Studies on optical absorption and photoluminescence of thioglycerol-stabilized CdS quantum dots

Nanoparticles of CdS were prepared at 303 K by aqueous precipitation method using CdSO4 and (NH4)2S in presence of the stabilizing agent thioglycerol. Adjustment of the thioglycerol (T) to ammonium sulphide (A) ratio (T:A) from 1:25 to 1:3.3 was done during synthesis and nanoparticles of different size were obtained. The prepared colloids were characterized by UV-vis and photoluminescence (PL) spectroscopic studies. Prominent first and second excitonic transitions are observed in the UV-vis spectrum of the colloid prepared with a T:A ratio of 1:3.3. Particle size analysis was done using XRD, high resolution TEM and dynamic light scattering and found to be approximately 3 nm. UV-vis and PL spectral features also agree with this particle size in colloid with T:A of 1:3.3. The band gap of CdS quantum dots has increased from the bulk value 2.4-2.9 eV. PL spectra show quantum size effect and the peak is shifted from 628 to 556 nm when the ratio of T:A was changed from 1:25 to 1:3.3. Doping of CdS with Zn2+ and Cu2+ is found to enhance the PL intensity. PL band shows blue-shift and red-shift on doping with Zn2+ and Cu2+, respectively. UV and PL spectral features of the CdS/Au hybrid nanoparticles obtained by a physical mixing of CdS and Au nanoclusters in various volume ratios is also discussed. Au red-shifts and rapidly quenches the PL of CdS. An additional low energy band approximately 650 nm is observed in the UV visible spectrum of the hybrid nanoparticles.     

Influence of cobalt concentration on structural and optical properties of ZnS quantum dots

ZnS and Co-doped ZnS nanoparticles have been prepared by simple chemical precipitation method. Zinc acetate, sodium sulfide, and cobalt nitrate have been used as precursors for the preparation of Co-doped ZnS quantum dots. The X-ray diffraction results revealed that the undoped and Co-doped ZnS quantum dots exhibit hexagonal structure. The average grain size of quantum dot was found to lie in the range of 2.6–3.8 nm. The surface morphology has been studied using scanning electron microscope. The compositional analysis results confirm the presence of Co, Zn and S in the sample. The optical properties of undoped and Co-doped ZnS quantum dots have been studied using absorption spectra. TEM results show that undoped and Co-doped ZnS nanoparticles exhibit a uniform size distribution with average size of 2.5–3.4 nm.     

Role of surface ligands in optical properties of colloidal CdSe/CdS quantum dots

In order to study the role of surface ligands in determining optical properties of colloidal quantum dots (QDs), we have selectively fabricated and studied CdSe/CdS core-shell QDs with strongly confined electron and hole states attached with commonly used surface ligands. Optical properties, viz. absorption and fluorescence of these QDs, are characterized from which salient changes have been observed for different ligand substitutions which, through theoretical analysis, can be associated with electronic structure properties of the QD-ligand composite systems, in particular localization of wave functions of electrons and holes in the QDs and the band matching of the HOMO-LUMO gap of the ligands. The findings can be utilized to facilitate the understanding and optimization of properties of QD biomarkers with functionalizing surface ligands for targeting cellular objects.     

Optical nonlinear absorption and refraction of CdS and CdS-Ag core-shell quantum dots

The CdS and CdS-Ag core-shell quantum dots (QDs) have been prepared. The nanostructures of the QDs were revealed by transmisson electron microscopy and absorption spectra, respectively. The third-order nonlinear optical properties of the core-shell QDs have been studied by using Z-scan technique with femtosecond pulses at the wavelength of 790 nm. The value of the effective nonlinear absorption coefficient beta(eff) of CdS-Ag QDs is measured to be about 16.8 cm/GW, which is about 400 times larger than that of bare CdS QDs of 3.9 x 10(-2) cm/GW. The nonlinear refraction index gamma of CdS-Ag QDs is about -2.3 x 10(-4) cm(2)GW, which is about 200 times larger than that of bare CdS QDs of 1.0 x 10(-6) cm(2)GW.     

Preparation and spectral properties of high-efficiency luminescent polyethylenimine-stabilized CdS quantum dots

Feasibility was demonstrated for obtaining ultrasmall colloidal CdS nanoparticles (with diameter about 2 nm) stabilized in aqueous solution by polyethylenimine with a narrow size distribution (~10%) and luminescing at 400-600 nm (quantum yield about 10%). Complexation between Cd^II and polyethylenimine is a necessary condition for formation of such nanoparticles.     

