The investigation of photoluminescence centers in porous alumina membranes

To investigate the origin of blue photoluminescence (PL) in porous alumina membrane, we have prepared an anodic aluminum oxide film with a pore diameter of 40 nm in oxalic acid solution by a two-step anodization process. Our results show that the as-prepared alumina membrane is amorphous and exhibits a broad emission band peaking at around 452 nm with two sub-peaks located at 443 nm and 470 nm. As indicated by the PL excitation spectra, there are two excitation peaks which can account for the sub-peaks observed in the PL emission band. We have proposed that the broad emission band with two sub-peaks can be attributed to two luminescence centers in porous alumina membranes, namely, oxygen defects and oxalic impurities. Furthermore, we have rationalized that the two emission centers show similar influence on the PL band in the blue region based on a simple model. PACS 81.07.-b; 81.16.Dn; 79.60.Jv     

多孔硅发光峰温度行为的研究

实验研究了多孔硅（Ｐｏｒｏｕｓ Ｓｉｌｉｃｏｎ）光致发光峰随测量温度的变化,发现发光峰位随温度的移动与发光峰的能量有关。随温度升高,发光峰波长较长的样品它们的发光峰都移向高能,面发光峰能量较高的样品它们的发光峰都移向低能,发光峰波长位于７４０ｎｍ附近的样品,它们的发光峰与测量温度无关。对上术结果的起源作了讨论。     

Effect of heat treatment on the structure of incorporated oxalate species and photoluminescent properties of porous alumina films formed in oxalic acid

The present work focuses on the use of IR spectroscopy and photoluminescence spectral measurements for studying the treatment temperature effect on the compositional and luminescent properties of oxalic acid alumina films. In line with the recent researches we have also found that heat treatment of porous alumina films formed in oxalic acid leads to considerable changes in their photoluminescence properties: upon annealing the intensity of photoluminescence (PL) increases reaching a maximum at the temperature of around 500 °C and then decreases. IR spectra of as-grown and heat-treated films have proved that PL emission in the anodic alumina films is related with the state of ‘structural’ oxalate species incorporated in the oxide lattice. These results allowed us to conclude that PL behavior of oxalic acid alumina films can be explained through the concept of variations in the bonding molecular orbitals of incorporated oxalate species including σ- and π-bonds.     

脉冲激光沉积法在多孔铝衬底上生长的ZnO薄膜的结构与光学性质

ZnO是一种性质优良很有前途的紫外光电子器件材料,多孔铝是一种良好的模板型衬底,试图将二者结合起来以制备出一种全新的光电功能材料。制备了三种不同孔径多孔铝衬底,采用脉冲激光沉积法,在真空背景下,在多孔铝衬底上生长了氧化锌薄膜。利用扫描电子显微镜、X射线衍射和光致荧光对样品进行了测试和分析。研究表明:利用不同孔径的多孔铝衬底生长的氧化锌薄膜的结构和光学性质差异很大。样品A的光致发光主要是394nm的紫外发射和498nm的蓝绿光发射;样品B的光致发光主要是417nm的紫光发射和466nm蓝光发射;样品C的光致发光主要是415nm的紫光发射和495nm的蓝绿光发射。由于薄膜是富锌的,随着在空气中氧化的进行,光谱发生变化。利用固体能带理论对光谱进行了全面的分析。     

Photoluminescence studies of porous silicon microcavities

Narrow photoluminescence peaks with a full-width at half-maximum of 14–20 nm are obtained from porous silicon microcavities (PSM) fabricated by the electrochemical etching of a Si multilayer grown by molecular beam epitaxy. The microcavity structure contains an active porous silicon layer sandwiched between two distributed porous silicon Bragg reflectors; the latter were fabricated by etching a Si multilayer doped alternatively with high and low boron concentrations. The structural and optical properties of the PSMs are characterised by scanning electron microscopy and photoluminescence (PL). The wavelength of the narrow PL peaks could be tuned in the range of 700–810 nm by altering the optical constants.     

