  We report an ultrahigh pyroelectric response in van der Waals bonded layers
of two-dimensional (2D) tellurium (Te) nanosheets (thickness, d = 4 to 5 nm) at
periodic on-off temperature oscillations. For the first time a large
pyroelectric coefficient, Pc ~ 3 mC.m-2.K-1, is observed which is eightfold
higher than the traditional state-of-the-art pyroelectrics (lead zirconate
titanate, PZT). The first-principles calculations point out that the breakdown
of centro-symmetry in the 1-3 Te-layers (P-3m1 space group) of a
non-centrosymmetry (higher-order symmetry of C2 space group) on an angular
twist in the Te-Te bonds of an exotic electronic state in 2D Te. The angular
Te-Te twisting elicits a surface-enhanced Raman band at 101 cm-1 (absent in
bulk Te). The stimulation of the Born effective charge, in-plane
piezoelectricity and thermal expansion coefficient are shown to tailor the
large pyroelectricity. Thus, 2D Te nanosheets present a new paradigm for the
wide application of pyroelectric materials for developing thermal energy-based
flexible electronics.
