Porous silicon/carbon composites as anodes for high-performance lithium-ion batteries

Silicon anodes are promising for use in lithium-ion batteries.However,their practical application is severely limited by their large volume expansion leading to irreversible material fracture and electrical disconnects.This study proposes a new top-down strategy for preparing microsize porous silicon and introduces polyacrylonitrile(PAN)for a nitrogen-doped carbon coating,which is designed to maintain the internal pore volume and lower the expansion of the anode during lithiation and delithiation.We then explore the effect of temperature on the evolution of the structure of PAN and the electrochemical behavior of the composite electrode.After treatment at 400℃,the PAN coating retains a high nitrogen content of 11.35 at%,confirming the presence of C—N and C—O bonds that improve the ionic-electronic transport properties.This treatment not only results in a more intact carbon layer structure,but also introduces carbon defects,and produces a material that has remarkable stable cycling even at high rates.When cycled at 4 A g^(-1),the anode had a specific capacity of 857.6 mAh g^(-1) even after 200 cycles,demonstrating great potential for high-capacity energy storage applications.

Ag Deposition Forms and Uniformity on Porous Silicon by Electrochemical Method

The electrochemical deposition technique was applied to achieve porous silicon （PS） surface passivated with Ag deposition for improving the properties of PS photoluminescence. The relation of Ag depositing forms to current density and the effect of PS hydrophilic surface on deposition uniformity were investigated. The experimental results indicated that there were two critical current densities （maximum and minimum） in which Ag was absent and electroplated on PS surface correspondingly, and the range of current density for deposition of Ag on porous silicon was from 50 μA/cm^2 to 400 μA/cm^2. The process of changing PS surface from hydrophobic into hydrophilic had positive effect on Ag deposition uniformity. Under the same experimental conditions, PS hydrophobic surface presented uneven Ag deposition.However, hydrophilic surface treated with SC-1 solution was even. Finally, the effect of PS surface passivation with Ag even deposition on photoluminescence intensity and stabilization of PS was studied. It was discovered that Ag passivation inhibited the degradation of PL intensity effectively. In addition, excessive Ag deposition had a quenching effect on room-temperature visible photoluminescence of PS.

Analysis and Design of an Accelerometer Fabricated with Porous Silicon as Sacrificial Layer

A piezoresistive silicon accelerometer fabricated by a selective,self-stopping porous silicon (PS) etching method using an epitaxial layer for movable microstructures is described and analyzed.The technique is capable of constructing a microstructure precisely.PS is used as a sacrificial layer,and releasing holes are etched in the film.TMAH solution with additional Si powder and (NH_4)_2S_2O_8 is used to remove PS through the small releasing holes without eroding uncovered Al.The designed fabrication process is full compatible with standard CMOS process.

Fabrication of Silicon Condenser Microphone Using Oxidized Porous Silicon as Sacrificial Layer

A new technique to fabricate silicon condenser microphone is presented.The technique is based on the use of oxidized porous silicon as sacrificial layer for the air gap and the heavy p+-doping silicon of approximately 15μm thickness for the stiff backplate.The measured sensitivity of the microphone fabricated with this technique is in the range from -45dB(5.6mV/Pa) to -55dB(1.78mV/Pa) under the frequency from 500Hz to 10kHz,and shows a gradual increase at higher frequency.The cut-off frequency is above 20kHz.

BLUE－VIOLET LIGHT EMISSION FROM POROUS SILICON

Porous silicon samples are made on Si wafers with different resistivities under different anod ic-react ion conditions. Visible photoluminescent spectra of porous silicon (PS) at room temperature are measured using a fluorescent spectrograph where blue-violet light is observed. The decision of the resistivity of Si substrates is provided.

