Spectroscopic Signatures for Interlayer Coupling in MoS₂–WSe₂ van der Waals Stacking

Stacking of MoS₂ and WSe₂ monolayers is conducted by transferring triangular MoS₂ monolayers on top of WSe₂ monolayers, all grown by chemical vapor deposition (CVD). Raman spectroscopy and photoluminescence (PL) studies reveal that these mechanically stacked monolayers are not closely coupled, but after a thermal treatment at 300 °C, it is possible to produce van der Waals solids consisting of two interacting transition metal dichalcogenide (TMD) monolayers. The layer-number sensitive Raman out-of-plane mode A²_1g for WSe₂ (309 cm^–1) is found sensitive to the coupling between two TMD monolayers. The presence of interlayer excitonic emissions and the changes in other intrinsic Raman modes such as E″ for MoS₂ at 286 cm^–1 and A²_1g for MoS₂ at around 463 cm^–1 confirm the enhancement of the interlayer coupling

Second Harmonic Generation from Artificially Stacked Transition Metal Dichalcogenide Twisted Bilayers

Optical second harmonic generation (SHG) is known as a sensitive probe to the crystalline symmetry of few-layer transition metal dichalcogenides (TMDs). Layer-number dependent and polarization resolved SHG have been observed for the special case of Bernal stacked few-layer TMDs, but it remains largely unexplored for structures deviated from this ideal stacking order. Here we report on the SHG from homo- and heterostructural TMD bilayers formed by artificial stacking with an arbitrary stacking angle. The SHG from the twisted bilayers is a coherent superposition of the SH fields from the individual layers, with a phase difference depending on the stacking angle. Such an interference effect is insensitive to the constituent layered materials and thus applicable to hetero-stacked bilayers. A proof-of-concept demonstration of using the SHG to probe the domain boundary and crystal polarity of mirror twins formed in chemically grown TMDs is also presented. We show here that the SHG is an efficient, sensitive, and nondestructive characterization for the stacking orientation, crystal polarity, and domain boundary of van der Waals heterostructures made of noncentrosymmetric layered materials

Ultrafast Transient Terahertz Conductivity of Monolayer MoS₂ and WSe₂ Grown by Chemical Vapor Deposition

We have measured ultrafast charge carrier dynamics in monolayers and trilayers of the transition metal dichalcogenides MoS₂ and WSe₂ using a combination of time-resolved photoluminescence and terahertz spectroscopy. We recorded a photoconductivity and photoluminescence response time of just 350 fs from CVD-grown monolayer MoS₂, and 1 ps from trilayer MoS₂ and monolayer WSe₂. Our results indicate the potential of these materials as high-speed optoelectronic materials

Large-Area Synthesis of Highly Crystalline WSe₂ Monolayers and Device Applications

The monolayer transition metal dichalcogenides have recently attracted much attention owing to their potential in valleytronics, flexible and low-power electronics, and optoelectronic devices. Recent reports have demonstrated the growth of large-size two-dimensional MoS₂ layers by the sulfurization of molybdenum oxides. However, the growth of a transition metal selenide monolayer has still been a challenge. Here we report that the introduction of hydrogen in the reaction chamber helps to activate the selenization of WO₃, where large-size WSe₂ monolayer flakes or thin films can be successfully grown. The top-gated field-effect transistors based on WSe₂ monolayers using ionic gels as the dielectrics exhibit ambipolar characteristics, where the hole and electron mobility values are up to 90 and 7 cm²/Vs, respectively. These films can be transferred onto arbitrary substrates, which may inspire research efforts to explore their properties and applications. The resistor-loaded inverter based on a WSe₂ film, with a gain of ∼13, further demonstrates its applicability for logic-circuit integrations

Observing Grain Boundaries in CVD-Grown Monolayer Transition Metal Dichalcogenides

Two-dimensional monolayer transition metal dichalcogenides (TMdCs), driven by graphene science, revisit optical and electronic properties, which are markedly different from bulk characteristics. These properties are easily modified due to accessibility of all the atoms viable to ambient gases, and therefore, there is no guarantee that impurities and defects such as vacancies, grain boundaries, and wrinkles behave as those of ideal bulk. On the other hand, this could be advantageous in engineering such defects. Here, we report a method of observing grain boundary distribution of monolayer TMdCs by a selective oxidation. This was implemented by exposing directly the TMdC layer grown on sapphire without transfer to ultraviolet light irradiation under moisture-rich conditions. The generated oxygen and hydroxyl radicals selectively functionalized defective grain boundaries in TMdCs to provoke morphological changes at the boundary, where the grain boundary distribution was observed by atomic force microscopy and scanning electron microscopy. This paves the way toward the investigation of transport properties engineered by defects and grain boundaries

