CdTe/CdS核壳结构量子点超快载流子动力学

在水相合成CdTe以及CdTe/CdS核壳结构量子点基础上, 利用基于抽运-探测技术的瞬态差分透射技术研究了CdTe量子点以及不同CdS壳层厚度的CdTe/CdS量子点的最低激子能态的超快激发与弛豫动力学. 研究表明:相比于CdTe,CdTe/CdS量子点的电子空穴由于空间分离,其所需的激发时间要长于电子空穴空间重叠态所需要的激发时间.随着壳层厚度的增加, 量子点表面的钝化有效地减少了表面态相关弛豫机理,并延长相对应的弛豫时间.     

Luminescence decay of broadband emission from CdS quantum dots

The stretched exponential photoluminescence decay of the energy-resolved broadband emission of purified and unpurified CdS quantum dots (QDs) made in reverse micelles is characterized as a function of photolysis time and thiol addition. Photolysis is found to proportionately increase both the lifetime and quantum yield of these QDs. This proportionality is consistent with a simple parallel channel model of the decay of the excited states. The ultimate QY of the purified sample is found to be as high as 24%, which is twice that previously reported for this preparation. At −70 °C both the QY and the lifetime increase by more than a factor of two, indicating that thermal quenching limits the QY at room temperature. Finally, the addition of alkanethiols is shown to red-shift and quench the emission while only modestly altering the lifetime. These thiolated QDs show an extremely large temperature dependence of QY, demonstrating stronger thermal quenching than the unfunctionalized QDs.     

Pump–probe absorption/gain spectroscopy of strongly confined semiconductor nanocrystals

The density matrix approach has been employed to analyze the pump–probe spectroscopic absorption spectra of small semiconductor nanocrystals popularly known as quantum dots (QDs) under the strong confinement regime (SCR) with sizes smaller than the bulk exciton Bohr radius such that the Coulombic interaction energy becomes negligible in comparison with the confinement energy. The average time rate of absorption has been obtained by incorporating the radiative and nonradiative decay processes as well as the inhomogeneous broadening arising due to nonuniform QD sizes. The analytical results are obtained for QDs duly irradiated by a strong near-resonant pump and a broadband weak probe. Numerical estimations have been made for: (i) isolated QDs and (ii) QD-arrays of GaAs and CdS. The results agree very well with the available experimental observations in CdS QDs. The results in the case of GaAs QDs can lead one to experimentally estimate absorption/gain spectra in the important III–V semiconducting microscopic structures.     

Abnormal Energy Dependence of Photoluminescence Decay Time in InGaN Epilayer

We employ photoluminescence (PL) and time-resolved PL to study exciton localization effect in InGaN epilayers.By measuring the exciton decay time as a function of the monitored emission energy at different temperatures,we have found unusual behaviour of the energy dependence in the PL decay process. At low temperature, the measured PL decay time increases with the emission energy. It decreases with the emission energy at 200K, and remains nearly constant at the intermediate temperature of 12OK. We have studied the dot size effect on the radiative recombination time by calculating the temperature dependence of the exciton recombination lifetime in quantum dots, and have found that the observed behaviour can be well correlated to the exciton localization in quantum dots. This suggestion is further supported by steady state PL results.     

Study of steady state and time resolved photoluminescence of thiol capped CdS nanocrystalline powders dispersed in N,N-dimethylformamide

The microwave (MW) assisted synthesis of thiol capped cadmium sulfide (CdS) nanocrystallites/quantum dots (QDs) was performed through the reaction of cadmium acetate with thiourea in N,N-dimethylformamide (DMF) by keeping the MW irradiation time fixed (40 s) in the presence of a thiol containing capping agent. Three capping agents, namely, benzyl mercaptan (BM), 1-butanethiol (BT) and 2-mercaptoethanol (ME) were used. The concentration of the precursors was varied to check the change in the average size of the thiol capped CdS nanocrystals formed. The nanocrystallites were characterized by usual procedure. The UV-vis absorption spectra and the photoluminescence (PL) spectra of the CdS nanocrystalline powders dispersed in DMF were studied. It was observed that with increase in concentration of the capping agent (BM), there is a shift in the nature of emission (PL) from trap associated PL to the band edge luminescence in the case of BM capped CdS nanocrystalline powders dispersed in DMF possibly due to better surface passivation. The relative PL quantum yield of the thiol capped CdS nanocrystalline powders dispersed in DMF was calculated under various experimental conditions. Time-correlated single-photon counting experiments were performed to study the time-resolved photoluminescence of the CdS nanocrystalline powders dispersed in DMF. The observed emission decay profiles have been simulated using the multiexponential model. The emission decay profiles for thiol capped CdS nanocrystalline powders dispersed in DMF depend on the nature of the capping agents (thiols) used to passivate the CdS nanocrystallites. The time resolved PL studies show that the average values of PL lifetime are related to the size and size distribution of the prepared thiol capped CdS nanocrystallites.     

