Photophysics of GaSe nanoparticles and nanoparticle aggregates
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Abstract
Polarized femtosecond transient absorption spectroscopy and time-resolved polarized emission spectroscopy have been employed to study the spectroscopy and dynamics of charge carriers in GaSe nanoparticles and nanoparticle aggregates. Transient absorptions in the visible and near infrared spectral regions are assigned in terms of a simple effective mass, particle-in-a-cylinder model. A particle size independent, z-polarized hole intraband transition maximizing at 600-650 nm is resolved from a particle size dependent x,y-polarized electron transition involving transfer of electrons from the conduction band to surface states. For the small (2.7 nm) and mid-sized (5.1 nm) particles, the electron charge transfer transition decays relatively rapidly (≈15 ps timescale) due to a direct to indirect (Γ to M) electron momentum relaxation. The hole intraband transition decays relatively slowly (400 - 900 ps) due to hole trapping. The same decay components are also present in the emission kinetics. Particle aggregation significantly shifts the lowest energy allowed transition in the small particles, thereby changing the nature (Γ versus M) of the initially excited state and altering the observed transient absorption spectra. For the large (11.8 nm) particles, the relative energetics of the electron Γ and M states are reversed and the fast decay of the electron transition is not observed.