Phonons In CdS Research Articles

Resonance Raman excitation profiles of CdS in pure CdS and CdSe/CdS core/shell quantum dots: CdS-localized excitons.

Resonance Raman excitation profiles have been measured for the longitudinal optical phonon in two sizes of CdS quantum dots and in CdSe/CdS core/shell quantum dots. In pure CdS, the resonance Raman cross section for the fundamental transition is sharply peaked around the lowest-energy excitonic transition and becomes weaker with higher-energy excitation even though the absorbance continues to increase to higher energies. This effect, also observed in earlier studies of CdSe quantum dots, is attributed largely to interferences among multiple excitonic transitions contributing to the resonance Raman polarizability. No variation in exciton-phonon coupling strength with exciton energy is required to explain the decrease in the ratio of the Raman cross section to absorption cross section at shorter wavelengths. In core/shell structures, the CdSe phonons are relatively strong when exciting on resonance with the lower excitonic transitions, in which the hole is largely localized to the CdSe core, but become nearly undetectable at excitation wavelengths >0.6 eV above the lowest exciton, where both electrons and holes are largely localized in the CdS shell. The CdS phonon Raman cross section exhibits a maximum 0.6-0.7 eV above the lowest exciton and then decreases at higher energies for the same reasons as in pure CdS.

Surface-enhanced Raman scattering by semiconductor nanostructures

Surface-enhanced Raman scattering by optical and surface phonons in CdS, GaN, and CuS nanocrystals, and AlN nanowires is detected and studied. It is found that the presence of metal (Ag, Au, and Pt) nanoclusters noticeably modifies the Raman spectra of the nanostructures and results in a resonant increase in the intensity of optical phonon modes in CdS and CuS nanocrystals or in the emergence of surface modes in GaN nanocrystals and AlN nanowires. It is shown that the frequencies of the surface optical phonon modes of the examined nanostructures are in good agreement with the theoretical values calculated within the framework of the dielectric continuum model.

Optical phonon modes in ordered core-shell CdSe/CdS nanorod arrays

We investigate the optical phonons in ``dot-in-a-rod'' core--shell CdSe/CdS nanorods at wavelengths resonant with the optical transitions either in the core or in the shell. At a wavelength above the CdS band gap, only CdS phonon modes were detected. In contrast, at excitation in resonance with the core transitions, we observed phonon modes of both the CdSe core and the CdS shell. In laterally ordered nanorod assemblies, the CdS longitudinal-optical phonon mode manifested a low-energy shoulder that could be related to higher-order longitudinal-optical phonon modes. Furthermore, we report on surface-optical modes that originate from the tracklike superstructure of the nanorod assemblies.

Pressure dependence of the optical and vibrational properties of hexagonal and cubic CdS films grown on GaAs

AbstractThe pressure dependence of the excitonic photoluminescence and Raman scattering in both hexagonal and cubic CdS films grown on GaAs substrates is reported. We found that the pressure coefficients of the cubic CdS films are equal to those of the hexagonal films within experimental uncertainties. The main difference between their optical and vibrational properties seems to lie in their exciton energies, the hexagonal phase having a slightly higher energy than the cubic phase. Strong enhancement in multi‐longitudinal optical (LO) phonon scattering has been observed by “pressure‐tuning” the exciton energy to resonate with the excitation laser line. As a result we have been able to determine the pressure coefficients of the higher order LO phonons in CdS for modes as high as 5LO. We found that the pressure coefficients of these higher order LO modes when normalized by the phonon frequency exhibit a systematic decrease with the order of the mode. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Surface-enhanced Raman spectroscopy of cadmium sulfide/cadmium selenide superlattices formed on gold by electrochemical atomic-layer epitaxy

The phonon properties of ultrathin CdS/CdSe superlattice films formed on gold by electrochemical atomic-layer epitaxy are characterized by means of surface-enhanced Raman spectroscopy (SERS). Substantial (15–25 cm −1) red-shifts in the CdS phonon frequencies are observed, whereas the CdSe frequencies are essentially unaltered, indicating that substantial crystallographic strain occurs in the former, but not the latter, superlattice component. The findings demonstrate the virtues of SERS for exploring the structure of such solid–solid interfaces with unique monolayer-level sensitivity.

