Copyright (c) 2009 All rights reserved. http://works.bepress.com/tunna_baruah Recent documents in Tunna Baruah en-us Sun, 31 May 2009 12:50:36 PDT 3600 http://works.bepress.com/tunna_baruah/5 http://works.bepress.com/tunna_baruah/5 Wed, 10 Oct 2007 10:48:51 PDT The development of environmentally friendly, inexpensive, lightweight solar cells would significantly enhance both sea- and land- based DoD operations and directly enhance global security through a reduction of competition for carbon-based fuels. For example, capturing one out of every 10000 photons from the sun would provide all the power the world currently uses. Approximately 75% of the solar radiation striking the upper atmosphere (1368 W/m2 = 8555 eV/sec/nm2 ) reaches the surface of the earth and most of this is in the form of photons with more than 1 eV of energy since water vapor and other atmospheric constituents effectively absorb energy below this threshold. In order to produce a one micron photovoltaic molecular film (composed of approximately 1000 layers of a molecule) with 10-20 percent efficiency, each molecule would have to create 1-3 eV/sec. Since the available solar radiation is in the 1-4 eV range, this implies ~1-3 electron-hole pairs per second which biological systems currently achieve. As such, various organic molecules, some of which are biologically inspired, have been proposed as alternative building blocks for solar energy materials. In the first phase of our challenge project, we have performed calculations on a molecular triad composed of a fullerene, a porphyrin and a carotenoid polyene. By calculating electronic structures, approximate excited states and respective dipole transition rates, we have simulated charge transfer dynamics in a collection of molecules exposed to an appropriate bath of solar photons. The resulting time constants associated with capture of solar radiation in the form of a charge-separated state have been determined. In the first phase of this challenge project, only electronic excitations were considered in the kinetic Monte Carlo modeling. Furthermore, the low symmetry in the molecular triad made it relatively easy to identify charge transfer excitations. In this phase of the project we report four new computationally intensive investigations that are aimed at validating and extending the formalism. First, to justify the use of an approximate excited state formalism, we have performed analogous calculations of excited states on a much larger selection of molecules and atoms and compared these results to experiment. Second we have performed calculations entirely analogous to our earlier molecular-triad results on highly idealized and high-symmetry fullerene tubules. Third we have used and extended a recently developed method[15] to calculate all electron-hole-phonon interactions in these carbon nanotubes as well as the light-harvesting molecular triad. Fourth, in analogy to symmetry-breaking methods used for density-functional treatments of ferro- and ferri- magnetic ordering in molecular magnets, we have developed and tested a massively parallel computational method, employing coarse and fine-grain strategies, for identifying self trapped ferroelectric excited states in highly symmetric carbon nanotubes. These techniques allow us to determine charge-transfer excitations and to determine whether excited-states are to be viewed within a Franck-Condon or Marcus-Hush picture. Tunna Baruah http://works.bepress.com/tunna_baruah/4 http://works.bepress.com/tunna_baruah/4 Wed, 10 Oct 2007 10:48:49 PDT The solar energy is an alternative source of renewable energy. The current inorganic solar cells mostly based on amorphous Si have been able to provide an efficiency of 10-20% for solar energy conversion. Organic solar cells have been proposed as another alternative mostly due to the high quantum efficiency of solar power conversion seen in natural world. They offer the possibility for lighter-weight and environmentally clean renewable energy sources. We present a first-principles study on a bio-mimetic light harvesting organic. This 207-atom molecular triad consists of a fullerene, a porphyrin and a carotenoid. In order to simulate this energy conversion it is necessary to determine the transition rates from density functional theory and then use a Monte-Carlo approach to simulate the photon induced charge-separation process. Charge transfer excited states have been calculated using a new density-functional based method. We have used NRLMOL and honey-bee algorithms to perform the electronic structure calculations on the NRL SGI-Altix system. The Monte-Carlo simulations have used a coarse grained approach in which many different starting conditions are studied on individual processors. We find that the molecule possesses a dipole moment of 180D in the charge-separated state. We show that an electric field and incident solar radiation are necessary for excitation into a charge-separated state. The necessary electric field can be realized through a solvated ionic solution or in the crystalline phase. Such excitation of one molecule in solution can quickly trigger a dipolar-induced cascade process leading to aligned chains and possibly solar-induced current. Tunna Baruah http://works.bepress.com/tunna_baruah/3 http://works.bepress.com/tunna_baruah/3 Wed, 10 Oct 2007 10:48:49 PDT We review some recent applications of density-functional theory to molecules and systems of molecules where the role of polarizabilities are particularly relevant. With respect to the implementation of density-functional theory, details related to numerics and basis sets are described. We then describe how self-consistent finite-field calculations may be used to separately extract the electronic polarizability tensor and infrared intensities. We review the relationship between second-harmonic vibrational polarizabilities and molecular infrared intensities. An efficient method for describing the polarization effects in system of molecules is included and a recent application of this method to a biomimetic light-harvesting complex is discussed. Tunna Baruah http://works.bepress.com/tunna_baruah/2 http://works.bepress.com/tunna_baruah/2 Wed, 10 Oct 2007 10:48:48 PDT We present a study on the electronic structure of a biology-inspired molecular triad which shows promises in replicating photosynthesis process in the laboratory. The triad contains three different units - \C60, porphyrin and beta-carotenoid. We present its geometrical and electronic structure, dipole moments, optical absorption spectrum and its polarizability calculated with an all-electron density functional approach. Such a study will be useful for further understanding of its photo-conversion properties. Tunna Baruah http://works.bepress.com/tunna_baruah/1 http://works.bepress.com/tunna_baruah/1 Wed, 10 Oct 2007 10:48:47 PDT We present the first ab-init study of the geometry, electronic structure, charged states, bonding and vibrational modes of the recently synthesized fullerene-like As@Ni12@As20 cluster which has icosahedral point symmetry [Science, 300, 778 (2003)]. We show that the molecule is vibrationally stable and will be electronically most stable in its -3 oxidation state in the condensed phase and in $-2$ state in the gas phase. We examine the bonding in this unusually structured molecule from charge trannsfer between atoms, infrared and Raman spectra, and charge density isosurfaces. Tunna Baruah