Exciton photoluminescence of ZnSe and CdS quantum dots in borosilicate glasses prepared by the sol-gel method

The spectra of exciton photoluminescence (PL) of ZnSe and CdS quantum dots (QDs) synthesized in borosilicate glasses by the sol-gel method are measured and analyzed. It is shown that the positions of the PL bands in the spectra of both systems are related to the spatial quantization of the energy of excitons in QDs. Significant differences in the conditions of this process in ZnSe and CdS QDs are revealed. It is ascertained that, at some critical concentration of the semiconductors (x _cr), exciton percolation levels are formed in both systems, which manifest themselves in an abrupt change in the shape and spectral position of the emission bands of both systems and the constancy of the noted band parameters with a further increase in the concentration. The values of the critical and mean radii of QDs in the stage of their Ostwald ripening are obtained for several concentrations of the doping semiconductors.     

自组织生长InAs/GaAs量子点发光动力学研究

介绍了最新发展的粒子数混合超快光谱测量技术,以及采用该技术对自组织生长InAs/GaAs量子点发光动力学的研究结果.实验发现,自组织InAs/GaAs量子点结构的发光寿命大约为1ns,与InAs层厚度关系不大;激子寿命与温度有一定的关系,但没有明显的实验证据表明与量子点的δ态密度有关;用粒子数混合技术,实验上可直接观察到量子点中载流子在激发态能级的态填充过程. 关键词：     

Optical anisotropy and the direction of polarization of exciton emissions in a semiconductor quantum dot:Effect of heavy-and light-hole mixing

The dependence of the directions of polarization of exciton emissions, fine structure splittings(FSS), and polarization anisotropy on the light-and heavy-hole(LH–HH) mixing in semiconductor quantum dots(QDs) is investigated using a mesoscopic model. In general, all QDs have a four-fold exciton ground state. Two exciton states have directions of polarization in the growth-plane, while the other two are along the growth direction of the QD. The LH–HH mixing does affect the FSS and polarization anisotropy of bright exciton states in the growth-plane in the low symmetry QDs(e.g., C_(2V),C_S, C_1), while it has no effect on the FSS and polarization anisotropy in high symmetry QDs(e.g., C_(3V), D_(2d)). When the hole ground state is pure HH or LH, the bright exciton states in the growth-plane are normal to each other. The LH–HH mixing affects the relative intensities and directions of bright exciton states in the growth-plane of the QD. The polarization anisotropy of exciton emissions in the growth-plane of the QD is independent of the phase angle of LH–HH mixing but strongly depends on the magnitude of LH–HH mixing in low symmetry QDs.     

CdS量子点超声水相制备、表征及与萘的相互作用

在室温条件下,用超声水相制备CdS量子点(QDs),并用透射电镜、X射线衍射、紫外光谱法、荧光光谱法等手段表征.探讨了初始pH、反应物配比、超声时间、熟化时间等因素对CdS量子点制备的影响.初步研究了该方法制备的CdS量子点与萘的相互作用,线性范围为0.002-0.025mg/mL,r<'2>=0.9965.CdS量子...     

Contribution of the electron-electron interaction to the optical properties of dense arrays of Ge/Si quantum dots

We present the results of an investigation of the light absorption due to interband and interlevel transitions and the photoconductivity in dense arrays of Ge quantum dots (QDs) in Si formed using the effect of self-organization during molecular-beam heteroepitaxy. It was found that the formation of charged exciton complexes composed of two holes and one electron, as well as of the be-exciton complexes in QDs of type II, leads to an increase in the energy of indirect (in real space) exciton transition, which is explained by the spatial separation of electron and hole. Self-consistent calculations of the wavefunctions for electrons and holes in exciton and in the exciton complexes showed that an electron in a single exciton is localized in the region of maximum stress for Si in the vicinity of the Ge pyramid apex, while a hole is localized near the pyramid base. In a be-exciton complex, electrons exhibit repulsion leading to their spatial separation. As a result, the second electron is bound at the boundary between Si and a continuous Ge layer in which the pyramid bases reside. The experimental data show that an increase in the charge carrier concentration in the ground state of QDs leads to a shortwave shift of the interband resonance and to the narrowing and shape change of the light absorption band, which is explained by depolarization of the external electromagnetic wave due to interaction with the collective charge density oscillations in the lateral direction of the array of Ge nanoclusters. It is established that the hole injection into an excited state of QDs leads to a longwave shift of the photoconductivity peak as a result of decay of the collective excitations and suppression of the depolarization effect.     

