Ultrahigh pyroelectricity in monoelemental two-dimensional tellurium

We report an ultrahigh pyroelectric response in weakly van der Waals bonded layers of two-dimensional (2D) tellurium (Te) nanosheets (thickness, $d=4$ to 6 nm) at periodic onoff temperature oscillations. A large pyroelectric coefficient, $p\phantom{\rule{4pt}{0ex}}\ensuremath{\sim}\phantom{\rule{4pt}{0ex}}3\ifmmode\times\else\texttimes\fi{}{10}^{3} \textmu{}\mathrm{C}/({\mathrm{m}}^{2}\phantom{\rule{0.16em}{0ex}}\mathrm{K})$, is observed, which is higher by eightfold than the traditional state of the art pyroelectrics, such as lead zirconate titanate, polymers, or hybrid nanocomposites. The first-principles calculations point out that the breakdown of the centrosymmetry starts from trilayers of Te atoms with a stable ground state energy and $C2$ point group ($P{3}_{1}21$ space group). It involves an angular twist in the Te-Te bonds of an exotic electronic state of 2D \ensuremath{\alpha}-Te layers. The stimulation of the Born effective charge, in-plane piezoelectricity, and thermal expansion coefficient are shown to be the key factors that facilitate giant pyroelectricity in \ensuremath{\alpha}-Te exfoliated thin layers. The lattice images and phonon bands (coupled to the electronic states) confirm that the \ensuremath{\alpha}-Te is anchored in a truncated lattice (rhomboid) along the basal plane. Thus the results present a unique paradigm for a wide range of applications of infrared imaging, detection, and waste thermal energy harvesting, and many more.