最新刊期
卷 25 ， 期 1 ， 2004
- 摘要：Based on a cluster model the adsorption of K on the graphite(0001)surface has been studied by using the charge self-consistent extended Hückel theory(EHT).The calculation is performed at four different coverages(Θ).The equilibrium adsorption heights obtained are in good agreement with experimental data and other theoretical values. The charge transfer ΔQ, adsorption energy ΔE, density of states(PDOS and TDOS),Mulliken population and the bonding nature of the K/graphite systems are presented. It is worth noting that the Linear Muffin-tin Orbital(LMTO)method with the local functional theory has attained quite reliable results that are in qualitative agreement with those of the EHT.The results also show strong sensitivity of the electronic structure on the very little change of K-graphite layer spacing.This implies that K-graphite system is not a stable“single-phase”system.It is found that the K-4s and carbon-σ hybrid states are mainly situated in the lowest energy range discussed,which is the reason why K-4s states cannot be found at the Fermi level for K-graphite systems in the X-ray photoelectron spectroscopy(XPS)and angle-resolved ultraviolet photoelectron spectroscopy(ARUPS)measurements.The empty 3d-orbitals are believed to play a central role in present calculations with two different methods.关键词：surface electronic structure;alkali metals;graphite;chemisorption;EHT;LMTO109|73|0更新时间：2020-08-11
- 摘要：The columnar structure of ZnO thin films with respect to the deposition angle prepared by single source chemical vapor deposition (SSCVD) on Si(100) substrate was examined. Growth was performed using a volatile precursor. It was found that the columnar structure depended on the angle between substrates and vapor sources during deposition. However, the XRD spectra indicated that the [002] crystallographic orientations of ZnO thin films were perpendicular to the substrate planes, irrespective of deposition angle and not aligned with columnar growth orientations as may be expected. The c-axis orientation of SSCVD ZnO thin films is easily aligned with the surface of substrate because of its surface anisotropy and the optimal growth is on its high-density face (002) with the lowest face free energy.关键词：ZnO thin film;columnar structure;c-axis orientation85|109|2更新时间：2020-08-11
- 摘要：Recently, there has been an increasing interest in nonlinear optical properties of semiconductor quantum well structures, because of their relevance for studying practical applications and as a probe for the electronic structure of mesoscopic media. In this paper, the nonlinear electro-optic effects in special asymmetric quantum wells are studied. Because of this kind of quantum well is more approximate a practical quantum well between the conduction band and valence band, and it can be realized very easily in experiment. The analytical expressions of electro-optic coefficient have been derived by compact density-matrix approach and the numerical results were presented for GaAs/AlGaAs asymmetric quantum wells. The shape of the quantum well varying with the parameter a and the parameter V0 were plotted. It is found that the asymmetry of the quantum wells enhances with the increase of parameter a. On the contrary, the asymmetry of the quantum wells decreases with the increase of the parameter V0. The maximum values of the electro-optic coefficient varying with the parameter a and the parameter V0 were plotted. The numerical results show the electro-optic coefficient enhances with the increases of the parameter a and the decrease of the parameter V0. Therefore, it is found that the electro-optic coefficient increases with the enhancement of the quantum wells’ asymmetry. The electro-optic coefficient as a function of the photon energy with different values of parameter a and different values of parameter V0 were also plotted. It can be observed that there are three peak values in the figures, respectively. And it is obvious that the larger the asymmetry of quantum wells is, the bigger the peak value is. It can be seen that with the increase of the asymmetry of quantum well, the peaks move to the low energy side. Moreover, the electro-optic coefficient obtained in this special quantum well is as large as 10-6 m/V. With the advances of nanofabrication technology recently, it is possible to fabricate such semiconductor quantum wells and it is possible for us to get better nonlinear materials in experiments.关键词：electro-optic effects;nonlinear;quantum well81|104|0更新时间：2020-08-11
- 摘要：The Kronig-Penney model has been widely used to explore the characteristics of electrons in a periodic potential as this model provides one with perhaps the simplest instance of Bloch states. This model and its relation with superlattices has also been used in recent times to provide an implementation of the physics of random and quasiperiodic systems. It is because of its importance and wide applicability that we focus our attention on the Kronig-Penney model to investigate the imperfect finite superlattices. There exist many theoretical methods developed for description of disordered systems such as Monte-Carlo simulation, truncated rate equation, perturbative approach, Wannier-Stark levels etc. However, provides an easy way (tranfer matrix) that is very valid for studying the electronic states of imperfect finite SLS. Tranfer matrix method can be turned out to be more convenient for computer than the conventional formalism, which is based on effective