11 research outputs found
Anisotropic Etching of Atomically Thin MoS<sub>2</sub>
Exposure to oxygen at 300â340
°C results in triangular etch pits with uniform orientation on
the surfaces of atomically thin molybdenum disulfide (MoS<sub>2</sub>), indicating anisotropic etching terminating on lattice planes.
The triangular pits grow laterally with oxidation time. The density
of pits scarcely depends on oxidation time, temperature, and MoS<sub>2</sub> thickness but varies significantly from sample to sample,
indicating that etching is initiated at native defect sites on the
basal plane surface rather than activated by substrate effects such
as charged impurities or surface roughness. Raman spectroscopy confirms
that oxygen treatment produces no molybdenum oxide (MoO<sub>3</sub>) below 340 °C. However, upon oxidation above 200 °C, the
Raman A<sub>1g</sub> mode upshifts and the linewidth decreases, indicating
p-type doping of MoS<sub>2</sub>. Oxidation at 400 °C results
in complete conversion to MoO<sub>3</sub>
Ambipolar Surface State Thermoelectric Power of Topological Insulator Bi<sub>2</sub>Se<sub>3</sub>
We measure gate-tuned thermoelectric
power of mechanically exfoliated
Bi<sub>2</sub>Se<sub>3</sub> thin films in the topological insulator
regime. The sign of the thermoelectric power changes across the charge
neutrality point as the majority carrier type switches from electron
to hole, consistent with the ambipolar electric field effect observed
in conductivity and Hall effect measurements. Near the charge neutrality
point and at low temperatures, the gate-dependent thermoelectric power
follows the semiclassical Mott relation using the expected surface
state density of states but is larger than expected at high electron
doping, possibly reflecting a large density of states in the bulk
gap. The thermoelectric power factor shows significant enhancement
near the electronâhole puddle carrier density âŒ0.5 Ă
10<sup>12</sup> cm<sup>â2</sup> per surface at all temperatures.
Together with the expected reduction of lattice thermal conductivity
in low-dimensional structures, the results demonstrate that nanostructuring
and Fermi level tuning of three-dimensional topological insulators
can be promising routes to realize efficient thermoelectric devices
Passivating Graphene and Suppressing Interfacial Phonon Scattering with Mechanically Transferred Large-Area Ga<sub>2</sub>O<sub>3</sub>
We demonstrate a large-area passivation layer for graphene
by mechanical
transfer of ultrathin amorphous Ga2O3 synthesized
on liquid Ga metal. A comparison of temperature-dependent electrical
measurements of millimeter-scale passivated and bare graphene on SiO2/Si indicates that the passivated graphene maintains its high
field effect mobility desirable for applications. Surprisingly, the
temperature-dependent resistivity is reduced in passivated graphene
over a range of temperatures below 220 K, due to the interplay of
screening of the surface optical phonon modes of the SiO2 by high-dielectric-constant Ga2O3 and the
relatively high characteristic phonon frequencies of Ga2O3. Raman spectroscopy and electrical measurements indicate
that Ga2O3 passivation also protects graphene
from further processing such as plasma-enhanced atomic layer deposition
of Al2O3
Self-Limiting Layer-by-Layer Oxidation of Atomically Thin WSe<sub>2</sub>
Growth of a uniform oxide film with
a tunable thickness on two-dimensional transition metal dichalcogenides
is of great importance for electronic and optoelectronic applications.
Here we demonstrate homogeneous surface oxidation of atomically thin
WSe<sub>2</sub> with a self-limiting thickness from single- to trilayers.
Exposure to ozone (O<sub>3</sub>) below 100 °C leads to the lateral
growth of tungsten oxide selectively along selenium zigzag-edge orientations
on WSe<sub>2</sub>. With further O<sub>3</sub> exposure, the oxide
regions coalesce and oxidation terminates leaving a uniform thickness
oxide film on top of unoxidized WSe<sub>2</sub>. At higher temperatures,
oxidation evolves in the layer-by-layer regime up to trilayers. The
oxide films formed on WSe<sub>2</sub> are nearly atomically flat.
Using photoluminescence and Raman spectroscopy, we find that the underlying
single-layer WSe<sub>2</sub> is decoupled from the top oxide but hole-doped.
Our findings offer a new strategy for creating atomically thin heterostructures
of semiconductors and insulating oxides with potential for applications
in electronic devices
Plasmon-Enhanced Terahertz Photodetection in Graphene
We report a large area terahertz
detector utilizing a tunable plasmonic resonance in subwavelength
graphene microribbons on SiC(0001) to increase the absorption efficiency.
