Boron-Doped, Nitrogen-Doped, and Codoped Graphene on Cu(111

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Boron, Nitrogen Doped and Co-Doped Graphene on Cu (111): a DFT+vdW Study Lara Ferrighi, Mario Italo Trioni, and Cristiana Di Valentin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp512522m • Publication Date (Web): 23 Feb 2015 Downloaded from http://pubs.acs.org on February 26, 2015

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The Journal of Physical Chemistry

Boron, Nitrogen Doped and Co-Doped Graphene on Cu (111): a DFT+vdW Study Lara Ferrighi1,*, Mario Italo Trioni2 and Cristiana Di Valentin1 1

Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca, via Cozzi 55, I-20125 Milano,

Italy 2

CNR-National Research Council of Italy, ISTM, via Golgi 19, I-20133 Milano, Italy

Abstract The electronic properties of free standing and Cu supported pristine, boron doped, nitrogen doped, and co-doped graphene have been studied by means of Density Functional Theory (DFT) with the vdW-DF2C09x functional. The effects of substitutional chemical doping, metal support, lattice parameter strain and their eventual interplay have been investigated. We find that only boron doped graphene strongly interacts with the copper substrate, due to chemical bonds between the boron atom and the underlying metal. The binding energy and charge transfer from Cu are also highly enhanced compared to both pristine and nitrogen doped supported graphene. The BN co-doped system behaves similarly to pristine graphene with a weakly physisorbed state and a small charge transfer from Cu. However, the presence of the non-metal dopants makes the co-doped sheet extremely tunable for redox purposes, with the boron site acting as an electron acceptor and the nitrogen site as an electron donor.

Keywords Chemical doping of carbon layer; Adsorption on copper metal Heteroatom doping effect Lattice parameter effect

*

Address: Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca, via Cozzi 55, I-20125 Milano, Italy; phone: +39 02 6448 5232 email address: [email protected]

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1. Introduction The possibility of tuning the electronic and catalytic properties of graphene (G) is an exciting challenge. A simple way of modifying the properties of graphene is to chemically dope the carbon sheet with selected non-metal heteroatoms, such as boron, nitrogen or sulfur, or a combination of them.1,2,3,4 The doping effect depends on the substitutional dopant: replacing a carbon with a boron atom (BG) will lead to a p-type doping, whereas replacing a carbon with a nitrogen atom (NG) will lead to an n-type doping, due to the difference in the number of electrons between B, C and N. Doping graphene with boron or nitrogen gives higher electrical conductivity,5 enhanced electro-catalytic activity towards oxygen reduction reaction (ORR),6,7,8,9,10 boosts hydrogen adsorption11 with respect to pure graphene and gives great potential for energy storage.12 In particular, co-doping G with both B and N (BNG) leads to a higher ORR activity and selectivity for 4-electrons path in alkaline media, compared to single doping.13 Chemical doping may have tremendous effects also on the electronic properties of graphene which can be tuned, for example, into a semiconductor by an appropriate amount of boron nitride: experimentally, a band gap opening has been observed for graphene doped with BN domains.14 These experimental findings have triggered several theoretical studies investigating the critical issues which relate directly to the band opening and found that the band gap increases with increasing concentration of BN.15,16,17,18 Another way to modify the properties of G is by supporting the carbon layer on substrates with electron donor or acceptor properties, which will induce n-type or p-type doping effects on G.19,20,21 Different metals can behave very differently, leading to weak (Cu, Ag or Au) or strong (Ni, Co, Pd or Ru) interactions with the supported carbon layer. In particular, the interaction of pure G with Cu is found to be rather weak, with binding energies and equilibrium distances that are typical of a physisorbed state, governed by dispersion forces. At the G-Cu interface a small charge transfer takes place (from Cu to G) and some mixing of graphene and metal states leads to a small band gap opening.20,22 Last, even a small change of the lattice parameter of G can also lead to a significant change in binding energies and equilibrium distances, charge transfers and doping levels, size of the band gap, thermal boundary conductance, and even a transition in the doping type (n↔p).20,23,24,25 The compression or expansion of the lattice parameter can therefore be an excellent tool for modifying a desired property in a controlled fashion,.


