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 MoS₂ and WS₂, the S-ZZ edges in the WS₂ monolayer flakes more easily decompose into sawtooth-like edges in M-ZZ type as compared to the MoS₂ monolayer; thus, the hexagonal morphology can be seen more often in WS₂. Moreover, the observed anisotropic short-range order in the MoS₂/WS₂ 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 MoSe₂ Quantum Dots and a Monolayer of MoS₂ or WSe₂

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 MoSe₂ quantum dots (QDs) and a MoS₂ or WSe₂ monolayer. The strong modification of photoluminescence and Raman spectra that includes the quenching of MoSe₂ QDs and the varied spectral weights of trions for the MoS₂ and WSe₂ 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 MoS₂, WS₂, and WSe₂, 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) MoS₂, 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-MoS₂ 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 SMo. 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-MoS₂ 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-MoS₂ 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-MoS₂. Consequently, the overall photocurrent of the hybrid 1L-MoS₂ photodetector was observed to be 250 times better than that of the pristine 1L-MoS₂ 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 WS₂ on gold foil by chemical vapor deposition. The limited tungsten and sulfur solubility in gold foil allows monolayer WS₂ film growth on gold surface. To ensure the coverage uniformity of monolayer WS₂ 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 WS₂-based field effect transistor reveals an electron mobility of 20 cm² V^–1 s^–1 with high on–off ratio of ∼10⁸ at room temperature, which is the highest reported value from previous reports of CVD-grown WS₂ samples. The on–off ratio of integrated multiple FETs on the large area WS₂ film on SiO₂ (300 nm)/Si substrate shows within the same order, implying reasonable uniformity of WS₂ FET device characteristics over a large area of 3 × 1.5 cm²

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 MoS₂ or MoSe₂ 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 MoS₂ 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-MoTe₂ thin films <i>via</i> the tellurization of Mo thin films. 1T′-MoTe₂ was initially grown and converted gradually to 2H-MoTe₂ over a prolonged growth time under a Te atmosphere. Maintaining excessive Te was essential for obtaining the stable stoichiometric 2H-MoTe₂ 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-MoTe₂ to 1T′-MoTe₂. The orientation of the 2H-MoTe₂ film was determined by the tellurization rate. Slow tellurization was the key for obtaining a highly oriented 2H-MoTe₂ film over the entire area, while fast tellurization led to a 2H-MoTe₂ 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 MoS₂ 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