12 research outputs found
Ultrathin 2D Photodetectors Utilizing Chemical Vapor Deposition Grown WS<sub>2</sub> With Graphene Electrodes
In
this report, graphene (Gr) is used as a 2D electrode and monolayer
WS<sub>2</sub> as the active semiconductor in ultrathin photodetector
devices. All of the 2D materials are grown by chemical vapor deposition
(CVD) and thus pose as a viable route to scalability. The monolayer
thickness of both electrode and semiconductor gives these photodetectors
∼2 nm thickness. We show that graphene is different to conventional
metal (Au) electrodes due to the finite density of states from the
Dirac cones of the valence and conduction bands, which enables the
photoresponsivity to be modulated by electrostatic gating and light
input control. We demonstrate lateral Gr–WS<sub>2</sub>–Gr
photodetectors with photoresponsivities reaching 3.5 A/W under illumination
power densities of 2.5 × 10<sup>7</sup> mW/cm<sup>2</sup>. The
performance of monolayer WS<sub>2</sub> is compared to bilayer WS<sub>2</sub> in photodetectors and we show that increased photoresponsivity
is achieved in the thicker bilayer WS<sub>2</sub> crystals due to
increased optical absorption. This approach of incorporating graphene
electrodes in lateral TMD based devices provides insights on the contact
engineering in 2D optoelectronics, which is crucial for the development
of high performing ultrathin photodetector arrays for versatile applications
Utilizing Interlayer Excitons in Bilayer WS<sub>2</sub> for Increased Photovoltaic Response in Ultrathin Graphene Vertical Cross-Bar Photodetecting Tunneling Transistors
Here
we study the layer-dependent photoconductivity in Gr/WS<sub>2</sub>/Gr vertical stacked tunneling (VST) cross-bar devices made
using two-dimensional (2D) materials all grown by chemical vapor deposition.
The larger number of devices (>100) enables a statistically robust
analysis on the comparative differences in the photovoltaic response
of monolayer and bilayer WS<sub>2</sub>, which cannot be achieved
in small batch devices made using mechanically exfoliated materials.
We show a dramatic increase in photovoltaic response for Gr/WS<sub>2</sub>(2L)/Gr compared to monolayers because of the long inter-
and intralayer exciton lifetimes and the small exciton binding energy
(both interlayer and intralayer excitons) of bilayer WS<sub>2</sub> compared with that of monolayer WS<sub>2</sub>. Different doping
levels and dielectric environments of top and bottom graphene electrodes
result in a potential difference across a ∼1 nm vertical device,
which gives rise to large electric fields perpendicular to the WS<sub>2</sub> layers that cause band structure modification. Our results
show how precise control over layer number in all 2D VST devices dictates
the photophysics and performance for photosensing applications
Revealing Strain-Induced Effects in Ultrathin Heterostructures at the Nanoscale
Two-dimensional
materials are being increasingly studied, particularly
for flexible and wearable technologies because of their inherent thickness
and flexibility. Crucially, one aspect where our understanding is
still limited is on the effect of mechanical strain, not on individual
sheets of materials, but when stacked together as heterostructures
in devices. In this paper, we demonstrate the use of Kelvin probe
microscopy in capturing the influence of uniaxial tensile strain on
the band-structures of graphene and WS<sub>2</sub> (mono- and multilayered)
based heterostructures at high resolution. We report a major advance
in strain characterization tools through enabling a single-shot capture
of strain defined changes in a heterogeneous system at the nanoscale,
overcoming the limitations (materials, resolution, and substrate effects)
of existing techniques such as optical spectroscopy. Using this technique,
we observe that the work-functions of graphene and WS<sub>2</sub> increase
as a function of strain, which we attribute to the Fermi level lowering
from increased p-doping. We also extract the nature of the interfacial
heterojunctions and find that they get strongly modulated from strain.
