Interpretation of the Raman spectra of ultrananocrystalline diamond

It has long been known that by slightly altering the deposition conditions for diamond in plasma-enhanced chemical vapor deposition (PECVD), a transition from a microcrystalline to a nanocrystalline diamond morphology can be affected. The method of this transition, however, is not clear. This work investigates that transition by using transmission electron microscopy (TEM), scanning electron microscopy (SEM), and Raman spectroscopy. These experiments show that far from being a continuous transition, there is competitive growth between microcrystalline and nanocrystalline diamonds. Additionally, this work confirms the interpretation that certain peaks in the Raman spectrum previously attributed to “nanocrystalline diamond” are indeed due to the presence of hydrogen at the grain boundaries. For ultrananocrystalline diamond (UNCD) films, we verify that none of the spectral features observed using visible Raman spectroscopy can be attributed to sp3-bonded carbon, although the sample is composed of ∼95% sp3-bonded carbon. Thus, the Raman signal in UNCD can be considered to be solely due to the disordered sp2-bonded carbon at the grain boundaries.