Multi-scale ordering of self-assembled InAs/GaAs(001) quantum dots

Ordering phenomena related to the self-assembly of InAs quantum dots (QD) grown on GaAs(001) substrates are experimentally investigated on different length scales. On the shortest length-scale studied here, we examine the QD morphology and observe two types of QD shapes, i.e., pyramids and domes. Pyramids are elongated along the [1–10] directions and are bounded by {137} facets, while domes have a multi-facetted shape. By changing the growth rates, we are able to control the size and size homogeneity of freestanding QDs. QDs grown by using low growth rate are characterized by larger sizes and a narrower size distribution. The homogeneity of buried QDs is measured by photoluminescence spectroscopy and can be improved by low temperature overgrowth. The overgrowth induces the formation of nanostructures on the surface. The fabrication of self-assembled nanoholes, which are used as a template to induce short-range positioning of QDs, is also investigated. The growth of closely spaced QDs (QD molecules) containing 2–6 QDs per QD molecule is discussed. Finally, the long-range positioning of self-assembled QDs, which can be achieved by the growth on patterned substrates, is demonstrated. Lateral QD replication observed during growth of three-dimensional QD crystals is reported.     

In situ preparation of fluorescent CdTe quantum dots with small thiols and hyperbranched polymers as co-stabilizers

A new strategy for in situ preparation of highly fluorescent CdTe quantum dots (QDs) with 3-mercaptopropionic acid (MPA) and hyperbranched poly(amidoamine)s (HPAMAM) as co-stabilizers was proposed in this paper. MPA and HPAMAM were added in turn to coordinate Cd²⁺. After adding NaHTe and further microwave irradiation, fluorescent CdTe QDs stabilized by MPA and HPAMAM were obtained. Such a strategy avoids the aftertreatment of thiol-stabilized QDs in their bioapplication and provides an opportunity for direct biomedical use of QDs due to the existence of biocompatible HPAMAM. The resulting CdTe QDs combine the mechanical, biocompatibility properties of HPAMAM and the optical, electrical properties of CdTe QDs together.     

Novel in-situ synthesis of Au/SnO2 quantum dots for enhanced visible-light-driven photocatalytic applications

In recent years, visible-light-driven metal–semiconductor nanocomposites have emerged as a suitable material for the decomposition of various water and air pollutants. In this work, a novel plasmonic Au nanoparticle (NP)/SnO₂ quantum dot (SQD) nanocomposite photocatalysts were prepared via a one-step solvothermal technique. The as-prepared plasmonic photocatalysts were characterized by various techniques, and the results established the formation of Au/SQD nanocomposites. The photocatalytic activity of the as-prepared plasmonic Au/SQD nanocomposites was examined by the degradation of Rhodamine B (RhB) at room temperature under visible light, and the Au/SQD photocatalyst, prepared using 1.0?g of tin chloride, exhibited a higher rate constant of RhB degradation than pristine SQDs. This exceptional improvement in catalytic performance under visible light is ascribed to a shift of the band gap from the ultraviolet to the visible region. The surface plasmon resonance effect of Au NPs and the synergistic coupling of the metal and the semiconductor QDs also played a vital role in enhancing the catalytic performance. The process of the photocatalytic degradation of RhB by the Au/SQD nanocomposites under visible light is described.     

Long-wavelength room-temperature luminescence from InAs/GaAs quantum dots with an optimized GaAsSbN capping layer

An extensive study on molecular beam epitaxy growth conditions of quaternary GaAsSbN as a capping layer (CL) for InAs/GaAs quantum dots (QD) was carried out. In particular, CL thickness, growth temperature, and growth rate were optimized. Problems related to the simultaneous presence of Sb and N, responsible for a significant degradation of photoluminescence (PL), are thereby solved allowing the achievement of room-temperature (RT) emission. A particularly strong improvement on the PL is obtained when the growth rate of the CL is increased. This is likely due to an improvement in the structural quality of the quaternary alloy that resulted from reduced strain and composition inhomogeneities. Nevertheless, a significant reduction of Sb and N incorporation was found when the growth rate was increased. Indeed, the incorporation of N is intrinsically limited to a maximum value of approximately 1.6% when the growth rate is at 2.0 ML s⁻¹. Therefore, achieving RT emission and extending it somewhat beyond 1.3 μm were possible by means of a compromise among the growth conditions. This opens the possibility of exploiting the versatility on band structure engineering offered by this QD-CL structure in devices working at RT.

