Carrier Relaxation Dynamics in GaSe Nanoparticles

Hole relaxation dynamics have been studied in approximately 4 nm diameter GaSe nanoparticles in solution. The particles are photoexcited with femtosecond pulses at 387.5 nm and subsequently probed at 500 to 700 nm. A strong absorption is observed in this range, which is assigned to hole intraband transitions. This assignment is made on the basis of the decay kinetics, compared with the emission decay kinetics. The assignment is also confirmed by carrier quenching studies in which the nanoparticles are capped with pyridine ligands that act as hole acceptors and thereby quench the transient absorption. The interfacial hole transfer time to the pyridine ligands is about 2.5 ps. The 500 to 700 nm absorption exhibits a 20 ps rise component, having a wavelength-dependent amplitude. This transient is assigned to a hole intraband relaxation. The comparison of this hole relaxation rate with carrier relaxation rates in other types of semiconductor nanoparticles is discussed.