Graphene films grown on Cu foils have been fluorinated with xenon difluoride (XeF(2)) gas on one or both sides. When exposed on one side the F coverage saturates at 25% (C(4)F), which is optically transparent, over 6 orders of magnitude more resistive than graphene, and readily patterned. Density functional calculations for varying coverages indicate that a C(4)F configuration is lowest in energy and that the calculated band gap increases with increasing coverage, becoming 2.93 eV for one C(4)F configuration. During defluorination, we find hydrazine treatment effectively removes fluorine while retaining graphene's carbon skeleton. The same films may be fluorinated on both sides by transferring graphene to a silicon-on-insulator substrate enabling XeF(2) gas to etch the Si underlayer and fluorinate the backside of the graphene film to form perfluorographane (CF) for which calculated the band gap is 3.07 eV. Our results indicate single-side fluorination provides the necessary electronic and optical changes to be practical for graphene device applications.

We show that the capacitance of single-walled carbon nanotubes (SWNTs) is highly sensitive to a broad class of chemical vapors and that this transduction mechanism can form the basis for a fast, low-power sorption-based chemical sensor. In the presence of a dilute chemical vapor, molecular adsorbates are polarized by the fringing electric fields radiating from the surface of a SWNT electrode, which causes an increase in its capacitance. We use this effect to construct a high-performance chemical sensor by thinly coating the SWNTs with chemoselective materials that provide a large, class-specific gain to the capacitance response. Such SWNT chemicapacitors are fast, highly sensitive, and completely reversible.

We explore the electronic response of single-walled carbon nanotubes (SWNT) to trace levels of chemical vapors. We find adsorption at defect sites produces a large electronic response that dominates the SWNT capacitance and conductance sensitivity. This large response results from increased adsorbate binding energy and charge transfer at defect sites. Finally, we demonstrate controlled introduction of oxidation defects can be used to enhance sensitivity of a SWNT network sensor to a variety of chemical vapors.

The adsorption of simple benzene derivatives composed of a benzene ring with NO 2 , CH 3 , or NH 2 functional groups on a semiconducting single-wall carbon nanotube is studied using the density-functional theory within the local-density approximation. The effects of molecular relaxation in the adsorption process are obtained, as well as the adsorption energies and equilibrium distances for several molecular locations and orientations on the surface. We find that all of these benzene derivatives are physisorbed mainly through the interaction of the orbitals of the benzene ring and those of the carbon nanotube. These aromatics do not change significantly the carbon nanotube's electronic structure, and therefore only small changes in the nanotube's properties are expected. This suggests that these benzene derivatives are suitable for noncovalent nanotube functionalization and molecule immobilization on nanotube surfaces.

We report polarized photoluminescence excitation spectroscopy of the negative trion in single charge tunable InAs/GaAs quantum dots. The spectrum exhibits a p-shell resonance with polarized fine structure arising from the direct excitation of the electron spin triplet states. The energy splitting arises from the axially symmetric electron-hole exchange interaction. The magnitude and sign of the polarization are understood from the spin character of the triplet states and a small amount of quantum dot asymmetry, which mixes the wavefunctions through asymmetric e-e and e-h exchange interactions.

We present a combination of theoretical calculations and experiments for the low-lying vibrational excitations of H and D atoms adsorbed on the Pt(111) surface. The vibrational band states are calculated based on the full three-dimensional adiabatic potential energy surface obtained from first principles calculations. For coverages less than three quarters of a monolayer, the observed experimental high-resolution electron peaks at 31 and 68meV are in excellent agreement with the theoretical transitions between selected bands. Our results convincingly demonstrate the need to go beyond the local harmonic oscillator picture to understand the dynamics of this system.Comment: In press at Phys. Rev. Lett - to appear in April 200

The spin of an electron in a self-assembled InAs/GaAs quantum dot molecule is optically prepared and measured through the trion triplet states. A longitudinal magnetic field is used to tune two of the trion states into resonance, forming a superposition state through asymmetric spin exchange. As a result, spin-flip Raman transitions can be used for optical spin initialization, while separate trion states enable cycling transitions for nondestructive measurement. With two-laser transmission spectroscopy we demonstrate both operations simultaneously, something not previously accomplished in a single quantum dot.

We demonstrate X-ray-diffraction-based composition estimation of β-(Al x Ga1− x )2O3 coherently grown on (010) β-Ga2O3. The relation between the strain along the [010] direction and the Al composition of the β-(Al x Ga1− x )2O3 layer was formulated using the stress–strain relationship in the monoclinic system. This formulation allows us to estimate the Al composition using the out-of-plane lattice spacing determined by conventional X-ray ω–2θ measurements. This method was applied to molecular-beam-epitaxy-grown coherent β-(Al x Ga1− x )2O3/Ga2O3 heterostructures, and the Al composition in β-(Al x Ga1− x )2O3 agrees closely with the composition determined directly by atom probe tomography.