Quenching of photoluminescence in cadmium selenide nanocrystals in external electric fields for different excitation photon energies

The effect of external electric fields on the photoluminescence of quantum-sized nanocrystals of cadmium selenide excited by photons of various energies is studied. Photoluminescence quenching by external electric fields is found to be different for nanoparticles with different shapes (quantum dots and nanorods) and does not depend on the exciting photon energy. The relationship between the strength of the external electric field and the degree of quenching is determined empirically for both types of nanoparticles. A possible mechanism for the effect of an external electric field on the excitation and quenching of photoluminescence in quantumsized nanoparticles is discussed.     

Effect of an electric field on photoluminescence of cadmium selenide nanocrystals

We have demonstrated a difference in the nature of the effect of a strong external electric field (>10⁵ V/cm) on the photoluminescence of cadmium selenide nanoparticles of different shapes. We have determined a correlation between the magnitude of the external electric field and the average photoluminescence decay time for two types of nanoparticles: "quantum dots" and nanorods. We discuss the mechanism for the effect of an electric field on the photoluminescence of both types of nanoparticles.     

The effect of nonadiabaticity on the efficiency of quantum memory based on an optical cavity

Photoluminescence of CdSe colloidal nanocrystals of different topologies in an external electric field has been studied. It has been found that quenching of photoluminescence, which takes place in quantum dots, is proportional to the square of the field, and in elongated nanocrystals quenching of photoluminescence is proportional to the square root. A physical model of the mechanism of quenching based on tunneling of free charges through potential barrier nanocrystal/matrix has been proposed.     

Quenching of photoluminescence from GaAs/AlGaAs single quantum well by an electric field at high temperature

Photoluminescence measurements at room temperature and at liquid nitrogen temperature on a GaAs/AlGaAs single quantum well structure subject to an electric field are performed to study (i) the photoluminescence quenching and (ii) the shift in the photoluminescence energy induced by the field. The observed shifts in the luminescence energies are explained successfully in terms of the field induced electron-hole separation model. For the quenching of the luminescence intensities, more work, particularly on nonradiative processes, is required to clarify the mechanism.     

Thermoelectronic mechanism of quenching the photoluminescence of ZnS — Cl phosphors by an electric field

Results are presented of an investigation of the quenching of the photoluminescence of ZnS — Cl by an electric field as a function of the specimen temperature, the intensity of the exciting radiation, and the electric-field intensity. The experimental data are explained on the basis of a thermoelectronic quenching mechanism.     

Zinc sulphide photoluminescence under weak alternating electric field conditions

The phenomenon of the quenching of photoluminescence by a weak alternating electric field was examined in the light of theory and verified by experiments on zinc sulphides. It is believed that the quenching is brought about by processes in the local regions of the crystal which correspond to energy barriers and that quenching involves the transition of valence electrons, which have obtained their energy from the electrons accelerated by the field, into the levels of luminescence centres. The kinetics of the processes in the barriers has been considered both for a reverse bias (a half-period of de-excitations) and for a direct bias (a half-period of recombination), and an expression for the average (time) quenching was obtained. The quenching characteristics calculated from the expression for quenching correlate well with the experimental data.     

Combined effects of electric field and hydrostatic pressure on electron states in asymmetric GaAs/(Ga,Al) triple quantum dots

The combined effects of an in-growth direction applied electric field and hydrostatic pressure on the exciton binding energy and photoluminescence energy transitions are reported in this work for triple vertically coupled quantum dots. The calculations have been carried out within the effective mass approximation, and using a variational procedure. The results show that the exciton binding energy and the photoluminescence energy transitions are functions of external probes like the hydrostatic pressure and the applied electric field.     

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)     

Aqueous Synthesis of Water-Soluble L-Cysteine-Modified Cadmium Sulfide Doped with Silver Ion/Zinc Sulfide Nanocrystals

The water-soluble silver ion-doped cadmium sulfide nanocrystals were synthesized by a co-precipitation technique in aqueous solution using L-cysteine as surface modifier, and then L-cysteine-modified cadmium sulfide doped with silver ion/zinc sulfide core/shell nanocrystals were prepared by zinc sulfide epitaxial coated on surface of silver ion-doped cadmium sulfide nanocrystals. The crystal structure, morphology, and spectral properties of cadmium sulfide doped with silver ion/zinc sulfide nanocrystals were characterized by X-ray power diffraction, transmission electron microscope, infrared spectrum, and photoluminescence spectrum. The results show that the photoluminescence quantum yield of cadmium sulfide doped with silver ion/zinc sulfide nanocrystals is improved greatly after doped with silver ion and coated with zinc sulfide shell. The cysteine modified on the surface of cadmium sulfide doped with silver ion/zinc sulfide nanocrystals renders the nanocrystals water-soluble and biocompatible.     