Photoluminescence properties of europium-doped porous silicon nanocomposites

We present new results concerning the photoluminescence properties of europium (Eu³⁺) incorporated in porous silicon (PS) matrix. Eu³⁺ ions were embedded in the matrix by simple impregnation of PS layers in chloride solution of europium. Complete and uniform penetration of Eu³⁺ into the pores is proved from RBS study.The PL spectrum shows the existence of several peaks superposed to the PL band of PS. These peaks are related to level transitions in Eu³⁺. The effect of the ray excitation on the PL shows that energy transfer is not the principal route for radiative recombination.A systematic study of the PL versus annealing temperature was performed. It was found that the optimised PL spectrum is found after annealing at 1000°C. Low-temperature study of the PL shows an important increase of the intensity and a broadness of the peaks due to the appearance of a second crystallographic site.     

Photoluminescent behavior of heat-treated porous alumina films formed in malonic acid

In the present work IR spectroscopy, electron probe microanalysis (EPMA) and photoluminescence (PL) spectral measurements were applied to study the effect of treatment temperature (T) on compositional and luminescent properties of malonic acid alumina films. Our studies have shown that the heat treatment of anodic alumina films at investigated temperatures from 100 up to 700 °C changes their photoluminescence spectra considerably. An increase in T results in the PL intensity growth. When reaching its maximum at 600 °C the luminescence intensity then decreases drastically with further T growth. The films heat-treated at 500 and 600 °C demonstrate asymmetrical PL band with Gaussian peaks at 437 and 502 nm. We proved that the malonic acid species incorporated into the alumina bulk during the film formation are responsible for photoluminescence band with its peak at 437 nm.     

8-羟基喹啉铝在多孔铝中的发光研究

利用多孔铝非常高的孔隙率,将8-羟基喹啉铝(Alq3)镶嵌到多孔铝中,得到Alq3/多孔铝镶嵌膜,研究了不同条件下制备的多孔铝镶嵌膜的荧光光谱。实验表明,Alq3在多孔铝中的发光峰位在490 nm左右,比其在固态粉末状态蓝移了许多。Alq3/多孔铝镶嵌膜的发光特性与多孔铝中嵌入Alq3分子的数量及聚集程度有关。当分子数量较多、聚集程度较大时,发光增强,光谱峰位红移,但聚集程度太大时,易发生荧光猝灭现象。数量较多时,由于Alq3分子大多以范德瓦尔斯力结合,聚体较少,所以峰位移动幅度不大。实验中还发现,Alq3因为处在小孔中,光学性质稳定,荧光光谱带宽超过100 nm,比一般染料大得多,这使Alq3/多孔铝镶嵌膜有可能在固体可调谐激光器方面得到新的应用;同时也为探究Alq3/多孔铝镶嵌膜在电致发光器件中的发光特性奠定了基础,为将其进一步推向实用提供了实验依据。     

Enhanced photoluminescence properties of morin and trypsin absorbed on porous alumina films with ordered pores array

For the first time, the photoluminescence (PL) spectra of anodic porous alumina (P-Al) films impregnated with essentially non-fluorescent morin and morin-trypsin (Try) were investigated and compared with those of liquid solutions. It was found that their PL positions are similar to that of dye-Al³⁺ in ethanol solution, and the PL intensity of embedded dye-Try is much higher than that of embedded dye alone. We infer that the appearance of the PL band detected here is due to the formation of morin-Al complex in the holes of P-Al with the inner wall involved, and a likely luminescent mechanism is proposed to elucidate the PL enhancement phenomena due to the coexistence of morin and Try in the P-Al pores, which is confirmed by UV and FTIR measurements. Moreover, it is also found that the PL intensity increases with the pore size.     

