Surface Review and Letters, Vol. 22, No. 6 (2015) 1550078 (6 pages) c World Scienti¯c Publishing Company ° DOI: 10.1142/S0218625X1550078X
STRONGLY AND WEAKLY INTERACTING CONFIGURATIONS OF HEXAGONAL BORON NITRIDE ON NICKEL ABBAS EBNONNASIR*, SUNEEL KODAMBAKA*,‡,¶ and CRISTIAN V. CIOBANU†,§,¶ *Department of Materials Science and Engineering, University of California Los Angeles, Los Angeles, California 90095, USA †Department
of Mechanical Engineering and Materials Science Program, Colorado School of Mines, Golden, Colorado 80401, USA ‡
[email protected] §
[email protected] Received 7 March 2015 Revised 20 July 2015 Accepted 23 July 2015 Published 4 September 2015
Using density functional theory calculations with van der Waals corrections, we have investigated the stability and electronic properties of monolayer hexagonal boron nitride (hBN) on the Ni(111) surface. We have found that hBN can bind either strongly (chemisorption) or weakly to the substrate with metallic or insulating properties, respectively. While the more stable con¯guration is the chemisorbed structure, many weakly bound (physisorbed) states can be realized via growth around an hBN nucleus trapped in an o®-registry position. This ¯nding provides an explanation for seemingly contradictory sets of reports on the con¯guration of hBN on Ni(111). Keywords: Hexagonal boron nitride; nickel; graphene; density functional theory.
With the recent advances in controllable growth of large-area, high-quality, two-dimensional (2D) layered materials (e.g. graphene1–4 and hexagonal boron nitride (hBN)5,6) on metallic substrates, the ¯eld has naturally progressed towards the design and/or investigation of heterostructures based on out-ofplane7–12 or in-plane13–15 stacking of these nanomaterials. Layered heterostructures of graphene on boron nitride o®er the best avenue of preserving the exotic properties of graphene and using them in nanoscale devices, because of the near-perfect epitaxial lattice match, small van der Waals (vdW) interactions between graphene and hBN, and most ¶ Corresponding
importantly because of the insulating nature of the underlying hBN layer(s). While only recently monolayer graphene been grown on7 or transferred8 onto hBN, the e®orts to grow graphene/hBN heterostructures on metallic substrates date back more than a decade.16–23 In particular, Oshima's group used chemical vapor deposition (CVD) to grow hBN on a Ni(111) substrate, followed by the CVD of graphene on top of hBN/Ni(111) with benzene as precursor.21 Experimental reports note that the CVD-deposited hBN is often chemically bound to the Ni substrate, with a 1 1 structure and a metallic character.21–23
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A. Ebnonnasir, S. Kodambaka & C. V. Ciobanu
Independent experimental groups con¯rm the structure and metallic properties of hBN/Ni,24 both of which are also qualitatively consistent with density functional theory (DFT) calculations in the local density approximation (LDA).25 However, there is also signi¯cant evidence of weakly bound (physisorbed) hBN on Ni(111),16–19 although the con¯guration of weakly bound hBN is not clear. The reason why both physisorbed and chemisorbed hBN on Ni(111) can be synthesized under very similar experimental conditions is presently not well understood. Naturally, in the physisorbed con¯gurations the vdW interactions play a determinant role and their consideration within the DFT framework is necessary. In order to understand the seemingly contradictory reports of physisorbed16–19 and chemisorbed21–23 hBN on Ni(111), we have used DFT calculations that account for vdW interactions to determine the binding and electronic structure of hBN/Ni(111), and have studied the binding energy as a function of the interplanar distance from the hBN layer to the top Ni(111) atomic plane. In the absence of any misorientation (strict epitaxy, no moire patterns), we ¯nd that a monolayer of hBN on Ni(111) has two local energy minima. The deeper minimum corresponds to strong chemical bonding with the substrate, with the