Spectroscopic Identification of the Charge Transfer State in Thiophene/Fullerene Heterojunctions: Electroabsorption Spectroscopy from GW/BSE Calculations.

Creation of charge transfer (CT) states in bulk heterojunction systems such as C60/polymer blends is an important intermediate step in the creation of carriers in organic photovoltaic systems. CT states generally have small oscillator strengths in linear optical absorption spectroscopy owing to limited spatial overlap of electron and hole wave functions in the CT excited state. Electroabsorption spectroscopy (EA) exploits changes in wave function character of CT states in response to static electric fields to enhance detection of CT states via nonlinear optical absorption spectroscopies. A 4 × 4 model Hamiltonian is used to derive splittings of even and odd Frenkel (FR) excited states and changes in wave function character of CT excited states in an external electric field. These are used to explain why FR and CT states yield EA lineshapes which are first and second derivatives of the linear optical absorption spectrum. The model is applied to ammonia-borane molecules and pairs of molecules with large and small B-N separations and CT or FR excited states. EA spectra are obtained from differences in linear optical absorption spectra in the presence or absence of a static electric field and from perturbative sum over states (SOS) configuration interaction singles χ(2) and χ(3) nonlinear susceptibility calculations. Good agreement is found between finite field (FF) and SOS methods at field strengths similar to those used in EA experiments. EA spectra of three C60/oligothiophene complexes are calculated using the SOS method combined with GW/BSE methods. For these C60/oligothiophene complexes, we find several CT states in a narrow energy range in which charge transfer from the thiophene HOMO level to several closely spaced C60 acceptor levels yields an EA signal around 10% of the signal from oligothiophene.

Charge transfer excitons in π-stacked thiophene oligomers and P3[Alkyl]T crystals: CIS calculations and electroabsorption spectroscopy.

Poly(3-alkylthiophenes) (P3[Alkyl]T) exhibit high mobility and efficiency of formation of polaronic charge carriers generated by light absorption, thus finding applications in field effect devices. Excited states of π-stacked dimers of tetra-thiophene oligomers (T4), infinite isolated polythiophene (PT) chains, and P3[Alkyl]T crystals are modeled using configuration interaction singles (CIS) calculations. Excited states in cofacial T4 dimers are mostly localized Frenkel states except for two low energy charge transfer (CT) exciton states, which become the ionization potential and electron affinity levels of T4 molecules at large dimer separation. The lowest excited states in infinite, isolated PT chains and P3[Alkyl]T crystals are intra-chain excitons where the electron and hole are localized on the same chain. The next lowest excited states are interchain, CT excitons in which the electron and hole reside on neighboring chains. The former capture almost all optical oscillator strength and the latter may be a route to efficient formation of polaronic charge carriers in P3[Alkyl]T systems. Changes in optical absorption energies of T4 dimers as a function of molecular separation are explained using CIS calculations with four frontier orbitals in the active space. Shifts in optical absorption energy observed on going from isolated chains to P3[Alkyl]T lamellar structures are already present in single-particle transition energies induced by direct π-π interactions at short range. The electroabsorption spectrum of T4 dimers is calculated as a function of dimer separation and states that are responsible for parallel and perpendicular components of the spectrum are identified.

Many-body theory of positron binding to polyatomic molecules.

Positron binding to molecules is key to extremely enhanced positron annihilation and positron-based molecular spectroscopy1. Although positron binding energies have been measured for about 90 polyatomic molecules1-6, an accurate ab initio theoretical description of positron-molecule binding has remained elusive. Of the molecules studied experimentally, ab initio calculations exist for only six; these calculations agree with experiments on polar molecules to at best 25 per cent accuracy and fail to predict binding in nonpolar molecules. The theoretical challenge stems from the need to accurately describe the strong many-body correlations including polarization of the electron cloud, screening of the electron-positron Coulomb interaction and the unique process of virtual-positronium formation (in which a molecular electron temporarily tunnels to the positron)1. Here we develop a many-body theory of positron-molecule interactions that achieves excellent agreement with experiment (to within 1 per cent in cases) and predicts binding in formamide and nucleobases. Our framework quantitatively captures the role of many-body correlations and shows their crucial effect on enhancing binding in polar molecules, enabling binding in nonpolar molecules, and increasing annihilation rates by 2 to 3 orders of magnitude. Our many-body approach can be extended to positron scattering and annihilation γ-ray spectra in molecules and condensed matter, to provide the fundamental insight and predictive capability required to improve materials science diagnostics7,8, develop antimatter-based technologies (including positron traps, beams and positron emission tomography)8-10, and understand positrons in the Galaxy11.

Dielectric Anisotropy of the GaP/Si(001) Interface from First-Principles Theory.

First-principles calculations of the dielectric anisotropy of the GaP/Si(001) interface are compared to the anisotropy extracted from reflectance measurements on GaP thin films on Si(001) [O. Supplie et al., Phys. Rev. B 86, 035308 (2012)PRBMDO1098-012110.1103/PhysRevB.86.035308]. Optical excitations from two states localized in several Si layers adjacent to the interface result in the observed anisotropy of the interface. The calculations show excellent agreement with experiment only for a gapped interface with a P layer in contact with Si and show that a combination of theory and experiment can reveal localized electronic states and the atomic structure at buried interfaces.

