Formation Of QDs Research Articles

In-situ template synthesis of a polymer/nanoparticles nanohybrid using hyperbranched poly(aryl ether ketone)

Nanostructured ZnS/hyperbranched poly(aryl ether ketone) (ZnS/HPAEK) hybrid material was synthesized by a facile hydrothermal reaction of HPAEK–Zn(Ac)2⋅2H2O in DMF. ZnS nanoparticles were prepared by heterogeneous stirring of carboxylic-functionalized hyperbranched poly(aryl ether ketone) (PCA-HPAEK)–Zn(Ac)2⋅2H2O in DMF with DMF solution of thiourea. The obtained nanocomposites were characterized using various techniques such as Fourier transform infrared spectroscopy (FTIR), high-resolution transmission electron microscopy (HRTEM), UV–Vis spectroscopy and photoluminescence. FTIR and HRTEM studies confirmed the formation of ZnS QDs with small particle size. And the results of photoluminescence measurement showed that the nanocomposites exhibited distinct luminescence properties. Differential scanning calorimetry and thermogravimetric analysis studies indicated that the nanocomposites exhibited excellent heat resistance. In general, a new ZnS/PCA-HPAEK nanohybrid was synthesized without ligand exchange, and had good optical properties and excellent heat resistance.

In situ formation of luminescent CdSe QDs in a metallohydrogel: a strategy towards synthesis, isolation, storage and re-dispersion of the QDs.

A one step, in situ, room temperature synthesis of yellow luminescent CdSe QDs was achieved in a metallohydrogel derived from a facially amphiphilic bile salt, resulting in a QD-gel hybrid. An ordered self-assembly and homogeneous distribution of the CdSe QDs in the hydrogel network was observed from optical and electron micrographs. The different excited state behavior of the CdSe QDs in the hybrid was revealed for the first time using time resolved spectroscopy. We also describe the successful isolation of the photoluminescent CdSe QDs from the gel followed by their re-dispersion in an organic solvent using suitable capping ligands.

Hydrogenated MoS2 QD-TiO2 heterojunction mediated efficient solar hydrogen production.

Herein, we report the development of a hydrogenated MoS2 QD-TiO2 (HMT) heterojunction as an efficient photocatalytic system via a one-pot hydrothermal reaction followed by hydrogenation. This synthetic strategy facilitates the formation of MoS2 QDs with an enhanced band gap and a proper heterojunction between them and TiO2, which accelerates charge transfer process. Hydrogenation leads to oxygen vacancies in TiO2, enhancing the visible light absorption capacity through narrowing its band gap, and sulfur vacancies in MoS2, which enhance the active sites for hydrogen adsorption. Due to the band gap reduction of hydrogenated TiO2 and the band gap enhancement of the MoS2 QDs, the energy states are rearranged to create a reverse movement of electrons and holes facilitated the charge transfer process which enhance life-time of photo-generated charges. The photocatalyst showed stable, efficient and exceptionally high noble metal free sunlight-induced hydrogen production with a maximum rate of 3.1 mmol g-1 h-1. The developed synthetic strategy also provides flexibility towards the shape of the MoS2, e.g. QDs/single or few layers, on TiO2 and offers the opportunity to design novel visible light active photocatalysts for different applications.

Ligand effect on the synthesis of emission-tunable near-infrared Ag2S quantum dots

The ligand effect on the formation of Ag2S QDs was investigated, and it was found that the mixed oleic acid and 1-octanethiol ligand made the synthesis of small Ag2S QDs more controllable. By modulating the ligand composition and growth time, the emission of Ag2S QDs could be tuned from 665 to 845 nm.