N atoms located directly above ¯rst-layer Ni atoms; the shallower minimum corresponds to a weaker bound con¯guration in which the boron atoms are directly above the top layer Ni atoms. This ¯nding reconciles the experiments showing chemically bonded hBN20–23 with those reporting physisorbed hBN monolayers on Ni(111).16–19 Furthermore, in order to assess the possibility that graphene may debond a strongly bound hBN layer from the Ni substrate (as proposed, e.g. in Refs. 21 and 26) we have also studied the binding energy and electronic properties of hBN/ Ni in the presence of a graphene layer above hBN. We have found that the interaction between graphene and the rest of the system is practically independent on the con¯guration of hBN on the substrate, and that the electronic properties of hBN (in either the weakly or the strongly bound con¯guration) remain virtually the same in the presence of the graphene layer. Our spin-polarized, zero total charge DFT calculations27 were performed in the generalized-gradient approximation (GGA) using the vdW functional of Dion et al.28 as parameterized by Klimeš et al.29 This
functional performs very well in reproducing the structural details and electronic properties of vdW bonded materials involving hBN and graphene.30 We used non-relativistic Troullier–Martins pseudopotentials,31 double- polarization (DZP) functions, and a mesh-cuto® of 200 Ry. The Brillouin zone was sampled with 30 30 1 and 50 50 1 -centered k-point grids for structural relaxations and band structure calculations, respectively; convergence with respect to k-point density was ensured separately for structure and electronic properties. All supercells are 1 1 and have a slab geometry with the graphene and/or hBN layers present only on one side of the sixlayer Ni(111) substrate, and with a vacuum thickness of 15 A. The residual atomic forces were smaller than 0.03 eV/ A after the structural relaxations. With these parameters, the calculated in-plane lattice constants were 2.560 A for Ni, 2.512 A for hBN and 2.485 A for graphene. We have used the lattice constant of hBN, 2.512 A, for all supercells. The reason for this choice is that straining the substrate32 rather than the graphene or hBN monolayers ensures that the e®ects of registry with respect to the substrate are not obfuscated by strain. We ¯rst describe our results concerning the monolayer hBN on Ni(111). We investigated all possible con¯gurations in which the in-plane forces on all atoms are zero by symmetry, and several starting con¯gurations for which they are not. In the former case, we found that the only strongly bound con¯guration is the N-top one, consistent with other studies34; for the latter case of arbitrary in-plane starting forces, the DFT relaxations push the system either in the N-top con¯guration, or into a physisorbed one. Therefore, we describe here only two con¯gurations for hBN on Ni(111): N-top and B-top, in which N and B atoms, respectively, are directly above ¯rst-layer Ni atoms, as shown in Fig. 1. If the starting con¯guration of this system is N-top, then the ¯nal state after relaxation remains in this N-top registry, while becoming atomically corrugated (rumpled), with the plane of B atoms lying 0.12 A below that of the N atoms; the mean distance from the hBN layer to the substrate is 2.17 A. On the other hand, for the B-top con¯guration, the relaxed structure is virtually °at and resides 3.01 A away from the substrate. These GGA þ vdW results are in line with previous LDA results25 as far as the interlayer distance and the ¯nal con¯guration are concerned. However, there are
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Strongly and Weakly Interacting Con¯gurations of hBN on Nickel
Fig. 1. (Color online) Binding energy of an hBN monolayer on Ni(111), for two di®erent registries (shown as insets): Nitrogen on top of a ¯rst-layer Ni atom (solid circles, con¯guration named N-top) and boron on top of a ¯rst-layer Ni atom (triangles, B-top). We also considered the binding of a °at hBN layer with Ni(111) (dashed curve).