CdS_xSe_(1-x)/ZnS(核/壳)量子点的光谱截面及其掺杂光纤的传光特性

测量了不同组份比例x的CdS_xSe_(1-x)/ZnS(核/壳)量子点的吸收谱和发射谱,确定了量子点的吸收系数、吸收截面和发射截面.量子点吸收截面随粒径的增大而增大、随x的增大而减小.采用紫外固化胶,制备了掺杂浓度为0.1~5mg/mL的CdS_(0.4)Se_(0.6)/ZnS量子点光纤,测量了不同掺杂浓度量子点光纤中473nm泵浦功率的吸收衰减速率.吸收衰减速率和吸收截面弱关联于掺杂浓度.测量了光致荧光光谱强度随光纤长度和量子点浓度的变化.量子点光纤的光致荧光峰值强度随掺杂浓度和光纤长度变化而变化,且存在一个与最大峰值强度对应的饱和掺杂浓度和光纤长度.本文的实验结果有助于进一步构建新型的CdS_xSe_(1-x)/ZnS量子点增益型光电子器件.     

Indium bound exciton luminescence in silicon

A high-resolution photoluminescence study is presented of the bound exciton line and its 4 meV low energy satellite in Si:In. The excitation and temperature dependence and the decay times suggest that the satellite line originates from the decay of an exciton bound to the indium acceptor. We tentatively correlate this line with excitations of the final state indium acceptors into a vibrational excited level.     

Photoluminescence quantum and surface states of excitons in ZnSe and CdS nanoclusters

The optical spectra of quantum dots (QDs) of CdS and ZnSe grown in borosilicate glass by the sol-gel method are obtained and analyzed. It is found that at concentrations of the two semiconductors x<0.06% the emission spectra are due to annihilation of free (internal) excitons in quantum states. The mean size of the quantum dots for a given concentration of ZnSe and CdS is calculated and found to be in good agreement with the X-ray data, and the exciton binding energy is calculated with allowance for the dielectric mismatch between the semiconductor and matrix. It is proposed that this mismatch may be the cause giving rise to the exciton percolation level that is observed in QD arrays for both systems at x>0.06%. The emission from the surface level of CdS QDs in the region ~2.7 eV, formed by the outer atoms with dangling bonds, is observed for the first time, as is the emission band from surface localized states. The relation between the position of the maximum of this band and the energy of the 1S state of the free exciton is established. It is shown that the properties of surface localized states are largely similar to the analogous properties of localized states of 3D (amorphous semiconductors, substitutional solid solutions of substitution) and 2D (quantum wells and superlattices) structures.     

Zeeman mapping of exciton localization in self-assembled CdSe quantum dots using diluted magnetic semiconductors

Localization of exciton wavefunctions in self-assembled quantum dots (QDs) has been investigated using CdSe QDs embedded in ZnMnSe. This system was chosen so as to make use of the giant Zeeman splitting in the diluted magnetic semiconductor (DMS) ZnMnSe, which enables one to map how the exciton wavefunction is distributed between the QDs and the surrounding matrix. Two series of CdSe QDs in ZnMnSe were prepared for this investigation by molecular beam epitaxy (MBE), either by varying the CdSe coverage while keeping a constant Mn concentration in ZnMnSe; or by varying the Mn concentration in the matrix while maintaining a constant CdSe coverage. Photoluminescence (PL) experiments show a systematic evolution of the CdSe QDs with increasing CdSe coverage; and also reveal the role of Mn in nucleating (“seeding”) the self-assembly of the QDs. By simultaneously measuring the Zeeman shifts of the PL peaks from both the CdSe QDs and their ZnMnSe matrix, we are able to extract information on exciton localization in the QDs and its dependence on the degree of development of the self-assembled CdSe QDs with increasing CdSe coverage.     

Effects of excitation spectral width on decay profile of weakly confined excitons

We report the effect due to a simultaneous excitation of several exciton states on the radiative decay profiles on the basis of the nonlocal response of weakly confined excitons in GaAs thin films. In the case of excitation of single exciton state, the transient grating signal has two decay components. The fast decay component comes from nonlocal response, and the long-lived component is attributed to free exciton decay. With an increase of excitation spectral width, the nonlocal component becomes small in comparison with the long-lived component, and disappears under irradiation of a femtosecond-pulse laser with broader spectral width. The transient grating spectra clearly indicates the contribution of the weakly confined excitons to the signal, and the exciton line width hardly changes by excitation spectral width. From these results, we concluded that the change of decay profile is attributed not to the many-body effect but to the effect of simultaneous excitation of several exciton states.     

