12 research outputs found
Impact of Polar Edge Terminations of the Transition Metal Dichalcogenide Monolayers during Vapor Growth
The
polar edges of two-dimensional monolayer transition metal dichalcogenides
(TMD) and their alloys are examined by combined theoretical (density
functional theory) and experimental approaches. For these polar edges,
the growth reaction energies between different edge terminations are
considered instead of the surface free energies. Due to different
energy evolutions during growth on the zigzag edges between MoS<sub>2</sub> and WS<sub>2</sub>, the S-ZZ edges in the WS<sub>2</sub> monolayer
flakes more easily decompose into sawtooth-like edges in M-ZZ type
as compared to the MoS<sub>2</sub> monolayer; thus, the hexagonal
morphology can be seen more often in WS<sub>2</sub>. Moreover, the
observed anisotropic short-range order in the MoS<sub>2</sub>/WS<sub>2</sub> alloys is originated from the freezed edge configurations
during growth, explainable by the growth kinetics and thermodynamics
of the Mo-ZZ-edges. The determination of the growing edge terminations
is of great importance for the controllable synthesis of the emergent
two-dimensional TMD materials
Observation of Charge Transfer in Heterostructures Composed of MoSe<sub>2</sub> Quantum Dots and a Monolayer of MoS<sub>2</sub> or WSe<sub>2</sub>
Monolayer transition metal dichalcogenides
(TMDs) are atomically
thin semiconductor films that are ideal platforms for the study and
engineering of quantum heterostructures for optoelectronic applications.
We present a simple method for the fabrication of TMD heterostructures
containing MoSe<sub>2</sub> quantum dots (QDs) and a MoS<sub>2</sub> or WSe<sub>2</sub> monolayer. The strong modification of photoluminescence
and Raman spectra that includes the quenching of MoSe<sub>2</sub> QDs
and the varied spectral weights of trions for the MoS<sub>2</sub> and
WSe<sub>2</sub> monolayers were observed, suggesting the charge transfer
occurring in these TMD heterostructures. Such optically active heterostructures,
which can be conveniently fabricated by dispersing TMD QDs onto TMD
monolayers, are likely to have various nanophotonic applications because
of their versatile and controllable properties
Simple Chemical Treatment to n‑Dope Transition-Metal Dichalcogenides and Enhance the Optical and Electrical Characteristics
The
optical and electrical properties of monolayer transition-metal
dichalcogenides (1L-TMDs) are critically influenced by two dimensionally
confined exciton complexes. Although extensive studies on controlling
the optical properties of 1L-TMDs through external doping or defect
engineering have been carried out, the effects of excess charges,
defects, and the populations of exciton complexes on the light emission
of 1L-TMDs are not yet fully understood. Here, we present a simple
chemical treatment method for n-dope 1L-TMDs, which also enhances
their optical and electrical properties. We show that dipping 1Ls
of MoS<sub>2</sub>, WS<sub>2</sub>, and WSe<sub>2</sub>, whether exfoliated
or grown by chemical vapor deposition, into methanol for several hours
can increase the electron density and also can reduce the defects,
resulting in the enhancement of their photoluminescence, light absorption,
and the carrier mobility. This methanol treatment was effective for
both n- and p-type 1L-TMDs, suggesting that the surface restructuring
around structural defects by methanol is responsible for the enhancement
of optical and electrical characteristics. Our results have revealed
a simple process for external doping that can enhance both the optical
and electrical properties of 1L-TMDs and help us understand how the
exciton emission in 1L-TMDs can be modulated by chemical treatments
Atomic Observation of Filling Vacancies in Monolayer Transition Metal Sulfides by Chemically Sourced Sulfur Atoms
Chemical
treatment using bis(trifluoromethane) sulfonimide (TFSI)
was shown to be particularly effective for increasing the photoluminescence
(PL) of monolayer (1L) MoS<sub>2</sub>, suggesting a convenient method
for overcoming the intrinsically low quantum yield of this material.
