CdSe/ZnSe quantum dot structures grown by molecular beam epitaxy with a CdTe submonolayer stressor

A procedure for formation of CdSe quantum dots (QDs) in a ZnSe matrix is suggested. The procedure is based on the introduction of a CdTe submonolayer stressor deposited on the matrix surface just before deposition of the material of the QDs. (For CdTe/ZnSe structure, the relative lattice mismatch is Δa/a ≈ 14%.) The stressor forms small strained islands at the ZnSe surface, thus producing local fields of high elastic stresses controlling the process of the self-assembling of the QDs. According to the data of transmission electron microscopy, this procedure allows a considerable increase in the surface density of QDs, with a certain decrease in their lateral dimensions (down to 4.5 ± 1.5 nm). In the photoluminescence spectra, a noticeable (～150 meV) shift of the peak to longer wavelengths from the position of the reference CdSe/ZnSe QD structure is observed. The shift is due to some transformation of the morphology of the QDs and an increase in the Cd content in the QDs. Comprehensive studies of the nanostructures by recording and analyzing the excitation spectra of photoluminescence, the time-resolved photoluminescence spectra, and the cathodoluminescence spectra show that the emission spectra involve two types of optical transitions, namely, the type-I transitions in the CdSeTe/ZnSe QDs and the type-II transitions caused mainly by the low cadmium content (Zn,Cd)(Se,Te)/ZnSe layer formed between the QDs.     

Optical characterisation of MOVPE grown vertically correlated InAs/GaAs quantum dots

Structures with vertically correlated self-organised InAs quantum dots (QDs) in a GaAs matrix were grown by the low-pressure metal-organic vapour phase epitaxy (MOVPE) and characterised by different microscopic techniques. Photoluminescence in combination with photomodulated reflectance spectroscopy were applied for characterisation of QDs structures. We show that combination of both methods allows detecting optical transitions originating both from QDs and wetting (separation) layers, which can be than compared with those obtained from numerical simulations. On the basis of obtained results, we demonstrate that photoreflectance spectroscopy is an excellent tool for characterisation of QDs structures wetting layers and for identification of spacer thicknesses in vertically stacked QDs structures.     

Optical properties of CdSe quantum dots grown on ZnSe and ZnBeSe by molecular beam epitaxy

We report detailed studies of the optical properties of CdSe quantum dots (QDs) grown on ZnSe and ZnBeSe by molecular-beam epitaxy (MBE). We performed steady-state and time-resolved photoluminescence (PL) measurements and observe that nonradiative processes dominate at room temperature (RT) in the CdSe/ZnBeSe QDs structures, though these nonradiative processes do not dominate in the CdSe/ZnSe QDs structures up to RT. We performed secondary ion-mass spectrometry (SIMS) measurement and propose that the oxygen incorporation in the ZnBeSe layers (possibly caused by the reactivity of Be) may contribute to the dominant nonradiative processes at high temperatures in the QDs grown on ZnBeSe.     

Formation and characterization of self-organized CdSe quantum dots

We have investigated the formation and characteristic of self-organized CdSe quantum dots (QDs) on ZnSe(001) surfaces with the use of photoluminescence (PL) and transmission electron microscopy (TEM). Coherent CdSe QDs are naturally formed on ZnSe surfaces, when the thickness of CdSe layers is around 2 ML. The plan-view TEM images exhibit that CdSe QDs have a relatively narrow distribution of QD size, and that the density of CdSe QDs is about 10¹⁰ cm⁻². The base structure of the CdSe dot is rhombic, which has the long axis of about 20 nm in length along direction. The temperature dependence of macro-PL spectra reveals that the behavior of self-organized CdSe QDs is quite different from that of ZnCdSe quantum well (QW), resulting from characteristic features of zero-dimensional structures of QDs. Moreover, the macro-PL results suggest the existence of QW-like continuous state lying over QD states. Micro-PL measurements show several numbers of high-resolved sharp lines from individual CdSe QDs. The linewidth broadening with temperature depends on peak energy position of the QDs. The linewidths of lower energy lines, corresponding to larger size QDs, are more temperature dependent.     

Band Gap Engineering Improves the Efficiency of Double Quantum Dot Upconversion Nanocrystals

Solution‐processed core/multishell semiconductor quantum dots (QDs) could be tailored to facilitate the carrier separation, promotion, and recombination mechanisms necessary to implement photon upconversion. In contrast to other upconversion schemes, upconverting QDs combine the stability of an inorganic crystalline structure with the spectral tunability afforded by quantum confinement. Nevertheless, their upconversion quantum yield (UCQY) is fairly low. Here, design rules are uncovered that enable to significantly enhance the performance of double QD upconversion systems, and these findings are leveraged to fabricate upconverting QDs with increased photon upconversion efficiency and reduced saturation intensities under pulsed excitation. The role of the intra‐QD band alignment is exemplified by comparing the upconversion process in PbS/CdS/ZnSe QDs with that of PbS/CdS/CdSe ones with variable CdSe shell thicknesses. It is shown that electron delocalization into the shell leads to a longer‐lived intermediate state in the QDs, facilitating further absorption of photons, and enhancing the upconversion process. The performance of these upconversion QDs under pulsed excitation versus continuous pumping is also compared; the reasons for the significant differences between these two regimes are discussed. The results show how one can overcome some of the limitations of previous upconverting QDs, with potential applications in biophotonics and infrared detection.     

