Piezoelectricity of single-atomic-layer Mo[S.sub.2] for energy conversion and piezotronics

The piezoelectric characteristics of nanowires, thin films and bulk crystals have been closely studied for potential applications in sensors, transducers, energy conversion and electronics (1-3).With their high crystallinity and ability to withstand enormous strain (4-6), two-dimensional materials are of great interest as high-performance piezoelectric materials. Monolayer Mo[S.sub.2] is predicted to be strongly piezoelectric, an effect that disappears in the bulk owing to the opposite orientations of adjacent atomic layers (7, 8). Here we report the first experimental study of the piezoelectric properties of two-dimensional Mo[S.sub.2] and show that cyclic stretching and releasing of thin Mo[S.sub.2] flakes with an odd number of atomic layers produces oscillating piezoelectric voltage and current outputs, whereas no output is observed for flakes with an even number of layers. A single monolayer flake strained by 0.53% generates a peak output of 15 mV and 20 pA, corresponding to a power density of 2 mW [m.sup.-2] and a 5.08% mechanical-to-electrical energy conversion efficiency. In agreement with theoretical predictions, the output increases with decreasing thickness and reverses sign when the strain direction is rotated by 90°. Transport measurements show a strong piezotronic effect in single-layer Mo[S.sub.2], but not in bilayer and bulk Mo[S.sub.2]. The coupling between piezoelectricity and semi-conducting properties in two-dimensional nanomaterials may enable the development of applications in powering nanodevices, adaptive bioprobes and tunable/stretchable electronics/optoelectronics.
Crystal structure and symmetry dictate the physical properties of a material and its interaction with external stimuli. Materials with polarization domains, such as Pb(Ti, Zr)[O.sub.3], or with non-centrosymmetric structure, such [...]