Tunable photoluminescence of porous silicon by liquid crystal infiltration

The photoluminescence (PL) of porous silicon films has been investigated as a function of the amount of liquid crystal molecules that are infiltrated into the constricted geometry of the porous silicon films. A typical nematic liquid crystal 4-pentyl-4′-cyanobiphenyl was employed in our experiment as the filler to modify the PL of porous silicon. It is found that the originally red PL of porous silicon films can be tuned to blue by simply adjusting the amount of liquid crystal molecules in the microchannels of the porous films. The chromaticity coordinates are calculated for the recorded PL spectra. The mechanism of the tunable PL is discussed. Our results have demonstrated that the luminescent properties of porous silicon films can be efficiently tuned by liquid crystal infiltration.     

多孔硅拉曼光谱的研究

采用电化学腐蚀的方法制备多孔硅。对不同实验条件下所得到的多孔硅的拉曼光谱进行了分析，确认多孔硅是具有纳米晶结构特征的材料，肯定了量子限制效应在多孔硅光致发光中的作用。     

Comparative investigation of photoluminescence of silicon wire structures and silicon oxide films

Photoluminescence spectra and their dependence on temperature as well as Raman scattering spectra and Atomic Force Microscopy investigations have been used to study the peculiarities of the red photoluminescence band in low-dimensional Si structures, such as porous silicon and silicon oxide films. It has been shown that the red photoluminescence band of porous silicon is complex and can be decomposed into two elementary bands. It was discovered that elementary band intensities depend very much on surface morphology of porous silicon. The same positions of the photoluminescence bands are also observed in silicon oxide films for different oxide composition. Comparative investigation of the PL temperature dependences in porous silicon and silicon oxide films indicates that silicon-oxide defect related mechanisms of some elementary photoluminescence bands are involved.     

有机吸附物对多孔硅微腔发光的影响

理论上,采用Bruggeman有效介质近似,研究了有机吸附物对多孔硅微腔的折射率及其光致发光谱的影响.实验上,采用计算机控制的电化学腐蚀法制备了多孔硅微腔样品,并利用机械泵油的蒸气分子与该微腔样品进行相互作用.研究发现,多孔硅微腔发射的窄化光致发光谱对泵油蒸气分子的吸附与脱附很敏感,与之伴随的是该窄化光致发光谱发生明显的峰位移动(可达71nm)和强度变化.结合Bruggeman近似和表面态对多孔硅发光的影响,对实验结果进行了定性解释.实验结果与理论模拟结果符合较好. 关键词： Bruggeman近似 吸附物 多孔硅微腔 光致发光谱     

Optical properties of neodymium incorporated in porous silicon

We report the optical properties of Nd-incorporated porous silicon. Photoluminescence and photoluminescence excitation measurements have been performed. Room temperature emission spectra from dried or annealed samples have been studied. While steady-state photoluminescence results indicate porous silicon light absorption by the Nd, the photoluminescence excitation shows a deficiency of energy transfer between porous silicon and Nd ions.     

Influence of Carbon Multiple Treatments on the Photoelectrical Properties of Porous Glasses

We have studied the influence of multiple carbon treatments on the properties of silica porous glasses. Each step of each carbon treatment started with filling the voids of porous glass with carbon. During the following anneal carbon interacted with the walls of the voids. It was shown that low dimensional silicon clusters were formed inside the voids as a result of this reaction. In the experiments the photoluminescence spectra and conductivity of carbon-processed specimens were measured. The size-distribution of voids in porous glasses was calculated from absorption—desorption isotherms. An original technique was proposed that allowed to obtain the size-distribution of silicon clusters from the positions of peaks in the photoluminescence spectra. Correlation between the photoluminescence intensity and the sizes of pores was revealed. The observed oscillations in the shapes of the photoluminescence spectra in subsequent cycles of carbon treatment are explained by changes of the number of clusters corresponding to definite peaks in the size distribution spectra.     

