854 research outputs found
The Stability of Polar Oxide Surfaces
The structures of the polar surfaces of ZnO are studied using ab initio calculations and surface x-ray diffraction. The experimental and theoretical relaxations are in good agreement. The polar surfaces are shown to be very stable; the cleavage energy for the (0001)-Zn and (0001Ě… )-O surfaces is 4.0J/m2 comparable to 2.32J/m2 for the most stable nonpolar (1010) surface. The surfaces are stabilized by an electronic mechanism involving the transfer of 0.17 electrons between them. This leads to 2D metallic surface states, which has implications for the use of the material in gas sensing and catalytic applications
Giant Intrinsic Carrier Mobilities in Graphene and Its Bilayer
We have studied temperature dependences of electron transport in graphene and
its bilayer and found extremely low electron-phonon scattering rates that set
the fundamental limit on possible charge carrier mobilities at room
temperature. Our measurements have shown that mobilities significantly higher
than 200,000 cm2/Vs are achievable, if extrinsic disorder is eliminated. A
sharp (threshold-like) increase in resistivity observed above approximately
200K is unexpected but can qualitatively be understood within a model of a
rippled graphene sheet in which scattering occurs on intra-ripple flexural
phonons
Graphene Spin Valve Devices
Graphene - a single atomic layer of graphite - is a recently-found
two-dimensional form of carbon, which exhibits high crystal quality and
ballistic electron transport at room temperature. Soft magnetic NiFe electrodes
have been used to inject polarized spins into graphene and a 10% change in
resistance has been observed as the electrodes switch from the parallel to the
antiparallel state. This coupled with the fact that a field effect electrode
can modulate the conductivity of these graphene films makes them exciting
potential candidates for spin electronic devices.Comment: 3 pages, 6 figure
Two Dimensional Electron and Hole Gases at the Surface of Graphite
We report high-quality two-dimensional (2D) electron and hole gases induced
at the surface of graphite by the electric field effect. The 2D carriers reside
within a few near-surface atomic layers and exhibit mobilities up to 15,000 and
60,000 cm2/Vs at room and liquid-helium temperatures, respectively. The
mobilities imply ballistic transport at micron scale. Pronounced Shubnikov-de
Haas oscillations reveal the existence of two types of carries in both 2D
electron and hole gases.Comment: related to cond-mat/0410631 where preliminary data for this
experimental system were reporte
Singular-phase nanooptics: towards label-free single molecule detection
Non-trivial topology of phase is crucial for many important physics phenomena
such as, for example, the Aharonov-Bohm effect 1 and the Berry phase 2. Light
phase allows one to create "twisted" photons 3, 4 , vortex knots 5,
dislocations 6 which has led to an emerging field of singular optics relying on
abrupt phase changes 7. Here we demonstrate the feasibility of singular
visible-light nanooptics which exploits the benefits of both plasmonic field
enhancement and non-trivial topology of light phase. We show that properly
designed plasmonic nanomaterials exhibit topologically protected singular phase
behaviour which can be employed to radically improve sensitivity of detectors
based on plasmon resonances. By using reversible hydrogenation of graphene 8
and a streptavidin-biotin test 9, we demonstrate areal mass sensitivity at a
level of femto-grams per mm2 and detection of individual biomolecules,
respectively. Our proof-of-concept results offer a way towards simple and
scalable single-molecular label-free biosensing technologies.Comment: 19 pages, 4 figure
On resonant scatterers as a factor limiting carrier mobility in graphene
We show that graphene deposited on a substrate has a non-negligible density
of atomic scale defects. This is evidenced by a previously unnoticed D peak in
the Raman spectra with intensity of about 1% with respect to the G peak. We
evaluated the effect of such impurities on electron transport by mimicking them
with hydrogen adsorbates and measuring the induced changes in both mobility and
Raman intensity. If the intervalley scatterers responsible for the D peak are
monovalent, their concentration is sufficient to account for the limited
mobilities achievable in graphene on a substrate.Comment: version 2: several comments are taken into account and refs adde
Strong suppression of weak (anti)localization in graphene
Low-field magnetoresistance is ubiquitous in low-dimensional metallic systems
with high resistivity and well understood as arising due to quantum
interference on self-intersecting diffusive trajectories. We have found that in
graphene this weak-localization magnetoresistance is strongly suppressed and,
in some cases, completely absent. This unexpected observation is attributed to
mesoscopic corrugations of graphene sheets which cause a dephasing effect
similar to that of a random magnetic field.Comment: improved presentation of the theory part after referees comments;
important experimental info added (see "note added in proof"
Two Dimensional Atomic Crystals
We report free-standing atomic crystals that are strictly 2D and can be
viewed as individual atomic planes pulled out of bulk crystals or as unrolled
single-wall nanotubes. By using micromechanical cleavage, we have prepared and
studied a variety of 2D crystals, including single layers of boron nitride,
graphite, several dichalcogenides and complex oxides. These atomically-thin
sheets (essentially gigantic 2D molecules unprotected from the immediate
environment) are stable under ambient conditions, exhibit high crystal quality
and are continuous on a macroscopic scale.Comment: 4 page
Thickness Estimation of Epitaxial Graphene on SiC using Attenuation of Substrate Raman Intensity
A simple, non-invasive method using Raman spectroscopy for the estimation of
the thickness of graphene layers grown epitaxially on silicon carbide (SiC) is
presented, enabling simultaneous determination of thickness, grain size and
disorder using the spectra. The attenuation of the substrate Raman signal due
to the graphene overlayer is found to be dependent on the graphene film
thickness deduced from X-ray photoelectron spectroscopy and transmission
electron microscopy of the surfaces. We explain this dependence using an
absorbing overlayer model. This method can be used for mapping graphene
thickness over a region and is capable of estimating thickness of multilayer
graphene films beyond that possible by XPS and Auger electron spectroscopy
(AES).Comment: 14 pages, 9 figure
Modeling electrolytically top gated graphene
We investigate doping of a single-layer graphene in the presence of
electrolytic top gating. The interfacial phenomena is modeled using a modified
Poisson-Boltzmann equation for an aqueous solution of simple salt. We
demonstrate both the sensitivity of graphene's doping levels to the salt
concentration and the importance of quantum capacitance that arises due to the
smallness of the Debye screening length in the electrolyte.Comment: 7 pages, including 4 figures, submitted to Nanoscale Research Letters
for a special issue related to the NGC 2009 conference
(http://asdn.net/ngc2009/index.shtml
- …