Size controlled preparation of CdTe nanoparticles by apoferritin

Cadmium telluride quantum dots (CdTe QDs) were synthesized in the cavity of horse spleen apoferritin and CdTe-apoferritin complex was fluorescent. Apoferritin is a popular bio-template to prepare various nanoparticles with narrow size distribution due to the confinement of the hollow protein shell. In this work, we controlled the diameters of CdTe NPs by changing the reaction conditions. Altering the molar ratio of Cd to Te from 1:0.05 to 1:0.2 and pH can change the diameters of NPs cores. The synthesized QDs were characterized by photoluminescence (PL) spectroscopy, UV–vis spectroscopy and transmission electron microscopy (TEM). The results showed that the PL intensity decreased when the Cd/Te molar ratio was decreased from 1:0.05 to 1:0.3 and pH was increased from 9.01 to 9.96. In addition, it was proven that the existence of apoferritin is necessary for the present synthetic method and the formation of CdTe QDs in the inner cavity of apoferritin.

Ex situ and in situ sensitized quantum dot solar cells

Although quantum dot‐sensitized solar cells (QDSCs) prepared with in situ sensitized wide‐band gap semiconductor nanorods are commonly reported, very few examples of the ex situ sensitization of these nanorods are known. They usually result in poor solar cell performance parameters. In this work, we have tested the photovoltaic application of both ex situ and in situ sensitized ZnO nanorods by linker‐assisted attachment and by the successive ionic layer adsorption and reaction (SILAR) technique, respectively. For the ex situ sensitization, instead of classical spherical QDs, we used quasi two‐dimensional CdSe nanoplatelets (NPLs) of controlled thicknesses, the photovoltaic application of which has never been reported before. Regarding the in situ sensitization, the photoanodes were characterized by XRD, UV–VIS absorption, and Raman spectroscopy during deposition, which helped to better understand the formation of CdSe QDs nanostructured ZnO surface. Devices incorporating in situ sensitized ZnO nanorods demonstrated superior performance compared to the best measured efficiencies of QDSCs prepared with ex situ sensitization, and their J–V characteristics were more reproducible.

Synthesis, Characterization, In Vivo Imaging, Hemolysis, and Toxicity of Hydrophilic Ag2S Near-Infrared Quantum Dots

Ag2S quantum dots (Qds) are a new type of near-infrared Qds and hold great potential for in vivo imaging. There is great demand for versatile, practical, and environmentally-friendly processes for formation of hydrophilic Ag2S Qds. The present study prepared hydrophilic Ag2S near-infrared Qds using a simple, easy, one-step method. The Qds were characterized by TEM, XRD, FT-IR, UV–Visible and PL spectroscopy and the effect of the reaction variables of time, temperature, precipitation agent feeding rate, and S:Ag molar ratio on the size and PL spectrum of the Ag2S Qds was investigated. The Qds exhibited near-infrared photoluminescence and good photostability, which was evaluated under different conditions. Cytotoxicity tests were carried out on human A549 and Hep G2 cell lines at concentrations of 6.25–200 µg/ml. MTT assay revealed that the Qds showed no significant toxicity. The effect of plasma and corona proteins on the hemolytic activity of Ag2S Qds in four concentrations was evaluated for rat and human red blood cells. In vivo imaging showed that the Ag2S Qds penetrated the body of mice with excellent brightness. The hydrophilic Ag2S Qds can be safely applied as near-infrared optical imaging probes for nanodiagnostics or in vivo imaging.

New Flexible Channels for Room Temperature Tunneling Field Effect Transistors.

Tunneling field effect transistors (TFETs) have been proposed to overcome the fundamental issues of Si based transistors, such as short channel effect, finite leakage current, and high contact resistance. Unfortunately, most if not all TFETs are operational only at cryogenic temperatures. Here we report that iron (Fe) quantum dots functionalized boron nitride nanotubes (QDs-BNNTs) can be used as the flexible tunneling channels of TFETs at room temperatures. The electrical insulating BNNTs are used as the one-dimensional (1D) substrates to confine the uniform formation of Fe QDs on their surface as the flexible tunneling channel. Consistent semiconductor-like transport behaviors under various bending conditions are detected by scanning tunneling spectroscopy in a transmission electron microscopy system (in-situ STM-TEM). As suggested by computer simulation, the uniform distribution of Fe QDs enable an averaging effect on the possible electron tunneling pathways, which is responsible for the consistent transport properties that are not sensitive to bending.

