Labelling of cells with quantum dots

Colloidal quantum dots are semiconductor nanocrystals well dispersed in a solvent. The optical properties of quantum dots, in particular the wavelength of their fluorescence, depend strongly on their size. Because of their reduced tendency to photobleach, colloidal quantum dots are interesting fluorescence probes for all types of labelling studies. In this review we will give an overview on how quantum dots have been used so far in cell biology. In particular we will discuss the biologically relevant properties of quantum dots and focus on four topics: labelling of cellular structures and receptors with quantum dots, incorporation of quantum dots by living cells, tracking the path and the fate of individual cells using quantum dot labels, and quantum dots as contrast agents.     

Three-dimensional optical control of individual quantum dots

We show that individual colloidal CdSe-core quantum dots can be optically trapped and manipulated in three dimensions by an infrared continuous wave laser operated at low laser powers. This makes possible utilizing quantum dots not only for visualization but also for manipulation, an important advantage for single molecule experiments. Moreover, we provide quantitative information about the magnitude of forces applicable to a single quantum dot and of the polarizability of an individual quantum dot.     

Photoluminenscence Blinking Dynamics of Colloidal Quantum Dots in the Presence of Controlled External Electron Traps

The effect of the external charge trap on the photoluminescence blinking dynamics of individual colloidal quantum dots is investigated with a series of colloidal quantum dot–bridge–fullerene dimers with varying bridge lengths, where the fullerene moiety acts as a well‐defined, well‐positioned external charge trap. It is found that charge transfer followed by charge recombination is an important mechanism in determining the blinking behavior of quantum dots when the external trap is properly coupled with the excited state of the quantum dot, leading to a quasi‐continuous distribution of ‘on' states and an early fall‐off from a power‐law distribution for both ‘on' and ‘off' times associated with quantum dot photoluminescence blinking.     

Bias-induced photoluminescence quenching of single colloidal quantum dots embedded in organic semiconductors

We demonstrate reversible quenching of the photoluminescence from single CdSe/ZnS colloidal quantum dots embedded in thin films of the molecular organic semiconductor N,N'-diphenyl-N,N'-bis(3-methylphenyl)-(1,1'-biphenyl)-4,4'-diamine (TPD) in a layered device structure. Our analysis, based on current and charge carrier density, points toward field ionization as the dominant photoluminescence quenching mechanism. Blinking traces from individual quantum dots reveal that the photoluminescence amplitude decreases continuously as a function of increasing forward bias even at the single quantum dot level. In addition, we show that quantum dot photoluminescence is quenched by aluminum tris(8-hydroxyquinoline) (Alq3) in chloroform solutions as well as in thin solid films of Alq3 whereas TPD has little effect. This highlights the importance of chemical compatibility between semiconductor nanocrystals and surrounding organic semiconductors. Our study helps elucidate elementary interactions between quantum dots and organic semiconductors, knowledge needed for designing efficient quantum dot organic optoelectronic devices.     

Semiconductor quantum dots

Three-dimensionally confined semiconductor quantum dots have emerged to be a versatile material system with unique physical properties for a wide range of device applications. With the advances in nanotechnology and material growth techniques for both epitaxial and colloidal quantum dots, recently the research has been shifted largely towards quantum dot based devices for practical applications. In this short review, we have tried to assemble a selection of recent advances in the areas of quantum dots for computing and communications, solid state lighting, photovoltaics, and biomedical applications that highlight the state of the art.     

Red, green and blue lasing enabled by single-exciton gain in colloidal quantum dot films

Colloidal quantum dots exhibit efficient photoluminescence with widely tunable bandgaps as a result of quantum confinement effects. Such quantum dots are emerging as an appealing complement to epitaxial semiconductor laser materials, which are ubiquitous and technologically mature, but unable to cover the full visible spectrum (red, green and blue; RGB). However, the requirement for high colloidal-quantum-dot packing density, and losses due to non-radiative multiexcitonic Auger recombination, have hindered the development of lasers based on colloidal quantum dots. Here, we engineer CdSe/ZnCdS core/shell colloidal quantum dots with aromatic ligands, which form densely packed films exhibiting optical gain across the visible spectrum with less than one exciton per colloidal quantum dot on average. This single-exciton gain allows the films to reach the threshold of amplified spontaneous emission at very low optical pump energy densities of 90?μJ?cm(-2), more than one order of magnitude better than previously reported values. We leverage the low-threshold gain of these nanocomposite films to produce the first colloidal-quantum-dot vertical-cavity surface-emitting lasers (CQD-VCSEL). Our results represent a significant step towards full-colour single-material lasers.     

