In this study we present modifications in a phonon confinement
model in order to obtain a better description for the Raman spectra
of spherical nanocrystals, namely: bare-core, core–shell, and
core–multishell. Our new interpretations allow investigating
the influences of the interfacial alloying and strain effects on the
vibrational spectra of core–shell nanocrystals. The robustness
of the modified phonon confinement model was confirmed by precisely
describing the Raman spectra of wurtzite CdSe/CdS core–shell
magic-sized quantum dots (CS-MSQDs) synthesized directly in aqueous
solution by a new route. The CdSe MSQD sample was used as template
to growth CdSe/CdS CS-MSQDs with different shell thickness by setting
the synthesis temperature. By using our modified model to fit the
Raman spectra of samples, we have obtained the size dimensions of
CS-MSQDs (core-diameter and shell-thickness), in excellent agreement
with the values obtained by the atomic force microscopy results, confirming
that the change in the synthesis temperature is a simple and efficient
way to control the CdS-shell thickness during the growth process.
Furthermore, we have confirmed the formation of an alloy layer (CdS
x
Se1‑x
) at the
interface of these CdSe/CdS CS-MSQDs and that the strain effects can
be neglected for the wurtzite structure.
Hexagonal and cubic GaN layers are grown on (001) GaAs substrates by means of molecular beam epitaxy. First order Raman spectra are taken from these layers at various incident laser wavelengths and temperatures. The T2 transverse-optical (TO) and longitudinal-optical (LO) frequencies of cubic GaN are determined, as well as the A1 TO and LO, E1 TO, and E2 frequencies of hexagonal GaN. The T2 TO frequency of cubic GaN lies between the A1 and E1 TO frequencies of hexagonal GaN as one expects comparing the lattice dynamics of zincblende and wurtzite type crystals. The T2 TO frequency is close to the calculated value but disagrees with a recently reported experimental value. For the hexagonal layer, all frequencies are close to those previously measured. A broad Raman structure below the A1 LO peak is interpreted in terms of a disturbed long range order of the hexagonal layer.
In this study, we report on how surface-passivated and nonpassivated cobalt ferrite nanoparticles (8 nm diameter), suspended as ionic magnetic fluids and aged under low pH conditions, revealed different behavior as far as the time evolution of the iron/cobalt cation distribution, crystal quality, coercivity, and saturation magnetization are concerned. Different techniques were used to perform a detailed study regarding the chemical stability, structural stability, and surface and magnetic properties of the suspended nanoparticles as a function of the aging time. Properties of surface-passivated and nonpassivated nanoparticles were investigated by transmission electron microscopy, X-ray diffraction, atomic absorption spectrometry, magnetic measurements, Raman spectroscopy, and Mössbauer spectroscopy. Our data showed that the employed nanoparticle surface passivation process, besides the formation of an iron-rich surface layer, modifies the nanoparticle core as well, improving the crystal quality while modifying the Fe/Co cation distribution and the nanoparticle dissolution rate profile. Magnetic data showed that the saturation magnetization increases for surface-passivated nanoparticles in comparison to the nonpassivated ones, though coercivity decreases after passivation. These two observations were associated to changes in the cation distribution among the available tetrahedral and octahedral sites.
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