| Summary: | 博士 === 國立中正大學 === 物理系 === 99 === In the first part of this thesis, I will present a study of the vibrational modes of acoustic
phonons and their corresponding eigenfrequencies in CdS/CdSe/CdS quantum-dot
quantum wells (QDQWs) obtained on the continuum model. The energy spectra of
the phonons in nanocrystals from the analytic solutions are checked by the Finite-
Element Method (FEM). Based on the spectrum of acoustic phonons and the Debye
model, the temperature dependences of the specific heat contributed from lattice
phonons are calculated to investigate their size-dependent effects. Lattice softening
is also demonstrated and the results qualitatively agree with the experimental observations
for fine particles and quantum dots. We found that the phonon density of
states of a QDQW is important for calculating specific heat, and, perhaps, also for
modifying the effective sound velocity in the nanocrystal.
In the second part of the thesis, I will describe an investigation of the Raman
light-to-vibration coupling coefficients Cαβ of the l=0 and the l=2 spheroidal phonon
modes of quasi-free spherical CdSe/CdS core/shell nanoparticles calculated. Based on
the Lamb model, the displacement vectors of acoustic phonon modes are obtained and
the Cαβ is also derived. The Raman scattering from quasi-free CdSe/CdS nanoparticles
with various inner radii is investigated. For the l=0 acoustic modes, the bond
polarizability model is adopted to calculate Cαβ, whose peak positions shift toward
lower frequencies with the increase of the inner radius. This could be accounted for
by the decrease of the averaged longitudinal and transverse sound velocities. Moreover,
the ratio of the coefficients Aαβγδ [Montagna and Dusi, Phys. Rev. B 52, 10080
(1995)] between layers characterizes behaviors of peak heights of Cαβ. For the l=2
modes based on the dipole-induced-dipole model, the behaviors of peak positions are
obtained by varying the values of vL and vT of materials in both layers. Because we
treat the core/shell nanoparticle as a whole, the behavior of Cαβ peak positions on a
CdSe/CdS core/shell nanoparticle is consistent with its dependence on the averaged
sound velocities of the whole nanoparticle. At the same time, it also agrees with the
calculated results for a CdSxSe1¡x nanoparticle [Risti´c et al., J. Appl. Phys. 104,
073519 (2008)]. However, we observed that some peaks reach dramatically high values
for given inner radii of the CdSe/CdS nanoparticles, which occur only in spherical
core/shell nanoparticles.
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