Chiral highly luminescent CdS quantum dots

Strongly white-emitting (lambda(max) = 495 +/- 10 nm) D- and L- penicillamine capped CdS nanoparticles, which show strong circular dichroism in the range 200-390 nm, have been prepared.     

Linear and nonlinear optical studies of CdS1−x Se x quantum dots

In this contribution we present and discuss our measurements on CdS_1?x Se _x quantum dots in a glass matrix. In linear absorption measurements we find the typical blue shift of the transitions with decreasing crystallite radius due to quantization. The luminescence shows a significant Stokes shift with respect to absorption, which is interpreted in terms of strong exciton-phonon coupling and allows to deduce the Huang-Rhys factor S. Under high excitation we find an additional emission band on the high energy side, which can be attributed to the recombination of an excited two electron-hole pair state to a one electron-hole pair state in agreement with theory. Pump and probe beam experiments give a bleaching but no hole burning. Finally we discuss some open questions especially concerning the high energy structures in the absorption spectrum.     

Structural characteristics of nanomaterials based on CdS quantum dots

CdS nanoparticles with sizes where a quantum-size effect is observed are structurally characterized in a detailed way. The following complex of structural methods is used to characterize the nanoparticles: electron diffraction; analytical, diffraction, and high-resolution transmission electron microscopy; and small-angle X-ray scattering.     

Dendron-tethered and templated CdS quantum dots on single-walled carbon nanotubes

CdS nanoparticles on the surface of single-walled carbon nanotubes (SWNTs) were templated and stabilized through the initial attachment of 1 --> 3 C-branched amide-based dendrons and were both photophysically and morphologically characterized. The CdS clusters were shown to be ca. 1.4 nm in diameter as calculated from their optical absorption spectra and exhibited reduced fluorescence emission intensity at 434 nm compared to that of CdS quantum dots stabilized by untethered dendrons due to partial emission quenching by the SWNT. Unchanged UV absorption behavior of these materials indicated that they are stable > 90 days at 25 degrees C.     

CdSe/CdS量子点荧光猝灭法测定芹黄素的研究

以巯基乙酸为稳定剂,在水溶液中合成了具有特殊光学性质的水溶性CdSe/CdS量子点。以该量子点为荧光探针,基于荧光猝灭法对芹黄素进行了定量检测。考察了缓冲体系、反应时间、量子点浓度等多种因素的影响。实验结果表明,在0.001 mol/L、pH为6.80的KH2PO4-Na2HPO4缓冲液中,当量子点浓度为1.2×10^-4mol/L、反应时间为20 min时,该方法的线性范围为0.16-27.02μg/mL,其线性回归方程为F0/F=0.99665＋0.11067ρ（μg/mL）,相关系数r=0.998,检出限为0.13μg/mL,并用于合成样品的分析。     

Absorption cross-section and related optical properties of colloidal InAs quantum dots

We report the absorption cross-section of colloidal InAs quantum dots of mean radii from 1.6 to 3.45 nm. We find excellent agreement between the measured results and calculated values based on a model of small-particle light absorption. The absorption cross-section per dot is 6.2 x 10(-16)R(3) cm(2) at 2.76 eV and 3.15 x 10(-16)R(1.28) cm(2) at the first-exciton absorption peak, with the dot radius R in nm. We find that the per-quantum-dot particle oscillator strength of the first-exciton transition is constant for all sizes studied. The radiative lifetime of the first exciton calculated from the oscillator strength increases with dot size and ranges from 4 ns for the smallest dots to 14 ns for the largest ones.     

