Influence of air annealing on the luminescence dynamics of HPHT nanodiamonds

• Impact of air-annealing on the dynamics of nanodiamond surface states was studied.
• Suppressing of surface non-diamond phase strongly affects luminescence decay speed.
• Results for sample annealed at 500 °C imply radical change of surface quality.

We present a study of the influence of air-annealing on the photoexcited charge carriers dynamics on the surface of nanodiamonds of sub-10 nm size. Samples were produced by high temperature annealing of high-pressure, high-temperature nanodiamonds. Measured photoluminescence spectra of annealed nanodiamonds show distinct blue shift and spectral narrowing compared to non-annealed reference. Time-resolved measurements revealed a change in relaxation process in the nanodiamond surface layer due to removal of non-diamond carbon phase on the surface. Spectro-temporal evolution takes place during the first 50 ps after photoexcitation suggesting that non-radiative states play a major role in the charge carrier relaxation. The acceleration of high energy states depopulation, observed for the annealed nanodiamonds, suggests gradual inhibition of non-radiative recombination channels (originally present in large numbers in non-annealed sample), which is in a good agreement with the picture of reduction of non-diamond carbon phase.

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