mass approximation and Bastard’s boundary condition. The barrier and well can be represented by matrixes and the wave functions at any two positions inside the superlattice are connected by a product of transfer matrixes. The sequence of the matrixes matches the arrangement of the barriers and wells. No matter how the barrier and well change, we need only to modify the matrixes of barrier and well. We mainly discuss the electronic states of finite SLS with intentional random barriers or well widths by means of a transfer matrix method. The transmittance and its localized wave function of SLS with random potentials were calculated and also the transmittance and electronic eigenfunction of SLS with random well widths were calculated. The results are compared with those obtained from ordered SLS. We can observe discrete localized states of finite SLS with intentional random barriers or well widths. Our simulated results indicate that electronic states are sensitive to both the intentional random barriers and well widths. Especially when well widths randomly vary in a range of (9.1～10.9) nm for SLS discussed in this paper, we find that the subband begins to disappear. In recent decades the physical community and scientists in related fields have shown an increasing interest in the structure and properties of disordered condensed systems. It’s clear that the method developed in this paper can be extended easily to solve some other kinds of problems caused by disordered systems such as localized states, extended states, mobility edges and so on.关键词：random potentials;random well widths;localized wave function88|101|0更新时间：2020-08-11
- 摘要：In recent years,the problem of a magnetopolaron has been of considerable interest.Larsen proposed a fourth-order perturbation method to calculate the ground state energy of two-dimensional polaron in a magnetic field.Employing Haga’s perturbation method,Ze et al.derived an effective Hamiltonian for the interface magnetopolaron in polar crystals at zero temperature,in which the interactions of both bulk LO phonons and interface phonons have been taken into account.Using the Green’s function method,Wei et al,studied the cyclotron resonance mass of an electron interacting with bulk longitudinal optical(BO)phonons as well as surface optical(SO)phonons in a polar crystals slab at finite temperature and found that the temperature dependence of magnetopolaron depends strongly on the strength of the magnetic field.Wang et al,investigated the magnetic field and temperature dependence of the properties of a magnetopolaron at the interface of polar-polar crystals in an external magnetic field by using the generalized Haga perturbation method.By using variational method of Pekar type,Zhou and co-workers studied the energy levels of strong coupling magnetopolaron in disk-shape quantum dot(QD)and quantum well(QW).Madkour et al. studied the effect of magnetic field on surface polaron states in a polar crystal interface and emphasized the result that the presence of magnetic field leads to a local potential for charge carriers at the interface.Based on Wigner-Brillouin theory,D.E.N.Brancus and G.Stan investigated the state energy of an anisotropic three-dimensional polaron in a magnetica field.One of our authors have done a lot of works on the proterties of the surface magnetopolaron.The properties of surface polaron in a magnetic field have been further discussed by the present authors and co-workers.Tokuda studied the property of the mean number of the optical and the piezoelectric polarons and the mean number of the acoustic and optical polarons via deformation potential by using improved variational method.However,the mean number of optical phonon of surface magnetopolarons at finite temperature has not been studied so far.In this paper,the properties of surface magnetopolaron which is a strong,weak-coupling of optical phonon was discussed by using Tokuda’s improved linear combination operator,the unitry transformation and the Lagrange multiplier methods.The mean number of optical phonons of the strong,weak-coupling magnetopolaron are calculated.Numerical calculations for KCl crystals,as an example,are performed.The relation of the vibration frequency and the mean number of optical phonons to magnetic field B,temperature T and Lagrange multiplier u are discussed.The result illustrates that the vibration frequency λ of surface magnetopolaron will increase with increasing magnetic field B,temperature T and Lagrange multiplier u;the mean number of optical phonons will increase with increasing B,T and u.关键词：surface magnetopolaron;mean number of optical phonon;finite temperature90|88|1更新时间：2020-08-11
