Impact of disorder on optical phonons confined in CdS nano-crystallites embedded in a SiO₂matrix

Abstract

Non-resonant Raman spectroscopy studies of a set of CdS films annealed at different temperatures were performed and showed a direct correlation between the width of the Raman peak produced by CdS-like optical phonons and the crystalline quality of the semiconductor phase probed by x-ray diffraction (XRD) and transmission electron microscopy (TEM). In order to decribe the Raman lineshape a model proposed by Trallero-Giner et al (1998 Phys. Rev. B 57 4664) was used, which considers optical phonons confined in small semiconductor spheres with a size distribution. The model is shown to give a good reproduction of the spectra of samples where the semiconductor phase is most crystalline. However, it required too large values of phonon damping to fit the spectra of several other samples, which, according to XRD and TEM data, do contain CdS nano-crystallites. This large broadening of the Raman peak was considered as inhomogeneous, i.e. associated with disorder. Numerical lattice dynamics calculations were performed for 2D binary clusters of arbitrary shape and three kinds of disorder were considered, (i) random variation of the Cd-S bond frequency from one nano-crystallite to another, (ii) cluster shape irregularities and (iii) fluctuations of the nearest-neighbour interaction constant within one cluster. It is shown that 'ensemble disorder' (i) can be responsible for a shoulder above the bulk CdS phonon frequency observed for some of our samples. The effect of shape disorder (ii) is similar to that of the size dispersion producing some inhomogeneous broadening of the peak. In addition, it gives rise to an extra low-frequency mode originating from the top of the acoustic band. The force constant's disorder (iii) is shown to result in a stronger asymmetric broadening of the Raman peak.

Quantum phenomena of ZnSe nanocrystals prepared by electron beam evaporation technique

“…Similar red-shift in bandgap energy, with increasing crystallites size has been reported for chemically deposited ZnSe thin films. Previous researchers [56] have reported two band gaps for chemically deposited ZnSe due to spin-orbit interaction. This red shift in the bandgap values with increasing particle size can also be predicted by the three dimensional quantum confinement model based on effective mass approximation ΔE g = E g eff -h , where E g eff is the effective band gap energy, E g is the bulk band gap energy, R is the nanoparticle radius, h is the Planck constant, μ is the reduced effective mass and m e * and m h * are the effective masses of electron and hole respectively [57].…”

Raman spectroscopy of optical phonon confinement in nanostructured materials

“…Subsequently, Roca et al 31 presented a rigorous continuum theory, which took into account the coupling between the vibrational amplitude and electrostatic potential to obtain the optical vibrational modes in a spherical quantum dot, and applied it to several systems. 32,33 Microscopic lattice dynamical calculations for nanoparticles containing up to a few thousand atoms have also been carried out and the changes in the phonon spectra as compared to the bulk have been discussed. 34 -36 This section briefly reviews all three approaches.…”

“…The theoretical spectra have been compared with those of CdSe dispersed in glass 32 and CdS embedded in fused quartz. 33 The fitting was found satisfactory only when the intrinsic linewidth of the phonon  was assumed to increase as the particle size was reduced. 32 Furthermore, the origin of additional intensity lying above the theoretical curve in the wings on the low-wavenumber side is not well understood.…”

Effect of grain size and hydrogen passivation on the electrical properties of nanocrystalline silicon films

“…Nevertheless, it is possible to obtain reasonably good fits to the nc-Si peak using the model explained in Ref. [11] (taking into account only the short-range part of the electronphonon interaction) as shown in Fig. 2.…”

“…It is perhaps less obvious that strong confinement models can apply to systems like nc-Si/a-Si where barriers between the crystalline core and the matrix are random, however, the sufficiently large difference that exists between the peaks of the phonon densities of states for c-Si (521 cm -1 ) and a-Si (480 cm -1 , see [10] for recent calculated results) makes it reasonable to assume that the relative displacement of two Si atoms vanishes at the NC matrix interface, at least for the first quantified phonon modes. Considering the perfect confinement of the optical phonon in a spherical shape 1 NC, the mean diameter can be evaluated from the relation [9,11], is too small to be reliably determined from the experimental spectra and the use of Eq. (1) is hardly possible.…”