Phonons In Nanocrystals Research Articles

Plasmon-Enhanced Vibrational Spectroscopy of Semiconductors Nanocrystals

A review of recent results and new data on the study of the optical response from semiconductor nanocrystals obtained using plasmon-enhanced optical spectroscopy, including surface enhanced Raman scattering (SERS) and plasmon-enhanced IR absorption, is presented. These methods are based on the amplification of the phonon response of semiconductor nanocrystals located in the field of localized surface plasmon resonance (LSPR) of metal nanostructures. Owing to the choice of a specific morphology of metal nanostructures, coincidence of the LSPR energy with the laser excitation energy and / or the energy of optical phonons in nanocrystals is provided. Resonant conditions ensure a significant increase in local electric fields and, as a result, a sharp increase in the Raman signal and IR absorption at the frequencies of surface optical phonons of nanocrystals. Amplification of the optical response makes it possible not only to detect monolayer coatings of nanocrystals, but also to study their crystal structure, phase and element compositions, and internal mechanical stresses. Application of Raman scattering (RS) in combination with atomic force microscopy with the use of a metallized probe has opened up new possibilities for analyzing the vibrational and electronic spectra of nanocrystals with nanometer spatial resolution

Nanoantenna structures for the detection of phonons in nanocrystals.

We report a study of the infrared response by localized surface plasmon resonance (LSPR) modes in gold micro- and nanoantenna arrays with various morphologies and surface-enhanced infrared absorption (SEIRA) by optical phonons of semiconductor nanocrystals (NCs) deposited on the arrays. The arrays of nano- and microantennas fabricated with nano- and photolithography reveal infrared-active LSPR modes of energy ranging from the mid to far-infrared that allow the IR response from very low concentrations of organic and inorganic materials deposited onto the arrays to be analyzed. The Langmuir–Blodgett technology was used for homogeneous deposition of CdSe, CdS, and PbS NC monolayers on the antenna arrays. The structural parameters of the arrays were confirmed by scanning electron microscopy. 3D full-wave electromagnetic simulations of the electromagnetic field distribution around the micro- and nanoantennas were employed to realize the maximal SEIRA enhancement for structural parameters of the arrays whereby the LSPR and the NC optical phonon energies coincide. The SEIRA experiments quantitatively confirmed the computational results. The maximum SEIRA enhancement was observed for linear nanoantennas with optimized structural parameters determined from the electromagnetic simulations. The frequency position of the feature’s absorption seen in the SEIRA response evidences that the NC surface and transverse optical phonons are activated in the infrared spectra.

Light-induced nonthermal population of optical phonons in nanocrystals

Raman spectroscopy is widely used to study bulk and nanomaterials, where information is frequently obtained from spectral line positions and intensities. In this study, we monitored the Raman spectrum of ensembles of semiconductor nanocrystals (NCs) as a function of optical excitation intensity (optical excitation experiments). We observe that in NCs the red-shift of the Raman peak position with increasing light power density is much steeper than that recorded for the corresponding bulk material. The increase in optical excitation intensity results also in an increasingly higher temperature of the NCs as obtained with Raman thermometry through the commonly used Stokes/anti-Stokes intensity ratio. More significantly, the obtained dependence of the Raman peak position on temperature in optical excitation experiments is markedly different from that observed when the same NCs are excited only thermally (thermal excitation experiments). This difference is not observed for the control bulk material. The inefficient diffusion of photogenerated charges in nanoparticulate systems, due to their inherently low electrical conductivity, results in a higher steady-state density of photoexcited charges and, consequently, also in a stronger excitation of optical phonons that cannot decay quickly enough into acoustic phonons. This results in a nonthermal population of optical phonons and thus the Raman spectrum deviates from that expected for the temperature of the system. Our study has major consequences to the general application of Raman spectroscopy to nanomaterials.