Investigation of structural and optical properties of nanoporous GaN film

The structural and optical characteristics of porous GaN prepared by Pt-assisted electroless etching under different etching durations are reported. The porous GaN samples were investigated by scanning electron microscopy (SEM), high-resolution X-ray diffraction (HR-XRD), photoluminescence (PL) and Raman scattering. SEM images indicated that the density of the pores increased with the etching duration; however, the etching duration has no significant effect on the size and shape of the pores. XRD measurements showed that the (0 0 0 2) diffraction plane peak width of porous samples was slightly broader than the as-grown sample, and it increased with the etching duration. PL measurements revealed that the near band edge peak of all the porous samples were red-shifted; however, the porosity-induced PL intensity enhancement was only observed in the porous samples; apart from that, two additional strain-induced structural defect-related PL peaks observed in as-grown sample were absent in porous samples. Raman spectra showed that the shift of E₂ (high) to lower frequency was only found in samples with high density of pores. On the contrary, the absence of two forbidden TO modes in the as-grown sample was observed in some of porous samples.     

Energy transfer in porous anodic alumina/rhodamine 110 nanocomposites

We have used porous anodic alumina (PAA) films as templates for embedding rhodamine 110 (Rh110) molecules and examined their photoluminescence (PL) properties in detail. The analysis of the polarization memory (PM) of PL strongly suggests that there is a significant energy transfer from PAA to Rh110 molecules. The effect of annealing the PAA layer on the PL properties of the nanocomposite has been studied. The results show that the energy transfer becomes more efficient in annealed PAA.     

多孔铝镶嵌8-羟基喹啉铝荧光光谱研究

利用多孔铝非常高的孔隙率,将8羟基喹啉铝(Alq3)镶嵌到多孔铝中,得到多孔铝Alq3镶嵌膜.研究了镶嵌膜的荧光光谱,并与Alq3在溶液状态下的荧光光谱进行比较,发现其荧光光谱与Alq3在乙醇溶液中的光谱相似,呈现单分子的发光特征,并且光谱线形更加对称.实验表明,多孔介质有机镶嵌膜有可能成为进一步发展Alq3在电致发光器件方面应用的新途径. 关键词： 多孔铝 8-羟基喹啉铝 光致发光光谱     

蓝光发射多孔硅RTO过程中的尺寸分离效应

对用水热腐蚀技术制备的、具有蓝光发射的多孔硅样品在快速热氧化(RTO)处理前后其光致发光谱、硅纳米颗粒的大小及尺寸分布变化进行了研究.实验发现,新鲜多孔硅样品经过RTO处理后,其光致发光谱整体蓝移并由单发光峰分裂为两个发光峰;与此对应,样品中的硅纳米颗粒在整体减小的同时出现尺寸分离现象.这一结果表明,多孔硅中的短波长发射也具有强烈的尺寸相关性. 关键词：     

Thermal annealing dependence of some optical properties of plasma-modified porous silicon

The photoluminescence and reflectance of porous silicon (PS) with and without hydrocarbon (CH_x) deposition fabricated by plasma enhanced chemical vapour deposition (PECVD) technique have been investigated. The PS samples were then, annealed at temperatures between 200 and 800 °C. The influence of thermal annealing on optical properties of the hydrocarbon layer/porous silicon/silicon structure (CH_x/PS/Si) was studied by means of photoluminescence (PL) measurements, reflectivity and ellipsometry spectroscopy. The composition of the PS surface was monitored by transmission Fourier transform infrared (FTIR) spectroscopy. Photoluminescence and reflectance measurements were carried out before and after annealing on the carbonized samples for wavelengths between 250 and 1200 nm. A reduction of the reflectance in the ultraviolet region of the spectrum was observed for the hydrocarbon deposited polished silicon samples but an opposite behaviour was found in the case of the CH_x/PS ones. From the comparison of the photoluminescence and reflectance spectra, it was found that most of the contribution of the PL in the porous silicon came from its upper interface. The PL and reflectance spectra were found to be opposite to one another. Increasing the annealing temperature reduced the PL intensity and an increase in the ultraviolet reflectance was observed. These observations, consistent with a surface dominated emission process, suggest that the surface state of the PS is the principal determinant of the PL spectrum and the PL efficiency.     

