Outstanding stretchability and thickness-dependent mechanical properties of 2D HfS, HfSe, and hafnium oxide.

We experimentally determine the elastic properties of 2D HfS and HfSe - two emerging nano-materials whose moderate energy bandgap and dielectric oxidized layer make them highly attractive for functional electronic and optoelectronic systems. We found that the average Young's moduli of HfS and HfSe nano-drumheads are relatively low (45.3 ± 3.7 GPa for a 12.2 nm thick HfS and 39.3 ± 8.9 GPa for a 13.4 nm thick HfSe) and depend on the thickness of the nano-drumhead (increasing with thickness for HfS and decreasing for HfSe). Moreover, both materials demonstrate outstanding stretchability (fracture strength and maximal strain of 5.7 ± 0.4 GPa and 12.2-14.3%, respectively, for HfS; fracture strength and maximal strain of 4.5 ± 1.4 GPa and 14.0-20.9%, respectively, for HfSe), which far exceeds the stretchability of other 2D materials and of polymers that are commonly used in flexible electronic applications. Finally, we describe the controlled oxidation of HfSe using a relatively simple laser treatment, which increased the Young's moduli of the thin oxidized layers to 182.6 ± 54.3 GPa. The extraordinary elastic properties of HfS and HfSe, together with their excellent electrical and optoelectrical properties, make these 2D materials highly attractive for use in strain engineering and in various stretchable electronic and optoelectronic applications, such as wearable devices.