Christian D. Grant

A technique to compare polythiophene solid-state dye sensitized TiO2 solar cells to liquid junction devices

In this communication, we report on a technique to fabricate solid-state polythiophenebased dye sensitized solar cells (DSSCs) that can be directly compared to analogous liquid junction devices. The device configuration is based on non-porous TiO2 thin films and one of the three undoped polythiophene hole conductors: poly[3-(11 diethylphosphorylundecyl) thiophene], P3PUT, poly(4-undecyl-2,2 0 -bithiophene), P4UBT, or poly(3-undecyl-2,2 0 -bithiophene), P3UBT. These polymers were spin coated and cast from organic solutions onto the TiO2 films. The dense TiO2 thin films (ca. 30 nm) were deposited on conductive glass via facile spray pyrolysis and sol–gel techniques. After that, cis-(SCN)2 Bis(2,2 0 bipyridyl-4,4 0 dicarboxylate) ruthenium(II) (a.k.a. Ru N3 dye) was adsorbed on the TiO2 surface, and the polythiophenes were utilized as hole conductors in a simplified solar cell geometry. The results were compared to the control DSSC device made with dense TiO2 and a liquid electrolyte, or

Characterization of nanocrystalline and thin film TiO2 solar cells with poly(3-undecyl-2,2′-bithiophene) as a sensitizer and hole conductor

We report on the use of poly(3-undecyl-2,2′-bithiophene) and three different types of TiO2 film layers to determine the dependence of photovoltaic performance on the morphology of the TiO2 films. It was observed that the TiO2 film morphology plays an important role in the performance of photovoltaic solar cells with polythiophene used as both sensitizer and hole conductor. The polymer was tested on a flat TiO2 layer made from a sol–gel process, a larger surface area TiO2 layer derived from an aqueous TiF4 solution, and a nanocrystalline thin TiO2 film. We observe over twice the improvement in short circuit current density in the nanocrystalline cells (jsc=233.6 μA cm−2) over that of the flat titania cells (89.8 μA cm−2). The best photocurrent density (448 μA cm−2) performance was from the TiF4 derived titania based cells when using the polymer. It was noted that soaking the nanocrystalline and TiF4 cells in the polymer for at least 24 h improves their overall performance, while no noticeable improvement is evident for the flat sol–gel derived cells. We explain our results by proposing that pore filling is likely to be easier in the TiF4 derived TiO2 due to larger pore sizes. Multiple reflections of light may also play an important role in the TiF4 titania films by enhancing the amount of light absorbed by the polymer. These observations suggest that the relative size of the sensitizer molecule and the pores of nanocrystalline films may be a critical factor to consider in designing photovoltaic devices such as solar cells based on nanoporous materials.

Silica anchored fluorescent organosilicon polymers for explosives separation and detection

The luminescent organosilicon copolymers poly(silafluorenyldiethynylspirobifluorene) and poly(tetrasilolediethynylspirobifluorene) have been covalently linked to a silica gel thin layer chromatography (TLC) support through the use of a trimethoxysilyl end group. Surface functionalization of silica with the fluorescent sensing polymer allows for more efficient quenching by the analyte, due to the small amount of fluorophore present, thus yielding enhanced detection sensitivity. The attachment of the sensing polymers onto a chromatographic support also allows for simultaneous separation of an explosive mixture, and component identification through the use of multiple sensing polymers. In a 1.0 mm2 area solution spotted onto the fluorescent silica plate, detection limits obtained for the explosives TNT (2,4,6-trinitrotoluene), DNT (2,6-dinitrotoluene), PA (picric acid), Tetryl (N-methyl-N,2,4,6-tetranitroaniline), HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine), RDX (1,3,5-trinitro-1,3,5-triazacyclohexane), and Cl-20 (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane) ranged from 4 to 750 pg mm−2. Since less than 350 pg of highly fluorescent polymer is required to coat each TLC plate, the relatively small amounts of explosive being detected still represent an excess of quencher over sensing fluorophore.

