Influence of successive electron and laser irradiation on the photoluminescence of porous silicon

The influence of electron irradiation on the light-emitting properties of p-and n-type porous silicon prepared by electrochemical etching is investigated. The dose and energy dependences of the electron-stimulated quenching of the photoluminescence (PL) are determined. It is shown that electron treatment of a porous silicon surface followed by prolonged storage in air can be used to stabilize the PL. The excitation of photoluminescence by a UV laser acting on sections of porous silicon samples subjected to preliminary electron treatment is discovered for the first time. The influence of the electron energy and the power of the laser beam on this process is investigated. The results presented are attributed to variation in the number of radiative recombination centers as a result of the dissociation and restoration of hydrogen-containing groups on the pore surface. Zh. Tekh. Fiz. 68, 58–63 (March 1998)     

Size-dependent photoluminescence properties under vacuum ultraviolet excitation of Zn2SiO4:Mn nanophosphors

The Zn₂SiO₄:Mn²⁺ nanophosphors with different particle sizes were synthesized via the hydrothermal method by adjusting the concentrations of surfactant and the hydrothermal temperature. The behavior of the photoluminescence as a function of phosphor particle sizes under vacuum ultraviolet excitation was investigated. Higher critical quenching concentration with decreasing particle size of the Zn₂SiO₄:Mn²⁺ nanophosphors was observed. This is ascribed to the hindrance of energy transfer between luminescence centers under vacuum ultraviolet excitation. The prolonged decay time in smaller samples provides further evidence that the energy transfer confinement has an effect on the photoluminescence properties.     

Luminescence properties of type-II quantum well light-emitting diodes formed with NPB and Alq3

The organic quantum well devices which are similar to the type-II quantum well of inorganic semiconductor have been fabricated. In the electroluminescence, the blue shift of spectrum with increasing applied voltage is observed, which is interpreted by exciton confinement effect and polarization effect, and the generation of exciton, including carrier injection and energy transfer, is discussed. This energy transfer from barrier to well is studied by photoluminescence and is interpreted in terms of Förster energy transfer. The electromodulation of photoluminescence demonstrates the quenching mainly comes from the dissociation of exciton in NPB and that in Alq₃ is very stable.     

Photoluminescence quenching through resonant energy transfer in blends of conjugated polymer with low-molecular acceptor

A model is proposed for photoluminescence quenching due to resonant energy transfer in a blend of a conjugated polymer and a low-molecular energy acceptor. An analytical dependence of the normalized photoluminescence intensity on the acceptor concentration is derived for the case of a homogeneous blend. This dependence can be described by two fitting parameters related to the Förster radii for energy transfer between conjugated segments of the polymer and between the conjugated polymer segment and the energy acceptor. Asymptotic approximations are obtained for the model dependence that make it possible to estimate the contribution from the spatial migration of excitons to the photoluminescence quenching. The proposed model is used to analyze experimental data on the photoluminescence quenching in a blend of the soluble derivative of poly(p-phenylene vinylene) and trinitrofluorenone [13]. The Förster radius for resonant energy transfer between the characteristic conjugated segment of poly(p-phenylene vinylene) and the energy acceptor is determined to be r _F = 2.6 ± 0.3 nm.     

纳米晶Y2SiO5：Eu的浓度猝灭研究

报道了分别用溶胶-凝胶法合成的Y₂SiO₅:Eu纳米晶和用高温固相法合成常规尺度的Y₂SiO₅:Eu材料的光致发光光谱和猝灭浓度的实验研究.结果表明:纳米Y_2-xEu_xSiO₅比常规尺度的Y_2-xEu_xSiO₅有更高的猝灭浓度和更高的发光亮度.理论分析认为这是由于在纳米材料中能量共振传递被阻断和猝灭中心在各个纳米晶内分布的涨落造成的.这个结果为高亮度的Y₂SiO₅:Eu纳米材料的实际开发应用展示了广阔前景.     

Influence of the ion synthesis and ion doping regimes on the effect of sensitization of erbium emission by silicon nanoclusters in silicon dioxide films

The photoluminescence spectra of erbium centers in SiO₂ films with ion-synthesized silicon nanoclusters under nonresonant excitation were investigated. Erbium was introduced into thermal SiO₂ films by ion implantation. The dependences of photoluminescence intensity on the dose, the order of ion implantation of Si and Er, the annealing temperature, and additional Ar⁺ and P⁺ ion irradiation regimes, i.e., factors determining the influence of radiation damage and doping on sensitization of erbium luminescence by silicon nanoclusters, were determined. It was found that the sensitization effect and its amplification due to doping with phosphorus are most pronounced under the conditions where nanoclusters are amorphous. The quenching of photoluminescence due to radiation damage in this case manifests itself to a lesser extent than for crystalline nanoclusters. The role of various factors in the observed regularities was discussed in the framework of the existing concepts of the mechanisms of light emission and energy exchange in the system of silicon nanoclusters and erbium centers.     

