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

Publication date: Available online 6 October 2017
Source:Diamond and Related Materials
Author(s): K.K. Mishra, Revati Rani, N. Kumar, T.R. Ravindran, K.J. Sankaran, I-Nan Lin
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 (MPECVD) method. Spherical ultranano grains with size 3–4nm in these films were investigated by high-resolution transmission electron microscopy (HRTEM). Raman spectroscopic studies of UNCD with a pressure transmitting medium (up to 16GPa) and uniaxial pressure (up to 28GPa) 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 4.7314J/mol-K, which is ~80% of the reported theoretical Cv value (5.568J/mol-K). Using our experimentally obtained mode Grüneisen parameters, GB contribution to the thermal expansion coefficient of UNCD is estimated to be 0.5×10⁻⁶K⁻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 90K were used to obtain the anharmonicity of GB phonon modes. The first order temperature coefficient of transpolyacetylene modes (ν1, ν2 and ν3) and E2g in-plane (G-band) modes of GB are obtained as −0.38×10⁻², 1.69×10⁻², −1.55×10⁻² and −1.41×10⁻²cm⁻¹K⁻¹, respectively.

Graphical abstract

http://ift.tt/2hRB8gC