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As the properties of graphene can be tuned by different routes, it is obviously important to highlight the possibility of a synergistic use of chemical doping, metal substrate and lattice parameter strain, and to elucidate the different role played by each of these factors. In addition, since doped graphene is usually grown on Cu foils, it is crucial to investigate the role of the different players because the interactions with the dopant and/or with the substrate could influence the growing process of (doped) G. While several theoretical works have been reported for both metal supported pure G26,27,28,29,30,31,32,33 or free standing chemically doped G (BG, NG and BNG),15,16,34,35,36,37,38 only few works are dealing with doped BG or NG (but not co-doped BNG) on Cu (111),39,40 despite the evident interest in unraveling the interplay between the dopant and the substrate.39 The kind of interactions and electronic properties of the (doped) graphene-metal interfaces are the consequence of several factors: dispersion forces and charge transfers between the carbon layer and the substrate (or viceversa), the hybridization between the graphene  orbitals and d metal orbitals, the lattice parameter match (or mismatch) between the carbon layer and the substrate as well as the relative position between the dopant and the substrate, just to mention the most relevant. All these effects are obviously strongly dependent on the doping type and on the metal. Here we present a Density Functional Theory study of B, N and BN doped graphene on Cu (111), comparing both a semi-empirical dispersion corrected DFT approach (DFT-D241) and an ab-initio van der Waals density functional (vdW-DF2C09x 30), and aim to untangle the effects on the electronic properties due to the substitutional doping, the lattice parameter strain or the metal substrate. 2. Computational details A 4×4 supercell with a total of 64 Cu atoms and 32 atoms in the graphene layer, is used, with a 6×6×1 Monkhorst-Pack k-points mesh, and a vacuum of about 20 Å in the direction perpendicular to the surface to avoid interaction between images. The dopant concentration corresponds to one dopant every 32 atoms in the graphene layer for BG and NG, and to two dopants (or one BN unit) for every 32 atoms in the graphene layer for BNG. The dopants in the repeated cells are considered to be far enough to have a negligible interaction. The PBE standard ultrasoft pseudopotentials are used, as implemented in the plane-wave based Quantum Espresso42 package, with energy cutoffs of 30 and 240 Ryd (for kinetic energy and charge density expansion, respectively). The projected and total density of states (PDOS and DOS) are calculated at a finer mesh of 36×36×1. Spin polarization is taken into account, but all systems considered in this work show no residual magnetization. 3. Results and Discussion


