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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.

We present recent developments in Raman probe of confined optical and acoustic phonons in nonpolar semiconducting nanowires, with emphasis on Si and Ge. First, a review of the theoretical spatial correlation phenomenological model widely used to explain the downshift and asymmetric broadening to lower energies observed in the Raman profile is given. Second, we discuss the influence of local inhomogeneous laser heating and its interplay with phonon confinement on Si and Ge Raman line shape. Finally, acoustic phonon confinement, its effect on thermal conductivity, and factors that lead to phonon damping are discussed in light of their broad implications on nanodevice fabrication. [ABSTRACT FROM AUTHOR]

Abstract: Results of Raman scattering investigations of optical phonons confined in the cross section of small diameter Si nanowires are discussed. Using low excitation intensity at 514.53nm to study Si as a prototypical material, we first demonstrate that the outcome of the phonon confinement is an asymmetric broadening and downshifting of the LO-TO phonon line observed at 520cm−1 in the bulk. The effect is important in Si wires with diameter d <10nm. We find good agreement between our data and an early theory due to Richter et al., provided we introduce an additional factor that sets the phonon confinement length. Furthermore, we examine the effects on the 520cm−1 lineshape from increasing the laser power density in the tight microRaman focal spot size (∼1μm). We find that a second asymmetric line broadening mechanism is also present that can be identified with an inhomogeneous temperature distribution set up in the nanowires. This distribution is driven by the Gaussian spatial intensity of the laser beam in the focal plane of the microRaman instrument. Using the thermal properties of the phonons in Si, we can explain semi-quantitatively the complex changes in the lineshape that we observe in both small and large diameter Si nanowires, i.e., with or without phonon confinement as an active consideration. [Copyright &y& Elsevier]