198 research outputs found
Crossover from Coulomb blockade to quantum Hall effect in suspended graphene nanoribbons
Suspended graphene nano-ribbons formed during current annealing of suspended
graphene flakes have been investigated experimentally. Transport measurements
show the opening of a transport gap around charge neutrality due to the
formation of "Coulomb islands", coexisting with quantum Hall conductance
plateaus appearing at moderate values of magnetic field . Upon increasing
, the transport gap is rapidly suppressed, and is taken over by a much
larger energy gap due to electronic correlations. Our observations show that
suspended nano-ribbons allow the investigation of phenomena that could not so
far be accessed in ribbons on SiO substrates.Comment: 5 pages and 5 figures, Accepted in Physical Review Letter
On-Demand Spin-Orbit Interaction from Which-Layer Tunability in Bilayer Graphene
Spin-orbit interaction (SOI) that is gate-tunable over a broad range is
essential to exploiting novel spin phenomena. Achieving this regime has
remained elusive because of the weakness of the underlying relativistic
coupling and lack of its tunability in solids. Here we outline a general
strategy that enables exceptionally high tunability of SOI through creating a
which-layer spin-orbit field inhomogeneity in graphene multilayers. An external
transverse electric field is applied to shift carriers between the layers with
strong and weak SOI. Because graphene layers are separated by sub-nm scales,
exceptionally high tunability of SOI can be achieved through a minute carrier
displacement. A detailed analysis of the experimentally relevant case of
bilayer graphene on a semiconducting transition metal dichalchogenide substrate
is presented. In this system, a complete tunability of SOI amounting to its
ON/OFF switching can be achieved. New opportunities for spin control are
exemplified with electrically driven spin resonance and topological phases with
different quantized intrinsic valley Hall conductivities.Comment: 8 pages, 3 figure
A ballistic pn junction in suspended graphene with split bottom gates
We have developed a process to fabricate suspended graphene devices with
local bottom gates, and tested it by realizing electrostatically controlled pn
junctions on a suspended graphene mono-layer nearly 2 micrometers long.
Measurements as a function of gate voltage, magnetic field, bias, and
temperature exhibit characteristic Fabry-Perot oscillations in the cavities
formed by the pn junction and each of the contacts, with transport occurring in
the ballistic regime. Our results demonstrate the possibility to achieve a high
degree of control on the local electronic properties of ultra-clean suspended
graphene layers, a key aspect for the realization of new graphene
nanostructures.Comment: 10 pages, 4 figure
Ambipolar Light-Emitting Transistors on Chemical Vapor Deposited Monolayer MoS2
We realize and investigate ionic liquid gated field-effect transistors (FETs)
on large-area MoS2 monolayers grown by chemical vapor deposition (CVD). Under
electron accumulation, the performance of these devices is comparable to that
of FETs based on exfoliated flakes. FETs on CVD-grown material, however,
exhibit clear ambipolar transport, which for MoS2 monolayers had not been
reported previously. We exploit this property to estimate the bandgap {\Delta}
of monolayer MoS2 directly from the device transfer curves and find {\Delta}
2.4-2.7 eV. In the ambipolar injection regime, we observe
electroluminescence due to exciton recombination in MoS2, originating from the
region close to the hole-injecting contact. Both the observed transport
properties and the behavior of the electroluminescence can be consistently
understood as due to the presence of defect states at an energy of 250-300 meV
above the top of the valence band, acting as deep traps for holes. Our results
are of technological relevance, as they show that devices with useful
optoelectronic functionality can be realized on large-area MoS2 monolayers
produced by controllable and scalable techniques
Progress in organic single-crystal field-effect transistors
Research on organic thin-film transistors tends to focus on improvements in device performance, but very little is understood about the ultimate limits of these devices, the microscopic physical mechanisms responsible for their limitations, and, more generally, the intrinsic transport properties of organic semiconductors. These topics are now being investigated through the study of transport in organic transistors realized using molecular single crystals of unprecedented chemical purity and structural quality. These studies are elucidating detailed microscopic aspects of the physics of organic semiconductors and corresponding devices and have also led to unforeseen high values for carrier mobility in these materials. Here, we discuss developments in this area and present a brief outlook on future goals that have come into experimental reac
Mono- and Bilayer WS2 Light-Emitting Transistors
We have realized ambipolar ionic liquid gated field-effect transistors based
on WS2 mono- and bilayers, and investigated their opto-electronic response. A
thorough characterization of the transport properties demonstrates the high
quality of these devices for both electron and hole accumulation, which enables
the quantitative determination of the band gap ({\Delta}1L = 2.14 eV for
monolayers and {\Delta}2L = 1.82 eV for bilayers). It also enables the
operation of the transistors in the ambipolar injection regime with electrons
and holes injected simultaneously at the two opposite contacts of the devices
in which we observe light emission from the FET channel. A quantitative
analysis of the spectral properties of the emitted light, together with a
comparison with the band gap values obtained from transport, show the internal
consistency of our results and allow a quantitative estimate of the excitonic
binding energies to be made. Our results demonstrate the power of ionic liquid
gating in combination with nanoelectronic systems, as well as the compatibility
of this technique with optical measurements on semiconducting transition metal
dichalcogenides. These findings further open the way to the investigation of
the optical properties of these systems in a carrier density range much broader
than that explored until now.Comment: 22 pages, 6 figures, Nano Letters (2014
Scanning photocurrent microscopy reveals electron-hole asymmetry in ionic liquid-gated WS2 transistors
We perform scanning photocurrent microscopy on WS2 ionic liquid-gated field
effect transistors exhibiting high-quality ambipolar transport. By properly
biasing the gate electrode we can invert the sign of the photocurrent showing
that the minority photocarriers are either electrons or holes. Both in the
electron- and the hole-doping regimes the photocurrent decays exponentially as
a function of the distance between the illumination spot and the nearest
contact, in agreement with a two-terminal Schottky-barrier device model. This
allows us to compare the value and the doping dependence of the diffusion
length of the minority electrons and holes on a same sample. Interestingly, the
diffusion length of the minority carriers is several times larger in the hole
accumulation regime than in the electron accumulation regime, pointing out an
electron-hole asymmetry in WS2
- …