CdS Semiconductor Nanocrystals Research Articles

Shape Dependence of Pressure-Induced Phase Transition in CdS Semiconductor Nanocrystals.

Understanding structural stability and phase transformation of nanoparticles under high pressure is of great scientific interest, as it is one of the crucial factors for design, synthesis, and application of materials. Even though high-pressure research on nanomaterials has been widely conducted, their shape-dependent phase transition behavior still remains unclear. Examples of phase transitions of CdS nanoparticles are very limited, despite the fact that it is one of the most studied wide band gap semiconductors. Here we have employed in situ synchrotron wide-angle X-ray scattering and transmission electron microscopy (TEM) to investigate the high-pressure behaviors of CdS nanoparticles as a function of particle shapes. We observed that CdS nanoparticles transform from wurtzite to rocksalt phase at elevated pressure in comparison to their bulk counterpart. Phase transitions also vary with particle shape: rod-shaped particles show a partially reversible phase transition and the onset of the structural phase transition pressure decreases with decreasing surface-to-volume ratios, while spherical particles undergo irreversible phase transition with relatively low phase transition pressure. Additionally, TEM images of spherical particles exhibited sintering-induced morphology change after high-pressure compression. Calculations of the bulk modulus reveal that spheres are more compressible than rods in the wurtzite phase. These results indicate that the shape of the particle plays an important role in determining their high-pressure properties. Our study provides important insights into understanding the phase-structure-property relationship, guiding future design and synthesis of nanoparticles for promising applications.

Сhiral recognition of cysteine molecules by chiral CdSe and CdS quantum dots

Here, we report the investigation of mechanism of chiral molecular recognition of cysteine biomolecules by chiral CdSe and CdS semiconductor nanocrystals. To observe chiral recognition process, we prepared enantioenriched ensembles of the nanocrystals capped with achiral ligand. The enantioenriched samples of intrinsically chiral CdSe quantum dots were prepared by separation of initial racemic mixture of the nanocrystals using chiral phase transfer from chloroform to water driven by L- and D-cysteine. Chiral molecules of cysteine and penicillamine were substituted for achiral molecules of dodecanethiol on the surfaces of CdSe and CdS samples, respectively, via reverse phase transfer from water to chloroform. We estimated an efficiency of the hetero- (d-L or l-D) and homocomplexes (l-L) formation by comparing the extents of corresponding complexing reactions. Using circular dichroism spectroscopy data we show an ability of nanocrystals enantiomers to discriminate between left-handed and right-handed enantiomers of biomolecules via preferential formation of heterocomplexes. Development of approaches for obtaining chiral nanocrystals via chiral phase transfer offers opportunities for investigation of molecular recognition at the nano/bio interfaces.

Microwave Induced Center-Doping of Transition Metals Ions in Aqueous CdS NCs with Special Optical Properties

Under microwave irradiation, we systematically studied the formation of transition metal ions (TM), Ag+, Mn2+, Cu2+, Fe2+, Co2+ and Ni2+, doped CdS semiconductor nanocrystals in a pure water solution. The center-doping strategy is essential for achieving efficient internal-doping to improve the optical properties and stability of doped nanocrystals. The resulting TM doped nanocrystals have a high photoluminescence quantum yield (~ 60%) and an amazingly large range of the visible spectrum (480 ~ 650 nm). For Mn2+ doped CdS nanocrystals, significant high photoluminescence quantum yield, between 20% and 50% even at more than 600 nm, was achieved and there is potential for them to be directly used as a consummate toner for multicolor sensing and encoding without ligand exchange. Highly emissive-doped nanocrystals synthesized under designed conditions have been found to have excellent stability; samples could be stored for months without precipitation. In practice, this approach has great potential to promote the metal doping of various semiconductor nanocrystals to realize the specific advantages of doped nanocrystals in water solution. Keywords: CdS, doping, microwave, nanocrystals, transition metal ions.

Tuning the Morphology of Au/CdS Nanocomposites through Temperature-Controlled Reduction of Gold-Oleate Complexes

A general synthetic strategy for controlling the shape of gold domains grown onto CdS semiconductor nanocrystals is presented. The colloidal growth of Au nanoparticles is based on the temperature-controlled reduction of Au-oleate complexes on the surface of CdS and allows for precise tuning of nanoparticle diameters from 2.5 to 16 nm simply by adjusting the temperature of the growth solution, whereas the shape of Au/CdS nanocomposites can be controllably switched between matchsticks and barbells via the reaction rate. Depending on the exact morphology of Au and CdS domains, fabricated nanocomposites can undergo evaporation-induced self-assembly on a substrate either through end-to-end coupling of Au domains, resulting in the formation of one-dimensional chains or via side-by-side packing of CdS nanorods, leading to the onset of two-dimensional superlattices.