Sub‐kilogram‐Scale One‐Pot Synthesis of Highly Luminescent and Monodisperse Core/Shell Quantum Dots by the Successive Injection of Precursors

The one‐pot synthesis of core/shell quantum dots (QDs) represents an attractive alternative to conventional synthesis techniques, where the core CdSe QDs are first purified and then an epitaxial shell of the desired thickness is obtained by the slow addition of shell precursors to a solution of the purified QDs at high temperature. We have developed a one‐pot synthesis procedure involving the successive injection of deliberately selected core‐ and shell‐forming reagents at appropriate temperatures. Sub‐kilogram quantities of highly luminescent and monodisperse core/shell QDs with desirable optical properties (full width at half maximum of photoluminescence (PL) band is ca. 30 nm) have been produced by the sequential growth of the core and shell in a controlled manner. This one‐pot method has also been extended to form water‐soluble core/double‐shell CdSe/ZnSe/ZnS QDs exhibiting high PL efficiency and stability.     

硒化锌量子点的热解组装及光学性质研究

报道了在无水无氧环境下利用热解反应将硒化锌量子点组装入介孔二氧化硅有序孔道内的方法,采用X射线衍射、透射电子显微成像及能量散射谱等测试方法分析了体系的微观结构,并采用紫外-可见光吸收光谱仪和荧光光谱仪研究了其光学性质.结果表明硒化锌量子点被组装在介孔二氧化硅的孔道内;与硒化锌体材料本征吸收谱相比,组装在介孔内的量子点的吸收光谱表现出显著的蓝移,这归结于量子尺寸效应.     

耦合GaN/AlxGa1-xN量子点中的激子特性

在有效质量近似下，运用变分方法，考虑到量子点内电子和空穴的三维束缚以及由压电极化和自发极化所引起的内建电场，对圆柱型耦合GaN量子点的光学性质及激子态做了研究。给出了激子结合能Eb、量子点发光波长λ、电子-空穴复合率和量子点高度L^GaN以及势垒层厚度L^AlGaN之间的函数关系。结果表明，量子点高度LG^GaN、势垒层厚度L^AlGaN的增加将导致激子结合能、电子-空穴复合率的降低，耦合量子点发光波长的增加。     

Bright CdSe quantum dot inserted in single ZnSe nanowires

We report the evidence of CdSe quantum dot (QD) insertion in single defect-free ZnSe nanowire. These nanowires have been grown by molecular beam epitaxy in vapour-liquid-solid growth mode catalysed with gold particles. We developed a two-step process allowing us to grow very thin (from 15 to 5 nm) defect-free ZnSe nanowire on top of a nanoneedle, where all defects are localised. The CdSe QDs are incorporated to the defect-free nanowires part. Owing to the extraction efficiency of the nanowires and the reduced number of stacking fault defects in the two-step-process nanowires, a very efficient photoluminescence is observed even on isolated single nanowire. Time-resolved photoluminescence and correlation photon give evidences that the bright photon emission is related to the CdSe QD.     

静压下（CdSe）_m／（ZnSe）_n－ZnSe超短周期超晶格量子阱的共振喇曼散射研究

在静压和液氮温度下观察到（ＣｄＳｅ）ｍ／（ＺｎＳｅ）ｎ短周期超晶格中重空穴激子的复合发光和多达４阶的类ＺｎＳｅＬＯ多声子喇曼散射，并观察到厚ＺｎＳｅ势垒层的带边发光和限制在厚势垒层中的类ＺｎＳｅＬＯ声子散射．结果表明，加压后（ＣｄＳｅ）ｍ／（ＺｎＳｅ）ｎ短周期超晶格中的类ＺｎＳｅ的１ＬＯ和２ＬＯ声子模频率分别以３．７６和７．１１ｃｍ－１／ＧＰａ的速率向高频方向移动，超晶格阱层光致发光峰的压力系数为５９．８ｍｅＶ／ＧＰａ．与（ＣｄＳｅ）ｍ／（ＺｎＳｅ）ｎ短周期超晶格共振时的类ＺｎＳｅ１ＬＯ声子模频率比与ＺｎＳｅ势垒层共振时的类ＺｎＳｅ１ＬＯ声子模频率低２．０ｃｍ－１，反映了（ＣｄＳｅ）ｍ／（ＺｎＳｅ）ｎ短周期超晶格中ＬＯ声子的限制效应     

