Thickness-Dependent Piezoelectric Property from Quasi-Two-Dimensional Zinc Oxide Nanosheets with Unit Cell Resolution

A quantitative understanding of the nanoscale piezoelectric property will unlock many application potentials of the electromechanical coupling phenomenon under quantum confinement.In this work,we present an atomic force microscopy-(AFM-)based approach to the quantification of the nanometer-scale piezoelectric property from single-crystalline zinc oxide nanosheets(NSs)with thicknesses ranging from 1 to 4 nm.By identifying the appropriate driving potential,we minimized the influences from electrostatic interactions and tip-sample coupling,and extrapolated the thickness-dependent piezoelectric coefficient(d_(33)).By averaging the measured d_(33) from NSs with the same number of unit cells in thickness,an intriguing tri-unit-cell relationship was observed.From NSs with 3n unit cell thickness(n=1,2,3),a bulk-like d_(33) at a value of~9 pm/V was obtained,whereas NSs with other thickness showed a~30%higher d_(33) of~12 pm/V.Quantification of d_(33) as a function of ZnO unit cell numbers offers a new experimental discovery toward nanoscale piezoelectricity from nonlayered materials that are piezoelectric in bulk.