quantitative di®erences, one of which being the larger value for the binding between hBN and the substrate in our system due the inclusion of vdW interactions. The total energy of each con¯guration relative to the energy of the unbound system (i.e. in which the hBN layer and the substrate are far apart) is shown in Fig. 1, as a function of the mean interlayer distance; this is the same as binding energy, but with the negative sign retained so as to depict the familiar well-pro¯le characteristic of bound states. For the N-top case shown in Fig. 1, the value of the rumpling is ¯xed at 0.12 A, i.e. at the value corresponding to the relaxed structure. The N-top con¯guration with the hBN layer kept atomically °at is also shown in Fig. 1. Despite such constraint, the N-top °at con¯guration is also strongly bound to the substrate. The binding energies and the structural parameters of the
hBN/Ni system are summarized in the ¯rst two rows of Table 1. Based on the interlayer distance, the N-top and B-top con¯gurations described in Fig. 1 correspond to chemical and physical bonding, respectively. The chemical bonding is strong and leads to a commensurate structure of hBN/Ni. On the contrary, the physisorbed hBN may not be unique, and a variety of weakly-bound con¯gurations other than B-top, commensurate, 1 1 structure can be expected. We note that the interlayer distance is not the sole indication of how strong the binding with the substrate is. Charge transfer at the interface is another indicator of the binding strength. Indeed, as shown in Fig. 2, the N-top con¯guration determines large electronic density transfer at the interface, consistent with strong, chemical bonding. On the same scale, the electron transfer density for the B-top con¯guration is signi¯cantly smaller than that in the N-top case: coupled with the large interlayer spacing (of the order of interlayer spacing in graphite), this indicates weak physical binding for the B-top con¯guration. As we have already noted, both the physisorbed16–19 and chemisorbed20–22 con¯gurations were observed experimentally, even though the latter is signi¯cantly more favorable energetically (Table 1). Based on the results in Fig. 1, we infer here why the physisorbed state survives to withstand experimental observation and characterization.16 From the minimum of the B-top curve (Fig. 1) to its intersection with the N-top (corrugated) curve, there is an energy barrier to climb of b ¼ 6 meV/atom. Only if such average barrier per atom is somehow overcome, could the system break away from the physisorbed minimum and fall into the deeper, chemisorbed one. We assume that temperature T is the only provider of energy variations and that an n-atom hBN cluster
Table 1. Binding energies, interlayer distances and atomic corrugation (rumpling) values for hBN/Ni(111) and gr/hBN/Ni(111). Binding energy (eV/atom) hBN-Ni hBN/Ni (N-top) hBN/Ni (B-top) gr/hBN/Ni (N-top) gr/hBN/Ni (B-top)
0.29 0.16 0.29 0.16
Interlayer distance ( A)
gr-hBN/Ni
hBN-Ni
0.17 0.16
2.17 3.01 2.33 3.07
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Rumpling ( A)
gr-hBN
hBN
gr
3.05 3.11
0.12 0.03 0.05 0.01
0.00 0.00
A. Ebnonnasir, S. Kodambaka & C. V. Ciobanu
(a)
B
N
Ni
(b)
B N
Ni
Fig. 2. (Color online) Charge transfer density at the hBN/ Ni(111) interface for (a) the N-top and (b) the B-top con¯gurations. The color scale is the same for both pictures.
remains °at. Under this assumption, the presence of a barrier (b ¼ 6 meV/atom) implies that if the n-atom nucleus happens to be in the physisorbed local minimum, then it will remain trapped in that local minimum as long as the thermal energy kB T (where kB is the Boltzmann constant) is not su±cient to take it past the barrier b per atom, i.e. as long as kB T nb . The larger the size n of the nucleus is, the harder it is to be taken away from the physisorbed state by temperature alone. As a rough estimate, at processing temperatures21 of T ¼ 1070 K, a nucleus of hBN containing more than kB T =b 15 atoms would be large enough to survive and grow into a monolayer domain in the physisorbed state. Given that often hBN is grown via CVD of borazine, this is only about three six-atom hBN rings in the minimum nucleus. This being said, there remains the question of why the nucleation may happen in the physisorbed state, when the chemisorbed state is markedly more stable. This is because there are numerous physisorbed states (most of them incommensurate) in which the system might \take o® ", but there is only one chemisorbed one (i.e. the N-top inset of Fig. 1). As a cautionary note, we note that these arguments are not likely to hold at the scale of large hBN sheets, since at such scales ripples can form33 a®ecting their planarity; further studies may be necessary in the