PACS

81.15.Hi (molecular beam epitaxy); 78.55.Cr (III-V semiconductors); 73.21.La (quantum dots)     

Terbium-doped ZnS quantum dots: Structural,morphological, optical,photoluminescence, and photocatalytic properties

Terbium (0, 2, and 4?at%)-doped ZnS quantum dots (QDs) were synthesized via a solvothermal method. The crystal structures of the synthesized QDs were determined to be zinc blend by X-ray powder diffraction (XRD) and Raman analyses. Transmission electron microscopy (TEM) studies revealed that particles with a mean size of 2–4?nm were formed. An X-ray photo electron spectroscopy (XPS) examination disclosed the existence of terbium with a trivalent state in the ZnS host lattice. The absorption bands of all QDs were located around 325?nm (3.81?eV) and were higher than that of the bulk ZnS band gap (3.67?eV), consistent with the quantum confinement effect. The photoluminescence spectra of the terbium-doped samples displayed five emission peaks at 467?nm (⁵D₄→⁷F₃), 491?nm (⁵D₄ → ⁷F₆), 460?nm (⁵D₄ – ⁷F₅), 484?nm (⁵D₄ – ⁷F₄), and 530?nm (⁵D₄ – ⁷F₃), respectively. The terbium-doped QDs exhibited a higher photocatalytic activity during the degradation of crystal violet dye under UV-light illumination compared to the undoped ZnS QDs. These interesting properties of terbium-doped ZnS QDs are potentially useful for both luminescent and photocatalysis applications.     

Multi-colored type-I Ag-doped ZnCdS/ZnS core/shell quantum dots with intense emission

Although doped quantum dots (d-dots) with intense and tunable emission have been studied for long time, there are still great efforts to prepare new ones with promising characteristics. In the present work, we used a mild/effective strategy for preparing high quality and aqueous-soluble Ag:ZnCdS/ZnS core/shell quantum dots (QDs) with N-acetyl-l-cysteine as the capping agent. Through investigating the experimental variables, the impurity-related emission intensity of the as-prepared samples was optimized. The capability of the present work on creating quantum structures with tunable emission across the entire visible spectrum was approved through simple but effective variation in Zn-to-Cd molar ratio. Indeed, as the Zn:Cd molar ratio changed from 2:0 to 0:2, the emission color was changed significantly from blue to red color with a satisfactory photoluminescence quantum yield. The quantum yield value reached ～41% for the as-prepared core/shell d-dots without any pre/post-treatment, which is a remarkable result for such aqueous-soluble structures. XRD, EDX, ICP, and TEM measurements were applied to determine the structural features of the QDs in a strong quantum confinement regime. The generality of preparation route, its biocompatibility, along with a multi-color emission, can create new opportunities especially for the white light emitting technologies, or multi-color bioimaging for theranostics.     

Bulk synthesis of homogeneous and transparent bulk core/multishell quantum dots/PMMA nanocomposites with bright luminescence

Core/multishell CdSe/CdS/CdS/Cd_0.75Zn_0.25S/Cd_0.5Zn_0.5S/Cd_0.25 Zn_0.75S/ZnS/ZnS QDs/PMMA bulk nanocomposites were successfully synthesized. To achieve a homogeneous and stable dispersion in MMA monomers, QDs were first modified with amphiphilic polymer of poly(ethylene glycol)‐oleate (PEG‐oleate). Subsequently, the transparent and homogeneous bulk composites were obtained by photopolymerization. The TEM analysis of ultramicrotome cut of composite demonstrates that QDs are well dispersed in the PMMA matrix. UV–vis transmission spectra show the nanocomposites still maintain high transmittance even with QDs content of up to 6 wt %. The thermogravimetric analysis (TGA) reveals the incorporation of the QDs significantly improves the thermal stability of composites, as evidenced by the retardation of a degradation initiation by 60°C. Photoluminescence spectra suggest that the nanocomposites exhibit bright fluorescence under UV excitation. X‐ray excited luminescence spectra show they have response to X‐ray. All of the experimental results indicate the great promising applications of core/multishell QDs/PMMA nanocomposites in optical communication devices and radiation detectors. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 1548–1553, 2013     

