Anomalous Stark shifts in single vertically coupled pairs of InGaAs quantum dots

Vertically coupled Stranski Krastanow quantum dots (QDs) are predicted to exhibit strong tunnelling interactions that lead to the formation of hybridised states. We report the results of investigations into single pairs of coupled QDs in the presence of an electric field that is able to bring individual carrier levels into resonance and to investigate the Stark shift properties of the excitons present. Pronounced changes in the Stark shift behaviour of exciton features are identified and attributed to the significant redistribution of the carrier wavefunctions as resonance between two QDs is achieved. At low electric fields coherent tunnelling between the two QD ground states is identified from the change in sign of the permanent dipole moment and dramatic increase of the electron polarisability, and at higher electric fields a distortion of the Stark shift is attributed to a coherent tunnelling effect between the ground state of the upper QD and the excited state of the lower QD.     

InAs/GaAs单量子点中电子/空穴自旋弛豫

利用分子束外延制备了三种类型量子点样品,它们分别是:未掺杂样品、n型Si调制掺杂样品和p型Be调制掺杂样品。在5 K温度下,采用共聚焦显微镜系统,测量了单量子点的光致发光谱和时间分辨光谱, 研究了单量子点中三种类型激子(本征激子、负电荷激子和正电荷激子)的电子/空穴自旋翻转时间。它们的自旋翻转时间常数分别为: 本征激子的自旋翻转时间约16 ns, 正电荷激子中电子的自旋翻转时间约2 ns, 负电荷激子中空穴的自旋翻转时间约50 ps。     

界面电子转移对量子点荧光闪烁行为的影响

利用激光扫描共聚焦显微系统分别测量了CdSe/ZnS量子点在SiO2玻片表面、铟锡氧化物(ITO)纳米粒子表面和聚甲基丙烯酸甲酯(PMMA)薄膜表面上的荧光闪烁行为.研究发现,不同界面环境中量子点的亮态发光持续时间的概率密度都服从指数修正的幂律分布P(t)∝t-αexp(-t/μ).与处于SiO2玻片表面的情况相比,在ITO表面上的单量子点具有非常短暂的亮态发光持续时间,而在PMMA表面的单量子点亮态发光持续时间最长.这种荧光闪烁行为的不同主要归因于量子点与三种材料之间的界面电子转移特性.     

Effect of Electrical Field on Colloidal CdSe/ZnS Quantum Dots

We fabricate the hybrid films of colloidal CdSe/ZnS quantum dots (QDs) and poly(9-vinylcarbazole) (PVK) sandwiched between two electrodes. The voltage and temperature dependences of the electroluminescence (EL) are measured. The quantum-confined Stark effect of colloidal QDs is clearly observed. To explore the mechanism in the QD EL, hybrid films are fabricated with different concentrations of colloidal QDs. Electrons and holes are proposed to be separately transported in QDs and PVK, respectively.     

Effects of a longitudinal magnetic field on spin injection and detection in InAs/GaAs quantum dot structures

Effects of a longitudinal magnetic field on optical spin injection and detection in InAs/GaAs quantum dot (QD) structures are investigated by optical orientation spectroscopy. An increase in the optical and spin polarization of the QDs is observed with increasing magnetic field in the range 0-2?T, and is attributed to suppression of exciton spin depolarization within the QDs that is promoted by the hyperfine interaction and anisotropic electron-hole exchange interaction. This leads to a corresponding enhancement in spin detection efficiency of the QDs by a factor of up to 2.5. At higher magnetic fields, when these spin depolarization processes are quenched, the electron spin polarization in anisotropic QD structures (such as double QDs that are preferably aligned along a specific crystallographic axis) still exhibits a rather strong field dependence under non-resonant excitation. In contrast, such a field dependence is practically absent in more 'isotropic' QD structures (e.g.?single QDs). We attribute the observed effect to stronger electron spin relaxation in the spin injectors (i.e.?wetting layer and GaAs barriers) of the lower-symmetry QD structures, which also explains the lower spin injection efficiency observed in these structures.     