Microwave induced center-doping of silver ions in aqueous CdS nanocrystals with tunable, impurity and visible emission

Under microwave radiation, Ag(+)-doped CdS semiconductor nanocrystals with high photoluminescence quantum yield (approximately 58%) and a surprisingly large optical window (480 to 630 nm) are formed controllably using a center-doping strategy and are optimized through a green approach in pure water solution.

A simple organic synthesis for CdS and Se-doped CdS nanocrystals

A simple and convenient organic synthesis method for preparation of CdS and Se-doped CdS semiconductor nanocrystals (NCs) was developed. Cadmium oxide and sulfur powder or thiourea all of which are inexpensive and easily available, were used as precursors of Cd and S. Cheap, stable and environmentally friendly paraffin and oleic acid were selected as the solvent. Compared with expensive trioctylphosphine (TOP) and trioctylphosphine oxide (TOPO) system, this procedure was simple, green and low-cost. By this new route, a series of CdS and Se-doped CdS NCs was obtained. Low amount of Se doped in CdS NCs could tailor their optical characteristics and emission peaks embodied red-shift with higher Se:S mole ratios. The absorption and room temperature luminescence spectra were used to follow the reaction process and to characterize the optical properties of as-prepared NCs, whose sharp and narrow emission peaks indicated that they had desirable dispersibility, uniformity and good fluorescence properties.

Solvothermal Fabrication of Monodisperse Zinc-blende CdS Nanocrystals

Abstract Nearly monodisperse CdS semiconductor nanocrystals have been successfully fabricated by a simple solvothermal route using cheap and low-cost normal inorganic compounds as precursors. The reaction temperature is demonstrated to be critical for the fabrication of the monodisperse nanocrystals. The nanocrystals were characterized by XRD, TEM, UV–vis absorption and photoluminescence spectra. Without involving special conditions, this method can be expected to make considerable contribution to the fabrication of various nanocrystals and nanocrystals-related research.

RNA-Templated Semiconductor Nanocrystals

The use of a biomolecular RNA template for the synthesis of CdS semiconductor nanocrystals is described. Transfer RNA, with a well-defined three-dimensional structure, is used as a scaffold and ligand system in the aqueous synthesis of CdS. We have found that nanocrystal structures can be modulated by the structure of the templating tRNA. When possessing a folded three-dimensional structure, tRNA-templated synthesis yields a single product. If the same tRNA is rendered unstructured through the introduction of destabilizing mutations, a range of products are observed. The results reported indicate that biomolecules can be used to systematically engineer the structures and properties of semiconductor-based materials, and that the synergy between the dimensions of nanostructures and biomolecules provides a means to tune the properties of materials with nanoscale precision.

Three-photon excited band edge and trap emission of CdS semiconductor nanocrystals

We report on three-photon excited band edge and trap emission of CdS semiconductor nanocrystals. While the band edge emission intensity clearly shows a cubic dependence on excitation intensity, demonstrating three-photon absorption process, the trap emission does not exhibit such a cubic dependence. A simple theoretical model based on the assumption that there exist a limited number of trap states in nanocrystals shows good agreement with the experiment, suggesting that the number of trap states play an important role in their emission intensity dependence of multiphoton excitation. The three-photon absorption cross section of CdS nanocrystals is measured to be ∼10−79 cm6 s2 photon−2, which is three to four orders of magnitude higher than those of the previously reported common UV fluorescent dyes.

Photoluminescence from CdS Quantum Dots in Silica Gel

CdS semiconductor nanocrystals were grown as quantum dots (QDs) inside a silica matrix obtained by the sol-gel method and assisted in the mother liquid by high power ultrasounds. Small-angle neutron scattering (SANS) accounts for a 3.6 nm crystal size homogeneously distributed. Optical excitation from the third harmonic of a Nd:YAG ns laser was focused on the sample to study the photoluminescence (PL) at room temperature. The PL spectrum shows radiative process from intrinsic transitions and a broad band corresponding to the traps. Variable stripe length (VSL) method was used to measure the optical gain spectra by the growth of the amplified luminescence. A broad optical gain spectrum produced by the biexciton-exciton transitions revealing the stimulated emission from the CdS QDs. It is also observed a red-shift of the PL emission crystal size-dependent.

