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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 3 n 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. 

Creating two-dimensional (2D) geometry from nonlayered catalytic materials may significantly advance electrocatalyst design. The 2D morphology of three-dimensional lattices (2D nonlayered materials) offer large structural distortions, massive surface dangling bonds, and coordinated-unsaturated surface atoms, which can induce high surface chemical activity and promote the chemisorption of reactants and fast interfacial charge transfer, thereby enhancing the electrocatalytic performance. In this article, we review typical strategies for structural engineering and manipulation of electronic states to enable the unique electrocatalytic advantages of 2D nonlayered materials. An overview is presented on recent research advances in the development of 2D nonlayered materials for catalyzing the representative electrochemical reactions that are essential to energy and sustainability, including hydrogen evolution, oxygen evolution, oxygen reduction, and CO 2 reduction. For each type of redox reactions, their unique catalytic performance and underlying mechanism are discussed. Important achievements and key challenges are also discussed. 

Abstract The emergence of memristive behavior in amorphous–crystalline 2D oxide heterostructures, which are synthesized by atomic layer deposition (ALD) of a few‐nanometer amorphous Al₂O₃layers onto atomically thin single‐crystalline ZnO nanosheets, is demonstrated. The conduction mechanism is identified based on classic oxygen vacancy conductive channels. ZnO nanosheets provide a 2D host for oxygen vacancies, while the amorphous Al₂O₃facilitates the generation and stabilization of the oxygen vacancies. The conduction mechanism in the high‐resistance state follows Poole–Frenkel emission, and in the the low‐resistance state is fitted by the Mott–Gurney law. From the slope of the fitting curve, the mobility in the low‐resistance state is estimated to be ≈2400 cm²V⁻¹s⁻¹, which is the highest value reported in semiconductor oxides. When annealed at high temperature to eliminate oxygen vacancies, Al is doped into the ZnO nanosheet, and the memristive behavior disappears, further confirming the oxygen vacancies as being responsible for the memristive behavior. The 2D heterointerface offers opportunities for new design of high‐performance memristor devices.