However, the underlying atomic mechanism of the PL enhancement has
remained elusive. Here, we report the microscopic origin of the defect
healing observed in TFSI-treated 1L-MoS<sub>2</sub> through a correlative
combination of optical characterization and atomic-scale scanning
transmission electron microscopy, which showed that most of the sulfur
vacancies were directly repaired by the extrinsic sulfur atoms produced
from the dissociation of TFSI, concurrently resulting in a significant
PL enhancement. Density functional theory calculations confirmed that
the reactive sulfur dioxide molecules that dissociated from TFSI can
be reduced to sulfur and oxygen gas at the vacancy site to form strongly
bound SMo. Our results reveal how defect-mediated nonradiative
recombination can be effectively eliminated by a simple chemical treatment
method, thereby advancing the practical applications of monolayer
semiconductors
Augmented Quantum Yield of a 2D Monolayer Photodetector by Surface Plasmon Coupling
Monolayer (1L) transition
metal dichalcogenides (TMDCs) are promising
materials for nanoscale optoelectronic devices because of their direct
band gap and wide absorption range (ultraviolet to infrared). However,
1L-TMDCs cannot be easily utilized for practical optoelectronic device
applications (e.g., photodetectors, solar cells, and light-emitting
diodes) because of their extremely low optical quantum yields (QYs).
In this investigation, a high-gain 1L-MoS<sub>2</sub> photodetector
was successfully realized, based on the surface plasmon (SP) of the
Ag nanowire (NW) network. Through systematic optical characterization
of the hybrid structure consisting of a 1L-MoS<sub>2</sub> and the
Ag NW network, it was determined that a strong SP and strain relaxation
effect influenced a greatly enhanced optical QY. The photoluminescence
(PL) emission was drastically increased by a factor of 560, and the
main peak was shifted to the neutral exciton of 1L-MoS<sub>2</sub>. Consequently, the overall photocurrent of the hybrid 1L-MoS<sub>2</sub> photodetector was observed to be 250 times better than that
of the pristine 1L-MoS<sub>2</sub> photodetector. In addition, the
photoresponsivity and photodetectivity of the hybrid photodetector
were effectively improved by a factor of ∼1000. This study
provides a new approach for realizing highly efficient optoelectronic
devices based on TMDCs
Synthesis of Centimeter-Scale Monolayer Tungsten Disulfide Film on Gold Foils
We report the synthesis of centimeter-scale monolayer WS<sub>2</sub> on gold foil by chemical vapor deposition. The limited tungsten and sulfur solubility in gold foil allows monolayer WS<sub>2</sub> film growth on gold surface. To ensure the coverage uniformity of monolayer WS<sub>2</sub> film, the tungsten source-coated substrate was placed in parallel with Au foil under hydrogen sulfide atmosphere. The high growth temperature near 935 °C helps to increase a domain size up to 420 μm. Gold foil is reused for the repeatable growth after bubbling transfer. The WS<sub>2</sub>-based field effect transistor reveals an electron mobility of 20 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> with high on–off ratio of ∼10<sup>8</sup> at room temperature, which is the highest reported value from previous reports of CVD-grown WS<sub>2</sub> samples. The on–off ratio of integrated multiple FETs on the large area WS<sub>2</sub> film on SiO<sub>2</sub> (300 nm)/Si substrate shows within the same order, implying reasonable uniformity of WS<sub>2</sub> FET device characteristics over a large area of 3 × 1.5 cm<sup>2</sup>
Enhanced Light Emission from Monolayer Semiconductors by Forming Heterostructures with ZnO Thin Films
Monolayer
transition-metal dichalcogenides (1L-TMDs) are atomically thin direct
band gap semiconductors, from which the emission of light is determined
by optical transitions of exciton complexes such as neutral excitons
and trions. While the quantum yields of 1L-TMDs are quite low, the
ability to control the populations of exciton complexes in 1L-TMDs
through various doping processes is an interesting advantage, and
provides ample possibilities for engineering the optical properties
of these semiconductor monolayers. Here we demonstrate a simple method
of controlling the populations of excitons and trions to enhance the
light emission of 1L-TMDs by having them form heterostructures with
ZnO thin films (TFs). 1Ls of MoS<sub>2</sub> or MoSe<sub>2</sub> showed
up to 17-fold increases in photoluminescence (PL) when they were placed
on ∼50 nm thick ZnO TFs. This enhancement of the PL was due
to charge exchanges occurring through the 1L-TMD/ZnO interface. The
PL enhancements and changes in the PL spectra of the 1L-TMDs were
greater when the 1L-TMD/ZnO heterostructures were subjected to 355
nm wavelength laser excitation than when they were excited with a
514 nm wavelength laser, which we attributed to the onset of energy
transfer by photoexcited excitons and/or the additional p-doping by
photoexcited holes in ZnO. The p-doping phenomenon and the enhanced
light emission of 1L-TMD/ZnO heterostructures were unambiguously visualized
in spatially resolved PL and Raman spectral maps. Our approach using
the 1L-TMD/ZnO TF heterostructure suggests that a rich variety of
options for engineering the optical properties of 1L-TMDs may be made
available by carrying out simple and intuitive manipulations of exciton
complexes, and these endeavors may yield practical applications for
1L-TMDs in nanophotonic devices
Selected Area Manipulation of MoS<sub>2</sub> via Focused Electron Beam-Induced Etching for Nanoscale Device Editing
We demonstrate direct-write patterning of single and
multilayer
MoS2 via a focused electron beam-induced etching (FEBIE)
process mediated with the XeF2 precursor. MoS2 etching is performed at various currents, areal doses, on different
substrates, and characterized using scanning electron and atomic force
microscopies as well as Raman and photoluminescence spectroscopies.
