Piezostrain-Driven\nBidirectional Enhancement of Optical\nSynaptic Plasticity in Wafer-Scale Co-Phased Tin Selenide Layers

Tin (Sn)-based two-dimensional (2D) materials exhibit\nintriguing\nmechanical and optoelectrical properties owing to their non-centrosymmetric\ncrystallinity and tunable band structures. A judicious integration\nof these individually decoupled properties is projected to introduce\nunparalleled functionalities into them, which remain largely unexplored.\nHerein, we develop wafer-scale tin selenide (SnSe<sub>2–<i>x</i></sub>, 0 < <i>x</i> < 1) 2D layers composed\nof thermodynamically stable coexisting phases of SnSe and SnSe<sub>2</sub> with distinct functionalities and identify a strong interplay\nbetween their mechanical and optoelectrical characteristics. Mechanically,\nthey display a strain-dependent piezoelectricity upon an anisotropic\ndeformation of convex vs concave bending. Optoelectrically, they present\nan optical pulse-induced potentiation and synaptic plasticity accompanying\na wavelength-tunable photoconduction upon visible to near-infrared\n(IR) illuminations. Harnessing these two independent features in a\ncoupled manner enables a drastic enhancement of their synaptic responsiveness\nby >40% with a piezostrain of <1%. These findings suggest opportunities\nfor atomically thin semiconductors in mechano-optical neuromorphic\ndevice applications.