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Raman analysis of the quantum dot morphology

A theoretical and experimental study of the spatial confinement of phonons in ternary CdSxSe1−x nanocrystals embedded in a glass matrix is reported. We discuss the morphological characteristics of the nanocrystals by analyzing the dependence of surface phonon modes on the geometrical parameters. The calculated frequencies are compared with experimental Raman spectra. A good theoretical and experimental correlation for 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. In this work, we explore theoretically and experimentally the connection between phonon surface modes (SOM) and the geometry of a semiconductor quantum dot (QD). By analyzing the behavior of SOM in ternary alloy based QD’s, we may determine the composition fluctuation and the geometrical parameter r, that measures the sphericity degree of deviation. To do this, we have extended the Comas et al. [1] model for binary spheroidal QDs to study ternary QD alloys of A 1−x A 2 B, and calculated the dependence of the A 1 B-like and A 2 B-like SO-modes as a function of x and r. Samples were prepared over SiO2-Na2CO3-B2O3-Al2O3 doped with CdO, S and Se. The mixture was melted in an aluminum crucible at 1200 ◦ C, for 2 h. 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 ta=3, 5, 6, 10, 12, 14, 16, 18, 20, and 22 h, respectively. As a result of these thermal treatments, the QD’s of CdSxSe1−x are formed over the glass matrix. The QD Raman spectra were recorded at room temperature. An argon-ion laser operating at 100 mW with the line λ = 514 nm was collected in back scattering Raman geometry. From a theoretical point of view, the quantities involved in the description of polar-optical phonons are derived from e(ω)∇ 2 ϕ =0 , where ϕ is the electrostatic phonon potential. For ternary alloys, e(ω) can be calculated as [2] e(ω )= e∞ + X1/(ω 2