Electronic structure of Fe- vs. Ru-based dye molecules

In order to explore whether Ru can be replaced by inexpensive Fe in dye molecules for solar cells, the differences in the electronic structure of Fe- and Ru-based dyes are investigated by X-ray absorption spectroscopy and first-principles calculations. Molecules with the metal in a sixfold, octahedral N cage, such as tris(bipyridines) and tris(phenanthrolines), exhibit a systematic downward shift of the N 1s-to-π* transition when Ru is replaced by Fe. This shift is explained by an extra transfer of negative charge from the metal to the N ligands in the case of Fe, which reduces the binding energy of the N 1s core level. The C 1s-to-π* transitions show the opposite trend, with an increase in the transition energy when replacing Ru by Fe. Molecules with the metal in a fourfold, planar N cage (porphyrins) exhibit a more complex behavior due to a subtle competition between the crystal field, axial ligands, and the 2+ vs. 3+ oxidation states.This work was supported by the National Science Foundation (NSF) under Award Nos. CHE-1026245, DMR-1121288 (MRSEC), DMR-0537588 (SRC), and by the (U.S.) Department of Energy (DOE) under Contract Nos. DE-FG02-01ER45917 (end station) and DE-AC02-05CH11231 (ALS). P. L. Cook acknowledges support from the University of Wisconsin System 2012-2013 Applied Research Grant. J. M. García-Lastra and A. Rubio acknowledge financial support from the European Research Council (ERC-2010-AdG-Proposal No. 267374), Spanish Grants (FIS2011-65702-C02-01 and PIB2010US-00652), Grupos Consolidados (IT-319-07), and European Commission project CRONOS (280879-2).Peer Reviewe

Perceptual Load-Dependent Neural Correlates of Distractor Interference Inhibition

The load theory of selective attention hypothesizes that distractor interference is suppressed after perceptual processing (i.e., in the later stage of central processing) at low perceptual load of the central task, but in the early stage of perceptual processing at high perceptual load. Consistently, studies on the neural correlates of attention have found a smaller distractor-related activation in the sensory cortex at high relative to low perceptual load. However, it is not clear whether the distractor-related activation in brain regions linked to later stages of central processing (e.g., in the frontostriatal circuits) is also smaller at high rather than low perceptual load, as might be predicted based on the load theory.We studied 24 healthy participants using functional magnetic resonance imaging (fMRI) during a visual target identification task with two perceptual loads (low vs. high). Participants showed distractor-related increases in activation in the midbrain, striatum, occipital and medial and lateral prefrontal cortices at low load, but distractor-related decreases in activation in the midbrain ventral tegmental area and substantia nigra (VTA/SN), striatum, thalamus, and extensive sensory cortices at high load.Multiple levels of central processing involving midbrain and frontostriatal circuits participate in suppressing distractor interference at either low or high perceptual load. For suppressing distractor interference, the processing of sensory inputs in both early and late stages of central processing are enhanced at low load but inhibited at high load

Conformational Disorder Enhances Electron Transfer Through Alkyl Monolayers: Ferrocene on Conductive Diamond

We have investigated the electron-transfer kinetics of ferrocene groups covalently tethered to surfaces of conductive diamond electrodes. Electrochemical measurements show that the rates are only weakly dependent on chain length but are strongly dependent on the surface coverage; these observations are contrary to what is commonly observed for self-assembled monolayers on gold, pointing to important mechanistic differences in electron transfer processes on covalently bonded materials. Molecular dynamics simulations show that dependence on chain length and molecular density can be readily explained in terms of dynamic crowding effects. At low coverage, conformational flexibility of surface-tethered alkyl chains allows the redox-active ferrocene group to dynamically approach the diamond surface, leading to facile electron transfer for all surface molecules. As the coverage is increased, steric crowding causes the average ferrocene-to-surface distance to increase, decreasing the electron transfer rate. Even at the most dense packings, the mismatch between the spacing of surface lattice sites and the optimum alkyl chain density leads to voids and inherent disorder that facilitates electron transfer. These results are significant in the design and optimization of electrocatalytically active surfaces on covalently bonded materials relevant for electro- and photocatalysis

Conformational Disorder Enhances Electron Transfer Through Alkyl Monolayers: Ferrocene on Conductive Diamond

We have investigated the electron-transfer kinetics of ferrocene groups covalently tethered to surfaces of conductive diamond electrodes. Electrochemical measurements show that the rates are only weakly dependent on chain length but are strongly dependent on the surface coverage; these observations are contrary to what is commonly observed for self-assembled monolayers on gold, pointing to important mechanistic differences in electron transfer processes on covalently bonded materials. Molecular dynamics simulations show that dependence on chain length and molecular density can be readily explained in terms of dynamic crowding effects. At low coverage, conformational flexibility of surface-tethered alkyl chains allows the redox-active ferrocene group to dynamically approach the diamond surface, leading to facile electron transfer for all surface molecules. As the coverage is increased, steric crowding causes the average ferrocene-to-surface distance to increase, decreasing the electron transfer rate. Even at the most dense packings, the mismatch between the spacing of surface lattice sites and the optimum alkyl chain density leads to voids and inherent disorder that facilitates electron transfer. These results are significant in the design and optimization of electrocatalytically active surfaces on covalently bonded materials relevant for electro- and photocatalysis

Conformational Disorder Enhances Electron Transfer Through Alkyl Monolayers: Ferrocene on Conductive Diamond

