Anabela G. Rolo

Growth and characterisation of cadmium sulphide nanocrystals embedded in silicon dioxide films

Abstract In this communication we report on the growth and the optical characterisation of CdS quantum dots in the diameter range of 4–10 nm embedded in silicon dioxide glass films grown by magnetron rf-sputtering technique with post-deposition annealing. Optical transmission measurements display a marked blue shift of the absorption band edge due to the quantum confinement of the electrons and holes in the CdS nanocrystals. The mean diameter of the nanocrystals was estimated from absorption spectra using a standard theoretical model. A study of the effect of the nanocrystal size on the Raman spectra has been made from an analysis of the first order LO mode lineshape. The increase in the linewidth observed with decreasing crystal size is in agreement with the results of a calculation based on the phonon confinement model. The asymmetric lineshape is well reproduced by the model, but they fail to reproduce the large low-frequency tail. The origin of this tail is discussed considering several possible sources proposed in the literature and is under study. Room temperature infrared reflectance measurements have also been obtained showing a diffuse band at about 230–300 cm−1, which closely corresponds to CdS optical polar phonons observed in the bulk crystals. However, we are unable to fit the observed spectra by assuming just this contribution. Instead, by including an additional contribution due to CdO we are able to well simulate the experimental spectra using a model based on the Maxwell-Garnet theory.

FIR absorption in CdSe quantum dot ensembles

Absorption of far-infrared (FIR) radiation in very small CdSe crystallites (quantum dots, QDs) via excitation of confined dipolar vibrational modes was studied, both theoretically and experimentally. Spatial quantisation of optical phonons was analysed in the framework of a continuum model, which couples the phonon amplitude to the electrostatic potential via the Frohlich interaction. The frequencies and oscillator strengths of the dipole-active optical vibration modes are presented calculated for a spherical QD, and for the first time for II-VI QDs these quantised modes were observed experimentally. Another effect studied in this work is the influence of the QD concentration on the FIR absorption of their ensemble. It is shown that the dipole-dipole interaction becomes very important in dense QD ensembles and leads to smearing of the structure in the CdSe phonon band as the QD concentration increases.

Third-Order Optical Nonlinearities in Thin Films of CdS Nanocrystals

The non-resonant nonlinear optical properties of CdS nanocrystals (NCs) embedded in silica (i) and forming matrix-free (MF) almost close-packed films (ii) were studied using the Z-scan technique. Among the group (i), samples containing NCs covered with a HgS and CuS shell were investigated. The highest value of the nonlinear refractive index n2 1⁄4 1.85 10– 6 cm2/W was measured for the MF films. This number exceeds by far the value typical for bulk CdS and cannot be explained in terms of thermally induced nonlinearity. Strong enhancement of local fluctuations of the electro-magnetic field in MF films where NCs form aggregates or clusters is suggested to explain this result. Within the group (ii), an enhancement of the nonlinearity was observed under pulsed irradiation for both CdS/HgS and CdS/CuS nanoparticles, compared to CdS NCs without a shell.

One-phonon Raman scattering from arrays of semiconductor nano-crystals

Abstract We re-consider Raman scattering by LO-type phonons in a small semiconductor crystal and derive a simple formula taking into account the higher order confined phonon modes, to describe the asymmetric Raman lineshapes observed experimentally. For a single crystalline sphere, we explicitly calculate the relative contributions of the confined modes with different values of the radial quantum number. For arrays of such spheres, we argue that, apart from a superposition of the above contributions from individual scatterers, there exists a collective scattering by a coherent mode which involves many spheres, unless their concentration is too small. This collective scattering is determined by an effective dielectric function of the composite medium and peaks near the Frohlich frequency. We apply these principles to the analysis and modelling of Raman spectra taken from CdS doped SiO2 films.

Confined Optical Vibrations in CdTe Quantum Dots and Clusters

Absorption of far-infrared (FIR) radiation and Raman scattering in CdTe nano-crystals (spherical quantum dots (QDs) of several nanometers in size) and Cd 54 Te 32 (SCH 2 CH 2 OH) 8- 32 clusters prepared by colloidal synthesis were studied. Raman spectra of both QDs and clusters are dominated by a peak situated slightly below the bulk CdTe LO phonon frequency. Experimental FIR transmittance spectra of films containing CdTe clusters show a broad band between the bulk TO and LO phonon frequencies, while there is only a single dip at the Frohlich frequency observed for larger QDs. These observations are in agreement with the results of our numerical lattice dynamics calculations and previously published theoretical results based on a continuous model. It is suggested that the lattice dynamics of very small clusters is similar to that of large crystallites in the sense that there still exists the dispersion of optical vibration frequencies within the LO-TO band of bulk material.

