Molecular beam epitaxy growth and characterization of self-assembled InAs quantum dots on (1 0 0) InAlAs/InP substrates

Self-assembled InAs quantum dots (QDs) on In_0.52Al_0.48As layer lattice matched to (1 0 0) InP substrates have been grown by molecular beam epitaxy (MBE) and evaluated by transmission electron microscopy (TEM) and photoluminescence (PL). TEM observations indicate that defect-free InAs QDs can be grown to obtain emissions over the technologically important 1.3–1.55 μm region. The PL peak positions for the QDs shift to low energy as the InAs coverage increases, corresponding to increase in QD size. The room temperature PL peak at 1.58 μm was observed from defect-free InAs QDs with average dot height of 3.6 nm.     

Formation of InAs quantum dots and wetting layers in GaAs and AlAs analyzed by cross-sectional scanning tunneling microscopy

We have used cross-sectional scanning-tunneling microscopy (X-STM) to compare the formation of self-assembled InAs quantum dots (QDs) and wetting layers on AlAs (1 0 0) and GaAs (1 0 0) surfaces. On AlAs we find a larger QD density and smaller QD size than for QDs grown on GaAs under the same growth conditions (500 °C substrate temperature and 1.9 ML indium deposition). The QDs grown on GaAs show both a normal and a lateral gradient in the indium distribution whereas the QDs grown on AlAs show only a normal gradient. The wetting layers on GaAs and AlAs do not show significant differences in their composition profiles. We suggest that the segregation of the wetting layer is mainly strain-driven, whereas the formation of the QDs is also determined by growth kinetics. We have determined the indium composition of the QDs by fitting it to the measured outward relaxation and lattice constant profile of the cleaved surface using a three-dimensional finite element calculation based on elasticity theory.     

Microstructural and optical properties of multiple closely stacked InAs/GaAs self-assembled quantum dot arrays

The microstructural and the optical properties of multiple closely stacked InAs/GaAs quantum dot (QD) arrays were investigated by using atomic force microscopy (AFM), transmission electron microscopy (TEM), and photoluminescence (PL) measurements. The AFM and the TEM images showed that high-quality vertically stacked InAs QD self-assembled arrays were embedded in the GaAs barriers. The PL peak position corresponding to the interband transitions from the ground electronic subband to the ground heavy-hole band (E₁-HH₁) of the InAs/GaAs QDs shifted to higher energy with increasing GaAs spacer thickness. The activation energy of the electrons confined in the InAs QDs increased with decreasing with GaAs spacer thickness due to the coupling effect. The present results can help to improve the understanding of the microstructural and the optical in multiple closely stafcked InAs/GaAs QD arrays.     

Thermodynamic analysis of the shape, anisotropy and formation process of InAs/InP(0 0 1) quantum dots and quantum sticks grown by metalorganic vapor phase epitaxy

This study describes the origin of the size and shape anisotropy of InAs/InP(0 0 1) quantum dots (QDs) grown by metalorganic vapor phase epitaxy (MOVPE). The geometry of the QDs is determined by carefully analyzing transmission electron microscopy (TEM) images. An analytical model adapted to our QD geometry is used to understand the formation mechanism of the QDs, and to describe the origin of their size dispersion. A shape transition from QDs to elongated quantum sticks (QS) is observed under As-poor growth conditions. This transition, driven by thermodynamics, is clearly described by our model.     

Synthesis and characterization of binary and ternary III–V quantum dots

Quantum dots of InP, GaP, GaInP₂, and GaAs with diameters ranging from 20–80 Å can be synthesized as well-crystallized nanoparticles with bulk zinc blende structure. The synthesis is achieved by heating appropriate organometallic precursors with stabilizers in high boiling solvents for several days to produce QDs, which can then be dissolved in nonpolar organic solvents to form transparent colloidal QD dispersions. The high sample quality of the InP and Gap QDs results in excitonic features in the absorption spectra; excitonic features could not be observed for GaAs or GaInP₂ QDs. The GaP and GaInP₂ QD colloids exhibit very intense (quantum yields of 15–25%) visible photoluminescence at room temperature. The photoluminescence for InP QDs preparations show two emission bands: one band is in the visible at the band edge of the QD, and a second band appears above 800 nm. The near-IR PL is attributed to deep traps, presumably phosphorus vacancies on the QD surface. This band can be removed after controlled addition of etchant; subsequently, very intense band-edge emission (quantum yield 30%), which is tunable with particle size, is obtained. The QDs were characterized by TEM, SAXS, AFM, powder X-ray diffraction, steady-state optical absorption and photoluminescence spectroscopy, ps to ns transient photoluminescence spectroscopy, and fs to ps pump-probe absorption (i.e., hole-burning) spectroscopy.     