Transfer of Thin Epitaxial Silicon Films by Wafer Bonding and Splitting of Double Layered Porous Silicon for SOI Fabrication

A double layered porous silicon with different porosity is formed on a heavy doped p type Si(111) substrate by changing current density during the anodizing.Then a high quality epitaxial mono crystalline silicon film is grown on the porous silicon using an ultra high vacuum electron beam evaporator.This wafer is bonded with other silicon wafer with a thermal oxide layer at room temperature.The bonded pairs are split along the porous silicon layer during subsequent thermal annealing.Thus the epitaxial Si film is transferred to the oxidized wafer to form a silicon on insulator structure.SEM,XTEM,spreading resistance probe and Hall measurement show that the SOI structure has good structural and electrical quality.

A Novel Technology for Post-Treating of Porous Silicon Thick Films in H_2O_2

The solution of H 2O 2 is proposed to post-treat thick porous silicon (PS) films.The prepared PS film as the cathode is applied about 10mA/cm 2 current in mixture of ethanol,HF,and H 2O 2 solutions,which is expected to improve the stability and the smoothness of the surface and the mechanical property of the thick porous silicon films.The microstructure of the PS thick films with thicknesse of 20μm and 70μm has been studied.The SEM images show significant improved smoothness on surface of PS films,and XRD spectra suggest the formation of oxide layer after post-treating in H 2O 2.

Porosity Determination Equation for Porous Silicon

Through the studying of the carriers moving of the porous and the definition of S BET ,the equation of the relationship among the porosity,the current density and the etching speed can be deduced.Here,it is shown that for porous silicon made from p type silicon,there is a universal relationship,it is possible to determine the change in porosity with respect to etching under a set etching current density.This relationship is checked against experimental data from several reports on these etching parameters,and they confirm the validity.

Theoretical Study of Interaction Between S2 and SiHx （x=1, 2, 3） in Porous Silicon

The interaction between S2 molecule and SiHx （x=1, 2, 3） in porous silicon is investigated using the B3LYP method of density functional theory with the lanl2dz basis set. The model of porous silicon doped with CH3, Si-O-Si and OH species is built. By analyzing the binding energy and electronic transfer, we conclude that the interaction of S2 molecule with SiHx （x=1, 2, 3） is much stronger than the interaction of S2 molecule with CH3 and OH, as S2 molecule is located in different sites of the model. Using the transition state theory, we study the Si2H6＋S2→H3SiH2SiS＋HS reaction, and the reaction energy barrier is 50.2 kJ/mol, which indicates that the reaction is easy to occur.

Pore structure of unidirectional solidified lotus-type porous silicon

Lotus-type porous silicon with elongated pores was fabricated by unidirectional solidification under pressurized hydrogen. Porosity, pore diameter, and pore length can be adjusted by changing solidification speed and hydrogen pressure. The porosity of the ingot is nearly constant under different solidification speeds, but decreases with the increase of hydrogen pressure. The overall porosities of ingots fabricated at different hydrogen pressures were evaluated through a theoretical model. Findings are in good agreement with experimental values. The average pore diameter and pore length increase simultaneously while the average pore aspect ratio changes slightly with the decreases of solidification speed and hydrogen pressure. The average pore length is raised from 7 to 24 mm and the pore aspect ratio is raised from 8 to 20 respectively with the average pore diameter promoted by about 0.3 mm through improving the superheat degree of the melt from 200 to 300 K.

Structural, Optical and Electrical Properties of ZnS/Porous Silicon Heterostructures

ZnS films are deposited by pulsed laser deposition on porous silicon （PS） substrates formed by electrochemical anodization of p-type （100） silicon wafer. Scanning electron microscope images reveal that the surface of ZnS films is unsmoothed, and there are some cracks in the ZnS films due to the roughness of the PS surface. The x-ray diffraction patterns show that the ZnS films on PS surface are grown in preferring orientation along cubic phase β-ZnS （111） direction. White light emission is obtained by combining the blue-green emission from ZnS films with the orange-red emission from PS layers. Based on the I-V characteristic, the ZnS/PS heterojunction exhibits the rectifying junction behaviour, and an ideality factor n is calculated to be 77 from the I-V plot.

