Synthesis and characterization of mesoporous silicon directly from pure silica sodalite single crystal

Mesoporous silicon granules with high surface area were synthesized directly from pure silica sodalite single crystals, with the starting shape retained. The sodalite single crystals were reduced by a magnesiothermal process in vacuum at 630 °C. The X-ray diffraction patterns indicate the presence of crystal silicon. Transmission electron microscopy studies reveal that the obtained silicon granules are composed of a monocrystalline surface with an island-like mesoporous internal structure. The results of N₂ adsorption and desorption analysis indicate that the surface area is around 308 m² g⁻¹ and the single point pore volume is 0.37 cm³ g⁻¹. The photoluminescence emission centered at 2.7 eV may be due to both an oxidized surface and quantum confinement effects. These results reveal that the silicon granules possess a different microstructure from those of etched silicon films. The present synthetic design correlates the microporous zeolite and mesoporous silicon together and gives a new way for enlarging the structural diversity of porous silicon crystal.     

Low mechanical pressure during electrochemical etching: induced modification in optical and structural properties of n-type porous silicon

In this study, n-type porous silicon (PS) layers are formed in the dark with the assistance of a low mechanical pressure during electrochemical etching process. Pressure-induced stress/strain modifies the resistivity of the silicon substrate to enhance the etching process. Under the same equivalent etching condition, pressure-assisted etching can yield PS layer with stronger room temperature photoluminescence intensity than the layer formed by ordinary electrochemical etching. The porosity of pressure-assisted etched PS layers is found to be much higher than that of ordinary etched layer. Fourier transformation infrared absorption spectroscopy and grazing incidence X-ray diffraction measurements and analysis show that application of the pressure during electrochemical etching promotes the degree of oxidation and reduces the crystallites size of the PS layer. The effect of the pressure during etching process on the surface topography of PS is revealed by scanning electron microscopy imaging.     

Nanostructured GaN on silicon fabricated by electrochemical and laser-induced etching

Nanostructured GaN layers have been fabricated by electrochemical and laser-induced etching (LIE) processes based on n-type GaN thin films grown on the Si (111) substrate with AlN buffer layers. The effect of varying current and laser power density on the morphology of the GaN layers is investigated. The etched samples exhibited a dramatic increase in photoluminescence intensity as compared to the as grown samples. The average diameter of the GaN crystallites was about 7-10 nm, as determined from the PL data The Raman spectra also displayed stronger intensity peaks, which were shifted and broadened as a function of etching parameters. A strong band at 522 cm^− 1 is from the Si (111) substrate, and a small band at 301 cm^− 1, due to the acoustic phonons of Si. Two Raman active optical phonons are assigned h-GaN at 139 cm^− 1 and 568 cm^− 1due to E2 (low) and E2 (high) respectively.     

Microstructural characterisation of metallurgical grade porous silicon nanosponge particles

Porous silicon finds numerous applications in the areas of bio-technology, drug delivery, energetic materials and catalysis. Recent studies by Vesta Sciences have led to the development of porous silicon nanosponge particles from metallurgical grade silicon powder through their own patented chemical etching process (Irish patent no. IE20060360). This discovery paves the way for a more economical production method for porous silicon. The study presented here studies the structural morphology of the porous silicon nanosponge particles using high resolution electron microscopy techniques combined with porisometry type measurements, where appropriate. The related surface pore structure is examined in detail using Scanning Electron Microscopy and Transmission Electron Microscopy techniques while the internal pore structure is explored using Focused Ion Beam milling and ultramicrotomed cross-sections. Three samples of the silicon particles were analysed for this study which include the starting metallurgical grade silicon powder and two samples that have been chemically etched. Analysis of the etched samples indicates a disordered pore structure with pore diameters ranging up to 15 nm on porous silicon particles ranging up to 5 μm in size. Crystallographic orientation did not appear to affect the surface pore opening diameter. Internal pore data indicated pore depths of up to 1 μm dependant on the particle size and etching conditions applied.     

