Electroluminescence of colloidal ZnSe quantum dots

The article reports a green chemical synthesis of colloidal ZnSe quantum dots at a moderate temperature. The prepared colloid sample is characterised by UV-vis absorption spectroscopy and transmission electron microscopy. UV-vis spectroscopy reveals as-expected blue-shift with strong absorption edge at 400 nm and micrographs show a non-uniform size distribution of ZnSe quantum dots in the range 1-4 nm. Further, photoluminescence and electroluminescence spectroscopies are carried out to study optical emission. Each of the spectroscopies reveals two emission peaks, indicating band-to-band transition and defect related transition. From the luminescence studies, it can be inferred that the recombination of electrons and holes resulting from interband transition causes violet emission and the recombination of a photon generated hole with a charged state of Zn-vacancy gives blue emission. Meanwhile electroluminescence study suggests the application of ZnSe quantum dots as an efficient light emitting device with the advantage of colour tuning (violet-blue-violet).     

胶体CdSe量子点的色度学特性研究

使用化学胶体法合成了CdSe量子点，研究了样品的荧光量子产率，发现随CdSe量子点粒径的增大，荧光量子产率存在先增大后减小的现象.研究了CdSe量子点的色度学特性，通过对不同尺寸、不同粒径分布范围的CdSe量子点色度坐标的计算，讨论了粒径分布范围对其荧光颜色饱和度的影响，解释了为何难以获得高颜色饱和度的绿光量子点.利用红、蓝两种不同尺寸的量子点配制出白光样品，提出了估计配色后样品色度坐标的经验公式，结果显示白光样品色度坐标的实验值与经验公式估计值基本一致.     

Preparation and optical characterization of nanocomposites with semiconductor colloidal quantum dots

A simple and effective technique for depositing thin films of semiconductor colloidal quantum dots (QD) on glass substrates is developed. Samples with CdSe/CdS/ZnS quantum dots are fabricated and investigated via luminescence microspectroscopy. Four-wave mixing signals are recorded at room temperature in a solution and in a thin film of quantum dots.     

Effects of nanocrystal shape on the physical properties of colloidal ZnO quantum dots

The effects of both nanocrystal shape and applied magnetic field on the electron energy spectra of colloidal ZnO quantum dots have been investigated in the frame of finite element method, using nonuniform triangular elements. Four shapes of quantum dots (spherical, ellipsoidal, rod-shaped, and lens-shaped) were studied. It was found that the physical properties of the semiconductor quantum dots could be manipulated by changing their size and/or their shape. The energies of an electron increase as one reduces the quantum dot shape symmetry from spherical towards the lens-shaped. The magnetic field effect strongly interplays with the nanocrystal size and the nanocrystal shape effects. Such interplay has been attributed to the competition of the quantum confinement effect introduced by the barrier potential and the quantum confinement effect introduced by the applied magnetic field.     

Chemical role of amines in the colloidal synthesis of CdSe quantum dots and their luminescence properties

The role of organic amines in the colloidal synthesis of CdSe quantum dots (QDs) has been studied. CdSe QDs were synthesized from the source solutions containing 5 vol% of amines having various alkyl chain lengths, stereochemical sizes and electron donation abilities. The role of the additional amines was evaluated on the basis of the photoluminescence (PL) properties such as PL wavelength and intensity of the obtained CdSe QDs. The observed PL spectra were explained by the fact that the amines behaved as capping ligands on the surface of the QDs in the product colloidal solution and complex ligands for cadmium in the source solutions. It was shown that the particle size was controlled by the diffusion process depending on the mass and stereochemical shape of the amines, and the luminescence intensity increased with the increasing electron donation ability and capping density of the amines.     

胶质红外量子点薄膜入射角对荧光特性的影响分析

胶质红外量子点优异的光学性能使其荧光特性具有广泛的应用前景,在实际应用中通常需要封装成薄膜形态以保持稳定的荧光特性。然而,分散形式的改变可能会导致量子点荧光效率降低以及荧光角度特性变化。因此,建立了红外量子点荧光强度探测系统,对胶质红外量子点薄膜在不同角度激发光入射时产生的荧光分布情况进行研究。实验及分析结果表明,激发光与样品表面夹角大小在10°～170°之间的较大范围内入射时,在反射及透射荧光区域均可探测到荧光峰值强度70%～80%以上的荧光出射,在这一范围内的反射荧光与透射荧光强度差异和荧光强度随激发光入射角度的变化规律分别与样品中量子点的浓度以及分布形态有关。同时,随着入射激光能量的增强,样品出射荧光强度对于入射角度变化的“平坦”范围进一步扩大。     

Purcell effect in triangular plasmonic nanopatch antennas with three-layer colloidal quantum dots

A model describing a plasmonic nanopatch antenna based on triangular silver nanoprisms and multilayer cadmium chalcogenide quantum dots is introduced. Electromagnetic-field distributions in nanopatch antennas with different orientations of the quantum-dot dipoles are calculated for the first time with the finite element method for numerical electrodynamics simulations. The energy flux through the surface of an emitting quantum dot is calculated for the configurations with the dot in free space, on an aluminum substrate, and in a nanopatch antenna. It is shown that the radiative part of the Purcell factor is as large as 1.7 × 10₂ The calculated photoluminescence lifetimes of a CdSe/CdS/ZnS colloidal quantum dot in a nanopatch antenna based on a silver nanoprism agree well with the experimental results.     