Determination of the dilation and vibrational contributions to the temperature shift of excitons in CdS: melting of semiconductors

An empirical method was used to separate the temperature shift curves of the A- and B-exciton energies in CdS. It was found that the lattice dilation contributed an amount −2.3 × 10−5 eV/K to the temperature shift, and the frequencies of the terms representing the electron–phonon interaction were found to correspond to the average frequencies of the phonons in CdS. The data on the semiconductors analyzed up to now by the empirical method (C, Si, Ge, CdS) were examined at the melting point temperatures of the materials and it was found that the combined results could be described by simple equations. It was found that the r.m.s. vibrational amplitude of the atoms in the above four materials was nearly identical at their melting temperatures.

Resonant Brillouin Scattering by LA Phonons in CdS

Resonant Brillouin scattering by longitudinal acoustic phonons has been observed for the first time in CdS by using mode converted phonons generated by partial reflection of acoustoelectric domains. Analysis based upon resonant Brillouin scattering theory and piezobirefringence theory shows a good agreement with the present observation, and predicts a weak cancellation at 750 nm.

Resonant Brillouin scattering by mode converted TA phonons in CdS

Resonant Brillouin scattering in CdS by piezoelectrically inactive TA phonons has been studied at room temperature in the region of photon energies very close to the fundamental absorption edge. The results have shown that the scattering efficiency depends strongly on the damping in the excited electronic states.

Comment on Keller's theory of Brillouin scattering

Keller has published a treatment of Brillouin scattering inside an anisotropic crystal with application to offaxis phonons in CdS, whereas the present authors have provided a treatment valid for a crystal of any symmetry and level of anisotropy, any direction of input light, scattered light, and sound (consistent with phase matching) and to either polarization of the light beams and any type of sound wave. Moreover, our treatment includes solid angle and demagnification corrections for an arbitrarily oriented exit crystal surface. Since the treatment of Keller does not use the dyadic Green's function appropriate to an anisotropic medium, his results are in disagreement with ours after we have omitted the surface corrections from our theory.

A new type of resonant enhancemebt of Brillouin scattering by amplified acoustic phonons in CdS

Resonant enhancement of Brillouin scattering efficiency was first observed under the configuration of both incident and scattered light polarizations parallel or perpendicular to the c-axis in CdS. A tentative explanation is also given.

Nanosecond acoustic strain pulses from CdS phonon masers

Observations of nanosecond strain pulses generated by a mode-locked CdS phonon maser are reported. Peak strains exceeding 5 × 10 -5 have been measured. An analogy is drawn between the observed mode-locked operation and a repetitive pulse generator.

Nanosecond acoustic strain pulses from CdS phonon masers

Nanosecond acoustic strain pulses, generated by a mode‐locked CdS phonon maser, have been observed through application of optical diffraction and spatial filtering techniques. Peak strains exceeding 5×10−5 have been measured. An analogy drawn between our device and a repetitive pulse generator has provided some insight into mode‐locked phonon maser operation. [Work supported by the National Research Council and Defense Research Board of Canada.]