By tailoring the orientation of the graphene ribbons with respect
to an array of subwavelength bimetallic electrodes, we achieve a condition
in which the plasmonic mode can be efficiently excited by an incident
wave polarized perpendicular to the electrode array, while the resulting
photothermal voltage can be observed between the outermost electrodes
Effects of Floquet Engineering on the Coherent Exciton Dynamics in Monolayer WS
Coherent optical manipulation of electronic bandstructures via Floquet Engineering is a promising means to control quantum systems on an ultrafast timescale. However, the ultrafast switching on/off of the driving field comes with questions regarding the limits of validity of the Floquet formalism, which is defined for an infinite periodic drive, and to what extent the transient changes can be driven adibatically. Experimentally addressing these questions has been difficult, in large part due to the absence of an established technique to measure coherent dynamics through the duration of the pulse. Here, using multidimensional coherent spectroscopy we explicitly excite, control, and probe a coherent superposition of excitons in the and valleys in monolayer WS. With a circularly polarized, red-detuned, pump pulse, the degeneracy of the and excitons can be lifted and the phase of the coherence rotated. We demonstrate phase rotations during the 100 fs driving pulse that exceed , and show that this can be described by a combination of the AC-Stark shift of excitons in one valley and Bloch-Siegert shift of excitons in the opposite valley. Despite showing a smooth evolution of the phase that directly follows the intensity envelope of the pump pulse, the process is not perfectly adiabatic. By measuring the magnitude of the macroscopic coherence as it evolves before, during, and after the pump pulse we show that there is additional decoherence caused by power broadening in the presence of the pump. This non-adiabaticity may be a problem for many applications, such as manipulating q-bits in quantum information processing, however these measurements also suggest ways such effects can be minimised or eliminated
Plasmon-Enhanced Terahertz Photodetection in Graphene
We report a large area terahertz
detector utilizing a tunable plasmonic resonance in subwavelength
graphene microribbons on SiC(0001) to increase the absorption efficiency.
By tailoring the orientation of the graphene ribbons with respect
to an array of subwavelength bimetallic electrodes, we achieve a condition
in which the plasmonic mode can be efficiently excited by an incident
wave polarized perpendicular to the electrode array, while the resulting
photothermal voltage can be observed between the outermost electrodes
Synthesis and Transfer of Large-Area Monolayer WS<sub>2</sub> Crystals: Moving Toward the Recyclable Use of Sapphire Substrates
Two-dimensional layered transition metal dichalcogenides (TMDs) show intriguing potential for optoelectronic devices due to their exotic electronic and optical properties. Only a few efforts have been dedicated to large-area growth of TMDs. Practical applications will require improving the efficiency and reducing the cost of production, through (1) new growth methods to produce large size TMD monolayer with less-stringent conditions, and (2) nondestructive transfer techniques that enable multiple reuse of growth substrate. In this work, we report to employ atmospheric pressure chemical vapor deposition (APCVD) for the synthesis of large size (>100 ÎŒm) single crystals of atomically thin tungsten disulfide (WS<sub>2</sub>), a member of TMD family, on sapphire substrate. More importantly, we demonstrate a polystyrene (PS) mediated delamination process <i>via</i> capillary force in water which reduces the etching time in base solution and imposes only minor damage to the sapphire substrate. The transferred WS<sub>2</sub> flakes are of excellent continuity and exhibit comparable electron mobility after several growth cycles on the reused sapphire substrate. Interestingly, the photoluminescence emission from WS<sub>2</sub> grown on the recycled sapphire is much higher than that on fresh sapphire, possibly due to <i>p</i>-type doping of monolayer WS<sub>2</sub> flakes by a thin layer of water intercalated at the atomic steps of the recycled sapphire substrate. The growth and transfer techniques described here are expected to be applicable to other atomically thin TMD materials
Direct Observation of 2D Electrostatics and Ohmic Contacts in Template-Grown Graphene/WS<sub>2</sub> Heterostructures
Large-area
two-dimensional (2D) heterojunctions are promising building
blocks of 2D circuits. Understanding their intriguing electrostatics
is pivotal but largely hindered by the lack of direct observations.
Here grapheneâWS<sub>2</sub> heterojunctions are prepared over
large areas using a seedless ambient-pressure chemical vapor deposition
technique. Kelvin probe force microscopy, photoluminescence spectroscopy,
and scanning tunneling microscopy characterize the doping in grapheneâWS<sub>2</sub> heterojunctions as-grown on sapphire and transferred to SiO<sub>2</sub> with and without thermal annealing. Both pân and nân
junctions are observed, and a flat-band condition (zero Schottky barrier
height) is found for lightly n-doped WS<sub>2</sub>, promising low-resistance
ohmic contacts. This indicates a more favorable band alignment for
grapheneâWS<sub>2</sub> than has been predicted, likely explaining
the low barriers observed in transport experiments on similar heterojunctions.
Electrostatic modeling demonstrates that the large depletion width
of the grapheneâWS<sub>2</sub> junction reflects the electrostatics
of the one-dimensional junction between two-dimensional materials
High-Quality Uniform Dry Transfer of Graphene to Polymers
In this paper we demonstrate high-quality, uniform dry
transfer
of graphene grown by chemical vapor deposition on copper foil to polystyrene.
The dry transfer exploits an azide linker molecule to establish a
covalent bond to graphene and to generate greater grapheneâpolymer
adhesion compared to that of the grapheneâmetal foil. Thus,
this transfer approach provides a novel alternative route for graphene
transfer, which allows for the metal foils to be reused