Dielectric properties of CdS nanoparticles synthesized by soft chemical route

CdS nanoparticles have been synthesized by a chemical reaction route using thiophenol as a capping agent. The frequency-dependent dielectric dispersion of cadmium sulphide (CdS) is investigated in the temperature range of 303-413 K and in a frequency range of 50 Hz-1 MHz by impedance spectroscopy. An analysis of the complex permittivity (ɛ′ and ɛ″) and loss tangent (tan δ) with frequency is performed by assuming a distribution of relaxation times. The scaling behaviour of dielectric loss spectra suggests that the relaxation describes the same mechanism at various temperatures. The frequency-dependent electrical data are analysed in the framework of conductivity and modulus formalisms. The frequency-dependent conductivity spectra obey the power law.     

Characterization of chemically synthesized CdS nanoparticles

II-VI semiconductor nanoparticles are presently of great interest for their practical applications such as zero-dimensional quantum confined materials and for their applications in optoelectronics and photonics. The optical properties get modified dramatically due to the confinement of charge carriers within the nanoparticles. Similar to the effects of charge carriers on optical properties, confinement of optical and acoustic phonons leads to interesting changes in the phonon spectra. In the present work, we have synthesized nanoparticles of CdS using chemical precipitation technique. The crystal structure and grain size of the particles are studied using XRD. The UV-visible absorption, photoluminescence and Raman spectra of the sample are recorded and discussed briefly     

Growth and optical properties of semiconductor nanocrystals in a glass matrix

Over the last 15 years nanocrystals embedded in a glass matrix have been a subject for studies of fundamental phenomena of quasiparticles (electrons, holes, excitons, phonons) quantum confinement in the nanosize semiconductor materials. Growth of the nanocrystals in a glass matrix is based on the thermodynamic process of the diffusion-controlled phase decomposition of oversaturated solid solutions. Three stages of the process in solutions prepared by co-melting, co-sputtering and ion-implantation techniques are discussed. It is shown that the growth technique makes it possible to vary the mean size of the particles, their size distribution and crystalline structure.

The optical properties of nanocrystals of various semiconductor compounds grown in different glass matrices are discussed. Attention is given to studies of a fine structure of optical spectra at resonant size-selective spectroscopy for both “strong” and “weak” confinement regimes. Energy spectra of confined acoustic and optical phonons in a “strong” confinement regime, studied by resonant Raman scattering, are discussed.     

Photoluminescence of CdS nanoparticles embedded in a starch matrix

CdS nanoparticles were synthesized by precipitation in aqueous solution using starch as the capping molecule, and the effect of the pH of the solution on the optical absorption, photoluminescence, and size of the nanoparticles was studied. Absorption spectra, obtained by photoacoustic spectroscopy, indicated that the band gap energy of the crystalline nanoparticles decreased from 2.68 eV down to 2.48 eV by increasing the pH of the solution from 9 up to 14. The X-ray diffraction analysis revealed that the CdS nanoparticles were of zinc blende structure, and that the particle size increased from 1.35 nm up to 2.45 nm with increasing pH. In addition, temperature-dependent photoluminescence (PL) measurements of the capped material showed a blue-shift of the emission peak for temperatures higher than 150 K, indicating the influence of starch on the formation of defect levels on the surface of the CdS nanoparticles.     

Third-order nonlinear optical properties of multiwalled carbon nanotubes modified by CdS nanoparticles

CdS nanoparticles were coated on the side wall of multiwalled carbon nanotubes (MWCNTs) by a wet chemical synthesis approach via noncovalent functionalization of MWCNTs with poly(diallyldimethylammonium chloride) (PDDA). The as-prepared material was characterized by X-ray diffraction (XRD), UV–vis absorption, fluorescence and transmission electron microscopy (TEM). The results indicated that CdS nanoparticles were uniformly coated on the surface of MWCNTs. Third-order optical nonlinearity of the as-prepared material was studied with the Z-scan technique with picosecond laser pulses at 532 nm. The Z-scan curve revealed that CdS nanoparticle-modified MWCNTs exhibited negative nonlinear refraction index and positive absorption coefficient. The real part and imaginary part of the third-order nonlinear susceptibility χ⁽³⁾ were calculated to be −4.9 × 10⁻¹² and 6.8 × 10⁻¹³ esu, respectively.     