Rhodamine B absorbed by anodic porous alumina: Stokes and anti-Stokes luminescence study

An organic dye, rhodamine B (RhB) solution, has been used to impregnate anodic porous alumina (PA) in order to form RhB/PA nanocomposites. The photoluminescence (PL) spectra of PA films impregnated with RhB are investigated and compared with those in liquid solution. The PL mechanism of RhB/PA nanocomposites has been investigated through the effect of energy excitation. We show the possibility of energy transfer from alumina nanocrystallites to RhB molecules. The interactions between chemical species in the internal surface of PA and the RhB molecules can play a key role in PL emission, which has been proved by the Fourier transform infrared (FTIR) measurements. Moreover, it is also found that the PL intensity of the nanocomposite increases with the PL of the PA layer. The effective cross section of RhB in PA has been estimated to be in the order of . An anti-Stokes PL (APL) has been observed from RhB/PA. The linear variation of the APL intensity with the laser power (I_APLαP^0.97) indicates that one photon is involved in emission process.     

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.     

Optical properties of low-dimensional structures formed by irradiation of laser

Some kinds of low-dimensional nanostructures can be formed by the irradiation of laser on a pure silicon sample and SiGe alloy sample. We have studied the photoluminescence (PL) of the hole-net structure of silicon and the porous structure of SiGe where the PL intensity at 706 nm and 725 nm wavelength increases obviously. The effect of intensity-enhancing in the PL peaks cannot be explained within the quantum confinement alone. We propose a mechanism on the increasing PL emission in the above structures, in which the trap state of the interface between SiO₂ and nanocrystal plays an important role.      

Low-dimensional structures formed by irradiation of laser

Some kinds of low-dimensional nanostructures can be formed by irradiation of laser on the pure silicon sample and the SiGe alloy sample. This paper has studied the photoluminescence (PL) of the hole-net structure of silicon and the porous structure of SiGe where the PL intensity at 706nm and 725nm wavelength increases obviously. The effect of intensity-enhancing in the PL peaks cannot be explained within the quantum confinement alone. A mechanism for increasing PL emission in the above structures is proposed, in which the trap states of the interface between SiO₂ and nanocrystal play an important role.     

Annealing and amorphous silicon passivation of porous silicon with blue light emission

The photoluminescence (PL) of the annealed and amorphous silicon passivated porous silicon with blue emission has been investigated. The N-type and P-type porous silicon fabricated by electrochemical etching was annealed in the temperature range of 700-900 °C, and was coated with amorphous silicon formed in a plasma-enhanced chemical vapor deposition (PECVD) process. After annealing, the variation of PL intensity of N-type porous silicon was different from that of P-type porous silicon, depending on their structure. It was also found that during annealing at 900 °C, the coated amorphous silicon crystallized into polycrystalline silicon, which passivated the irradiative centers on the surface of porous silicon so as to increase the intensity of the blue emission.     

Elaboration and characterization of organic–inorganic quantum wells impregnated on porous anodic alumina nanostructures

An organic–inorganic quantum well embedded on porous anodic alumina was synthesized and studied by MEB, AFM, optical absorption and photoluminescence. The morphology determined by the MEB and the AFM, shows that the size of the pores is about 10 nm for alumina evaporated on glass substrate and about 35 nm for alumina template prepared in sulfuric acid (H₂SO₄). The optical properties are characterized from absorption and photoluminescence spectra measured at room and low temperature. The measured spectral characteristics demonstrate the influence of the pore size on the emission of the organic–inorganic quantum well ((C₁₂H₂₅NH₃)₂PbI₄). An obvious blueshifted photoluminescence (PL) of (C₁₂H₂₅NH₃)₂PbI₄ in nanometer-sized pores was observed. It results in a better quantum confinement.