Model study of coherent quantum dynamics of hole states in functionalized semiconductor nanostructures

Abstract

Functionalization of semiconductor nanocrystals can be achieved by anchoring organic ligands to the surface dangling bonds. The resulting surface complexes often introduce electronic states in the semiconductor band gap. These interband states sensitize the host material for photoabsorption at frequencies characteristic of the molecular adsorbates, leading to the well-known process of photoexcitation and subsequent femtosecond interfacial electron transfer. This paper investigates the relaxation dynamics of hole states, energetically localized deep in the semiconductor band gap, after the ultrafast electron-hole pair separation due to interfacial electron transfer. Mixed quantum-classical methods, based on mean-field nuclear dynamics approximated by ab initio density functional theory molecular dynamics simulations, reveal superexchange hole tunneling between adjacent adsorbate molecules in a model study of functionalized TiO2-anatase nanostructures. It is shown that electronic coherences can persist for hundreds of picoseconds under cryogenic and vacuum conditions, despite the partial intrinsic decoherence induced by thermal ionic motion, providing results of broad theoretical and experimental interest.

Photoinduced electron transfer processes in dye-semiconductor systems with different spacer groups

“…[31][32][33][34] To this end, a variety of different methods have been developed and employed. 31, The electronic structure of dye molecules adsorbed at semiconductor substrates, in particular titanium dioxide, has been studied employing cluster models of nanoparticles 37,52,55,62,64,65 or the a) Present address: Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, USA.…”

Force field parameters for large-scale computational modeling of sensitized TiO 2 surfaces

“…A more thorough description, including details concerning the ab initio Molecular Dynamics (MD) simulations and the analysis of structural and electronic properties of the models can be found in previous work. [30][31][32][33][34] …”

“…Especially, considering that the TiO 2 frequencies correspond to the characteristic vibrational period of 40 fs, which is an order of magnitude larger than the ultrafast time scale associated with the primary electron injection event. [30][31][32][33][34] In addition, the vibrational frequencies of the catechol molecule are only slightly affected by photo-excitation of the molecule to the S 1 electronic state. wavenumber (100 cm Figure 3 shows the evolution of the time-dependent adsorbate population, after instantaneously populating the catechol-LUMO.…”

“…18,20,23,35,36,[48][49][50] In addition, both the structure of the reconstructed catechol/TiO 2 surface and the underlying dynamical processes after photoexcitation of catechol/TiO 2 surface complexes have been characterized at the ab initio level. [30][31][32][33][34] While these previous quantum mechanical studies have provided valuable insight into the nature of dry sensitized TiO 2 , studies addressing the role of the solvent and electrolyte environment on recombination redox processes, or the synergistic effects between multiple adsorbates, require large scale simulations beyond the limitations of purely ab initio methods.…”

“…However, attempts to achieve further improvements in efficiency have been hindered by the lack of fundamental understanding of the molecular and electronic processes associated with the constituent operational steps. This problem continues to be a standing challenge, despite significant research effort reported in recent years, including experimental, 2-18 computational [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36] and theoretical [37][38][39][40][41][42][43][44] studies. It is therefore essential to develop new experiments and theoretical studies to construct realistic models of dye-sensitized TiO 2 interfaces and gain insight on the design of surface-sensitization with optimum photon-to-current conversion efficiency.…”