Hydrophilic Character of Single-Layer MoS2 Grown on Ag(111)

Interface coupling in Au-supported MoS2-WS2 heterobilayers grown by pulsed laser deposition

Van der Waals heterostructures of transition metal dichalcogenides (TMDs) are promising systems for engineering functional layered 2D materials with tailored properties. In this work, we study the growth of WS₂/MoS₂ and MoS₂/WS₂ heterobilayers by pulsed laser deposition (PLD) under ultra-high vacuum conditions. Using Au(111) as growth substrate, we investigated the heterobilayer morphology and structure at the nanoscale by in situ scanning tunneling microscopy. Our experiments show that the heterostructure growth can be controlled with high coverage and thickness sensitivity by tuning the number of laser pulses in the PLD process. Raman spectroscopy complemented our investigation, revealing the effect of the interaction with the metallic substrate on the TMD vibrational properties and a strong interlayer coupling between the MoS₂ and WS₂ layers. The transfer of the heterobilayers on a silica substrate via a wet etching process shows the possibility to decouple them from the native metallic substrate and confirms that the interlayer coupling is not substrate-dependent. This work highlights the potential of the PLD technique as a method to grow TMD heterostructures, opening to new perspectives in the synthesis of complex 2D layered materials.

Universal epitaxy of non-centrosymmetric two-dimensional single-crystal metal dichalcogenides

The great challenge for the growth of non-centrosymmetric 2D single crystals is to break the equivalence of antiparallel grains. Even though this pursuit has been partially achieved in boron nitride and transition metal dichalcogenides (TMDs) growth, the key factors that determine the epitaxy of non-centrosymmetric 2D single crystals are still unclear. Here we report a universal methodology for the epitaxy of non-centrosymmetric 2D metal dichalcogenides enabled by accurate time sequence control of the simultaneous formation of grain nuclei and substrate steps. With this methodology, we have demonstrated the epitaxy of unidirectionally aligned MoS₂ grains on a, c, m, n, r and v plane Al₂O₃ as well as MgO and TiO₂ substrates. This approach is also applicable to many TMDs, such as WS₂, NbS₂, MoSe₂, WSe₂ and NbSe₂. This study reveals a robust mechanism for the growth of various 2D single crystals and thus paves the way for their potential applications.

Ambient Pressure Chemical Vapor Deposition of Flat and Vertically Aligned MoS2 Nanosheets

Molybdenum disulfide (MoS₂) got tremendous attention due to its atomically thin body, rich physics, and high carrier mobility. The controlled synthesis of large area and high crystalline monolayer MoS₂ nanosheets on diverse substrates remains a challenge for potential practical applications. Synthesizing different structured MoS₂ nanosheets with horizontal and vertical orientations with respect to the substrate surface would bring a configurational versatility with benefit for numerous applications, including nanoelectronics, optoelectronics, and energy technologies. Among the proposed methods, ambient pressure chemical vapor deposition (AP-CVD) is a promising way for developing large-scale MoS₂ nanosheets because of its high flexibility and facile approach. Here, we show an effective way for synthesizing large-scale horizontally and vertically aligned MoS₂ on different substrates such as flat SiO₂/Si, pre-patterned SiO₂ and conductive substrates (TaN) benefit various direct TMDs production. In particular, we show precise control of CVD optimization for yielding high-quality MoS₂ layers by changing growth zone configuration and the process steps. We demonstrated that the influence of configuration variability by local changes of the S to MoO₃ precursor positions in the growth zones inside the CVD reactor is a key factor that results in differently oriented MoS₂ formation. Finally, we show the layer quality and physical properties of as-grown MoS₂ by means of different characterizations: Raman spectroscopy, scanning electron microscopy (SEM), photoluminescence (PL) and X-ray photoelectron spectroscopy (XPS). These experimental findings provide a strong pathway for conformally recasting AP-CVD grown MoS₂ in many different configurations (i.e., substrate variability) or motifs (i.e., vertical or planar alignment) with potential for flexible electronics, optoelectronics, memories to energy storage devices.

Photoluminescence of monolayer MoS2 modulated by water/O2/laser irradiation

The low photoluminescence (PL) quantum yields of transition metal dichalcogenide monolayers have been a limiting factor for their optoelectronic applications. Various and even inconsistent mechanisms have been proposed to modulate their PL efficiencies. Herein, we use PL/Raman microspectroscopy and the corresponding in situ mapping, atomic force microscopy, and field-effect transistor (FET) characterization to investigate the changes in the structural and optical properties of monolayer MoS₂. Relatively low power density (<4.08 × 10⁵ W cm^-2) of laser irradiation in ambient air can cause a slight PL suppression effect on monolayer MoS₂, whereas relatively high power density (∼1.02 × 10⁶ W cm^-2) of laser irradiation brings significant PL enhancement. Experiments under different atmospheres reveal that the laser-irradiation-induced enhancement only occurs in the atmosphere containing O₂ and is more remarkable in pure O₂. In addition, physically adsorbed water can also induce PL enhancement of monolayer MoS₂. FET devices suggest that the adsorbed water produces a p-doping effect on MoS₂, and the laser irradiation in ambient air generates an n-doping effect, and both types of doping can enhance the PL intensity. The island-shaped defects caused by laser irradiation can be stabilized by oxygen atoms and act as trapping centers for excited trions or electrons, thus reducing the non-radiative recombination ratio and enhancing the PL intensity. The physically adsorbed water works in a similar way. A low power density of laser irradiation can sweep away the originally adsorbed H₂O on the surface, thus reducing the PL.