Counting the Electrons Hopping in Ultrafast Time Scales in an Ag–CdTe Hybrid Nanostructure

Abstract

In a metal–semiconductor hybrid nanostructure, exchange of carriers between the two constituents is a crucial process determining its utility for various applications. Although the transfer of carriers from the metal to semiconductor (or vice versa) shows up in several ways, it is not easy to get a quantitative estimate of the process. In this paper, we show that it is possible to obtain a fair estimate of the number of carriers hopping from or to a metal nanoparticle by performing transient absorption measurement near the localized surface plasmon resonance peak. For demonstrating this concept, a specific Ag–CdTe hybrid nanostructure was prepared such that plasmon resonance of the Ag nanoparticles could be excited well below the band gap of the CdTe quantum dots. Using a simple Drude model and the changes in the number density of free electrons, we show that the transient optical response of the hybrid is governed by both free-electron temperature and the number of carriers transported in and out of the metal nanoplates. It is also shown that using the linear absorption spectra as reference, it is possible to estimate the number of carriers hopping between the two constituents of the hybrid. The estimate of the number of electrons hopping at ultrafast time scales can provide a way to optimize the design of a metal–semiconductor hybrid system for specific applications.