STRONGLY AND WEAKLY INTERACTINGCONFIGURATIONS 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

‡kodambaka@ucla.edu§cciobanu@mines.edu

Received 7 March 2015Revised 20 July 2015Accepted 23 July 2015

Published 4 September 2015

Using density functional theory calculations with van der Waals corrections, we have investigatedthe 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 sub-strate with metallic or insulating properties, respectively. While the more stable con¯guration is thechemisorbed structure, many weakly bound (physisorbed) states can be realized via growth aroundan hBN nucleus trapped in an o®-registry position. This ¯nding provides an explanation forseemingly 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) lay-

ered 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-of-

plane7–12 or in-plane13–15 stacking of these nanoma-

terials. 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 epi-

taxial lattice match, small van der Waals (vdW)

interactions between graphene and hBN, and most

importantly because of the insulating nature of the

underlying hBN layer(s). While only recently mono-

layer graphene been grown on7 or transferred8 onto

hBN, the e®orts to grow graphene/hBN hetero-

structures 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) sub-

strate, followed by the CVD of graphene on top of

hBN/Ni(111) with benzene as precursor.21 Experi-

mental 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

¶Corresponding authors.

Surface Review and Letters, Vol. 22, No. 6 (2015) 1550078 (6 pages)°c World Scienti¯c Publishing CompanyDOI: 10.1142/S0218625X1550078X

1550078-1

Independent experimental groups con¯rm the struc-

ture 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 (physi-

sorbed) hBN on Ni(111),16–19 although the con¯gu-

ration of weakly bound hBN is not clear. The reason

why both physisorbed and chemisorbed hBN on

Ni(111) can be synthesized under very similar exper-

imental conditions is presently not well understood.

Naturally, in the physisorbed con¯gurations the vdW

interactions play a determinant role and their con-

sideration within the DFT framework is necessary.

In order to understand the seemingly contradic-

tory reports of physisorbed16–19 and chemisorbed21–23

hBN on Ni(111), we have used DFT calculations that

account for vdW interactions to determine the bind-

ing 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 misori-

entation (strict epitaxy, no moir�e 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 bond-

ed 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 calcu-

lations27 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 pseudopoten-

tials,31 double-� polarization (DZP) functions, and

a mesh-cuto® of 200Ry. 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 six-

layer 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 gra-

phene or hBN monolayers ensures that the e®ects of

registry with respect to the substrate are not obfus-

cated 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¯g-

uration is the N-top one, consistent with other stud-

ies34; 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¯gura-

tion of this system is N-top, then the ¯nal state after

relaxation remains in this N-top registry, while be-

coming 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

GGAþ 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

A. Ebnonnasir, S. Kodambaka & C. V. Ciobanu

1550078-2

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 corre-

spond to chemical and physical bonding, respectively.

The chemical bonding is strong and leads to a com-

mensurate 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, com-

mensurate, 1� 1 structure can be expected.

We note that the interlayer distance is not the sole

indication of how strong the binding with the sub-

strate 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 physi-

sorbed16–19 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 mini-

mum 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 6meV/atom. Only if such

average barrier per atom is somehow overcome, could

the system break away from the physisorbed mini-

mum 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

Fig. 1. (Color online) Binding energy of an hBN mono-layer on Ni(111), for two di®erent registries (shown asinsets): Nitrogen on top of a ¯rst-layer Ni atom (solid cir-cles, con¯guration named N-top) and boron on top of a¯rst-layer Ni atom (triangles, B-top). We also consideredthe binding of a °at hBN layer with Ni(111) (dashed curve).

Table 1. Binding energies, interlayer distances and atomic corrugation (rum-pling) values for hBN/Ni(111) and gr/hBN/Ni(111).

Binding Interlayer Rumpling

energy (eV/atom) distance (�A) (�A)

hBN-Ni gr-hBN/Ni hBN-Ni gr-hBN hBN gr

hBN/Ni (N-top) 0.29 2.17 0.12hBN/Ni (B-top) 0.16 3.01 0.03gr/hBN/Ni (N-top) 0.29 0.17 2.33 3.05 0.05 0.00gr/hBN/Ni (B-top) 0.16 0.16 3.07 3.11 0.01 0.00

