Nanocrystalline diamond: Effect of confinement, pressure, and heating on phonon modes

Micro- and nanocrystalline systems exhibit properties that differ markedly from bulk systems. Diamond, a prototypical system, demonstrates a broadening, shift, and emergence of Raman phonon modes that are believed to originate from finite-size effects. Such information should be useful in constraining confinement models developed to describe the state of these mesoscopic systems. For example, previous investigations have analyzed crystallite size and stresses in scientifically and technologically relevant environments, including chemical-vapor-deposition diamond films and diamond nanocomposites. We have experimentally measured the effect on the diamond Raman phonon modes due to confinement, pressure, and heating effects. At ambient pressure, we present Raman measurements for diamond crystallites ranging from 6 nm to 10 μm, which were synthesized by both static and dynamic techniques. The Raman spectra obtained from the statically synthesized samples exhibit a characteristic strong and narrow diamond band, while those dynamically synthesized exhibit both diamond and graphiticlike features. A redshift of the diamond Raman band is observed for decreasing particle size. However, the pressure dependence of the phonon is about the same as that for the bulk system up to 30 kbar for crystallite sizes between 6 and 10 nm. Our measurements also indicate that heating effects from the incident laser dramatically affect the measured Raman spectra. This result leads us to an explanation for discrepancies among previously published results. We show that crystallite size and stress information cannot be determined without compensating for heating effects. Lastly, the phonon confinement model is able to explain the shifts of the Raman modes with size.