Photoluminescence Properties of Porous Silicon Based on FZ(H) Si Wafer

The photoluminescence (PL) properties of porous silicon (PS) have been studied based on n-type single-crystal (111) silicon wafers (80-90 Omega .cm in the resistivity). The porous silicon layers (PSL) were created by anodizing the wafers with a denuded zone of 20-40 mum formed by neutron transmutation doping (NTD) and thermal treatment at 940 degreesC for 4 h and then 700 degreesC for 2 h, two-step heating of the floating-zone silicon (FZ Si) grown in a hydrogen (H,) ambience. By surface modification with stannic chloride or amine immersion and rapid thermal oxidation (RTO), the PL peak position from the PS can be qualitatively controlled factitiously. The as-prepared PS shows an orange-yellow luminescence, while the modified samples emit red, green and blue luminescence. Mechanisms for the different colors of the PL are discussed. Fourier transform infrared (FTIR) is carried out to analyze the differences in the structural configuration of the PS samples.

The light-enhanced NO_2 sensing properties of porous silicon gas sensors at room temperature

The NO2 gas sensing behavior of porous silicon（PS） is studied at room temperature with and without ultraviolet（UV） light radiation.The PS layer is fabricated by electrochemical etching in an HF-based solution on a p ＋-type silicon substrate.Then,Pt electrodes are deposited on the surface of the PS to obtain the PS gas sensor.The NO2 sensing properties of the PS with different porosities are investigated under UV light radiation at room temperature.The measurement results show that the PS gas sensor has a much higher response sensitivity and faster response-recovery characteristics than NO2 under the illumination.The sensitivity of the PS sample with the largest porosity to 1 ppm NO2 is 9.9 with UV light radiation,while it is 2.4 without UV light radiation.We find that the ability to absorb UV light is enhanced with the increase in porosity.The PS sample with the highest porosity has a larger change than the other samples.Therefore,the effect of UV radiation on the NO2 sensing properties of PS is closely related to the porosity.

Plasmonic characteristics of suspended graphene-coated wedge porous silicon nanowires with Ag partition

We investigate a graphene-coated nanowire waveguide(GCNW) composed of two suspended wedge porous silicon nanowires and a thin Ag partition. The plasmonic characteristics of the proposed structure in terahertz(THz) frequency band are simulated by the finite element method(FEM). The parameters including the gap between the nanowires and Ag partition, the height of the nanowire, the thickness of the Ag partition, and the Fermi level of graphene, are optimized. The simulation results show that a normalized mode field area of ~10-4 and a figure of merit of ~100 can be achieved. Compared with the cylindrical GCNW and isolated GCNW, the proposed wedge GCNW has good electric field enhancement.A waveguide sensitivity of 32.28 is obtained, which indicates the prospects of application in refractive index(RI) sensing in THz frequency band. Due to the adjustable plasmonic characteristics by changing the Fermi level(EF), the proposed structure has promising applications in the electro-optic modulations, optical interconnects, and optical switches.

Preparation and room temperature NO_2-sensing performances of porous silicon/V_2O_5nanorods

In this paper, porous silicon/V205 nanorod composites are prepared by a heating process of as-sputtered V film on porous silicon （PS） at 600 ℃ for different times （15, 30, and 45 min） in air. The morphologies and crystal structures of the samples are investigated by field emission scanning electron microscope （FESEM）, x-ray diffractometer （XRD）, x-ray photoelectron spectroscopy （XPS）, and Raman spectrum （RS）. An improved understanding of the growth process of V205 nanorods on PS is presented. The gas sensing properties of samples are measured for NO2 gas of 0.25 ppm-3 ppm at 25 ℃. We investigate the effects of the annealing time on the NO2-sensing performances of the samples. The sample obtained at 600 ℃ for 30 min exhibits a very strong response and fast response-recovery rate to ppm level NO2, indicating a p-type semiconducting behavior. The XPS analysis reveals that the heating process for 30 rain produces the biggest number of oxygen vacancies in the nanorods, which is highly beneficial to gas sensing. The significant NO2 sensing performance of the sample obtained at 600 ℃ for 30 rain probably is due to the strong amplification effect of the heterojunction between PS and V205 and a large number of oxygen vacancies in the nanorods.