We observe that the strain-enhanced charge transfer with the substrate
plays a dominant role, causing the heterostructures to behave differently
from two-dimensional materials in their isolated forms
Revealing Defect-State Photoluminescence in Monolayer WS<sub>2</sub> by Cryogenic Laser Processing
Understanding the stability of monolayer
transition metal dichalcogenides
in atmospheric conditions has important consequences for their handling,
life-span, and utilization in applications. We show that cryogenic
photoluminescence spectroscopy (PL) is a highly sensitive technique
to the detection of oxidation induced degradation of monolayer tungsten
disulfide (WS<sub>2</sub>) caused by exposure to ambient conditions.
Although long-term exposure to atmospheric conditions causes massive
degradation from oxidation that is optically visible, short-term exposure
produces no obvious changes to the PL or Raman spectra measured at
either room temperature or even cryogenic environment. Laser processing
was employed to remove the surface adsorbents, which enables the defect
states to be detected via cryogenic PL spectroscopy. Thermal cycling
to room temperature and back down to 77 K shows the process is reversible.
We also monitor the degradation process of WS<sub>2</sub> using this
method, which shows that the defect related peak can be observed after
one month aging in ambient conditions
Negative Electro-conductance in Suspended 2D WS<sub>2</sub> Nanoscale Devices
We
study the <i>in situ</i> electro-conductance in nanoscale
electronic devices composed of suspended monolayer WS<sub>2</sub> with
metal electrodes inside an aberration-corrected transmission electron
microscope. Monitoring the conductance changes when the device is
exposed to the electron beam of 80 keV energy reveals a reversible
decrease in conductivity with increasing beam current density. The
response time of the electro-conductance when exposed to the electron
beam is substantially faster than the recovery time when the beam
is turned off. We propose a charge trap model that accounts for excitation
of electrons into the conduction band and localized trap states from
energy supplied by inelastic scattering of incident 80 keV electrons.
These results show how monolayer transition metal dichalcogenide 2D
semiconductors can be used as transparent direct electron detectors
in ultrathin nanoscale devices
Biexciton Formation in Bilayer Tungsten Disulfide
Monolayer
transition metal dichalcogenides (TMDs) are direct band
gap semiconductors, and their 2D structure results in large binding
energies for excitons, trions, and biexcitons. The ability to explore
many-body effects in these monolayered structures has made them appealing
for future optoelectronic and photonic applications. The band structure
changes for bilayer TMDs with increased contributions from indirect
transitions, and this has limited similar in-depth studies of biexcitons.
Here, we study biexciton emission in bilayer WS<sub>2</sub> grown
by chemical vapor deposition as a function of temperature. A biexciton
binding energy of 36 ±4 meV is measured in the as-grown bilayer
WS<sub>2</sub> containing 0.4% biaxial strain as determined by Raman
spectroscopy. The biexciton emission was difficult to detect when
the WS<sub>2</sub> was transferred to another substrate to release
the stain. Density functional theory calculations show that 0.4% of
tensile strain lowers the direct band gap by about 55 meV without
significant change to the indirect band gap, which can cause an increase
in the quantum yield of direct exciton transitions and the emission
from biexcitons formed by two direct gap excitons. We find that the
biexciton emission decreases dramatically with increased temperature
due to the thermal dissociation, with an activation energy of 26 ±
5 meV. These results show how strain can be used to tune the many-body
effects in bilayered TMD materials and extend the photonic applications
beyond pure monolayer systems
High-Performance All 2D-Layered Tin Disulfide: Graphene Photodetecting Transistors with Thickness-Controlled Interface Dynamics
Tin
disulfide crystals with layered two-dimensional (2D) sheets are grown
by chemical vapor deposition using a novel precursor approach and
integrated into all 2D transistors with graphene (Gr) electrodes.
The Gr:SnS<sub>2</sub>:Gr transistors exhibit excellent photodetector
response with high detectivity and photoresponsivity. We show that
the response of the all 2D photodetectors depends upon charge trapping
at the interface and the Schottky barrier modulation. The thickness-dependent
SnS<sub>2</sub> measurements in devices reveal a transition from the
interface-dominated response for thin crystals to bulklike response
for the thicker SnS<sub>2</sub> crystals, showing the sensitivity
of devices fabricated using layered materials on the number of layers.
