We investigate the vibrational properties of ultrananocrystalline diamond (UNCD) using molecular dynamics simulations. We compare the vibrational spectra of two UNCD models of average grain size 2 and 4 nm with single crystal diamond and an isolated nanodiamond (ND) particle. The vibrational spectra of the ND particle and UNCD models exhibit the effect of phonon confinement as well as undercoordinated atoms at the surface/interfaces. This is further reflected in the specific heat of UNCD models and the ND particle that showed enhancements over that of single crystal diamond. The excess specific heat in UNCD models in comparison to single crystal diamond is found to be maximum at approximately 350 K.
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