金－多孔硅光致发光和红外光谱研究

报道了金－多孔硅的稳态光致发光，瞬态光致发光和傅里叶变换红外光谱的研究，讨论了金在多孔硅表面吸附产生的表面电子态对多孔硅光致发光特性的影响。     

发射可见光多孔硅的拉曼光谱

本文报导了多孔硅的拉曼散射和光致发光的研究。给出了多孔硅的拉曼和光致发光谱之间的对应关系,根据拉曼峰的移动,估算了多孔硅量子线横截面的平均尺度为2.1～4.2nm。     

Synchrotron investigations of the specific features in the electron energy spectrum of silicon nanostructures

The Si L _{2, 3} x-ray absorption near-edge structure (XANES) spectra of porous silicon nanomaterials and nanostructures with epitaxial silicon layers doped by erbium or containing germanium quantum dots are measured using synchrotron radiation for the first time. A model of photoluminescence in porous silicon is proposed on the basis of the results obtained. According to this model, the photoluminescence is caused by interband transitions between the energy levels of the crystalline phase and oxide phases covering silicon nanocrystals. The stresses generated in surface silicon nanolayers by Ge quantum dots or clusters with incorporated Er atoms are responsible for the fine structure of the spectra in the energy range of the conduction band edge and can stimulate luminescence in these nanostructures.     

Luminescence of porous silicon films with an europium-containing complex

We obtained porous silicon films modified at room temperature by an Eu³⁺-containing polymer complex. The most intense photoluminescence of Eu³⁺ implanted in the porous silicon was observed at the wavelengths of 611, 618, 691, and 704 nm. In this case, the intensity of the intrinsic photoluminescence of strongly irradiated specimens of porous silicon decreased, while the intensity of weakly emitting films multiply increased. An investigation of the photoexcitation spectra made it possible to establish the effect of Eu³⁺-containing complexes on the mechanism underlying the excitation of photoluminescence of porous silicon. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 4, pp. 499–501, July–August, 1997.     

Laser-induced etching of silicon

Conventional fabrication method of porous silicon is anodisation of single crystal silicon in hydrofluoric acid. In this report, we show that it is possible to fabricate porous silicon by laser-induced etching. An earlier report by us has demonstrated the dependence of porous silicon photoluminescence characteristic on the etching laser wavelength [1]. Here we used 780 nm line from a diode laser as the etching source, and the optimum etching conditions were obtained. A simple model was proposed to explain the etching process. Scanning Electron Microscope (SEM) images of the samples support the proposed process.     

多孔硅的后处理及其发光特性

采用一种新颖而简便的方法,改善多孔硅的发光特性。该方法包括酸处理和阴极还原两步。实验证明通过对多孔硅进行酸处理,能有效提高多孔硅的发光强度;通过对多孔硅进行阴极还原处理,能明显改善多孔硅的发光稳定性,而且发光强度也得到了提高。综合酸处理和阴极还原两技术的特点,对所制备的多孔硅立即先进行酸处理,然后再对其进行阴极还原处理,结果表明该方法能较好地提高多孔硅的发光效率和发光稳定性。而且还对其发光机制进行了探讨。     

Effect of surface modification by thermally oxidization and HF etching on UV photoluminescence emission of porous silicon

Photoluminescence of porous silicon (PS) is instable due perhaps to the nanostructure modification in air. The controllable structure modification processes on the as-prepared PS were conducted by thermal oxidization and/or HF etching. The PL spectra taken from thermally oxidized PS showed a stable photoluminescence emission of 355 nm. The photoluminescence emission taken from both of PS and oxidized porous silicon (OPS) samples etched with HF were instable, which can be reversibly recovered by the HF etching procedure. The mechanism of UV photoluminescence is discussed and attributed to the transformation of luminescence centers from oxygen deficient defects to the oxygen excess defects in the thermal oxidized PS sample and surface absorbed silanol groups on PS samples during the chemical etched procedure.     

Multiparametric sensor based on DBR porous silicon for detection of ethanol gas

We are here concerned with fabrication possibility of multiparametric gas sensor based on porous silicon. In order to use the porous silicon as a gas sensor, we made the DBR (distributed Bragg reflector) porous silicon onto silicon wafer and monitored the change of three parameters during exposure of DBR porous silicon to ethanol gas. The sensing parameters were the shift of reflectance peak, the PL (photoluminescence) intensity, and the electrical conductance. As a result, the spectra of reflectance and PL shifted toward longer wavelength. The electrical conductivity increases rapidly. After removing the gas, all sensing parameters return exactly to the initial value.     