Sunflower-type nanogels carrying a quantum dot nanoprobe for both superior gene delivery efficacy and tracing of human mesenchymal stem cells

Sunflower-type nanogels carrying the QD 655 nanoprobe can be used for both gene transfection and bioimaging of hMSCs. The entry of sunflower-type nanogels into hMSCs can be possibly controlled by changing the formation of QDs. The physico-chemical properties of sunflower-type nanogels internalized by hMSCs were confirmed by AFM, SEM, TEM, gel retardation, and ζ-potential analyses. The bioimaging capacity was confirmed by confocal laser microscopy, Kodak imaging, and Xenogen imaging. Specifically, we investigated the cytotoxicity of sunflower-type nanogels via SNP analysis. Internalization of sunflower-type nanogels does not cause malfunction of hMSCs.

Enhanced photovoltaic property by forming p-i-n structures containing Si quantum dots/SiC multilayers.

Si quantum dots (Si QDs)/SiC multilayers were fabricated by annealing hydrogenated amorphous Si/SiC multilayers prepared in a plasma-enhanced chemical vapor deposition system. The thickness of amorphous Si layer was designed to be 4 nm, and the thickness of amorphous SiC layer was kept at 2 nm. Transmission electron microscopy observation revealed the formation of Si QDs after 900°C annealing. The optical properties of the Si QDs/SiC multilayers were studied, and the optical band gap deduced from the optical absorption coefficient result is 1.48 eV. Moreover, the p-i-n structure with n-a-Si/i-(Si QDs/SiC multilayers)/p-Si was fabricated, and the carrier transportation mechanism was investigated. The p-i-n structure was used in a solar cell device. The cell had the open circuit voltage of 532 mV and the power conversion efficiency (PCE) of 6.28%.PACS81.07.Ta; 78.67.Pt; 88.40.jj

Characterization and Optimization of the Fluorescence of Nanoscale Iron Oxide/Quantum Dot Complexes

In this paper, nanoscale iron oxide/quantum dot (QD) complexes were formed in an efficient and versatile reaction that relied on the nucleation of chalcogenides on preformed iron oxide nanocrystals. Iron oxide nanocrystals acted as seeds for the growth of CdSe quantum rods (QRs), CdSe QDs, and CdSe@ZnS QDs. A zinc sulfide shell was added to protect the CdSe core in the complex chemically and provide a reasonable fluorescence quantum yield (∼5%). High-resolution transmission electron microscopy revealed that QDs shared an interface with iron oxide, yielding structures that resemble pincushions with QDs or QRs studding the surface of the iron oxide. These complexes only formed under specific conditions of temperature, injection rate, and surfactant composition that minimized the formation of unbound QDs. As a superparamagnetic material, iron oxide provided a high purity (∼89%) of complexed materials without unbound QDs. The quantitative photoluminescence quantum yields of the purified complexes correlated with the number of QDs per iron oxide. These nanoscale complexes retained the size-dependent optical and magnetic properties of each component.

Structural evolution and photoluminescence of annealed Si-rich nitride with Si quantum dots prepared by plasma enhanced chemical vapor deposition

Silicon-rich nitride films were deposited by plasma enhanced chemical vapor deposition. Silicon quantum dots (Si QDs) were formed by post-thermal annealing processing verified using the High-Resolution Transmission Electron Microscope. The 1100 °C thermal annealing leads to the nucleation of silicon atoms, the growth of Si QDs, and the rearrangement of Si 2p and N 1s elements. The structural evolution of silicon-rich nitride thin film with post annealing promotes the formation of Si QDs and Si3N4 matrix. We also investigated the effect of the NH3-to-SiH4 ratio R on the photoluminescence (PL) of SiNx with Si QDs. We found that the broad blue luminescence originates from both quantum confined effect and radiative defects. The intensity of the PL was changed by adjusting the NH3 flow rate. The increase of R could limit the transformation of Si QDs from amorphous to crystalline status, meanwhile lead to the alteration of distribution of defect states. These can help to understand the annealing-dependent characteristics, the PL mechanisms of silicon-rich nitride and to optimize the fabrication process of Si QDs embedded in nitride.

