Optical Properties Of Quantum Dots Research Articles

Copolymer nanosphere encapsulated CdS quantum dots prepared by RAFT copolymerization: synthesis, characterization and mechanism of formation

Cadmium sulfide (CdS) quantum dots (QDs) encapsulated in block copolymer spheres were synthesized by an aqueous emulsion polymerization process. First, stable dispersions of CdS QDs in water were prepared using a polymer dispersant, either poly(acrylic acid) or a random copolymer having an average of ten acrylic acid and five butyl acrylate units. These polymer dispersants were prepared by reversible addition-fragmentation chain transfer polymerization. Then, the CdS QDs dispersed in water were encapsulated in a polystyrene shell using an emulsion polymerization process. Spectroscopic and microscopic techniques were used to characterize the resulting nanocomposites. Optical properties of QDs in polymer microspheres were investigated by UV-vis and fluorescence spectroscopic studies. Particle sizes of all CdS QD samples were calculated from absorption edges using Henglein's empirical curve. Transmission electron microscopy was used to determine the size and morphology of CdS QD samples. These observations were used to elucidate the mechanism of formation of the resulting well-defined polymer-encapsulated CdS nanoparticles.

Quantum Dots for Electro-Optic Devices

We discuss the use of quantum dots (QDs) in electro-optic devices. The focus of the article is on key problems in this area: the electronic and optical properties of QDs, dispersing QDs in polymers and the requirements for both, and QDs and polymer composites in devices such as solar cells, light-emitting devices, and detectors. The accompanying article by Geyer et al. in this issue of ACS Nano demonstrates an excellent example of a detector device, which also points to future directions for research in this field.

Effects of an InGaAs Cap Layer on the Optical Properties of InAs Quantum Dot Molecules

Self-assembled InAs quantum dot molecules are grown on GaAs substrates without following any special protocols by using metal-organic chemical vapor deposition. The effects of indium composition and the thickness of the InGaAs cap layer on the optical properties of InAs quantum dot molecules are investigated by photoluminescence. With increasing indium composition and thickness of the InGaAs cap layer, the ground-state wavelength of the emission spectrum redshifts and the peak intensity decreases. In addition, the structural and optical properties of quantum dots and quantum dot molecules are comparatively studied, and the results show that when quantum dots turn into quantum dot molecules, the emission wavelength red shifts.

The medicinal use of realgar (As4S4) and its recent development as an anticancer agent

Arsenicals have been known as poisons and paradoxically as therapeutic agents. In the early 1970s, Chinese physicians from Harbin revived the medicinal use of arsenicals as anticancer agents. Notable success was observed in the treatment of acute promyelocytic leukemia (APL) with arsenic trioxide (ATO). The FDA approved ATO injection in the year 2000 for the treatment of APL. In contrast, the clinical use of the other arsenical, realgar (As₄S₄), is currently much less established, though it has also long been used in medical history. According to ancient medical records and recent findings in clinical trials, realgar was found as effective as ATO, but with relatively good oral safety profiles even on chronic administration. These give realgar an advantage over ATO in maintenance treatment. Though there is increasing understanding on the mechanisms of action and metabolic profiles of ATO, similar aspects of realgar are unclear to date. We outline the use of realgar in traditional medicines, especially in traditional Chinese medicines (TCM) from ancient times to present. The clinical and experimental observations on realgar as a therapeutic agent are described with an emphasis on those findings that may imply the rationale and future directions of realgar as a potential anticancer drug candidate. There is an increasing understanding in the mechanisms of action of realgar as an antileukemic agent. However, there is still sparse information on its metabolism and toxicity profiles. Realgar is poorly soluble in water. Recently, several types of realgar nanoparticles (NPs) have been developed. Some of these realgar NPs also possess the unique optical properties of quantum dots. The activities and bioavailability of realgar NPs are much influenced by their sizes, making realgar an interesting biomedical and pharmaceutical research candidate.