Structural and optical studies of CdS nanocrystals embedded in silicon dioxide films

Silicon dioxide films doped with nanocrystallites of CdS with a mean radius value in the range of 20 to 40 A have been grown using the magnetron rf-sputtering technique. We have studied the combined effects of rf-power deposition, the number of semiconductor chips on the SiO target, and post-annealing temperature on the CdS crystal size. The optical transmission spectra display a marked blue shift of 2 the absorption band edge of up to 400 meV which was attributed to quantum size confinement effects. The luminescence spectra show a narrow emission peak tentatively explained as being due to the recombination of electrons on shallow donor levels with confined valence band holes. q 1998 Elsevier Science S.A.

Structural properties of Ge nano-crystals embedded in SiO2 films from X-ray diffraction and Raman spectroscopy

Abstract SiO 2 films containing small particles of Ge were grown using the r.f.-magnetron sputtering technique. The films were studied by means of X-ray diffraction (XRD) and Raman spectroscopy. XRD studies revealed diamond structure of Ge particles in the films grown at temperatures higher than 500°C. The dependence of the average size of Ge nanocrystals, determined by fitting the X-ray spectra (13–63 A), on the substrate temperature, r.f.-power and the fraction of semiconductor in the target was determined. For higher-temperature grown films containing crystalline Ge particles, a pronounced peak due to confined optical phonons was observed in Raman spectra, while for those grown at lower temperature, there is just a broad band seen below 300 cm −1 . A theoretical model is applied to describe the contribution of optical phonons confined in small Ge spheres.

HRTEM and GIXRD studies of CdS nanocrystals embedded in Al2O3 films produced by magnetron RF-sputtering

In this paper we report on the structural properties of as-grown CdS nanoparticles embedded in Al2O3 films produced by a magnetron RF-sputtering technique. Grazing incidence X-ray diffraction together with high-resolution transmission electron microscopy (HRTEM) and electron diffraction were used to study the crystallinity and morphology of the CdS nanocrystals. Depending on the deposition parameters, elongated or spherical nanocrystals were grown. HRTEM shows evidence of the growth of CdS nanocrystals at room temperature with sizes in the range of 3–8 nm, and indicates that the nanocrystals formed in the cubic phase during the early stages of the deposition process. Stress-free films were formed under selected deposition conditions. r 2002 Elsevier Science B.V. All rights reserved.

Ferroelectric characterization of aligned barium titanate nanofibres

We report the synthesis, structural and ferroelectric characterization of continuous well-aligned nanofibres of barium titanate produced by the electrospinning technique. The fibres with average diameter of 150–400 nm consist of connected nanoparticles of BaTiO3 stacked together to form the shape of a long filament. The tetragonal phase in the obtained nanofibres was revealed by the x-ray diffraction and Raman spectroscopy and has been also confirmed by the second harmonic generation (SHG) and piezoresponse force microscopy (PFM). The temperature dependence of the SHG in the vicinity of the paraelectric–ferroelectric phase transition suggests that barium titanate nanofibres are indeed ferroelectric with an apparent glass-like state caused by metastable polar nanoregions. The existence of domain structure and local switching studied by PFM present clear evidence of the polar phase at room temperature.

Low-temperature fabrication of layered self- organized Ge clusters by RF-sputtering

In this article, we present an investigation of (Ge + SiO2)/SiO2 multilayers deposited by magnetron sputtering and subsequently annealed at different temperatures. The structural properties were investigated by transmission electron microscopy, grazing incidence small angles X-ray scattering, Rutherford backscattering spectrometry, Raman, and X-ray photoelectron spectroscopies. We show a formation of self-assembled Ge clusters during the deposition at 250°C. The clusters are ordered in a three-dimensional lattice, and they have very small sizes (about 3 nm) and narrow size distribution. The crystallization of the clusters was achieved at annealing temperature of 700°C.