Flying qubits and entangled photons

Efficient generation of polarized single photons or entangled photon pairs is crucial for the implementation of quantum key distribution (QKD) systems. Self organized semiconductor quantum dots (QDs) are capable of emitting on demand one polarized photon or an entangled photon pair upon current injection. Highly efficient single‐photon sources consist of a pin structure inserted into a microcavity where single electrons and holes are funneled into an InAs QD via a submicron AlOx aperture, leading to emission of single polarized photons with record purity of the spectrum and non‐classicality of the photons. A new QD site‐control technique is based on using the surface strain field of an AlO_x current aperture below the QD. GaN/AlN QD based devices are promising to operate at room temperature and reveal a fine‐structure splitting (FSS) depending inversely on the QD size. Large GaN/AlN QDs show disappearance of the FSS. Theory also suggests QDs grown on (111)‐oriented GaAs substrates as source of entangled photon pairs.     

Optical and structural characterization of a self-aligned single electron transitor structure by cathodoluminescence microscopy

We report about optical and structural investigations of a self-aligned single electron transistor (SET) structure using cathodoluminescence-(CL) and transmission electron microscopy (TEM). The SET structures were fabricated by MBE growth of GaAs/AlAs on different prepatterned GaAs (1 0 0) substrates. This technique for the in situ formation of nanoscopic semiconductor heterostructures is presently a widely used and promising approach for the fabrication of low-dimensional systems like quantum wires and quantum dots (QD). The active region of the SET structure consists of a GaAs/AlGaAs-QD formed by thickness modulation of a single quantum well (SQW) during the MBE growth. The position and the size of the QD is defined by the design of the substrate pattern. The thickness modulation of the GaAs-SQW is evidenced by TEM investigations. The lateral confinement potential given by the thickness modulation of GaAs-SQW is directly imaged by CL microscopy.     

复合荧光CdSe量子点-脂质体的制备与表征

通过脂质体方法成功地将三辛基氧化膦(TOPO)包覆的CdSe发光量子点从非极性有机溶剂转移到生物相容性的水溶液中.分别通过透射电镜(TEM)、荧光Mapping图像,以及光致发光(PL)光谱进行表征.TEM照片显示制备的CdSe核量子点为球形,具有良好的单分散特性,平均粒径约为3nm.CdSe-脂质体复合体的平均尺寸大约20nm,TEM清楚地显示了CdSe量子点被诱捕在脂质体中.荧光Mapping显示了CdSe-脂质体复合体的发光强度分布.脂质体方法转移TOPO包覆的CdSe量子点,借助了磷脂的双分子链与CdSe表面的TOPO配体之间的疏水相互作用,在CdSe的第一配体层外部形成第二配体层,保留了CdSe的存在环境,光致发光光谱表明,量子点-脂质复合体基本保持了CdSe核量子点的发射效率.     

Raman spectroscopy of very small Cd1−xCoxS quantum dots grown by a novel protocol: direct observation of acoustic‐optical phonon coupling

Glass‐embedded Cd_1−xCo_xS quantum dots (QDs) with mean radius of R ≈ 1.70 nm were successfully synthesized by a novel protocol on the basis of the melting‐nucleation synthesis route and herein investigated by several experimental techniques. Incorporation of Co²⁺ ions into the QD lattice was evidenced by X‐ray diffraction and magnetic force microscopy results. Optical absorption features with irregular spacing in the ligand field region confirmed that the majority of the incorporated Co²⁺ ions are under influence of a low‐symmetry crystal field located near to the Cd_1−xCo_xS QD surface. Electron paramagnetic resonance data confirmed the presence of Co²⁺ ions in a highly inhomogeneous crystal field environment identified at the interface between the hosting glass matrix (amorphous) and the crystalline QD. The acoustic‐optical phonon coupling in the Cd_1−xCo_xS QDs (x ≠ 0.000) was directly observed by Raman measurements, which have shown a high‐frequency shoulder of the longitudinal optical phonon peak. This effect is tuned by the size‐dependent sp‐d exchange interaction due to the magnetic doping, causing variations in the coupling between electrons and longitudinal optical phonon. Copyright © 2013 John Wiley & Sons, Ltd.     