THE EFFECT OF In0.1Ga0.9As UNDERLYING LAYER ON THE STRUCTURAL PROPERTIES OF SELF-ASSEMBLED In0.5Ga0.5As QUANTUM DOTS

The effect of a thin In0.1Ga0.9As underlying layer on the structural properties of single layer In0.5Ga0.5As quantum dots (QDs) was investigated using atomic force microscopy (AFM), transmission electron microscopy (TEM) and high-resolution X-ray diffraction (HR-XRD) characterization. The size of dots formed on the surface is uniform but the density increases with the addition of In0.1Ga0.9As underlying between In0.5Ga0.5As QDs and GaAs buffer layer. This is consistent with the TEM characterization. The existence of thin underlying layer has caused the dots to have different crystal orientation as shown in TEM characterization. From the HR-XRD characterization, broad peak of In0.1Ga0.9As underlying layer and QDs has been observed. The wider width of the layer peak than the expected one has been attributed to the strain-relaxation-induced defects. The growth of a thin In0.1Ga0.9As underlying layer in the In0.5Ga0.5As/GaAs structures strongly affects the structural properties, which was also believed to influence the optical properties of QDs.

Ionic liquid assisted electrospinning of quantum dots/elastomer composite nanofibers

Quantum dots (QDs)/elastomer (VM) composite nanofibers have been fabricated via electrospinning method with the assistance of small amount (1 wt%) of ionic liquid. Without ionic liquid, polymer solution underwent an electrospraying process within the electric field and only individual droplets rather than continuous fibers were observed. Both fixed electrode and rotating disk electrode were used to collect the products. The latter one turned out to be much more advanced in collecting separated, aligned and narrow-size distributed composite nanofibers. With fixed electrode, even though nanofibers were obtained initially, the as-spun fibers were easily to merge together due to the flexible non-crystalline nature of the VM chains and finally formed a condensed thin film. Strong fluorescent emission was observed in the composite nanofibers with a QD loading of 3 and 5 wt%, respectively. The optical property of QDs was not degraded after dispersing in the polymer solution as evidenced by the UV–Vis absorption at 562 nm and 592 nm, and strong photoluminescent emission at 612 nm. In addition, differential scanning calorimetry (DSC) analysis revealed a strong interaction between ionic liquid and the polymer chains, which well explains the function of the ionic liquid on producing fiber structure of VM. An enhanced thermal stability of the elastomer in the composite nanofibers is observed as compared to that of the pure elastomer fibers.

Effect of Different Types of Surface Ligands on the Structure and Optical Property of Water-Soluble PbS Quantum Dots Encapsulated by Amphiphilic Polymers

Successful transfer of near-infrared emitting quantum dots (QDs) into water is critical for many biological applications. In this paper, PbS QDs capped by several types of most commonly used capping ligands are transferred from an organic solvent into water via poly(maleic anhydride-alt-1-octadecene)-co-poly(ethylene glycol). It is found that the variation of the structure and optical property of QDs upon water transfer highly depends on the type of capping ligands because of their different interactions with their surroundings (such as solvent molecules, polymers, and QD surface). This is in clear contrast to the common concept that amphiphilic polymers will not disturb the ligand structure on the QD surface during the transfer process, and thus the type of ligands will not bear any direct relevance with the variation of QDs and their properties. This work demonstrates that when oleic acid (OA) or OA/trioctylphosphine (TOP) ligands are used, the amphiphilic polymer approach is able to maintain the initia...

Quantum Dot Sensitized Solar Cells with Improved Efficiency Prepared Using Electrophoretic Deposition

Quantum dot sensitized solar cells (QDSSC) may benefit from the ability to tune the quantum dot optical properties and band gap through the manipulation of their size and composition. Moreover, the inorganic nanocrystals may provide increased stability compared to organic sensitizers. We report the facile fabrication of QDSSC by electrophoretic deposition of CdSe QDs onto conducting electrodes coated with mesoporous TiO(2). Unlike prior chemical linker-based methods, no pretreatment of the TiO(2) was needed, and deposition times as short as 2 h were sufficient for effective coating. Cross-sectional chemical analysis shows that the Cd content is nearly constant across the entire TiO(2) layer. The dependence of the deposition on size was studied and successfully applied to CdSe dots with diameters between 2.5 and 5.5 nm as well as larger CdSe quantum rods. The photovoltaic characteristics of the devices are greatly improved compared with those achieved for cells prepared with a linker approach, reaching efficiencies as high as 1.7%, under 1 sun illumination conditions, after treating the coated electrodes with ZnS. Notably, the absorbed photon to electron conversion efficiencies did not show a clear size-dependence indicating efficient electron injection even for the larger QD sizes. The electrophoretic deposition method can be easily expanded and applied for preparations of QDSSCs using diverse colloidal quantum dot and quantum rod materials for sensitization.

