37 research outputs found
Controlled Growth, Patterning and Placement of Carbon Nanotube Thin Films
Controlled growth, patterning and placement of carbon nanotube (CNT) thin
films for electronic applications are demonstrated. The density of CNT films is
controlled by optimizing the feed gas composition as well as the concentration
of growth catalyst in a chemical vapor deposition process. Densities of CNTs
ranging from 0.02 CNTs/{\mu}m^2 to 1.29 CNTs/{\mu}m^2 are obtained. The
resulting pristine CNT thin films are then successfully patterned using either
pre-growth or post-growth techniques. By developing a layered photoresist
process that is compatible with ferric nitrate catalyst, significant
improvements over popular pre-growth patterning methods are obtained.
Limitations of traditional post-growth patterning methods are circumvented by
selective transfer printing of CNTs with either thermoplastic or metallic
stamps. Resulting as-grown patterns of CNT thin films have edge roughness (< 1
{\mu}m) and resolution (< 5 {\mu}m) comparable to standard photolithography.
Bottom gate CNT thin film devices are fabricated with field-effect mobilities
up to 20 cm^2/Vs and on/off ratios of the order of 10^3. The patterning and
transfer printing methods discussed here have a potential to be generalized to
include other nanomaterials in new device configurations
Localization, Coulomb interactions and electrical heating in single-wall carbon nanotubes/polymer composites
Low field and high field transport properties of carbon nanotubes/polymer
composites are investigated for different tube fractions. Above the percolation
threshold f_c=0.33%, transport is due to hopping of localized charge carriers
with a localization length xi=10-30 nm. Coulomb interactions associated with a
soft gap Delta_CG=2.5 meV are present at low temperature close to f_c. We argue
that it originates from the Coulomb charging energy effect which is partly
screened by adjacent bundles. The high field conductivity is described within
an electrical heating scheme. All the results suggest that using composites
close to the percolation threshold may be a way to access intrinsic properties
of the nanotubes by experiments at a macroscopic scale.Comment: 4 pages, 5 figures, Submitted to Phys. Rev.
Passivating graphene and suppressing interfacial phonon scattering with mechanically transferred large-area Ga2O3
Quantum Matter and Optic
Electronic Structure of Carbon Nanotube Ropes
We present a tight binding theory to analyze the motion of electrons between
carbon nanotubes bundled into a carbon nanotube rope. The theory is developed
starting from a description of the propagating Bloch waves on ideal tubes, and
the effects of intertube motion are treated perturbatively in this basis.
Expressions for the interwall tunneling amplitudes between states on
neighboring tubes are derived which show the dependence on chiral angles and
intratube crystal momenta. We find that conservation of crystal momentum along
the tube direction suppresses interwall coherence in a carbon nanorope
containing tubes with random chiralities. Numerical calculations are presented
which indicate that electronic states in a rope are localized in the transverse
direction with a coherence length corresponding to a tube diameter.Comment: 15 pages, 10 eps figure
Dynamical Mean-Field Theory of Electron-Phonon Interactions in Correlated Systems: Application to Isotope Effects on Electronic Properties
We use a recently developed formalism (combining an adiabatic expansion and
dynamical mean-field theory) to obtain expressions for isotope effects on
electronic properties in correlated systems. As an example we calculate the
isotope effect on electron effective mass for the Holstein model as a function
of electron-phonon interaction strength and doping. Our systematic expansion
generates diagrams neglected in previous studies, which turn out to give the
dominant contributions. The isotope effect is small unless the system is near a
lattice instability. We compare this to experiment.Comment: 6 pages, 4 figures; added discussion of isotope effect away from half
fillin
Diffusive Charge Transport in Graphene on SiO2
We review our recent work on the physical mechanisms limiting the mobility of
graphene on SiO2. We have used intentional addition of charged scattering
impurities and systematic variation of the dielectric environment to
differentiate the effects of charged impurities and short-range scatterers. The
results show that charged impurities indeed lead to a conductivity linear in
density in graphene, with a scattering magnitude that agrees quantitatively
with theoretical estimates [1]; increased dielectric screening reduces
scattering from charged impurities, but increases scattering from short-range
scatterers [2]. We evaluate the effects of the corrugations (ripples) of
graphene on SiO2 on transport by measuring the height-height correlation
function. The results show that the corrugations cannot mimic long-range
(charged impurity) scattering effects, and have too small an
amplitude-to-wavelength ratio to significantly affect the observed mobility via
short-range scattering [3, 4]. Temperature-dependent measurements show that
longitudinal acoustic phonons in graphene produce a resistivity linear in
temperature and independent of carrier density [5]; at higher temperatures,
polar optical phonons of the SiO2 substrate give rise to an activated, carrier
density-dependent resistivity [5]. Together the results paint a complete
picture of charge carrier transport in graphene on SiO2 in the diffusive
regime.Comment: 28 pages, 7 figures, submitted to Graphene Week proceeding
Non-Drude THz conductivity of graphene due to structural distortions
Quantum Matter and Optic
Suppression of the vortex glass transition due to correlated defects with a persistent direction perpendicular to an applied magnetic field
It is found on the basis of the lowest Landau level approach for the
Ginzburg-Landau model that, in bulk type II superconductors with strong line
disorder directed {\it perpendicularly} to an applied field, the continuous
vortex-glass transition is depressed to the low limit in the limit of weak
{\it point} disorder. An anomalous resistive broadening in twin-free YBCO with
columnar defects in a field parallel to the layers is discussed based on this
theoretical finding. Other phenomena which, we argue, arise indirectly from
this mechanism in type II superconductors including correlated defects are also
discussed.Comment: 5 pages, 3 figures, Fig.2 and text were modified. To appear in Phys.
Rev. B (Rapid Comunication