Alternative stacking sequences in hexagonal boron nitride

Abstract

The relative orientation of successive sheets, i.e. the stacking sequence, in layered two-dimensional materials is central to the electronic, thermal, and mechanical properties of the material. Often different stacking sequences have comparable cohesive energy, leading to alternative stable crystal structures. Here we theoretically and experimentally explore different stacking sequences in the van der Waals bonded material hexagonal boron nitride (h-BN). We examine the total energy, electronic bandgap, and dielectric response tensor for five distinct high symmetry stacking sequences for both bulk and bilayer forms of h-BN. Two sequences, the generally assumed AA' sequence and the relatively unknown (for h-BN) AB (Bernal) sequence, are predicted to have comparably low energy. We present a scalable modified chemical vapor deposition method that produces large flakes of virtually pure AB stacked h-BN; this new material complements the generally available AA' stacked h-BN.In recent years there has been a dramatic resurgence in interest in van der Waals bonded layered materials, including graphite, boron-nitride, and transition metal dichalcogenides. 1,2 These materials display strong intraplane (typically covalent) bonding and weak van der Waals interplane bonding, which facilitates exfoliation into mono-layer or few-layer forms, and further allows custom stack-ups or laminations of sheets with different chemical composition or crystallographic orientation. 1-8 Even for a material composed of identical sheets, the stacking order of the successive sheets or layers, which may be translationally and/or rotationally shifted, can profoundly influence the overall physical properties. 9-12 For example, for naturally occurring graphite the usual stacking sequence is Bernal (AB) stacking, but rhombohedral (ABC) stacking is also possible, which has a completely different electronic band structure. 13,14 Hexagonal boron nitride (h-BN) is structurally very similar to graphite, with successively stacked (and van der Waals bonded) sheets of hexagonally arranged sp 2bonded boron and nitrogen. 5 However, unlike graphite, h-BN is purely synthetic, with a wide electronic band gap (hence the nickname "white graphite"). [15][16][17] Virtually all synthesis methods for h-BN lead to an AA' stacking sequence, where atoms in one layer all lie directly above atoms in the next layer. 4,[18][19][20] Successive layers are rotated such that all nitrogens lie above borons, and all borons lie above nitrogens. 20 Potential alternative stacking sequences for h-BN are of great theoretical and experimental interest.Here we explore these alternative h-BN stacking sequences. We employ Density Functional Theory (DFT) to determine the total energy, electronic band structure, and dielectric tensor elements for five different high-symmetry h-BN stackings. We find that Bernal (AB) stacked h-BN has a total energy comparable to, and indeed a bit lower than,

Electron beam-induced nanopores in Bernal-stacked hexagonal boron nitride

“…Stacking the layers of h-BN in different ways yields different material properties without altering the structure within each layer [6][7][8][9]. The trivial stacking sequence [ Figure 1(a)] is named the AA stacking, where there is no in-plane shift or rotation between consecutive lay- Figure 1.…”

“…Common synthesis methods yield a bilayer stacking sequence [ Figure 1(b)], named the AA stacking, where there is a 60 • rotation between consecutive layers, which results in columns of alternating B and N atoms in the bulk [10]. An alternative stacking sequence, where there is a shift but no rotation between consecutive layers [ Figure 1(c,d)], named the AB stacking, had been observed in rare cases until recently [11][12][13] robust and reproducible method of growing AB stacked h-BN (AB-h-BN) [9]. Many computed properties of ABh-BN are similar to those of AA -h-BN, such as interlayer distance (AB: 3.09Å vs. AA : 3.11Å), indirect band gap (AB: 4.43 eV vs. AA : 4.41 eV) and dielectric tensor [9].…”

“…An alternative stacking sequence, where there is a shift but no rotation between consecutive layers [ Figure 1(c,d)], named the AB stacking, had been observed in rare cases until recently [11][12][13] robust and reproducible method of growing AB stacked h-BN (AB-h-BN) [9]. Many computed properties of ABh-BN are similar to those of AA -h-BN, such as interlayer distance (AB: 3.09Å vs. AA : 3.11Å), indirect band gap (AB: 4.43 eV vs. AA : 4.41 eV) and dielectric tensor [9]. Therefore the new AB-h-BN can be used interchangeably with AA -h-BN in many materials applications.…”

“…The most commonly observed edge type in h-BN is the nitrogen-terminated zigzag edge (N-edge) [10,[19][20][21][22][23][24][25], shown in Figure 2. By the application of a high-energy electron beam in a transmission electron microscopy (TEM) chamber, it is possible to fabricate the N-edge almost exclusively among all edge types [9,[26][27][28][29]. In this study, we investigate the spin configurations of the N-edge in single-layer h-BN and build a statistical model to make predictions about the magnetic properties of real edges.…”

“…We then discuss what happens to the N-edge in multilayer h-BN. Because of the lack of relative rotation between the consecutive layers in AB-h-BN, multilayer N-edges can be formed [9,29], which is not possible in AA -h-BN because of the 60 • rotation between consecutive layers [26]. Additionally, it has been observed that in AA -h-BN, edge relaxations occur and give rise to covalent bonding between the edge atoms in the neighboring layers [30], which is not the case for AB-h-BN [29].…”