Enhanced Photocurrent Generation in Proton‐Irradiated “Giant” CdSe/CdS Core/Shell Quantum Dots

Group II–VI quantum dots (QDs) possess tunable electrical and optical properties that make them very attractive for high‐tech applications and power generation. The effects of proton irradiation on both the structural and physical properties of “giant” CdSe/CdS core–shell QDs (g‐CS QDs) are investigated. These experiments shed light on photoelectron delocalization in g‐CS QDs, where current linkages and strong variations in optical emission result from the spatial extension of the photoelectron wavefunctions over the conduction bands of CdSe and CdS. Monte Carlo simulations of ion–matter interactions show that the damaging rates can be set from the energy of impinging protons to promote the formation of structural defects in the core or shell. The formation of nanocavities is demonstrated after irradiation doses higher than ≈10¹⁷ H⁺ cm^?2, while a continuous decrease in luminescence intensity is observed for increasing proton fluencies. This feature is accompanied by a concomitant lifetime decrease marking the rise of nonradiative phenomena and the occurrence of greater photocarrier transfers between CdS and CdSe. Current‐to‐voltage characterizations evidence that proton implantation can be implemented to enhance the photocurrent generation in g‐CS QDs. This increase is attributed to the delocalization of photoelectrons in the CdS shell, whose improvement is found to promote electron–hole pair separation.     

Förster Resonance Energy Transfer and Harvesting in II–VI Fractional Monolayer Structures

We report on Förster resonance energy transfer in the dense arrays of epitaxial quantum dots (QDs), formed by fractional monolayer CdSe insertions within a ZnSe matrix. In such arrays comprising the QDs of different sizes, the energy transfer can take place between the ground levels of small QDs and the excited levels of large radiating QDs, when these states are in resonance. This mechanism provides directional excitation of a limited number of the large QDs possessing the excited levels. It reveals itself by the shrinkage of photoluminescence (PL) bands and the appearance of the narrow single excitonic lines in micro-PL spectra. The strong shortening of characteristic PL decay times in the energy-donating QDs is observed when the distance between them and the energy-accepting QDs decreases. Photoluminescence excitation spectroscopy demonstrates the switching of the dominant energy transfer mechanism at the energy predicted by theoretical modeling of the excitonic levels in the QD arrays. Our results pave the way for engineering of the architecture of excitonic levels in the QD arrays to realize efficient nano-emitters.     

Excitonic recombination near the mobility edge in CdSe/ZnSe nanostructures

The low-temperature photoluminescence and Raman scattering in CdSe/ZnSe nanostructures with individual CdSe inserts of 1.5 and 3.0 monolayers in nominal thickness were studied. The energy position of the photoluminescene band is governed by interdiffusion of Cd and Zn into the insert regions, whereas the shape of this band is controlled by strong interaction of localized excitons with optical phonons in the Zn_1?xCd_xSe solid-solution insert. Multiphonon processes of excitonic relaxation with involvement of acoustic phonons at the Brillouin zone edges are also important. The results obtained are interpreted in the context of a model for the effective excitonic mobility edge.     

Analysis of mechanisms of carrier emission in the p-i-n structures with In(Ga)As quantum dots

With the help of the photocurrent spectroscopy, the mechanism of emission of charge carriers from energy levels of the (In,Ga)As/(Al,Ga)As quantum dots of different design are studied. Thermal activation is shown to be the main mechanism of carrier emission from the quantum dots for the isolated layer of quantum dots separated by wide (Al,Ga)As spacer layers. At a small width of the (Al,Ga)As spacer layer, when electron binding of separate layers of the quantum dots in the vertical direction takes place, the role of the tunneling mechanism of carrier emission between the vertically coupled quantum dots increases.     

CdSe四脚状纳米颗粒的光学性质

主要研究了高产率CdSe四脚状纳米颗粒的时间分辨光学性质.结果表明在氯仿溶液中和均匀涂布在绝缘硅片上的CdSe四脚状纳米颗粒室温下的光致发光(PL)衰减曲线均可以用一个双e指数的衰减函数来拟合,荧光寿命短的部分对应快衰减,而寿命长的部分对应慢衰减.通过分析荧光寿命与PL峰的能量的关系可知,快衰减和慢衰减分别来自于非局域态载流子和局域态载流子的贡献.通过比较溶液中和硅片上两种情况的快衰减和慢衰减系数的比值,得出结论:CdSe四脚状纳米颗粒的PL快衰减和慢衰减分别来源于纳米颗粒中非局域态和颗粒表面缺陷中局域态的贡献.     

