Jj. Cavaleri et al., ULTRAFAST CHARGE-CARRIER DYNAMICS OF SNO2 NANOCLUSTERS - A REFINED-INTERPRETATION OF THE ELECTRON-HOLE KINETICS IN METAL-OXIDES, JOURNAL OF PHYSICAL CHEMISTRY B, 102(8), 1998, pp. 1341-1346
Femtosecond experiments on 15 nm diameter SnO2 nanoclusters measure th
e elementary charge carrier reactions of electron trapping and electro
n-hole recombination. From the early time transient absorption data, a
n electron-trapping time of 200 +/- 20 fs is determined. In addition,
an experimental scheme to determine the effect of electron thermalizat
ion on the relaxation of photoexcited elections is presented. Excess e
xcitation energy above the conduction band increases the decay time to
500 +/- 50 fs indicating that thermalization plays an important role
in the electron-trapping kinetics. The dynamics of charge carrier reco
mbination are investigated by an ultraviolet pump intensity study. A s
econd-order rate constant of (1.0 +/- 0.3) x 10(-10) cm(3)/s is found
to fit all of the decays. The early time decay kinetics in metal oxide
nanoclusters do not agree with a recently proposed fractal kinetic st
udy but are consistent with trapped electron/free hole recombination.
The assignment of the early time transient absorption at 620 nm to tra
pped electrons is supported by comparing the transient absorption kine
tics to ground state recovery results in both SnO2 and TiO2 nanocluste
rs.