Light-Emitting Diodes Based on All-Quantum-Dot Multilayer Films and the Influence of Various Hole-Transporting Layers on the Performance

We present a systematic analysis of the exciton-recombination zone within all-quantum-dot(QD) multilayer films using sensing QD layers in QD-based light-emitting diodes(QLEDs),and demonstrate the all-QD multilayer films with different sequences of layers prepared by inserting a sensing blue QD layer denoted as B at various positions within four red QD multilayers denoted as R.We also use different hole transporting layers(PVK,CBP as well as poly-TPD) to prevent the formation of leakage current and to improve the luminance.The results show that the total EL emission is mostly at the fourth(60%) and fifth(40%) QD monolayers,adjacent to ITO.This presents both decreasing current density and increasing brightness with different hole transporting layers,thus resulting in more efficient performance.     

利用N型半导体纳米材料抑制单量子点的荧光闪烁特性

利用N型半导体纳米材料氧化铟锡(ITO)作为单CdSe/ZnS量子点的基质来抑制单量子点的荧光闪烁特性. 实验采用激光扫描共聚焦显微成像系统测量了单量子点荧光的亮、暗态持续时间的概率密度分布的指数截止的幂律特性, 并与直接吸附在SiO₂玻片上的单CdSe/ZnS量子点的荧光特性进行比较. 研究发现处于ITO中的单量子点比SiO₂玻片上的单量子点荧光亮态持续时间提高两个数量级, 掺杂于ITO中的单量子点的荧光寿命约减小为SiO₂玻片上的单量子点的荧光寿命的41%, 并且寿命分布宽度变小50%.     

White light-emitting quantum dot diodes and tuning of luminescence processes

A novel white light-emitting diode based on a large Stokes shift (~200 nm) and using pure green light-emitting CdSeS quantum dots (QDs) with an Ag/ZnSnO/QDs/spiro-TPD/ITO structure has been fabricated in which ZnSnO and spiro-TPD are served as the electron and hole transport layer, respectively. The large Stokes shift of the CdSeS QDs excludes potentially Förster resonance energy transfer process, which allows spiro-TPD to act as both an emitter and hole transport layer. The devices exhibit a wide EL spectrum consisting of three components: blue emission from spiro-TPD, green emission from QD band–band recombination, and red emission from QD surface-state recombination. We further found that as the intensity ratios among these three components vary with bias the color of the QD light-emitting diodes is tunable. The device displays a good white light-emitting characteristic with CIE coordinates of (0.281, 0.384) at an appropriate bias.     

The influence of ligand type on self-organization and optical properties of cadmium selenide quantum dots

A method for successive replacement of organic shells of colloidal cadmium selenide quantum dots (QDs) of different sizes is proposed. It is found that the spectral parameters of QD samples depend on the type of organic shells. It is shown that the structural morphology is independent of the QD size and is determined by the chemical composition of the organic shell. Spectral analysis of the luminescence of QD-based superstructures shows that the luminescence wavelength and intensity strongly depend on the degree of QD surface passivation.     

InAs 单量子点精细结构光谱

在5 K下,采用光致发光光谱和时间分辨光谱研究了不同单量子点的精细结构和对应发光光谱的偏振性、单激子/双激子发光光谱和相应发光动力学。给出InAs单量子点发光光谱所对应能级的精细结构及激子本征态的偏振特性。当精细结构能级劈裂为零时, 激子的本征态为简并的圆偏振态。而当精细结构能级劈裂大于零时,一般在几十到几百μeV,激子的本征态为非简并的线偏振态。相对于单激子发光寿命,激子-激子间的散射使单激子的复合发光寿命减小。     

Quantum Dots Luminescence Collection Enhancement and Nanoscopy by Dielectric Microspheres

In recent years, dielectric microspheres have been used in conjunction with optical microscopes to beat the diffraction limit and to obtain superresolution imaging. The use of microspheres on quantum dots (QDs) is investigated, for the first time, to enhance the light coupling efficiency. The enhancement of the QD luminescence collection in terms of extraction and directionality is demonstrated, as well as the enhancement of spatial resolution. In particular, it is found that a dielectric microsphere, placed on top of an epitaxial QD, increases the collected radiant energy by about a factor of 42, when a low numerical aperture objective is used. Moreover, if two or more QDs are present below the microsphere, the modification of the far field emission pattern allows selective collection of the luminescence from a single QD by simply changing the collection angle. Dielectric microspheres present a simple and efficient tool to improve the QD spectroscopy, and potentially QD-based devices.     