Green synthesis of yellow emitting PMMA–CdSe/ZnS quantum dots nanophosphors

We herein report the fabrication of highly fluorescent yellow emitting nanophosphors using CdSe/ZnS quantum dots (QDs) dispersed in polymethyl methacrylate (PMMA). The QDs were synthesised via a simple, non-phosphine and one pot synthetic method in the absence of an inert atmosphere. The as-prepared nanocrystallites were characterised by Fourier transform infrared spectroscopy (FTIR), ultraviolet–visible (UV–vis) and photoluminescence spectroscopy, energy-dispersive spectroscopy (EDS), Raman spectroscopy, transmission electron microscopy (TEM) and high resolution TEM (HRTEM) microscopy. Optical analysis confirmed that the as-synthesised CdSe/ZnS QDs were of high quality with sharp absorption peaks, bright luminescence, narrow emission width and high PL quantum yield (up to 74%). The electron microscope images showed that the QDs are small and spherical in shape with narrow size distributions while the HRTEM micrograph confirmed the high crystallinity of the material. The Raman analysis of the QDs revealed the formation of core–shell structure and the energy dispersive spectroscopy confirmed the presence of the corresponding elements (i.e., Cd, Se, Zn and S). The dispersion of the core–shell QDs in PMMA matrix led to the red-shifting of the emission position from 393 nm in the neat PMMA to 592 nm in the nanocomposite. The fabricated highly fluorescent yellow emitting PMMA–CdSe/ZnS core–shell QDs polymer nanocomposite film display excellent optical properties without loss of luminescence. Furthermore, the as-synthesised organic soluble CdSe/ZnS QDs were successfully converted into highly water soluble QDs after ligand exchange with mercaptoundecanoic acid (MUA) without the loss of their emission properties. The simplicity of the method and the quality of the as-synthesised nanocomposite make it a promising material for the large scale fabrication of diverse optical devices.     

Absorption in laser structures with coupled and uncoupled quantum dots in an electric field at room temperature

The absorption of uncoupled and tunnel-coupled vertically correlated quantum dots (QDs), measured at room temperature, has been experimentally compared. It is revealed that matching of the laser wavelength and Stark shift for laser structures with tunnel-coupled QDs leads to resonant absorption with formation of bound and antibound exciton states with a splitting energy of ～62 meV between them in QD molecules. For these states, an external field causes a large linear Stark shift (up to 68 meV). For uncoupled QDs, one resonant absorption peak with the formation of an exciton (for which the Stark shift does not exceed 13 meV) is observed.     

Engineering laser gain spectrum using electronic vertically coupled InAs-GaAs quantum dots

Continuous large-broad laser gain spectra near 1.3 /spl mu/m are obtained using an active region of electronic vertically coupled (EVC) InAs-GaAs quantum dots (QDs). A wide continuous electroluminescence spectrum, unlike that from conventional uncoupled InAs QD lasers, was obtained around 230 nm (below threshold) with a narrow lasing spectrum. An internal differential quantum efficiency as high as 90%, a maximum measured external differential efficiency of 73% for a stripe-length of L=1 mm, and a threshold current density for zero total optical loss as low as 7 A/cm/sup 2/ per QD layer were achieved.     

Optical detection of asymmetric quantum-dot molecules in double-layer InAs/GaAs structures

Self-assembled quantum dots (QDs) in double-layer InAs/GaAs structures are studied by resonant photoluminescence and photoluminescence excitation spectroscopy. A weakly correlated (50%) double-layer system with an array of vertically coupled QDs (asymmetric quantum-dot molecules) was formed in a structure consisting of the 1.8-monolayer-thick first and the 2.4-monolayer-thick second InAs layers separated by 50 monolayers of GaAs. The nature of discrete quantum states in this system was studied and resonances corresponding to vertically coupled QDs were clearly observed for the first time.     

Effect of swift heavy ion (SHI) irradiation on the structural and optical properties of N implanted CVT grown ZnSe single crystals

Present study reports the variation of structural and optical properties of N implanted ZnSe single crystals grown by a Chemical Vapor Transport (CVT) technique due to120 MeV Au ion irradiation. The grazing incidence X-ray diffraction (GIXRD) results show that the full width at half maximum (FWHM) increases on irradiation. The surface morphology of irradiated sample shows the pits and islands by AFM studies. The optical absorption cut off wavelength is 493 nm for as grown ZnSe whereas for the implanted and irradiated samples cut off wavelength shift towards red region. The photoluminescence spectrum shows the emission wavelength is at 592 nm whereas for the implanted-irradiated samples PL emission shift towards red region. The intensity of defect level emission decreases due to Au irradiation. The FT-Raman spectrum shows the peak at 252 cm⁻¹ due to LO mode of Zn–Se lattice vibrations. The X-ray absorption near edge structure (XANES) study was performed for the as grown and irradiated samples. The detailed investigation on the structural and optical properties of the N implanted and Au irradiated ZnSe single crystal was carried out.