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.     

Novel Two-Step Approach for the Synthesis of Cadmium Selenide/Zinc Sulfide Core/Shell Nanocomposites with Precursor Injection Technique

This work describes the synthesis of cadmium selenide/zinc sulfide core/shell nanoparticles from the cadmium oxide precursor through a convenient, two-step approach. This modified novel synthesis procedure for cadmium selenide/zinc sulfide nanoparticles in trioctylphosphine oxide and trioctyl phosphine provides better control over growth dynamics. The outer zinc sulfide shell provides efficient confinement of electron and hole wave functions inside the nanocrystals as well as high photochemical stability. The materials have been characterized using a range of optical and structural techniques. The high resolution transmission electron microscope micrographs of the cadmium selenide/zinc sulfide core/shell nanoparticles show well-defined spherical particles with an increase in diameter as compared to the parent cadmium selenide material. Thus, the present simulation and its extension can give insight to the understanding of the formation of core/shell or other heterostructures in different kinds of self-assembled aggregates.     

All-Optical Photorefractive Effect in Carbazole-Based Azo-Side Group Polymer

We observe the photorefractivity without bias voltage or prepoling in a bifunctional photorefractive polymer. The maximum two-beam coupling gain is measured to be 126cm^-1 at zero bias voltage. The sample is considered to be poled by the photoinduced longitudinal electric field, which is formed due to the light intensity gradient along the light path. The expression of the electric field was deduced. The energy transfer direction between two writing beams and light intensity dependence of the two-beam coupling gain coefficient is predicted to be consistent with the experimental results. Furthermore, the dependence of the two-beam coupling gain coefficient on external applied electrical field is measured and this experiment verifies the existence of the photoinduced longitudinal electric field.     

Optical spectroscopy of GaAsGaAlAs quantum wells under an external electric field

We have studied by means of low temperature photoluminescence (PL) and photocurrent spectroscopy the effects of an external electric field on the excitons in GaAs quantum wells confined between GaAlAs. Increasing the field causes a Stark shift of the excitons toward lower energies with a simultaneous quenching in the PL intensity. At moderate fields, we find very good agreement (better than 0.5 meV) between the light- and heavy-hole exciton energies obtained by PL and photocurrent measurements. A significant deviation in energy of the PL relative to the photocurrent is observed at high fields, manifesting the increase in the contributions of impurity-bound excitons to the PL lineshape. A detailed PL study of the Stark shift as a function of well thickness has also been performed. The results show an increasing Stark shift with increasing well thickness, amounting to 110 meV for a 230 Å-wide well at a field of 10⁵ V/cm. For very wide wells (∼ 1000 Å) the behavior of bulk GaAs is recovered: the excitons become ionized before large Stark shifts can be observed. Variational calculations have been carried out and shown to account for the experimental observations of both the Stark shift and the quenching of the PL. In this light, we will discuss the mechanisms governing the optical properties of quantum wells under an external electric field.     

用巯基乙酸作稳定剂制备CdSe纳米晶的光学性质

以巯基乙酸为稳定剂制备了CdSe纳米晶，通过尺寸选择沉淀得到2nm到3nm之间不同尺寸的纳米晶，利用室温光吸收，光致发光（PL）和光致发光激发（PLE）谱来研究了CdSe纳米团簇的光学性质。紫外－可见吸收谱给了具有清晰激光特征的尖锐吸收边，这表明样品的尺寸分布很窄。光致发光研究表明，样品有两个发射带，一个具有较高能量位于吸收边，来自电子－空穴对从最低激发态能级弛豫后的辐射复合，另一个低能发射带归属于基质与纳米晶界面存在的俘获中心。PLE谱中有2个吸收带，分别是S－S和P－P跃迁。最后还给出了不同激发能量下的发光特性。     

Photoluminescence of Er<Superscript>3+</Superscript> ions in layers of quasi-ordered silicon nanocrystals in a silicon dioxide matrix

The spectra and kinetics of photoluminescence from multilayered structures of quasi-ordered silicon nanocrystals in a silica matrix were studied for undoped samples and samples doped with erbium. It was shown that the optical excitation energy of silicon nanocrystals could be effectively transferred to Er³⁺ ions, which was followed by luminescence at a wavelength of 1.5 µm. The effectiveness of energy transfer increased as the size of silicon nanocrystals decreased and the energy of exciting light quanta increased. The excitation of erbium luminescence in the structures was explained based on dipole-dipole interaction (the Förster mechanism) between excitons in silicon nanocrystals and Er³⁺ ions in silica surrounding them.     