Critical Parameters for the Scale-Up Synthesis of Quantum Dots

The optical properties of quantum dots strongly depend on the synthesis conditions including the reagent grade of the chemicals. We developed a synthesis set-up allowing the reproducible preparation of quantum dots with a high control over chemical and physical parameters. Here, we report on the effects of the injection speed, the stirring velocity and the reagent grade of the chemicals on the photoluminescence behaviour of CdSe quantum dots. Additionally we compared the commonly used hot-injection method with a novel one-pot approach leading to a slightly larger particle size distribution of the resulting high quality quantum dots in the latter case.

Effect of Mechanical Strain on the Optical Properties of Quantum Dots: Controlling Exciton Shape, Orientation, and Phase with a Mechanical Strain

We show how a nanomechanical strain can be used to dynamically reengineer the optics of quantum dots, giving a tool to manipulate mechanoexciton shape, orientation, fine structure splitting, and optical transitions, transfer carriers between dots, and interact qubits for quantum processing. Most importantly, a nanomechanical strain reengineers both the magnitude and phase of the exciton exchange coupling to tune exchange splittings, change the phase of spin mixing, and rotate the polarization of mechanoexcitons, providing phase and energy control of excitons.

Morphology and optical properties of single- and multi-layer InAs quantum dots

An understanding of the structural and optical properties of quantum dots (QDs) is critical for their use in optical communication devices. In this study, single- and multi-layer self-organized InAs QDs grown on (001) GaAs substrates by molecular beam epitaxy (MBE) were investigated. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images show that the lateral size of multi-layer InAs QDs are larger and flatter than single-layer InAs QDs, which are oval-shaped. The change in shape and size may be attributed to the presence of InGaAs spacer layers in multi-layer InAs QDs. Reciprocal spacer mapping and fast Fourier transformation images clearly show that InGaAs spacer layers present in the multi-layer InAs QDs structures help to release the strain originally existing in the QDs. In addition, the photoluminescence peak of the multi-layer InAs QDs is broader than QD in the single-layer one, which implies that the multi-layer InAs QDs size variation is more random than the single-layer one and this corresponds with the HAADF-STEM images. These results prove that spacer layers release strain influencing the morphology and optical properties of the QDs.

New opportunities in multiplexed optical bioanalyses using quantum dots and donor–acceptor interactions

This review highlights recent trends in the development of multiplexed bioanalyses using quantum dot bioconjugates and donor-acceptor interactions. In these methods, multiple optical signals are generated in response to biorecognition through modulation of the photoluminescence of populations of quantum dots with different emission colors. The donor-acceptor interactions that have been used include fluorescence resonance energy transfer, bioluminescence resonance energy transfer, charge transfer quenching, and quenching via proximal gold nanoparticles. Assays for the simultaneous detection of between two and eight target analytes have been developed, where spectral deconvolution is an important tool. Target analytes have included small molecules, nucleic acid sequences, and proteases. The unique optical properties of quantum dots offer several potential advantages in multiplexed detection, and a large degree of versatility, for example, one pot multiplexing at the ensemble level, where only wavelength discrimination is required to differentiate between detection channels. These methods are not being developed to compete with array-based technologies in terms of overall multiplexing capacity, but rather to enable new formats for multiplexed bioanalyses. In particular, quantum dot bioprobes based on donor-acceptor interactions are anticipated to provide future opportunities for multiplexed biosensing within living cells.

Performance study of InAs/GaAs quantum dot covered by graded In xGa 1 − xAs layer

InAs/GaAs quantum dots (QDs) with graded In x Ga 1 − x As strained-reducing layer (SRL) are grown by metal-organic chemical vapor deposition, the effects of Indium (In) composition and thickness in In x Ga 1 − x As on QD morphological characteristics and optical properties are investigated. Compared with In x Ga 1 − x As SRL with fixed In content, gradient In x Ga 1 − x As SRL can further improve the growth quality of InAs QDs, enhance luminescence intensity and extend emission spectrum toward longer wavelength.

