This project continues the development of computational methods to study electronic excitations in semiconductor nanostructures. Fully atomistic simulations will investigate ultrafast quantum dynamical phenomena and coherent control of photoinduced electronic excitations in TiO2, SnO2 and ZnO surfaces functionalized with molecular adsorbates for solar energy conversion. The projects include the characterization of electron-hole pair separation and recombination pathways, photoexcitation methods for laser manipulation of electronic excitations, photoinduced electronic dynamics and trapping mechanisms, and the integration of the proposed research with an outreach educational program for minority students and collaborations with research groups in South America.
Intellectual Merit: These studies of quantum electronic relaxation, control and rectification of interfacial electron transfer in fully atomistic models of functionalized semiconductors combine rigorous computational approaches based on large-scale ab initio molecular dynamics and nonequilibrium Green?s function (NEGF) methods as well as direct comparisons with a wide range of experiments.
Broad Impact: Studies of quantum dynamics in semiconductor nanostructures will impact a wide range of applications in molecular electronics, photooptic devices, imaging and memory units, as well as photocatalysis, electrochemistry and artificial photosynthesis based on semiconductor materials with common photoinduced electronic excitations. The integration of the proposed research program with the Science, Technology and Research Scholars (STARS) program will naturally broaden the participation of underrepresented minorities in the physical sciences. Dissemination of research findings through the proposed Wiki interface will benefit the whole scientific community, facilitating the rapid exchange and distribution of results, software developments and pedagogical materials in the public domain.
’ (NSF ECCS Award # 1028066) by Batista (PI, Yale University) has established an interdisciplinary research program including Rego (University of Florianopolis, Brazil), Brudvig (Yale University) and Schmuttenmaer (Yale University), in a joint theoretical and experimental effort to advance our understanding of electronic relaxation processes at semiconductor surfaces functionalized with adsorbate sensitizers. The project has offered an opportunity to sustain an international co-mentorship program that exposes students to computational modeling and state-of-the-art laser technology by leading groups in the U.S. and Brazil. Research has been focused on functionalization of semiconductor surfaces with organic or inorganic molecules which is a promising technique to manufacture robust and inexpensive materials suitable for large scale technological applications, including devices for solar energy conversion and artificial photosynthesis. An important goal has been the characterization of conductivity in nanoporous semiconductor films at the molecular level, and the study of photoinduced electron-hole pair relaxation pathways associated with interfacial electron transfer. These are critical aspects that usually determine the overall efficiency of dye-sensitized solar cells. One of our findings was the discovery that the DC conductivity of mesoscopic TiO2 surfaces can be properly described, over the complete range of experimentally accessible temperatures, by a simple fluctuation induced tunneling conduction model. The model accounts for the Arrhenius behavior at high-temperatures and tunneling under cryogenic temperatures, as shown in the figure below. A practical methodology for simulations of transient electronic dynamics has been developed and applied to model photoexcitation processes in a wide range of semiconductor surfaces functionalized by molecular adsorbates. In one of these studies, the PI and Rego combined ab initio DFT molecular dynamics simulations and quantum dynamics propagation of transient electronic excitations to investigate visible light sensitization of TiO2-anatase surfaces by covalent attachment of derivatized aluminum tris-(8-hydroxyquinoline) (Alq3) complexes, as shown in the figure below. These studies introduced for the first time hydroxylated arylquinoline and benzoquinoline ligands have been introduced as robust anchoring groups to form Alq3/TiO2 molecular assemblies. In studies of quantum control, the PI and co-workers have investigated a general coherent control scenario to suppress or accelerate tunneling of quantum states decaying into a continuum, as shown in the figure below. The methodology is based on deterministic, or stochastic, sequences of unitary pulses that affect the underlying interference phenomena responsible for quantum dynamics, without inducing decoherence, or collapsing the coherent evolution of the system. The influence of control sequences on the ensuing quantum dynamics was analyzed by using perturbation theory to first order in the control pulse fields and compared to dynamical decoupling protocols and to sequences of pulses that collapse the coherent evolution and induce quantum Zeno (QZE) or quantum anti-Zeno effects (AZE). The analysis revealed a subtle interplay between coherent and incoherent phenomena and demonstrated that dynamics analogous to the evolution due to QZE or AZE can be generated from stochastic sequences of unitary pulses when averaged over all possible realizations. Considering that current laser technology can produce a wide range of pulses with ultrashort time resolution and extremely high-peak power, it is natural to expect that the quantum control techniques explored in our studies should raise significant experimental interest. The research and educational program was integrated with an outreach component that offered mentorship and research opportunities to underrepresented students from Yale and high-school students from Connecticut. Through partnership with advanced graduate students and post-docts from the Batista group, selected students benefited from research training and participation in computational modeling studies. The mentoring/tutoring experience gained benefited the students’ academic performance and allowed them to achieve success in higher education. In addition, the research program maintained a long-term international collaboration with Brazil by hosting visiting graduate students for training and long-distance collaboration. Locally, the research program allowed the PI to modernize the undergraduate curriculum through the development of a new course on Alternative Energy with frontier-research interdisciplinary topics and computational modeling assignments, including the design and computational modeling of sensitized semiconductors for solar-energy conversion, photocatalysis of waste-pollutants, and atomistic modeling molecular electronics assemblies.