Exciton emission dynamics in single InAs/GaAs quantum dots due to the existence of plasmon-field-induced metastable states in the wetting layer

A very long lifetime exciton emission with non-single exponential decay characteristics has been reported for single InA-s/GaAs quantum dot(QD) samples,in which there exists a long-lived metastable state in the wetting layer(WL)through radiative field coupling between the exciton emissions in the WL and the dipole field of metal islands.In this article we have proposed a new three-level model to simulate the exciton emission decay curve.In this model,assuming that the excitons in a metastable state will diffuse and be trapped by QDs,and then emit fluorescence in QDs,a stretchedlike exponential decay formula is derived as I(t)=At~(β-1)e~(-(rt)β),which can describe well the long lifetime decay curve with an analytical expression of average lifetime     

Mechanism of fluorescence energy transfer in aqueous solution cadmium sulfide quantum dots

Cadmium sulfide (CdS) quantum dots (QDs) prepared by a convenient chemical method have been characterized using absorption, fluorescence, and photoluminescence excitation techniques. The photoluminescence excitation studies show that there is an electron transfer from the surface adsorbate (thiourea) to CdS QDs in aqueous solution. The excitation band with peak maximum at 5.8 eV is assigned to the electronic transitions in the chemisorbed thiourea, whereas the excitation band between 3.45 and 3.7 eV corresponds to the band-to-band transition within the nanocrystalline CdS host. The absorption spectroscopy of the CdS QD solutions shows a strong absorption peak which is generated from thiourea. The band-edge fluorescence of the CdS QDs has also been investigated. It is shown that the fluorescence property of the CdS QDs can be enhanced by adding cadmium chloride (CdCl₂) solution.     

CdTe,核 壳型CdTe/CdS及CdTe/ZnS量子点的合成及表征

在水相中制备了半导体CdTe纳米晶,核 壳型CdTe/CdS和CdTe/ZnS纳米晶（即量子点;QDs）.利用扫描隧道显微镜（STM）和荧光光谱(FS)对合成的纳米晶量子点进行了研究,并且根据FS的数据进行了量子效率的计算.STM的结果表明合成的量子点直径约为3 nm并且分布良好.为了提高量子效率,对Cd2+浓度和Cd2+∶S2－比例等反应条件进行了研究,结果表明随着回流时间的增加,核 壳型量子点CdTe/CdS的量子效率总体上呈下降趋势.CdTe/CdS在pH8.5,Cd2+∶S2－=10∶1（摩尔比）时可获得80.0%的最大量子效率.同时制备了核 壳型量子点CdTe/ZnS,其最大发射波长由551 nm（CdTe）红移到635 nm（CdTe/ZnS）表明量子点的尺寸在增长,但是量子效率下降到14.4%. 当前研究的量子点可适用于生物标记,生物成像,以及基于共振能量转移的生物传感研究.     

Room-temperature exciton storage in elongated semiconductor nanocrystals

The excited state of colloidal nanoheterostructures consisting of a spherical CdSe nanocrystal with an epitaxially attached CdS rod can be perturbed effectively by electric fields. Field-induced fluorescence quenching coincides with a conversion of the excited state species from the bright exciton to a metastable trapped state (dark exciton) characterized by a power-law luminescence decay. The conversion is reversible so that up to 10% of quenched excitons recombine radiatively post turn-off of a 1 micro s field pulse, increasing the delayed luminescence by a factor of 80. Excitons can be stored for up to 10(5) times the natural lifetime, opening up applications in optical memory elements.     

Shape dependent electronic structure and exciton dynamics in small In(Ga)As quantum dots

We present a study of the primary optical transitions and recombination dynamics in InGaAs self-assembled quantum nanostructures with different shape. Starting from the same quantum dot seeding layer, and depending on the overgrowth conditions, these new nanostructures can be tailored in shape and are characterized by heights lower than 2 nm and base lengths around 100 nm. The geometrical shape strongly influences the electronic and optical properties of these nanostructuctures. We measure for them ground state optical transitions in the range 1.25–1.35 eV and varying energy splitting between their excited states. The temperature dependence of the exciton recombination dynamics is reported focusing on the intermediate temperature regime (before thermal escape begins to be important). In this range, an important increase of the effective photoluminescence decay time is observed and attributed to the state filling and exciton thermalization between excited and ground states. A rate equation model is also developed reproducing quite well the observed exciton dynamics.