Session B36: Focus Session: New Energy II

Theories of plasmon enhanced optical processes important in solar energy

This talk will focus on the development of electronic structure methods that can be combined with electrodynamics calculations to describe enhancement in chemical reaction rates that arise from plasmon excitation of noble metal nanoparticles. Two types of enhancement are described: passive and active. In the passive case, the nanoparticle produces an enhanced electromagnetic field that acts externally to the nanoparticle to enhance a photochemical process, while in the active case, plasmon excitation leads to electron transfer to or from the noble metal nanoparticle. Examples and applications of each type will be described.   

Porphyrin Molecular Multilayer Thin-Films on Gold (111) Electrodes for Electro-optical Applications

We have developed a Layer-by-Layer (LbL) method for the fabrication of thin-film molecular multilayers on gold (111) electrodes. Copper(I) catalyzed azide-alkyne cycloaddition (CuAAC) coupling reactions were used for surface attachment and subsequent LbL deposition of porphyrin building blocks. The electrochemical and photophysical properties of the thin-films can be tuned through synthetic modification of the individual components, resulting in new porphyrin multilayers for applications in light harvesting and molecular electronics. Herein, we demonstrate the reproducible growth trends and optical properties of these films. Multilayer growth was followed by UV-Vis absorption and reflectance spectroscopy. Film thickness (FT) and optical constants were obtained from spectroscopic ellipsometry. Topology and surface roughness was examined by TM-AFM, while the copper content was quantified by XPS. The redox characteristics were studied by electrochemical methods, whereas the conductance of individual porphyrin constructs was examined by STM using the molecular break junction method. The multilayers show consistent linear growth in absorbance and FT over tens of layers and continuity in their molecular structure.   

Nonadiabatic Excited-State Molecular Dynamics (NA-ESMD): Numerical tests of convergence and parameters

Nonadiabatic molecular dynamics simulations, involving multiple coupled potential energy surfaces, often requires a large number of independent trajectories in order to achieve the desired convergence of the results, and simulation relies on different parameters that should be tested and compared. In addition to influencing the speed of the simulation, the chosen parameters combined with frequently implemented approximations can lead to unanticipated changes in the accuracy of the simulated dynamics. We have previously developed a nonadiabatic excited state molecular dynamics (NA-ESMD) methodology employing Tully's fewest switches surface hopping (FSSH) algorithm. In this study, we seek to investigate the impact of the number of trajectories and the various parameters on the simulation of the photoinduced dynamics of distyrylbenzene (a small oligomer of polyphenylene vinylene) within our developed framework. Various user-defined parameters are analyzed: classical and quantum integration time steps, and the number of trajectories used for statistical averaging. Common approximations such as reduced number of nonadiabatic coupling terms and the classical path approximation (CPA) are also investigated.   

Probing Nanointerfaces of Nanoparticle-Based Solar Energy Conversion: Molecular Dynamics on the Angstrom Scale

Charge-induced surface chemistry following solar-illumination of nanometer-scale semiconductor particles is one possible route to low-cost solar energy conversion. Because of their high surface to volume ration interfaces play a major role in the performance of this technology and thus the physics of charges interacting with adsorbed molecules is of central interest. In this talk, after a brief review of related work, we will focus on our advances in understanding molecular-dissociation-dynamics at the interface of nanometer-scale TiO$_{2}$ crystals and large organic molecules. Clearly the standard probes of beam-based molecule dynamics are not easily adaptable to small nanointerfaces. Instead our work uses STM imaging to examine dynamics of fragments following tip-induced electron injection into organic molecules on TiO$_{2}$ (110) and on $\sim $10nm nanocrystals (110). Our experiments have used halogenated anthracene to probe the efficiency and fragment trajectory following dissociative electron capture. Our work has examined nanoparticle synthesis, adsorbate-molecule orientation, thermal and injected-electron chemistry, and adsorbate charge-mediated fragmentation trajectories.   

Band gap engineering iron pyrite for sustainable solar energy conversion

In the quest to develop sustainable materials for solar energy conversion, iron pyrite (FeS2) holds great promise as a solar absorber. The electronic band gap of FeS2, however, is not well matched to the solar spectrum. Here we explore chemical doping as a strategy to engineer the band gap of FeS2, as has been successfully demonstrated with other semiconducting materials. Using first-principles calculations, we first establish the relationship between pressure, lattice distortions, and the electronic structure of FeS2, and rationalize the results in terms of distortions in the crystal-field splitting of Fe. We then investigate the effects of doping FeS2 with transition metal elements, wherein our strategy is guided by the knowledge of band gap dependence on local distortions.   

Photochemistry of chemisorbed and physisorbed O$_{2}$ on reduced rutile TiO$_{2}$(110)

The ultraviolet (UV) photon-stimulated reactions of oxygen on TiO$_{2}$(110) are studied. For chemisorbed O$_{2}$, the photochemistry depends on the O$_{2}$ coverage. For small coverages, only $\sim $14{\%} desorbs while the rest either dissociates during UV irradiation, or remains molecularly adsorbed on the surface. For the maximum coverage of chemisorbed oxygen, the fraction of O$_{2}$ that photodesorbs is $\sim $40{\%}. However when physisorbed O$_{2}$ is also present, $\sim $70{\%} of the initially chemisorbed O$_{2}$ photodesorbs. Experiments using O$_{2}$ isotopologues show that UV irradiation results in exchange of atoms between the chemisorbed and physisorbed oxygen. Annealing chemisorbed oxygen to 350 K maximizes these exchange reactions. The exchange products photodesorb in the plane perpendicular to the bridge-bonded oxygen rows at an angle of 45\r{ }. Remarkably, the chemisorbed species is stable under multiple cycles of UV irradiation with physisorbed O$_{2}$, and the atoms in the chemisorbed species can be changed from $^{18}$O to $^{16}$O and then back to $^{18}$O via the exchange reactions. The results show that annealing oxygen adsorbed on TiO$_{2}$(110) to $\sim $350 K produces a stable chemical species with interesting photochemical properties. Possible forms for the photoactive species include O$_{2}$ adsorbed in a bridging oxygen vacancy or tetraoxygen.   

Energy Materials in Extreme Environments

The critical shortage of abundant, affordable, and clean energy calls upon novel materials with extreme properties for energy production, storage, conversion, and transfer that are superior to materials that now exist or are in use today. Twenty-first century energy technology also demands enhanced performance from existing materials under extreme environments of pressure, temperature, chemistry, radiation, and electromagnetic fields. Investigating the behavior of materials in extreme pressures and temperatures, in particular, provides the fundamental knowledge needed to address these problems. These studies are leading to the discovery of both new materials with enhanced performance as well as new physical and chemical phenomena, and take advantage of advances at national x-ray, infrared, neutron, and laser facilities. An important example is the continued study of hydrogen-rich materials, from investigations of transformations in pure hydrogen, which has now been pressurized well above 300 GPa, to the discovery of new hydrogen storage materials and hydriding reactions induced by extreme conditions. Other examples include studies of carbon-based materials, which are also deepening our understanding of carbon sources and cycling within the planet.