Photophysical processes stimulated in nanoporous silicon by high-power laser radiation

Photoprocesses initiated on the surface of porous silicon irradiated with laser radiation with wavelengths (λ = 266, 337, and 532 nm) in a wide range of intensities (up to 2 × 10⁷W/cm²) were investigated. Laser-induced luminescence and laser mass-spectrometry were used as experimental procedures. X-ray reflection was used to determine the parameters of the porous silicon films. The photoluminescence spectra obtained at different wavelengths and low intensities were analyzed. This analysis showed that for an optically thin layer of porous silicon the luminescence spectrum does not depend on the wavelength of the exciting radiation. This indicates the existence of a separate system of levels in porous silicon that are responsible for the luminescence. The behavior of the photoluminescence spectra as a function of the intensity q of the exciting radiation was investigated. It was shown that the luminescence intensity is a nonlinear function of q. At high intensities of the exciting radiation, the luminescence intensity saturates and a short-wavelength shift of the spectra is observed; this is due to the high concentrations of photoexcited carriers. This increases the probability of the experimentally observed nonequilibrium photodesorption of H₂ and Si from the surface of porous silicon.     

Porous silicon bandgap broadening at natural oxidation

Emission and excitation photoluminescence spectra of porous silicon thin layers have been investigated at natural oxidation. The shift of both types of spectra to high-energy region with time has been shown. Analysis of excitation spectra points out the indirect behavior of electron transitions responsible for visible photoluminescence, which remains unaltered at natural oxidation. The value of optical bandgap is estimated in each case. It is shown that the optical bandgap broadens during oxidation due to size reduction of silicon nanocrystallites.     

Photoluminescence quenching of porous silicon by noble metal adsorbates

Ｐｈｏｔｏｌｕｍｉｎｅｓｃｅｎｃｅ ｑｕｅｎｃｈｉｎｇ ｏｆ ｐｏｒｏｕｓ ｓｉｌｉｃｏｎ ｂｙ ｎｏｂｌｅ ｍｅｔａｌ ａｄｓｏｒｂａｔｅｓ￥ＧＵＯＨｅｎｇｑｕｎ（ＤｅｐａｒｔｍｅｎｔｏｆＡｐｐｌｉｅｄＰｈｙｓｉｃｓ，ＨｕａｑｉａｏＵｎｉｙｅｒｓｉ...     

Effect of contact with air on the photoluminescence spectrum of porous silicon

A study has been made of the transformation of photoluminescence (PL) spectra of porous silicon (PS) induced by its ageing, including the early stages of contact with air. The sample was prepared under conditions that minimized this contact, and spectral measurements were carried out in a high vacuum or in liquid nitrogen. The PS PL spectra obtained under continuous measurement in high vacuum are always dominated by one emission band of PS nanoelements, which shifts toward shorter wavelengths with ageing by 150 nm. At 80 K, the band intensity is considerably higher than at 300 K, and this difference grows with ageing. Exposure of a sample to air for a few tens of seconds is long enough to strongly transform its time-resolved PL spectra, which is evidence of a change in the sample surface. The effect of immersion of PS samples in liquid nitrogen on PL spectra is associated not only with their cooling, but also with the field of adsorbed nitrogen molecules, whose influence becomes weaker with increasing thickness of the oxidized near-surface layer. The variation of the spectral properties and kinetics of the long-wavelength PS PL band with temperature, medium (liquid nitrogen or vacuum), and exposure time suggest that these factors affect carrier migration between silicon nanoelements.     

Correlation of excitation-wavelength dependent photoluminescence with the fractal microstructures of porous silicon

The correlation of the excitation-wavelength dependent photoluminescence with the fractal microstructures of porous silicon has been investigated. As the excitation wavelength increases from 340 to 650 nm, the photoluminescence of porous silicon redshifts from 500 to 780 nm. The excitation-wavelength dependent photoluminescence suggests the existence of a size distribution for the large number of silicon nanocrystallites in porous silicon. Using scanning electron microscopy and computer simulation, we have investigated the fractal features of the microstructures of porous silicon. Our results have demonstrated that the fractal features in the microstructures of porous silicon indicate the existence of a size distribution for the silicon nanocrystallites in porous silicon. The recorded excitation-wavelength dependent photoluminescence of porous silicon can be interpreted in terms of the bond-order-length-strength correlation theory.