Boris Yakobson1
Rice University1
(1) Why the 2D transition metal dichalcogenides grow better if salt is added to the mix? First-principles calculations of the atomistics behind-the-scenes, from the eutectic effect to gas-phase chemistry, reveal the mechanisms and suggest ways to better “cooking recipes” [Z. Liu et al. Nature, 2018: <b>556</b>, 355 || J.ACS Nano, 2021: <b>15</b>, 10525 || JACS, 2022: <b>144</b>, 7497]. (2) How to ensure that the produced 2D-layers are coveted single crystals and not a patchwork of inverted domains, disrupted by the grain boundaries? Vicinal plane substrates’ surface steps dictate the direction of 2D nucleation by virtue of edge-epitaxy. Theory reveals the key role of structural complementarity [Nano Lett. 2019: <b>19</b>, 2027], and thus predicts which substrates work best, e.g. for h-BN [T.-A. Chen et al. Nature, 2020: <b>579</b>, 219 || Appl. Mater. Today, 2023: <b>30</b>, 101734]. (3) This edge-step epitaxy offers a way to produce free, weakly-bonded borophene on insulator [ACS Nano, 2021: <b>15</b>, 18347], a potential breakthrough in the field, if experimentally realized.<br/>(1`) We explore with DFT a hetero-bilayer of realistic 2D components: ferroelectric In<sub>2</sub>Se<sub>3</sub> and the β-Sb (known for its strong spin−orbit coupling). The ±<b><i>P</i></b> of the In<sub>2</sub>Se<sub>3</sub> induces distinctly different electronics in the bilayer: if <b><i>P</i></b> points “inward”, it is a trivial insulator; switched “outward”, its state is nontrivial topological, Z<sub>2</sub> = 1 — inviting future multifunctional devices’ applications [Nano Lett. 2021: <b>21</b>, 785].<br/>(2`) In another example, the often-overlooked (blocked by the substrate) flexibility of 2D materials leads to unexpected behavior of the ferroelectrics. Introduced “ferro-flexo” coupling ∼<i>P</i><i>×k</i> in the Ginzburg-Landau-Devonshire <i>j</i><sup>4</sup>-energy, allows one to predict the spontaneous curvature <i>k</i><i><sub>s</sub></i> and the rise of Curie temperature, domain wall width, etc. for InP, CuInP<sub>2</sub>S<sub>6</sub> and In<sub>2</sub>Se<sub>3</sub>. Polarization switches do bend the layer, converting electrical signal to movement as an actuator, with efficient work-cycle [ACS Nano, 2023: <b>17</b>, 5121].