We present a multiscale ab initio study of electronic and transport properties of two-dimensional graphene after epoxide functionalization via ozone treatment. The orbital rehybridization induced by the epoxide groups triggers a strong intervalley scattering and changes dramatically the conduction properties of graphene. By varying the coverage density of epoxide defects from 0.1 to 4%, charge conduction can be tuned from a diffusive to a strongly localized regime, with localization lengths down to a few nanometers long. Experimental results supporting the interpretation as a metal-insulator transition are also provided.

Focused surface-acoustic waves ͑SAWs͒ provide a way to reach intense acoustic fields for electroand optoacoustic applications on semiconductors. We have investigated the focusing of SAWs by interdigital transducers ͑IDTs͒ deposited on ͑100͒-oriented GaAs substrates. The focusing IDTs have curved fingers designed to account for the acoustic anisotropy of the substrate. Different factors that affect focusing, such as the aperture angle and the configuration of the IDT fingers, were systematically addressed. We show that the focusing performance can be considerably improved by appropriate choice of the IDT metal pads, which, under appropriate conditions, create an acoustic waveguide within the IDT. We demonstrate the generation of narrow ͑full width at half maximum of approx 15 m͒, high-frequency ͑0.5 GHz͒, continuous SAW beams with vertical displacement as high as 4 nm collimated over distances that exceed 100 m.

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The interaction of strong surface acoustic wave ͑SAW͒ fields with the electronic band structure of GaAs quantum wells ͑QW's͒ is investigated using spatially resolved photoluminescence ͑PL͒ spectroscopy. The optical studies are accompanied by k•p and tight-binding ͑TB͒ calculations of the SAW effects on the electronic band structure. The SAW induces a time-dependent coupling between the heavy-͑hh͒ and light-hole ͑lh͒ states in the valence band of the QW's, which leads to an anticrossing of their energy levels for high SAW intensities. The coupling alters the strength and polarization of the optical transitions and can be reproduced by calculations of the optical transition matrix elements. Spatially resolved PL measurements of the SAW-induced ambipolar transport of electrons and holes provide evidence of a reduction of the transport efficiency for high SAW fields, which is attributed to a decrease of the hole mobility as the hh and lh levels approach each other. This conclusion is supported by TB calculations that show a significant enhancement of the heavy-hole effective mass under these conditions. In addition, the mobility may also be reduced by the squeezing of the wave functions towards the QW interfaces induced by strong piezoelectric fields, which makes the transport more sensitive to potential fluctuations induced by interface roughness and defects in the barrier layers.

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