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
To join NDL as a graduate student or postdoctoral research visit
the web-page HERE.
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Devices and Circuits visit the web-page HERE.