界面計算科学グループ

CMSセミナー2012

Simulation of charge transfer: from oxides to organic semiconductors to proteins

Dr. Jochen Blumberger
【Date & Time】14 December 2012 (Fri), 15:30-16:30
【Place】Auditorium, 1F, WPI-MANA bldg. Namiki site
【Speaker】Dr. Jochen Blumberger
【Affiliation】Dept. of Physics and Astronomy, University College London, UK
【Title】Simulation of charge transfer: from oxides to organic semiconductors to proteins
【Abstract】
Charge transfer and transport plays a crucial role in many energy converting processes, both in the inorganic world as well as in biology. In this talk I will review some of the progress we have made in the computation of the parameters that determine the thermodynamics, kinetics and the mechanism of charge transport in condensed phase systems based on constrained and fragment orbital density functional theory methods. Applications we will discuss are hole transfer between oxygen vacancies in MgO, electron transfer in fullerene materials and in a deca-heme 'wire'-protein. We find that while for proteins, charge hopping models usually provide a good description, this is not the case for organic semiconductors. For the latter, the dynamics of the excess charge should not be integrated out, as is typically done in hopping models, but solved explicitly, e.g. via non-adiabatic dynamics methods. Oxide materials form an intermediate case, where charge hopping models are valid for tunneling distances that are larger than a certain critical defect separation but need to be replaced by an appropriate scattering (Green’s function) approach for smaller distances. We expect that our findings will guide future modeling efforts of gate dielectrics in transistors, organic semiconductors for organic photovoltaics, and electron transporting proteins for use in biofuel cells.
【Contact】Dr. Yoshitaka Tateyama(館山佳尚)

Towards an atomistic picture of the active interface in dye sensitized solar cells

Prof. Joost VandeVondele
【Date & Time】6 July 2012 (Fri), 16:00-16:45
【Place】The 2nd conference room (1F), Sengen site
【Speaker】Prof. Joost VandeVondele
【Affiliation】Nanosimulations, ETH Zurich
【Title】Towards an atomistic picture of the active interface in dye sensitized solar cells
【Abstract】
In order to optimize the performance of dye sensitized solar cells (DSSC), detailed insight in the atomistic processes at the sensitized solid/liquid interface is very valuable. We will present results of computer simulations based on atomistic models and density functional theory. The aim of these simulations is to provide a atomistic model of the interface, which takes all important components (oxide, dye, electrolyte), explicitly into account.
In particular, we have computed various binding modes of the N3 dye on anatase (101), and have compared computed with experimental IR spectra. These results allow for predicting the most stable surface conformation, and clarifies the various carboxylate-Ti interactions. Extensive classical simulations of the TiO2-acetonitrile interface provide an explanation for the good performance of the acetonitrile/I3 -/I- electrolyte. Finally, we have studied the interaction of the redox mediator with the N3 dye in solution, suggesting an efficient pathway for dye regeneration and an alternative mechanism for the formation of I3-.
【Contact】Dr. Yoshitaka Tateyama(館山佳尚)

Electronic and structural defects in transition-metal-oxide semiconductors

Prof. Anderson Janotti
【Date & Time】29 June 2012 (Fri), 16:00-17:00
【Place】4F room 409/410, Collaborative Bldg., Namiki site
【Speaker】Prof. Anderson Janotti
【Affiliation】Materials Department, University of California, Santa Barbara
【Title】Electronic and structural defects in transition-metal-oxide semiconductors
【Abstract】
There is great interest in developing transition-metal oxides such as TiO2 and SrTiO3 for semiconductor devices applications. As with any semiconductor, one of the main challenges in the early stages of development is to control the conductivity by doping and reducing the concentration of native defects and unintentional impurities, thus, minimizing their deleterious effects. These transition-metal oxides significantly differ from conventional semiconductors (Si, Ge, and GaAs) with respect to their electronic structure and charge transport. While in conventional semiconductors, electron and holes originated from doping are delocalized and result in high carrier mobility, excess electrons in TiO2 and SrTiO3 occupy degenerate conduction bands derived from Ti d states, leading to a variety of interesting physical phenomena; for instance, is often argued that transport in TiO2 occurs through hopping of small polarons, giving rise to rather low electron mobility. Here we will present results of density functional theory and hybrid functional for electronic and structural defects in TiO2 and SrTiO3. We will discuss the effects of charge localization in the form of small polarons in TiO2 and the consequences for optical and transport measurements. Shortcomings of the conventional density functional methods in describing charge localization will also be addressed.
【Contact】Dr. Naoto Umezawa(梅澤直人)
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