Fabrication of large area high density, ultra-low reflection silicon nanowire arrays for efficient solar cell applications

High density vertically aligned and high aspect ratio silicon nanowire (SiNW) arrays have been fabricated on a Si substrate using a template and a catalytic etching process. The template was formed from polystyrene (PS) nanospheres with diameter 30–50 nm and density 10¹⁰/cm², produced by nanophase separation of PS-containing block-copolymers. The length of the SiNWs was controlled by varying the etching time with an etching rate of 12.5 nm/s. The SiNWs have a biomimetic structure with a high aspect ratio (∼100), high density, and exhibit ultra-low reflectance. An ultra-low reflectance of approximately 0.1% was achieved for SiNWs longer than 750 nm. Well-aligned SiNW/poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS) heterojunction solar cells were fabricated. The n-type silicon nanowire surfaces adhered to PEDOT:PSS to form a core-sheath heterojunction structure through a simple and efficient solution process. The large surface area of the SiNWs ensured efficient collection of photogenerated carriers. Compared to planar cells without the nanowire structure, the SiNW/PEDOT:PSS heterojunction solar cell exhibited an increase in short-circuit current density from 2.35 mA/cm² to 21.1 mA/cm² and improvement in power conversion efficiency from 0.4% to 5.7%.      

Optical and microstructural investigations of porous silicon

Raman scattering and photoluminescence (PL) measurements on (100) oriented n-type crystalline silicon (c-Si) and porous silicon (PS) samples were carried out. PS samples were prepared by anodic etching of c-Si under the illumination of light for different etching times of 30, 60 and 90 min. Raman scattering from the optical phonon in PS showed the redshift of the phonon frequency, broadening and increased asymmetry of the Raman mode on increasing the etching time. Using the phonon confinement model, the average diameter of Si nanocrystallites has been estimated as 2.9, 2.6 and 2.3 nm for 30, 60 and 90 min samples, respectively. Similar size of Si crystallites has been confirmed from the high resolution transmission electron microscopy (HRTEM). Using 2TO phonon mode intensity, we conjectured that the disordered Si region around the pores present in 30 min PS dissolved on etching for 90 min. The photoluminescence (PL) from PS increased in intensity and blue shifted with etching time from 2.1–2.3 eV. Blue shifting of PL is consistent with quantum confinement of electron in Si nanocrystallites and their sizes are estimated as 2.4, 2.3 and 2.1 nm for 30, 60 and 90 min PS, respectively which are smaller than the Raman estimated sizes due to temperature effect. Unambiguous dominance of quantum confinement effect is reported in these PS samples.     

Raman temperature dependence analysis of carbon-doped titanium dioxide nanoparticles synthesized by ultrasonic spray pyrolysis technique

Carbon-doped titanium dioxide nanoparticles were prepared by ultrasonic spray pyrolysis technique using titanium tetra-ethoxide as a precursor and glucose as a dopant. The as-synthesized nanoparticles were then characterized using high resolution transmission electron microscopy for the structural properties and temperature dependence Raman spectroscopy for the optical properties. High resolution transmission electron microscopy analysis shows that the ultrasonic synthesized carbon-doped titanium dioxide nanoparticles have an interplanar d-spacing of 0.352 nm, a value close to 0.374 nm of the pure undoped anatase titanium dioxide (bulk). The fast Fourier transform (FFT) of the selected electron diffraction images, of the selected areas, display the fact that only the main diffraction (reflection) plane of Miller indices (101) in titanium dioxide is responsible for diffraction pattern. Raman spectroscopy confirms the titanium dioxide polymorph to be anatase with the intense phonon frequency at 153 cm⁻¹ blue-shifted from 141 cm⁻¹ due to both carbon doping and particle size decrease. The temperature dependence analysis of the spectra shows an initial linear dependence of the Raman shift for the E _g(1) mode at 152.7 cm⁻¹ with increase in temperature up to a critical temperature 450 °C, after which we observe a decrease with increase in temperature. The other Raman modes [B _1g and E _g(3)] exhibit a different temperature dependence in that the B _1g displays a somewhat sinusoidal behavior and the E _g(3) mode shows a linear decrease of Raman shift with an increase in temperature. Temperature dependence analysis of peak width for the E _g(1) indicates the peak width of the as prepared nanoparticle to be 20 cm⁻¹ which is far much larger than that for single crystal of 7 cm⁻¹ at room temperature.     

