Femtosecond polarized transient absorption results are obtained for InSe and GaSe nanoparticles. The results indicate that the transient absorption spectrum of large GaSe particles is dominated by a size-independent, z-polarized hole intraband transition. The small particle spectra exhibit the same z-polarized hole transition and a much more intense x,y-polarized absorption that is assigned to a charge transfer transition from the conduction band to particle surface (edge) states. The intensity of this transition depends on the momentum state (Γ or M) of the electron, and Γ to M electron momentum relaxation results in a 15 ps absorption decay. These results are used to interpret analogous results obtained for mixed GaSe-InSe nanoparticle aggregates, also in the solution phase. The static absorption spectrum of the mixed aggregates exhibits a strong interparticle charge transfer absorption band at an energy slightly higher than the InSe bandgap. Photoexcitation of this band results in a polarized transient absorption spectrum and transient absorption kinetics characteristic of InSe valence band holes and GaSe conduction band electrons. This result indicates that with small GaSe particles, direct InSe to GaSe electron transfer occurs upon photoexcitation.
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.
The spectroscopic characteristics of GaSe nanoparticle aggregates are reported. We find that the lowest energy absorption band shifts slightly to the red and sharpens, while the next band shifts to the blue as the concentration is increased. The spectral changes are reversible and are interpreted in terms of the particles forming strongly interacting aggregates in high concentration, room temperature solutions. The absorption spectra can be semiquantitatively modeled in terms of the lowest two transitions observed in bulk GaSe, quantum confinement effects and dipolar coupling between excited state monomers. From these calculations, the lowest excited state interparticle coupling is estimated to be about -250 cm-1. Polydisperse samples have larger energy differences but comparable couplings between adjacent particles in the aggregates. As a result, the spectroscopic effects of aggregation are less pronounced in these samples. The nanoparticle aggregate spectra are reminiscent of J-aggregate spectra of organic dyes.
Conference Committee Involvement (1)
Physical Chemistry of Interfaces and Nanomaterials V
15 August 2006 | San Diego, California, United States