键合氢对GDa—Si:H薄膜光致发光性质的影响

测量了在不同衬底温度下制备的GDa-Si:H膜的光致发光谱和光吸收谱。实验发现键合氢含量的增加导致光致发光峰强度、发光峰能量、发光带半宽度、Stokes位移和热淬灭的增大。由此导出:(1)键合氢不仅能消除无辐射复合中心,而且能产生辐射复合中心;(2)随着键合氢含量的增加电子-声子相互作用增强,带尾宽度缓慢变窄。 关键词：     

Reversible quenching of luminescence in ZnO films by electric field action

We report the effect of the external electric field on the photoluminescence (PL) properties of ZnO films grown by a pulsed laser deposition method. The PL quenching of bound excitons under the electric field was attributed to a decrease in the capture cross section of the radiative centers. In addition, the change in the surface/grain boundaries charge induced a degradation of the 3.33 eV emission line over the whole sample, which remained even after voltage removal. Besides the PL degradation, this emission at 3.33 eV demonstrates the change in the thermal quenching process, where the activation energy of exciton detachment corresponds to its binding energy. All behaviors were restored to the initial state by application of the voltage with opposite polarity. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Photoinduced processes in nanocrystals of cadmium selenide in an external electric field

The dependence of photoinduced processes in cadmium selenide nanocrystals in an external electric field on the energy of the exciting photons is established. A relationship between the photoinduced buildup of photoluminescence and its quenching in an external electric field in quantum sized cadmium selenide nanocrystals is demonstrated. The mechanisms for the quenching of the photoluminescence and for the influence of the exciting photon energy on the photoinduced processes are discussed.     

Carrier dynamics in type-II GaAsSb/GaAs quantum wells

Time-resolved photoluminescence (PL) characteristics of type-II GaAsSb/GaAs quantum wells are presented. The PL kinetics are determined by the dynamic band bending effect and the distribution of localized centers below the quantum well band gap. The dynamic band bending results from the spatially separated electron and hole distribution functions evolving in time. It strongly depends on the optical pump power density and causes temporal renormalization of the quantum well ground-state energy occurring a few nanoseconds after the optical pulse excitation. Moreover, it alters the optical transition oscillator strength. The measured PL lifetime is 4.5 ns. We point out the critical role of the charge transfer processes between the quantum well and localized centers, which accelerate the quantum well photoluminescence decay at low temperature. However, at elevated temperatures the thermally activated back transfer process slows down the quantum well photoluminescence kinetics. A three-level rate equation model is proposed to explain these observations.     

Physical implications of activation energy derived from temperature dependent photoluminescence of InGaN-based materials

Physical implications of the activation energy derived from temperature dependent photoluminescence(PL) of In Ga Nbased materials are investigated, finding that the activation energy is determined by the thermal decay processes involved.If the carrier escaping from localization states is responsible for the thermal quenching of PL intensity, as often occurs in In Ga N materials, the activation energy is related to the energy barrier height of localization states. An alternative possibility for the thermal decay of the PL intensity is the activation of nonradiative recombination processes, in which case thermal activation energy would be determined by the carrier capture process of the nonradiative recombination centers rather than by the ionization energy of the defects themselves.     

富勒烯C60掺杂的共轭高聚物混合薄膜的光谱性质

测量了不同富勒烯C60掺杂浓度的聚对亚苯基亚乙烯衍生物（MEH-PPV）混合薄膜的紫外-可见吸收光谱、稳态荧光和时间分辨荧光光谱。观察到MEH-PPV吸收峰受到明显的抑制以及荧光峰的猝灭。证明混合薄膜内，由于л-л共轭体系的强相互作用在基态发生MEH-PPV向C60的电子转移、形成电荷转移络合物。在光激发下会发生MEH-PPV向C60的激发传递进一步抑制了发光跃迁过程，导致了MEH-PPV荧光的强猝灭效应。     

晶体离子质量差异对激活离子声子参助能量传递几率的影响

在已有理论模型的基础上讨论了基质晶体离子质量差异对激活离子声子参助能量传递几率的影响。结果表明基质离子的质量差异除了导致声子频率的变化从而影响其声子参助能量传递几率,还直接改变其传递几率的显式。对单声子参助能量传递过程,其传递几率必须乘以质量差异因子D²,对双声子参助能量传递过程其传递几率则乘以因子D⁴。文中给出了两种不同质量离子组成的晶体的因子D的表示式和p种不同质量的离子组成的晶体的因子D的一般表达式。     

Silver aggregates and twofold-coordinated tin centers in phosphate glass: A photoluminescence study

