Relaxation of Photoexcited Electrons at a Nanostructured Si(111) Surface

Abstract

Relaxation pathways of photoinduced electronic redistribution at nanostructured semiconductor surfaces are obtained from time-dependent density matrix and ab initio electronic structure methods, giving electronic changes in energy and space over time. They are applied to a Ag cluster on a Si(111) surface, initially photoexcited by a short pulse, and show that the Ag cluster adds surface-localized states that enhance electron transfer. Results on the time evolution of population density distributions in energy and in space, for valence and conduction bands, explore the energy band landscape of a Si slab, with various relaxation pathways ending up in a charge-separated state, with a hole in the Si slab and an electron in the adsorbed Ag cluster. Calculated electronic relaxation times for Si(111)/H are of the same order as experimental values for similar semiconductor systems.

Generalized Response Theory for a Photoexcited Many-Atom System

“…This presentation derives some new relations giving density and current responses to applied fields in terms of time-correlation functions (TCFs) of an excited system and also illustrates some of the general results on optical properties and conductivity of excited silicon slabs, from our recent work. [22][23][24][25][26][27][28][29][30][31] Light absorption moves the physical system away from thermal equilibrium (eq) and into a new state which for steady light excitation can reach a steady state (ss) involving dissipation in a medium. This results from continuous light excitation in the primary or p-region of interest concurrent with its de-excitation as the medium, or s-region, absorbs energy and is electronically excited in a time scale much faster than the alternative de-excitation by light emission.…”

Charge Transfer Mechanism in Titanium-Doped Microporous Silica for Photocatalytic Water-Splitting Applications

“…LU+2 behaves as an electron trap, which slows its relaxation rate. Ultimately, LU+7 → LU relaxation occurs at a rate of 0.8141 ps −1 , which is computed according to Equation (22). A summary of relaxation rates can be found in Table 1.…”

“…were used to compose the density matrix for each instant of time in such a way that it matches the initial conditions Equation (14) [6][7][8]22]. The electron in the conduction band and the hole in the valence band are indicated by e and h, respectively.…”

“…In addition, index e was used with the appreciation that equations for holes are the same as for electrons. Utilizing the above information, the charge density distribution, rate of energy dissipation, and rate of charge transfer were calculated as follows [6][7][8]22,33].…”

“…This information helps to identify orbital transitions due to excitations, which are of interest. Molecular dynamics [19] are simulated, which leads to couplings [20] and eventually to electron dynamics of photoexcitation [21] that then provides excitation lifetimes [22] and details of the charge transfer meachanism [6][7][8]. These lifetimes are an integral part of making conclusions about a material and its photocatalytic properties.…”