An alternative to Western blot analysis using RNA aptamer-functionalized quantum dots

To make full use both of optical properties of quantum dots (QDs) and of specific interactions between aptamers and their ligands of interest, we employed QD-conjugated RNA aptamer interactions with histidine tag. QDs offer revolutionary fluorescence performance due to their long-term photostability, brilliant colors, fixability, and narrow, symmetrical emission spectra, and aptamers are known to specifically bind to their target molecules, including metal ions, small molecules, and macromolecules. In this study, we have synthesized RNA aptamer-functionalized QDs, and demonstrated their application to specific protein detection, as an alternative to the conventional Western blot analysis. We observed that our RNA aptamer-functionalized QD system dramatically reduced the time and effort required for conventional Western blot analysis, whereas the selectivity was comparable to that of the conventionally available anti-histidine tag antibody and the sensitivity was comparable to that of the Coomassie blue staining method. In principle, owing to the remarkable optical properties of QDs and a wide versatility of aptamers for selection, our system can harness the high brightness, stability and reusability to quantitatively detect aptamer-recognizable proteins. Furthermore, multiplex detection for several proteins on a single blot can be achieved by our new method, which thus may be able to facilitate and simplify the routinely used protein detection procedure, and make a variety of proteomics analysis possible.

Growth and Characterization of Self-Assembled InAs/InP Quantum Dot Structures

InAs quantum dots (QDs) are grown on InP or lattice matched GaInAsP buffers using horizontal flow metal-organic chemical vapor deposition (MOCVD) at a pressure of 180 mbar. A range of techniques, such as photoluminescence (PL), atomic force microscopy, and plan-view transmission electron microscopy is used to characterize the QD and other semiconductor layers. The effects of different growth parameters, such as V/III ratio and growth time, and the effects of buffer layers, interlayers, and cap layers are investigated and the optimized growth conditions are discussed. In the case of the QDs grown on InP buffers, the As/P exchange reaction is found to be prominent. A very thin (0.6 nm) GaAs interlayer grown between the buffer and the QD layers consumes segregated indium and minimizes the As/P exchange reaction. As a result, the QD PL emission energy increases, the PL intensity improves, and the PL linewidth decreases. The experimental results show that by changing the thickness of a GaAs interlayer (0.3-0.6 nm), the emission wavelength/energy of the QDs grown on a lattice matched GaInAsP buffer can be tuned over a wide range covering 1550 nm. However, further increase in the thickness of the GaAs interlayer results in the agglomeration of the QDs and the deterioration of the QD optical properties. Detailed microscopy studies show that capped QDs have higher density and are smaller in size on average compared to uncapped QDs, which undergo coalescence during cooling of the sample after growth. Overall, the QDs grown for shorter time with a smaller V/III ratio (approximately 8) show improved PL intensity and narrower PL linewidth.

Quantification of Water Solubilized CdSe/ZnS Quantum Dots in Daphnia magna

The relative transparency of Daphnia magna (daphnia) and the unique optical properties of quantum dots (QDs) were paired to study the accumulation potential and surface coating effects on uptake of amphiphilic polymer coated CdSe/ZnS QDs. Fluorescence confocal laser scanning microscopy was used to visualize and spectrally distinguish QDs from competing autofluorescent signals arising from the daphnia themselves and their food sources. QDs were found to accumulate within the digestive tracts of daphnia, as well as, in some cases, adhere to the carapace, antennae, and thoracic appendages. After 48 h of gut clearance with and without feeding, QD fluorescence signal was still apparent in the digestive tracts of daphnia, and inductively coupled plasma mass spectrometry (ICP-MS) measurements confirmed that 36-53% of the initial uptake was retained. As surface charge and pegylation can influence the uptake of nanoparticles, uptake of QDs coated with two different amphililic polymers and their polyethylene glycol (PEG) coated counterparts was also examined. Fluorescence microscopy and ICP-MS measurements revealed differences in uptake after 24 h of exposure which were attributed to particle surface coating and stability.

