872 research outputs found
Efros-Shklovskii variable range hopping in reduced graphene oxide sheets of varying carbon sp2 fraction
We investigate the low temperature electron transport properties of
chemically reduced graphene oxide (RGO) sheets with different carbon sp2
fractions of 55 to 80 %. We show that in the low bias (Ohmic) regime, the
temperature (T) dependent resistance (R) of all the devices follow
Efros-Shklovskii variable range hopping (ES-VRH) R ~ exp[(T(ES)/T)^1/2] with
T(ES) decreasing from 30976 to 4225 K and electron localization length
increasing from 0.46 to 3.21 nm with increasing sp2 fraction. From our data, we
predict that for the temperature range used in our study, Mott-VRH may not be
observed even at 100 % sp2 fraction samples due to residual topological defects
and structural disorders. From the localization length, we calculate a bandgap
variation of our RGO from 1.43 to 0.21 eV with increasing sp2 fraction from 55
to 80 % which agrees remarkably well with theoretical prediction. We also show
that, in the high bias regime, the hopping is field driven and the data follow
R ~ exp[(E(0)/E)^1/2] providing further evidence of ES-VRH.Comment: 13 pages, 6 figures, 1 tabl
Coulomb Blockade and Hopping Conduction in Graphene Quantum Dots Array
We show that the low temperature electron transport properties of chemically
functionalized graphene can be explained as sequential tunneling of charges
through a two dimensional array of graphene quantum dots (GQD). Below 15 K, a
total suppression of current due to Coulomb blockade through GQD array was
observed. Temperature dependent current-gate voltage characteristics show
Coulomb oscillations with energy scales of 6.2-10 meV corresponding to GQD
sizes of 5-8 nm while resistance data exhibit an Efros-Shklovskii variable
range hopping arising from structural and size induced disorder.Comment: The document will be appeared in Physics Review
Space charge limited conduction with exponential trap distribution in reduced graphene oxide sheets
We elucidate on the low mobility and charge traps of the chemically reduced
graphene oxide (RGO) sheets by measuring and analyzing temperature dependent
current-voltage characteristics. The RGO sheets were assembled between source
and drain electrodes via dielectrophoresis. At low bias voltage the conduction
is Ohmic while at high bias voltage and low temperatures the conduction becomes
space charge limited with an exponential distribution of traps. We estimate an
average trap density of 1.75x10^16 cm^-3. Quantitative information about charge
traps will help develop optimization strategies of passivating defects in order
to fabricate high quality solution processed graphene devices.Comment: 6 pages, 3 figures, 1 tabl
High yield fabrication of chemically reduced graphene oxide field effect transistors by dielectrophoresis
We demonstrate high yield fabrication of field effect transistors (FET) using
chemically reduced graphene oxide (RGO) sheets suspended in water assembled via
dielectrophoresis. The two terminal resistances of the devices were improved by
an order of magnitude upon mild annealing at 200 0C in Ar/H2 environment for 1
hour. With the application of a backgate voltage, all of the devices showed FET
behavior with maximum hole and electron mobilities of 4.0 and 1.5 cm2/Vs
respectively. This study shows promise for scaled up fabrication of graphene
based nanoelectronic devices.Comment: 8 pages, 6 figure
ISO LWS Spectra of T Tauri and Herbig AeBe stars
We present an analysis of ISO-LWS spectra of eight T Tauri and Herbig AeBe young stellar objects.