RECTIFYING EFFECT OF POLYANILINE(PANI)/N-TYPE POROUS SILICONE HETEROJUNCTION

Heterojunctions between polyaniline (PANI) and n-type porous silicon (PS), Al/PS-PANI/Au cell, were fabricated, and the rectifying parameters of this heterojunction diode were measured as a function of the preparation conditions of PANI and PS, the electronic structure of PANI as well as cell structure. The rectifying parameters of Al/PS-PANI/Au cell were determined to be gamma = 1.8x10(1) similar to 1.0x10(5) for the rectifying ratio at 3V, n = 3 similar to 12 for the ideal factor, j(0) = 8.0x10(-5) similar to 5.6x10(-2) mA/cm(2) for the reversed saturated current density, and phi(0) = 0.67 similar to 0.83 V for the barrier height, respectively. The best rectifying heterojunction diode made between PANI and n-type PS with higher rectifying factor (gamma = 1.0x10(5) at 3V), output current (>1500 mA/cm(2) at 3V) and lower ideal factor (n = 3.3) was obtained by preventing the oxidation of PS before evaporating Al electrode.

Influence of polarized bias and porous silicon morphology on the electrical behavior of Au-porous silicon contacts

This paper reports the surface morphology and I-V curves of porous silicon （PS） samples and related devices. The observed fabrics on the PS surface were found to affect the electrical property of PS devices. When the devices were operated under different external bias （10 V or 3 V） for 10 min, their observed obvious differences in electrical properties may be due to the different control mechanisms in the A1/PS interface and PS matrix morphology.

Fabrication of CoFe_2O_4 ferrite nanowire arrays in porous silicon template and their local magnetic properties

CoFe_2O_4 ferrite nanowire arrays are fabricated in porous silicon templates. The porous silicon templates are prepared via metal-assisted chemical etching with gold（Au） nanoparticles as the catalyst. Subsequently, CoFe_2O_4 ferrite nanowires are successfully synthesized into porous silicon templates by the sol–gel method. The magnetic hysteresis loop of nanowire array shows an isotropic feature of magnetic properties. The coercivity and squareness ratio（M_r/M_s） of ensemble nanowires are found to be 630 Oe（1 Oe = 79.5775 A·m^（-1） and 0.4 respectively. However, the first-order reversal curve（FORC） is adopted to reveal the probability density function of local magnetostatic properties（i.e., interwire interaction field and coercivity）. The FORC diagram shows an obvious distribution feature for interaction field and coercivity. The local coercivity with a value of about 1000 Oe is found to have the highest probability.

Experimental Study on Mechanical Properties of Micro-Structured Porous Silicon Film

Mechanical properties of micro-structured porous silicon film （PS） were studied combining X-ray diffraction with micro-Raman spectroscopy. The micro-structured porous silicon samples with different porosities ranging from 30.7700 to 96.2500 were obtained by chemical etching. Lattice parameters of the samples were measured using X-ray diffraction and its maximal change is up to （1.000.） This lattice mismatch with the bulk silicon substrate may introduce residual stress to the porous film. The residual stress measurement by micro-Raman spectroscopy reveals that the maximum of tensile residual stress has reached GPa level in the porous film. Moreover, the lattice mismatch and its corresponding residual stress are increasing with the porosity of PS, but average （elastic） modulus is about 14.5 GPa, one order of magnitude lower than that of substrate Si. The mechanical properties of PS have a close relation with its micro-pore structure.

Self-diffractionin Porous Silicon/PMMA-DR1 Composite Fil ms

Two-layer structure consisting of PS/PMMA-DR1 composite film planar waveguide layer on porous silicon cladding layer was fabricated in our experiment. The induced grating based on the third nonlinear optical properties was formed by interaction of two Nd∶YAG laser beams at 1064nm in the porous silicon/PMMA-DR1 waveguide. The diffraction efficiency of the first order diffracted light is measured to be about 0.2% of the total output.