Condensable InP quantum dots solid

Colloidal thioacetic acid-capped InP quantum dots (InP-TAA QDs) and their spin coated-films have been examined in comparison with those of myristic acid-capped InP (InP-MA) QDs. While the QDs are far away from each other in the InP-MA QD films, even in a InP-MA QD film cured at 250 °C, upon thermal annealing a film of InP-TAA QDs at 250 °C, the indium thioacetate groups on the surfaces QDs likely condensed, thus resulting in the QD film that consists of individual and proximally packed InP QDs. The structures of the films of InP-MA QDs or InP-TAA QDs were characterized by means of TEM, XRD, and XPS. The current through the film of InP-TAA QDs cured at 250 °C was about 2–5 orders of magnitude higher than that of the film of InP-MA film annealed by the sample conditions. We, newly in this letter, define this kind of materials architecture as the condensable QDs solid concept.     

Near‐field optical enhancement by lead‐sulfide quantum dots and metallic nanoparticles for SERS

There is a growing interest in using quantum dots (QDs) and metallic nanoparticles (NPs), both for luminescence enhancement and surface‐enhanced Raman scattering (SERS). Here, we study the electromagnetic‐field enhancement that can be generated by lead‐sulfide (PbS) QDs using three‐dimensional finite‐element simulations. We investigate the field enhancement associated with combinations of PbS QDs with metallic NPs and substrates. The results show that high field enhancement can be achieved by combining PbS QDs with metallic NPs of larger sizes. The ideal size for Ag NPs is 25 nm, providing a SERS enhancement factor of ~5*10⁸ for light polarization parallel to the NP dimer axis and a gap of 0.6 nm. For Au NPs, the bigger the size, the higher is the field for the studied diameters, up to 50 nm. The near‐field values for PbS QDs above metallic substrates were found to be lower compared to the case of PbS QD‐metal NP dimers. This study provides the understanding for the design and application of QDs for the enhancement of near‐field phenomena. Copyright © 2013 John Wiley & Sons, Ltd.     

Numerical investigation on threading dislocation bending with InAs/GaAs quantum dots

The threading dislocations (TDs) in GaAs/Si epitaxial layers due to the lattice mismatch seriously degrade the performance of the lasers grown on silicon. The insertion of InAs quantum dots (QDs) acting as dislocation filters is a pretty good alternative to solving this problem. In this paper, a finite element method (FEM) is proposed to calculate the critical condition for InAs/GaAs QDs bending TDs into interfacial misfit dislocations (MDs). Making a comparison of elastic strain energy between the two isolated systems, a reasonable result is obtained. The effect of the cap layer thickness and the base width of QDs on TD bending are studied, and the results show that the bending area ratio of single QD (the bending area divided by the area of the QD base) is evidently affected by the two factors. Moreover, we present a method to evaluate the bending capability of single-layer QDs and multi-layer QDs. For the QD with 24-nm base width and 5-nm cap layer thickness, taking the QD density of 10¹¹ cm^-2 into account, the bending area ratio of single-layer QDs (the area of bending TD divided by the area of QD layer) is about 38.71%. With inserting five-layer InAs QDs, the TD density decreases by 91.35%. The results offer the guidelines for designing the QD dislocation filters and provide an important step towards realizing the photonic integration circuits on silicon.     