Experimental and TD-DFT study of optical absorption of six explosive molecules: RDX, HMX, PETN, TNT, TATP, and HMTD.

Time dependent density function theory (TD-DFT) has been utilized to calculate the excitation energies and oscillator strengths of six common explosives: RDX (1,3,5-trinitroperhydro-1,3,5-triazine), β-HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine), TATP (triacetone triperoxide), HMTD (hexamethylene triperoxide diamine), TNT (2,4,6-trinitrotoluene), and PETN (pentaerythritol tetranitrate). The results were compared to experimental UV-vis absorption spectra collected in acetonitrile. Four computational methods were tested including: B3LYP, CAM-B3LYP, ωB97XD, and PBE0. PBE0 outperforms the other methods tested. Basis set effects on the electronic energies and oscillator strengths were evaluated with 6-31G(d), 6-31+G(d), 6-31+G(d,p), and 6-311+G(d,p). The minimal basis set required was 6-31+G(d); however, additional calculations were performed with 6-311+G(d,p). For each molecule studied, the natural transition orbitals (NTOs) were reported for the most prominent singlet excitations. The TD-DFT results have been combined with the IPv calculated by CBS-QB3 to construct energy level diagrams for the six compounds. The results suggest optimization approaches for fluorescence based detection methods for these explosives by guiding materials selections for optimal band alignment between fluorescent probe and explosive analyte. Also, the role of the TNT Meisenheimer complex formation and the resulting electronic structure thereof on of the quenching mechanism of II-VI semiconductors is discussed.

Nanosecond time-resolved and steady-state infrared studies of photoinduced decomposition of TATB at ambient and elevated pressure.

The time scale and/or products of photoinduced decomposition of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) were investigated at ambient pressure and compared with products formed at 8 GPa. Ultrafast time-resolved infrared and steady-state Fourier transform IR (FTIR) spectroscopies were used to probe TATB and its products after photoexcitation with a 5 ns pulse of 532 nm light. At ambient pressure, transient spectra of TATB indicate that the molecule has significantly decomposed within 60 ns; transient spectra also indicate that formation of CO(2), an observed decomposition product, is complete within 30-40 mus. Proof of principle time-resolved experiments at elevated pressures were performed and are discussed briefly. Comparison of steady-state FTIR spectra obtained at ambient and elevated pressure (ca. 8 GPa) indicate that the decomposition products vary with pressure. We find evidence for water as a decomposition product only at elevated pressure.

Anomalous Photoluminescence in CdSe Quantum‐Dot Solids at High Pressure Due to Nonuniform Stress

The application of static high pressure provides a means to precisely control and investigate many fundamental and unique properties of nanoparticles. CdSe is a model quantum-dot system, the behavior of which under high pressure has been extensively studied; however, the effect of nonuniform stresses on this system has not been fully appreciated. Photoluminescence data obtained from CdSe quantum-dot solids in different stress environments varying from purely uniform to highly nonuniform are presented. Small deviations from a uniform stress distribution profoundly affect the electronic properties of this system. In nonuniform stress environments, a pronounced flattening of the photoluminescence enegy is observed above 3 GPa. The observations are validated with theoretical calculations obtained using an all-atom semiempirical pseudopotential technique. This effect must be considered when investigating other potentially pressure-mediated phenomena.

Ab initio calculation of ionization potential and electron affinity of six common explosive compounds

