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 report the first observation of the n-type nature of hydrogenated graphene on SiO(2) and demonstrate the conversion of the majority carrier type from electrons to holes using surface doping. Density functional calculations indicate that the carrier type reversal is directly related to the magnitude of the hydrogenated graphene's work function relative to the substrate, which decreases when adsorbates such as water are present. Additionally, we show by temperature-dependent electronic transport measurements that hydrogenating graphene induces a band gap and that in the moderate temperature regime [220-375 K], the band gap has a maximum value at the charge neutrality point, is tunable with an electric field effect, and is higher for higher hydrogen coverage. The ability to control the majority charge carrier in hydrogenated graphene, in addition to opening a band gap, suggests potential for chemically modified graphene p-n junctions.

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Epitaxial thin films of La2NiMnO6, a ferromagnetic semiconductor, have been fabricated on different substrates by pulsed laser deposition. The x-ray diffraction and Raman scattering observations reveal that the films are single crystalline and have an orthorhombic structure. The magnetic properties of the films, including the coercive field, remanent magnetization, and Curie temperature, are strongly dependent on the choice of the substrate. The optimized films exhibit a magnetic moment of 4.63μB∕f.u. at 5K, with a Curie temperature close to 280K. The film characteristics are promising for potential device applications in information storage, spintronics, and sensors.

Thin films of the double perovskite La 2 NiMnO 6 ͑LNMO͒ have been grown on various lattice-matched substrates ͑SrTiO 3 , LaAlO 3 , NdGaO 3 , and MgO͒ by pulsed laser deposition under different oxygen background pressure ͑25-800 mTorr͒ conditions. The out-of-plane lattice constant of the LNMO film decreases with increasing pressure, which is likely caused by reduction in the defect concentration and improved structural ordering, before leveling off at higher oxygen concentrations. The scanning transmission electron microscopy results confirm that the films are epitaxial, and the interface is sharp and coherent. While few defects are observed in a film grown at a high oxygen pressure ͑800 mTorr͒, a film grown at a lower pressure ͑100 mTorr͒ clearly shows the formation of defects that extend throughout the thickness except for a very thin layer near the interface. The Raman spectra of the films are dominated by two broad peaks at around 540 and 685 cm −1 , which are assigned to the antisymmetric stretching and symmetric stretching modes of MnO 6 and NiO 6 octahedra, respectively. The Raman peaks of the LNMO thin films grown in the 800 mTorr background O 2 are blueshifted in comparison to those of bulk LNMO, and the shift increases with decreasing film thickness, indicating the increased influence of strain. The critical thickness for strain relaxation, as determined from the Raman spectra, is between 40 and 80 nm. However, the strain is observed to have negligible influence on the magnetic properties of films grown at high oxygen pressures. In contrast, films grown at low pressures exhibit degraded magnetic properties, which can be attributed to a combination of increased B-site cation disorder and the concentration of Mn 3+ and Ni 3+ Jahn-Teller ions caused by oxygen or cation related defects. With increasing oxygen pressure during growth, the paramagnetic-ferromagnetic transition temperature ͑ϳ280 K͒ becomes sharper and the saturation magnetization at low temperatures is enhanced. Based on the electron energy loss spectroscopy studies, the Mn and Ni ions in LNMO thin films are determined to be mixed-valent Mn 3+ / Mn 4+ , and charge transition disproportionation of the Mn 4+ +Ni 2+ → Mn 3+ +Ni 3+ type likely occurs with increasing oxygen deficiency.