- 摘要：With the development of solid science theory and experimental technology, the properties of the bound magnetic polarons(BMP)in crystals have been of considerable interest. Many investigators studied the properties of the BMP by means of various theoretical methods and experimental methods. Wollf et al. investigated the BMP in semimagnetic semiconductors and the BMP in dilute magnetic semiconductors by the optical experiments.Heiman et al. observed the variational model of the bound magnetic polaron and Roman spin flip line shape. Larsen studied the upper bound to the polaron ground state in a magnetic field using the Feyman path-integral method; Zhou solved the effective-mass equation and calculated the magnetopolaron binding energy in the ground state and in the excited state as well as the resonance energy of the magnetoplaron by means of the variational method. Wang and his co-workers studied the ionization energy of bound polaron in a magnetic field in asymmetric polar semiconductor heterostructures by using a modified Lee-Low-Pines variational method. Chen et al. investigated the properties of the impurity-bound polarons in a parabolic quanrum dot and in anisortopic quantum dots in magnetic fields and then studied impurity-bound polarons in double quantum wells in magnetic fields. Umehara presented a theory for the BMP in diluted magnetic semiconductors by a modified molecular-field approximation; Recently,Nagaku studied the bound excitonic magnetopolarons. In the early 1970s, Huybrechts investigated the internal excited state of the bulk optical polaron by using linear combination operator method. Some properties of the surface magnetopolaron in polyatomic polar crystals were studied by means of a linear combination operator and a perturbation method by one of the authors. However,no one studied the BMP with the linear combination operator and perturbation method so far. In this paper,we investigated the vibration frequency and the ground state energy of the BMP with this method. Using the parameters and the coupling constant of the AgCl crystal and the RbCl cvrystal for the weak coupling and the strong coupling, we calculated the vibration frequency and the ground state energy of the BMP. The results showed that the vibration frequency of the BMP increase with increasing the magnetic field, and the ground state energy does nearly not change when the magnetic field is increasing.关键词：weak coupling;strong coupling;bound magnetopolaron;ground state energy109|116|1更新时间：2020-08-11
- 摘要：Blue organic electroluminescent devices with a novel structure by using a new hole transporting material have been fabricated. The structure is ITO/CuPc/J0503/JBEM:perylene/TPBi/Alq3/LiF/Al, in which a (hole-blocking) layer was inserted between electron-transporting layer and emitting layer. For comparison, devices with a conventional structure have also been made, and the structure is ITO/CuPc/J0503/JBEM:perylene/Alq3/LiF/Al. Here CuPc(Copper phthalocyanine)and LiF are hole and electron injection layers,respectively,J0503 hole-transporting layer(HTL), JBEM(9,10-bis(3’5’-diaryl)phenyl anthracene)emitting-layer(EML), TPBi(1, 3, 5-tri(pheny1-2-benzimidazole)-benzene)hole-blocking layer(HBL), and Alq3(tris(8-quinolinolato)aluminium complex)electron-transporting layer(ETL). Devices with different EML thickness of each structure have also been prepared in order to investigate the effect of EML thickness on the performance. The results indicate that the devices with the novel structure show improved efficiency and luminance as compared to the conventional ones. However, the improvements depend on the thickness of EML. Small differences are found in both structures with the EML thickness larger than the diffusion length of excitons because the blocking-effect from HBL has little effect on the increasing density of excitons.While the novel devices with thickness approximate to the diffusion length of excitons show much higher efficiency and luminance than that of the conventional ones having the same EML thickness because of the good confinement to both charge carriers and excitons. When the thickness of EML is too small, probably, strong interface effect leads to break the performances of the novel devices.关键词：blue organic electroluminescent;hole-blocking layer;thickness of emitting layer106|65|2更新时间：2020-08-11
- 摘要：Over the last two decades, much attention has been paid to mesomophic compounds containing heterocyclic units. Most of these investigated mesogens are 1,4-disubstituted six-mengered aromatic, heteroaromatic or aliphatic rings as basic units, which unit have almost linearity as required according to the classical concept of calamitic mesogens. Mesomorphic compounds containing five membered heterocycles have been less investigated. Liquid crystalline 1,3,4-oxadiazoles were described only in recent years. In connection with the considerable theoretical and technological interest in banana shaped achiral molecules and electron transport materials used in light-emitting diodes(LED), substituted 1,3,4-oxadiazoles become more significant. Due to the electron deficient property and poor hole accepting character of 1,3,4-oxadiazole ring, certain low molecular weight aromatic 1,3,4-oxadiazole derivatives facilitate both electron injection and transport. Aromatic 1,3,4-oxadiazoles, such as 2-(4-tert-buty-phenel)-5-biphenyl-1,3,4-oxadizole(PBD), have been synthesized and widely used as electron-conducting/hole-blocking materials in multiplayer devices. According to H.Tokohisa et al’s recent investigations of liquid crystalline 1,3,4-oxadiazole derivatives, the carrier mobility is higher in the more ordered smectic phase than that in the less ordered phase. In addition, the present increasing interest in liquid crystalline oxadiazoles is due to their photoconducting properties. Here two novel liquid crystals based on 2,5-diphenyl-1,3,4-oxadiazole as mesogenic group, namely 2-[(4-methoxy) phenyl]-5-{[4-n-methoxy) phenoxy] propyloxy} phenyl-1,3,4-oxadiazole (E3) and 2-[(4-methoxy) phenyl]-5-{[4-n-methoxy) phenoxy] hexyloxy} phenyl-1,3,4-oxadiazole (E6) have been synthesized. Their liquid crystalline behaviors were studied by differential scanning calorie-meter (DSC), polarizing microscopy (POM) as well as wide angle X-ray diffraction (WAXD). The results showed that both E3 and E6 exhibited semetic phases. UV-Vis spectra and cyclic voltammetry were used to investigate their energy level parameters. The highest occupied molecular orbital (HOMO), the lowest unoccupied molecular orbital (LUMO) and the energy gap obtained showed that the HOMO level of E3 and E6 is relative low, the LUMO level is relative high while the energy gap is relative high, which indicated that they are not only benefit for the electrons injection but also for the block of holes and excitons. It can be predicted that the electron transporting material with liquid crystalline group will play an important role in the improvement of the properties organic electroluminescent devices.关键词：electroluminescence;liquid crystalline;oxadiazoles derivatives118|83|2更新时间：2020-08-11