Strongly and Weakly Interacting Con¯gurations of hBN on Nickel

1550078-3

remains °at. Under this assumption, the presence of a

barrier (�b ¼ 6meV/atom) implies that if the n-atom

nucleus happens to be in the physisorbed local mini-

mum, then it will remain trapped in that local mini-

mum as long as the thermal energy kBT (where kB is

the Boltzmann constant) is not su±cient to take it

past the barrier �b per atom, i.e. as long as

kBT � n�b. 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 1070K, a nucleus of

hBN containing more than kBT=�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 physi-

sorbed 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 summa-

rized 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 dis-

tance 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 rum-

pling 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 par-

ticular 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 inter-

face. We have also analyzed the band structure

(Fig. 4) of the gr/hBN/Ni system with hBN in the B-

top 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

N

Ni

B

B

N

Ni

B

(a)

(b)

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.

A. Ebnonnasir, S. Kodambaka & C. V. Ciobanu

1550078-4

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 gra-

phene layer on top of hBN/Ni does not signi¯cantly

change the properties of the hBN/Ni interface irre-

spective 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 nu-

cleus around which the physisorbed layer can grow.

This explains the seemingly contradictory reports of

strongly and weakly bound hBN on Ni from experi-

ments with similar parameters of the CVD growth.

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 inter-

esting 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 Organi-

zation at Colorado School of Mines.

References

1. X. S. Li, W. W. Cai, J. H. Ann, S. Kim, J. Nah, D. X.Yang, R. Piner, A. Velamakanni, I. Jung, E. Tutuc,

Fig. 4. (Color online) Electronic structure of the gra-phene/hBN/Ni(111) heterostructure, with the hBN layer inthe B-top registry. The � and d character of the bands areindicated by the colors (inset legend). The structure of hBNis now physisorbed and insulating, with a band gap greaterthan 4 eV at the K-point.

Fig. 3. (Color online) Electronic structure of the gra-phene/hBN/Ni(111) heterostructure, with the N-top reg-istry for hBN/Ni and graphene Bernal-stacked on hBN.The � and d character of the bands are indicated by thecolors (inset legend). The structure of hBN is metallic, sincepz states of B and N are present across the Fermi level — asshown both in the band structure and in projected densityof states (PDOS). In this con¯guration, hBN is chemicallybonded to Ni (N-top con¯guration), at a distance of 2.17�A.

Strongly and Weakly Interacting Con¯gurations of hBN on Nickel

1550078-5

S. K. Banerjee, L. Colombo and R. S. Ruo®, Science324 (2009) 1312.

2. S. Bae, H. Kim, Y. Lee, X. F. Xu, J. S. Park, Y. Zeng,J. Balakrishnan, T. Lei, H. R. Kim, Y. I. Song, Y. J.Kim, K. S. Kim, B. Ozyilmaz, J. H. Ahn, B. H. Kongand S. Iijima, Nature Nanotechnol. 5 (2010) 574.

3. Z. Z. Sun, Z. Yan, J. Yao, E. Beitler, Y. Zhu and J. M.Tour, Nature 468 (2010) 549.

4. A. Reina, S. Thiele, X. T. Jia, S. Bhaviripudi, M. S.Dresselhaus, J. A. Schaefer and J. Kong, Nano Res. 2(2009) 509.

5. K. H. Lee, H. J. Shin, J. Lee, I. Y. Lee, G. H. Kim, J. Y.Choi and S. W. Kim, Nano Lett. 12 (2012) 714.

6. L. Song, L. Ci, H. Lu, P. B. Sorokin, C. Jin, J. Ni, A. G.Kvashin, D. G. Kvashin, J. Lou, B. I. Yakobson and P.M. Ajayan, Nano Lett. 10 (2010) 3209.

7. W. Yang, G. Chen, Z. Shi, C.-C. Liu, L. Zhang, G. Xie,M. Cheng, D. Wang, R. Yang, D. Shi, K. Watanabe, T.Taniguchi, Y. Yao, Y. Zhang and G. Zhang, NatureMater. 12 (2013) 792.

8. J. M. Xue, J. Sanchez-Yamagishi, D. Bulmash, P.Jacquod, A. Deshpande, K. Watanabe, T. Taniguchi,P. Jarillo-Herrero and B. J. Leroy, Nature Mater. 10(2011) 282–285.

9. M. Wang, S. K. Jang, W. J. Jang, M. Kim, S. Y. Park,S. W. Kim, S. J. Kahng, J. Y. Choi, R. S. Ruo®, Y. J.Song and S. Lee. Adv. Mater. 25 (2013) 2746.