Infrared photoluminescence from lead sulfide quantum dots in glasses enriched in sulfur

Formation of PbS QDs in the glasses containing a small amount of lead oxide was examined. Oversaturation of sulfur only was sufficient to promote the formation of PbS QDs in the glasses. Upon thermal treatment, absorption of PbS QDs was tuned from 824nm to 2213nm. Infrared photoluminescence, especially, mid-infrared photoluminescence from PbS QDs was observed in the range of 1008nm to 2182nm. Ostwald ripening of PbS QDs occurred when the heat-treatment temperature was 530°C or 540°C, and led to the decrease in the absorption coefficients and splitting of the photoluminescence bands.

Electroluminescence Devices Based on Si Quantum Dots/SiC Multilayers Embedded in PN Junction

We deposited a p-i-n structure device with alternative amorphous Si (a-Si) and amorphous SiC (a-SiC) multilayers as an intrinsic layer in a plasma-enhanced chemical vapor deposition (PECVD) system. A KrF pulsed excimer laser-induced crystallization of a-Si/a-SiC stacked structures was used to prepare Si quantum dots (Si QDs)/SiC multilayers. The formation of Si QDs with an average size of 4 nm was confirmed by Raman spectra, whereas the layered structures were revealed by cross-sectional transmission electron microscopy. Electroluminescence (EL) devices containing Si QDs/SiC multilayers embedded in a p-n junction were fabricated, and the device performance was studied and compared with the reference device without the p-i-n structure. It was found that the turn-on voltage was reduced and that luminescence efficiency was significantly enhanced by using the p-i-n device structure. The recombination mechanism of carriers in a Si-QD-based EL device was also discussed, and the improved device performance can be attributed to the enhanced radiative recombination probability in a p-i-n EL device.

Au@poly(acrylic acid) plasmons and C60 improve the light harvesting capability of a TiO2/CdS/CdSeS photoanode

A quantum dot solar cell (QDSC) was developed using a photoanode with CdS and CdSeS QDs as co-sensitizers, fullerene (C60) clusters as electron conduits and Au nanoparticles capped by poly(acrylic acid) or Au@PAA as plasmons, which were all tethered to TiO2 as the semiconducting scaffold. The formation of CdSeS QDs and hexagon-shaped Au@PAA, each with a face-centered cubic lattice, was confirmed by high-resolution transmission electron microscopy. Evidence for rapid electron injection from CdS or CdSeS to C60 and Au@PAA, as well as for inter-dot electron transfer from CdS to CdSeS, were obtained from fluorescence quenching and emission decay lifetime analyses. The fastest electron deactivation pathway was achieved in the TiO2/CdS/CdSeS/C60/Au@PAA electrode, which demonstrated the ability of C60 and Au@PAA to serve as efficient electron sinks due to their high electronic conductivities and their favorably poised Fermi levels. Au@PAA particles induced absorbance increments of CdS and CdSeS QDs, thus indicating that plasmonic improvements could be exploited in the photoanode using the same. High electron transfer rates and plasmonic effects translated into a high overall power conversion efficiency of 3.61% for a QDSC constructed with this composite as the photoanode, aqueous S2− as the electrolyte and carbon-nanotube-based counter electrode. The efficiencies of cells without Au@PAA and C60 were lower by ∼19% and 23%, respectively, confirming the synergy between C60 and Au@PAA in producing larger photocurrents. Aligned conduction bands in the TiO2/CdS/CdSeS/C60/Au@PAA electrode that permitted electron transfer by cascade, broader spectral utilization by using CdS and CdSeS and near field plasmonic enhancement by Au@PAA due to its proximity with the QDs, cumulatively manifested in an incident photon to current conversion efficiency of 48%, which is 25% greater than that of the cell without Au@PAA. Our study shows that significant improvements in QDSC performances can be realized by simply developing more efficient designs for photoanode configurations.

Photophysical and structural characterisation of in situ formed quantum dots.

