Abstract
We demonstrate the growth of overlapping grain boundaries in continuous, polycrystalline hexagonal boron nitride (h-BN) monolayer films via scalable catalytic chemical vapor deposition. Unlike the commonly reported atomically stitched grain boundaries, these overlapping grain boundaries do not consist of defect lines within the monolayer films but are composed of self-sealing bilayer regions of limited width. We characterize this overlapping h-BN grain boundary structure in detail by complementary (scanning) transmission electron microscopy techniques and propose a catalytic growth mechanism linked to the subsurface/bulk of the process catalyst and its boron and nitrogen solubilities. Our data suggest that the overlapping grain boundaries are comparatively resilient against deleterious pinhole formation associated with grain boundary defect lines and thus may reduce detrimental breakdown effects when polycrystalline h-BN monolayer films are used as ultrathin dielectrics, barrier layers, or separation membranes.
Highlights
Hexagonal boron nitride (h-BN) is a two-dimensional (2D) insulator with a wide application profile,[1] including its use as an thin dielectric in electronics,[2,3] a tunnel barrier in spintronics,[4,5] an encapsulation and barrier layer in electronics and metallurgy,[6] and a suspended separation membrane in nanofluidics.[7,8] All these applications require scalable manufacturing of h-BN films, whereby in particular pinhole-free continuous h-BN films of controlled layer number are desired
Current literature reports the nature of the asgrown grain boundary (GB) in h-BN as atomically stitched, composed of defect lines within a h-BN monolayer (Figure 1a).[22−34] Preferential pinhole formation is reported at such atomically stitched GBs,[30] and due to their structure, they are intrinsically prone to electrical breakdown,[35,36] chemical attack, or mechanical failure,[26,34] all of which may render monolayer h-BN films ineffective in their envisaged applications as an ultrathin dielectric, barrier layer, or separation membrane
Recent reports on 2D materials other than h-BN suggest the possibility to form a different type of GB, namely, overlapping GBs, where two merging 2D monolayer grains form a bilayer region of limited width at the GB instead of a defect line within the monolayer.[37−40] Overlapping GBs remain unexplored for hBN films far
Summary
Hexagonal boron nitride (h-BN) is a two-dimensional (2D) insulator with a wide application profile,[1] including its use as an thin dielectric in electronics,[2,3] a tunnel barrier in spintronics,[4,5] an encapsulation and barrier layer in electronics and metallurgy,[6] and a suspended separation membrane in nanofluidics.[7,8] All these applications require scalable manufacturing of h-BN films, whereby in particular pinhole-free continuous h-BN films of controlled layer number are desired. Recent reports on 2D materials other than h-BN suggest the possibility to form a different type of GB, namely, overlapping GBs, where two merging 2D monolayer grains form a bilayer region of limited width at the GB instead of a defect line within the monolayer.[37−40] Overlapping GBs remain unexplored for hBN films far. We confirm that in overlapping GBs in h-BN monolayer films two h-BN monolayer grains merge via the self-sealing formation of a turbostratic bilayer region of limited width and thereby without formation of a defect line within the monolayer (Figure 1b). We characterize this overlapping GB structure in detail, identify catalytic CVD conditions that result in such GB structure, and propose. We aim at expanding the understanding of GB formation and control in heteroelemental 2D materials such as h-BN, which directly links to their properties and various application profiles
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