High pressure Raman spectroscopic studies on ultrananocrystalline diamond thin films: Anharmonicity and thermal properties of the grain boundary

Abstract

Ultrananocrystalline diamond (UNCD) thin film has been attracted widely to scientific community due to its attractive thermal and mechanical properties. Herein, we report the high-pressure behavior of grain boundary (GB) originated phonon modes and their correlation with thermal properties in UNCD film grown by microwave plasma enhanced chemical vapor deposition method. Spherical ultranano grains with size 3-4 nm in these films were investigated by high-resolution transmission electron microscopy (HRTEM). Raman spectroscopic studies of UNCD with a pressure transmitting medium (up to 16 GPa) and uniaxial pressure (up to 28 GPa) revealed the presence of grain boundary phonons which are found to harden upon compression. GB phonons contribution to the specific heat of UNCD at room temperature is estimated to be 4.7314 J/mol-K, which is similar to 80% of the reported theoretical molar specific heat C-v value (5.568 J/mol-K). Using our experimentally obtained mode Grtineisen parameters, GB contribution to the thermal expansion coefficient of UNCD is estimated to be 0.5 x 10(-6) K-1. Grain boundaries of UNCD are found to be highly stable and Raman spectra showed reversible behavior upon release of pressure. Phonon spectra as a function of temperature down to 90 K were used to obtain the anharmonicity of GB phonon modes. The first order temperature coefficient of trans-polyacetylene modes (v(1), v(2) and v(3)) and E-2g in-plane (G-band) modes of GB are obtained as -0.38 x 10(-2), 1.69 x 10(-2), -1.55 x 10(-2) and -1.41 x 10(-2) cm(-1) K-1, respectively.