Session H4: Condensed Matter II

Recent advances in organic semiconducting materials

Organic semiconductors have attracted attention due to their low cost, easy fabrication, and tunable properties. Applications of organic materials in thin-film transistors, solar cells, light-emitting diodes, sensors, and many other devices have been actively explored. Recent advances in organic synthesis, material processing, and device fabrication led to significant improvements in (opto)electronic device performance. However, a number of challenges remain. These range from lack of understanding of basic physics of intermolecular interactions that determine optical and electronic properties of organic materials to difficulties in controlling film morphology and stability. In this presentation, current state of the field will be reviewed and recent results related to charge carrier and exciton dynamics in organic thin films will be presented.\\[4pt] In collaboration with Whitney Shepherd, Mark Kendrick, Andrew Platt, Oregon State University; Marsha Loth and John Anthony, University of Kentucky.   

Thermal transport and structural transitions in biological molecules

Structural transitions appear everywhere: proteins fold, nanotubes collapse, DNA denatures, ice melts, and so on. In biology, these transitions play a role in processes such as transcription and also determine protein function. Yet, at the same time, they give examples of highly {\it nonlinear processes} that are challenging to model and understand. I will discuss one such transition - the denaturation of DNA, where its double stranded form unravels into two single strands. There are many models that can describe certain aspects of this transition equally well, such as the fraction of bound base pairs versus temperature. I will show, however, that two well-known models yield drastically different predictions for thermal transport. The latter can then be used to ``peek inside'' DNA and understand what is happening during the denaturation transition. Thus, on the one hand, thermal transport gives a method to probe structural transitions in biological molecules and other materials. On the other hand, molecular systems and materials with nonlinear structural transitions also give opportunities for developing novel thermal devices.   

Theoretical Reconstruction of Realistic Dynamics of Polymer Melts from Soft Sphere Representation

A theoretical method to reconstruct realistic dynamics of polymer melts from highly coarse-grained descriptions has been developed from a first-principle approach. Starting from the Liouville equation and exploiting the Mori-Zwanzig projection operator formalism we have derived the generalized Langevin equations for the coarse-grained representations of polymer melts. Each polymer chain is coarse-grained at two levels: at the monomer level and at the molecular level as a soft sphere. By enforcing equivalence between the two descriptions in the long time regime where the internal dynamics is completely relaxed we derived an analytical rescaling formalism. Change in entropy and change in friction are the two corrections that need to be accounted. The rescaling factors explicitly depend on the thermodynamic and molecular parameters of the system simulated. After applying our reconstruction method, the dynamics obtained from mesoscale simulations of polyethylene and polybutadiene, represented as soft-colloidal particles, show quantitative agreement with experiments and atomistic simulations. The predictive power of the method is demonstrated for samples of long polyethylene chains.   

Gamma Radiation Tolerance of Magnetic Tunnel Junctions

Determining the radiation tolerance of magnetic tunnel junctions (MTJ), which are the storage elements of non-volatile magnetoresistive random access memories (MRAM), is important for investigating their potential application in space. In this effort, the effect of gamma radiation on MTJs with MgO tunnel barriers was studied. Experimental and control groups of samples were characterized by \textit{ex situ} measurements of the magnetoresistive hysteresis loops and I-V curves. The experimental group was exposed to gamma rays from a $^{60}$Co source. The samples initially received a dose of 5.9 Mrad (Si) after which they were again characterized electrically and magnetically. Irradiation was then continued for a cumulative dose of 10 Mrad and the devices re-measured. The result shows no change in magnetic properties such as coercivity or exchange coupling due to irradiation. After correcting for differences in temperature at the time of testing, the tunneling magnetoresistance was also found to be unchanged. Thus, it has been determined that MgO-based MTJs are highly tolerant of gamma radiation, particularly in comparison to silicon field-effect transistors which have been shown to degrade with gamma ray exposure even as low as 100 Krad [Zhiyuan Hu. et al., IEEE trans. on Nucl. Sci., vol. 58, 2011].   