微腔中CdSe量子点荧光增强效应

文章主要研究了CdSe量子点微腔结构,微腔结构包括上下分布式布拉格反射镜(DBR),中间的有源层为溶解在聚甲基丙烯酸甲酯(PMMA)中的CdSe胶体量子点.采用传递矩阵法模拟微腔的反射光谱,对实验测试曲线进行较好的拟合.通过测试微腔结构的光致荧光(PL)光谱,其半峰宽(FWHM)由未加入微腔的CdSe量子点样品的27.9 nm,减小到微腔结构的7.5 nm,在微腔中的量子点,由于腔模式的出现,其发光谱的品质因数增加了3.6倍,达到了荧光增强的效果. 关键词： CdSe量子点 微腔效应 荧光增强     

Time-gated biological imaging by use of colloidal quantum dots

The long (but not too long) fluorescence lifetime of CdSe semiconductor quantum dots was exploited to enhance fluorescence biological imaging contrast and sensitivity by time-gated detection. Significant and selective reduction of the autofluorescence contribution to the overall image was achieved, and enhancement of the signal-to-background ratio by more than an order of magnitude was demonstrated.     

Optical sectioning by multiexcitonic ladder climbing in colloidal quantum dots

Optical sectioning is performed by collecting the fluorescent emission of two-exciton states in colloidal quantum dots. The two-exciton state is created by two consecutive resonant absorption events, thus requiring unprecedented low excitation energy and peak powers as low as 10(5) W/cm(2). The depth resolution is shown to be equivalent to that of standard multiphoton microscopy, and it was found to deteriorate only slowly as saturation of the two-exciton state is approached, owing to signal contribution from higher excitonic states.     

Transport properties of quantum dots

Linear and nonlinear transport through a quantum dot that is weakly coupled to ideal quantum leads is investigated in the parameter regime where charging and geometrical quantization effects coexist. The exact eigenstates and spins of a finite number of correlated electrons confined within the dot are combined with a rate equation. The current is calculated in the regime of sequential tunneling. The analytic solution for an Anderson impurity is given. The phenomenological charging model is compared with the quantum mechanical model for interacting electrons. The current-voltage characteristics show Coulomb blockade. The excited states lead to additional fine-structure in the current voltage characteristics. Asymmetry in the coupling between the quantum dot and the leads causes asymmetry in the conductance peaks which is reversed with the bias voltage. The spin selection rules can cause a ‘spin blockade’ which decreases the current when certain excited states become involved in the transport. In two-dimensional dots, peaks in the linear conductance can be suppressed at low temperatures, when the total spins of the corresponding ground states differ by more than 1/2. In a magnetic field, an electron number parity effect due to the different spins of the many-electron ground states is predicted in addition to the vanishing of the spin blockade effect. All of the predicted features are consistent with recent experiments.     

Synthesis and luminescence of CdS quantum dots capped with a silica precursor

A novel synthesis method for efficiently luminescing CdS nanocrystals is reported using (3-mercaptopropyl)-trimethoxysilane (MPS) as a capping agent in THF or MeOH. The synthesis yields particles capped with a precursor for coating the particle with a silica shell and offers control over a wide range of particle sizes (2-) by varying the MPS concentration. The temperature dependence of the luminescence and the luminescence life time was studied and explained in terms of a donor-acceptor recombination process in which both electron and hole are trapped at low temperature. In addition to varying the emission color by controlling the particle size, nanocrystals were doped with Zn or Cu resulting in a blue shift or a red shift of the emission.     

Warm-white light-emitting diodes integrated with colloidal quantum dots for high luminous efficacy and color rendering

Warm-white LEDs (WLEDs) with high spectral quality and efficiency are required for lighting applications, but current experimental performances are limited. We report on nanocrystal quantum dot (NQD) hybridized WLEDs with high performance that exhibit a high luminous efficacy of optical radiation exceeding 350lm/W(opt) and a high color rendering index close to 90 at a low correlated color temperature <3000K. These spectrally engineered WLEDs are obtained using a combination of CdSe/ZnS core/shell NQD nanophosphors integrated on blue InGaN/GaN LEDs.     