量子点背光技术的研究进展

量子点材料因具有发光波长可调,色度纯,量子效率高等优异特性而受到广泛关注,在光致发光高色彩显示方面有着巨大的应用潜力。本文综述了量子点背光技术的研究进展,主要对比了QDs On-Chip、QDs On-Surface及QDs On-Edge 3种量子点背光主流技术的基本原理及结构,并分析了它们在液晶显示领域的应用,未来前景及面临的挑战;然后介绍了几种新型的量子点背光技术,并对两种量子点背光新技术进行重点说明:一种是采用低温注塑成型工艺将量子点与高分子材料均匀混合为一体,用于制备直下式背光的量子点体散射型结构扩散板;另一种新技术是采用丝网印刷或喷墨打印工艺将量子点转印至导光板表面,形成应用于侧入式背光的量子点网点微结构导光板。这两种背光都具有制备工艺简单、成本低、生产效率高等特点,对高色域液晶显示的研究及发展意义深远。     

A comprehensive study of the effect of <Emphasis Type="Italic">in situ</Emphasis> annealing at high growth temperature on the morphological and optical properties of self-assembled InAs/GaAs QDs

We investigate the effect of in situ annealing during growth pause on the morphological and optical properties of self-assembled InAs/GaAs quantum dots (QDs). The islands were grown at different growth rates and having different monolayer coverage. The results were explained on the basis of atomic force microscopy (AFM) and photo-luminescence (PL) measurements. The studies show the occurrence of ripening-like phenomenon, observed in strained semiconductor system. Agglomeration of the self-assembled QDs takes place during dot pause leading to an equilibrium size distribution. The PL properties of the QDs are affected by the Indium desorption from the surface of the QDs during dot pause annealing at high growth temperature (520°C) subsiding the effect of the narrowing of the dot size distribution with growth pause. The samples having high monolayer coverage (3.4 ML) and grown at a slower growth rate (0.032 ML s⁻¹) manifested two different QD families. Among the islands the smaller are coherent defect-free in nature, whereas the larger dots are plastically relaxed and hence optically inactive. Indium desorption from the island surface during the in situ annealing and inhomogeneous morphology as the dots agglomerate during the growth pause, also affects the PL emission from these dot assemblies.     

Ultrasensitive PbS‐Quantum‐Dot Photodetectors for Visible–Near‐Infrared Light Through Surface Atomic‐Ligand Exchange

A surface atomic‐ligand exchange method is applied the first time in the construction of photodetectors (PDs) based on PbS quantum dots (QDs) for ultrasensitivity. The device thus produces a high photosensitivity to visible and near‐infrared light with a photoresponsivity up to 7.5 × 10³ A W^?1 and a high stability in air. In particular, these PbS‐QD‐based PDs show the capability of following a pulse light with a frequency up to 100 kHz well at a relatively fast response time/recovery time of ≈4/40 μs, much faster than most previous QD‐based PDs. The short response time is attributed to modification for the surface of the PbS‐QDs by cetyltrimethylammonium bromide treatment, which effectively improves the contact between the QDs and the Au electrodes, leading to extracting a high carrier mobility (≈0.142 cm² V^?1 s^?1). These findings show the great potential of PbS‐QDs as high‐speed nano‐photodetectors, and, more importantly, demonstrate the importance of the surface atomic‐ligand exchange method in the construction of QD‐based devices.     

Surface-initiated ring-opening metathesis polymerization (SI-ROMP) to attach a tethered organic corona onto CdSe/ZnS core/shell quantum dots

Core–shell CdSe/ZnS quantum dots (QDs) are useful as tunable photostable fluorophores for multiple applications in industry, biology, and medicine. However, to achieve the optimum optical properties, the surface of the QDs must be passivated to remove charged sites that might bind extraneous substances and allow aggregation. Here we describe a method of growing an organic polymer corona onto the QD surface using the bottom-up approach of surface-initiated ring-opening metathesis polymerization (SI-ROMP) with Grubbs catalyst. CdSe/ZnS QDs were first coated with mercaptopropionic acid by displacing the original tri-octylphosphine oxide layer, and then reacted with 7-octenyl dimethyl chlorosilane. The resulting octenyl double bonds allowed the attachment of ruthenium alkylidene groups as a catalyst. A subsequent metathesis reaction with strained bicyclic monomers (norbornene-dicarbonyl chloride (NDC), and a mixture of NDC and norbornenylethylisobutyl-polyhedral oligomeric silsesquioxane (norbornoPOSS)) allowed the construction of tethered organic homo-polymer or co-polymer layers onto the QD. Compounds were characterized by FT-IR, 1H-NMR, X-ray photoelectron spectroscopy, differential scanning calorimetry, and transmission electron microscopy. Atomic force microscopy showed that the coated QDs were separate and non-aggregated with a range of diameter of 48–53 nm.     

