Jiayou Feng

Synthesis and Photoluminescence Properties of Porous Silicon Nanowire Arrays

Herein, we prepare vertical and single crystalline porous silicon nanowires (SiNWs) via a two-step metal-assisted electroless etching method. The porosity of the nanowires is restricted by etchant concentration, etching time and doping lever of the silicon wafer. The diffusion of silver ions could lead to the nucleation of silver nanoparticles on the nanowires and open new etching ways. Like porous silicon (PS), these porous nanowires also show excellent photoluminescence (PL) properties. The PL intensity increases with porosity, with an enhancement of about 100 times observed in our condition experiments. A “red-shift” of the PL peak is also found. Further studies prove that the PL spectrum should be decomposed into two elementary PL bands. The peak at 850 nm is the emission of the localized excitation in the nanoporous structure, while the 750-nm peak should be attributed to the surface-oxidized nanostructure. It could be confirmed from the Fourier transform infrared spectroscopy analyses. These porous SiNW arrays may be useful as the nanoscale optoelectronic devices.

The fabrication of large-scale sub-10-nm core-shell silicon nanowire arrays

A combination of template-assisted metal catalytic etching and self-limiting oxidation has been successfully implemented to yield core-shell silicon nanowire arrays with inner diameter down to sub-10 nm. The diameter of the polystyrene spheres after reactive ion etching and the thickness of the deposited Ag film are both crucial for the removal of the polystyrene spheres. The mean diameter of the reactive ion-etched spheres, the holes on the Ag film, and the nanowires after metal catalytic etching exhibit an increasing trend during the synthesis process. Two-step dry oxidation and post-chemical etching were employed to reduce the diameter of the silicon nanowires to approximately 50 nm. A self-limiting effect was induced by further oxidation at lower temperatures (750°C ~ 850°C), and core-shell silicon nanowire arrays with controllable diameter were obtained.

The synthesis and photoluminescence properties of selenium-treated porous silicon nanowire arrays

Here we prepared vertical and single crystalline porous silicon nanowire (SiNW) arrays using the silver-assisted electroless etching method. The selenization was carried out by annealing the samples in vacuum with selenium atmosphere. The selenization treatment at 700u2009°C is useful for investigating the photoluminescence (PL) properties of porous SiNWs, with an enhancement of 30 times observed. The observed PL peaks blue-shift to 650 nm and the decomposition of the spectrum reveals that three PL bands with different origins are obtained. It is proved that selenization treatment could remove the Si-H bonds on the surface and form Si-Se bonds, which could increase the absorbance of the SiNWs and also enhance the stability of the PL intensity. These Se-treated porous SiNWs may be useful as nanoscale optoelectronic devices.

Indirect to direct band gap transition in ultra-thin silicon films

Free standing silicon layers undergo a transition from indirect to direct band gap semiconductor, which predicts a new possible way in silicon band gap engineering. The thickness and crystal orientation of the exposed surface are crucial. Our simulations reveal that the (100) films with thickness of ∼1.05 nm and (110) films with thickness of ∼1.14 nm could maintain the direct band gap structure. However, the (111) films always show indirect band gap structure even if the monolayer is constructed. The electron states density calculations were also carried out and the transition of the band gap structure is considered to be determined by the quantum confinement and surface termination conditions. The momentum matrix element calculations were also carried out, approving the effective direct band gap transitions for these ultra-thin films.

Formation of β-C3N4 phase in C-N films deposited by reactive ionized cluster beam method

Abstract Carbon nitride thin films have been prepared by the reactive ionized cluster beam (RICB) method using mixed beam of activated carbon and nitrogen atoms and atom clusters produced from polyethylene and NH3. The structure of the films has been characterized by electron diffraction analysis that shows strong evidences suggesting the formation of crystalline β-C3N4 phase embedded in the amorphous film matrix. Rutherford backscattering measurements show that average nitrogen content is up to 40%. X-ray photoelectron spectroscopy indicates that carbon and nitrogen form a weak polarized covalent bond in these C-N thin films.

First-Principles Study of the Band Gap Structure of Oxygen-Passivated Silicon Nanonets

A net-like nanostructure of silicon named silicon nanonet was designed and oxygen atoms were used to passivate the dangling bonds. First-principles calculation based on density functional theory with the generalized gradient approximation (GGA) were carried out to investigate the energy band gap structure of this special structure. The calculation results show that the indirect–direct band gap transition occurs when the nanonets are properly designed. This band gap transition is dominated by the passivation bonds, porosities as well as pore array distributions. It is also proved that Si–O–Si is an effective passivation bond which can change the band gap structure of the nanonets. These results provide another way to achieve a practical silicon-based light source.

Reactive partially ionized beam deposition of AlN thin films

Aluminum nitride films were synthesized on Si(111) wafer and quartz by the reactive partially ionized beam (RPIB) deposition technique. Under specific experimental conditions, polycrystalline and preferential crystalline AlN films of hexagonal structure were obtained by this method. The correlation between experimental parameters and the resulting structure as well as the stoichiometry of the AlN film are discussed.

Micro-arc oxidization fabrication and ethanol sensing performance of Fe-doped TiO2 thin films

In-situ pure TiO2 and Fe-doped TiO2 thin films were synthesized on Ti plates via the micro-arc oxidation (MAO) technique. The as-fabricated anatase TiO2 thin film-based conductometric sensors were employed to measure the gas sensitivity to ethanol. The results showed that Fe ions could be easily introduced into the MAO-TiO2 thin films by adding precursor K4(FeCN)6·3H2O into the Na3PO4 electrolyte. The amount of doped Fe ions increased almost linearly with the concentration of K4(FeCN)6·3H2O increasing, eventually affecting the ethanol sensing performances of TiO2 thin films. It was found that the enhanced sensor signals obtained had an optimal concentration of Fe dopant (1.28at%), by which the maximal gas sensor signal to 1000 ppm ethanol was estimated to be 7.91 at 275°C. The response time was generally reduced by doped Fe ions, which could be ascribed to the increase of oxygen vacancies caused by Fe3+ substituting for Ti4+.