Step voltage with periodic hold-up etching: A novel porous silicon formation

A novel etching method for preparing light-emitting porous silicon (PS) is developed. A gradient steps (staircase) voltage is applied and hold-up for different periods of time between p-type silicon wafers and a graphite electrode in HF based solutions periodically. The single applied staircase voltage (0–30 V) is ramped in equal steps of 0.5 V for 6 s, and hold at 30 V for 30 s at a current of 6 mA. The current during hold-up time (0 V) was less than 10 μA. The room temperature photoluminescence (PL) behavior of the PS samples as a function of etching parameters has been investigated. The intensity of PL peak is initially increased and blue shifted on increasing etching time, but decreased after prolonged time. These are correlated with the study of changes in surface morphology using atomic force microscope (AFM), porosity and electrical conductance measurements. The time of holding-up the applied voltage during the formation process is found to highly affect the PS properties. On increasing the holding-up time, the intensity of PL peak is increased and blue shifted. The contribution of holding-up the applied steps during the formation process of PS is seen to be more or less similar to the post chemical etching process. It is demonstrated that this method can yield a porous silicon layer with stronger photoluminescence intensity and blue shifted than the porous silicon layer prepared by DC etching.     

直流电化学腐蚀法制备多孔硅的表面形貌研究

采用正交实验，直流电化学腐蚀法制备多孔硅。用原子力显微镜对表面进行观察，研究电化学腐蚀参数对其表面形貌的影响。氢氟酸浓度（CHF）升高，使临界电流密度（JPS）增大，有利于多孔硅的形成。电流密度（J）增大，多孔硅的孔隙率和孔径随之变大，而其纳米粒径将变小。腐蚀时间（t）越长，孔径越大，孔越深。     

Preparation and characterization of polystyrene/polycarbonate composite hollow microspheres by microencapsulation method

Polystyrene/polycarbonate (PS/PC) composite hollow microspheres were successfully prepared via microencapsulation method. Fourier-transform infrared spectroscopy, scanning electron microscopy (SEM), differential scanning calorimetry, and thermogravimetric analysis were used for the characterization of the obtained hollow microspheres. SEM images showed that there were a big cavity and some small cavities inside the composite hollow microspheres, and the hollow microspheres prepared at 42 °C presented better morphology and smaller size distribution compared with that prepared at higher temperature of solvent evaporation. The tap density of the composite hollow microspheres increased from 0.28 g cm⁻³ at PS/PC composite concentration of 5 wt% in oil phase to 0.42 g cm⁻³ at concentration of 11.7 wt%. The mean diameter of the composite hollow microspheres ranged from 13 to 528 μm. It increased with an increase in the concentration of composite in oil phase and decreased with increasing the second rotating speed. Thermal analysis showed that the composite hollow had thermal stability below 358 °C.     

Effects of macropore size on structural and electrochemical properties of hierarchical porous carbons

Hierarchical porous carbons (HPCs) were synthesized by a colloid crystal template method with phenolic resin as carbon source and triblock copolymer Pluronic F127 as a soft template. The obtained HPCs with tunable macropore size of 242–420 nm exhibit large BET surface areas (~900 m² g⁻¹) and large pore volumes (~1.2 cm³ g⁻¹). With an increase in the diameters of silica template, the BET surface areas and pore volumes of HPCs decrease. The electrochemical properties of the HPCs with various macropore sizes used as supercapacitor electrodes materials were evaluated using cyclic voltammetry, galvanostatic charge–discharge, and electrochemical impedance spectroscopy techniques. The results show the HPC with the macropore size of 242 nm possesses the largest specific capacitance among the HPCs. The excellent capacitive behavior of HPC-242 can be attributed to its faster ion transport behavior and better ion-accessible surface area.     

