The notable transformation of the electronic properties of transition-metal dichalcogenides (TMDs) when reduced to a single X-M-X plane (X: chalcogen; M: metal) [1] makes them suitable for flexible, innovative optoelectronic devices, [2][3][4] and transistors. [5] Like graphene, few-layer TMDs can also withstand surprisingly large mechanical deformations, [6][7][8][9] which, coupled to the material's electronic structure, would enable the observation of nondissipative topological transport, provided a periodic modulation of strain is attained. [10][11][12][13] TMD monolayers (MLs) and nanostructures are also important for their catalytic role in the cost-effective production of hydrogen. [14][15][16] These examples share the need to achieve spatial control of the material's properties, over sample regions with size ranging from the nano [14,16] to the micrometer [16] scale lengths.In this study, we present a route toward the patterning of TMDs based on the effects of low-energy proton irradiation [17] on the structural and electronic properties of bulk WS 2 , WSe 2 , WTe 2 , MoS 2 , MoSe 2 , and MoTe 2 . Suitable irradiation conditions trigger the production and accumulation of H 2 just beneath the first X-M-X basal plane, leading to the localized exfoliation of the topmost monolayer and to the formation of spherically shaped domes. Structural and optical characterizations confirm that these domes are typically one ML-thick and contain H 2 at pressures in the 10-100 atm range, depending on their size. Such high pressures induce strong and complex strain fields acting on the curved X-M-X planes, that are evaluated by means of a mechanical model. The domes' morphological characteristics can be tuned by lithographically controlling the area of the sample basal plane participating in the hydrogen production process. This results in the unprecedented fabrication of robust domes with controlled position/density and sizes tunable from the nanometer to the micrometer scale, that, by virtue of their inherently strained nature and geometry, might prompt a variety of applications.The samples, consisting of thick (tens to hundreds of MLs) TMD flakes, were obtained by mechanical exfoliation, deposited on Si substrates, and afterwards proton-irradiated using a Kaufman source (see the Experimental Methods). Differently from the other works in the literature concerning protonirradiation of TMDs-where beams with energies ≥10 5 eV are used, [18] aiming at the controlled formation of defects in the irradiated samples-here we irradiate the flakes with low energy At the few-atom-thick limit, transition-metal dichalcogenides (TMDs) exhibit strongly interconnected structural and optoelectronic properties. The possibility to tailor the latter by controlling the former is expected to have a great impact on applied and fundamental research. As shown here, proton irradiation deeply affects the surface morphology of bulk TMD crystals. Protons penetrate the top layer, resulting in the production and progressive accumulation of molecular hydr...

The variegated family of two-dimensional (2D) crystals has developed rapidly since the isolation of its forerunner: Graphene. Their planeconfined nature is typically associated with exceptional and peculiar electronic, optical, magnetic, and mechanical properties, heightening the interest of fundamental science and showing promise for applications. Methods for tuning their properties on demand have been pursued, among which the application of mechanical stresses, allowed by the incredible mechanical robustness and flexibility of these atomically thin materials. Great experimental and theoretical efforts have been focused on the development of straining protocols and on the evaluation of their impact on the peculiar properties of 2D crystals, revealing a novel, alluring physics. The relevance held by strain for 2D materials is introduced in Sec. I. Sections II and III present the multiplicity of methods developed to induce strain, highlighting the peculiarities, effectiveness, and drawbacks of each technique. Strain has largely widened the 2D material phase space in a quasi-seamless manner, leading to new and rich scenarios, which are discussed in Secs. IV-VI of this work. The effects of strain on the electronic, optical, vibrational, and mechanical properties of 2D crystals are discussed, as well as the possibility to exploit strain gradients for single-photon emission, non-linear optics, or valley/spintronics. Quantitative surveys of the relevant parameters governing these phenomena are provided. This review seeks to provide a comprehensive state-of-the-art overview of the straining methods and strain-induced effects, and to shed light on possible future paths. The aims and developments, the tools and strategies, and the achievements and challenges of this research field are widely presented and discussed.

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