Surface phonons modes: a tool to determine the quantum dot morphology

Abstract

We report here theoretical and experimental studies on the spatial confinement of phonons in ternary CdS x Se 1−x nanocrystals embedded in a glass matrix formed by the composites SiO 2 -Na 2 CO 3 -B 2 O 3 -Al 2 O 3 doped with CdO, S and Se. We determined the morphologic characteristics of the nanocrystals by analyzing the dependence of surface phonon modes on the geometrical parameters. The calculated frequencies are compared with values from Raman spectra of CdS x Se 1−x nanocrystals grown under different thermal treatments. A good correlation between experimental and calculated CdS-like and CdSe-like surface optical modes is observed. Raman selection rules and their connection with the nature of the surface optical phonons is discussed.Keywords: Quantum dots; Raman scatteringThe quantum confinement effects on the optical properties of nanocrystallites or quantum dots (QD's) grown over semiconductor-doped glasses are subject of current researches. The interest in these materials arises from their great potential for use in device applications due to their nonlinear optical properties. Experimental and theoretical studies showing the confinement effects on the QD electronic states and their energy levels are now well established. Nonetheless, the size effects on their phonon modes and on the electron-phonon interaction are still somewhat controversial. In this theoretical scenario, one aspect has not yet been sufficiently addressed: how the geometrical shape effects can modify the electronic structure and other physical properties in small QD's. In II-VI QD's, the morphological effects can modify the energies, symmetry properties of the wave functions and, thus, their optical transitions.In this work, we explore theoretically and experimentally the connection between the surface optical (SO) phonon modes and the geometrical shape of a semiconductor QD. By analyzing the behavior of SO modes in ternary alloy based QD's, we may determine the alloy composition fluctuation and the geometrical parameter r, that measures the degree of deviation from sphericity. To do this, we have extended the Comas et al. [1] model for binary spheroidal dots to study ternary alloys of A 1 1−x A 2 x B QD type. Thus, we calculated the dependence of the A 1 B-like and A 2 B-like SO-modes as a function of x and r. Our extended theory recovers the binary compound limits x → 0 and x → 1 of Ref.[1] and verifies the conditions ω TO < ω SO < ω LO .Samples were prepared over the glass matrix SiO 2 -Na 2 CO 3 -B 2 O 3 -Al 2 O 3 doped with CdO, S and Se. The mixture was melted in an aluminum crucible at 1200 • C, for 2h. Then, it was quickly cooled down to room temperature. In order to enhance the diffusion of the Cd 2+ , S 2− , and Se 2− ions, we have performed 600 • C thermal treatments on each samples using different annealing times of t a =3, 5, 6, 10, 12, 14, 16, 18, 20, and 22 h, respectively. As a result of these thermal treatments, the QD's of CdS x Se 1−x are formed over the glass matrix. The QD Raman spectra were recorded at room tempe...

Resonant Raman scattering in CdSxSe1−xnanocrystals: effects of phonon confinement, composition, and elastic strain

“…Modes with angular momentum number l p = 2 are supposed to be Raman active in a spherical particle from the well‐known symmetry arguments 25. Usually they were considered in the modeling of the Raman spectra of nearly spherical nanoparticles 7, 11–13. Let us note that Eqn (8) takes into account only electrostatic confinement but not the mechanical boundary conditions.…”

“…As known from the broad literature devoted to Al x Ga 1− x As nanostructures, important effects like the surface segregation in pseudo‐binary alloys can be probed by studying their phonon‐related properties by means of far‐infrared (FIR) and Raman spectroscopies 9, 10. Raman studies of CdS x Se 1− x NCs have been performed before,7, 11–15 demonstrating the usefulness of Raman spectroscopy for the determination of the alloy composition in NCs. However, most of these studies were performed on relatively large nanoparticles (NC radius > 3 nm) where the contribution of optical phonons with significant nonzero wavenumbers is not readily seen in the Raman spectra.…”

“…In addition, there are demands for the synthesis of semiconductor nanoparticles buried in robust, transparent matrices such as silica 5. The traditional melting‐nucleation method emerges as a suitable approach, as it allows the fabrication of CdS x Se 1− x NCs with controllable size and Se fraction inside borosilicate glass matrices 6, 7…”

Phonon Confinement and Related Effects in Three-Dimensional Assemblies of Cubic Cadmium Selenide Quantum Dots Synthesized by Conventional Chemical and Sonochemical Routes

“…Such issues range from pretty “basic” questions, such as the very direction of the shift of a particular nanocrystal vibrational mode with the change in the QD size, to the question of evolution of the exciton–phonon coupling strength with QD size. − In the latest period, however, it seems that the interest in issues related to phonon spectra of ultrasmall semiconductor nanocrystals has been renewed, due to the apparent “phonon bottleneck” problem ,,, of the mechanism of charge carrier relaxation in these low-dimensional systems. Most of the studies within this research area have been devoted either to colloidal nanocrystals dispersed in liquids (see refs − and references therein), nanocrystals embedded in glass or other (usually isolating) matrices (see refs − and references therein), and nanocrystalline powders, etc. (see refs − and references therein).…”