Tellurium-Assisted Low-Temperature Synthesis of MoS₂ and WS₂ Monolayers

Chemical vapor deposition (CVD) is a scalable method able to synthesize MoS₂ and WS₂ monolayers. In this work, we reduced the synthesis temperature by 200 °C only by introducing tellurium (Te) into the CVD process. The as-synthesized MoS₂ and WS₂ monolayers show high phase purity and crystallinity. The optical and electrical performance of these materials is comparable to those synthesized at higher temperatures. We believe this work will accelerate the industrial synthesis of these semiconducting monolayers

Super-stretchable Graphene Oxide Macroscopic Fibers with Outstanding Knotability Fabricated by Dry Film Scrolling

Graphene oxide (GO) has recently become an attractive building block for fabricating graphene-based functional materials. GO films and fibers have been prepared mainly by vacuum filtration and wet spinning. These materials exhibit relatively high Young’s moduli but low toughness and a high tendency to tear or break. Here, we report an alternative method, using bar coating and drying of water/GO dispersions, for preparing large-area GO thin films (<i>e.g.</i>, 800–1200 cm² or larger) with an outstanding mechanical behavior and excellent tear resistance. These dried films were subsequently scrolled to prepare GO fibers with extremely large elongation to fracture (up to 76%), high toughness (up to 17 J/m³), and attractive macroscopic properties, such as uniform circular cross section, smooth surface, and great knotability. This method is simple, and after thermal reduction of the GO material, it can render highly electrically conducting graphene-based fibers with values up to 416 S/cm at room temperature. In this context, GO fibers annealed at 2000 °C were also successfully used as electron field emitters operating at low turn on voltages of <i>ca.</i> 0.48 V/μm and high current densities (5.3 A/cm²). Robust GO fibers and large-area films with fascinating architectures and outstanding mechanical and electrical properties were prepared with bar coating followed by dry film scrolling

Electric-Field-Assisted Directed Assembly of Transition Metal Dichalcogenide Monolayer Sheets

Directed assembly of two-dimensional (2D) layered materials, such as transition metal dichalcogenides, holds great promise for large-scale electronic and optoelectronic applications. Here, we demonstrate controlled placement of solution-suspended monolayer tungsten disulfide (WS₂) sheets on a substrate using electric-field-assisted assembly. Micrometer-sized triangular WS₂ monolayers are selectively positioned on a lithographically defined interdigitated guiding electrode structure using the dielectrophoretic force induced on the sheets in a nonuniform field. Triangular sheets with sizes comparable to the interelectrode gap assemble with an observed preferential orientation where one side of the triangle spans across the electrode gap. This orientation of the sheets relative to the guiding electrode is confirmed to be the lowest energy configuration using semianalytical calculations. Nearly all sheets assemble without observable physical deformation, and postassembly photoluminescence and Raman spectroscopy characterization of the monolayers reveal that they retain their as-grown crystalline quality. These results show that the field-assisted assembly process may be used for large-area bottom-up integration of 2D monolayer materials for nanodevice applications

Direct Synthesis of van der Waals Solids

The stacking of two-dimensional layered materials, such as semiconducting transition metal dichalcogenides (TMDs), insulating hexagonal boron nitride (hBN), and semimetallic graphene, has been theorized to produce tunable electronic and optoelectronic properties. Here we demonstrate the direct growth of MoS₂, WSe₂, and hBN on epitaxial graphene to form large-area van der Waals heterostructures. We reveal that the properties of the underlying graphene dictate properties of the heterostructures, where strain, wrinkling, and defects on the surface of graphene act as nucleation centers for lateral growth of the overlayer. Additionally, we show that the direct synthesis of TMDs on epitaxial graphene exhibits atomically sharp interfaces. Finally, we demonstrate that direct growth of MoS₂ on epitaxial graphene can lead to a 10³ improvement in photoresponse compared to MoS₂ alone