6 research outputs found
Intrinsic Electronic Transport Properties of High-Quality Monolayer and Bilayer MoS<sub>2</sub>
We
report electronic transport measurements of devices based on
monolayers and bilayers of the transition-metal dichalcogenide MoS<sub>2</sub>. Through a combination of in situ vacuum annealing and electrostatic
gating we obtained ohmic contact to the MoS<sub>2</sub> down to 4
K at high carrier densities. At lower carrier densities, low-temperature
four probe transport measurements show a metalāinsulator transition
in both monolayer and bilayer samples. In the metallic regime, the
high-temperature behavior of the mobility showed strong temperature
dependence consistent with phonon-dominated transport. At low temperature,
intrinsic field-effect mobilities approaching 1000
cm<sup>2</sup>/(VĀ·s) were observed for both monolayer
and bilayer devices. Mobilities extracted from Hall effect measurements
were several times lower and showed a strong dependence on density,
likely caused by screening of charged impurity scattering at higher
densities
Photoresponse of an Electrically Tunable Ambipolar Graphene Infrared Thermocouple
We explore the photoresponse of an
ambipolar graphene infrared
thermocouple at photon energies close to or below monolayer grapheneās
optical phonon energy and electrostatically accessible Fermi energy
levels. The ambipolar graphene infrared thermocouple consists of monolayer
graphene supported by an infrared absorbing material, controlled by
two independent electrostatic gates embedded below the absorber. Using
a scanning infrared laser microscope, we characterize these devices
as a function of carrier type and carrier density difference controlled
at the junction between the two electrostatic gates. On the basis
of these measurements, conducted at both mid- and near-infrared wavelengths,
the primary detection mechanism can be modeled as a thermoelectric
response. By studying the effect of different infrared absorbers,
we determine that the optical absorption and thermal conduction of
the substrate play the dominant role in the measured photoresponse
of our devices. These experiments indicate a path toward hybrid graphene
thermal detectors for sensing applications such as thermography and
chemical spectroscopy
Electronic Transport of Encapsulated Graphene and WSe<sub>2</sub> Devices Fabricated by Pick-up of Prepatterned hBN
We report high quality graphene and
WSe<sub>2</sub> devices encapsulated
between two hexagonal boron nitride (hBN) flakes using a pick-up method
with etched hBN flakes. Picking up prepatterned hBN flakes to be used
as a gate dielectric or mask for other 2D materials opens new possibilities
for the design and fabrication of 2D heterostructures. In this Letter,
we demonstrate this technique in two ways: first, a dual-gated graphene
device that is encapsulated between an hBN substrate and prepatterned
hBN strips. The conductance of the graphene device shows pronounced
FabryāPeĢrot oscillations as a function of carrier density,
which implies strong quantum confinement and ballistic transport in
the locally gated region. Second, we describe a WSe<sub>2</sub> device
encapsulated in hBN with the top hBN patterned as a mask for the channel
of a Hall bar. Ionic liquid selectively tunes the carrier density
of the contact region of the device, while the hBN mask allows independent
tunability of the contact region for low contact resistance. Hall
mobility larger than 600 cm<sup>2</sup>/(VĀ·s) for few-layer p-type
WSe<sub>2</sub> at 220 K is measured, the highest mobility of a thin
WSe<sub>2</sub> device reported to date. The observations of ballistic
transport in graphene and high mobility in WSe<sub>2</sub> confirm
pick-up of prepatterned hBN as a versatile technique to fabricate
ultraclean devices with high quality contact
Disorder Imposed Limits of Mono- and Bilayer Graphene Electronic Modification Using Covalent Chemistry
A central question in graphene chemistry is to what extent
chemical
modification can control an electronically accessible band gap in
monolayer and bilayer graphene (MLG and BLG). Density functional theory
predicts gaps in covalently functionalized graphene as high as 2 eV,
while this approach neglects the fact that lattice symmetry breaking
occurs over only a prescribed radius of nanometer dimension, which
we label the S-region. Therefore, high chemical conversion is central
to observing this band gap in transport. We use an electrochemical
approach involving phenyl-diazonium salts to systematically probe
electronic modification in MLG and BLG with increasing functionalization
for the first time, obtaining the highest conversion values to date.
We find that both MLG and BLG retain their relatively high conductivity
after functionalization even at high conversion, as mobility losses
are offset by increases in carrier concentration. For MLG, we find
that band gap opening as measured during transport is linearly increased
with respect to the <i>I</i><sub><i>D</i></sub>/<i>I</i><sub><i>G</i></sub> ratio but remains
below 0.1 meV in magnitude for SiO<sub>2</sub> supported graphene.
The largest transport band gap obtained in a suspended, highly functionalized
(<i>I</i><sub><i>D</i></sub>/<i>I</i><sub><i>G</i></sub> = 4.5) graphene is about 1 meV, lower
than our theoretical predictions considering the quantum interference
effect between two neighboring S-regions and attributed to its population
with midgap states. On the other hand, heavily functionalized BLG
(<i>I</i><sub><i>D</i></sub>/<i>I</i><sub><i>G</i></sub> = 1.8) still retains its signature
dual-gated band gap opening due to electric-field symmetry breaking.
We find a notable asymmetric deflection of the charge neutrality point
(CNP) under positive bias which increases the apparent on/off current
ratio by 50%, suggesting that synergy between symmetry breaking, disorder,
and quantum interference may allow the observation of new transistor
phenomena. These important observations set definitive limits on the
extent to which chemical modification can control graphene electronically
Graphene-Based Thermopile for Thermal Imaging Applications
In
this work, we leverage grapheneās unique tunable Seebeck coefficient
for the demonstration of a graphene-based thermal imaging system.
By integrating graphene based photothermo-electric detectors with
micromachined silicon nitride membranes, we are able to achieve room
temperature responsivities on the order of ā¼7ā9 V/W
(at Ī» = 10.6 Ī¼m), with a time constant of ā¼23 ms.
The large responsivities, due to the combination of thermal isolation
and broadband infrared absorption from the underlying SiN membrane,
have enabled detection as well as stand-off imaging of an incoherent
blackbody target (300ā500 K). By comparing the fundamental
achievable performance of these graphene-based thermopiles with standard
thermocouple materials, we extrapolate that grapheneās high
carrier mobility can enable improved performances with respect to
two main figures of merit for infrared detectors: detectivity (>8
Ć 10<sup>8</sup> cm Hz<sup>1/2</sup> W<sup>ā1</sup>) and
noise equivalent temperature difference (<100 mK). Furthermore,
even average graphene carrier mobility (<1000 cm<sup>2</sup> V<sup>ā1</sup> s<sup>ā1</sup>) is still sufficient to detect
the emitted thermal radiation from a human target
Efficiency of Launching Highly Confined Polaritons by Infrared Light Incident on a Hyperbolic Material
We
investigated phononāpolaritons in hexagonal boron nitrideīøa
naturally hyperbolic van der Waals materialīøby means of the
scattering-type scanning near-field optical microscopy. Real-space
nanoimages we have obtained detail how the polaritons are launched
when the light incident on a thin hexagonal boron nitride slab is
scattered by various intrinsic and extrinsic inhomogeneities, including
sample edges, metallic nanodisks deposited on its top surface, random
defects, and surface impurities. The scanned tip of the near-field
microscope is itself a polariton launcher whose efficiency proves
to be superior to all the other types of polariton launchers we studied.
Our work may inform future development of polaritonic nanodevices
as well as fundamental studies of collective modes in van der Waals
materials