Sabas G. Abuabara

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

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.

Coherent control of tunnelling dynamics in functionalized semiconductor nanostructures: a quantum-control scenario based on stochastic unitary pulses

The feasibility of achieving coherent control of tunnelling dynamics, associated with electronic excitations in functionalized semiconductor nanostructures, is investigated. The coherent control scenario is based on the application of frequent stochastic unitary pulses, affecting the interference phenomena between electronic wave-packet components and consequently the overall electronic tunnelling dynamics without collapsing the coherent quantum evolution of the system. It is shown that tunnelling can be inhibited and eventually halted by sufficiently frequent pulse fields that exchange energy with the system but do not affect the potential energy tunnelling barriers. Further, the proposed stochastic quantum-control scenario is demonstrated more generally as applied to an archetype model, a particle tunnelling in a quartic double-well potential.

Multiple unitary-pulses for coherent-control of tunnelling and decoherence

This paper investigates the feasibility of using sequences of unitary pulses for achieving coherent-control of quantum dynamical phenomena, including quantum control of electron tunnelling in archetype model systems and control of decoherence in a quantum dot system coupled to a thermal bath. The proposed dynamical decoupling scenario is based on the repetitive application of 2π pulses, affecting the interference phenomena between wave-packet components. The pulses affect the overall relaxation dynamics without collapsing the coherent-quantum evolution of the system. It is shown that both bound-to-bound state tunnelling and bound-to-continuum tunnelling processes can be inhibited and eventually halted by sufficiently frequent pulse fields that exchange energy with the system but do not affect the potential energy tunnelling barriers. Furthermore, the proposed quantum-control scenario is demonstrated as applied to manipulating the electronic quantum dynamics in a quantum dot coupled to a free standing quasi two-dimensional phonon cavity. The reported results are therefore particularly relevant to the understanding of coherent optical manipulation of electronic excitations in semiconductor devices where performance is limited by quantum tunnelling and decoherence.

Force field parameters for large-scale computational modeling of sensitized TiO2 surfaces

Force field parameters for large scale computational modeling of sensitized TiO2-anatase surfaces are developed from ab initio molecular dynamics simulations and geometry optimization based on Density Functional Theory (DFT). The resulting force field, composed of Coulomb, van der Waals and harmonic interactions, reproduces the ab initio structures and the phonon spectra density profiles of TiO2-anatase nanostructures functionalized with catechol, a prototype of an aromatic linker commonly used to sensitize TiO2 nanoparticles with Ru(II)-polypyridyl dyes. In addition, simulations of interfacial electron injection and electron-hole relaxation dynamics demonstrate the capabilities of the resulting molecular mechanics force-field, as applied in conjunction with mixed quantum-classical methods, for modeling quantum processes that are critical for the overall efficiency of sensitized-TiO2 solar cells.