Photoluminescence of colloidal CdSe nano-tetrapods and quantum dots in oxygenic and oxygen-free environments

The effects of oxygenic versus oxygen-free environments on colloidal CdSe nano-tetrapods and quantum dots (QDs) were studied using both continuous and time-resolved photoluminescence (PL) measurements. The decays of PL intensities for tetrapods and QDs in oxygen-free solution (chloroform) and in air (on silicon) can be well fitted by a bi-exponential function. Based on the emission-energy dependence of carrier lifetimes and the amplitude ratio of the fast-decay component to the slow-decay component, the fast and slow PL decays of CdSe nanocrystals are attributed to the recombination of delocalized carriers in the core states and localized carriers in the surface states, respectively. The PL intensities of CdSe nano-tetrapods and QDs were found to be five times and an order of magnitude higher in air than in vacuum, respectively, which is explained by the passivation of surface defects by the polar gas (oxygen) absorption. The lower enhancement in PL intensities of CdSe nano-tetrapods is explained by the special morphology of the tetrapods.     

Confocal microscopy of electrostatic properties of Si quantum dots and silica surfaces by charge-sensitive dye molecules

An aminonaphthylethenylpyridinium dye (ANEPPS) has been used to investigate the photoinduced charging of SiO₂ surfaces via fluorescence shifts. Confocal microscopy allows for a localization of the charge distributions of better than 250 nm. The surface becomes negatively charged upon photoexcitation. This effect is enhanced by silicon nanocrystals (quantum dots) of less than 20 nm in diameter. Experiments suggest that ANEPPS occupies the SiO₂ surface in an oriented way.     

Magneto-optical properties of self-assembled InAs quantum dots for quantum information processing

Semiconductor quantum dots have been intensively investigated because of their fundamental role in solid-state quantum information processing. The energy levels of quantum dots are quantized and can be tuned by external field such as optical, electric, and magnetic field. In this review, we focus on the development of magneto–optical properties of single In As quantum dots embedded in Ga As matrix, including charge injection, relaxation, tunneling, wavefunction distribution,and coupling between different dimensional materials. Finally, the perspective of coherent manipulation of quantum state of single self-assembled quantum dots by photocurrent spectroscopy with an applied magnetic field is discussed.     

Spin-flip limited exciton dephasing in CdSe/ZnS colloidal quantum dots

The dephasing time of the lowest bright exciton in CdSe/ZnS wurtzite quantum dots is measured from 5 to 170 K and compared with density dynamics within the exciton fine structure using a sensitive three-beam four-wave-mixing technique unaffected by spectral diffusion. Pure dephasing via acoustic phonons dominates the initial dynamics, followed by an exponential zero-phonon line dephasing of 109 ps at 5 K, much faster than the ~10 ns exciton radiative lifetime. The zero-phonon line dephasing is explained by phonon-assisted spin flip from the lowest bright state to dark-exciton states. This is confirmed by the temperature dependence of the exciton lifetime and by direct measurements of the bright-dark-exciton relaxation. Our results give an unambiguous evidence of the physical origin of the exciton dephasing in these nanocrystals.     

Magnetic properties of quantum dots and rings

Exact many-body methods as well as current-spin-density functional theory are used to study the magnetism and electron localization in two-dimensional quantum dots and quasi-one-dimensional quantum rings. Predictions of broken-symmetry solutions within the density functional model are confirmed by exact configuration interaction (CI) calculations: In a quantum ring the electrons localize to form an antiferromagnetic chain which can be described with a simple model Hamiltonian. In a quantum dot the magnetic field localizes the electrons as predicted with the density functional approach. Received 5 December 2000     

Effects of absorption saturation in colloidal quantum dots as fluorophores for multiphoton fluorescence microscopy

It is shown that colloidal semiconductor nanocrystals (quantum dots), which are promising fluorophores for multiphoton fluorescence microscopy, exhibit a two-photon absorption saturation effect at moderate powers (not exceeding 10 mW) of femtosecond pumping radiation. An analytical expression for the power of two-photon fluorescence of quantum dots as a function of the average pumping power is obtained. With this expression, the deviation of the found dependence from the quadratic law is explained by two factors, i.e., a large two-photon absorption cross section of quantum dots and their slow (compared to the typical pumping pulse-repetition period) relaxation to an unexcited state. Using an LSM 510 Carl Zeiss laser scanning microscope equipped with a Ti:Sa tunable femtosecond laser, a series of model experiments is performed to reveal the saturation effect in a solution of commercially available quantum dots. Good agreement is obtained between the measured dependence of the power of two-photon fluorescence on the average pump power and the theoretical calculation results. It is also experimentally demonstrated that, under fluorescence saturation conditions, the spatial resolution of the method of multiphoton fluorescence microscopy is lost; this effect is analyzed numerically.     

Nonlinear transport properties of quantum dots

The influence of excited levels on nonlinear transport properties of a quantum dot weakly coupled to leads is studied using a master-equation approach. A charging model for the dot is compared with a quantum mechanical model for interacting electrons. The currentvoltage curve shows Coulomb lockade and additional finestructure that is related to the excited states of the correlated electrons. Unequal coupling to the leads causes asymmetric conductance peaks. Negative differential conductances are predicted due to the existence of excited states with different spins.