Date of Award

12-2012

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Physics

Advisor

Rao, Apparao M

Committee Member

Skove , Malcolm

Committee Member

He , Jian

Committee Member

Richardson , Kathleen

Abstract

Study of nanomaterials has gained interest of researchers from various fields of science and technology due to their unique electronic and vibrational properties as compared to their bulk counterparts. In particular, carbon nanotechnology has evolved rapidly over the past few decades and nowadays, carbon nanotubes are used in various fields such as energy storage, electronics etc. However, the quest for new properties of this material is never ending and the invention of graphene generated enormous interest in the scientific community due to its excellent properties such as strength, high electron mobility, thermal conductivity etc. In this thesis, I aim at gaining better understanding of the electronic properties of carbon nanostructures and also discuss the effect of impurities on the vibrational properties of Bismuth nanorods.
In the case of SWNTs, I have studied the effect of surrounding environment on their electronic properties, in particular Sub-nm SWNTs. Due to their unique electronic and vibrational properties, single walled carbon nanotubes (SWNTs) with sub-nanometer diameters d ∼ 0.5-0.9 nm have recently gained interest in the carbon community. Using UV-Vis-NIR spectroscopy and ultra-centrifugation, we have conducted a detailed study of the π plasmon energy (present at∼5-7 eV) in sub-nm SWCNTs as a function of the size of the bundle. We find that the energy of the π plasmon peak E varies with the bundle diameter Dh as E = (0.023 eV )∗ln(Dh/do) + 5.3 7 eV, where do = 0.5 nm and corresponds to the smallest tube diameter. This is compared with the same data for HiPCo and Carbolex SWCNTs of larger diameter (1-1.4 nm) confirming a clear dependence of E on the bundle size, which is present in addition to the previously reported dependence of E on SWCNT diameter d.
In case of graphene, the carbon atoms at the edges of graphene sheet contribute to its electronic properties. This effect becomes more prominent in confined structures such as graphene nanoribbons (GNRs) and graphene quantum dots (GQDs). In case of GQDs, previous reports showed that they exhibit a strong photoluminescence (PL) in visible region upon excitation. However, currently no experimental evidence is reported for the origin of PL in these quantum dots. In this work, based on a combination of synthesis, annealing and PL measurements of GQDs, carbon nano-onions (CNOs) and GNRs, we found the PL of GQDs to be independent of its suspension medium and the chirality of its edges. Since GQDs can also be understood as a highly conjugated aromatic molecules, for comparison, this study also discusses the PL spectra of aromatic molecules, which collectively with the PL spectra of GQDs, GNRs and carbon nano-onions serve as a basis for future theoretical and experimental studies of PL in carbon nanostructures.
Lastly, we studied the effect of impurities on the vibrational properties of Bismuth nanorods. Theoretical calculations predict that Bi should undergo a semimetal-to-semiconductor transition as at least one of its dimensions becomes < 50 nm. This prediction was experimentally confirmed by infrared (IR) absorption spectra, which is largely underlain by transitions between the L (electron) and T (hole) pockets of the Fermi surface. In this work, however, we report that in our nanosize samples, the observed IR peak positions are practically independent of temperature, which is hard to reconcile with the predicted behavior of the L-T transition. To help elucidate the origin of these IR peaks, we performed a careful analysis of the IR spectra of Bi nanorods, as well as those of bulk Bi, Bi samples prepared under different conditions and Bi2(CO3)O2 using Fourier transform infrared and photoacoustic spectroscopy measurements. We propose that the observed IR peaks in Bi nanorods arise from the oxygen-carbon containing secondary phases formed on the surface of Bi rather than from the Bi itself. We believe that secondary phases must be taken into account on a general basis in modeling the IR spectra of Bi and that the scenario that ascribes these IR peaks solely to the L-T transitions may not be correct. The results reported herein may also impact the research of Bi-based thermoelectric nanostructures and bulk materials

Included in

Physics Commons

Share

COinS