Two-dimensional materials represent the ultimate in device miniaturization as the width of the active component is limited to a single layer of the material. By combining control of conductivity with ferroelectric switchable electric dipoles, high-density memory technologies may be enabled. However, the presence of free charges in ultrathin systems tends to screen polarization, removing ferroelectricity. It has been recently shown that by sliding one layer of a van der Waals material over another, an out-of-plane ferroelectricity can be generated, this may enable decoupling of in-plane conductivity and ferroelectricity, but sliding ferroelectricity has only been seen in bilayer materials.
Shalom and colleagues studied three layers of WSe2 nanosheets. They used the side-band Kelvin probe mode of an atomic force microscope to map the room-temperature surface potential (left panel; scale bar, 0.5 μm). This measured potential jumps across two different interface types — one where there are two different potentials, and one with three different potentials (top right panel). One of these line mappings represents sliding between two WSe2 layers, and so depending on slide direction two different polarizations. However, the other line mapping represents the case where there are two active interfaces — the sliding between top-layer WSe2 and middle-layer WSe2, and the sliding between middle-layer WSe2 and bottom-layer WSe2. These generate two decoupled polarizations, as each polarization can be up or down, this can be summed to three different values (up–up, up–down and down–down; left panel). Support for this interpretation was provided by density functional theory calculations (bottom right panel) of a trilayer system, with the potential drop across the trilayer agreeing with experiment, while also showing minimal coupling between interfaces, and so the polarizations are decoupled.
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