Brillouin-scattering cross sections of off-axis phonons in CdS

Incorporating the rotational contribution to the direct photoelastic effect and the angular deviation of the Poynting vector from the wave vector of the diffracted light a Brillouin-scattering theory, valid for a general anisotropic scattering kinematics in a hexagonal crystal, is derived. From the basic theory Brillouin-scattering cross sections of off-axis pure transverse (${T}_{1}$), quasitransverse (${T}_{2}$), and quasilongitudinal ($L$) phonons are calculated in the cases where the optic axis lies in the scattering plane and the incident light is polarized either parallel or perpendicular to this plane. The frequency dependence and the angular dependence of the cross section for visible light (${\ensuremath{\lambda}}_{0}=6328$ \AA{}) in CdS are evaluated for a number of important cases. The main emphasis of the numerical calculations is devoted to the ${T}_{1}$ mode for which the scattering cross section has one, two, or four branches. For certain scattering geometries the cross section equals zero for selected phonon frequencies and off-axis angles. It is shown that an experimental determination of the relative signs of the symmetric photoelastic tensor components conveniently can be based on a localization of the zeros for the $L$-phonon scattering cross section. The formulas derived in the present paper are very useful for an analysis of the frequency spectrum and the angular distribution of the phonons in acoustoelectrically active or inactive off-axis domains in hexagonal crystals like CdS and ZnO.

Resonant Brillouin scattering by amplified phonons in CdS

Dispersion spectra of the Brillouin scattering cross section are investigated in the range of photon energy 2.0-2.4 eV at room temperature by making use of intense acoustic phonons amplified through the acoustoelectric effect in CdS. We have found a deep and narrow minimum of the scattering cross section at a photon energy of about 2.22 eV and a steep increase near the fundamental absorption edge. These features are explained in terms of resonant enhancement and cancellation based on Loudon's theory. Theoretical calculation by taking into account the exciton effect shows an excellent agreement with experimental data. Scattering of the virtual intermediate states associated with the $p$-like $A$, $B$, and $C$ valence bands was found to give rise to the resonant enhancement. The dominant contribution comes from the scattering process of the exciton of the $B$ state to the $A$ state by the deformation potential ${C}_{6}$ and the recombination of the $A$ state to emit the scattered photons in the present configuration.

Light Diffraction Studies of CdS Phonon Masers: Off-Axis Cavity Structures

Light diffraction experiments on CdS phonon masers whose cavity normal is oriented at 15, 30, 45, 60, and 75° to the crystallographic c axis are presented and compared with theory. It is shown that in general the acoustic waves do not propagate along the direction of the cavity normal. An interpretation of the observations requires that one takes into account the acoustic anisotropy and the effect of mode conversion at the cavity walls.

Ultrasonic wave propagation in CdS phonon maser structures : Vrba, J., Haering, R.R., Canadian journal of physics51 (15 July 1973) 1350–1358

Ultrasonic wave propagation in CdS phonon maser structures : Vrba, J., Haering, R.R., Canadian journal of physics51 (15 July 1973) 1350–1358

Voltage modulation of CdS phonon masers

We have investigated the voltage tuning of b-axis CdS phonon masers. Voltage modulation is found to be a fairly complex phenomenon involving amplitude as well as frequency modulation of the phonon maser output. Near threshold, the frequency modulation is typically of the order of 10 kHz/V. The response is flat up to frequencies of the order of the reciprocal of the acoustic transit time. At the higher driving voltages, the tuning is reduced. The device appears promising as a FM communications transmitter.

Light Diffraction Studies of CdS Phonon Masers

Light diffraction experiments on c- and b-axis CdS phonon masers are presented and compared with the theoretical analysis presented in the previous paper. It is shown that in general the active acoustic waves do not propagate along the normal to the plane parallel sides of the crystal. The agreement with theory is good. Strain amplitudes as large as 10−4 have been observed.

Electric field modulated Raman scattering in CdS

We have observed electric field modulated Raman scattering by A 1 LO phonons in CdS. The field induced scattering is observed with a geometry in which Raman scattering by A 1 LO phonons is normally allowed. The interference of the field induced and allowed terms in the transition susceptibility leads to a modulated Raman scattering intensity proportional to the applied field. This is contrasted with data previously reported on field induced Raman scattering by E 1 LO phonons in a configuration in which the Raman scattering is normally forbidden and in which there is no interference between linear wavevector dependent and field induced terms in the transition susceptibility. Electric field effects on Raman scattering by TO phonons and by 2 LO phonons is also discussed.

Resonant Brillouin Scattering by Acoustoelectrically Amplified Phonons in CdS

Resonant Brillouin Scattering by Acoustoelectrically Amplified Phonons in CdS