基于聚合物-量子点共混的量子点发光二极管

在ITO玻璃上制备了ITO/poly(3,4-ethylene dioxythiophene)∶poly(styrene sulphonate)(PEDOT∶PASS)/poly(N,N-bis(4-butylphenyl)-N,N-bis(phenyl)benzidine(poly-TPD)/QD/1,3,5-Tri(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl(TPBi)/Li F/Al结构的量子点发光二极管(QD-LED)。通过优化量子点的浓度,发现浓度为30 mg/m L时的器件性能最优,最大外量子效率(EQE)为0.83%,最大发光亮度为4 076 cd/m2。为了进一步提高QD-LED的发光效率,将QD掺入聚合物poly(N-vinylcarbazole)(PVK)和1,3-Bis(5-(4-(tert-butyl)phenyl)-1,3,4-oxadiazol-2-yl)benzene(OXD-7)中,以使得注入的电子和空穴更加平衡,同时还有助于能量传递,降低QD团聚及修饰QD薄膜表面,减少激子猝灭效应等。为此,通过旋涂和蒸镀两步法制备ITO/PEDOT∶PASS/poly-TPD/(PVK∶OXD-7)∶QD/TPBi/Li F/Al结构的器件,改变(PVK∶OXD-7)∶QD比例(1∶1,1∶3,1∶5,0∶1),发现(PVK∶OXD-7)∶QD为1∶3时的QD-LED具有最优性能,最大EQE为1.97%,相当于非掺杂器件的2.3倍,并且发光峰没有发生偏移。     

Optically induced mechanical interaction between semiconductor quantum dots under an electronic resonance condition

Assuming an electronic resonance condition, we study the shape-dependence of the radiation force (RF) on a semiconductor quantum dot (QD) floating in medium and the optically induced mechanical interaction (OIMI) between two QDs with Maxwell stress tensor (MST) method, where the response fields are calculated by the improved discrete dipole approximation (DDA). Main results are as follows: (1) Properties of the RF on an isolated QD drastically change due to its shape and polarization of an incident light, which can be used for shape-selective manipulation. (2) Anomalous OIMI between two QDs arises depending on the spatial structures of internal fields, which results from the interaction between polarizations in respective QDs when they are near each other.     

Anti-Reflection Characteristics of Si Nanowires for Enhanced Photoluminescence from CdTe/CdS Quantum Dots

CdTe/CdS quantum dots(QDs) are fabricated on Si nanowires(NWs) substrates with and without Au nanoparticles(NPs). The formation of Au NPs on Si NWs can be certified as shown in scanning electron microscopy images. The optical properties of samples are also investigated. It is interesting to find that the photoluminescence(PL) intensity of Cd Te/Cd S QD films on Si nanowire substrates with Au NPs is significantly increased,which can reach 8-fold higher than that of samples on planar Si without Au NPs. The results of finite-difference time-domain simulation indicate that Au NPs induce stronger localization of electric field and then boost the PL intensity of QDs nearby. Furthermore, the time-resolved luminescence decay curve shows the PL lifetime, which is about 5.5 ns at the emission peaks of QD films on planar, increasing from 1.8 ns of QD films on Si NWs to4.7 ns after introducing Au NPs into Si NWs.     

Observation of Overhauser shift in a self-assembled InAlAs quantum dot

We report the polarization-dependent energy shift of excitonic emission in a self-assembled InAlAs/AlGaAs quantum dot (QD). The energy shift is well known as Overhauser shift and was observed in a naturally formed GaAs QD using monolayer fluctuation of a quantum well. However, there has been no observation so far in a self-assembled QD, which is suitable for formation of vertically coupled QDs. We demonstrate that the magnitude of the Overhauser shift is enhanced by the photo-injection of the highly polarized electron and is controllable by the polarization of the excitation light in a self-assembled InAlAs QD.     

等离子体增强InAs单量子点荧光辐射的研究

通过测量光致发光(PL)谱、PL时间分辨光谱及不同激发功率下PL发光强度,研究了低温(5 K)下等离子体对InAs单量子点PL光谱的增强效应.采用电子束蒸发镀膜技术在InAs量子点样品表面淀积了5 nm厚度的金膜,形成纳米金岛膜结构.实验发现,金岛膜有利于量子点样品发光强度的增加,最大PL强度增加了约5倍,其主要物理机理是金岛膜纳米结构提高了量子点PL光谱的收集效率.     

Optical Stark effect in a quantum dot: ultrafast control of single exciton polarizations

We report the first experimental study of the optical Stark effect in single semiconductor quantum dots (QD). For below band gap excitation, two-color pump-probe spectra show dispersive line shapes caused by a light-induced blueshift of the excitonic resonance. The line shape depends strongly on the excitation field strength and is determined by the pump-induced phase shift of the coherent QD polarization. Transient spectral oscillations can be understood as rotations of the QD polarization phase with negligible population change. Ultrafast control of the QD polarization is demonstrated.     

