In situ study of two-dimensional dendritic growth of hexagonal boron nitride

Abstract

Hexagonal boron nitride, often entitled the ‘white graphene’ because of its large band gap, is one of the most important two-dimensional (2D) materials and frequently investigated in context with stacked arrays of single 2D layers, so called van der Waals heterostructures. Here, we concentrate on the growth of hBN on the coinage metal surface Cu(1 1 1). Using low energy electron microscopy and diffraction, we investigate the self-terminated growth of the first layer in situ and in real time. Most prominently, we find dendritic structures with three strongly preferred growth branches that are mostly well aligned with the Cu(1 1 1) substrate and exhibit a three-fold symmetric shape. The observation of dendritic structures is very surprising since hBN was found to grow in compact, triangular-shaped islands on many other metal substrates, in particular, on transition metal surfaces where it shows a much stronger interaction to the surface. We explain the unexpected dendritic growth by an asymmetry of the bonding energy for the two possible ways a borazine molecule can attach to an existing hBN island, namely either with one of its boron or one of its nitrogen atoms. We suggest that this asymmetry originates from different dehydrogenation states of the adsorbed borazine molecules and the hBN islands. We call this mechanism ‘Dehydrogenation Limited Aggregation’ since it is generic in the sense that it is merely based on different dehydrogenation energies for the involved building blocks forming the 2D layer.

Equilibrium shape of single-layer hexagonal boron nitride islands on iridium

“…Crystallographic analysis, taking into account image rotation in LEEM (e.g., see ref. 21 for technical details), reveals that the hBN edges are of ZZ type. The orientation of hBN island edges can be described by unit vectors which are perpendicular to the edges, as shown in Fig.…”

“…Single-layer hBN has been grown via CVD on a wide range of metal substrates, e.g., on Ru, Rh, Ni, Ir, Pd, Pt, Cu and Fe 7,10–18 . Initially, hBN islands are often zig-zag (ZZ) terminated triangles, with possible exceptions for some growth conditions 19–21 . The triangles exhibit two dominant orientations, which originate from the bi-elemental hBN unit cell 12 .…”

Recent Progress on Two-Dimensional Materials

“…On such a substrate, the energetically most preferred orientations of a 2D material are not degenerated and, thus, uniformly aligned 2D islands may be grown under the optimized experimental condition. of a 2D material near a step edge is superior to that on a terrace, because of strong adhesion between the active step edges and the edge of 2D materials 1654,[1656][1657][1658][1659] .…”

“…Both experimental and theoretical studies have showed that, the nucleation of 2D materials near a step edge of a high-index surface is energetically more preferred than that on a low index terrace 1658,1659,1666 . Generally, step edges of a high index surfaces may along any direction and can be classified into straight step edges and tilted ones 1655 .…”

Long Vertical Distance Bonding of the Hexagonal Boron Nitride Monolayer on the Cu(111) Surface

“…However, this assumption has not been proven, yet. The first study on the growth process of hBN was conducted by Felter et al [183]. They investigated the growth of hBN on Cu (111) in low energy electron microscopy (LEEM) experiments and proposed a model for the growth process of hBN domains.…”