Whispering gallery modes in photoluminescence and Raman spectra of a spherical microcavity with CdTe quantum dots: anti-Stokes emission and interference effects

We have studied the photoluminescence and Raman spectra of a system consisting of a polystyrene latex microsphere coated by CdTe colloidal quantum dots. The cavity-induced enhancement of the Raman scattering allows the observation of Raman spectra from only a monolayer of CdTe quantum dots. Periodic structure with very narrow peaks in the photoluminescence spectra of a single microsphere was detected both in the Stokes and anti-Stokes spectral regions, arising from the coupling between the emission of quantum dots and spherical cavity modes.     

A study of photoluminescence properties and performance improvement of Cd-doped ZnO quantum dots prepared by the sol-gel method

ABSTRACT: In the present work, ZnO quantum dots (QDs) have been prepared by the sol-gel method, and the performance of the QDs have been improved. The effect of Cd concentration on the structural and luminescent properties of the QDs, as well as the effect of the mass ratio of trioctylphosphine oxide (TOPO)/octadecylamine (ODA), has been investigated. The ZnO and Cd-doped ZnO QDs have hexagonal wurtzite structures and are 3~6 nm in diameter. When the Cd content was increased, the QD particle size was reduced; this effect was confirmed in the corresponding ultraviolet-visible (UV) spectra. The fluorescence intensity was simultaneously enhanced significantly. Both the UV and fluorescence spectra were blue-shifted. The luminous intensity was further enhanced when the QDs were modified with TOPO/ODA. FTIR and XRD techniques proved that the polymer successfully coated the surfaces of the QDs. A TOPO/ODA mass ratio of 1:2 was determined to result in the best optical performance among the different ratios examined. The results showed that the described synthetic method is appropriate for the preparation of doped QDs with a high fluorescence quantum efficiency.     

Direct electrochemiluminescence of CdTe quantum dots based on room temperature ionic liquid film and high sensitivity sensing of gossypol

A new method with high sensitivity was developed to determine gossypol content using CdTe quantum dot (QD) electrochemiluminescence (ECL) with a room temperature ionic liquid (RTIL) modified glassy carbon (GC) electrode. It was found that use of RTIL film on the GC electrode can greatly enhance the ECL intensity of CdTe QDs, and the ECL peak potential and ECL onset potential were both shifted positively. Under optimal conditions, the quenching effect of gossypol on the ECL emission of CdTe QDs was observed, and ECL intensity showed a good linear relationship in the gossypol concentration range of 5.0 × 10⁻⁷ to 5.0 × 10⁻⁹ M with a detection limit of 5.0 × 10⁻⁹ M. The proposed method was used to detect gossypol in cottonseed oil with satisfactory results. As a result, the introduction of an RTIL-modified electrode can extend the analytical applications of QD ECL systems.     

MoO3/Cd0.5Zn0.5S复合材料光催化降解甲基橙

以MoO3、Cd0.5Zn0.5S、聚乙烯吡咯烷酮（PVP）为原料，水热一锅法制备了复合材料MoO3/Cd0.5Zn0.5S，通过XRD、XPS、SEM、UV-Vis DRS及PL对复合材料的结构、形貌以及光学性能进行了表征。可见光照射下，MoO3/Cd0.5Zn0.5S复合材料对甲基橙（MO）、罗丹明B、亚甲基蓝、孔雀石绿、酸性品红等染料具有光催化降解能力，其中对MO的光催化活性最佳。结果表明，可见光照射60 min，0.67 L/g 10%Cd0.5Zn0.5S/MoO3对MO的降解率达到98.0%，反应速率常数为0.06725 min-1，分别为MoO3和Cd0.5Zn0.5S 光降解MO的169倍和31倍。     

Effects of annealing on bandgap and surface plasmon resonance enhancement in Au/SnO2 quantum dots

