Raman Scattering from n-Graphene Layers (nGLs; n = 1, 2, 3 ...)
Open Access
- Author:
- Gupta, Awnish K
- Graduate Program:
- Physics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- March 06, 2009
- Committee Members:
- Prof Peter C Eklund, Dissertation Advisor/Co-Advisor
Peter C Eklund, Committee Chair/Co-Chair
Vincent Henry Crespi, Committee Member
Milton Walter Cole, Committee Member
Angela Lueking, Committee Member - Keywords:
- Incommensuratation
Raman Scattering
Nano Ribbions
nGLs
Graphene
Defect Scattering - Abstract:
- This thesis addresses the Raman scattering from graphene and n-Graphene Layers(nGLs), where n represent the number of layers in the few-graphene-layer flakes. We follow the scattering process to 3rd order and observe many new Raman peaks which participate in double resonant (DR) Raman scattering. Many of the Raman bands exhibit linear dispersion, i.e., dω/dE=constant, where ω is the Raman peak frequency and E is the excitation photon frequency. This dispersive behavior stems from DR which involeved the dispersion of the electronic and phonon states. We find dispersion in the range of -40< dω/dE <180 cm-1. We used dispersive Raman peaks of graphene to map out phonon dispersion of graphene and compare it with theoretical calculations. We also find that the frequencies of these Raman peaks changes systematic with n which indicate that they are sensitive to distant interlayer interactions (Chapter 5). We studied the effect of the substrate on the Raman scattering of nGLs by performing Raman scattering on suspended nGL films. We find suspended nGLs to show significant differences in Raman scattering of nGLs, presumably intrinsic properties that are not perturbed by the underlying substrate. We find an increased phonon life time for Raman peaks in suspended nGLs (Chapter 6). We measured the effect of surface corrugation on the linewidth of G-band (ΓG = γph-ph + γe-ph + γdisorder) in nGLs by preparing samples on substrates of varying roughness. Under the assumption that only Γdisorder varies with the substrate roughness, we estimated the intrinsic corrugation of suspended nGLs (Chapter 7). TEM and Raman scattering show that a typical graphene edge prepared by micromechanical cleavage from HOPG is, on average very straight, but at short range, meanders by about ~ ± 2 nm and thereby presents a mixture of zigzag and armchair edge symmetries. Nevertheless, we find that these “real” edges exhibit polarized scattering as if the absorption and re-radiation of the photons were made by a line antenna aligned along the average direction of the edge. Scanning the excitation beam across the edge allows the observation of the onset of the G-band with distance, as well as a D-band localized within ~ 70 nm of the edge (Chapter 8). We discuss Raman data collected on very narrow (ribbon width ~ 2-3 nm) one- or two-layer graphene nanoribbons (GNRs). New G-bands, more similar to those observed in 1.2 - 1.6 nm diameter single-walled nanotubes than those observed in graphene. We discuss the activation of these new Raman modes in narrow GNRs in terms of transverse phonon confinement. Interestingly, several of these GNRs were observed to NOT exhibit a D-band (Chapter 9). Our Raman scattering studies on skewed bi-layer graphene (SkBL) reveal an altogether different Raman spectrum from that seen in commensurately stacked bilayers (CBL) or monolayer graphene. We find that SkBL activates a new band (“I-band”) for sp2 bonded carbon near ~1350 cm-1 which has two components I1 and I2: I1 is dispersive at ~50 cm-1/eV, while I2 exhibits little or no ispersion. These Raman peaks are not due to the ordinary D-band scattering in sp2 carbons which is normally associated with defects. Within a double-resonant model of Raman scattering these unusual features are consistent with a skewed bilayer coupling, wherein one layer imposes a weak but well ordered perturbation on the other. The discrete Fourier structure of the skewed interlayer interaction potential explains the unusual non-dispersive peak near 1350 cm-1 (Chapter 10). We also investigated the effect of curvature in graphene and find that curvature in graphene induced a significant D-band in Raman scattering. We also showed that these fold can be treated as a one-dimensional defects. These experimental were also supported by theoretical calculations (Chapter 11).