Papers by Keyword: Electronic Transport

Abstract: Resistivity, r (T), and Hall coefficient in weak (B < 1 T) magnetic fields, R (T), are investigated in Ca₂Si and CaSi₂ films at temperatures T between ~ 20 - 300 K. In CaSi₂, r (T) is typical of metals increasing with T within the whole temperature range. On the other hand, the resistivity of Ca₂Si is pertinent of semiconductors. Namely, it is activated below T ~ 200 K, exhibiting different slopes of ln r vs. T ^-1 plots at lower and higher T, and a weak increase between T ~ 200 - 300 K. Both materials demonstrate a complex dependence of R (T), including a change of the sign. Transport properties above have been analyzed assuming two groups of charge carriers, electrons and holes, contributing them.

Abstract: In this paper, the electronic transport in epitaxial graphene (EG) grown on the Si face of 8° off-axis 4H-SiC has been investigated, using both electrical characterization of macroscopic devices and conductive atomic force microscopy (CAFM). In particular, current measurements on linear transmission line model (TLM) structures with different orientations showed a current transport anisotropy related to steps orientation, with the resistance of EG in the direction orthogonal to the steps ~2× higher than in the parallel direction. Two dimensional morphology and current maps in EG over the stepped SiC surface were obtained by CAFM and revealed a local resistance increase of EG over the (11-2n) facets with respect to the (0001) basal planes. This effect allows to account for the observed macroscopic current transport anisotropy and can be explained in terms of a different interface nature between EG and SiC on the two faces, leading to a locally different substrate induced doping of EG.

Abstract: A novel doping scheme for graphene was recently realized experimentally by creating different vacancy complexes doped with a transition metal (TM) atom [nanoLett. 12, 141 (2012)]. This provides a new reliable way to modifying the electronic structure and transport property of graphene. Here, we show, by performing first-principles calculations, that the defect complex of TM@V₄ (a TM atom doped tetra-vacancy) in zigzag graphene nanoribbons (ZGNRs) can lead to a 100% spin-polarized electron transport in a wide energy range around the Fermi energy. Analyses show that this is due to the particular atomic structure of the TM@V₄ complex regardless of the species of the TM atom.

Abstract: The structure, carrier concentration and low temperature magnetoresistance of Tantalum-doped Tin oxide thin films prepared by RF magnetron sputtering method have been investigated. Hall coefficient is negative at all measuring temperatures, which confirms the n-type characteristic of the films with metallic characteristic. The low temperature magnetoresistance measurement show negative magnetoresistance at all measured temperatures from 2 to 30 K. However, weak-localization theories can not explain the behavior of the negative magnetoresistance of our samples. We fitted the magnetoresistance data of the samples with semiempirical expression that takes into account the third order sd exchange Hamiltonians. The theory and experiment are in excellent agreement. This strongly suggests that the magnetoresistance in SnO₂: Ta film originates from the scattering of conduction electrons due to localized magnetic moments.

Abstract: Two dimensional maps of the electronic conductance in epitaxial graphene (EG) grown on SiC were obtained by conductive atomic force microscopy (CAFM). The correlation between morphological and electrical maps revealed the local conductance degradation in EG over the SiC substrate steps or at the junction between monolayer (1L) and bilayer (2L) graphene regions. The effect of steps strongly depends on the charge transfer phenomena between the step sidewall and graphene, whereas the resistance increase at 1L/2L junction is a purely quantum mechanical effect, due to the weak coupling between 1L and 2L electron wavefunctions.

Abstract: When an STM tip is brought close to a nano sized sample then it can deliver (or draw) a current that is determined by the Landauer-Büttiker formalism in terms of the scattering matrix that gives partial local density of states. We show that, very paradoxically, the interference related term in this formula vanish in a quantum regime making semi-classical formula for injectance exact in some regime. We explicitly show how evanescent modes are responsible for this. This can have useful implications to experimentalists as semi-classical formulas are much simpler.

Abstract: We report electron transport properties of iron filled multiwalled carbon nanotubes (MWCNT) with outer diameters of 30 to 80 nm and lengths of 1 to 10 μm. Our study is combined with a structural investigation of the iron core using transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). It was found that high current densities of 1.8x10⁷ A/cm² increase the conductivity of the MWCNT by a factor of two at 300 K, while the Fe core disappears probably forming defect states in the carbon shells. The enhanced diffusion of iron is most probably the result of local heating of the iron followed by implantation of iron atoms in the nanotube layers.

Abstract: Geometry structure, electronic structure and electronic transport properties of saturated hexagonal single crystalline GaN nanowires in the [001] growth direction have been investigated based on generalized gradient approximation (GGA) of density functional theory (DFT) and non-equilibrium green's function (NEGF) method. The results show, there is a contraction of the bond lengths of the saturated GaN nanowires after optimization; the nanowires have direct band gap, and band gap decreases with the increase of the cross section of nanowires; the electronic density of state and electronic transmission spectra of two-probe system have their own pulse-type sharp peaks with almost the same location of electron energy; the curves of I-V characteristics of the three saturated GaN nanowires are symmetric over the entire bias-voltage range, and they are semiconducting.

Abstract: The electronic transport of the single molecule via different anchoring groups is studied using density functional theory in conjunction with the nonequilibrium Green’s function. The results show that the electronic transport properties are strongly dependent on the anchoring groups. Asymmetric electrical response for opposite biases is observed resulting in significant rectification in current. The transmission coefficients and spatial distributions of molecular orbitals under various external biases voltage are analyzed, and it suggests that the asymmetry of the coupling between the molecule and the electrodes with external bias leads to rectifying performance.

Abstract: Based on nonequilibrium Green’s function and first-principles calculation, we investigate the transport properties of the molecule device with a donor-acceptor molecular complex sandwiched between two electrodes. Numerical results show that a negative differential resistance under applied bias can be observed. The mechanism of negative differential resistance is mainly induced by the orbital match of molecule and electrodes as well as intermolecular charge transfer.