Structural phases of disordered carbon materials
Dallas, Timothy E. J.
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The purpose of this study is to characterize the structural and optical properties of a number of disordered carbon materials using the non-destructive optical techniques of Raman and photoluminescence spectroscopies. The samples were produced with many different growth techniques and included microcrystalline graphites, amorphous carbons, and synthetic diamond films. A new CCD-based, multichannel Raman system was set up and used to measure spectra with very high signal-to-noise in a very short period of time. Micro-Raman spectroscopy was used to determine the bonding structure of a pulse-laser annealed graphite sample. The extent of crystalline structure was quantitatively determined in both the planar and stacking directions. A follow-up study on a series of graphitic amorphous carbon (GAC) samples confirmed models by showing evidence that GAC contained very small hexagonal clusters interconnected by odd-membered rings. Annealing the GAC increased the size of the hexagonal clusters and reduced the amount of disorder. A shift in the energy position of the main phonon band of graphite has been attributed to a finite-size effect. Disorder induced vibrational modes in carbons can vary in intensity and peak position depending upon the excitation wavelength used for Raman scattering. This resonance Raman phenomena was used to help detect difference in the Raman spectra of GAC, amorphous carbon, and disordered synthetic diamond. Continuous wave (CWPL) and time-resolved (TRPL) photoluminescence spectroscopies were used to study low level point defects and structures in synthetic polycrystalline diamond films. A series of sharp peaks observed in the CWPL spectrum of a film produced by Arc-Jet chemical vapor deposition have been attributed to tungsten atoms incorporated during growth. Broadband PL in the red wavelength region was determined to be a combination of emissions from vibronics and amorphous carbon structure.