Self-organized ZnSe quantum dots: synthesis and characterization

Self-organized ZnSe quantum dots (Q-ZnSe) were grown on indium tin oxide substrate using wet chemical technique without or in presence of copper and manganese dopants. The structural, morphological and luminescence properties of the as grown Q-dot films have been investigated, using X-ray diffraction, transmission electron microscopy, atomic force microscopy and optical and luminescence spectroscopy. Composition of the samples were analyzed using atomic absorption spectroscopy. The quantum dots have been shown to deposit in a compact, uniform and organized array on the indium tin oxide substrate. The size dependent blue shift in the experimentally determined absorption edge has been compared with the theoretical predictions based on the effective mass and tight binding approximations. It is shown that the experimentally determined absorption edges depart significantly from the theoretically calculated values. The photoluminescence properties of the undoped as well as doped Q-ZnSe have also been discussed.     

Controlled synthesis and characterization of ZnSe quantum dots

Adopting improved metal-organic "Green method," Colloidal ZnSe quantum dots were synthesized by using cheap and low toxic zinc oxide (ZnO) in an organic solvent system of 1-hexadecylamine (HDA), lauric acid (LA) and tri-n-octylphosphine (TOP). The effects of HDA dosage, injection temperature, growth temperature and time on the microstructure and optical properties of ZnSe were studied by means of X-Ray diffraction(XRD), transmission electron microscopy (TEM), spectrofluorometers and ultraviolet spectrophotometer, respectively. The results showed that ZnSe quantum dots with the best range of the size evolution were obtained under the condition of injection at 280 degrees C and growth at 240 degrees C by choosing the optimal parameters of ZnO:HDA:LA= 1:2.1:5.2 and TOPSe = 1 mol/L. Its size became larger and the emission peak shifted obviously to red with increasing the growth time. Meanwhile, the obtained ZnSe was of the wurtzite structure, had good uniformity and fluorescent characteristics.     

Synthesis and characterization studies of ZnSe quantum dots

ZnSe QDs have been synthesized by wet chemical, template free process by zinc acetate and elemental selenium powder in presence of ethylene glycol, hydrazine hydrate and a defined amount of water at 90 °C. The product was in strong quantum confinement regime, having yield as high as 50 %. The transmission electron microscopy image indicated that the particles were well dispersed and spherical in shape. The X-ray diffraction analysis showed that the ZnSe nanoparticles were of the Cubic structure, with average particle diameter of about 3.50 nm. The FTIR characteristic indicates that the N₂H₄ molecule has intercalated into the complex and formed a molecular precursor.     

碳量子点的合成、性质及其应用

碳量子点(CQDs,C-dots or CDs)是一种新型的碳纳米材料,尺寸在10nm以下,具有良好的水溶性、化学惰性、低毒性、易于功能化和抗光漂白性、光稳定性等优异性能,是碳纳米家族中的一颗闪亮的明星。自从2006年[1]报道了碳量子点(CQDs)明亮多彩的发光现象后,世界各地的研究小组开始对CQDs进行了深入的研究。最近几年的研究报道了各种方法制备的CQDs在生物医学、光催化、光电子、传感等领域中都有重要的应用价值。这篇综述主要总结了关于CQDs的最近的发展,介绍了CQDs的合成方法、表面修饰、掺杂、发光机理、光电性质以及在生物医学、光催化、光电子、传感等领域的应用。     

Wurtzite ZnSe nanowires: growth, photoluminescence, and single-wire Raman properties

Wurtzite ZnSe nanowires were prepared on GaAs substrates in a metal-organic chemical vapour deposition system. Electron microscopy shows that they are smooth and uniform in size. Both transmission electron microscopy and x-ray diffraction reveal the wurtzite structure of the nanowires, which grows along the [Formula: see text] direction. Raman scattering studies on individual nanowires were performed in the back-scattering geometry at room temperature. Besides the commonly observed longitudinal and transverse optical phonon modes, a possible surface mode located at 233?cm(-1) is also observed in the Raman spectrum. A peak located at 2.841?eV was clearly observed in the photoluminescence spectra of the nanowires, which can be assigned to near band edge emissions of wurtzite ZnSe.     

One-step synthesis of water-soluble ZnSe quantum dots via microwave irradiation

A facile strategy has been developed for the synthesis of glutathione-capped ZnSe quantum dots (QDs) in aqueous media. The reaction was carried out in air atmosphere with a single step by using Na₂SeO₃, a stable and commercial Se source, to replace the commonly adopted NaHSe or H₂Se. Moreover, microwave irradiation improved the photoluminescence quantum yield (PLQY) as well as lowered the trap emission of as-prepared ZnSe QDs. The obtained QDs performed strong band-edge luminescence (PLQY reached 18%), narrow size distribution (full width at half maximum was 26-30 nm) and weak trap emission without post-treatments. The results of transmission electron microscopy and X-ray diffraction demonstrated the small particle size (2-3 nm), good monodispersity and ZnSe(S) alloyed structure of as-prepared QDs. The experimental variables including precursors and stabilizer amounts as well as pH value had significant influence on the PL properties of the ZnSe QDs.     