Some of the objects are in the embedded phase of star-formation, whereas others have cleared their environs
but are still surrounded by a circumstellar disk. Fine-structure lines of [OI] and [CII] are most likely excited by
far-ultraviolet photons in the circumstellar environment rather than high-velocity outflows, based on comparisons
of observed line strengths with predictions of photon-dominated and shock chemistry models. A subset of our
stars and their ISO spectra are adequately explained by models constructed by Chiang & Goldreich (1997) and
Chiang et al. (2001) of isolated, passively heated, flared circumstellar disks. For these sources, the bulk of the
LWS flux at wavelengths longward of 55 µm arises from the disk interior which is heated diffusively by reprocessed
radiation from the disk surface. At 45 µm, water ice emission bands appear in spectra of two of the coolest stars,
and are thought to arise from icy grains irradiated by central starlight in optically thin disk surface layers
Schottky diode via dielectrophoretic assembly of reduced graphene oxide sheets between dissimilar metal contacts
We demonstrate the fabrication of reduced graphene oxide (RGO) Schottky diodes via dielectrophoretic (DEP) assembly of RGO between two dissimilar metal contacts. Titanium (Ti) was used to make a Schottky contact, while palladium (Pd) was used to make an Ohmic contact. From the current-voltage characteristics, we obtain rectifying behavior with a rectification ratio of up to 600. The ideality factor was high (4.9), possibly due to the presence of a large number of defects in the RGO sheets. The forward biased turn-on voltage was 1V, whereas the reverse biased breakdown voltage was -3.1 V, which improved further upon mild annealing at 200 degrees C and can be attributed to an increase in the work function of RGO due to annealing
Two- to one-dimensional crossover in graphene quantum dot arrays observed in reduced graphene oxide nanoribbons
We investigate how the electron transport properties of graphene quantum dot (GQD) arrays transition from two dimensions (2D) to one dimension (1D) in lithographically defined reduced graphene oxide nanoribbons (RGONRs). From the low-temperature electron transport measurements of 200-, 100-, and 50-nm-wide RGONRs, we find that the energy barrier for charge transport increases with decreasing RGONR width in both the Coulomb blockade and the variable-range hopping regime. Different charge transport parameters for 200-nm RGONR are in agreement with 2D transport while these parameters show a gradual transition to 1D transport in 50-nm RGONR
Spectral Energy Distributions of T Tauri and Herbig Ae Disks: Grain Mineralogy, Parameter Dependences, and Comparison with ISO LWS Observations
We improve upon the radiative, hydrostatic equilibrium models of passive
circumstellar disks constructed by Chiang & Goldreich (1997). New features
include (1) account for a range of particle sizes, (2) employment of
laboratory-based optical constants of representative grain materials, and (3)
numerical solution of the equations of radiative and hydrostatic equilibrium
within the original 2-layer (disk surface + disk interior) approximation. We
explore how the spectral energy distribution (SED) of a face-on disk depends on
grain size distributions, disk geometries and surface densities, and stellar
photospheric temperatures. Observed SEDs of 3 Herbig Ae and 2 T Tauri stars,
including spectra from the Long Wavelength Spectrometer (LWS) aboard the
Infrared Space Observatory (ISO), are fitted with our models. Silicate emission
bands from optically thin, superheated disk surface layers appear in nearly all
systems. Water ice emission bands appear in LWS spectra of 2 of the coolest
stars. Infrared excesses in several sources are consistent with vertical
settling of photospheric grains. While this work furnishes further evidence
that passive reprocessing of starlight by flared disks adequately explains the
origin of infrared-to-millimeter wavelength excesses of young stars, we
emphasize how the SED alone does not provide sufficient information to
constrain particle sizes and disk masses uniquely.Comment: Accepted to ApJ, 35 pages inc. 14 figures, AAS preprin
SPHS: Smoothed Particle Hydrodynamics with a higher order dissipation switch
We present a novel implementation of Smoothed Particle Hydrodynamics (SPHS)
that uses the spatial derivative of the velocity divergence as a higher order
dissipation switch. Our switch -- which is second order accurate -- detects
flow convergence before it occurs. If particle trajectories are going to cross,
we switch on the usual SPH artificial viscosity, as well as conservative
dissipation in all advected fluid quantities (for example, the entropy). The
viscosity and dissipation terms (that are numerical errors) are designed to
ensure that all fluid quantities remain single-valued as particles approach one
another, to respect conservation laws, and to vanish on a given physical scale
as the resolution is increased. SPHS alleviates a number of known problems with
`classic' SPH, successfully resolving mixing, and recovering numerical
convergence with increasing resolution. An additional key advantage is that --
treating the particle mass similarly to the entropy -- we are able to use
multimass particles, giving significantly improved control over the refinement
strategy. We present a wide range of code tests including the Sod shock tube,
Sedov-Taylor blast wave, Kelvin-Helmholtz Instability, the `blob test', and
some convergence tests. Our method performs well on all tests, giving good
agreement with analytic expectations.Comment: 21 pages; 15 Figures. Submitted to MNRAS. Comments welcom
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