Surface-Enhanced Nitrate Photolysis on Ice

Heterogeneous nitrates photolysis is the trigger for many chemical processes occurring in the polar boundary layer and is widely believed to occur in a quasi-liquid layer (QLL) at the surface of ice. The dipole forbidden character of the electronic transition relevant to boundary layer atmospheric chemistry and the small photolysis/photoproducts quantum yields in ice (and in water) may confer a significant enhancement and interfacial specificity to this important photochemical reaction at the surface of ice. Using amorphous solid water films at cryogenic temperatures as models for the disordered interstitial air/ice interface within the snowpack suppresses the diffusive uptake kinetics thereby prolonging the residence time of nitrate anions at the surface of ice. This approach allows their slow heterogeneous photolysis kinetics to be studied providing the first direct evidence that nitrates adsorbed onto the first molecular layer at the surface of ice are photolyzed more effectively than those dissolved within the bulk. Vibrational spectroscopy allows the ~3-fold enhancement in photolysis rates to be correlated with the nitrates’ distorted intramolecular geometry thereby hinting at the role played by the greater chemical heterogeneity in their solvation environment at the surface of ice than in the bulk. A simple 1D kinetic model suggests 1-that a 3(6)-fold enhancement in photolysis rate for nitrates adsorbed onto the ice surface could increase the photochemical NO[subscript 2] emissions from a 5(8) nm thick photochemically active interfacial layer by 30%(60)%, and 2-that 25%(40%) of the NO[subscript 2] photochemical emissions to the snowpack interstitial air are released from the top-most molecularly thin surface layer on ice. These findings may provide a new paradigm for heterogeneous (photo)chemistry at temperatures below those required for a QLL to form at the ice surface

Bandgap Engineering of Quantum Semiconductor Microstructures

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Bis(trifluoromethyl)methylene addition to vinyl-terminated SAMs : a gas-phase C–C bond-forming reaction on a surface

The authors acknowledge the Scottish Funding Council for a SPIRIT Studentship (M.A.) and the EPSRC (EP/K000411/1) for financial support, and D.O’H. acknowledges the European Research Council for an Advanced Grant and the Royal Society for a Wolfson Research Merit Award.Vinyl-terminated self-assembled monolayers (SAMs) on silicon oxide substrates were chemically modified by the addition of a bis(trifluoromethyl)methylene group in a rare gas-phase C–C bond-forming reaction to directly generate films carrying terminal CF3 groups. The vinyl-terminated films were treated with hexafluoroacetone azine (HFAA) for modification. The films were characterized with ellipsometry, contact angle measurements, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). In this study, we find that for optimized conditions clean reactions occur on a surface between SAMs with terminal olefins and HFAA, and the product is consistent with bis(trifluoromethyl)cyclopropanation formation after nitrogen extrusion.Publisher PDFPeer reviewe

Laser-induced selective area tuning of GaAs/AlGaAs quantum well microstructures for two-color IR detector operation

Selective area laser annealing of GaAs/AlxGa1 12xAs quantum well infrared photodetector (QWIP) material has been investigated as a possible route towards the fabrication of two-color low-cost focal plane array devices. Tuning of the wavelength response of the material has been achieved as a consequence of the quantum well intermixing (QWI) effect. A 90 s irradiation with a continuous wave Nd:yttrium\u2013aluminum\u2013garnet laser, at the peak temperature of 850\u200a\ub0C, resulted in the 40 nm blueshift of the QW photoluminescence peak from 832 to 792 nm. This corresponded to the 0.7 \u3bcm redshift of the wavelength response of the investigated QWIP microstructure in the 8 \u3bcm optical absorption region. The amplitude of this shift is consistent with the literature data obtained for similar material processed directly by rapid thermal annealing (RTA) or by a two-step process involving particle implantation and RTA. We have examined the laser-QWI approach for direct writing of arrays of a two-band gap material. The preliminary results indicate the feasibility of this approach for fabricating linear arrays with a period of 0.8 mm.NRC publication: Ye