These results show that SnS<sub>2</sub> has photosensing performance
when combined with Gr electrodes that is comparable to other 2D transition
metal dichalcogenides of MoS<sub>2</sub> and WS<sub>2</sub>
Doping Graphene Transistors Using Vertical Stacked Monolayer WS<sub>2</sub> Heterostructures Grown by Chemical Vapor Deposition
We study the interactions in graphene/WS<sub>2</sub> two-dimensional (2D) layered vertical heterostructures with
variations in the areal coverage of graphene by the WS<sub>2</sub>. All 2D materials were grown by chemical vapor deposition and transferred
layer by layer. Photoluminescence (PL) spectroscopy of WS<sub>2</sub> on graphene showed PL quenching along with an increase in the ratio
of exciton/trion emission, relative to WS<sub>2</sub> on SiO<sub>2</sub> surface, indicating a reduction in the n-type doping levels of WS<sub>2</sub> as well as reduced radiative recombination quantum yield.
Electrical measurements of a total of 220 graphene field effect transistors
with different WS<sub>2</sub> coverage showed double-Dirac points
in the field effect measurements, where one is shifted closer toward
the 0 V gate neutrality position due to the WS<sub>2</sub> coverage.
Photoirradiation of the WS<sub>2</sub> on graphene region caused further
Dirac point shifts, indicative of a reduction in the p-type doping
levels of graphene, revealing that the photogenerated excitons in
WS<sub>2</sub> are split across the heterostructure by electron transfer
from WS<sub>2</sub> to graphene. Kelvin probe microscopy showed that
regions of graphene covered with WS<sub>2</sub> had a smaller work
function and supports the model of electron transfer from WS<sub>2</sub> to graphene. Our results demonstrate the formation of junctions
within a graphene transistor through the spatial tuning of the work
function of graphene using these 2D vertical heterostructures
Chemical Vapor Deposition Growth of Two-Dimensional Monolayer Gallium Sulfide Crystals Using Hydrogen Reduction of Ga<sub>2</sub>S<sub>3</sub>
Two-dimensional
gallium sulfide (GaS) crystals are synthesized
by a simple and efficient ambient pressure chemical vapor deposition
(CVD) method using a single-source precursor of Ga<sub>2</sub>S<sub>3</sub>. The synthesized GaS structures involve triangular monolayer
domains and multilayer flakes with thickness of 1 and 15 nm, respectively.
Regions of continuous films of GaS are also achieved with about 0.7
cm<sup>2</sup> uniform coverage. This is achieved by using hydrogen
carrier gas and the horizontally placed SiO<sub>2</sub>/Si substrates.
Electron microscopy and spectroscopic measurements are used to characteristic
the CVD-grown materials. This provides important insights into novel
approaches for enlarging the domain size of GaS crystals and understanding
of the growth mechanism using this precursor system
Photoluminescence Segmentation within Individual Hexagonal Monolayer Tungsten Disulfide Domains Grown by Chemical Vapor Deposition
We
show that hexagonal domains of monolayer tungsten disulfide
(WS<sub>2</sub>) grown by chemical vapor deposition (CVD) with powder
precursors can have discrete segmentation in their photoluminescence
(PL) emission intensity, forming symmetric patterns with alternating
bright and dark regions. Two-dimensional maps of the PL reveal significant
reduction within the segments associated with the longest sides of
the hexagonal domains. Analysis of the PL spectra shows differences
in the exciton to trion ratio, indicating variations in the exciton
recombination dynamics. Monolayers of WS<sub>2</sub> hexagonal islands
transferred to new substrates still exhibit this PL segmentation,
ruling out local strain in the regions as the dominant cause. High-power
laser irradiation causes preferential degradation of the bright segments
by sulfur removal, indicating the presence of a more defective region
that is higher in oxidative reactivity. Atomic force microscopy (AFM)
images of topography and amplitude modes show uniform thickness of
the WS<sub>2</sub> domains and no signs of segmentation. However,
AFM phase maps do show the same segmentation of the domain as the
PL maps and indicate that it is caused by some kind of structural
difference that we could not clearly identify. These results provide
important insights into the spatially varying properties of these
CVD-grown transition metal dichalcogenide materials, which may be
important for their effective implementation in fast photo sensors
and optical switches