Morphology response to strain field interferences in stacks of highly ordered quantum dot arrays

Twofold stacked InGaAs/GaAs quantum dot (QD) layers are grown on GaAs(001) substrates patterned with square arrays of shallow holes. We study the surface morphology of the second InGaAs QD layer as a function of pattern periodicity. Comparing our experimental results with a realistic simulation of the strain energy density E(str) distribution, we find that the second InGaAs QD layer sensitively responds to the lateral strain-field interferences generated by the buried periodic QD array. This response includes the well-known formation of vertically aligned QDs but also the occurrence of QDs on satellite strain energy density minima. Our calculations show that base size and shape as well as lateral orientation of both QD types are predefined by the E(str) distribution on the underlying surface.     

Phase transfer of agglomerated nanoparticles: deagglomeration by adsorbing grafted molecules and colloidal stability in polymer solutions

We aimed to develop liposomes for loading both cisplatin and quantum dots (QDs) for both drug delivery and bioimaging. The resultant quantum-dot-liposomes (QDLs) with cisplatin were characterized using dynamic light scattering, transmission electron microscopy (TEM), encapsulation efficiencies, and fluorescence intensity. QDLs composed of CdSe or CdSe/ZnS QDs represented a size of about 100?nm. The QDLs were prepared at a high QD loading efficiency of nearly 100?%. Most QDs were located within the liposomal bilayers as evidenced by TEM. Slow and sustained cisplatin release from QDLs was achieved. The cellular uptake of QDLs demonstrated effective internalization and significant fluorescence in melanoma cells. The signal derived from QDLs could be observed by different wavelength settings. The cisplatin-containing QDLs revealed higher cytotoxic activity compared to an equal dose of free cisplatin. CdSe/ZnS QDLs were intravenously administered to mice, and the biodistribution was observed with an in vivo imaging system. Significant fluorescence signal and cisplatin accumulation were detected in the brain and skin, as verified by ex vivo imaging and drug distribution. Liposomal inclusion could reduce the reticuloendothelial system uptake of QDs and cisplatin. QDLs evaluated in this study represent a new potential method for theranostic purposes.     

Synthesis and photoluminescence properties of aqueous CdWO4 quantum dots with WO6 luminescence center

Aqueous CdWO₄ QDs were synthesized by the reaction of CdCl₂ and Na₂WO₄ in the presence of mercaptoacetic acid (TGA) as capping reagent. The crystal morphology, particle size and its distribution of as-prepared products were characterized by transmission electron microscopy (TEM, SAED) atomic force microscopy (AFM), high-resolution transmission electron microscopy (HRTEM), and photon correlation spectroscopy (PCS), respectively. Qualitative assays for functional groups on the QDs’ surface were measured by fourier transform infrared spectroscopy (FTIR). Photoluminescence properties of QDs were studied by photoluminescence spectroscopy (PL). The results showed that the single QD with diameter of about 8 ± 2 nm was single-crystal. The particle size distribution of QDs was normal. Infrared absorption bands of carboxylic group on the surface of CdWO₄ QDs were observed around 1610-1550 cm⁻¹ (nonsymmetrical vibration of -COO⁻) and 1400 cm⁻¹ (symmetric vibration of C-O). With reaction-time going, PL peak position shifted from 498 to 549 nm and intensity of PL increased first and then decreased. PL peak position of QDs was blue-shift compared with 570 nm WO₆⁶⁻ luminescence center of bulk CdWO₄.     

Mn-including InAs quantum dots fabricated by Mn implantation

Mn-including InAs quantum dots (QDs) were fabricated by Mn-ion implantation and subsequent annealing. The optical, compositional, and structural properties of the treated samples were analyzed by photoluminescence (PL) and microscopy. Energy dispersive X-ray (EDX) results indicate that Mn ions diffused from the bulk GaAs into the InAs QDs during annealing, and the diffusion appears to be driven by the strain in the InAs QDs. The temperature dependence of the PL of Mn-including InAs QD samples exhibits QDs PL characteristics. At the same time, the heavy Mn-including InAs QD samples have ferromagnetic properties and high T_c.