- 摘要：Recently organic phosphors have been demanded the attention of the organic electroluminescence community because their efficiencies can be as quadruple as that of the fluorescent materials. The research was focused on the new phosphorescence dyes and host materials. The materials with high-efficient phosphorescence were relatively rare, and to date, only a few of them was found suitable for EL application such as Pt, Ir, Re, and Os complexes. The luminescence of these organometallic complexes is from metal to ligand charge transfer (MLCT) triplet state. As we know, ruthenium(Ⅱ) complex c.a. tris(bipyridyl)ruthenium (Ru(bpy)32+) was the most typical MLCT molecule polypyridine, which have been widely studied in the past half-century. The photoluminescence (PL) efficiency of those Ru complex in solution can be in the range of 7% to 50% by substituting with modified ligands. As a luminescence materials, the electrochemically generated luminescence (ECL) in a liquid cell has been extensively studied, but there are few reports on the use of these materials as emitters in a solid-state devices because Ru(bpy)32+ can not be evaporated to form the film, and also can not be doped into semiconductive polymers. Considering that Ru(bpy)32+ is a water-soluble molecule, the polyelectrolytes, which generally are water-soluble, may function as host of Ru(bpy)3 to form a molecular-dispersion thin film, subsequently used for LEDs fabrication. Here we reported the properties of EL devices fabricated by doping Ru(bpy)32+ into the polyelectrolytes. We have found that the type of opposite charge polymer used as host influences dramatically on the performance of the devices.关键词：polymer electroluminescence device;phosphorescence materials;red light emission83|78|2更新时间：2020-08-11
- 摘要：The use of bridging ligand such as diisocyanides in the synthesis of polynulcear silver and gold complexes leads to the formation of dimeric, trimeric, polymeric structures in the solid state. These organometallic coordination compounds exhibit interesting luminescence properties, which can be studied for the potential applications for the molecular-based optical materials or devices. Much research has been reported on the synthesis and spectroscopic properties of organometallic compounds of silver (or copper) with the diisocyanide ligand, 1,8-diisocyano-p-menthane(L). It was found that both Ag and Cu can form organometallic coordination polymer with L:{[M(L)2][BF4]}n (M=Ag, Cu), but the crystalline state is different. The molecular weight of the two organometallic polymer have been measured and it is found that the Cu polymer has larger molecular weight and longer polymeric chain, while the Ag polymer shows smaller molecular weight and shorter chain length. By means of the molecular assembly, some heterometallic coordination polymers with 1,8-diisocyano-p-menthane have been synthesized with the different ratio of silver and copper: {[CuxAg1-x(L)2][BF4]}n (x=1.0,(0.95,0.78),0.52,0.19,0.05,0) whose composition have been determined with X-rays fluorescence spectra, IR spectra and NMR spectra. Silver and copper were introduced into one kind of coordination polymer systems for their similar reaction conditions. From the solubility of product, it can be preliminary predict that the molecular lengths of heterometallic coordination polymers decrease with the increasing of copper content. We further studied the corresponding photophysical properties (absorption spectra and luminescence spectra) of these heterometallic Cu-Ag coordination polymers in detailes. The absorption spectra and luminescence spectra of them both exhibit band shift compared with luminescence of pure Cu and Ag polymers, which indicates that the introduction of copper and silver with different ratio has great influence on the polymer chain structure and the corresponding absorption and emission properties.关键词：heterometallic copper-silver;organometallic coordination polymers;photophysical properties86|105|0更新时间：2020-08-11
- 摘要：A theoretical study on three 2-styrylbenzothiazole compounds with better activity of killing moulds by semi-empirical method is given in this paper. Their geometric configurations have been optimized by semi-empirical method RHF/AM1. The results of optimization show that most of the atoms are in the same plane. Their smallest vibrational frequencies are 13.50, 9.66, 7.53 cm-1 and there is no imaginary frequency in vibrational analysis. Energy differences between HOMO and LUMO are 7.838, 7.706 and 7.288 eV. ΔE is so small that the electrons can be easily excited. Their fluorescence spectra have also been calculated by RHF/CIS. The values are essentially consistent with experimental values. All the spectra data are calculated by ideal gas models, but the experimental values are determined in solutions. So it is permitted that there are some differences between them.关键词：2-styrylbenzothiazole compounds;quantum chemistry;fluorescence spectra;CIS96|97|4更新时间：2020-08-11
