Theoretical Study of Electron Transport and Trapping in Solvated Titanium Dioxide Nanoparticles

摘要

This thesis consists of two parts of work: the advances on the efficient and robust implementation of our quantum chemistry program Jaguar for large DFT calculations, and theoretical modeling of electron transport and trapping in solvated TiO_2 nanoparticles. In first half, We implemented an OpenMP/MPI hybrid parallelization implementation of the pseudospectral algorithm, developed a fragment based initial guess methodology which is suitable for addressing systems with significant delocalization, and improved numerical robustness with regard to the converged wavefunction. The parallel scalability is enhanced greatly from ∼16 to ∼128, which enables a hybrid-DFT calculation for a system with up to 5000 basis functions. Those advances enable Jaguar to be used in a wide range of materials science problems that previously were not accessible to our approach.In second half, by employing a large cluster, a hybrid DFT model (B3LYP), and a realistic treatment of solvent, we proposed an atomically detailed model for the electronic states involved in transport and trapping, and obtain good agreement with experiment for properties such as the energetics of the trapping states and the barriers to hopping conduction by electrons. The results suggest the existence of energetically shallow electron trapping states at (sub) surface region induced by the presence of small cations and the continuum solvent effect. The barrier heights imply that concerted ambipolar diffusion of the Li+/e − can occur under thermal activation, but it is energetically disfavored for proton/e− pair. Those results advance our understanding of the effect of the cation and solvent on the density of states (DOS) of electron traps, and establish the plausibility that ambipolar model plays a role in electron transport through TiO_2 film in DSSC.