ZnS薄膜参数对有机/无机复合发光器件特性的影响

半导体量子点(QDs)具有发光效率高和发光波长可调等特点。采用胶体CdSe QDs作电致发光器件的有源材料,TPD(N,N′-biphenyl-N,N′-bis-(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine)作空穴传输层,ZnS作电子传输层,研究了有机/无机复合发光器件ITO/TPD/CdSe QDs/ZnS/Ag的电致发光特性。TPD和CdSe QDs薄膜采用旋涂方法、ZnS薄膜采用磁控溅射方法沉积,器件表面平整。CdSe QDs的光致发光和电致发光谱峰位波长均位于～580 nm,属于量子点的带边激子发光。我们与以前的ITO/ZnS/CdSe QDs/ZnS/Ag发光器件结构进行了对比,发现新的器件结构的电致发光谱没有观察到QDs表面态的发光,而且新器件的发光强度是ITO/ZnS/CdSe QDs/ZnS/Ag结构的～10倍。发光效率的提高归因于碰撞激发与载流子注入两种发光机制并存的结果：一方面电子经过ZnS 层加速后,碰撞激发CdSe QDs发光;另一方面,空穴从TPD层注入CdSe QDs 与QDs中激发的电子复合发光。我们进一步研究了ZnS电子加速层厚度对发光特性的影响,选择ZnS薄膜的厚度分别是80,120 和160 nm,发现随着ZnS层厚度增大,器件启亮电压升高,EL强度增大,但是击穿电压降低。EL峰位随着ZnS厚度的减小发生明显蓝移,对上述实验现象进行了机理解释。     

Si quantum dot structures and their applications

This paper presents briefly the history of emission study in Si quantum dots (QDs) in the last two decades. Stable light emission of Si QDs and NCs was observed in the spectral ranges: blue, green, orange, red and infrared. These PL bands were attributed to the exciton recombination in Si QDs, to the carrier recombination through defects inside of Si NCs or via oxide related defects at the Si/SiOx interface. The analysis of recombination transitions and the different ways of the emission stimulation in Si QD structures, related to the element variation for the passivation of surface dangling bonds, as well as the plasmon induced emission and rare earth impurity activation, have been presented.The different applications of Si QD structures in quantum electronics, such as: Si QD light emitting diodes, Si QD single union and tandem solar cells, Si QD memory structures, Si QD based one electron devices and double QD structures for spintronics, have been discussed as well. Note the significant worldwide interest directed toward the silicon-based light emission for integrated optoelectronics is related to the complementary metal-oxide semiconductor compatibility and the possibility to be monolithically integrated with very large scale integrated (VLSI) circuits. The different features of poly-, micro- and nanocrystalline silicon for solar cells, that is a mixture of both amorphous and crystalline phases, such as the silicon NCs or QDs embedded in a α-Si:H matrix, as well as the thin film 2-cell or 3-cell tandem solar cells based on Si QD structures have been discussed as well. Silicon NC based structures for non-volatile memory purposes, the recent studies of Si QD base single electron devices and the single electron occupation of QDs as an important component to the measurement and manipulation of spins in quantum information processing have been analyzed as well.     

Lateral electric field effects on quantum size confinement in cylindrical quantum dot under parabolic potential

Within the effective mass approximation, we investigated theoretically the ground-state energy of a single particle and the binding energy of the neutral donor impurity (D⁰) affected by a lateral electric field in a parabolic quantum dot (QD). The results show that the electron and the hole ground-state energy and the band to band transition energies shift to lower values (red shift) by increasing the field intensity. The quantum Stark shift (QSS) for the electron increases rapidly in the quasi spherical QD (QSQD) by increasing the lateral field, whereas for the hole it increases monotony. In the cylindrical QDs (CQDs), we found that the QSS for electron and hole increase monotonically. The quantum size, lateral electric field and impurity position effect on the binding energy of neutral donor (D⁰) is studied. Unexpected behavior of D⁰ in quantum well limit (QW), the binding energy of D⁰ is increasing (blue shift) with increasing QD radius R$R$ at the presence of a lateral electric field. It appears that for a fixed size of the QD, the off-center binding energy decreases when the impurity ion is displaced from the center to the QD borders, while it is shifted to lower energy with increasing the field.