Lateral patterning of multilayer InAs/GaAs(001) quantum dot structures by in vacuo focused ion beam

We report on the effects of patterning and layering on multilayer InAs/GaAs(001) quantum dot structures laterally ordered using an in vacuo focused ion beam. The patterned hole size and lateral pattern spacing affected the quantum dot size and the fidelity of the quantum dots with respect to the lateral patterns. 100% pattern fidelity was retained after six layers of dots for a 9.0 ms focused ion beam dwell time and 2.0 μm lateral pattern spacing. Analysis of the change in quantum dot size as a function of pattern spacing provided a means of estimating the maximum average adatom surface diffusion length to be approximately 500 nm, and demonstrated the ability to alter the wetting layer thickness via pattern spacing. Increasing the number of layers from six to 26 resulted in mound formation, which destroyed the pattern fidelity at close pattern spacings and led to a bimodal quantum dot size distribution as measured by atomic force microscopy. The bimodal size distribution also affected the optical properties of the dots, causing a split quantum dot photoluminescence peak where the separation between the split peaks increased with increasing pattern spacing.     

异质外延自组织量子点弹性应变场分布的研究

通过有限元法,在ANSYS环境下,利用二维模型对自组织应变外延异质结透镜形量子点的应力应变分布情况进行了系统分析。分析过程分别考虑了孤立量子点系统、单层多量子点系统以及量子点超晶格系统3种情况,结果表明单层和超晶格多量子点系统衬底之间存在的长程相互作用力对量子点及其周围的应力应变分布有显著影响。在计算应变对多量子点系统的电子结构的影响时,必须将多量子点系统整体考虑。     

Photoluminescence imaging of focused ion beam induced individual quantum dots

We report on scanning microphotoluminescence measurements that spectrally and spatially resolve emission from individual InAs quantum dots that were induced by focused ion beam patterning. Multilayers of quantum dots were spaced 2 μm apart, with a minimum single dot emission line width of 160 μeV, indicating good optical quality for dots patterned using this technique. Mapping 16 array sites, at least 65% were occupied by optically active dots and the spectral inhomogeneity was within 30 meV.     

DNA-based programming of quantum dot valency, self-assembly and luminescence

The electronic and optical properties of colloidal quantum dots, including the wavelengths of light that they can absorb and emit, depend on the size of the quantum dots. These properties have been exploited in a number of applications including optical detection, solar energy harvesting and biological research. Here, we report the self-assembly of quantum dot complexes using cadmium telluride nanocrystals capped with specific sequences of DNA. Quantum dots with between one and five DNA-based binding sites are synthesized and then used as building blocks to create a variety of rationally designed assemblies, including cross-shaped complexes containing three different types of dots. The structure of the complexes is confirmed with transmission electron microscopy, and photophysical studies are used to quantify energy transfer among the constituent components. Through changes in pH, the conformation of the complexes can also be reversibly switched, turning on and off the transfer of energy between the constituent quantum dots.     

基于BIMF-GLCM分析的印刷网点异常状态诊断方法

目的为了实现印刷生产过程中网点异常状态的智能诊断,提出一种基于二维经验模式分解(BEMD)的网点特征提取方法。方法通过对网点图像的BEMD分析,获取了其二维本征模式分量,并利用灰度共生矩阵(GLCM)对其进行特征提取,构建印刷网点的特征表示向量。结果依托支持向量机决策方法开展分类实验,所提出的方法能够准确诊断出网点压力不当、水墨不均等异常状态,网点分类实验的正确率达到90%以上。结论 BIMF-GLCM分析对于网点特性有着很好的表征能力,相关研究为印刷网点智能诊断特征集的构建提供了有效方法。     