Au/SnO₂ quantum dots (AuSQDs) were synthesized, and the effects of annealing on their structural and optical properties were examined. Significant changes were observed in the bandgap and surface plasmon resonance (SPR) of the AuSQDs after thermal treatment at different temperatures (400, 500, and 600 °C). The properties of the as-prepared and annealed samples were characterized via X-ray diffraction analysis, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy, and diffuse reflectance spectroscopy. Annealing reduced the bandgap from 3.03 to 2.33 eV and increased the crystallinity while maintaining an average crystallite size below 10 nm. XPS valence band (VB) profiles provided information regarding the VB edge potentials, which helped to determine the conduction band edge potentials. An enhancement in the SPR of the Au nanoparticles was observed for AuSQD-500, which had the smallest bandgap among the samples investigated.     

CdTe/PEDOT-PSS hybrid microspheres: Facile fabrication and multiple-color pH-sensing

CdTe/PEDOT-PSS hybrid microspheres have been attained via the direct polymerization of EDOT-PSS on mercapopropionic acid capped CdTe quantum dot aggregates. The obtained hybrid microspheres provide multiple-color absorption and emission properties as a function of the pH value of the medium, which indicate the presence of both physical and electronic contact between the PEDOT and CdTe. The pH-dependent absorption and emission properties of the CdTe/PEDOT-PSS hybrid microspheres were relative to the doping-dedoping behavior of conducting PEDOT-PSS and insensitive to the morphology or distribution of the hybrid microspheres. This study gives a route for facile fabrication of semiconductor nanoparticle/conducting polymer microspheres applicable in optoelectronic and photovoltaic devices.     

Synthesis and Luminescence Properties of Core/Shell ZnS:Mn/ZnO Nanoparticles

In this paper the influence of ZnO shell thickness on the luminescence properties of Mn-doped ZnS nanoparticles is studied. Transmission electron microscopy (TEM) images showed that the average diameter of ZnS:Mn nanoparticles is around 14 nm. The formation of ZnO shells on the surface of ZnS:Mn nanoparticles was confirmed by X-ray diffraction (XRD) patterns, high-resolution TEM (HRTEM) images, and X-ray photoelectron spectroscopy (XPS) measurements. A strong increase followed by a gradual decline was observed in the room temperature photoluminescence (PL) spectra with the thickening of the ZnO shell. The photoluminescence excitation (PLE) spectra exhibited a blue shift in ZnO-coated ZnS:Mn nanoparticles compared with the uncoated ones. It is shown that the PL enhancement and the blue shift of optimum excitation wavelength are led by the ZnO-induced surface passivation and compressive stress on the ZnS:Mn cores.     

Functionalized silicon quantum dots by N-vinylcarbazole: synthesis and spectroscopic properties

Silicon quantum dots (Si QDs) attract increasing interest nowadays due to their excellent optical and electronic properties. However, only a few optoelectronic organic molecules were reported as ligands of colloidal Si QDs. In this report, N-vinylcarbazole - a material widely used in the optoelectronics industry - was used for the modification of Si QDs as ligands. This hybrid nanomaterial exhibits different spectroscopic properties from either free ligands or Si QDs alone. Possible mechanisms were discussed. This type of new functional Si QDs may find application potentials in bioimaging, photovoltaic, or optoelectronic devices.     

Facile one-step synthesis of cerium oxide-carbon quantum dots/RGO nanohybrid catalyst and its enhanced photocatalytic activity

Improvement in the catalytic activity of nanostructured cerium oxides (CeOx) was attained by incorporating in-situ generated heteroatom-(N and S)-doped carbon quantum dots/reduced graphene oxide (HDCQD@RGO) nanohybrid catalyst for the degradation of organic pollutants. The CeOx-HDCQD@RGO nanohybrid catalyst was synthesized by a facile, one-pot hydrothermal eco-friendly process. The HDCQD plays a vital role to improve the interaction between CeOx and RGO and function as a sensitizer for the electron-transfer process with CeOx. The size of CeOx and HDCQD are 10 nm and 5 nm, respectively. The nanostructured CeOx possesses multiple oxygen vacancies; this helps to generate the active oxygen and hydroxyl radicals. The active hydroxyl radical generation by the photocatalytic process with the help of nanostructured CeOx results in the improved photodegradation of organic pollutants. In addition, the prepared CeOx-HDCQD@RGO nanohybrid catalyst exhibits an improved water oxidation reaction. The CeOx-HDCQD@RGO nanohybrid performs as an excellent bifunctional catalyst for energy and environmental applications. The method presented here is a facile and environmentally friendly, scalable synthesis of the highly efficient catalyst.     