Monolayer behavior and Langmuir-Blodgett manipulation of CdS quantum dots

Cadmium sulfide (CdS) quantum dots (QDs) were prepared and surface modified by dodecanthiol or mercaptosuccinic acid (MSA) to render a surface with alkyl chains (C(12)-CdS) or carboxylic acid groups (MSA-CdS), respectively. Due to the hydrophobic property of C(12)-CdS, the nanoparticles disperse well in chloroform and stay stable at the air/water interface. However, 3-dimensional (3D) aggregative domains and particle-free pores were formed in the monolayer due to poor particle-water interaction. For the MSA-CdS nanoparticles, the surface was hydrophobized through physical adsorption of a cationic surfactant, cetyltrimethylammonium bromide (CTAB). The capped MSA on the CdS plays an important role in enhancing the adsorption of CTAB and improving the stability of the QDs at the air/water interface. Due to the reversible adsorption of CTAB on MSA-CdS, a hydrophilic area can be exposed in the water-contacting region of a nanoparticle when it stays at the air/water interface. Thus, the CTAB-MSA-CdS QD behaves as an amphiphilic compound at the air/water interface and has properties superior to those of C(12)-CdS QDs in fabrication of layer-by-layer 2D structure of particulate films. The distinct behaviors of the two QDs at the air/water interface and the related effect on the properties of LB films were studied using a number of methods, including pressure-area (pi-A) isotherm, relaxation and hysteresis experiments, in-situ observation of Brewster angle microscopy (BAM), the postdeposition analysis of atomic force microscopy (AFM), and UV-vis spectroscopy.     

Ultrasensitive cysteine sensing using citrate-capped CdS quantum dots

The importance of cysteine (Cys) in biological systems has stimulated a great deal of efforts in the development of analytical methods for the determination of this amino acid. In this work, a novel fluorescent probe for Cys based on citrate (Cit)-capped CdS quantum dots (QDs) is reported. The Cit-capped CdS QDs fluorescent probe offers good sensitivity and selectivity for detecting Cys. A good linear relationship was obtained from 1.0 × 10⁻⁸ mol L⁻¹ to 5.0 × 10⁻⁵ mol L⁻¹ for Cys. The detection limit was calculated as 5.4 × 10⁻⁹ mol L⁻¹. The proposed method was applied to detect Cys in human urine samples, which showed satisfactory results. This assay is based on both the lability of Cit and the strong affinity of thiols to the surface of CdS QDs. The addition of Cys improved the passivation of the surface traps of CdS QDs and enhanced the fluorescence intensity.     

Isotachophoretic purification of nanoparticles: tuning optical properties of quantum dots

Synthesized nanoparticles often require fine fractionation according to shape, dimension, mass, chemical composition, charge, and other properties in order to become suitable for practical use. Quantum dots (QDs) are luminescent nanocrystals with narrow emission peaks. This property has been widely utilized for the multiplexed sensing and barcoding of microparticles. QDs with narrower emission peaks are preferred for such applications. The width of the emission peaks can be significantly reduced after purification. A newly developed preparative isotachophoretic method employs the dependence of spectral properties and electrophoretic mobility on the diameter of QDs. Separated fractions of QDs revealed narrower emission peaks (72% of the original width) and improved quantum yield (two-fold). The usefulness of the developed isotachophoresis for purification and analysis of other nanostructures, for example, plasmonic nanoparticles and nanobioconjugates, is expected, too.     

Synthesis,characterizations, and optical properties of copper selenide quantum dots

We demonstrate the synthesis of copper selenide quantum dots (QDs) by element directed, inexpensive, straight forward wet chemical method which is free from any surfactant or template. Copper selenide QDs have been synthesized by elemental copper and selenium in the presence of ethylene glycol, hydrazine hydrate, and a defined amount of water at 70 °C within 8 h. The product is in strong quantum confinement regime, phase analysis, purity and morphology of the product has been well studied by X-ray diffraction (XRD), UV–Visible spectroscopy (UV–Vis), Photo-luminescent spectroscopy (PL), Fourier transform infrared spectroscopy (FTIR), Transmission electron microscopy (TEM), High resolution transmission electron microscopy (HRTEM), and by Atomic force microscopy (AFM) techniques. The absorption and photoluminescence studies display large “blue shift”. TEM and HRTEM analyses revealed that the QDs diameters are in the range 2–5 nm. Due to the quantum confinement effect copper selenide QDs could be potential building blocks to construct functional devices and solar cell. The possible mechanism is also discussed.     

Probing cytotoxicity of CdSe and CdSe/CdS quantum dots

Toxicities of CdSe and CdSe/CdS quantum dots(QDs) synthesized by ultrasound-assisted methods were investigated in vitro and in vivo.Five human cell lines were used to assess the cytotoxicity of as-prepared CdSe and CdSe/CdS by assays of MTT viability,red blood cell hemolysis,flow cytometry,and fluorescence imaging.The results show that these QDs may be cytotoxic by their influence in S and G2 phases in cell cycles.The cytotoxicity of QDs depends on both the physicochemical properties and related to target cells.