Colloidal quantum dot photovoltaics: the effect of polydispersity

The size-effect tunability of colloidal quantum dots enables facile engineering of the bandgap at the time of nanoparticle synthesis. The dependence of effective bandgap on nanoparticle size also presents a challenge if the size dispersion, hence bandgap variability, is not well-controlled within a given quantum dot solid. The impact of this polydispersity is well-studied in luminescent devices as well as in unipolar electronic transport; however, the requirements on monodispersity have yet to be quantified in photovoltaics. Here we carry out a series of combined experimental and model-based studies aimed at clarifying, and quantifying, the importance of quantum dot monodispersity in photovoltaics. We successfully predict, using a simple model, the dependence of both open-circuit voltage and photoluminescence behavior on the density of small-bandgap (large-diameter) quantum dot inclusions. The model requires inclusion of trap states to explain the experimental data quantitatively. We then explore using this same experimentally tested model the implications of a broadened quantum dot population on device performance. We report that present-day colloidal quantum dot photovoltaic devices with typical inhomogeneous linewidths of 100-150 meV are dominated by surface traps, and it is for this reason that they see marginal benefit from reduction in polydispersity. Upon eliminating surface traps, achieving inhomogeneous broadening of 50 meV or less will lead to device performance that sees very little deleterious impact from polydispersity.     

Optical properties of HgTe colloidal quantum dots

Room temperature photodetection with HgTe colloidal quantum films is reported between 2 and 5 μm for particles of sizes between ~5 and ~12 nm diameter, and photodetection extends to 7 μm at 80 K. The size-tuning of the absorption of HgTe colloidal quantum dots, their optical cross section and the infrared absorption depth of films are measured. The tuning with radius is empirically given by [see formula in text] where R is in nm. The optical cross section of the colloidal dots at 415 nm is approximately proportional to their volume and given by σ(Hg)(415) = 2.6 ± 0.4 10(-17) cm(2)/mercury atom. The size-dependent optical cross section at the band edge ~1.5 10(-15) cm(2) is consistent with the expected oscillator strength of the quantum dots. The absorption depth of HgTe colloidal dot films is short, about 1-2 μm, which is an advantage for thin film devices. These properties agree rather well with the expectation from the k · p model. HgTe colloidal quantum dot thin films show a strong tuning with temperature with a large positive thermal shift between 0.4 and 0.2 meV K(-1), decreasing with decreasing size within the size range studied and this is attributed primarily to electron-phonon effects.     

Empirical pseudo-potential studies on electronic structure of semiconducting quantum dots

Theoretical investigations of electronic structure of quantum dots is of current interest in nanophase materials. Empirical theories such as effective mass approximation, tight binding methods and empirical pseudo-potential method are capable of explaining the experimentally observed optical properties. We employ the empirical pseudo-potential to calculate the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) as a function of shape and size of the quantum dots. Our studies explain the building up of the bulk band structure when the size of the dot is much larger than the bulk Bohr exciton radius. We present our investigations of HOMO-LUMO gap variation with size, for CdSe, ZnSe and GaAs quantum dots. The calculated excitonic energies are sensitive to the shape and size of quantum dots and are in good agreement with experimental HOMO-LUMO gaps for CdSe quantum dots. The agreement improves as experimentally observed lattice contraction is incorporated in pseudo-potential calculations for ZnSe quantum dots. Electronic structure evolution, as the size of quantum dot increases, is presented for CdSe, ZnSe and GaAs quantum dots.     

Visible colloidal nanocrystal silicon light-emitting diode

We herein demonstrate visible electroluminescence from colloidal silicon in the form of a hybrid silicon quantum dot-organic light emitting diode. The silicon quantum dot emission arises from quantum confinement, and thus nanocrystal size tunable visible electroluminescence from our devices is highlighted. An external quantum efficiency of 0.7% was obtained at a drive voltage where device electroluminescence is dominated by silicon quantum dot emission. The characteristics of our devices depend strongly on the organic transport layers employed as well as on the choice of solvent from which the Si quantum dots are cast.     

