron microscope (TEM). The room temperature photoluminescence spectrum of these ZnO nanostructures is presented. The results show that as-grown ZnO nanomaterials have a hexagonal wurtzite crystalline structure. Besides nanosaws, nanobelts and nanowires, complex ZnO nanotrees have also been observed in synthesized products. The study provides a new simple route to construct 1D and quasi-1D ZnO nanomaterials, which can probably be extended to fabricate other oxide nanomaterials with high melting point and doped oxide nanomaterials.     

Polarization anisotropy in the optical properties of silicon ellipsoids

A new real space quantum mechanical approach with local field effects included is applied to the calculation of the optical properties of silicon nanocrystals. Silicon ellipsoids are studied and the role of surface polarization is discussed in details. In particular, surface polarization is shown to be responsible for a strong optical anisotropy in silicon ellipsoids, much more pronounced with respect to the case in which only quantum confinement effects are considered. The static dielectric constant and the absorption spectra are calculated, showing that the perpendicular and parallel components have a very different dependence on the ellipsoid aspect ratio. Then, a comparison with the classical dielectric model is performed, showing that the model only works for large and regular structures, but it fails for thin elongated ellipsoids.     

SERS activity of Au nanoparticles coated on an array of carbon nanotube nested into silicon nanoporous pillar

A novel composite structure, Au nanoparticles coated on a nest-shaped array of carbon nanotube nested into a silicon nanoporous pillar array (Au/NACNT/Si-NPA), was fabricated for surface-enhanced Raman scattering (SERS). The morphology of the Au/NACNT/Si-NPA composite structure was characterized with the aid of scanning electron microscopy, X-ray diffraction instrumentation and Transmission electron microscopy. Compared with SERS of rhodamine 6G (R6G) adsorbed on SERS-active Au substrate reported, the SERS signals of R6G adsorbed on these gold nanoparticles were obviously improved. This was attributed to the enlarged specific surface area for adsorption of target molecules brought by the nest-shaped CNTs structure.     

pH-controlled silicon nanowires fluorescence switch

Covalently immobilizing photoinduced electronic transfer (PET) fluorophore 3-[N, N-bis(9-anthrylmethyl)amino]-propyltriethoxysilane (DiAN) on the surface of silicon nanowires (SiNWs) resulted a SiNWs-based fluorescence switch. This fluorescence switch is operated by adjustment of the acidity of the environment and exhibits sensitive response to pH at the range from 8 to 10. Such response is attributed to the effect of pH on the PET process. The successful combination of logic switch and SiNWs provides a rational approach to assemble different logic molecules on SiNWs for realization of miniaturization and modularization of switches and logic devices.     

The influence of hydrogen on the growth of gallium catalyzed silicon oxide nanowires

In this paper, we report that amorphous silicon oxide nanowires can be grown in a large quantity by chemical vapor deposition with molten gallium as the catalyst in a flow of mixture of SiH₄, H₂ and N₂ at 600 °C. Meanwhile, when we grow these nanowires under the same conditions but without H₂, octopus-like silicon oxide nanostructures are obtained. The reasons and mechanisms for the growth of these nanowires and nanostructures are discussed. Blue light emission is observed from SiO_x nanowires, which can be attributed to defect centers of high oxygen deficiency. These SiO_x nanowires may find applications in nanodevices and reinforcing composites.     

Effect of temperature and silicon resistivity on the elaboration of silicon nanowires by electroless etching

The morphology of silicon nanowire (SiNW) layers formed by Ag-assisted electroless etching in HF/H₂O₂ solution was studied. Prior to the etching, the Ag nanoparticles were deposited on p-type Si(1 0 0) wafers by electroless metal deposition (EMD) in HF/AgNO₃ solution at room temperature. The effect of etching temperature and silicon resistivity on the formation process of nanowires was studied. The secondary ion mass spectra (SIMS) technique is used to study the penetration of silver in the etched layers. The morphology of etched layers was investigated by scanning electron microscope (SEM).     

Doping in silicon nanocrystals

The absorption and, for the first time, the emission spectra of doped silicon nanocrystals have been calculated within a first-principles framework including geometry optimization. Starting from hydrogenated silicon nanocrystals, simultaneous n- and p-type doping with boron and phosphorous impurities have been considered. We found that the B-P co-doping results to be easier than simple B- or P-doping and that the two impurities tend to occupy nearest neighbours sites inside the nanocrystal itself. The co-doped nanocrystals bandstructure presents band edge states that are localized on the impurities and are responsible of the red-shifted absorption threshold with respect to that of pure un-doped nanocrystals in fair agreement with the experimental outcome. The emission spectra show a Stokes shift with respect to the absorption due to the structural relaxation after the creation of the electron-hole pair. Moreover, the absorption and emission spectra have been calculated for a small co-doped nanocrystal beyond the single particle approach by introducing the self-energy correction and solving the Bethe-Salpeter equation scheme. Our procedure shows the important role played by the many-body effects.     

Trench-template fabrication of indium and silicon nanowires prepared by thermal evaporation process

In this study, we report on the trench-template assisted fabrication of nanowires for thermally evaporated indium and silicon thin films on quartz substrate. Length of the nanowires is completely dependent on the length of the trench, whereas the diameter of the nanowires is dependent on the thickness of the thin film. The diameter of nanowire increases from 200 nm to 1 μm when the thickness was increased from 15 to 60 nm. It is observed that nanowires diameter is invariably controlled by material deposition thickness. Average crystallite sizes for 60 nm indium and silicon deposition inside the trench are 120 and 35 nm, respectively. Nanowire surface plasmon peak shift as compared to the same thickness untemplated continuous thin film is more for thinner nanowires. This technique of nanowire fabrication is shown to be versatile in nature.     