Conjugated polymer-semiconductor quantum dot (QD) composites are attracting increasing attention due to the complementary properties of the two classes of materials. We report a convenient method for in situ formation of QDs, and explore the conditions required for light emission of nanocomposite blends. In particular we explore the properties of nanocomposites of the blue emitting polymer poly[9,9-bis(3,5-di-tert-butylphenyl)-9H-fluorene] together with cadmium sulphide (CdS) and cadmium selenide (CdSe) precursors. We show the formation of emissive quantum dots of CdSe from thermally decomposed precursor. The dots are formed inside the polymer matrix and have a photoluminescence quantum yield of 7.5%. Our results show the importance of appropriate energy level alignment, and are relevant to the application of organic-inorganic systems in optoelectronic devices.

Surface structure and optical property of amorphous carbon nanotubes hybridized with cadmium selenide quantum dots

Amorphous carbon nanotubes (α-CNTs) were synthesized by a chemical reaction between ferrocene and ammonium chloride at low temperature. The as-synthesized α-CNTs were then hybridized with cadmium selenide quantum dots (CdSe QDs) through a simple chemical process. Raman spectra reveal the amorphous nature of the α-CNTs surface. X-ray diffraction pattern confirmed the amorphous phase of carbon and the formation of CdSe QDs crystalline phase. Field emission scanning electron microscopy and transmission electron microscopy (TEM and HRTEM) indicate that the successfully formed hybridized α-CNTs–CdSe QDs possess an average outer diameter in the range of 110–130 nm. The CdSe QDs fall in the size range of 15–40 nm. UV–visible spectroscopy showed quantum confinement effect due to the attachment of CdSe QDs on the surface of α-CNTs.

Improved photovoltaic properties of Si quantum dots/SiC multilayers-based heterojunction solar cells by reducing tunneling barrier thickness

Si quantum dots (Si QDs)/SiC multilayers were fabricated by annealing hydrogenated amorphous Si/SiC stacked structures prepared in plasma enhanced chemical vapor deposition (PECVD) system. The microstructures were examined by transmission electron microscopy (TEM) and Raman spectroscopy, and results demonstrate the formation of Si QDs. Moreover, p-i-n devices containing Si QDs/SiC multilayers were fabricated, and their photovoltaic property was investigated. It was found that these devices show the good spectral response in a wide wavelength range (400–1200 nm). And it was also observed that by reducing the thickness of SiC layer from 4 to 2 nm, the external quantum efficiency was obviously enhanced and the short circuit current density (Jsc) was increased from 17.5 to 28.3 mA/cm2, indicating the collection efficiency of photo-generated carriers was improved due to the reduced SiC barriers.

Synthesis and Characterization of CdTe Quantum Dots Capped by N-Acetyl-L-Cysteine

The highly luminescent cadmium telluride quantum dots (CdTe QDs) have been synthesized by using N-acetyl-L-cysteine (NAC) as stabilizer in aqueous solution. Experiments indicate that the QDs nanocrystals could grow larger as the extension of reaction time, causing the red shift of emission spectra. The typical product was analyzed by fluorescence spectroscopy, high-resolution transmission electron microscope (HRTEM), x-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). The morphological and structural characterization confirmed the formation of monodisperse CdTe QDs with several nanometers in size.

Bio-templated CdSe quantum dots green synthesis in the functional protein, lysozyme, and biological activity investigation

Bifunctional fluorescence (CdSe Quantum Dots) – protein (Lysozyme) nanocomposites were synthesized at room temperature by a protein-directed, solution-phase, green-synthetic method. Fluorescence (FL) and absorption spectra showed that CdSe QDs were prepared successfully with Lyz. The average particle size and crystalline structure of QDs were investigated by high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD), respectively. With attenuated total reflection-fourier transform infrared (ATR-FTIR) spectra and thermogravimetric (TG) analysis, it was confirmed that there is interaction between QDs and amide I, amide II groups in Lyz. FL polarization was measured and FL imaging was done to monitor whether QDs could be responsible for possible changes in the conformation and activity of Lyz. Interestingly, the results showed Lyz still retain the biological activity after formation of QDs, but the secondary structure of the Lyz was changed. And the advantage of this synthesis method is producing excellent fluorescent QDs with specifically biological function.