- 摘要：Red emitting phosphors (Y, Gd)BO3:Eu, with mean size of 1～2 μm were prepared by a sol-gel process combined with a furnace firing. For the sol-gel procedure, tri-n-butyl borate B(OC4H9)3 was mixed and stirred with a nitric solution of Y2O3, Gd2O3, Eu2O3 having a mole proportion of 0.45:0.50:0.05 in ethanol. The (Y, Gd, Eu)BO3 gel precursor prepared from solution was sintered at a certain temperature range of 800～1200℃. The prepared orthoborates were characterized with X-ray diffraction, scanning electron microscopy and photoluminescence. Conventional solid state reactions require higher sintering temperatures above 1200℃ for pure (Y, Gd)BO3:Eu phase. However, the sol-gel process allows a significant decrease of the sintering temperature and the phosphor sintered at 800℃ had pure orthoborate phase. The effect of preparation conditions such as amount of B(OC4H9)3 and sintering temperature on the crystallinity, morphology and luminescence characteristics of phosphor particles were investigated. The results show that 10%～20% excess of B(OC4H9)3 is favorable to get pure orthoborate phase and control the morphology of particles. Also, the particle size and VUV luminescence of phosphors are dependent on sintering temperature. The particles obtained by sintering at 1000 and 1100℃ had narrow distribution and the aggregation of particles appeared at 1200℃. The PL intensity of the phosphor prepared by sol-gel process reached the maximum value at 1100℃ and was 20% higher than that of the phosphor prepared at 1200℃ by conventional solid state reaction. By sol-gel process, small-sized (Y, Gd)BO3:Eu with narrow distribution, high luminescent efficiency and good morphology can be prepared in a wide range of reaction condition.关键词：orthoborate;sol-gel process;PDP;phosphor85|106|4更新时间：2020-08-11
- 摘要：Trivalent europium-activated Y2O3 has attracted much attention as red emitting phosphor in commercial application on fluorescent lighting and displaying. With the development of nanotechnology, the optical properties of nanocrystalline (NC) Y2O3:Eu3+ have also been investigated extensively for its potential application on high resolution images and for fundamental researches such as local environment probing because Eu3+ is supersensitive to its surroundings. As well known, Y2O3:Eu3+ phosphor can absorb the UV light through a charge transfer band (CTB) or host excitation band and then generates red color fluorescence peaking at 611 nm. Some investigations on CTB of bulk and NC Y2O3:Eu3+ have been reported. There exist S6 and C2 crystallographic sites in cubic Y2O3. The S6 site has inversion symmetry center in which electric dipole transition is forbidden. Due to the absence of the inversion symmetry center, the C2 site makes dominant contribution to the 611 nm emission which corresponds to the electronic dipole transition of 5D0→7F2 in Y2O3:Eu3+. Hence, the generally observed CTB and host excitation band in the excitation spectra by monitoring the red color fluorescence from 5D0→7F2 transition merely corresponds to the C2 site. Although the S6 site has almost no contribution to the red color fluorescence, it possibly competes with the C2 site for energies under UV excitation. To our knowledge, there is no report on the UV excitation properties of the S6 site in bulk and nanocrystalline Y2O3:Eu3+. The S6 site allows the magnetic dipole transition 5D0→7F1 of Eu3+, which provides the possibility to study the UV excitation properties of the S6 site. The CTB and host excitation band for Eu3+ at the S6 site in bulk and nanocrystalline Y2O3:Eu3+ is mainly investigated. The NC Y2O3:0.01 Eu3+ was prepared by fast thermal decomposition of metal nitrate solution. The advantage of this method is that pure cubic phased Y2O3:Eu3+ nanocrystals can be obtained at a relatively lower temperature than by any other methods. The particle sizes were determined to be 7 nm from a survey of the transmission electron microscopy micrographs. The bulk Y2O3:0.01 Eu3+ powders was formed by annealing as-prepared corresponding nanoparticles at (1250)℃ in air. It was determined to be 2～3 μm by field-emission scanning electron microscopy (FE-SEM). Both the samples were identified as cubic structure from XRD. The spectra were carried out at room temperature with a Hitachi F-4500 florescence spectrometer using a Xe lamp as the excitation source. Increases of emission intensities for Eu3+ at the S6 site relative to that at the C2 site have been observed as UV excitation wavelength decreases from 200～300 nm in both bulk and nanocrystalline cubic Y2O3:Eu3+. It indicates that the two kinds of sites have different charge transfer states and host lattice excitation responds. Decomposition of excitation spectra shows that the charge transfer band(CTB) of Eu3+ at the S6 site is located at the high-energy side of the C2 site and the host prefers transferring energy to the S6 site. Compared with the bulk material, the CTBs for the two sites both shift toward the red and the number ratio of S6 to C2 sites is smaller in nanocrystalline Y2O3:Eu3+. Above results are discussed.关键词：lattice site;charge transfer band;energy transfer;red-shift96|83|1更新时间：2020-08-11