The optical properties of silver species in various oxidation and aggregation states and of tin centers in melt-quenched phosphate glasses have been assessed by optical absorption and photoluminescence (PL) spectroscopy. Glasses containing silver and tin, or either dopant, were studied. Emission and excitation spectra along with time-resolved and temperature-dependent PL measurements were employed in elucidating the different emitting centers observed and investigating on their interactions. In regard to silver, the data suggests the presence of luminescent single Ag⁺ ions, Ag⁺-Ag⁺ and Ag⁺-Ag⁰ pairs, and nonluminescent Ag nanoparticles (NPs), where Ag⁺-Ag⁰→Ag⁺-Ag⁺ energy transfer is indicated. Tin optical centers appear as twofold-coordinated Sn centers displaying PL around 400 nm ascribed to triplet-to-singlet electronic transitions. The optically active silver centers were observed in glasses where 8 mol% of both Ag₂O and SnO, and 4 mol% of Ag₂O were added. Heat treatment (HT) of the glass with the high concentration of silver and tin leads to chemical reduction of ionic silver species resulting in a large volume fraction of silver NPs and the vanishing of silver PL features. Further characterization of such heat-treated glass by transmission electron microscopy and X-ray photoelectron spectroscopy appears consistent with silver being present mainly in nonoxidized form after HT. On the other hand, HT of the glass containing only silver results in the quenching of Ag⁺-Ag⁰ pairs emission that is ascribed to nonradiative energy transfer to Ag NPs due to the positioning of the pairs near the surface of NPs during HT. In this context, an important finding is that a faster relaxation was observed for this nanocomposite in relation to a heat-treated glass containing both silver and tin (no silver pairs) as revealed by degenerate four-wave mixing spectroscopy. Such result is attributed to Ag NP→Ag⁺-Ag⁰ plasmon resonance energy transfer. The data thus indicates that energy transfer between Ag⁺-Ag⁰ pairs and NPs is bi-directional.     

Photoluminescence of semiconducting glasses: the role of negative-U centers, and pressure effects

A theoretical model of photoluminescence (PL) in semiconducting glasses is suggested, which is related to negative-U centers and their (two-particle) excitations and from which characteristic PL energies and frequency dependence of the PL intensity are found. An associated mechanism of an anomalous low-temperature thermal quenching of PL is revealed. Pronounced pressure-induced effects in PL are predicted.     

Influence of surface quenching on luminescence dynamics of ion centers in nanocrystals

In this paper, the influence of surface quenching on decay curve and quantum efficiency of ion centers in nanoparticles were discussed by considering energy transfer between doped ions and surface quench centers.With isotropic and continuum approximation, energy transfer rate of an ion at position r within a spherical nanoparticle to all the surface quench centers was calculated. Quantum efficiency and decay curve under nonselective excitation were then calculated numerically by integrating over the sample. The calculated decay curve was fitted to the experimental one measured in Eu³⁺-doped YVO₄ nanoparticles. The ratio of the quantum efficiency of YVO₄:Eu³⁺ nanoparticle (R=13.5 nm) to that of the bulk is 17%, estimated with the parameters obtained from the fitting.     

Energy transfer probe for the characterization of luminescent photonic crystals morphology

We evaluated energy transfer probe for the characterization of luminescent photonic crystals morphology. It demonstrates a monomolecular filming of fluorescent chelated complexes inside photonic crystal voids. Either direct fluorescence quenching of excited fluorescent donors by the acceptors in case of low concentration of fluorescent centers or fluorescence quenching accelerated by energy migration over fluorescent donor ions in case of 100% of sites occupied by donors is demonstrated in 2D space contrarily to the powder samples of the same complexes where energy transfer occurs in 3D space.     

Photoluminescence quenching of porous silicon by noble metal adsorbates

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Temperature and excitation dependence of stimulated emission and spontaneous emission in InGaN epilayer

Temperature and excitation dependent photoluminescence(PL) of InGaN epilayer grown on c-plane Ga N/sapphire template by molecular beam epitaxy(MBE) has been systematically investigated. The emission spectra of the sample consisted of strong multiple peaks associated with one stimulated emission(SE) located at 430 nm and two spontaneous emissions(SPE) centered at about 450 nm and 480 nm, indicating the co-existence of shallow and deep localized states.The peak energy of SE exhibiting weak s-shaped variation with increasing temperature revealed the localization effect of excitons. Moreover, an abnormal increase of the SPE intensity with increasing temperature was also observed, which indicated that the carrier transfer between the shallow and deeper localized states exists. Temperature dependent time-resolved PL(TRPL) demonstrated the carrier transfer processes among the localized states. In addition, a slow thermalization of hot carriers was observed in InGaN film by using TRPL and transient differential reflectivity, which is attributed to the phonon bottleneck effect induced by indium aggregation.     

Resonant electronic energy transfer from excitons confined in silicon nanocrystals to oxygen molecules

We demonstrate efficient resonant energy transfer from excitons confined in silicon nanocrystals to molecular oxygen (MO). Quenching of photoluminescence (PL) of silicon nanocrystals by MO physisorbed on their surface is found to be most efficient when the energy of excitons coincides with triplet-singlet splitting energy of oxygen molecules. The dependence of PL quenching efficiency on nanocrystal surface termination is consistent with short-range resonant electron exchange mechanism of energy transfer. A highly developed surface of silicon nanocrystal assemblies and a long radiative lifetime of excitons are favorable for achieving a high efficiency of this process.