We have investigated the electron-transfer kinetics of ferrocene groups covalently tethered to surfaces of conductive diamond electrodes. Electrochemical measurements show that the rates are only weakly dependent on chain length but are strongly dependent on the surface coverage; these observations are contrary to what is commonly observed for self-assembled monolayers on gold, pointing to important mechanistic differences in electron transfer processes on covalently bonded materials. Molecular dynamics simulations show that dependence on chain length and molecular density can be readily explained in terms of dynamic crowding effects. At low coverage, conformational flexibility of surface-tethered alkyl chains allows the redox-active ferrocene group to dynamically approach the diamond surface, leading to facile electron transfer for all surface molecules. As the coverage is increased, steric crowding causes the average ferrocene-to-surface distance to increase, decreasing the electron transfer rate. Even at the most dense packings, the mismatch between the spacing of surface lattice sites and the optimum alkyl chain density leads to voids and inherent disorder that facilitates electron transfer. These results are significant in the design and optimization of electrocatalytically active surfaces on covalently bonded materials relevant for electro- and photocatalysis

Hafnium sulfate prenucleation clusters and the Hf(18) polyoxometalate red herring.

In prior studies, aqueous Hf sulfate-peroxide solutions were spin-coated, dehydrated, patterned by electron-beam lithography, ion-exchanged (OH(-) for SO4(2-)), and finally converted to HfO2 hard masks via annealing. The atomic-level details of the underlying aqueous chemistries of these processes are complex and yet to be understood. Yet a thorough understanding of this specific chemical system will inspire development of design rules for other aqueous-precursor-to-solid-state metal oxide systems. Often-observed crystallization of the Hf18 polyoxometalate from aqueous Hf sulfate-peroxide precursor solutions has led us to believe that Hf18 may represent an important intermediate step in this process. However, via detailed solution studies described here (small-angle X-ray scattering, electrospray ionization mass spectrometry, and Raman spectroscopy), we ascertained that Hf18 is in fact not a prenucleation cluster of Hf sulfate coatings. Rather, the Hf tetramers, pentamers, and hexamers that are the core building blocks of Hf18 are robustly persistent over variable compositions and aging time of precursor solutions, and therefore they are likely the rudimentary building blocks of the deposited thin-film materials. These Hf clusters are capped and linked by sulfate and peroxide anions in solution, which probably prevents crystallization of Hf18 during the rapid dehydration process of spin-coating. In fact, crystallization of Hf18 from the amorphous gel coating would be detrimental to formation of a high-density conformal coating that we obtain from precursor solutions. Therefore, this study revealed that the well-known Hf18 polyoxometalate is not likely to be an important intermediate in the thin-film process. However, its subunits are, confirming the universal importance of deriving information from the solid state, albeit judiciously and critically, to understand the solution state

Hafnium sulfate prenucleation clusters and the Hf(18) polyoxometalate red herring.

In prior studies, aqueous Hf sulfate-peroxide solutions were spin-coated, dehydrated, patterned by electron-beam lithography, ion-exchanged (OH(-) for SO4(2-)), and finally converted to HfO2 hard masks via annealing. The atomic-level details of the underlying aqueous chemistries of these processes are complex and yet to be understood. Yet a thorough understanding of this specific chemical system will inspire development of design rules for other aqueous-precursor-to-solid-state metal oxide systems. Often-observed crystallization of the Hf18 polyoxometalate from aqueous Hf sulfate-peroxide precursor solutions has led us to believe that Hf18 may represent an important intermediate step in this process. However, via detailed solution studies described here (small-angle X-ray scattering, electrospray ionization mass spectrometry, and Raman spectroscopy), we ascertained that Hf18 is in fact not a prenucleation cluster of Hf sulfate coatings. Rather, the Hf tetramers, pentamers, and hexamers that are the core building blocks of Hf18 are robustly persistent over variable compositions and aging time of precursor solutions, and therefore they are likely the rudimentary building blocks of the deposited thin-film materials. These Hf clusters are capped and linked by sulfate and peroxide anions in solution, which probably prevents crystallization of Hf18 during the rapid dehydration process of spin-coating. In fact, crystallization of Hf18 from the amorphous gel coating would be detrimental to formation of a high-density conformal coating that we obtain from precursor solutions. Therefore, this study revealed that the well-known Hf18 polyoxometalate is not likely to be an important intermediate in the thin-film process. However, its subunits are, confirming the universal importance of deriving information from the solid state, albeit judiciously and critically, to understand the solution state

High Areal Capacity Si/LiCoO 2

Freestanding nanofiber mat Li-ion battery anodes containing Si nanoparticles, carbon black, and poly(acrylic acid) (Si/C/PAA) are prepared using electrospinning. The mats are compacted to a high fiber volume fraction (≈0.85), and interfiber contacts are welded by exposing the mat to methanol vapor. A compacted+welded fiber mat anode containing 40 wt % Si exhibits high capacities of 1484 mA h g-1 (3500 mA h g-1Si ) at 0.1 C and 489 mA h g-1 at 1 C and good cycling stability (e.g., 73 % capacity retention over 50 cycles). Post-mortem analysis of the fiber mats shows that the overall electrode structure is preserved during cycling. Whereas many nanostructured Si anodes are hindered by their low active material loadings and densities, thick, densely packed Si/C/PAA fiber mat anodes reported here have high areal and volumetric capacities (e.g., 4.5 mA h cm-2 and 750 mA h cm-3 , respectively). A full cell containing an electrospun Si/C/PAA anode and electrospun LiCoO2 -based cathode has a high specific energy density of 270 Wh kg-1 . The excellent performance of the electrospun Si/C/PAA fiber mat anodes is attributed to the: i) PAA binder, which interacts with the SiOx surface of Si nanoparticles and ii) high material loading, high fiber volume fraction, and welded interfiber contacts of the electrospun mats