Iron-Chalcogenide Based Solar Absorbers

Earth abundant, non-toxic solar absorbers are greatly desirable to reduce solar cell production cost. FeS$_{2}$ pyrite, with a band gap of $\sim $0.9 eV, is well known for outstanding absorption properties, yet significant photoconversion has never been achieved. Our computational and experimental study recognizes the failure mechanism of iron pyrite as an instability with respect to other Fe$_{x}$S (0.5$<$x$\le $1) metallic compositions. A set of design rules emerges for the realization of high absorption transition metal-chalcogenide absorbers. Fe$_{2}$MS$_{4}$ (M=Si,Ge) are proposed as viable candidates, and merit for solar absorber application discussed.   

Representation of Polymer melts as liquids of soft-colloid chains

Descriptions at various levels of coarse-graining are of great interest in understanding the complex structure and dynamics of polymer liquids, as relevant processes take place at a wide variety of length and time scales. In this talk we present a theory for a set of effective interaction potentials that map a polymer melt onto a liquid of soft-colloid chains, with each soft colloid representing the center of mass of a large subsection of a polymer chain. Molecular dynamics simulations using the effective potentials are shown to reproduce the predicted static structure, which has previously been tested against microscopic simulations. Because the theory provides analytical results and is based in first principles liquid state integral equation theory, the thermodynamics of the coarse-grained system using the effective potentials can be shown to agree with the thermodynamics of monomer-level descriptions across a range of thermodynamic states. The theory thus is able to represent of both the long-range structure and thermodynamic state of the system while retaining many-body physics on a range of large submolecular length scales.   

Thermodynamic Consistency in Highly Coarse-Grained Models of Polymer Melts

One of the main obstacles to the widespread application of coarse-graining methods in materials science is the lack of thermodynamics consistency between various hierarchical levels of description. Oftentimes, effective potentials that are optimized to reproduce one property of the system, such as the pair distribution function, fail to reproduce other properties of interest. Here we present a quantitative coarse-grained model of a polymer melt that preserves thermodynamic consistency and reproduces the correct equation of state of the system. Comparison is made to molecular dynamics simulations of various models and to results from integral equation theory. We also discuss some of the implications coarse-graining has for the entropy and free energy of the system due to a reduction of the degrees of freedom in the model.   

Dusting and time dependence of x-ray triboluminescence of adhesive tape

The intensity of x-ray triboluminescence from a continuously running belt of adhesive tape is highly time dependent, with a shape roughly like 1/t. Aiming to uncover mechanisms underlying time dependence and prolonging high-intensity x-ray production we investigated Zinc Oxide dusting which led to an entirely different regime of intensity as a function of time, hinting at glue flow and beta radiation damage as possible reasons for decreased intensity over time.   

First principles study of optical and electronic properties of anthradithiophene based organic conductors

Electronic band structures based on first principles density functional theory are reported for functionalized anthradithiophene (ADT) derivatives, such as ADT-TES-F (donor) and ADT-TIPS-CN (acceptor).The addition of side groups such as triethylsilylethynyl (TES) to the ADT backbone induces a change in the morphology from a herringbone to a planar crystal structure in which improved intermolecular $\pi$-orbital overlap increases carrier mobility. A comparison of the band gaps and effective masses deduced from theoretical calculations may indicate which side groups promote increased optical response and electrical conduction. We compare our results to available experimental results such as photoluminescence and photocurrent measurements.   