Multilayer Graphene–WSe₂ Heterostructures for WSe₂ Transistors

Two-dimensional (2D) materials are drawing growing attention for next-generation electronics and optoelectronics owing to its atomic thickness and unique physical properties. One of the challenges posed by 2D materials is the large source/drain (S/D) series resistance due to their thinness, which may be resolved by thickening the source and drain regions. Recently explored lateral graphene–MoS₂− and graphene–WS₂, heterostructures shed light on resolving the mentioned issues owing to their superior ohmic contact behaviors. However, recently reported field-effect transistors (FETs) based on graphene–TMD heterostructures have only shown n-type characteristics. The lack of p-type transistor limits their applications in complementary metal-oxide semiconductor electronics. In this work, we demonstrate p-type FETs based on graphene–WSe₂ lateral heterojunctions grown with the scalable CVD technique. Few-layer WSe₂ is overlapped with the multilayer graphene (MLG) at MLG–WSe₂ junctions such that the contact resistance is reduced. Importantly, the few-layer WSe₂ only forms at the junction region while the channel is still maintained as a WSe₂ monolayer for transistor operation. Furthermore, by imposing doping to graphene S/D, 2 orders of magnitude enhancement in <i>I</i>_on/<i>I</i>_off ratio to ∼10⁸ and the unipolar p<i>-</i>type characteristics are obtained regardless of the work function of the metal in ambient air condition. The MLG is proposed to serve as a 2D version of emerging raised source/drain approach in electronics

Selectively Plasmon-Enhanced Second-Harmonic Generation from Monolayer Tungsten Diselenide on Flexible Substrates

Monolayer two-dimensional transition-metal dichalcogenides (2D TMDCs) exhibit promising characteristics in miniaturized nonlinear optical frequency converters, due to their inversion asymmetry and large second-order nonlinear susceptibility. However, these materials usually have very short light interaction lengths with the pump laser because they are atomically thin, such that second-harmonic generation (SHG) is generally inefficient. In this paper, we fabricate a judiciously structured 150 nm-thick planar surface consisting of monolayer tungsten diselenide and sub-20 nm-wide gold trenches on flexible substrates, reporting ∼7000-fold SHG enhancement without peak broadening or background in the spectra as compared to WSe₂ on as-grown sapphire substrates. Our proof-of-concept experiment yields effective second-order nonlinear susceptibility of 2.1 × 10⁴ pm/V. Three orders of magnitude enhancement is maintained with pump wavelength ranging from 800 to 900 nm, breaking the limitation of narrow pump wavelength range for cavity-enhanced SHG. In addition, SHG amplitude can be dynamically controlled <i>via</i> selective excitation of the lateral gap plasmon by rotating the laser polarization. Such a fully open, flat, and ultrathin profile enables a great variety of functional samples with high SHG from one patterned silicon substrate, favoring scalable production of nonlinear converters. The surface accessibility also enables integration with other optical components for information processing in an ultrathin and flexible form

Photoluminescence Enhancement and Structure Repairing of Monolayer MoSe₂ by Hydrohalic Acid Treatment

Atomically thin two-dimensional transition-metal dichalcogenides (TMDCs) have attracted much attention recently due to their unique electronic and optical properties for future optoelectronic devices. The chemical vapor deposition (CVD) method is able to generate TMDCs layers with a scalable size and a controllable thickness. However, the TMDC monolayers grown by CVD may incorporate structural defects, and it is fundamentally important to understand the relation between photoluminescence and structural defects. In this report, point defects (Se vacancies) and oxidized Se defects in CVD-grown MoSe₂ monolayers are identified by transmission electron microscopy and X-ray photoelectron spectroscopy. These defects can significantly trap free charge carriers and localize excitons, leading to the smearing of free band-to-band exciton emission. Here, we report that the simple hydrohalic acid treatment (such as HBr) is able to efficiently suppress the trap-state emission and promote the neutral exciton and trion emission in defective MoSe₂ monolayers through the <i>p</i>-doping process, where the overall photoluminescence intensity at room temperature can be enhanced by a factor of 30. We show that HBr treatment is able to activate distinctive trion and free exciton emissions even from highly defective MoSe₂ layers. Our results suggest that the HBr treatment not only reduces the <i>n</i>-doping in MoSe₂ but also reduces the structural defects. The results provide further insights of the control and tailoring the exciton emission from CVD-grown monolayer TMDCs

Monolayer MoSe₂ Grown by Chemical Vapor Deposition for Fast Photodetection

Monolayer molybdenum disulfide (MoS₂) has become a promising building block in optoelectronics for its high photosensitivity. However, sulfur vacancies and other defects significantly affect the electrical and optoelectronic properties of monolayer MoS₂ devices. Here, highly crystalline molybdenum diselenide (MoSe₂) monolayers have been successfully synthesized by the chemical vapor deposition (CVD) method. Low-temperature photoluminescence comparison for MoS₂ and MoSe₂ monolayers reveals that the MoSe₂ monolayer shows a much weaker bound exciton peak; hence, the phototransistor based on MoSe₂ presents a much faster response time (<25 ms) than the corresponding 30 s for the CVD MoS₂ monolayer at room temperature in ambient conditions. The images obtained from transmission electron microscopy indicate that the MoSe exhibits fewer defects than MoS₂. This work provides the fundamental understanding for the differences in optoelectronic behaviors between MoSe₂ and MoS₂ and is useful for guiding future designs in 2D material-based optoelectronic devices