Optical properties of citrate-stabilized CdS nanoparticles

Citrate-stabilized CdS nanoparticles of size 4 nm are obtained by varying the sulfide:citrate ion concentration in a simple aqueous synthesis method. The optical absorption and photoluminescence properties of the nanoparticles are studied. The size of the crystallites is found to be less affected by sulfide:citrate ratio. At lower concentrations of S²⁻, trap state emission is favoured and at higher concentrations excitonic transition is predominant as shown by optical absorption and photoluminescence spectra. Effective surface capping and optimum concentration of S²⁻ leads to the quenching of surface-defect-related emission. Increase in citrate ion concentration is found to increase the intensity of photoluminescence band arising from trap state emission revealing the role of sulfide:citrate ratio on surface modification of CdS nanocrystals. The nanoparticles are hexagonal as shown by the X-ray diffraction and selected area electron diffraction pattern.     

Tuning the optical properties of Fe-Au core-shell nanoparticles with spherical and spheroidal nanostructures

Present research interest is to highlight on the manufacturing of core-shell nanoparticles because of core activity with unique properties and surface modification by a shell in the diverse fields (e.g. optoelectronic, catalysis and magneto-optics). In addition, the combined optical properties of magnetic-plasmonic core-shell NPs make them ideal candidates for many applications in biomedical fields. The influence of Fe-core and Au-shell for the formation of the core-shell viz. spherical and spheroidal nanostructures is studied using the discrete dipole approximation method. DDA is an approximation method and its accuracy is compared to Mie theory results for spherical core-shell NPs as Mie theory gives the exact solution to spherical targeted NPs. DDA calculations are further extended to spheroidal core-shell nanostructures. It is observed that the localized surface plasmon resonance (LSPR) peak position in considered core-shell nanostructures is enhanced by changing the cores and shell thickness in the core-shell spherical nanostructures and aspect ratio as well as shell thickness in spheroidal core-shell nanostructures. The absorption spectra are found between 363–788 nm wavelength ranges and can be tuned into UV-visible-near-infrared region of the electromagnetic (EM) spectrum in accordance with desired applications. It has been found that the Fe@hollow@Au and prolate core-shell nanostructures show enhancement to LSPR peaks, bandwidth and their corresponding intensities in comparison to other considered spherical and spheroidal core-shell nanostructures. Tunability in core size, shell thickness, aspect ratio, and configuration will open new potential uses of suitable magnetic-plasmonic core-shell nanostructures in cancer therapy, tissue engineering, drug delivery, and many more of biomedical fields.     

Structural and optical properties of monodispersed ZnS/CdS/ZnO and ZnO/ZnS/CdS nanoparticles

Excellent luminescence properties of ZnS/CdS/ZnO and ZnO/ZnS/CdS nanocrystallites synthesized through a simple chemical method at room temperature are reported. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM), UV–visible absorption and photoluminescence techniques were used to characterize the undoped ZnS, CdS and ZnO and the novel ZnS/CdS/ZnO and ZnO/ZnS/CdS nanoparticles. The optical properties of ZnS/CdS/ZnO and ZnO/ZnS/CdS nanoparticles reflect a combinational effect of the photoluminescent properties of ZnS, CdS and ZnO.     