Monofunctional Quantum Dot Probes for Single-Molecule Imaging

Recently, it has been shown that the optical properties of quantum dot (QDs) nanoparticles enable novel experiments at the single molecule level in live cells, thereby opening new prospects for the understanding of cellular processes. One difficulty with these experiments is that the complex biological environment imposes stringent design requirements on QD probes, necessitating the development of smaller, low valency and more biocompatible QDs. In this work, we present our efforts towards minimizing the size and controlling the surface functionality of QDs. We show that an engineered peptide surface coating and a purification method based on gel electrophoresis are sufficient to produce compact monofunctional QDs covalently conjugated to streptavidin (SAV), biotin and antibodies. To prove the monofunctionality of the QD-SAV probes, we apply novel single-molecule assay following complexation of the QDs with a fluorescent dye biotin derivative. Counting the photobleaching steps of the fluorescent dye gives us direct access to the number of binding sites present on the QD surface. We then apply these QD probes to the targeting and tracking of individual biotinylated membrane proteins expressed in living HeLa cells. We analyze the diffusion properties of these membrane proteins and compare the measurements to those obtained using commercially available QD probes. Overall, these monofunctional QD probes should be useful for studying a wide-range of biophysical phenomena, down to the single molecule level in live cells.

Effect of different cap layers on the structure and optical properties of InAs/GaAs self-assembled quantum dot

Self-assembled InAs/GaAs quantum dot structures with different cap layers are grown by metal-organic chemical vapor deposition.The structure and optical properties of quantum dots are investigated using atomic force microscopy and photoluminescence.The cap layers sandwiched between quantum dots are composed of a low-temperature layer and a high-temperature layer.The comparative studies on low-temperature cap layer show that In graded InGaAs layer structure improves the uniformity of quantum dots,decreases coalescent islands and enhances photoluminescence intensity.Emission wavelength shifts from 1256.0 nm to 1314.4 nm when the thickness of graded InGaAs low-temperature cap layer increases form 6.8 nm to 12 nm.The research on high-temperature cap layer structure indicates that In graded InGaAs layer can increase photoluminescence intensity.

Delivering quantum dots to cells: bioconjugated quantum dots for targeted and nonspecific extracellular and intracellular imaging

Bioconjugated nanomaterials offer endless opportunities to advance both nanobiotechnology and biomedical technology. In this regard, semiconductor nanoparticles, also called quantum dots, are of particular interest for multimodal, multifunctional and multiplexed imaging of biomolecules, cells, tissues and animals. The unique optical properties, such as size-dependent tunable absorption and emission in the visible and NIR regions, narrow emission and broad absorption bands, high photoluminescence quantum yields, large one- and multi-photon absorption cross-sections, and exceptional photostability are the advantages of quantum dots. Multimodal imaging probes are developed by interfacing the unique optical properties of quantum dots with magnetic or radioactive materials. Besides, crystalline structure of quantum dots adds scope for high-contrast X-ray and TEM imaging. Yet another unique feature of a quantum dot is its spacious and flexible surface which is promising to integrate multiple ligands and antibodies and construct multi-functional probes for bioimaging. In this critical review, we will summarize recent advancements in the preparation of biocompatible quantum dots, bioconjugation of quantum dots, and applications of quantum dots and their bioconjugates for targeted and nonspecific imaging of extracellular and intracellular proteins, organelles and functions (181 references).

Theoretical study on strain compensation layer for growth of quantum dots

The optical properties of quantum dots have a close relationship with the size fluctuation，density，strain filed distribution of the dots and the spacer layer thickness. InAs/GaAs quantum dot with GaNXAs1-X strain compensation layers （SCL） is theoretically investigated for improving the crystal quality. The reduction effects of the spacer thickness are discussed quantitatively. The influence of the location and the N concentration of the GaNXAs1-X SCL on compensation of the strain formed on quantum dots （QDs） and the system is also discussed. The reduction effect of SCL on strain of system is analyzed and the vertical alignment probability between the adjacent layers is calculated. Our results can provide a theoretical basis for finding the optimal properties of SCL to realize the high quality multi-QD layer.