| dc.description.abstract | Raman Spectroscopy Studies on Novel Materials
Raman spectroscopy is a powerful probe to study vibrational and structural properties of solids. This thesis includes spectroscopic studies of novel materials having interesting physical, structural, and electronic properties. The thesis reports experimental results on the following systems:
Single-Wall Carbon Nanotubes (SWNTs)
Semiconducting Nanorods and Nanoparticles (CdSe, ZnSe, and ZnS)
Metallic Lanthanum Hexaboride (LaB?)
Chapter 1
Includes a brief description of the theory of Raman scattering along with the effects of high pressure manifested in Raman spectra. The systems studied are broadly divided into three categories: single-wall carbon nanotubes, semiconducting nanoparticles, and metallic LaB?. A brief review of the literature on SWNTs pertaining to geometric structure, electronic structure, vibrational properties, and mechanical properties is presented. The vibrational properties of semiconducting nanoparticles are discussed in terms of the phonon confinement model. The crystal structure of LaB? is also explained briefly.
Chapter 2
Discusses the experimental setups used for Raman measurements, namely Spex Raman spectrometer and Dilor XY Raman spectrometer. The high-pressure techniques including diamond alignment, gasket preparation, and sample loading inside the diamond anvil cell are also discussed.
Chapter 3
Single-Wall Carbon Nanotubes (SWNTs)
SWNTs can be thought of as rolled graphite sheets which assemble themselves in a bundle forming a two-dimensional triangular lattice possessing a very high degree of flexibility. The vibrational spectrum of SWNTs is divided into two main regimes:
Low-frequency radial breathing mode (RBM) around ~180 cm?¹ associated with radial motion of carbon atoms.
High-frequency tangential modes (T) around ~1590 cm?¹ associated with motion of carbon atoms along the axial as well as circumferential direction.
This chapter is based on various studies done on SWNTs and is divided into six parts:
Part (A): Effect of Hydrostatic Pressure
Response of SWNT bundles to hydrostatic pressures up to 26 GPa using Raman spectroscopy. As pressure increases, RBM and tangential modes shift to higher frequencies with reduction in peak intensities and increase in linewidths. Tangential modes soften around 10 GPa indicating a phase transition. On decompression, Raman spectra are completely recovered, revealing remarkable mechanical resilience of SWNT bundles.
Part (B): Structural Phase Transition
In-situ high-pressure X-ray experiments using synchrotron radiation confirm that SWNT bundles lose translational coherence around 10 GPa. Translational symmetry of the triangular lattice is lost and regained upon decompression.
Part (C): Non-Hydrostatic Pressure
In-situ non-hydrostatic pressure Raman experiments up to 30 GPa show mechanical resilience even under non-hydrostatic conditions. A phase transition is inferred at ~4 GPa.
Part (D): Effect of Liquid Media
Comparison of Raman spectra of SWNT bundles soaked in alcohol and water. Both RBM and tangential modes show blue shifts when soaked. High-pressure results using water and alcohol as pressure-transmitting media are discussed.
Part (E): Surface-Enhanced Resonance Raman Scattering
Experiments on isolated semiconducting SWNTs adsorbed on roughened silver surfaces show large enhancement in Raman signals of RBM and tangential modes, analyzed by vibrational pumping model.
Part (F): Intercalation Effects
Effect of intercalation of CdSe and ZnS quantum dots in interstitial channels of SWNT bundles. SWNT lattice expands with CdSe doping and contracts with ZnS doping. Raman experiments show blue shift with CdSe and red shift with ZnS. Explanation involves van der Waals interaction and electronic density of states.
Chapter 4
Semiconducting Nanostructures
Raman studies on CdSe nanotubes and ZnSe nanorods grown by Triton X-100 surfactant-assisted preparation.
CdSe nanotubes show confined LO phonon at 207.5 cm?¹ and surface phonon mode at 198 cm?¹ (Frohlich mode, l = 1).
ZnSe nanorods show surface phonon mode at 237 cm?¹ and LO/TO modes blueshifted due to compressive strain.
ZnS Nanoparticles
Raman spectra of ZnS nanoparticles capped with 1-thioglycerol for sizes 18 Å, 25 Å, and 35 Å.
All sizes show l = 1 surface phonon mode at ~330 cm?¹.
35 Å particles also show confined LO Raman mode at ~345 cm?¹.
Results analyzed using phonon confinement model and wavevector quantization.
Chapter 5
Metallic LaB?
High-pressure Raman and X-ray diffraction studies reveal a phase transition at 10 GPa from cubic to orthorhombic phase. Ab-initio electronic band structure calculations using full-potential linear augmented plane wave method show that this transition is driven by interception of Fermi level by electronic band minimum, demonstrating a Lifshitz transition (Electronic Topological Transition, ETT). | |
