We report the synthesis and evidence of graphene fluoride, a two-dimensional wide bandgap semiconductor derived from graphene. Graphene fluoride exhibits hexagonal crystalline order and strongly insulating behavior with resistance exceeding 10 GΩ at room temperature. Electron transport in graphene fluoride is well described by variable-range hopping in two dimensions due to the presence of localized states in the band gap. Graphene obtained through the reduction of graphene fluoride is highly conductive, exhibiting a resistivity of less than 100 kΩ at room temperature. Our approach provides a new path to reversibly engineer the band structure and conductivity of graphene for electronic and optical applications.

Results are presented from an experimental and theoretical study of the electronic properties of back-gated graphene field effect transistors (FETs) on Si/SiO(2) substrates. The excess charge on the graphene was observed by sweeping the gate voltage to determine the charge neutrality point in the graphene. Devices exposed to laboratory environment for several days were always found to be initially p-type. After approximately 20 h at 200 degrees C in approximately 5 x 10(-7) Torr vacuum, the FET slowly evolved to n-type behavior with a final excess electron density on the graphene of approximately 4 x 10(12) e/cm(2). This value is in excellent agreement with our theoretical calculations on SiO(2), where we have used molecular dynamics to build the SiO(2) structure and then density functional theory to compute the electronic structure. The essential theoretical result is that the SiO(2) has a significant surface state density just below the conduction band edge that donates electrons to the graphene to balance the chemical potential at the interface. An electrostatic model for the FET is also presented that produces an expression for the gate bias dependence of the carrier density.

No abstract

We report new cross section measurements, believed to be the first involving synchrotron radition together with a 'hot-wire' diode detector, in which we have observed the doubly excited series of C a I at a much improved signal to noise ratio using the radiation from the DORIS I I electron accelerator and the H 4 S Y L 4 B facilities. The data provide " e n information on the 'vanishing width' effect in Ca I and are compared with several calculations of the cross section reported in recent literature. The best representation of the detailed structure appears to be achieved by the most recent combination of MQDT with the R-matrix method. All the calculations ( R matrix and MBPT) underestimate the cross section in the 3d5p peak by 20-40%.

We describe a new study of the odd-parity doubly excited spectrum of Ca I, in which a tunable coherent vuv source was used to scan over an energy range previously investigated by classical spectroscopy using synchrotron radiation. A vast improvement in the quality of the data, especially as regards the available spectral resolution, results from the use of a four-wave mixing source and enables the comparison between theory and experiment to be pushed further than ever before for doubly excited states in alkaline-earth spectra. Some remaining limitations of the theoretical computations are brought to light and discussed.