Ultrafast photoluminescence spectroscopy of H- and O-terminated nanocrystalline diamond films

We use femtosecond photoluminescence spectroscopy to study the light-induced changes in the sub-gap energy states of nanocrystalline diamond samples (thickness similar to 500 and 1000 nm) prepared on a spectral-grade fused silica substrate by microwave plasma enhanced chemical vapour deposition technique. The decay of photoluminescence in the visible spectral interval excited by blue femtosecond light pulses (405 nm,70 Is) shows that photoexcited charge carrier dynamics depend strongly on the ambient air pressure and on the light irradiation by the laser pulses. Specifically, at lower ambient air pressure (0.5-300 Pa) the irradiation leads to the peak photoluminescence intensity increase and to its faster decay. At higher air pressures (>600 Pa) the photoluminescence intensity slightly decreases with no change in decay rates. O- and H-termination of nanocrystalline diamond films had negligible effect on their photoluminescence dynamics. The photoluminescence decay curves are described very well by the power-law decay reflecting the importance of the carrier trapping in the dynamics. Based on our results we propose a model of surface and sub-surface structure of nanodiamond films. (C) 2011 Elsevier B.V. All rights reserved.