future to elucidate the details of registry transitions for monolayer sheets and °akes. In order to assess the possibility that an additional graphene layer on the hBN/Ni system may alter the electronic properties of hBN and its binding to the substrate, we turn our attention to the graphene/ hBN/substrate system in which graphene is placed above the hBN layer in a Bernal con¯guration. We have also studied other di®erent relative registries of the graphene monolayer with respect to hBN/Ni, with the same conclusions. The results are summarized in the last two rows of Table 1. As seen in the table, the graphene layer does not a®ect much the binding of the hBN to the substrate, neither in the B-top nor in the N-top con¯guration. The binding energy of graphene to hBN/Ni is signi¯cant, 0.16– 0.17 eV/atom, and is due to the dispersion forces. However, graphene does not detach the hBN layer from the substrate, but rather adjusts itself at a distance of 3.1 A from the hBN layer. In the N-top con¯guration, the B atoms are pulled out a bit, resulting in a decreased corrugation of the hBN sheet (Table 1). Contrary to an early proposal suggesting that this corrugation decrease leads to the debonding of hBN from the substrate,35 we have found that the hBN moves only slightly away from the substrate and does not debond. Furthermore, as we have shown in Fig. 1 (the curve labeled \N-top °at"), even if rumpling were set to zero, the binding of hBN to the substrate remains very strong, comparable to that in the N-top corrugated con¯guration. When analyzing the electronic properties, in particular band structure, we ¯nd that in the N-top con¯guration the hBN layer is metallic (Fig. 3), as there are -d hybridized bands corresponding to hBN and Ni that cross the Fermi level. The hybridized bands shown in Fig. 3 are indicative of chemical bonding between hBN and the substrate, and are nearly identical to those of the hBN/Ni system without graphene; this shows that graphene does not change the electronic structure at the hBN/Ni interface. We have also analyzed the band structure (Fig. 4) of the gr/hBN/Ni system with hBN in the Btop con¯guration and nearly equally spaced between graphene and the nickel substrate (refer to last row of Table 1). As shown in Fig. 4, the hBN layer is now insulating with a large band gap ( 4 eV). The band structure shown in Fig. 4 for the B-top con¯guration is very similar to that obtained for the B-top hBN/Ni
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Strongly and Weakly Interacting Con¯gurations of hBN on Nickel
Fig. 3. (Color online) Electronic structure of the graphene/hBN/Ni(111) heterostructure, with the N-top registry for hBN/Ni and graphene Bernal-stacked on hBN. The and d character of the bands are indicated by the colors (inset legend). The structure of hBN is metallic, since pz states of B and N are present across the Fermi level — as shown both in the band structure and in projected density of states (PDOS). In this con¯guration, hBN is chemically bonded to Ni (N-top con¯guration), at a distance of 2.17 A.
system in the absence of the graphene layer. Based on the electronic structure analysis, and on the rather small structural changes induced by graphene (Table 1), we conclude that the presence of a graphene layer on top of hBN/Ni does not signi¯cantly change the properties of the hBN/Ni interface irrespective of the registry and binding (chemical or physical) that hBN has with the substrate. Since the mere presence of the graphene layer does not alter the electronic properties of hBN/Ni in N-top or B-top registries (much less change the binding21,23 between hBN and the Ni substrate from chemical to physical), it follows that a more likely explanation for the debonding reports of Oshima et al.21,23 should stem from unintended processes happening during the CVD growth of graphene on hBN/Ni. In conclusion, we have shown that hBN has two adsorption con¯gurations on Ni(111), one chemical and one physical. Even though the chemically bonded hBN is more energetically favorable, the weakly bound can also be achieved when a small cluster is trapped in the B-top con¯guration, acting as a nucleus around which the physisorbed layer can grow. This explains the seemingly contradictory reports of strongly and weakly bound hBN on Ni from experiments with similar parameters of the CVD growth.
Fig. 4. (Color online) Electronic structure of the graphene/hBN/Ni(111) heterostructure, with the hBN layer in the B-top registry. The and d character of the bands are indicated by the colors (inset legend). The structure of hBN is now physisorbed and insulating, with a band gap greater than 4 eV at the K-point.
We have also found that the presence of a graphene layer on top of hBN/Ni does not signi¯cantly alter the binding and the electronic properties of the hBN/ Ni interface. The presence of two adsorption states with the same crystalline orientation with respect to the substrate but with markedly di®erent electronic properties (one metallic, one insulating), while interesting in itself, could ¯nd potential applications in nanoscale electromechanical switching devices that toggle between the two states when stimulated externally.
Acknowledgments The authors gratefully acknowledge funding from National Science Foundation (A. E. and C. V. C) through Grant No. CMMI-0846858 and from the O±ce of Naval Research (A. E. and S. K.) through ONR Award No. N00014-12-1-0518 (Dr. Chagaan Baatar). Supercomputer time for this project was provided by the Golden Energy Computing Organization at Colorado School of Mines.
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