Nanosecond dynamics of the near-infrared photoluminescence of Er-doped SiO2 sensitized with Si nanocrystals

We report on an observation of a fast 1.5 microm photoluminescence band from Er3+ ions embedded in an SiO2 matrix doped with Si nanocrystals, which appears and decays within the first microsecond after the laser excitation pulse. We argue that the fast excitation and quenching are facilitated by Auger processes related to transitions of confined electrons or holes between the space-quantized levels of Si nanocrystals dispersed in SiO2. We show that a great part--about 50%--of all Er dopants is involved in these fast processes and contributes to the submicrosecond emission.     

Luminescence and photosensitization properties of ensembles of silicon nanocrystals in terms of an exciton migration model

The relaxation processes that occur in ensembles of coupled silicon nanocrystals are described by a quantitative model that takes into account the energy transfer between them during exciton migration. This model is used to study the formation of singlet oxygen during the photoexcitation of silicon nanocrystals in porous silicon layers under various external conditions. It is experimentally found that, upon fine milling of as-deposited porous silicon films, the photoluminescence decay time increases despite an increase in the concentration of point defects. The photosensitized activity of ensembles of silicon nanocrystals degrades monotonically during their photostimulated oxidation. These experimental results agree completely with the conclusions of the model and are explained by a decrease in the number of exciton migration ways between nanocrystals when the granule size of a porous silicon powder decreases and by an increase in the efficiency of nonradiative recombination during the photostimulated oxidation of the nanocrystals. Our data indicate that nanocrystalline silicon is a promising material for the methods of nontoxic photodynamic therapy of oncological diseases.     

Electric-field-induced photovoltaic effect of azo–TiOPc composites

The electric-field-induced photovoltaic effects of TiOPc, DBCL and their composites in ITO/sample/Pt cells are described. The positive photovoltaic responses of TiOPc and DBCL are strongly enhanced when a positive external electric field is applied to the ITO electrode. However, only minimal discrepancies are found in the positive photovoltaic responses of the composites whenever an external electric field is applied. This is attributed to a photoinduced charge transfer between DBCL and TiOPc. An energy diagram of the charge-transfer process is proposed. Received: 12 July 2000 / Accepted: 29 March 2001 / Published online: 27 June 2001     

Effect of the external electric field on the kinetics of recombination of photoexcited carriers in a ZnSe/BeTe type II heterostructure

The kinetics of the radiative recombination of photoexcited electrons and holes for a spatially direct transition in a ZnSe/BeTe type II heterostructure in an external electric field has been analyzed. A strong decrease (more than two orders of magnitude) in the photoluminescence intensity, as well as a decrease in the duration of the relaxation of the direct transition, is observed when the electric field is applied. The energy levels and wavefunctions of electrons and holes in the ZnSe/BeTe heterostructure subjected to the electric field have been numerically calculated. It has been shown that the observed decrease in the photoluminescence intensity and duration of the relaxation of the direct transition is due to both an increase in the radiative recombination time and an increase in the rate of escape of photoexcited holes from the above-barrier level in the ZnSe layer to the BeTe layer.     

Effect of nonuniform permittivity of a solid-state matrix on the spectral width of erbium ion luminescence

The Stark splitting of the energy levels of Er³⁺ ions implanted in a structure made up of alternating layers of silicon dioxide and quasi-ordered silicon nanocrystals is calculated. The level splitting is caused by the electric field of the image charges induced at the interfaces between layers with different permittivities. The splitting was established to increase as the contrast in permittivity between the silicon dioxide and silicon nanocrystal layers increases, as well as when the erbium ions approach the layer interface. The results obtained offer an adequate explanation of the experimentally observed additional broadening of the erbium photoluminescence band (0.8 eV) with increasing characteristic size of the silicon nanocrystals.