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 the… Show more
“…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.…”
Section: Theory and Comparison With Experimentsmentioning
confidence: 99%
“…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.…”
Section: Introductionmentioning
confidence: 99%
“…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…”
Optical phonon modes, confined in CdS x Se 1−x nanocrystal (NC) quantum dots (≈2 nm in radius) grown in a glass matrix by the melting-nucleation method, were studied by resonant Raman scattering (RRS) spectroscopy and theoretical modeling. The formation of nanocrystalline quantum dots (QDs) is evidenced by the observation of absorption peaks and theoretically expected resonance bands in the RRS excitation spectra. This system, a ternary alloy, offers the possibility to investigate the interplay between the effects of phonon localization by disorder and phonon confinement by the NC/matrix interface. Based on the concept of propagating optical phonons, which is accepted for two-mode pseudo-binary alloys in their bulk form, we extended the continuous lattice dynamics model, which has successfully been used for nearly spherical NCs of binary materials, to the present case. After determining the alloy composition for NCs (that was evaluated with only 2-3% uncertainty using the bulk longitudinal optical phonon wavenumbers) and the NC size (using atomic force microscopy and optical absorption data), the experimental RRS spectra were described rather well by this theory, including the line shape and polarization dependence of the scattering intensity. Even though the presence of a compressive strain in the NCs (introduced by the matrix) masks the expected downward shift owing to the phonons' spatial quantization, the asymmetric broadening of both Raman peaks is similar to that characteristic of NCs of pure binary materials. Although with some caution, we suggest that both CdSe-like and CdS-like optical phonon modes indeed are propagating within the NC size unless the alloy is considerably heterogeneous.
“…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.…”
Section: Theory and Comparison With Experimentsmentioning
confidence: 99%
“…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.…”
Section: Introductionmentioning
confidence: 99%
“…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…”
Optical phonon modes, confined in CdS x Se 1−x nanocrystal (NC) quantum dots (≈2 nm in radius) grown in a glass matrix by the melting-nucleation method, were studied by resonant Raman scattering (RRS) spectroscopy and theoretical modeling. The formation of nanocrystalline quantum dots (QDs) is evidenced by the observation of absorption peaks and theoretically expected resonance bands in the RRS excitation spectra. This system, a ternary alloy, offers the possibility to investigate the interplay between the effects of phonon localization by disorder and phonon confinement by the NC/matrix interface. Based on the concept of propagating optical phonons, which is accepted for two-mode pseudo-binary alloys in their bulk form, we extended the continuous lattice dynamics model, which has successfully been used for nearly spherical NCs of binary materials, to the present case. After determining the alloy composition for NCs (that was evaluated with only 2-3% uncertainty using the bulk longitudinal optical phonon wavenumbers) and the NC size (using atomic force microscopy and optical absorption data), the experimental RRS spectra were described rather well by this theory, including the line shape and polarization dependence of the scattering intensity. Even though the presence of a compressive strain in the NCs (introduced by the matrix) masks the expected downward shift owing to the phonons' spatial quantization, the asymmetric broadening of both Raman peaks is similar to that characteristic of NCs of pure binary materials. Although with some caution, we suggest that both CdSe-like and CdS-like optical phonon modes indeed are propagating within the NC size unless the alloy is considerably heterogeneous.
“…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).…”
The effects of phonon confinement and exciton−phonon coupling (EPC) in three-dimensional (3D) assemblies of CdSe quantum dots with zincblende structure, synthesized by colloidal chemical and sonochemical routes as well as in postdeposition thermally treated samples, were studied by resonant Raman scattering (RRS) technique. Combining RRS results with those from structural analysis based on X-ray diffraction and optical spectroscopy, insights into the factors determining the intricate trends in the positions of bands due to firstand higher-order vibrational transitions upon crystal size reduction were gained. The band shifts with respect to the bulk material were found to result from subtle balance between phonon confinement effects and lattice contraction due to internal strain. Accurate assignments of the surface optical (SO) phonon modes were made on the basis of the dielectric continuum model of Ruppin and Englman. The magnitude of EPC (measured, e.g., through the Huang−Rhys parameter) and its size dependence in the synthesized 3D QD assemblies were studied as well. It was found that the EPC in the studied low-dimensional material appears to be much weaker than in the case of the corresponding bulk specimen, but a decreasing trend was observed upon postdeposition thermal annealing treatment, which is accompanied by an average crystal size increase. In strongly confined nanocrystal assemblies, it is larger than in those with weaker confinement effects. The relative importance of the two EPC mechanisms, the long-range one (Froḧlich interaction) vs the short-range one (due to the optical deformation potential) was discussed and related to the appearance of bands due to transverse optical (TO) phonon modes in the studied material. Combination modes involving acoustic phonons and the 1LO mode were observed for the first time in 3D QD assembly deposited in thin film form.
This work gives the evidence of the lattice contraction in CdSe nanocrystals (NCs) grown in a glass matrix. The CdSe NCs were investigated by atomic force microscopy (AFM), optical absorption (OA), and Raman spectroscopy. The average size of CdSe NCs can be estimated by AFM images. Using the OA spectra and the effective-mass approximation, it was also possible to estimate the average sizes of CdSe NCs, which agree very well with the AFM data. These results showed that the CdSe NCs grow with increasing time of heat treatment. The blue shift of the longitudinal optical (LO) modes and surface optical (SO) phonon modes with an increase in the average radius of the NCs, shown in the Raman spectra, was explained by the lattice contraction in CdSe NCs caused by thermodynamic interactions at the interface with the host glass matrix.
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