Improving the efficiency of ITO/nc-TiO2/CdS/P3HT:PCBM/PEDOT:PSS/Ag inverted solar cells by sensitizing TiO2 nanocrystalline film with chemical bath-deposited CdS quantum dots

An improvement in the power conversion efficiency (PCE) of the inverted organic solar cell (ITO/nc-TiO₂/P3HT:PCBM/PEDOT:PSS/Ag) is realized by depositing CdS quantum dots (QDs) on a nanocrystalline TiO₂ (nc-TiO₂) film as a light absorption material and an electron-selective material. The CdS QDs were deposited via a chemical bath deposition (CBD) method. Our results show that the best PCE of 3.37% for the ITO/nc-TiO₂/CdS/P3HT:PCBM/PEDOT:PSS/Ag cell is about 1.13 times that (2.98%) of the cell without CdS QDs (i.e., ITO/nc-TiO₂/P3HT:PCBM/PEDOT:PSS/Ag). The improved PCE can be mainly attributed to the increased light absorption and the reduced recombination of charge carriers from the TiO₂ to the P3HT:PCBM film due to the introduced CdS QDs.     

Theoretical and experimental studies of (In,Ga)As/GaP quantum dots

(In,Ga)As/GaP(001) quantum dots (QDs) are grown by molecular beam epitaxy and studied both theoretically and experimentally. The electronic band structure is simulated using a combination of k·p and tight-binding models. These calculations predict an indirect to direct crossover with the In content and the size of the QDs. The optical properties are then studied in a low-In-content range through photoluminescence and time-resolved photoluminescence experiments. It suggests the proximity of two optical transitions of indirect and direct types.     

核壳结构半导体纳米晶体CdS/ZnS的制备及其光学性质

通过反胶束法成功制备了CdS及CdS/ZnS核壳结构量子点,采用紫外-可见吸收光谱(UV-Vis)、透射电子显微镜(TEM)、能量色散型X射线能谱(EDX)以及荧光光谱(PL)等对其结构和性能进行了研究。与CdS量子点相比,CdS/ZnS量子点的粒径明显变大,本文估算了粒子的粒径,TEM图像很好的印证了估算的结果;结合EDX的测试结果可以推断CdS/ZnS量子点的结构是ZnS包裹在CdS表面的核壳结构;CdS/ZnS量子点的荧光强度与CdS量子点相比有了明显的提高,原因主要是壳层的ZnS消除了CdS量子点表面上存在的无辐射复合中心。     

Effect of phase transition on SiO2-coated CsPbBr3/Cs4PbBr6 quantum dots: Air-stability and quantum efficiency improvement

To develop white light-emitting diodes (WLEDs) with wide color gamut for displays, compared with nitride-based phosphors and traditional core-shell quantum dots (QDs) such as CdSe, InP, CuInS₂, all-inorganic perovskite QDs CsPbX₃ (X = Cl, Br, I) were more promising luminescent materials due to tunable wavelength, narrow emission spectrum and high quantum efficiency. However, when QDs were made into solid form (powders or films), poor air-stability and drastic decrease of quantum efficiency would be observed in CsPbBr₃. These drawbacks would restrict their practical applications. To resolve these issues, in this paper, we proposed a new concept that zero-dimensional perovskite QDs powders Cs₄PbBr₆ with outstanding quantum efficiency and long lifetime up to three months could be successfully prepared via silica-coated method and crystal phase transition in low-temperature synthesis. This phenomenon of phase transition would be discussed in detail and the quantum efficiency could be improved from 31.41% to 45.87%. Moreover, green LEDs with high color purity of 92% and luminous efficiency of 88.59 lm/W could also be achieved by using this material. Therefore, our proposed perovskite QDs powders Cs₄PbBr₆ had extreme potential for displays applications.