A case study: Te in ZnSe and Mn-doped ZnSe quantum dots

Photoluminescence (PL) behavior of ZnSe(1-y)Te(y) quantum dots is investigated by varying Te concentration as well as size. The striking effect of quantum confinement is the observation of isoelectronic center-related emission at room temperature in lieu of near-band-edge emission that dominates the optical scenario. ZnSe(0.99)Te(0.01) quantum dots were also doped by Mn(2+) ions. The Mn(2+) ion-related d-d transition is drastically suppressed by Te isoelectronic centers. Incorporation of Mn(2+) at substitutional sites in ZnSe(0.99)Te(0.01) quantum dots is also confirmed by the electron paramagnetic resonance measurements. Effect of Te isoelectronic impurity on the emission behavior is more pronounced than that of Mn(2+) ions. A subtle blueshift in the orange d-d transition is a sign of a decrease in crystal field strength. PL and photoluminescence excitation measurements on Zn(1-x)Se(0.99)Te(0.01)Mn(x) quantum dots indicate that the transition probability from the lowest unoccupied molecular orbital to Te levels is substantially larger than that to Mn(2+) d-d levels.     

Lattice expansion in ZnSe quantum dots

Solid ZnSe quantum dots (QDs) have been prepared via chemical route. The QDs have been characterized by X-Ray Diffraction (XRD), Transmission Electron Microscopy (TEM), Photoluminescence (PL) and Ultraviolet-Visible Spectroscopy (Uv-Vis). The QD sizes were found to vary from 2.5 to 9.5 nm. The XRD measurement reveals an increase in the interplanar spacing in QDs as compared to their bulk counterpart. This observation is further supported by Rietveld analysis which establishes the formation of single phase zinc blende ZnSe QDs and confirms 3.9% lattice expansion. Calculations based upon the thermodynamical theory yield 8.7% concentration of vacancies due to the lattice expansion. We observe various peaks in the PL spectra which may arise either due to the QD size variations or the defects due to the vacancies.     

Synthesis, nonlinear optical properties and photoluminescence of ZnSe quantum dots in stable solutions

ZnSe quantum dots (QDs) were synthesized by using a novel oleic acid-controlled hydrothermal route. The as-synthesized QDs were easily dispersed in nonpolar solvents to form highly stable homogenous solutions, on the basis of which their optical properties were systematically investigated. It was found that the QDs had multiple optical properties of both obvious optical nonlinearity with a frequency-doubled Nd:YAG laser (532 nm) used as inducing beam and a strong photoluminescence emission at ca. 458 nm, suggesting that they have potential applications in complex functional optical devices.     

Rare-earth ion-doped perovskite quantum dots: synthesis and optoelectronic properties

Journal of Materials Science: Materials in Electronics - Rare-earth Nd3+ ions were doped into CsPbBr3 perovskite quantum dots (QDs) by solution-processed method at room temperature. By controlling...     

Luminescent properties and excitation mechanism of ZnSe quantum dots embedded in ZnS Matrix

In this paper, we report alternative-current thin film electroluminescence structure with ZnSe quantum dots embedded in ZnS matrix as light-emitting center, i.e., ITO/SiO_x (100 nm)/[ZnS (10 nm)/ZnSe (1 nm)]₃₀/SiO_x (100 nm)/Al. Blue emissions at 390 and 477 nm are obtained in its alternative-current electroluminescent spectra. By studying its luminescent spectroscopy and brightness oscillogram of the device, we found that blue emission came from defect states at ZnSe/ZnS interface and the excitation mechanism was hot-electron impact.     

Green chemical approaches to ZnSe quantum dots: preparation,characterisation and formation mechanism

ZnSe quantum dots (QDs) and flower-shaped nanocrystals (NCs) were successfully synthesised via a cheap, green and nontoxic route, using environmentally friendly N, N-dimethyl-oleoyl amide as the solvent of Se. The experimental results show that the as-prepared ZnSe QDs with a zinc-blende structure have a narrow size distribution and without resorting to any as-synthetic size-selective procedure. A systematic study of the ZnSe QDs formation process indicates the following properties: variation of some reaction parameters allows us to tune the particle sizes and plays a greater role in the determination of the monodisperse characterisation, for example, these parameters include the amount of ligand and precursors, the injection temperature of Se solution. These size tunabilities interpreted well by the growth kinetics. Another interesting result is that the ZnSe QDs aggregate to flower-shaped NCs, and the flower-shaped NCs also have size-dependence quantum effects as the prepared disperse ZnSe QDs.     