Influence of annealing on the electrochemical behavior of finemet amorphous and nanocrystalline alloy

The electrochemical corrosion behavior of finemet alloy at various heat treatment temperatures was investigated. Thermal behavior and structural changes were studied using differential scanning calorimetry and X-ray diffractometry, respectively. The electrochemical corrosion of amorphous and annealed samples was investigated in 0.10 M NaOH solution using electrochemical impedance spectroscopy and linear sweep voltammetery. Changes in morphology of the samples before and after corrosion were characterized using optical microscope. The results showed that structural relaxation and nanocrystallization during the heat treatment improved corrosion behavior of the alloy. The heat-treated alloy at 650 °C showed a corrosion rate of 1.37 × 10⁻⁸ A cm⁻² and a positive shift of +417 mV in the corrosion potential compared to the amorphous alloy. Also, the heat-treated alloy at 650 °C showed a higher charge transfer resistance up to 50 kΩ due to corrosion resistance, compared with amorphous sample that showed a charge transfer resistance of 0.5 kΩ.     

Structural properties of silver particulate films deposited on softened polymer blends of polystyrene/poly (2-vinyl pyridine)

Results of the structural studies of silver particulate films deposited at a rate of 0.4 nm/s on polymeric blends of polystyrene/poly (2-vinyl pyridine), PS/P2VP held at a temperature 457 K by evaporation in a vacuum of 8 × 10⁻⁶ Torr are reported here. The morphology of silver particulate films, characterized by their size, size distribution, shape and inter-particle separation, was observed to modify due to blending of PS with P2VP and amount of silver deposited. The red shift in the plasmon resonance indicates the effect of blending P2VP with PS. Scanning electron microscopy was used to study the change in morphology of the silver nanoparticles in correlation with the optical properties of silver particulate films on PS/P2VP blends. The silver nanoparticles on the thin layers of polymer blends exhibited much smaller, narrower dispersion and wide size distribution due to blending of PS with P2VP.     

Preparation and characterizations of tungsten oxide electrochromic nanomaterials

Nanoscaled tungsten oxide thin films were fabricated by galvanostatic electrodeposition. The effect of preparation parameters such as tungsten ions concentration, pH, current density and annealing on the properties and performance of WO₃ thin films electrochromic materials was investigated. XRD, SEM–EDS, TEM, FTIR, UV–VIS spectrophotometry, and electrochemical measurements were utilized to characterize the structural and compositional properties as well as the electrochromic behaviour of the prepared thin films. Triclinic WO₃ structure was prepared at 0.1 M W⁺ and current density of 0.5 mA cm⁻², while at 0.2 M W⁺ and 1 mA cm⁻², orthorhombic structure was revealed. High energy gap of 3.5 eV with diffusion coefficient of 6.81 × 10⁻¹¹ cm² S⁻¹ and coloration efficiency of 62.68 cm² C⁻¹ were obtained for the films prepared at pH 2, 1 mA cm⁻², and 0.1 M W⁺.     

Comparison of Ni/Ti and Ni ohmic contacts on <Emphasis Type="Italic">n</Emphasis>-type 6H–SiC

Ni (50 nm)/Ti (10 nm) and Ni (50 nm) contact structures were deposited by vacuum evaporation on n-type 6H–SiC with various doping level. Prior to deposition, part of the substrates had been subjected to plasma cleaning. To achieve ohmic character, the samples were annealed in vacuum. Electrical parameters of the contacts were determined by measuring specific contact resistances. The results were similar for both contact structures; we have not found any influence of plasma cleaning. The lowest value was 1.4 × 10⁻⁴ Ω cm² for substrate with doping level of 1.9 × 10¹⁹ cm⁻³. Using XPS depth profiling it was found, that the titanium layer was shifted upon annealing of the Ni/Ti structures from the interface towards contact surface and that this layer consists of TiC. Between the TiC layer and the substrate was a layer of nickel silicides and carbon. In the plasma-cleaned Ni/Ti/SiC samples, increased content of nickel at the expense of carbon was detected just below the TiC layer. We suggest the snowplow effect of dopants in the SiC substrate upon annealing of the structures as a main factor in ohmic contact formation.     

