J. Benziger

Effects of film morphology and gate dielectric surface preparation on the electrical characteristics of organic-vapor-phase-deposited pentacene thin-film transistors

Organic vapor phase deposition was used to grow polycrystalline pentacene channel thin-film transistors. Substrate temperature, chamber pressure during film deposition, and growth rate were used to vary the crystalline grain size of pentacene films on O2-plasma treated SiO2 from 0.2 to 5 μm, leading to room-temperature saturation regime field-effect hole mobilities (μeff) from 0.05±0.02 to 0.5±0.1 cm2/V s, respectively. Surface treatment of SiO2 with octadecyltrichlorosilane (OTS) prior to pentacene deposition resulted in μeff⩽1.6 cm2/V s, and drain current on/off ratios of ⩽108 at room temperature, while dramatically reducing the average grain size. X-ray diffraction studies indicate that the OTS treatment decreases the order of the molecular stacks. This suggests an increased density of flat-lying molecules, accompanying the improvement of the hole mobility at the pentacene/OTS interface.

Approaches and technical challenges to high temperature operation of proton exchange membrane fuel cells

Water loss and the coincident increase in membrane resistance to proton conduction are significant barriers to high performance operation of traditional proton exchange membrane fuel cells at elevated temperatures where the relative humidity may be reduced. We report here approaches to the development of high temperature membranes for proton exchange membrane fuel cells; composite perfluorinated sulfonic acid membranes were prepared to improve water retention, and non-aqueous proton conducting membranes were prepared to circumvent the loss of water. Experimental results of composite membranes of Nafion and zirconium phosphate show improved operation at elevated temperatures. Imidazole impregnated membranes poisoned the electrocatalysts. Cesium hydrogen sulfate membranes were not able to produce appreciable current. A brief analysis of temperature requirements for CO tolerance and a framework for understanding water loss from fuel cell membranes are presented.

Silicon Oxide Nafion Composite Membranes for Proton-Exchange Membrane Fuel Cell Operation at 80-140°C

Silicon oxide/Nafion composite membranes were studied for operation in hydrogen/oxygen proton-exchange membrane fuel cells (PEMFCs) from 80 to 140°C. The composite membranes were prepared either by an impregnation of Nafion 115 via sol-gel processing of tetraethoxysilane or by preparing a recast film, using solubilized Nafion 115 and a silicon oxide polymer/gel Tetraethoxysilane, when reacted with water in an acidic medium, undergoes polymerization to form a mixture of SiO 2 and siloxane polymer with product hydroxide and ethoxide groups. This material is referred to as SiO s /-OH/-OEt. When Nafion is used as the acidic medium, the SiO 2 /siloxane polymer forms within the membrane. All composite membranes had a silicon oxide content of less than or equal to 10 wt %. The silicon oxide improved the water retention of the composite membranes, increasing proton conductivity at elevated temperatures Attenuated total reflectance-Fourier transform infrared spectroscopy and scanning electron microscopy experiments indicated an evenly distributed siloxane polymer of SiO 2 /-OH/-OEt in the composite membranes. At a potential of 0.4 V, silicon oxide/Nafion 115 composite membranes delivered four times the current density obtained with unmodified Nafion 115 in a H 2 /O 2 PEMFC at 130°C and a pressure of 3 atm. Furthermore, silicon oxide-modified membranes were more robust than the control membranes (unmodified Nafion 115 and recast Nafion), which degraded after high operation temperature and thermal cycling.

Exciton diffusion lengths of organic semiconductor thin films measured by spectrally resolved photoluminescence quenching

We demonstrate spectrally resolved photoluminescence quenching as a means to determine the exciton diffusion length of several archetype organic semiconductors used in thin film devices. We show that aggregation and crystal orientation influence the anisotropy of the diffusion length for vacuum-deposited polycrystalline films. The measurement of the singlet diffusion lengths is found to be in agreement with diffusion by Forster transfer, whereas triplet diffusion occurs primarily via Dexter transfer.