Scanning transmission electron microscopy reveals a sub-40 nm etching
resolution and the progression of point defects and lateral etching
of the consequent unsaturated bonds. The results confirm that the
electron beam-induced etching process is minimally invasive to the
underlying material in comparison to ion beam techniques, which damage
the subsurface material. Single-layer MoS2 field-effect
transistors are fabricated, and device characteristics are compared
for channels that are edited via the selected area etching process.
The source–drain current at constant gate and source–drain
voltage scale linearly with the edited channel width. Moreover, the
mobility of the narrowest channel width decreases, suggesting that
backscattered and secondary electrons collaterally affect the periphery
of the removed area. Focused electron beam doses on single-layer transistors
below the etching threshold were also explored as a means to modify/thin
the channel layer. The FEBIE exposures showed demonstrative effects
via the transistor transfer characteristics, photoluminescence spectroscopy,
and Raman spectroscopy. While strategies to minimize backscattered
and secondary electron interactions outside of the scanned regions
require further investigation, here, we show that FEBIE is a viable
approach for selective nanoscale editing of MoS2 devices
Phase-Engineered Synthesis of Centimeter-Scale 1T′- and 2H-Molybdenum Ditelluride Thin Films
We report the synthesis of centimeter-scale, uniform 1T′- and 2H-MoTe<sub>2</sub> thin films <i>via</i> the tellurization of Mo thin films. 1T′-MoTe<sub>2</sub> was initially grown and converted gradually to 2H-MoTe<sub>2</sub> over a prolonged growth time under a Te atmosphere. Maintaining excessive Te was essential for obtaining the stable stoichiometric 2H-MoTe<sub>2</sub> phase. Further annealing under a lower partial pressure of Te at the same temperature, followed by a rapid quenching, led to the reverse phase transition from 2H-MoTe<sub>2</sub> to 1T′-MoTe<sub>2</sub>. The orientation of the 2H-MoTe<sub>2</sub> film was determined by the tellurization rate. Slow tellurization was the key for obtaining a highly oriented 2H-MoTe<sub>2</sub> film over the entire area, while fast tellurization led to a 2H-MoTe<sub>2</sub> film with a randomly oriented <i>c</i>-axis
Metal–Insulator–Semiconductor Diode Consisting of Two-Dimensional Nanomaterials
We
present a novel metal–insulator–semiconductor (MIS)
diode consisting of graphene, hexagonal BN, and monolayer MoS<sub>2</sub> for application in ultrathin nanoelectronics. The MIS heterojunction
structure was fabricated by vertically stacking layered materials
using a simple wet chemical transfer method. The stacking of each
layer was confirmed by confocal scanning Raman spectroscopy and device
performance was evaluated using current versus voltage (<i>I</i>–<i>V</i>) and photocurrent measurements. We clearly
observed better current rectification and much higher current flow
in the MIS diode than in the p–n junction and the metal–semiconductor
diodes made of layered materials. The <i>I</i>–<i>V</i> characteristic curve of the MIS diode indicates that current
flows mainly across interfaces as a result of carrier tunneling. Moreover,
we observed considerably high photocurrent from the MIS diode under
visible light illumination