Tuning electronic structures and optical properties in CdSe/CdS Dot-in-rod colloidal nanostructures: Atomistic tight-binding theory

Using the atomistic tight-binding theory, in this study, the electronic structures and optical properties of CdSe/CdS dot-in-rod nanocrystals are modified by engineering the core and rod diameters. The ground electron states favor an s-like orbital, while the ground hole states are formed by light hole-like characteristics. The quantum confinement effect is attributed to the reduction in excitonic band gaps as the core and rod diameters increase. Changing the core and rod sizes produces excitonic band gaps across the visible light spectra. The highest oscillation strength, excitonic-binding energy, and dark-bright excitonic splitting are witnessed at a 3.0 nm CdSe core diameter. The oscillation strength, excitonic-binding energy, and dark-bright excitonic splitting decrease as the CdS rod diameters increase. This study highlights the importance of theoretically understanding and controlling the active materials’ structural parameters in optoelectronic technology.     

Growth mechanism and optical property of CdS nanoparticles synthesized using amino-acid histidine as chelating agent under sonochemical process

Using amino-acid histidine as chelating agent, CdS nanoparticles have been synthesized by sonochemical method. It is found that by varying the ultrasonic irradiation time, we can tune the band gap and particle size of CdS nanoparticles. The imidazole ring of histidine captures the Cd ions from the solution, and prevents the growth of the CdS nanoparticles. The deviation in the linear relation in between cube of radius of nanoparticles and ultrasonic irradiation time confirms the growth of CdS nanoparticles occur via two process; one is the diffusion process of the reactants as well as reaction at the surface of the crystallite. CdS nanoparticles synthesized using histidine as organic chelating agent have band edge emission at 481 nm and have greater photoluminescence intensity with blue-shift to higher energy due to typical quantum confinement effect.     

Effect of deformation of a polymer matrix on the optical properties of CdS nanocrystals incorporated into it

A study is made of the photodegradation kinetics of luminescence and absorption of cadmium sulfide nanocrystals incorporated into a plastic polymer matrix. The reversibility effect of photodegradation under mechanical deformation (stretching) of a polymer film is revealed. It is suggested that the photodegradation reversibility is related to the extraction of electrons, photoexcited in nanocrystals, from deep-lying traps in the surrounding polymer matrix during its deformation. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 1, pp. 101–104, January–February, 2000.     

Formation and optical properties of CdSSe semiconductor nanocrystals in the silicate glass matrix

Technological conditions providing the formation of CdS _x Se_1−x semiconductor crystal grains with sizes ranging from 2 to 8 nm in a silicate glass matrix have been determined. As the temperature of forming annealing increases, the size of crystal grains increases without changes in their crystal structure and composition. The observed short-wavelength shift of the optical absorption edge indicates that the quantum confinement affects the energy band structure of the nanocrystals. Intense luminescence of the samples is due to radiative transitions involving defects at the semiconductor nanocrystal-silicate matrix interface or intrinsic defects of nanocrystals.     

尺寸相关的CdS纳米颗粒的拉曼散射特性

采用反胶束法,合成了具有不同尺寸的CdS纳米颗粒。利用透射电镜(TEM)和高分辨透射电镜(HR-TEM)以及紫外-可见光吸收谱(UV/vis)对这些纳米颗粒的结构特性进行了表征和分析。利用拉曼光谱仪测量了这些具有不同尺寸的CdS纳米颗粒的拉曼特性。研究结果表明：当纳米颗粒尺寸小于一定值时,拉曼峰出现了蓝移,大于一定值时出现了红移,这些不同的结果是与纳米颗粒的尺寸效应以及纳米颗粒结构中具有各向异性的电子－声子耦合作用有关。     

Effect of surface on the optical structure and thermal properties of organically capped CdS nanoparticles

