Outstanding stretchability and thickness-dependent mechanical properties of 2D HfS2, HfSe2, and hafnium oxide.

We experimentally determine the elastic properties of 2D HfS2 and HfSe2 - 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 HfS2 and HfSe2 nano-drumheads are relatively low (45.3 ± 3.7 GPa for a 12.2 nm thick HfS2 and 39.3 ± 8.9 GPa for a 13.4 nm thick HfSe2) and depend on the thickness of the nano-drumhead (increasing with thickness for HfS2 and decreasing for HfSe2). Moreover, both materials demonstrate outstanding stretchability (fracture strength and maximal strain of 5.7 ± 0.4 GPa and 12.2-14.3%, respectively, for HfS2; fracture strength and maximal strain of 4.5 ± 1.4 GPa and 14.0-20.9%, respectively, for HfSe2), 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 HfSe2 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 HfS2 and HfSe2, 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.