10. Z. Liu, L. Song, S. Z. Zhao, J. Q. Huang, L. L. Ma, J. N.Zhang, J. Lou and P. M. Ajayan, Nano Lett. 11 (2011)2032.

11. R. Cort�es, D. P. Acharya, C. V. Ciobanu, E. Sutterand P. Sutter, J. Phys. Chem. C 117 (2013) 20675.

12. A. Ramasubramaniam, D. Naveh and E. Towe, NanoLett. 11 (2011) 1070.

13. L. Ci, L. Song, C. Jin, D. Jariwala, D. Wu, Y. Li, A.Srivastava, Z. F. Wang, K. Storr, L. Balicas, F. Liuand P. M. Ajayan, Nature Mater. 9 (2010) 430.

14. P. Sutter, R. Cort�es, J. Lahiri and E. Sutter, NanoLett. 12 (2012) 4869.

15. S. Jun, X. Li, F. Meng and C. V. Ciobanu, Phys. Rev.B. 83 (2011) 153407.

16. A. Nagashima, N. Tejima, Y. Gamou, T. Kawai and C.Oshima, Phys. Rev. Lett. 75 (1995) 3918.

17. A. Nagashima, N. Tejima, Y. Gamou, T. Kawai and C.Oshima, Phys. Rev. B 51 (1995) 4606.

18. A. Nagashima, N. Tejima, Y. Gamou, T. Kawai, M.Terai, M. Wakabayashi and C. Oshima, Int. J. Mod.Phys. B 10 (1996) 3517.

19. A. Nagashima, N. Tejima, Y. Gamou, T. Kawai and C.Oshima, Surf. Sci. 357–358 (1996) 307.

20. E. Rokuta, Y. Hasegawa, K. Suzuki, Y. Gamou and C.Oshima, Phys. Rev. Lett. 79 (1997) 4609.

21. C. Oshima, A. Itoh, E. Rokuta, T. Tanaka, K.Yamashita and T. Sakurai, Solid State Commun. 116(2000) 37.

22. C. Oshima, N. Tanaka, A. Itoh, E. Rokuta, K.Yamashita and T. Sakurai, Surf. Rev. Lett. 7 (2000)521.

23. T. Tanaka, A. Ito, A. Tajima, E. Rokuta and C.Oshima, Surf. Rev. Lett. 10 (2003) 721.

24. W. Auwarter, T. J. Kreutz, T. Greber and J. Oster-walder, Surf. Sci. 429 (1999) 229.

25. M. N. Huda and L. Kleinman, Phys. Rev. B. 74 (2006)075418.

26. T.Kawasaki,T. Ichimura,H.Kishimoto,A.A.Akbar,T.Ogawa and C. Oshima, Surf. Rev. Lett. 9 (2002) 1459.

27. J. M. Soler, E. Artacho, J. D. Gale, A. Garcia, J.Junquera, P. Ordejón and D. S. Sǎnchez-Portal, J.Phys., Condens. Matter 14 (2002) 2745.

28. M. Dion, H. Rydberg, E. Schr€oder, D. C. Langreth andB. I. Lundqvist, Phys. Rev. Lett. 92 (2004) 246401.

29. J. Klimeš, D. R. Bowler and A. Michaelides, Phys. Rev.B 83 (2011) 195131.

30. G. Graziano, J. Klimeš, F. Fernandez-Alonso and A.Michaelides, J. Phys., Condens. Matter 24 (2012)424216.

31. N. Troullier and J. L. Martins, Phys. Rev. B 43 (1991)1993; ibid Phys. Rev. B 43 (1991) 8861.

32. G. Giovannetti, P. A. Khomyakov, G. Brocks, V. M.Karpan, J. van den Brink and P. J. Kelly, Phys. Rev.Lett. 101 (2008) 026803.

33. S. K. Singh, M. Neek-Amal, S. Costamagna and F. M.Peeters, Phys. Rev. B 87 (2013) 184106.

34. G. B. Grad, P. Blaha, K. Schwarz, W. Auwarter andT. Greber, Phys. Rev. B 68 (2003) 085404.

35. C. Oshima and A. Nagashima, J. Phys., Condens.Matter 9 (1997) 1.

A. Ebnonnasir, S. Kodambaka & C. V. Ciobanu

1550078-6