Luminescence and Resonance Raman Spectroscopy of Indirect Excitons in AgBr Nanocrystals
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ABSTRACT AgBr nanocrystals with radii R 2.5 to 5 nm comparable with the exciton Bohr radius are produced in inverse micelles. As compared with bulk AgBr, the exciton emission exhibits a substantial blue shift and enhanced intensity due to the spatial confinement. Besides luminescence, first- and second-order resonance Raman scattering is discovered the occurrence of a zero-phonon process clearly revealing mixing of L with r point states. From time-resolved measurements, exciton lifetimes of the order of 500 ps are found. They are close to the radiative lifetime in the bulk demonstrating that nonradiative processes are negligible in these nanocrystals. In agreement with state mixing, the total decay rate is found proportional to R- 2 . A small shift of the TO(L) Raman line with excitation photon energy is analyzed in terms of the wavevector dependent interaction of the quantized exciton states with the dispersive phonon. 1. INTRODUCTION Because of their fundamental and expected practical importance, quantum size effects of excitons in nanocrystals have recently attracted increasing attention (for references see e.g. [1, 2, 3]). The small crystalline particles have sizes of the order of a few exciton Bohr radii so that the unit cell is already fully developed as in the volume crystal, but the energy spectra are still discrete because of the spatial confinement. While the main body of theoretical and experimental work in this field refers to excitonic transitions in II-VI and I-VII semiconductors like e.g. CdSe and CuCl having direct energy gaps, it is only recently that indirect gap nanocrystals are being studied. Strong impact here came from the discovery of red and blue luminescence in porous silicon (for a recent review see e.g. Ref. [4]), other systems lately under intense study being Ge, GaP and AgBr. Quantum size effects in AgBr nanocrystals were first reported by groups at Kodak (USA)[5, 6, 7] and Fuji Photo (Japan)[8]. The crystallites are produced either in reversed micelles in solution or in a gelatine matrix both solidified at low temperatures. Compared with bulk AgBr, the main effect of the spatial confinement consists of a blue shift of the exciton emission. It is accompanied by strong enhancement of emitted intensity which is believed to originate from either indirect-direct state mixing, resulting in relaxed wavevector selection rules or from the absence of the impurities that is to be anticipated in the small particles from statistical considerations [5, 8]. An important observation that makes AgBr quantum crystallites unique compared to all other indirect-gap materials is the occurrence of resonant Raman scattering (RRS) under 289 Mat. Res. Soc. Symp. Proc. Vol. 358 0 1995 Materials Research Society
excitation in the free exciton (FE) region[9, 10]. Actually this is very similar to bulk AgBr, where the lowest exciton state is situated at point L in the Brillouin zone, and phononassisted absorption and emission associated with various momentum-conserving phonons (TA(L), TO(L), LA(L), LO(L)) give ris
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