Cadmium sulfide (CdS) nanoparticles were synthesized by a novel wet chemical route with various organic thiol stabilizers. Systematic experimental studies, including X-ray diffraction (XRD), ultraviolet–visible (UV–vis) spectroscopy, Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), photoluminescence (PL) spectroscopy, and time-resolved photoluminescence (TRPL), have evidenced that the stability, crystallinity, and optical properties of the CdS nanoparticles are affected by the organic groups which generate significant effects in surface reconstruction. Particle size was evaluated from UV–vis spectroscopy using the effective mass approximation (EMA) method and from XRD patterns based on Scherrer?s formula. The S–H vibrations are not detectable in the infrared (IR) spectra of any of the bound ligands, which are expected for thiols covalently bound to the surface of nanoparticles. PL studies reveal that the emission from the nanostructures is not much influenced by the surface states, indicating a good passivation of the particle?s surface. The time-resolved measurements reveal a biexponential decay behavior. The fast decay component is attributed to the recombination of core states, while the slow decay component of PL is associated with the charge-carrier recombination process with the involvement of surface states.     

Synthesis, surface morphology, and optical and structural properties of CdS and ZnS nanoparticles

Synthesis of CdS and ZnS nanoparticles in reverse micelles and organic solvents has been carried out. Particles with a hexagonal structure 2–5 nm in size are formed during synthesis. Maintaining the reaction mixture at room temperature leads to the formation of nanoparticles with a cubic structure 100–150 nm in size. The changes in the optical properties of CdS and ZnS nanoparticles, depending on the synthesis method and conditions and on the precursors used, have been investigated. The luminescence characteristics of local surface defects of nanoparticles depend weakly on nanoparticle sizes. The dependence of the fluorescence and phosphorescence intensity of nanoparticle surface defects on the polarity of surrounding solution is demonstrated; thus, these particles can be used as polarity indicators.     

Linear and nonlinear optical properties of luminescent ZnO nanoparticles embedded in PMMA matrix

ZnO nanoparticles embedded in the PMMA matrix were prepared by wet chemical synthesis. The optical band gap of the ZnO nanoparticles decreases with increase in NaOH concentration. The photoluminescence spectra of the ZnO colloids show strong UV, green and blue emissions. The optical absorptive nonlinearity of the ZnO:PMMA composites was analyzed using an open aperture Z-scan technique which shows optical limiting type nonlinearity due to the two photon absorption in ZnO. The efficiency of limiting is found to increase with decrease in the band gap. ZnO:PMMA shows a negative value for nonlinear refractive index n₂ and the magnitude of n₂ increases with decrease of band gap. Stability as well as the mechanical properties of the nanoparticles embedded in the PMMA matrix makes it more suitable for device fabrication as compared to the ZnO nanoparticles dispersed in solution.     

Atomic motion in Se nanoparticles embedded into a porous glass matrix

By neutron diffraction it was shown that nanostructured Se confined within a porous glass matrix exists in a crystalline as well as in an amorphous state. The spontaneous crystallization of crystalline Se from confined amorphous phase was observed. The root-mean-square amplitudes of the atomic motions in the bulk as well as in confinement are found to be essentially different in a basal plane and in the perpendicular direction along the hexagonal axis. The atomic motions in the confined Se differ from the atomic motions in the bulk at low temperatures. The results shows an unusual “freezing" of the atomic motion along the chains, while the atomic motions in the perpendicular plane still keep. This “freezing" is accompanied by the deformation of nanoparticles and the appearance of inner stresses. This effect is attributed to the interaction of confined nanoparticle with the cavity walls.     

Synthesis and optical characterization of monodispersed Mn doped CdS nanoparticles

Monodispersed Mn²⁺ doped CdS nanoparticles with average size as small as 1.8 nm have been synthesized through chemical method. The nanostructures of the prepared nanoparticles have been confirmed through X-ray diffraction (XRD), ultraviolet-visible (UV-vis) absorption and transmission electron microscope (TEM) measurements. The photoluminescence emission covering 450-650 nm of the visible region is observed under ultraviolet light excitation, from Mn²⁺ doped CdS nanoparticles dispersed in dimethyl sulfoxide (DMSO).