Monte Carlo random walk simulation of transient absorption kinetics using reflectance and absorption of electrons at Au/TiO₂ nanoparticle boundaries

To understand plasmon-induced charge-transfer mechanisms between a photo-excited gold (Au) nanoparticle and a TiO2 nanoparticle, a Monte Carlo random walk (MCRW) simulation was applied to reproduce the charge recombination kinetics in the nanocrystalline (Au/TiO2)2) assemblies reported previously based on transient absorption spectroscopy. The Au/TiO2 assemblies consist of a confined electron diffusion space within a tiny TiO2 nanoparticle, making it possible to study electron diffusion transport through MCRW simulation. In this simulation algorithm, the electron diffusion starts at the coordinate origin of a rectangle, and the next direction of movement is obtained by calculating the coordinate matrix and random offset so that the electron is reflected on three boundaries and absorbed when it reaches the other boundary. By simulation programming, the histogram which indicates the occurrence frequency of the step accumulation number up to the right boundary was obtained. From 100 to 100000 steps under condition of 10000 iteration, that is, changing the steps but keeping the iteration times to ensure that all particles experience absorption in the simulation. Comparing the trace of 10(6) particles position with that of 10(4) under 1000 simulations, the electron density was found to saturate other than the region near the right boundary, where electrons disappear by the absorption process during the electron diffusion process. Finally, by fitting curves, it is confirmed that the tendency of the simulated response reproduced the transient absorption kinetics.