- 摘要：Since Bhargava firstly reported in 1994 that the luminescence quantum efficiency was up to 18% in ZnS:Mn nanocrystalline, numerous studies on ZnS nanocrystalline were emerged. However, most of these studies focused on the luminescence of doped ZnS nanoparticles, but the study about the effect of annealing on crystal-phase transition and photoluminescence in ZnS nanoparticles was relatively rare. In this paper, some efforts were made. The cubic undoped and Eu-doped ZnS nanoparticles were prepared by co-precipitation, and the samples were achieved by the annealing of as-made nanoparticles at different temperatures. For the comparison, the phosphors were also made by the annealing of ZnS raw powders (bulk material for commercial use) under the same condition. During the annealing of nanoparticles, the particles grown up dramatically and the average diameter increased from 11 nm to 3～5 μm. The structural phase transition (from cubic to wurtzite) was studied by X-ray diffraction patterns and the experiment of difference thermal analysis (DTA). The result indicated that the phase transition temperature, in the samples annealed with ZnS nanoparticles, was 618℃, which decreased greatly due to surface effect than the value (800℃) reported by the reference. The luminescent properties of the samples were studied and compared to the phosphors annealed with ZnS raw powders. Two broad emission bands were observed in the samples annealed above 900℃. One was centered at 460 nm and the other at 520 nm. While in the phosphors annealed with ZnS raw powders, only 460 nm-centered emission was detected. The 520 nm-centered emission was irrelevant with the crystal phase transition and independent on the 460 nm-centered emission. So it maybe originated from different recombination centers. But in the measurement of time-resolved spectra and lifetime, they differed little from each other. However, it can be observed that the peak of the emission band of time-resolved spectra red-shifted as the delay time varied from 5～25 μs, which was the characteristics of recombination of donor-acceptor pairs. The normalized emission intensity decayed following with a bi-exponential fitting function, ηe-t/τ1+(1-η)e-t/τ2. For 520 nm-centered emission, τ1, τ2 and η were determined to be 64, 250 μs and 0.47 respectively. For 460 nm-centered emission, τ1, τ2 and η were determined to be 68, 251 μs and 0.57. The decay time constants had little change with detected wavelength, but the ratio η changed with wavelength. The 460 nm-centered emission was generally ascribed to self-actived luminescence, and the 520 nm-centered emission was interpreted as the formation of new defaults induced by the preparation of nanoparticles or of some new dopants brought by the annealing. Doping of europium did not induce new luminescent centers, but caused the luminescent efficiency of defects to increase greatly.关键词：nanocrystalline;phase transition;self-actived luminescence107|115|3更新时间：2020-08-11
- 摘要：It was observed that Zn2+ and Mn2+ additives played the activators and quenchers for the orange emission of colloidal ZnS:Mn2+ nanoparticles, respectively. The time decay curves of Mn2+ 580 nm emission are nonexponential and become slower as Zn2+ is added, faster as Mn2+ added. On the basis of Langmuir isotherm model, the adsorption-desorption equilibrium constant K is obtained to be 2.9×104(mol/L)-1. The activation process resulted from the formation of monolayer ZnS outside of nanoparticles, which blocked the nonradiative pathways related to the dangling bonds of lone pairs on surface S2-. It is worth noting that the blue emission is also activated on introduction of Zn2+, indicating the original quenchers eliminated by the passivation of the surface through forming the ZnS shell can quench both the blue and the orange emission bands. It is not appropriate to describe the quenching process aroused by the addition of Mn2+ using the standard Stern-Volmer model. Considering Poisson statistics and assuming that one Mn2+ is sufficient for 100% quenching for orange emission, the reasonable linear plot is obtained. The average size of nanoparticles is 2.7 nm less than that calculated within the framework of effective mass approximation (EMA) model. The plausible reasons are as follows:1. the increase of Eg owing to quantum confinement effect is overestimated by EMA;2. the assumption that one Mn2+ is sufficient for 100% quenching of orange emission does not consist with the actual situation on the ZnS:Mn2+ colloids. The quenching centers induced by the addition of Mn2+ may probably be Mn2+ themselves adsorbed on the surface of nanoparticles, which interact with the interior Mn2+ to perform Mn-Mn energy migration within short distance. Such opinion and the formation of the manolayer of ZnS on the surface is supported by the fact that the orange emission intensity is almost unchanged with the addition of Mn2+ after the activation process has taken place due to the addition of Zn2+. Surface or surface states play a crucial role on the optical properties of nanoparticles due to the relatively larger surface-to-volume ratio compared to the bulk. We expect the results may help to approach the nature of the mechanisms that surface or surface states dramatically affect the optical properties of nanoparticles.关键词：ZnS:Mn2+ nanoparticles;surface modification;activation;quenching centers;photoluminescence92|100|3更新时间：2020-08-11
