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.     

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     

Surface-enhanced Raman spectroscopy of semiconductor nanostructures

We review our recent results concerning surface-enhanced Raman scattering (SERS) by confined optical and surface optical phonons in semiconductor nanostructures including CdS, CuS, GaN, and ZnO nanocrystals, GaN and ZnO nanorods, and AlN nanowires. Enhancement of Raman scattering by confined optical phonons as well as appearance of new Raman modes with the frequencies different from those in ZnO bulk attributed to surface optical modes is observed in a series of nanostructures having different morphology located in the vicinity of metal nanoclusters (Ag, Au, and Pt). Assignment of surface optical modes is based on calculations performed in the frame of the dielectric continuum model. It is established that SERS by phonons has a resonant character. A maximal enhancement by optical phonons as high as 730 is achieved for CdS nanocrystals in double resonant conditions at the coincidence of laser energy with that of electronic transitions in semiconductor nanocrystals and localized surface plasmon resonance in metal nanoclusters. Even a higher enhancement is observed for SERS by surface optical modes in ZnO nanocrystals (above 10⁴). Surface enhanced Raman scattering is used for studying phonon spectrum in nanocrystal ensembles with an ultra-low areal density on metal plasmonic nanostructures.     

Quantum confinement effect in the vibrational spectrum of CdSxSe1−x quantum dots in a fluorophosphate glass matrix

The vibrational spectra of mixed cadmium sulfoselenide nanocrystals in a fluorophosphate glass matrix are investigated by Raman spectroscopy. The asymmetry of the lines of the fundamental modes of nanocrystals is experimentally observed in the region of lattice vibrations, which is interpreted as a quantum confinement effect. In the framework of the model of confined phonons, the contribution of the band states to the Raman scattering spectrum is calculated and the size of nanocrystalline regions is estimated. The results obtained are in good agreement with the data on the low-frequency Raman scattering in these objects.     

Nanostructuring-induced modification of optical properties of p-GaAs (1 0 0)

A pulsed anodic etching method has been utilized for nanostructuring of p-type GaAs (1 0 0) surface, using HCl-based solution as electrolyte. The resulting porous GaAs layer is characterized by atomic force microscopy (AFM), room temperature photoluminescence (PL), Raman spectroscopy and optical reflectance measurements. AFM imaging reveals that the porous GaAs layer is consisted of a pillar-like of few nm in width distributed between more-reduced size nanostructures. In addition to the “infrared” PL band of un-etched GaAs, a strong “green” PL band is observed in the etched sample. The broad visible PL band of a high-energy (3.82 eV) excitation is found to compose of two PL band attributed to excitons confinement in two different sizes distribution of GaAs nanocrystals. The quantum confinement effects in GaAs nanocrystallites is also evidenced from Raman spectroscopy through the pronounced appearance of the transverse optical (TO) phonon line in the spectra of the porous sample. Porosity-induced a significant reduction of the specular reflection, in the spectral range (400–800 nm), is also demonstrated.     

Size-dependent photo-induced shift of the first exciton band in CdTe quantum dots in glass prepared by a two-stage heat-treatment process

CdTe nanocrystals were grown from commercially available RG850 Schott filter glass by two-step heat-treatment process which almost doubles the particle to matrix volume fraction. A calculation shows that a quantized-state effective mass model in the strong confinement regime might be used to deduce the average radius for the nanocrystals larger than 2 nm in radius from the energetic position of the first exciton peak in optical absorption spectrum. Size-induced shift of ∼360 meV in the first exciton peak position was observed. The steady state photoluminescence spectra exhibit a broad band red shifted relative to the first exciton band, which indicates the existence of shallow trap states. The non-linear optical properties of CdTe nanocrystals were studied by room temperature resonant photoabsorption spectroscopy. The differential absorption spectra had three-lobed structure whose size-dependent evolution was explained by bleaching of the absorption, red shift and broadening in the Gaussian absorption band used to fit the first exciton peak. A maximum red shift of 2.32 meV for the average nanocrystal radius of 4.65 nm was estimated by fitting the photomodulation spectra with a combination of first and second derivative Gaussian absorption bands. We presume that the red shift is induced by the electric field of trapped charges in surface states. Internal electric field strengths of 23 and 65 kV/cm were predicted for the average nanocrystal radii of 3.95 and 4.65 nm, respectively, with the help of second-order perturbation theory in the strong confinement limit.     

Controlled synthesis, characterization and optical properties of CdS nanocrystalline thin films via chemical bath deposition (CBD) route

In this study, the CdS nanocrystalline thin films obtained from an ammonia-free chemical bath deposition process. The crystallites with a size range of 10–20 nm in diameter with zinc blend (cubic) and wurtzite (hexagonal) crystal structure and strong photoluminescence were prepared from the mixture solutions of: cadmium chloride dihydrate as a cadmium source, thiourea as a sulfur source and sodium citrate dihydrate as a complexing agent for cadmium ions. The well-cleaned glass used as a substrate for thin films deposition. The obtained samples were characterized by the techniques such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), atomic force microscopy (AFM) and fluorescence spectroscopy. Also, the effect of two parameters such as pH and temperature of reaction on the synthesis of CdS nanocrystals was studied. Finally, it was found the CdS nanocrystals showed sharp excitation features and strong “band-edge” emission.     

Raman properties embeddedof GaSb nanoparticlesin SiO2 films

The Raman shifts of nanocrystalline GaSb excited by an Ar^ ion laser at wavelengths 514.5, 496.5, 488.0, 476.5,and 457.9nm are studied by an SPEX-1403 laser Raman spectrometer respectively, and they are explained by phonon confinement, tensile stress, resonant Raman scattering and quantum size effects. The Stokes and anti-Stokes Raman spectra of GaSb nanocrystals strongly support the Raman feature of GaSb nanocrystals. The calculated optical spectra compare well with experimental data on Raman scattering GaSb nanocrystals.     

Electrical and optical properties of colloidal semiconductor nanocrystals in aqueous environments

Microscale and larger semiconductor crystals have electronic and optical properties that depend on their bulk band structures. When these crystals are reduced into the nanoscale, they enter a new regime in which the electrical and optical properties are no longer influenced solely by their bulk band structures, but are influenced by the crystallite size and shape. In this paper, dimensional confinement and proximity phenomena are examined for colloidal semiconductor nanocrystals in several cases of practical importance. Specifically, we determine the effective binding potentials of selected quantum dots in aqueous environments in various colloidal semiconductor nanocrystals and correlate them with experimentally obtained absorption spectra. We also study fluorescence resonance energy transfer (FRET) between semiconductor crystals connected by short peptide chains as well as the shift in photoluminescence spectra of CdTe nanowires made from a chain of CdTe quantum dots.     

Auger relaxation processes in semiconductor nanocrystals and quantum wells

Auger recombination rates in mesoscopic semiconductor structures have been studied as a function of energy band parameters and heterostructure size. It is shown that nonthreshold Auger processes stimulated by the presence of heteroboundaries become the dominant nonradiative recombination channel in nanometer size semiconductor structures. The size dependence of luminescence quantum yields in nanostructures and microcrystals are discussed. Auger-like collisions of electrons and heavy holes are shown to serve as “accelerators” of thermalization processes in semiconductor quantum dots.