Relationship between Crystalline Order and Exciton Diffusion Length in Molecular Organic Semiconductors

One of the most fundamental properties of both organic and inorganic semiconductors is charge mobility. It has been unambiguously shown that the mobility in both of these materials systems is strongly linked to the degree of long range order—thatis,moreextendedcrystallinityleadstoalargercharge mobility, which ultimately determines such extrinsic properties as seriesresistance andresponse tocurrentand optical pulses. An equally fundamental property for organic semiconductors is the molecular excited state-, or exciton-, diffusion length which characterizes energy transport within these more correlated solids. While it has been predicted that exciton transport should also be linked to the extent of crystalline order, to our knowledge nosuchdependencehasyetbeenestablished.Here,weaccurately measure the exciton diffusion length of the archetypal organic semiconductor, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) and clearly show its relationship to thin-film crystal morphology. As in the case of charge mobility, we show that the exciton transport diffusion length is a monotonic function of the extent of crystalline order. This study provides insight into the control and ultimately the tunability of the exciton diffusion lengthinorganic systems, whichiscrucial forthemanagementof energy transport in a wide range of important organic electronic devices.

First evidence of pep solar neutrinos by direct detection in Borexino.

G. Bellini, J. Benziger, D. Bick, S. Bonetti, G. Bonfini, D. Bravo, M. Buizza Avanzini, B. Caccianiga, L. Cadonati, F. Calaprice, C. Carraro, P. Cavalcante, A. Chavarria, D. D’Angelo, S. Davini, A. Derbin, A. Etenko, K. Fomenko, 4 D. Franco, C. Galbiati, S. Gazzana, C. Ghiano, M. Giammarchi, M. Goeger-Neff, A. Goretti, L. Grandi, E. Guardincerri, S. Hardy, Aldo Ianni, Andrea Ianni, D. Korablev, G. Korga, Y. Koshio, D. Kryn, M. Laubenstein, T. Lewke, E. Litvinovich, B. Loer, F. Lombardi, P. Lombardi, L. Ludhova, I. Machulin, S. Manecki, W. Maneschg, G. Manuzio, Q. Meindl, E. Meroni, L. Miramonti, M. Misiaszek, 4 D. Montanari, 7 P. Mosteiro, V. Muratova, L. Oberauer, M. Obolensky, F. Ortica, K. Otis, M. Pallavicini, L. Papp, L. Perasso, S. Perasso, A. Pocar, J. Quirk, R.S. Raghavan, G. Ranucci, A. Razeto, A. Re, A. Romani, A. Sabelnikov, R. Saldanha, C. Salvo, S. Schönert, H. Simgen, M. Skorokhvatov, O. Smirnov, A. Sotnikov, S. Sukhotin, Y. Suvorov, R. Tartaglia, G. Testera, D. Vignaud, R.B. Vogelaar, F. von Feilitzsch, J. Winter, M. Wojcik, A. Wright, M. Wurm, J. Xu, O. Zaimidoroga, S. Zavatarelli, and G. Zuzel

Science and technology of Borexino: a real-time detector for low energy solar neutrinos

Abstract Borexino, a real-time device for low energy neutrino spectroscopy is nearing completion of construction in the underground laboratories at Gran Sasso, Italy (LNGS). The experiments goal is the direct measurement of the flux of 7 Be solar neutrinos of all flavors via neutrino–electron scattering in an ultra-pure scintillation liquid. Seeded by a series of innovations which were brought to fruition by large-scale operation of a 4-ton test detector at LNGS, a new technology has been developed for Borexino. It enables sub-MeV solar neutrino spectroscopy for the first time. This paper describes the design of Borexino, the various facilities essential to its operation, its spectroscopic and background suppression capabilities and a prognosis of the impact of its results towards resolving the solar neutrino problem. Borexino will also address several other frontier questions in particle physics, astrophysics and geophysics.

