The conversion of light into free electron-hole pairs constitutes the key process in the fields of photodetection and photovoltaics. The efficiency of this process depends on the competition of ...different relaxation pathways and can be greatly enhanced when photoexcited carriers do not lose energy as heat, but instead transfer their excess energy into the production of additional electron-hole pairs through carrier-carrier scattering processes. Here we use optical pump-terahertz probe measurements to probe different pathways contributing to the ultrafast energy relaxation of photoexcited carriers. Our results indicate that carrier-carrier scattering is highly efficient, prevailing over optical-phonon emission in a wide range of photon wavelengths and leading to the production of secondary hot electrons originating from the conduction band. As hot electrons in graphene can drive currents, multiple hot-carrier generation makes graphene a promising material for highly efficient broadband extraction of light energy into electronic degrees of freedom, enabling high-efficiency optoelectronic applications. PUBLICATION ABSTRACT
Cooperativity in Ion Hydration Tielrooij, K.J; Garcia-Araez, N; Bonn, M ...
Science (American Association for the Advancement of Science),
05/2010, Letnik:
328, Številka:
5981
Journal Article
Recenzirano
Despite prolonged scientific efforts to unravel the effects of ions on the structure and dynamics of water, many open questions remain, in particular concerning the spatial extent of this effect ...(i.e., the number of water molecules affected) and the origin of ion-specific effects. A combined terahertz and femtosecond infrared spectroscopic study of water dynamics around different ions (specifically magnesium, lithium, sodium, and cesium cations, as well as sulfate, chloride, iodide, and perchlorate anions) reveals that the effect of ions and counterions on water can be strongly interdependent and nonadditive, and in certain cases extends well beyond the first solvation shell of water molecules directly surrounding the ion.
We study electron mobilities in nanoporous and single-crystal titanium dioxide with terahertz time domain spectroscopy. This ultrafast technique allows the determination of the electron mobility ...after carrier thermalization with the lattice but before equilibration with defect trapping states. The mobilities reported here for single-crystal rutile (1 cm2/(V s)) and porous TiO2 (10(-2) cm2/(V s)) therefore represent upper limits for electron transport at room temperature for defect-free materials. The large difference in mobility between bulk and porous samples is explained using Maxwell-Garnett effective medium theory. These results demonstrate that electron mobility is strongly dependent on the material morphology in nanostructured polar materials due to local field effects and cannot be used as a direct measure of the diffusion coefficient.
One of the important factors limiting solar-cell efficiency is that incident photons generate one electron-hole pair, irrespective of the photon energy. Any excess photon energy is lost as heat. The ...possible generation of multiple charge carriers per photon (carrier multiplication) is therefore of great interest for future solar cells. Carrier multiplication is known to occur in bulk semiconductors, but has been thought to be enhanced significantly in nanocrystalline materials such as quantum dots, owing to their discrete energy levels and enhanced Coulomb interactions. Contrary to this expectation, we demonstrate here that, for a given photon energy, carrier multiplication occurs more efficiently in bulk PbS and PbSe than in quantum dots of the same materials. Measured carrier-multiplication efficiencies in bulk materials are reproduced quantitatively using tight-binding calculations, which indicate that the reduced carrier-multiplication efficiency in quantum dots can be ascribed to the reduced density of states in these structures.
We study hydrated model membranes, consisting of stacked bilayers of 1,2-dioleoyl-
sn-glycero-3-phosphocholine lipids, using terahertz time-domain spectroscopy and infrared spectroscopy. Terahertz ...spectroscopy enables the investigation of water dynamics, owing to its sensitivity to dielectric relaxation processes associated with water reorientation. By controlling the number of water molecules per lipid molecule in the system, we elucidate how the interplay between the model membrane and water molecules results in different water dynamics. For decreasing hydration levels, we observe the appearance of new types of water dynamics: the collective bulklike dynamics become less pronounced, whereas an increased amount of both very slowly reorienting (i.e., irrotational) and very rapidly reorienting (i.e., fast) water molecules appear. Temperature-dependent measurements reveal the interconversion between the three distinct types of water present in the system.
Resistance-nodulation-division (RND) superfamily efflux pumps promote antibiotic resistance in Gram-negative pathogens, but their role in Gram-positive pathogens, including methicillin-resistant ...Staphylococcus aureus (MRSA) is undocumented. However, recent
selections for resistance of S. aureus to an antimicrobial fatty acid, linoleic acid, and an antibiotic, rhodomyrtone, identified H121Y and C116R substitution variants, respectively, in a TetR family regulator, FarR, promoting increased expression of the RND pump FarE. Hypothesizing that
selection pressures have also promoted the emergence of FarR variants, we searched available genome data and found that strains with FarR
from human and bovine hosts have emerged sporadically in clonal complexes (CCs) CC1, CC30, CC8, CC22, and CC97, whereas multiple FarR variants have occurred within CC5 hospital-associated (HA)-MRSA. Of these, FarR
and FarR
were exclusive to CC5, while FarR
, FarR
, and FarR
also occurred in nonrelated CCs, primarily from bovine hosts. Within CC5, FarR
and FarR
strains were polyphyletic, each exhibiting two emergence events. FarR
and FarR
were individually sufficient to confer increased expression of FarE and enhanced resistance to linoleic acid (LA). Isolates with FarR
were most closely related to S. aureus N315 MRSA and exhibited increased resistance independently of FarR
. Accumulation of pseudogenes and additional polymorphisms in FarR
strains contributed to a multiresistance phenotype which included fosfomycin and fusidic acid resistance in addition to increased linoleic acid resistance. These findings underscore the remarkable adaptive capacity of CC5 MRSA, which includes the polyphyletic USA100 lineage of HA-MRSA that is endemic in the Western hemisphere and known for the acquisition of multiple resistance phenotypes.
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