Lattice-strain-induced slow electron cooling due to quasi-type-II behavior in type-I CdTe/ZnS nanocrystals

Abstract

Detailed analysis on charge separation energetics and dynamics for CdTe/ZnS nanocrystals have been carried out with varying shell thickness to elucidate quasi-type-II behavior in a standard type-I system. Redshift in the absorption–photoluminescence spectra and increase of the excited state lifetime in the core/shell nanocrystals with a thick ZnS shell (2 and 4 ML of ZnS) indicate quasi-type-II behavior caused by charge separation. Separation of charge arises as the lattice strain at the core/shell interface alters the conduction band energy levels for both the core and the shell in an opposite way, extending the electronic wave function toward the shell. To find out the energetics of the charge separation, the steady-state spectra were analyzed in the realm of Marcus theory to reveal charge separation occurring in the inverted region with −ΔG°_ET > λ. Slow electron cooling as observed from ultrafast transient absorption measurements with increasing shell thickness also confirms electron being decoupled from the hole as the electronic wave function spreads out to the shell. Consistent with the Marcus theory analysis, the separation of charge is clearly exhibited in the nanocrystal with the highest ZnS shell thickness because the excitonic bleach shows a slower electron cooling rate and increased amplitude of a slow recovery component in the red region of transient absorption spectrum.