We report the experimental determination of the elastic properties of suspended multilayer WSe2, a promising two-dimensional (2D) semiconducting material combined with high optical quality. The suspended WSe2 membranes have been fabricated by mechanical exfoliation of bulk WSe2 and transfer of the exfoliated multilayer WSe2 flakes onto SiO2/Si substrates pre-patterned with hole arrays. Then, indentation experiments have been performed on these membranes with an atomic force microscope. The results show that the 2D elastic modulus of the multilayer WSe2 membranes increases linearly while the prestress decreases linearly as the number of layers increases. The interlayer interaction in WSe2 has been observed to be strong enough to prevent the interlayer sliding during the indentation experiments. The Young's modulus of multilayer WSe2 (167.3 ± 6.7 GPa) is statistically independent of the thickness of the membranes, whose value is about two thirds of other most investigated 2D semiconducting transition metal dichalcogenides, namely, MoS2 and WS2. Moreover, the multilayer WSe2 can endure ∼12.4 GPa stress and ∼7.3% strain without fracture or mechanical degradation. The 2D WSe2 can be an attractive semiconducting material for application in flexible optoelectronic devices and nano-electromechanical systems.

This report presents a simple and efficient method of layer thinning and p-type doping of WSe 2 with vapor XeF 2 . With this approach, the surface roughness of thinned WSe 2 can be controlled to below 0.7 nm at an etched depth of 100 nm. By selecting appropriate vapor XeF 2 exposure times, 23-layer and 109-layer WSe 2 can be thinned down to monolayer and bilayer, respectively. In addition, the etching rate of WSe 2 exhibits a significant dependence on vapor XeF 2 exposure pressure and thus can be tuned easily for thinning or patterning applications. From Raman, photoluminescence, X-ray photoelectron spectroscopy (XPS), and electrical characterization, a p-doping effect of WSe 2 induced by vapor XeF 2 treatment is evident. Based on the surface composition analysis with XPS, the causes of the p-doping effect can be attributed to the presence of substoichiometric WO x (x < 3) overlayer, trapped reaction product of WF 6 , and nonstoichiometric WSe x (x > 2). Furthermore, the p-doping level can be controlled by varying XeF 2 exposure time. The thinning and p-doping of WSe 2 with vapor XeF 2 have the advantages of easy scale-up, high etching selectivity, excellent controllability, and compatibility with conventional complementary metal-oxide-semiconductor fabrication processes, which is promising for applications of building WSe 2 devices with versatile functionalities.

Two-dimensional crystals with angstrom-scale pores are widely considered as candidates for a next generation of molecular separation technologies aiming to provide extreme, exponentially large selectivity combined with high flow rates. No such pores have been demonstrated experimentally. Here we study gas transport through individual graphene pores created by low intensity exposure to low kV electrons. Helium and hydrogen permeate easily through these pores whereas larger species such as xenon and methane are practically blocked. Permeating gases experience activation barriers that increase quadratically with molecules’ kinetic diameter, and the effective diameter of the created pores is estimated as ∼2 angstroms, about one missing carbon ring. Our work reveals stringent conditions for achieving the long sought-after exponential selectivity using porous two-dimensional membranes and suggests limits on their possible performance.

The epitaxial growth of single-crystal thin films relies on the availability of a single-crystal substrate and a strong interaction between epilayer and substrate. Previous studies have reported the roles of the substrate (e.g., symmetry and lattice constant) in determining the orientations of chemical vapor deposition (CVD)-grown graphene, and Cu( 111) is considered as the most promising substrate for epitaxial growth of graphene single crystals. However, the roles of gas-phase reactants and graphene−substrate interaction in determining the graphene orientation are still unclear. Here, we find that trace amounts of oxygen is capable of enhancing the interaction between graphene edges and Cu(111) substrate and, therefore, eliminating the misoriented graphene domains in the nucleation stage. A modified anomalous grain growth method is developed to improve the size of the as-obtained Cu(111) single crystal, relying on strongly textured polycrystalline Cu foils. The batch-to-batch production of A3-size (∼0.42 × 0.3 m 2 ) single-crystal graphene films is achieved on Cu(111) foils relying on a self-designed pilot-scale CVD system. The as-grown graphene exhibits ultrahigh carrier mobilities of 68 000 cm 2 V −1 s −1 at room temperature and 210 000 cm 2 V −1 s −1 at 2.2 K. The findings and strategies provided in our work would accelerate the mass production of high-quality misorientation-free graphene films.