Phonon confinement effects in the Raman spectrum of nanodiamond

Nanodiamonds (ND) exhibit unique properties due to their small size and high surface-to-volume ratio compared to bulk diamonds. A reduction in crystal size also affects ND Raman spectra. The confinement of optical phonons in nanocrystals $(<10\text{ }\text{nm})$ results in asymmetrically broadened Raman lines, which are shifted toward lower wavenumbers. The phonon confinement model (PCM) relates the observed changes in the Raman spectra to the crystal size and can be used for size characterization at the nanoscale. While the PCM was successfully applied to a variety of materials including Si and BN, results remained unsatisfactory in the case of ND. In order to improve the agreement between the predictions of the model and experimental Raman spectra of ND, effects such as crystal size distribution, lattice defects, and the energy dispersion of the phonon modes were taken into consideration and incorporated into the PCM. This work has shown that phonon wave vectors from small vibrational domains lead to a broad shoulder peak at $\ensuremath{\sim}1250\text{ }{\text{cm}}^{\ensuremath{-}1}$, that is often observed in the Raman spectrum of ND.

Specific heat of CdS/CdSe/CdS quantum-dot quantum wells

The vibrational modes of acoustic phonons and their corresponding eigenfrequencies in CdS/CdSe/CdS quantum-dot quantum wells (QDQWs) are obtained by taking on the continuum model. The energy spectra of the phonons in nanocrystals from the analytic solutions are checked by the finite-element method. 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 the size-dependent effects. The 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 plays an important role in the calculation of specific heat, and the modification of the effective sound velocity in the nanocrystal is also suggested.

Resonance-enhanced dipolar interaction between terahertz photons and confined acoustic phonons in nanocrystals

Taking advantage of the specific core-shell charge separation structure in the CdSe∕CdTe core-shell type-II nanocrystals, we experimentally observed and verified the existence of the resonance-enhanced dipolar interaction between terahertz photons and their corresponding confined acoustic phonons. From the measured terahertz transmission spectra, we found that the photon frequency of the terahertz resonant absorption is inversely proportional to the diameter D of the nanocrystals and agrees with that of dipolar active l=1, n=0 confined acoustic modes. The corresponding absorption cross section shows a D4 dependence, supporting a charged simple-harmonic-oscillator model. These facts verify the occurrence of dipolar interaction between terahertz photons and confined terahertz acoustic phonons.

Size dependence of confined acoustic phonons in CuCl nanocrystals

Persistent spectral hole burning spectroscopy was used to study the size dependence of the confined acoustic phonons in CuCl nanocrystals embedded in silicate glass, NaCl, and KCl. It is found that the energies of the confined acoustic phonons in the nanocrystals in glass and KCl are almost the same, but twice larger than those in NaCl, which were obtained from the Stokes-side acoustic phonon holes with the same excitation energies. The confined acoustic phonons in the nanocrystals in glass and KCl can be well explained in terms of the lowest-frequency vibrational modes calculated on a sphere model with a free boundary condition. However, the energies of the confined acoustic phonons in the nanocrystals in NaCl are lower than the frequencies of the lowest-frequency vibrational modes predicted by a cube model with a free boundary condition. This observation shows that the energies of the confined acoustic phonons in CuCl nanocrystals depend on the size, the shape, and the boundary condition of the nanocrystals.

Raman Scattering and Photoluminescence of Mn‐Doped Nanocrystals of ZnS Formed by the Pressure Cycle Method

AbstractZn1−xMnxS (x = 0.043) undergoes a pressure‐induced phase transition at 14.1 GPa from the zincblende to the NaCl structure. We have measured Raman scattering for a pressure released sample after compressing up to 16 GPa in a diamond anvil cell. The LO peak has shifted to lower energy compared with a virgin sample, which indicates the quantum confinement of LO phonons in nanocrystals formed by the pressure cycle. The diameter of nanocrystals is estimated to be 8 nm according to the spatial correlation model, in agreement with the value estimated from X‐ray diffraction data. The observed decrease of photoluminescence intensity and the shortening of the decay time for the 4T1 → 6A1 transition within the Mn2+ ion are ascribed to the formation of nonradiative centers in the nanocrystals.