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.

Patterning at resolution below the diffraction limit for projection optical lithography has been demonstrated using evanescent near-field optical lithography with broadband illumination (365–600 nm). Linewidths of 50 nm and gratings with 140 nm period have been achieved. Ultrathin photoresist layers in conjunction with conformable photomasks are employed and a reactive ion etching process using SF6 has been developed to transfer the patterns to a depth of more than 100 nm into silicon. Full electromagnetic field simulations of the exposure process show that a high contrast image is present within the resist layer, and that the exposure is dominated by one polarization for the grating structures studied.

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