Effect of pH, Aging, and Drying on the Size of CdS Nanocrystallites, Monolithicity, and Transparency of Nanocrystalline CdS-Doped Silica Xerogels

CdS semiconductor nanocrystals are prepared in SiO2 matrix xerogels by a sol–gel method using ethanolic tetraethylorthosilicate [TEOS, Si(OC2H5)4], catalyst (HCl or NH4OH), cadmium acetate [Cd (CH3COO)2 ⋅ 2 H2O], and thiourea [SC (NH2)2] and drying the alcogels at room temperature and heating up to 250°C. The effect of alcosol pH on the CdS crystallite size in silica matrix, color, monolithicity, and transparency of the xerogels is studied by varying the pH from 1 to 10, and maintaining the CdS/SiO2 molar ratio constant at 0.0025. It was found that the color of samples varied from light yellow to orange and transparency decreased from 85 to 15% with the variation of alcosol pH from 1 to 10, respectively. The cracked sample were found for pH 8, and transparent monolithic yellow-colored samples in the pH range 3–7. The quantum size effect was observed in the optical absorption spectra of the samples. The threshold peak decreased from 520 to 250 nm, energy increased from 2.4 to 4.9 eV, and the CdS crystallite size decreased from 6 to 1 nm with the variation of pH of the alcosol from 10 to 1, respectively. In the case of the effect of aging, a minimum of 5 days aging is required for monolithic samples and the CdS crystallite to increase in size with and remain constant, even when the aging period is increased for 10 days. Monolithic samples were obtained with drying and heating rates of 0.4 wt.% loss/h and >50°C/h drying and heating rates, respectively. The density of the CdS crystallites doped-silica xerogels decreased from 1.57 to 1.30 gm/cm3 with an increase in pH from 1 to 10, respectively. From XRD spectra, the CdS crystallite structure was found to be a hexagonal wurtzite structure. The intensity of the peaks increased and the breadth decreased with the variation of pH of the alcosol from 1 to 10 because of an increase in CdS crystallite size.

Optical properties of excitons in CdS semiconductor-insulator quantum wires

The characteristic features of the luminescence spectra of CdS semiconductor nanocrystals, crystallized in hollow channels in a dielectric template, are explained in terms of excitonic transitions in semiconductor-insulator quantum wires. The excitonic transition energies agree with the values calculated taking into account the effects of size quantization and the “dielectric enhancement of excitons” — the large increase in the electron-hole attraction as a result of the difference between the permittivities of the semiconductor and insulator. The theoretically computed binding energies of excitons in CdS quantum wires with a diameter of 10 nm reach 170 meV. It is shown that the excitonic transition energy is constant for a wide range of wire diameters.

Characterization and Surface Charge Measurement of Self-Assembled CdS Nanoparticle Films

The self-assembly of CdS semiconductor nanocrystals (Q-CdS) as well-defined monolayer and multilayer films on glass, indium tin oxide, and gold surfaces was carried out. These were characterized by ultraviolet−visible spectroscopy, ellipsometry, and scanning tunneling microscopy. The adsorption of the polymeric anion hexametaphosphate (HMP) on the Q-CdS particle surface produced a negative surface charge. The diffuse double-layer interaction forces between a silica probe and the Q-CdS/HMP particle film-covered gold substrate immersed in aqueous solutions were measured using an atomic force microscope. The surface charge and electrostatic potential of Q-CdS particles were obtained by theoretical fits of the force data to solutions of the complete nonlinear Poisson−Boltzmann equation with a knowledge of the silica probe surface potential.

Optical properties of Mn-doped CdS nanocrystals

We report on optical properties of CdS semiconductor nanocrystals doped with manganese ions which have been successfully prepared using the inverted micelle technique. The effects of quantum confinement on the emission properties of the manganese ions are studied by means of absorption, continuous and time resolved luminescence as well as photoluminescence excitation spectra at room temperature and at 10 K. The Mn2+ emission band associated to the 4T1 → 6A1 transition dominates the continuous luminescence spectrum and is the result of an energy transfer from the nanocrystals to the internal Mn2+ levels. Time resolved experiments clearly show that the Mn2+ lifetime is not affected by confinement in the nanocrystals and is of the same order of magnitude as in bulk CdS.