Christian D Grant Physical and Life Sciences Chemical Sciences Division Lawrence Livermore National Laboratory, 7000 East Ave, Livermore, CA 94550, USA Email grant29@llnl.gov Abstract: The vertical and adiabatic ionization potential (IP V and IP A ) and vertical electron affinity (EA V ) for six explosives, hexogen (RDX), octogen (HMX), triacetone triperoxide (TATP), hexamethylene triperoxide diamine (HMTD), 2,4,6-trinitrotoluene (TNT), and pentaerythritol tetranitrate (PETN), have been studied using ab initio computational methods. The IP V was calculated using MP2, CBS-QB3, and Koopmans’ theory, while the IP A was calculated with B3LYP, CAM-B3LYP, ωB97XD, B2PLYP, and MP2 using the ∆E method for the ground state B3LYP optimized geometries. IP A s of RDX and TNT were also calculated using CBS-QB3 with relaxed geometries of the ions. Of the methods tested, B3LYP and B2PLYPD provided superior and more consistent results for calculating the IP compared to CBS-QB3 level IP A calculations and experimental data (where available). CBS-QB3 was used as a benchmark for calculating the EA V as experimental data has not been reported. For calculations of the EA V , B3LYP performed the worst while MP2 and B2PLYPD predicted values closest to those made by CBS-QB3. Basis set effects were evaluated using 6-31+G(d,p), 6-311+G(d,p), and 6-311+G(3df,2p) for both IP and EA. 6-31+G(d,p) gave satisfactory results for calculating IP while 6-311+G(3df,2p) had improved results for calculating the EA. The four nitro-containing compounds have exothermic reduction potentials while the peroxides are endothermic. In addition, it was determined that RDX, HMX, TATP, and HMTD had unstable geometries in their reduced forms. The results should be useful in developing detection and screening methods including ionization methods for mass spectroscopy and fluorescence quenching methods of detection.

Longitudinal Plasma Resonance Shifts in Gold Nanoparticle Aggregates

Recent interest in colloidal gold focuses on understanding the tunability of the longitudinal and transverse plasma resonance. It was reported that the reduction of HAuCl4 by Na2S produces gold nanoparticles with an optical absorption in the near infrared. This absorption blue shifts during the course of the reaction. X-ray photoemission spectroscopy (XPS) measurements on this system indicated that there was little sulfur present in the system. A small angle x-ray scattering (SAX) experiment was used to monitor the reaction while simultaneously the UV-VIS spectrum was measured. During the reaction the fractal dimension decreased from 4.154 ± 0.850 to 0.624 ± 0.146. The decrease in fractal dimension coincided with the blue shift in the longitudinal plasma resonance from the near IR to the visible. This suggests a change from reaction limited colloid aggregation (RLCA) to diffusion limited colloid aggregation (DLCA), caused the shift in the plasma resonance.

X-ray diffraction of electrodeposited nanocrystalline nickel under high pressure

We studied the compressibility of monolithic fully dense electrodeposited nanocrystalline Ni (∼29 nm grain size) under both quasihydrostatic and nonhydrostatic conditions up to a nominal pressure of 50 GPa using angle-dispersive x-ray diffraction. We obtained an equation of state consistently and unambiguously from each measured reflection. The apparent bulk modulus measured under nonhydrostatic conditions is larger than that of the corresponding coarse-grained material under either type of compression, but is nearly the same when measured under quasihydrostatic conditions. These results may be consistent with a strength, but not necessarily a bulk modulus, that is enhanced in the nanomaterial relative to its coarse-grained counterparts.

Ultrafast electronic relaxation in colloidal gold (I) sulfide nanoparticles

Ultrafast electronic relaxation dynamics in Au2S colloidal nanoparticles have been studied using fs transient absorption spectroscopy. The electronic absorption spectrum of the nanoparticles exhibits a broad featureless absorption with increasing intensity from the near-IR into the visible and UV, indicating that Au2S is an indirect bandgap semiconductor. The electronic relaxation dynamics have been measured with 390 nm excitation and probing at 790 and 850 nm. The transient absorption decay profiles can be fit to a double exponential with time constants of 600 fs and 23 ps. The fast decay can be assigned to trapping of electrons from the conduction band to shallow trap states or from shallow traps to deep traps, while the long decay is assigned to recombination from shallow or deep trap states. The overall fast relaxation can be attributed to a high density of intrinsic or surface trap states. This fast decay is non-radiative and consistent with no observable luminescence at room temperature. EXAFS data show a 20% decrease in the first coordination shell for nanoparticles relative to bulk, which suggests a large number of surface dangling bounds that can contribute to a high density of surface trap states.