- 摘要：Porous silicon (PS) was prepared by common electrochemical etching in HF-based electrolytes, and then annealed in plasma of hydrogen at different temperature(250～420℃)in this article. Atomic Force Microscope (AFM) was employed for imaging surface structure at an atomic level. Afterwards, photoluminescence (PL) measurement was applied. Different annealing temperature leads to great change of surface morphology and photoluminescence spectrum of porous silicon. Besides the band caused by quantum size effect, there also exists the efficient blue and violet band in PL spectra of PS. An asymmetric emission band appeared in PL spectrum of PS annealed at 250℃. It could be separated to three components by curve fitting. If these components can be attributed to the recombination of non-equilibrium carriers via three impurity centers in suboxide layer, the PL spectrum could be (explained). The surface composition of PS, which was subjected to the annealing treatment at 420～450℃, is considered to be SiOx:Si(H,O). In its PL spectrum, a new violet emission band not reported in literature before appeared. Its mechanism is still in study.关键词：porous silicon;thermal annealing;violet emission band;blue emission band;atomic force microscopy91|61|0更新时间：2020-08-11
- 摘要：Silica gels have lots of unique capabilities. In this paper organic dyes doped silica gels were prepared by sol-gel method in order to find the effect of the catalysts on structure and optical properties of the composite. From the emission spectra of coumarin 102(C102) in different conditions, the emission peak of PL shifted to short wavelength in both the composite material films compared with solution. Most of dye molecules were being monomers in ethanol, and few of them were being dimmer. As water joined it was found that a red-shift of emission peak appears. This showed that the gather degree became higher. When in sol catalyzed by acid the dye molecules were wrapped in the hole together with water and ethanol, so the concentration in hole was higher than average. On the other hand, hydrolysis product began to polymerize after aging two days, and the product was water. So the proportion of water added in composite. These two aspects resulted in red shift. The pH degree was very high in sol catalyzed by alkali, this restrain dye molecules polymerize. Doped in silica gels, the C102 molecules were isolated and adsorbed in the tiny pores of the inorganic network steadily. Almost all of them existed in the form of monomer, so the spectra were similar to that of a single molecule. In structure, the sample catalyzed by acid had flat surface and dense structure, but it had hackly surface and open structure catalyzed by the alkali. All the results showed that the sample catalyzed by acid can be better used as a solid laser material.关键词：silica gel matrix;fluorescent spectrum;monomer80|81|0更新时间：2020-08-11
- 摘要：A numerical simulation study on fluid velocity field in melt was reported in this paper by using Finite Differential Method (FDM) during the growth of Nd:YAG crystals by Cz (Czochralski) method. First, the mathematical model of Nd:YAG crystal growth was given out. The momentum conservation equation of melt, the continuity equation of melt, the heat conservation equation of melt, the mass conservation of melt, the heat conservation equation of crystal and the heat conservation equation of crucible were included in the model. Then, the dimensionless equations were given for solving the equations by the numerical method. Finally, the boundary conditions were given and the equations were solved by FDM. The simulation program was given by above method and then the changes of fluid velocity field in melt were described by simulation program. The effects of processing conditions on fluid velocity field in melt were analyzed. The effects of simulation time, the effects of crystal rotation speed during the crystal growth, the effects of the crystal diameter and the effects of the crucible size were considered. The results of simulation indicated: 1. The forced convection in the melt is weaker than the natural convection during the process of simulation, so that it can’t be shown in the figure of fluid velocity field, and the forced convection will become weaker and weaker with the simulation time increased. 2. As the rotation speed of crystal is increased, the natural convection in the melt will become stronger and stronger, and the effective area of natural convection will become bigger than it is in initial cases. 3. The natural convection in the melt will become weaker when either the diameter of crystal or the ratio of the crucible diameter to its depth is increased. Although the above method is employed to simulate the fluid velocity field during the crystal growth of Nd:YAG, it can also be used to simulate some other single crystal growth by Cz method, such as the growth of silicon single crystal, the GaAs single crystal and the GGG single crystal and so on. It is essential to adapt the parameters of the equations and the simulation program to the new materials.关键词：crystal growth;Cz method;velocity field;numerical simulation;finite differential method (FDM)97|70|2更新时间：2020-08-11
