Aluminum transformed to noble-metal-like catalyst for activating molecular hydrogen
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miconductor nanocrystals (NCs) are promising materials for applications in light-emitting diodes, field-effect transistors, photodetectors, and solar cells, including generation-III devices that exploit effects such as carrier multiplication. Semiconductor nanocrystals have a variety of novel properties that can be controlled by the dimensions, shapes, and internal structures of the NCs, but poor understanding of charge transport in films generated from these particles hinders progress in realizing NC-based devices. Recently, researchers at Los Alamos National Laboratory, New Mexico, have fabricated optical fieldeffect transistors (OFETs) based on lead sulfide (PbS) NCs and have used them to elucidate the charge-transport mechanism in the dark and under illumination.
Through spectrally resolved studies of gate-voltage–dependent photoconduction, they demonstrated the existence of a mid-gap band (MGB) which plays an important role in charge conductance both in dark and under illumination. As reported in the September 27 online edition of Nature Communications (DOI: 10.1038/ncomms1492), P. Nagpal and V.I. Klimov fabricated OFET devices, with a channel comprising PbS NCs treated with ethanedithiol, and gold gate and source electrodes (see Figure). While hole injection was much more efficient than electron injection in the dark, illumination of these devices with monochromatic light resulted in a dramatic increase in the source–drain current even at low light intensities, demonstrating a higher mobility of charges responsible for photoconduction compared to carriers responsible for charge transport in dark. In the case of white light illumination, application of a negative gate voltage resulted in an overall increase in photocurrent across b
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(a) A schematic of the optical field-effect transistor made from PbS nanocrystals (NCs) treated with ethanedithiol. (b) “Dark” conductance in NC films is mediated by midgap band states whereas photoconductance is dominated by photogenerated holes transported by quantum-confined NC states. MGB is mid-gap band, CB is conduction band, and VB is valence band. Reproduced with permission from Nat. Commun. (DOI: 10.1038/ncomms1492). © 2011 Macmillan Publishers Ltd.
Aluminum transformed to noblemetal-like catalyst for activating molecular hydrogen
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ctivation of molecular hydrogen plays an essential role in many industrial processes such as the synthesis of ammonia, the hydrogenation of organic compounds in petroleum refining.
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MRS BULLETIN
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VOLUME 36 • DECEMBER 2011
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As current processes typically rely on the use of expensive noble metal catalysts or aluminum which is susceptible to oxidation, there is significant interest in identifying cheaper and more efficient ways to activate molecular hydrogen under relatively mild conditions. As reported in the November issue of Nature Materials (DOI: 10.1038/ nmat3123; p. 884), Y.J. Chabal of the
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the entire spectrum. Simultaneously, two new spectral features at ~0.9 eV and ~1.5 eV were detected in the photocu
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