Non-catalytic growth of high aspect-ratio ZnO nanowires by thermal evaporation

High-density and high aspect-ratio ZnO nanowires were grown on Si(100) substrates by the thermal evaporation of metallic zinc powder without the use of metal catalysts or additives. The as-grown nanowires had diameters in the range of 60-100 nm with lengths 5-15 μm. Detailed structural characterization indicated that the obtained nanowires are single-crystalline with a perfect hexagonal facet and surfaces. The room temperature PL spectrum exhibited strong UV emission, affirming that the as-grown products have good optical properties. The possible growth mechanism for the formation of hexagonal-faceted and perfect surface ZnO nanowires is also discussed.     

Electrochemical albumin sensing based on silicon nanowires modified by gold nanoparticles

Si nanowires (SiNWs) were modified by Au nanoparticles (AuNPs) using a self-assembled monolayer of aminopropyltriethoxysilane (APTES) and used for direct sensing of the bovine serum albumin (BSA). It was shown that repeated thermal treatment of the sensor greatly enhanced the reliability of the SiNW sensor by increasing the electrical conductivity largely from carbonization of the APTES molecules and from bringing the AuNPs in intimate contact with the SiNW surface. The AuNP-modified SiNW array sensor was able to detect 1-7 μM of BSA. The sensor exhibited a good sensitivity over the tested concentration range and linear behavior. It is expected that the proposed label-free biosensor can be further developed to selectively detect and quantify biomolecules other than BSA.     

Scanning tunneling microscopy investigations of silicon carbide nanowires

Scanning tunneling microscopy (STM) images have been obtained from the surfaces of silicon carbide nanowires produced in the thermolysis-induced carbonization of halocarbons (combustion synthesis). The morphology of the nanowires shows trench-like features perpendicular to the fibres’ axis, which is assigned to the existence of microfacets on their sidewalls. For the first time high-resolution STM images of the SiC nanowires are presented. The results are in agreement with the previous reports suggesting the presence of microfacets on the SiC whiskers’ surface.     

Use of tin as a surfactant material for the growth of thin silver films on silicon oxide

In this letter, we report on the use of tin as an effective surfactant material for silver growing on silicon oxide. We observed that submonolayers of Sn pre-deposited on SiO₂ result in earlier film coalescence and formation of smoother Ag layers. We suggest that Sn atoms reduce the Ag-adatom mobility resulting in experimentally observed increased island density and decreased film roughness. Angle-resolved X-ray photoelectron spectroscopy reveals that Sn remains under the Ag layer giving circumstantial evidence that at later stages of Ag film growth Sn does not influence the interlayer transport.     

An analysis on synthesizing large scales of one-dimensional silicon nano-structures by simple evaporation of sulfur-contained powders

The growth mechanism for synthesizing large scales of one-dimensional silicon nano-structures (silicon nano-wires (SiNWs) or silicon oxide nano-wires (SiO₂-NWs)) by a simple evaporation of sulfur-contained powders on silicon wafer is discussed. A novel sulfide-assisted mechanism referring to oxygen-assisted mechanism is proposed. Amongst this simple method, sulfide or pure sulfur can both assist the formation of SiNWs. The growth is fast and some SiNWs are easily oxidized to be amorphous structure of SiO₂-NWs under the low-vacuum system. The simple method suggests a useful route to achieve plenty of one-dimensional silicon nano-structures for further research.     

Comparison of confinement characters between porous silicon and silicon nanowires

Confinement character and its effects on photoluminescence (PL) properties are theoretically investigated and compared between porous silicon (p-Si) and silicon nanowires (Si-NWs). The method is based on the application of the tight-binding technique using the minimal sp³-basis set, including the second-nearest-neighbor interactions. The results show that the quantum confinement (QC) is not entirely controlled by the porosity, rather it is mainly affected by the average distance between pores (d). The p-Si is found to exhibit weaker confinement character than Si-NWs. The confinement energy of charge carriers decays against d exponentially for p-Si and via a power-law for Si-NWs. This latter type of QC is much stronger and is somewhat similar to the case of a single particle in a quantum box. The excellent fit to the PL data demonstrates that the experimental samples of p-Si do exhibit strong QC character and thus reveals the possibility of silicon clustering into nano-crystals and/or nanowires. Furthermore, the results show that the passivation of the surface dangling bonds by the hydrogen atoms plays an essential role in preventing the appearance of gap states and consequently enhances the optical qualities of the produced structures. The oscillator strength (OS) is found to increase exponentially with energy in Si-NWs confirming the strong confinement character of carriers. Our theoretical findings suggest the existence of Si nanocrystals (Si-NCs) of sizes 1-3 nm and/or Si-NWs of cross-sectional sizes in the 1-3 nm range inside the experimental p-Si samples. The experimentally-observed strong photoluminescence from p-Si should be in favor of an exhibition of 3D-confinement character. The favorable comparison of our theoretical results with the experimental data consolidates our above claims.     

Interplay between phonon confinement effect and anharmonicity in silicon nanowires

Getting light out of silicon is a difficult task since the bulk silicon has an indirect energy electronic band gap structure. It is expected that this problem can be circumvented by silicon nanostructuring, since the quantum confinement effect may cause the increase of the silicon band gap and shift the photoluminescence into the visible energy range. The increase in resulting structural disorder also causes the phonon confinement effect, which can be analyzed with a Raman spectroscopy. The large phonon softening and broadening, observed in silicon nanowires, are compared with calculated spectra obtained by taking into account the anharmonicity, which is incorporated through the three and four phonon decay processes into Raman scattering cross-section. This analysis clearly shows that the strong shift and broadening of the Raman peak are dominated by the anharmonic effects originating from the laser heating, while confinement plays a secondary role.