- 摘要：The light emitting intensity of light-emitting diode (LED) improved so much in these years that it is profitable enough to be used for design LED solid lighting. This LED light have been proved to be having many excellent virtues and was predicted to be having the possibility of leading the lighting industry into a new revolution. So in order to meet the design and optimization requirement of developing LED light, an effective method that is befitting in this work should be found out. In this article, the traditional LED’s light distribution is obtained through carrying out simulation of LED’s optical encapsulation structure by means of Monte Carlo method but not geometry optics method. The Monte Carlo LED simulation model is abstracted as dot light source, reflecting bowl, interface of epoxy resin and air. Light is treated as photons’ physical reactions with every possible part of whole LED’s optical encapsulation structure after emitting from the dot light source. These reactions include photons’ reflection and refraction on interface, absorption by epoxy resin material or wear down by interface of optical structure. Every stochastically emitted photo follows the physical optics and geometry optics rules and finally gets out of encapsulation and then is collected in a screen. When a large number of photons’ travel routes and locations of reaction with screen were determined, the light distribution pattern of correspondent LED’s optical encapsulation structure would be presented. And the average illumination intensity can be figured out through statistic method. Here, some kinds of traditional LED’s optical encapsulation structure were simulated using Monte Carlomethod. And experiments were performed using the correspondent LED’s optical encapsulation structure with the same conditions just like what were used in simulation. The Φ5, Φ12 dome-top and Φ5 plane-top LED tubes were simulated and experimentally measured for comparison. Some conclusions can be drawn that the simulation results fit the experimentation results well in the light distribution. This testified that Monte Carlo method is one of the effective tools for LED light’s optical design. There are some differences between simulation and experiment results in the patterns’ distribution scale because there are still some conditions’ differences just as characteristics of encapsulation materials and shapes of encapsulation structure. It is accurate enough for analyzing and finding out the relations between light distribution and correspondent optical structure and directing the right ways for designing more effective LED with new optical structure. The optimization and construction of random figures, the validation of simulation results, the comparison of different optical structure and the structure optimization were discussed in this article.关键词：Monte Carlo method;computer simulation;light-emitting diode (LED) light;optical encapsulated structure design95|86|2更新时间：2020-08-11
- 摘要：In recent years, the high power lasers have been widely applied in military, industry, medicine, such as in the strong laser weapon, laser detonator, laser guidance, precise machine processes, light storage, light pumps, laser chemistry, laser medicine and so on. Because of the asymmetry of semiconductor laser output beam, the beam must be shaped by optical system in some cases. The far-field characteristics of laser out put must be understood during designing the optical element and optical coupling.Furthermore, optical-field characteristics of laser cavity can be obtained by correct far-field modal with measure data. In this project,the Helmholtz equation was solved by the cleavage facet boundary condition of quantum-well laser. So the density distribution of far-field and divergence angle of beam were obtained. The theoretical curves and data obtained by computation were ploted also. The curve of the far-field distribution of laser and the measure data by the semiconducter laser all-parameter measurement meter, including the far-field intensity profile and the beam divergence angle. The experimental results are in agreement with the theory. The laser sample used in the experimental measurement is 82# laser diode, CW output power as 2.01W, operating current as 10.53A, operating voltage as 1.7V, slope efficiency as 1.01W/A, center wavelength as 801.8nm, spectral width 2 nm, emitting dimensions 150μm×1μm, divergence angle of perpendicular junction plan as 28.8°, divergence angle of parallel junction plane as 7.5°, the experimental measurement performed with a separation distance of 50 mm from the laser cavity surface. The results showed that the theoretical far-field intensity distribution curve is in accordance with the experimental measurement data basically.关键词：high power laser diode;beam quality;far-field characteristics100|98|4更新时间：2020-08-11
- 摘要：Researchers and designers who work with color displays often confronts with the color gamut transform between different display devices. However, there are still deviations in the transformed gamut for other factors. This paper demonstrates the principle in the color transformation and the way the color transformed enveloped is plotted when quantization form is present. Simultaneously, the analysis of error on three-dimensional representation of the color gamut has been given. This paper demonstrates the effect of quantization error on the transformation based on analyzing the color gamut deviation profoundly. The conclusion can be utilized to appraise the quality of the color gamut transformation and can tell whether it is possible to obtain a color of a given chromaticity at a required display panel. Such interdependencies are easily recognized when full three-dimensional representations are available.关键词：color display;quantization error;color gamut transformation93|115|3更新时间：2020-08-11
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