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.

Static and Ultrafast Optical Response of Two Ag Nanospheres Glued by a CdTe Quantum Dot

“…The reduction in C ext of the Ag\CdTe/Ag hybrid colloid is mainly caused by the random orientations of the particles. The origin of ultrafast optical response of the bare metal NP colloid has been reported by several groups [49][50][51][52][53]. When a ultrashort pulse excites a metal nanoparticle at its LSPR, the free-electrons in the particles are set to oscillate in phase with the applied field.…”

“…Thus the increase in the relaxation time for the Ag\CdTe/Ag hybrid colloid cannot be explained by an improvement in lattice quality. At low temperatures, the electron-phonon relaxation time also depends on its temperature itself, increasing linearly with it [50]. Thus if the increase in the temperature of Ag in the Ag\CdTe/Ag hybrid nanostructure is more, then it can also show an increased electron-phonon relaxation time.…”

“…If ∆ε m is the small change in real part of dielectric constant due to the change in electron temperature, then the change in transmission at the probe wavelength (∆T /T ) of a colloid having randomly oriented ellipsoidal particle can be written as [50,53],…”

“…Signature of such hot-electron transfer from metal NP to semiconductor QD has been observed in several hybrid nanostructures [59,60,62]. Even if there is no direct contact between the metal and semiconductor, hot-carrier transport still occurs via the linking polymer/molecule [30,50]. In addition to the above mentioned processes, a direct energy transfer between metal and semiconductor is also possible, if the LSPR of metal nanoparticle overlaps with the absorption spectrum of semiconductor QD [63,64].…”

“…Charge or energy transfer between the constituents of a hybrid strongly depends on the final superstructure formed by the metal-semiconductor nanostructures [30,50,65].…”