Reflectance anisotropy of the anatase Tio2(0 0 1)-(4 × 1) surface.

The dielectric functions of bulk anatase TiO(2) and its (4 × 1) reconstructed (0 0 1) surface were calculated using a hybrid density functional theory method. The bulk dielectric function is compared to data obtained from synchrotron radiation reflectivity measurements in the energy range 0-20 eV and to dielectric functions from two Bethe-Salpeter calculations. There is agreement between predictions of the two theoretical methods, except at the absorption threshold, where there is some shift of spectral weight to lower energy in the Bethe-Salpeter calculations. All features observed in the dielectric function derived from synchrotron reflectivity data are reproduced in calculations. There are some differences in relative peak intensities for dielectric functions derived from experiment and theory. A dispersionless surface state is found which is localised on the topmost oxygen ion in the reconstructed surface. The reflectance anisotropy spectrum of the TiO(2)(0 0 1)-(4 × 1) surface shows strong features in the energy range to 12 eV but does not show any features caused by sub-bandgap surface state transitions.

Transition levels of defects in ZnO: total energy and Janak's theorem methods.

Transition levels of defects are commonly calculated using either methods based on total energies of defects in relevant charge states or energy band single particle eigenvalues. The former method requires calculation of total energies of charged, perfect bulk supercells, as well as charged defect supercells, to obtain defect formation energies for various charge states. The latter method depends on Janak's theorem to obtain differences in defect formation energies for various charge states. Transition levels of V(Zn), V(O), and V(ZnO) vacancy defects in ZnO are calculated using both methods. The mean absolute deviation in transition level calculated using either method is 0.3 eV. Relative computational costs and accuracies of the methods are discussed.

Detection of organics using porphyrin embedded nanoporous organosilicas.

Molecularly imprinted polymers and silica have been studied as receptor binding site mimics for use in a wide range of separation, catalysis, and detection applications employing transduction mechanisms including conductometric, amperometric, and capacitance. Porphyrins are also well known as sensor components due to the extreme sensitivity of their spectrophotometric characteristics to changes in their immediate environment. We have developed periodic mesoporous organosilicas (PMO) which incorporate a porphyrin into the material for use as an optical indicator of target binding. This material combines the stability, selectivity, and high density of binding sites characteristic of the molecularly imprinted PMO with the sensitivity and selectivity of the porphyrin. We demonstrate binding of p-nitrophenol, p-cresol, 2,4,6-trinitrotoluene, and RDX by the porphyrin-embedded PMOs with selective adsorption of TNT over the other analytes. In addition, the binding of each of the organics by the PMO results in unique changes in the spectrophotometric characteristics of the incorporated porphyrin. These changes can be observed by visual inspection or through the use of fluorescence spectra collected in 96-well format.

Immobilization and hybridization of DNA in a sugar polyacrylate hydrogel.

Using a non-contact microarrayer, amine-terminated probe oligonucleotides representing 20-, 50-, and 70-mer fragments of the fliC gene were covalently coupled into three-dimensional regions in a "sugar polyacrylate" hydrogel based on poly(6-acryloyl-beta-O-methyl galactopyranoside-co-aminopropyl methacrylamide). The arrayer deposited the solution containing ssDNA probes in discrete regions on the surface of the gel (i.e. as a droplet with a ca. 450 microm diameter), allowing penetration and attachment of the ss DNA within the three dimensional region of the gel. The attachment was mediated by the homobifunctional crosslinker bis-succinimidyl suberate. Confocal microscopy showed the density of attached probe DNA was greatest in the interior-most regions of the gel volume. Target ssDNA (20- and 70-mer) was able to diffuse through the gel and undergo successful hybridization with the probes. For target ssDNA in the concentration range 0.19 microM to 6.0 microM, there was a linear correlation between DNA concentration and the fluorescence of the gel region where hybridization occurred.

Molecularly inherent voltage-controlled conductance switching.

Molecular electronics has been proposed as a pathway for high-density nanoelectronic devices. This pathway involves the development of a molecular memory device based on reversible switching of a molecule between two conducting states in response to a trigger, such as an applied voltage. Here we demonstrate that voltage-triggered switching is indeed a molecular phenomenon by carrying out studies on the same molecule using three different experimental configurations-scanning tunnelling microscopy, crossed-wire junction, and magnetic-bead junction. We also demonstrate that voltage-triggered switching is distinctly different from stochastic switching, essentially a transient (time-dependent) phenomenon that is independent of the applied voltage.

Detection of proteins cross-linked within galactoside polyacrylate-based hydrogels by means of a quantum dot fluororeagent.

