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Raman Spectroscopy of Nanostructures and Bio-Inorganic Hybrid Structures

Raman spectroscopy is the inelastic scattering of photons (light) by phonons (crystal vibrations). It is a fast, relatively simple, and non-destructive materials characterization technique, which is used in the research laboratories and industry. In addition to conventional applications as a tool for identifying specific materials in complex structures, Raman spectroscopy can be used for extracting information about the phonon properties of nanoscale objects. Due to changes in the acoustic and optical phonon dispersion induced by spatial confinement, Raman spectra of nanostructures undergo strong modification compared to constituent bulk materials. As a result, detail investigation of Raman spectrum of inorganic, organic and hybrid nanostructures can provide valuable structural information as well as insight to the specifics of the carrier transport, heat propagation, optical response, and strain distribution in such structures (see Figure 1). Professor Balandin's Nano-Device Laboratory (NDL) research group has leading expertise in the theory and modeling of confined phonons in nanostructures, i.e. nano-phononics, and carries out experimental Raman spectroscopy research focusing on nanostructures, biological objects and hybrid organic-inorganic systems.

Figure 1. Raman spectrum of Ge/Si quantum dot superlattice.
Analysis of the peaks positions allows us to extract information
about the strain and stress in the system. NDL, 2004.

NDL is equipped with the state-of-the-art S1000 Renishaw micro-Raman spectrometer, which includes grating stage, Leica microscope with a set of objectives, automatic XYZ scanning stage, CCD camera, 488 nm (514.5 nm) Ar-ion laser, a 325 nm He-Cd ultraviolet (UV) laser with corresponding UV optics set. The Raman spectrometer system also includes NeXT filter, which allows us to probe confined acoustic phonon modes with small wave numbers, color video and a specially designed fiber optic input/output for coupling the system to the Signaton device-probe station. The extended scanning option provides capability for measuring the photoluminescence (PL) spectra (see Figure 2). A set of polarises allows us to carry out polarization dependent studies. Additional systems upgrades provide the 2D high-resolution Raman mapping and the global imaging capability. See also description of the NDL experimental facilities .

Figure 2. Extended Raman scann of the CdS quantum dor array.

NDL researchers have used the micro-Raman spectroscopy to characterize a variety of different nanostructures, materials and biological objects. For example, Raman spectroscopy provides a wealth of information about the electronic states and phonon dispersion in carbon nanotubes. Each part of the Raman spectrum, the radial breathing mode (RBM), the disorder induces mode (D mode) and the high-energy mode (HEM), can be used to access different properties of single-wall carbon nanotubes (see Figure 3). The radial breathing mode is unique signature in the Raman spectrum of the single-wall carbon nanotubes. In the high-energy range around 1600 cm-1 single-wall carbon nanotubes show a characteristic double-peak structure.

Figure 3. Raman spectrum of the carbon nanotubes.
Note a RBM peak at 158 (1/cm). NDL, 2004.

The unique vibrational modes observed in Raman (Brillouin) spectra of viruses can be used to monitor the process of virus growth and functionalization (see Figure 4). Information, which can be obtained with the help of Raman spectroscopy, is particularly valuable since other direct characterization techniques, such as transmission electron microscopy (TEM), are difficult to carry out with viruses and other biological objects and require special treatment of the samples. For details see A.A. Balandin and V.A. Fonoberov, Vibrational Modes of Nano-Template Viruses published in the Journal of Biomedical Nanotechnology .

Figure 4. Raman scattering from TMV viruses.

To help in interpretation of the measured Raman spectra, Nano-Device Laboratory (NDL) research group carries out theoretical and computer modeling of confined acoustic and phonon modes in nanostructures and calculates exact phonon dispersion relation in different nanoscale objects. We have recently calculate the Raman spectrum from the three-dimensional regimented array of GeSi quantum dots on Si. Our analysis was based on a numerical solution of the elasticity equation for the whole quantum dot superlattice. We have shown that the three-dimensional acoustic phonon folding due to structure regimentation and small feature size leads to unique signatures in Raman spectra, which cannot be predicted using Lamb-type models. Details of these calculations can be found in our paper published in the J. Superlattices and Microstructures. In another theoretical project related to Raman spectroscopy of nanostructures, we derived, within the dielectric-continuum approximation, an integral equation that defines interface and confined polar optical-phonon modes in nanocrystals with wurtzite crystal structure (see Figure 5). Details of the derivation can be found in the paper by V.A. Fonoberov and A.A. Balandin published in the Physical Review B journal.

Figure 5. Calculated phonon modes in wurtzite nanocrystals.

More information on the projects currently under way in the Nano-Device Laboratory (NDL) can be found HERE. To join NDL as a graduate student or postdoctoral research visit the web-page HERE. To learn more about course offering in the field of Materials, Devices and Circuits visit the web-page HERE.

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