Advanced research into the growth mechanism and optical properties of wurtzite ZnSe quantum dots

Zinc selenide (ZnSe) quantum dots (QDs) with the hexagonal wurtzite structure were successfully prepared using a safe, controllable ethylenediamine-mediated solvothermal method in the absence of surfactants. This new synthesis process of the wurtzite ZnSe QDs was described and the growth mechanism of QDs was proposed. The room-temperature photoluminescence (PL) spectrum of the wurtzite ZnSe QDs (about 4 nm) showed a strong near-band-edge emission peak at 422 nm. The near-band-edge emission peak was blue-shifted compared to that of the bulk ZnSe due to the quantum confinement effects; the peak also displayed a progressive red-shift with increasing the excitation power and an associated reduction in peak energy of up to 300 meV. Band gap renormalization in the electron–hole plasma regime might be used to explain this phenomenon. No previous published research regarding the observed excitation-power-dependent PL properties of the wurtzite ZnSe QDs had been found. Our experimental results contributed valuable insights into the optical properties of the wurtzite ZnSe QDs; with potential applications in optoelectronics and other areas where advanced uniformly-structured nanocrystalline semiconductor materials were finding increased use.     

Effects of multiple organic ligands on size uniformity and optical properties of ZnSe quantum dots

The effects of multi-ligands on the formation and optical transitions of ZnSe quantum dots have been investigated. The dots are synthesized using 3-mercapto-1,2-propanediol and polyvinylpyrrolidone ligands, and have been characterized by X-ray diffraction, transmission electron microscopy (TEM), UV–visible absorption spectroscopy, photoluminescence spectroscopy, and Fourier transform infrared spectroscopy. TEM reveals high monodispersion with an average size of 4 nm. Polymer-stabilized, organic ligand-passivated ZnSe quantum dots exhibit strong UV emission at 326 nm and strong quantum confinement in the UV–visible absorption spectrum. Uniform size and suppressed surface trap emission are observed when the polymer ligand is used. The possible growth mechanism is discussed.     

Investigation on one-pot hydrothermal synthesis,structural and optical properties of ZnS quantum dots

The advantage of hydrothermal synthesis of semiconductor quantum dots (QDs) over the control of particles size, morphology and stability is reported here. In a typical synthesis procedure, the zinc and sulfur precursor molar ratio of 1:3 was used in an aqueous solution at 150 °C. The cubic phase of ZnS with average particles size of 5 nm was confirmed and estimated from the X-ray diffraction (XRD) analysis. The composition and purity of the sample were analyzed from (energy dispersive-ray analysis) EDAX and (X-ray photoelectron spectroscopy analysis) XPS spectra. The absorption spectrum shows the large shift in the absorption band over 90 nm due to the quantum confinement of carriers. The emission spectrum of quantum dots carry more evidence on the presence of shallow trap, deep trap in the band gap of the material responsible for weak emission in the spectral region of 450–500 nm. High resolution transmission electron microscope and scanning electron microscope studies reveal the structural and morphological features of ZnS with slightly distorted spherical morphology. We found that the coordinating ability of solvent strongly influences the reaction process and morphology of the products.     

Synthesis of ZnSe quantum dots and ZnSe-ZnS core/shell nanostructures

Colloidal ZnSe nanocrystals were synthesized in hot mixtures of long-chain alkylamines, fatty acids, and alkylphosphines. It was possible to tune the size of nanocrystals by varying the reaction time. Transmission electron microscope images showed the presence of spherical ZnSe nanocrystals and X-ray diffraction pattern of ZnSe nanocrystals showed the existence of both the crystalline phase, namely, wurtzite and zinc blende. The ZnSe nanocrystals were then passivated with higher band gap ZnS; this lead to a 2.6-fold enhancement in the integrated photoluminescence intensity of ZnSe nanocrystals. We also synthesized the reverse type core/shell ZnS/ZnSe nanocrystals. These exhibited a significant red shift in the absorption edge after coating with a thin ZnSe shell.     

Surface characterization of GSH-CdTe quantum dots

The surface characterization of CdTe QDs synthesized by a novel procedure using glutathione (GSH), low temperatures (60–90 °C) and K₂TeO₃ as the –Te precursor is reported. Fluorescence of the produced QDs is stable in the pH range 6–13 and QDs inside eukaryotic cells are highly fluorescent. The surface composition of GSH-CdTe QDs with different spectroscopic properties and particle size distributions was determined by XPS. The XPS analysis indicated that the QDs are essentially CdTe, although all nanoparticles contain 12–24% of CdO (and in one case also TeO₂). GSH decomposes with reaction time releasing small amounts of S⁻² ions that react with Cd(Te) to yield Cd(Te)S in a smaller amount than that of CdTe. Finally, the use of QDs in fluorescence mediated immunodetection of bacterial pathogens has been evaluated.