Temperature and Pore Size Dependence of a Nanoporous Platinum Based Hydrogen Senor

In this study, hydrogen sensing properties of nanoporous Pt films have been investigated for different pore sizes at various temperatures (25--100\r{ }C) and hydrogen concentrations (100-- 1000ppm). The nanoporous thin films were fabricated by a method of cosputtering, dealloying and coarsening. Cu$_{x}$Pt$_{1-x}$ thin films of thickness 150nm were formed by magnetron sputtering of Cu and Pt. These films were dealloyed in concentrated sulfuric acid to remove Cu. Coarsening of the dealloyed films at various temperatures produced nanoporous Pt thin films of different pore sizes. The morphologies of the nanoporous Pt films were studied by Scanning Electron Microscopy (SEM). Hydrogen sensing properties of the nanoporous Pt film were measured using a resistance transient method. It was found that the sensor response of the nanoporous Pt films was approximately 3.5{\%} at 1000ppm H$_{2}$ for a pore size of 35nm at room temperature. The detection limit was lower than 100 ppm at room temperature and the sensor showed repeatability.   

What happens to the energy of recrystallization of amorphous Si?

Si ICs require annealing a-Si to regain x-Si without nucleating crystallites ahead of the recrystallization front. Accepted models of a-Si (Gilmer et al. 2001) describe the state as a dense array of ``I-V pairs,'' linked 5 and 7 member rings that can revert to x-Si by a rebonding and small motion of 2 neighboring atoms to make 6 member rings. The defect energy is $\sim $ 3.5 eV, enough to melt between 6 and 7 x-Si atoms if it were abruptly delivered into the small region where the 2 odd rings intersect. Explosive crystallization of crystallites would be expected where 1 defect reverted in the a-Si. If the 2 atoms of the defect were to revert according to a configuration coordinate model like that of the Glyde-Flynn model of atomic diffusion, the energy would be abruptly deposited where the rings intersect by the atoms coming down from the saddle point. An alternative that would avoid the local release of atomic kinetic energy would be for the constraining bonds of the 2 odd rings to be momentarily weakened by an accumulation of h+s and e-s in localized bonding and antibonding states about the defect intersection long enough for the rebonding and motion of the 2 atoms to occur with little kinetic energy. To explain why this happens at steps on the recrystallization front and not in the bulk of the a-Si, one concludes that the localized states are confined to steps than in the bulk.   

Computer Modeling of the Self Assembly of Tyrosine

We used Scanning Tunneling Microscopy (STM) results to obtain information on bond symmetry, spacing, and general orientation of amino acids adsorbed on a graphite surface. One goal of our research is to compliment these experimental results using two computer programs , Igor and HyperChem, to model energetic behaviors between two molecules. This computational approach is used for a better understanding of the geometry and binding of the amino acid molecules. Previously, computer modeling and STM scans on Methionine, a neutral and non-polar amino acid with a Sulfide functional group, resulted in good agreements between experiment and modeling. These results were promising and led us to attempt the same for Tyrosine, a neutral and polar amino acid with a hydroxyphenyl functional group. Our STM results for Tyrosine showed a monolayer film with Tyrosine molecules ordered in parallel rows angled alternating at 110 degrees. For this configuration our approach to modeling was modified from that of Methionine. We have run computer simulations with HyperChem using the Amber94 force field, modeling the molecules in their neutral and zwitterionic states and using a single sheet of graphite as the substrate. I will be presenting first results of these calculations.   

Shell Model of BaTiO$_3$ derived from {\it{ab-initio}} DFT Calculations

A shell model for ferroelectric perovskites fitted to properties of first-principles density functional theory (DFT) is strongly affected by approximations made in the exchange-correlation functional within DFT and in general not as accurate as a shell model derived from experimental data. We have developed an isotropic shell model for BaTiO$_3$ based on the PBEsol exchange-correlation functional, which was specifically designed for solids and gives overall good agreement for the lattice parameters of all BaTiO$_3$ phases. Our shell model reproduces the sequence of phases of BaTiO$_3$ (rhombohedral, orthorhombic, tetragonal, cubic) and shows good agreement with experimental lattice constants at all temperatures, however the phase transition temperatures are too low. The energy scale can be improved by a simple scaling of the {\it{ab-initio}} potential energy surface. The polarization in the shell model is qualitatively correct but too small by approximately 30\% compared to the experimental value.