Observation of pressure-induced direct-to-indirect band gap transition in InP nanocrystals

We investigate the quantum size effects in the pressure-induced direct-to-i ndirect band gap transition in InP nanocrystals. Hydrostatic pressures of u p to 13 GPa are applied to two different sizes of InP nanocrystal samples i n a diamond anvil cell. The band gap pressure dependence and the nature of the emitting states are studied by photoluminescence (PL) and fluorescence line narrowing (FLN) techniques at 10 K. Pressure-dependent FLN spectra sho w that the nature of the emitting states at pressures up to 9 GPa is simila r to that at ambient pressure, suggesting that no direct-to-indirect transi tion happens below 9 GPa. For both sizes, the PL peak energy exhibits a str ong blueshift with rising pressure until approximately 9 to 10 GPa. Above t his pressure, the PL peak position slightly shifts red. Beyond 12 GPa, the band gap emission intensity becomes extremely weak and trap emission domina tes the PL spectra. As the pressure is released, both the luminescence inte nsity and the peak position recover in a fully reversible manner. The chang e in the sign of the band gap energy pressure dependence and the disappeara nce of the band edge luminescence indicate the pressure-induced direct-to-i ndirect band gap transition. Contrary to theoretical calculations, no subst antial reduction of the transition pressure is observed in the nanocrystal cases compared to the bulk transition pressure. (C) 2000 American Institute of Physics. [S0021-9606(00)70113-9].