Measurement of the solar 8B neutrino rate with a liquid scintillator target and 3 MeV energy threshold in the Borexino detector

G. Bellini, J. Benziger, S. Bonetti, M. Buizza Avanzini, B. Caccianiga, L. Cadonati, F. Calaprice, C. Carraro, A. Chavarria, A. Chepurnov, F. Dalnoki-Veress, D. D’Angelo, S. Davini, H. de Kerret, A. Derbin, A. Etenko, K. Fomenko, D. Franco, C. Galbiati, S. Gazzana, C. Ghiano, M. Giammarchi, M. Goeger-Neff, A. Goretti, E. Guardincerri, S. Hardy, Aldo Ianni, Andrea Ianni, M. Joyce, G. Korga, D. Kryn, M. Laubenstein, M. Leung, T. Lewke, E. Litvinovich, B. Loer, P. Lombardi, L. Ludhova, I. Machulin, S. Manecki, W. Maneschg, G. Manuzio, Q. Meindl, E. Meroni, L. Miramonti, M. Misiaszek, 11 D. Montanari, V. Muratova, L. Oberauer, M. Obolensky, F. Ortica, M. Pallavicini, L. Papp, L. Perasso, S. Perasso, A. Pocar, R.S. Raghavan, G. Ranucci, A. Razeto, A. Re, P. Risso, A. Romani, D. Rountree, A. Sabelnikov, R. Saldanha, C. Salvo, S. Schönert, H. Simgen, M. Skorokhvatov, O. Smirnov, A. Sotnikov, S. Sukhotin, Y. Suvorov, 9 R. Tartaglia, G. Testera, D. Vignaud, R.B. Vogelaar, F. von Feilitzsch, J. Winter, M. Wojcik, A. Wright, M. Wurm, J. Xu, O. Zaimidoroga, S. Zavatarelli, and G. Zuzel

Diffusion and Interfacial Transport of Water in Nafion

Water absorption, membrane swelling, and self-diffusivity of water in 1100 equivalent weight Nafion were measured as functions of temperature and water activity. Free volume per water at 80 °C, determined from water uptake and volume expansion data, decreases with water content in the membrane from 12 cm(3)/mol at λ = 0.5 H(2)O/SO(3) to 1.5 cm(3)/mol at λ = 4. The change in free volume with water content displays a transition at λ = 4. Limiting water self-diffusivity in Nafion was determined by pulsed gradient spin echo NMR at long delay times. The limiting self-diffusivity increases exponentially with water activity; the rate of increase of diffusivity with water content shows a transition at λ = 4. The tortuosity of the hydrophilic domains in Nafion decreased from 20 at low membrane water activity to 3 at λ = 4. It suggested a change in the connectivity of the hydrophilic domains absorbed water occurs at λ ∼ 4. The diffusivity results were employed to separate the contributions of diffusional and interfacial resistance for water transport across Nafion membranes, which enabled the determination of the interfacial mass transport coefficients. A diffusion model was developed which incorporated activity-dependent diffusivity, volume expansion, and the interfacial resistance, and was used to resolve the water activity profiles in the membrane.

The effect of the phase composition of model VPO catalysts for partial oxidation of n-butane

X-ray diffraction, Raman spectroscopy, 3’P MAS-NMR and spin-echo NMR indicated that model vanadium phosphorus oxide (VPO) precursors and catalysts contained various minor phases depending oxboth the synthetic approach and P/V ratios used. Raman spectroscopy revealed the presence of a number of micro-crystalline and amorphous V(W) and V(V) phases not evident by XRD. The presence of VOPO, phases was detrimental to the performance of the VP0 catalysts for KN-butane oxidation. The best model organic VP0 catalyst contained only vanadyl pyrophosphate with the highest degree of stacking order and virtually no VOPO, phase impurity. Raman spectroscopy detected vanadyl metaphosphate. VO(PO,),, in the catalysts derived from aqueous precursors possessing P/V ratios greater than I. Pure vanadyl metaphosphate catalyst was inactive in n-butane oxidation. s’P NMR demonstrated the absence of vanadyl metaphosphate and other impurity phases in the best catalyst derived from organic precursors at P/V = 1.18. The experimental data strongly indicate that the best VP0 catalysts for n-butane oxidation contain only vanadyl pyrophosphate with well-ordered stacking of the (200) planes.