Showing papers by "Peter J. Rossky published in 2016"

Relating Chromophoric and Structural Disorder in Conjugated Polymers

Abstract: The optoelectronic properties of amorphous conjugated polymers are sensitive to conformational disorder and spectroscopy provides the means for structural characterization of the fragments of the chain which interact with light - "chromophores". A faithful interpretation of spectroscopic conformational signatures, however, presents a key challenge. We investigate the relationship between the ground state optical gaps, the properties of the excited states, and the structural features of chromophores of a single molecule poly(3-hexyl)-thiophene (P3HT), using quantum-classical atomistic simulations. Our results demonstrate that chromophoric disorder reflects an interplay between excited state de-localization and electron-hole polarization, and is controlled by torsional disorder that is specifically associated with the presence of side chains. Within this conceptual framework, we predict and explain a counter-intuitive spectral signature of P3HT: a red-shifted absorption, despite shortening of chromophores, with increasing temperature.

Communication: Isotopic effects on tunneling motions in the water trimer

Abstract: We present results of ring polymer molecular dynamics simulations that shed light on the effects of nuclear quantum fluctuations on tunneling motions in cyclic [H2O]3 and [D2O]3, at the representative temperature of T = 75 K. In particular, we focus attention on free energies associated with two key isomerization processes: The first one corresponds to flipping transitions of dangling OH bonds, between up and down positions with respect to the O-O-O plane of the cluster; the second involves the interchange between connecting and dangling hydrogen bond character of the H-atoms in a tagged water molecule. Zero point energy and tunneling effects lead to sensible reductions of the free energy barriers. Due to the lighter nature of the H nuclei, these modifications are more marked in [H2O]3 than in [D2O]3. Estimates of the characteristic time scales describing the flipping transitions are consistent with those predicted based on standard transition-state-approximation arguments.

Chemical Understanding of the Mechanisms Involved in Mitigation of Charged Impurity Effects by Polar Molecules on Graphene

Abstract: It is well-known that the transport properties of monolayer graphene are degraded by charged impurities present between graphene and either a given substrate or air. Such impurities cause charge scattering of holes and electrons in graphene. In previous work, our group has used both fluoropolymer thin films and polar vapor molecules to dramatically improve graphene field-effect transistor (FET) device characteristics, including Dirac voltage and mobility. We attributed the graphene device improvements to mitigation of charged impurities and defects due to electrostatic interaction with the dipoles of the applied fluoropolymers and polar molecules. In this work, we present theoretical support to this hypothesis, in the form of computational chemical simulations involving the interaction of polar molecules and impurities on a graphene sheet. We examine two types of impurities which may occur at graphene interfaces: ionic impurities and molecular dipole impurities. Upon introduction of polar vapor molecules ...