Protein toxins have been immobilized in a galactoside polyacrylate hydrogel in a microarray format. The large pore size and solution-like environment of these novel hydrogels allow for easy penetration of large proteins and detection reagents. Confocal microscopy provided three-dimensional visualization of dye-labeled toxins cross-linked within the gel and of streptavidin-coated quantum dot (QD) fluorophores used to visualize the toxins after incubation with biotinylated anti-toxin antibodies. Fluorescence microscopy was utilized to visualize arrays of toxins detected by a biotinylated antibody and then exposure to streptavidin-conjugated QDs. The intensity of the QD fluorescence was quantified, and binding to two toxins on three types of hydrogels was examined.

Characterization and performance evaluation of in vivo and in vitro produced monoclonal anti-TNT antibodies for the detection of TNT.

In this paper are the experimental results used to characterize four distinct monoclonal anti-TNT antibodies (in vivo and in vitro cloned) for potential use in a field-portable immunosensor. Direct and competitive enzyme-linked immunosorbent assays (ELISA) were performed to determine their affinity for TNT and a fluorescently labeled analog of TNT for use in an immunosensor. Effective concentrations (EC(50)), inhibition concentration (IC(50)) and cross-reactivity measurements to related nitroaromatics (e.g., 2,4,6-trinitrobenzene [TNB], methyl-2,4,6-trinitrophenyl nitramine [tetryl], 2-amino-4,6-dinitrotoluene [2A-4,6-DNT], 2,4-dinitrotoluene [2,4-DNT] and 1,3-dinitrotoluene [1,3-DNT]) were measured. Final characterization of the monoclonal antibodies was based on performance (measured by fluorescence dose response) using a fluorescence-based microcapillary displacement assay. Analytical techniques showed a high degree of affinity for TNT and varying degrees of cross-reactivity with each respective monoclonal antibody. Microcapillary displacement immunoassays with each of the antibodies resulted in detection capabilities at the lowest applied TNT concentration (10 ng/ml).

Detection of 2,4,6-trinitrotoluene in environmental samples using a homogeneous fluoroimmunoassay.

We have tested both soil and water environmental samples for 2,4,6-trinitrotoluene (TNT) using a simple homogeneous assay. This assay is based on changes in fluorescence emission intensity when a fluorescently labeled TNT analogue, bound to an anti-TNT antibody, is competitively displaced by TNT. Fluoroimmunoassay results for TNT concentrations in diluted acetone extracts prepared from archived soils were in good agreement with the results from high-performance liquid chromatography analysis of the same sample extracts. In addition, assays of TNT-spiked environmental well water gave results essentially identical with assays conducted in a TNT-spiked laboratory buffer. The homogeneous fluoroimmunoassay, which is rapid, simple, sensitive, and amenable to high throughput screening, shows promise for near real-time evaluation of TNT contamination in environmental samples.

Ligand effects on charge transport in platinum(II) acetylides.

To investigate the electrical characteristics of organometallic complexes as molecular conductors, organometallic pi-conjugated molecules of the type trans-[PtL2(CCC6H4SAc-4)2], where L = PCy3, PBu3, PPh3, P(OEt)3, P(OPh)3, were synthesized and characterized by NMR, IR, UV, and X-ray spectroscopies. For the three complexes (L = PCy3, PPh3, and P(OEt)3) that could be measured using a cross-wire junction technique, the current-voltage (I-V) characteristics of a molecular monolayer of these complexes showed no ligand effect, despite spectroscopic evidence that electronic interaction between the phosphine ligands and the pi-system does occur. It was concluded that the tunneling efficiency across the molecule is the determining factor for conduction in this metal-molecule-metal system. It was also shown that the incorporation of a transition metal in pi-conjugated molecular wires does not adversely affect charge transport compared to all-carbon pi-conjugated molecular wires.

Analysis of aqueous 2,4,6-trinitrotoluene (TNT) using a fluorescent displacement immunoassay.

We report a rapid, simple, and sensitive assay that is potentially amenable to high throughput screening for analysis of 2,4,6-trinitrotoluene (TNT) present in aqueous solutions. The assay is based on the change in fluorescence emission intensity of a fluorescently labeled TNT analogue pre-bound to an anti-TNT antibody that occurs upon its competitive displacement by TNT. The assay can be performed in both cuvette- and 96-well plate-based formats. TNT at a level of 0.5 micro g L(-1) (0.5 ppb) was detected in phosphate buffered saline; detection improved to 0.05 micro g L(-1) (0.05 ppb) for TNT dissolved in artificial seawater.

Demonstration of four immunoassay formats using the array biosensor.

The ability of a fluorescence-based array biosensor to measure and quantify the binding of an antigen to an immobilized antibody has been demonstrated using the four different immunoassay formats: direct, competitive, displacement, and sandwich. A patterned array of antibodies specific for 2,4,6-trinitrotoluene (TNT) immobilized onto the surface of a planar waveguide and used to measure signals from different antigen concentrations simultaneously. For direct, competitive, and displacement assays, which are one-step assays, measurements were obtained in real time. Dose-response curves were calculated for all four assay formats, demonstrating the array biosensor's ability to quantify the amount of antigen present in solution.