We report the dc and rf performance of graphene rf field-effect transistors,where the graphene films are grown on copper by using the chemical vapour deposition (CVD) method and transferred to SiO2/Si substrates.Composite materials,benzocyclobutene and atomic layer deposition Al2O3 are used as the gate dielectrics.The observation of n- and p-type transitions verifies the ambipolar characteristics in the graphene layers.While the intrinsic carrier mobility of CVD graphene is extracted to be 1200cm2/V·s,the parasitic series resistances are demonstrated to have a serious impact on device performance.With a gate length of 1 μm and an extrinsic transconductance of 72 mS/mm,a cutoff frequency of 6.6 GHz and a maximum oscillation frequency of 8.8 GHz are measured for the transistors,illustrating the potential of the CVD graphene for rf applications.
MA PengJIN ZhiGUO Jian-NanPAN Hong-LiangLIU Xin-YuYE Tian-ChunWANG HongWANG Guan-Zhong
The carrier doping effects on the magnetic properties of defective graphene with a hydrogen chemisorbed single-atom vacancy(H-GSV)are investigated by performing extensive spin-polarized first-principles calculations.Theoretical results show that the quasi-localized pz-derived states around the Fermi level are responsible for the weakened magnetic moment(MM)and magnetic stabilized energy(MSE)of the H-GSV under carrier doping.The mechanism of reduced MSE in the carrier doped H-GSV can be well understood by the Heisenberg magnetic coupling model due to the response of these p_(z)-derived states to the carrier doping.Within the examined range of carrier doping concentration,the total MM of H-GSV is always larger than 1.0μ_(B) with μ_(B) representing the Bohr magneton,which is mainly contributed by the localized sp^(2) states of the unsaturated C atom around the vacancy.These findings of H-GSV provide fundamental insight into defective graphene and help to understand the related experimental observations.
LEI Shu-LaiLI BinHUANG JingLI Qun-XiangYANG Jin-Long
Picene, which attracts the great interest of researchers, not only can be used to fabricate thin film transistors with high hole mobilities, but also is the parent material of a new type organic superconductor. Here, we investigate the electronic properties of individual picene molecules directly adsorbed on Cu（111） surface by a combination of experimental scanning tunneling microscopy/spectroscopy measurements and theoretical calculations based on the density functional theory. At low coverage, the picene molecules exhibit mono-dispersed adsorption behavior with the benzene ring planes parallel to the surface. The highest occupied state around -1.2 V and the lowest unoccupied state around 1.6 V with an obvious energy gap of the singly adsorbed picene molecule are identified by the dI/dV spectra and maps. In addition, we observe the strong dependence of the dI/dV signal of the unoccupied states on the intramolecular positions. Our first-principles calculations reproduce the above experimental results and interpret them as a specific molecule-substrate interaction and energy/spatial distributions of hybrid states mainly derived from different molecular orbitals of picene with some intermixing between them. This work provides direct information on the local electronic structure of individual picene on a metallic substrate and will facilitate the understanding the dependence of electron transport properties on the coupling between molecules and metal electrodes in single-molecule devices.
To advance hierarchical equations of motion as a standard theory for quantum dissipative dynamics, we put forward a mixed Heisenberg-SchrSdinger scheme with block-matrix implementation on efficient evaluation of nonlinear optical response function. The new approach is also integrated with optimized hierarchical theory and numerical filtering algorithm. Different configurations of coherent two-dimensional spectroscopy of model excitonic dimer systems are investigated, with focusing on the effects of intermolecular transfer coupling and bi-exciton interaction.