Anodic etching of p-type cubic silicon carbide

p-Type cubic silicon carbide was anodically etched using an electrolyte of HF:HCl:H₂O. The etching depth was determined versus time with a fixed current density of 96.4 mA cm^–2. It was found that the etching was very smooth and very uniform. An etch rate of 22.7 nm s^–1 was obtained in a 1:1:50 HF:HCl:H₂O electrolyte.     

Formation of In nanoparticles on InP wafers by laser-assisted etching

Indium nanoparticles were formed by laser etching an InP (100) wafer in a 10% chlorine–helium atmosphere maintained at ~5–8 × 10⁻⁵ Torr. The wafer was irradiated by a homogenized ultraviolet beam with a series of 50–4500 pulses at a fluence of 230 mJ/cm². The surface was also irradiated using fluences from 50 to 340 mJ/cm² with 600 pulses. The irradiated surfaces were studied using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and Raman spectroscopy. Raman spectroscopy confirmed that the irradiated surface layer remains crystalline. According to EDS analysis, the surface particles are composed primarily of indium. SEM images show that the number of pulses and the pulse intensity can control the size distribution of the particles.     

Synthesis of mesoporous silicon directly from silicalite-1 single crystals and its response to thermal diffusion of ZnO clusters

This article describes the preparation of mesoporous silicon granules with a layered structure directly from silicalite-1 single crystals. The silicalite-1 single crystals were thermally reduced in vacuum at 630 °C, with the original shape retained. The samples are confirmed as crystalline silicon by X-ray diffraction and transmission electron microscope. The silicon granule is composed of a monocrystalline surface and polycrystalline layered interior. A surface area of around 66 m² g⁻¹ and the pore size centered at 3.7 nm were obtained from nitrogen porosimetry, BET and BJH analysis. The ZnO clusters have been loaded into the porous silicon granule by thermal diffusion method. The photoluminescence emission centered at 3.44 eV originates from the small particles of ZnO and the band at 2.81 eV may be due to both an oxidized surface and quantum confinement effects. The microstructure in this silicon granule is very different from those in etched samples. The synthetic design demonstrates an interesting way from the microporous zeolite to mesoporous silicon and enlarges the structural diversity of porous silicon crystal.     

Investigations on the structural, optical and electrical properties of Nb-doped SnO2 thin films

Niobium-doped tin oxide thin films were deposited on glass substrates by the chemical spray pyrolysis method at a substrate temperature of 400 °C. Effects of Nb doping on the structural, electrical and optical properties have been investigated as a function of niobium concentration (0–2 at.%) in the spray solution. X-ray diffraction patterns showed that the films are polycrystalline in nature and the preferred growth direction of the undoped film shifts to (200) for Nb-doped films. Atomic force microscopy study shows that the surface morphology of these films vary when doping concentration varies. The negative sign of Hall coefficient confirmed the n-type conductivity. Resistivity of ~4.3 × 10⁻³ Ω cm, carrier concentration of ~5 × 10¹⁹ cm⁻³, mobility of ~25 cm² V⁻¹ s⁻¹ and an average optical transmittance of ~70% in the visible region (500–800 nm) were obtained for the film doped with 0.5 at.% niobium.     

Characterization of carbon coatings on SiC monofilaments using Raman spectroscopy

The axial residual stresses in the carbon coatings deposited onto different silicon carbide monofilaments have been determined experimentally using Raman spectroscopy. The stress-dependent band shift for the carbon G-band at around 1600 cm⁻¹, due to symmetric in-plane stretching mode of graphite, has been found to be −1.6 cm⁻¹/GPa. Using this calibration, the axial residual stresses in carbon coatings can be estimated from measured band shifts between the broken end and middle of the monofilaments. It was found that the stresses in the coatings of all monofilaments were compressive and between −440 and −810 MPa. Modelling indicated that this was consistent with the coating stress arising from the difference in coefficients of thermal expansion of carbon and the underlying silicon carbide. The coating stress was measured as a function of distance from the broken monofilament end. It was found that the distance for the stress to build up varied greatly, from 40 μm in Ultra-SCS to 500 μm in SM1140+. This suggests there are significantly different shear stresses between the coatings and underlying silicon carbide in the different monofilaments.