Ultrathin Pinhole-Free Hexagonal Boron Nitride Dielectrics by the Repeated Stacking of Liquid-Liquid Assembled Monolayers

In our previous work, liquid-liquid assembled 2D-materials monolayers demonstrate exceptional flatness, packing density, and nanosheet alignment compared with top-down deposited films.¹ Accordingly, these monolayers demonstrate much improved electrical conductivity and optical transparency over conventional top-down deposited 2D-material networks. Statistical analysis of these monolayers demonstrated high packing density and flatness, with total covered area approaching 90 % and over 70 % covered by a single nanosheet thickness. In contrast, the random deposition of nanosheets can be statistically predicted by the Poisson distribution and drop drying defects (e.g. the coffee ring effect) can further increase film roughness and pinhole density. Hence, stacking our liquid-liquid assembled monolayers to form pinhole-free dielectric films should require fewer layers than currently achievable by top-down deposition methods. <br/><br/>In the present work, the liquid-phase exfoliated insulating 2D material hexagonal boron nitride (hBN) was assembled into neatly-tiled monolayers by their assembly at the interface of the immiscible solvents hexane and water. AFM height measurement and statistical analysis of these monolayers has revealed their remarkable flatness and packing density in line with our previous work. The sequential stacking of these monolayers resulted in a linear increase in thickness with stacking, indicating an additive stacking process. The exceptional flatness, nanosheet allignment and packing density of these stacked monolayers of hBN is characteristic of nacre. We demonstrate that these films are ideal for the fabrication of dielectric layers in flexible electronic devices by deposition of gold contacts and subsequently probing their electrical properties. Further, liquid-liquid assembled monolayers of reduced graphene oxide were utilised as contacts in the fabrication of all-2D materials-based flexible and transparent capacitors.<br/><br/>This work demonstrates for the first time the ability to assemble and stack exceptionally flat and aligned 2D materials monolayers via the liquid-liquid interface, and has enabled nacre mimetic pinhole-free layers of hBN and fabrication of heterostacked capacitor devices. The remarkable control of monolayer deposition and the ability to stack dissimilar 2D materials by this liquid-liquid assembly process provides unique insights and opportunities towards future solution-based processing of 2D materials for applications in flexible and large-area electronics.<br/><br/><u>References</u><br/>1. Neilson, J. M.; Avery, M. P.; Derby, B. Tiled Monolayer Films of 2D Molybdenum Disulfide Nanoflakes Assembled at Liquid/Liquid Interfaces<i> Appl. Mater. Interfaces</i> 2020, 12, 22, 25125–25134