Synthesis of atomically thin hexagonal boron nitride films on nickel foils by molecular beam epitaxy

Hexagonal boron nitride (h-BN) is a layered two-dimensional material with properties that make it promising as a dielectric in various applications. We report the growth of h-BN films on Ni foils from elemental B and N using molecular beam epitaxy. The presence of crystalline h-BN over the entire substrate is confirmed by Raman spectroscopy. Atomic force microscopy is used to examine the morphology and continuity of the synthesized films. A scanning electron microscopy study of films obtained using shorter depositions offers insight into the nucleation and growth behavior of h-BN on the Ni substrate. The morphology of h-BN was found to evolve from dendritic, starshaped islands to larger, smooth triangular ones with increasing growth temperature. The previous decade has seen extensive research efforts focused on the novel properties of two-dimensional materials and their associated potential applications. This surge in interest was instigated by the isolation of monolayer graphene for the first time, 1 and has rapidly spread to other materials. 2 One such material, hexagonal boron nitride (h-BN), has been the subject of particular attention. This intense research interest has been driven by the suitability of h-BN for integration into heterostructures with other two-dimensional materials, such as graphene. 3 The first, and perhaps most intuitive way to integrate h-BN and graphene is in vertically stacked heterostructures. In this configuration, the h-BN acts as an insulating layer between electrically active graphene sheets, as well as offering the capability to tune the properties of the graphene layers through moiré effects and other interlayer interactions. 4 This scheme is enhanced by the atomically smooth surface and homogeneous charge potential offered by h-BN, as illustrated by the order of magnitude higher charge a) Author’s current address: Materials Department, University of California, Santa Barbara, California 93106-5050, USA b) Author to whom correspondence should be addressed. Electronic mail: [email protected] 2 carrier mobility in graphene supported by h-BN when compared to SiO2. 3,5,6 Furthermore, h-BN is isomorphic to graphene with a small lattice mismatch of 1.7%, which is commonly regarded as a prerequisite for the growth of defect free epitaxial heterostructures. The structural similarities between the two materials also allow for a second type of heterostructure in which graphene and h-BN are incorporated in a single, laterally patterned monolayer, offering yet more intriguing possibilities. 7,8 Hexagonal boron nitride’s large direct band-gap, low dielectric constant, outstanding thermal, mechanical, and chemical stability 9–13 add to its attractiveness for various other research and technological uses. Similar to graphene synthesis, optimizing the crystalline quality of h-BN films and establishing control over their thickness while using a scalable synthesis method has proved challenging. Chemical vapor deposition (CVD) is the most thoroughly explored method for the growth of h-BN films. The synthesis of monoand few-layer h-BN by CVD on catalytic singleor polycrystalline transition metals such as Ni, 14–17 Cu, 18 and Pt 19,20 has been reported. Moreover, using metal organic precursors, synthesis of thicker films of h-BN on dielectrics such as Al2O3 and 6H-SiC has been achieved. 21,22 Different forms of physical vapor deposition (PVD), such as pulsed laser deposition, 23 and reactive magnetron sputtering, 24 have also been used for h-BN thin film growth. While these studies exhibit the progress being made in h-BN growth, a single scalable synthesis method which combines highcrystalline quality with absolute thickness control remains elusive. Among the various approaches to synthesizing h-BN films, molecular beam epitaxy (MBE) offers precise control over growth conditions. Whereas CVD growth relies on the decomposition of a molecular precursor, the rate of which may change dramatically with the exposed surface area of the catalytic substrate, 25 MBE faces no such constraints. The absence of catalytic processes in MBE growth is crucial for vertically stacked heterostructures, where the non-catalytic surfaces of graphene, h-BN or other dielectrics will serve as substrates for growth. The precise fluxes and resulting growth rates also offered by MBE facilitate the deposition of sub-monolayer films, making the production of laterally patterned monolayer heterostructures feasible. Similar considerations make MBE ideal for fundamental studies of h-BN crystal growth, the results from which may also benefit synthesis by other methods. However, despite the sub-monolayer precision it allows, the few existing reports of boron nitride growth by MBE focus on the cubic phase or thicker h-BN films for optoelectronic purposes. 26,27 These experiments have only