PbS量子点能级结构的尺寸和配体依赖性及其对异质结电池性能的影响

通过电化学循环伏安测试和吸收光谱测试, 确定了有机配体(油酸)和原子配体(四正丁基碘化铵, TBAI)钝化的不同粒径(2.6~4.5 nm)PbS量子点的导带和价带能级, 并研究了量子点尺寸对PbS/TiO₂异质结电池(空气气氛中制备)性能的影响。结果表明：PbS量子点的能级结构受其粒径大小和表面配体特性的影响。当PbS量子点尺寸从2.6 nm增加至4.5 nm时, 油酸包覆PbS量子点的导带底从-3.67 eV减小到-4.0 eV, 价带顶从-5.19 eV增加到-4.97 eV; 而对于TBAI配体置换的PbS量子点, 其导带底和价带顶则分别从-4.15 eV和-5.61 eV变化至-4.51 eV和-5.46 eV。粒径为3.9 nm的PbS量子点所制备的电池性能最优, 其能量转化效率达到2.32%, 这可归因于其适宜的禁带宽度、结晶质量和良好的PbS/TiO₂界面能级匹配度。     

Photo-induced suppression of plasmonic emission enhancement of CdSe/ZnS quantum dots

Emission of semiconductor quantum dots can be increased via two fundamentally different processes: (i) surface plasmon resonances (plasmonic emission enhancement) and (ii) irradiation with light (photo-induced fluorescence enhancement). In this paper we theoretically and experimentally study the mutual impacts of these processes on each other in quantum dot solids. We show that when thin films of colloidal quantum dots are placed in the vicinity of Au nano-islands, the plasmonic enhancement of the radiative decay rates of quantum dots and Forster energy transfer can hinder the photo-induced fluorescence enhancement of these films. This in turn leads to significant suppression of their plasmonic emission enhancement when they are irradiated with a laser beam. We investigate the impact of the sizes and shapes of the metallic nanoparticles in this process and theoretically analyze how plasmons and energy transfer can hinder the electrostatic barrier responsible for photo-induced fluorescence enhancement.     

Principles of conjugating quantum dots to proteins via carbodiimide chemistry

The covalent coupling of nanomaterials to bio-recognition molecules is a critical intermediate step in using nanomaterials for biology and medicine. Here we investigate the carbodiimide-mediated conjugation of fluorescent quantum dots to different proteins (e.g., immunoglobulin G, bovine serum albumin, and horseradish peroxidase). To enable these studies, we developed a simple method to isolate quantum dot bioconjugates from unconjugated quantum dots. The results show that the reactant concentrations and protein type will impact the overall number of proteins conjugated onto the surfaces of the quantum dots, homogeneity of the protein-quantum dot conjugate population, quantum efficiency, binding avidity, and enzymatic kinetics. We propose general principles that should be followed for the successful coupling of proteins to quantum dots.     

Transient reflection: a versatile technique for ultrafast spectroscopy of a single quantum dot in complex environments

Increasingly complex structures such as optical antennas or cavities are coupled to self-assembled quantum dots to harvest their quantum-optical properties. In many cases, these structures pose a problem for common methods of ultrafast spectroscopy used to write and read out the state of the quantum dot. We present a pure far-field method that only requires optical access to the quantum dot and does not impose further restrictions on sample design. We demonstrate Rabi oscillations and perturbed free induction decay of single GaAs quantum dots that have a dipole moment as small as 18 D. Our method will greatly facilitate ultrafast spectroscopy of complex quantum-optical circuits.     

Lead‐Chalcogenide Colloidal‐Quantum‐Dot Solids: Novel Assembly Methods,Electronic Structure Control,and Application Prospects

The quest for novel semiconductors with easy, cheap fabrication and tailorable properties has led to the development of several classes of materials, such as semiconducting polymers, carbon nanotubes, hybrid perovskites, and colloidal quantum dots. All these candidates can be processed from the liquid phase, enabling easy fabrication, and are suitable for different electronic and optoelectronic applications. Here, recent developments in the field of colloidal‐quantum‐dot solids are discussed, with a focus on lead‐chalcogenide systems. These include novel deposition methods; the recent growing understanding of their fundamental properties, driven by major successes in the control of the nanostructured assembly and surface chemistry; and selected reports on lab‐scale devices showing the technological prospects of these fascinating class of materials.