NANOCRYSTALLINE DIAMOND - EFFECT OF CONFINEMENT, PRESSURE, AND HEATING ON PHONON MODES

Micro-and nanocrystalline systems exhibit properties that differ marke dly from bulk systems. Diamond, a prototypical system, demonstrates a broadening, shift, and emergence of Raman phonon modes that are believ ed to originate from finite-size effects. Such information should be u seful in constraining confinement models developed to describe the sta te of these mesoscopic systems. For example, previous investigations h ave analyzed crystallite size and stresses in scientifically and techn ologically relevant environments, including chemical-vapor-deposition diamond films and diamond nanocomposites. We have experimentally measu red the effect on the diamond Raman phonon modes due to confinement, p ressure, and heating effects. At ambient pressure, we present Raman me asurements for diamond crystallites ranging from 6 nm to 10 mu m, whic h were synthesized by both static and dynamic techniques. The Raman sp ectra obtained from the statically synthesized samples exhibit a chara cteristic strong and narrow diamond band, while those dynamically synt hesized exhibit both diamond and graphitelike features. A redshift of the diamond Raman band is observed for decreasing particle size. Howev er, the pressure dependence of the phonon is about the same as that fo r the bulk system up to 30 kbar for crystallite sizes between 6 and 10 nm. Our measurements also indicate that heating effects from the inci dent laser dramatically affect the measured Raman spectra. This result lends us to an explanation for discrepancies among previously publish ed results. We show that crystallite size and stress information canno t be determined without compensating for heating effects. Lastly, the phonon confinement model is able to explain the shifts of the Raman mo des with size.