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The carbon atoms can lie on Cu(111) in different ways, but the top-fcc stacking (with C atoms directly on top of Cu atoms in the first and third layers) is considered to be the most stable one 27,43 and therefore the only one considered in this study. On the other hand, replacing a carbon atom with either boron or nitrogen can be done for two not equivalent carbons on the graphene sheet, namely on top of Cu (BtG or NtG ) or in a hollow fcc position (BhG or NhG). For the BN co-doped sheet we have calculated the stability of the possible configurations within the same ring, namely placing N in para, meta or ortho positions with respect to B, and found that the most stable arrangement (by about 1.3 eV) is the ortho position38,35 and will be the only one considered in the following (BNG). The BNG sheet can be placed on Cu(111) either with B on top of Cu and N in hollow position (named BtNhG ) or with B in hollow position and N on top of Cu (named BhNtG ). All configurations for the doped graphene considered in this work are reported in Figure 1. The Cu surface is modeled by four layer slabs with 16 atoms per metal layer, where the top two layers are allowed to fully relax as well as the different adsorbates, namely graphene (G), boron-doped (BG) and nitrogen-doped (NG), and boron and nitrogen co-doped (BNG) graphene. The adsorption of pure and doped graphene on Cu(111) has been modelled using both the widely used dispersion corrected DFT-D2 with the PBE44 functional (PBE-D2) and the most recent van der Waals density functional vdW-DF2C09x, which gives the best overall performances for adsorption distances of graphene on metal surfaces.30,45 In particular, the vdW-DF2C09x is built with the vdW-DF246 correlation and the C09x exchange47 combining the improvements on both the equilibrium distances and binding energies with respect to vdW-DF48 and vdW-DF2. Experimentally, as a consequence of the mismatch between G (2.46 Å) and Cu (2.56 Å) lattice parameters, graphene grows on Cu(111) forming Moiré patterns,49 where G is mostly aligned with the Cu(111). From a theoretical point of view, when adsorbing carbon layers on metals it is possible (a) either to adapt the metal to the graphene lattice parameter, or (b) adapt the graphene sheet to the metal lattice parameter. The former (a) will resemble more closely the experimental conditions where G retains its lattice parameter, while the latter (b) represents a strain on the G lattice parameter, thus a stretching of the carbon network. Both vdW-DF2C09x and PBE-D2 give a value for the G lattice parameter of 2.47 Å, which well reproduce the experimental value of 2.46 Å, and a value for the Cu lattice parameter of 2.52 Å, which is slightly underestimated with respect to the experimental value of 2.56 Å. The PBE functional, which is the underlying functional of the dispersion corrected PBED2, gives a value for the Cu lattice parameter of 2.57 Å, in better agreement with the experiments. To investigate the effect of the lattice parameter strain on the adsorption and electronic properties of supported graphene we have considered different approaches: the first is to adapt the metal to the G lattice parameter as obtained with vdW-DF2C09x and PBE-D2 (2.47 Å), which corresponds to zero ACS Paragon Plus Environment

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strain on G; the second is to adapt G to the metal lattice parameter as obtained with vdW-DF2C09x and PBE-D2 (2.52 Å), which corresponds to a 2% strain on G; and the third is to adapt G to the metal lattice parameter as obtained with PBE (2.57 Å), which corresponds to a 4% strain on G. Accordingly, when the lattice parameter was obtained with vdW-DF2C09x, we have calculated the adsorption energies with the vdW-DF2C09x functional, and when the lattice parameter was obtained with PBED2 or PBE, we have calculated the adsorption energies with the PBE-D2 approach.


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Figure 1: Top and side views for the different models of pure, doped and co-doped graphene adsorbed on Cu(111) considered in this work. Top row: labeling of B atoms (in green), C atoms (in gray), N atoms (in blue) and Cu atoms (in brown) on the left and pure graphene (G) on the right. Second row: BG with B in hollow position (BhG on the left) and BG with B in top position (BtG on the right). Third row: NG with N in hollow position (NhG on the left) and NG with N in top position (NtG on the right). Bottom row: BNG with B in hollow position and N in top position (BhNtG on the left) and BNG with B in top position and N in hollow position (BtNhG on the right). In this way we aim at determining i) the effect of the dopant, ii) the effect of the metal, iii) the influence of the lattice parameter strain, and iv) the effect of using different theoretical approaches to include long-range dispersion forces on the properties of G.

3.1 Adsorption on Cu (111) The adsorption of G on different (111) metal substrates has been previously investigated at different levels of theory, finding that the interaction of G with Cu is rather weak, with equilibrium binding distances suggesting a physisorbed state governed by dispersion interactions between the delocalized  states of G and the underlying Cu donating electrons to G (n-type doping). For G on Cu, it is indeed expected a small Fermi-level shift towards more negative values, as it is well represented in the literature. 27,28,29,30 In our study, the adsorption energy of C atoms is calculated as: Eads = {EG@Cu(111) − [ECu(111) + EG]}/NC where EG@Cu(111), ECu(111) and EG are the energies of interacting G-Cu(111) system, clean metal slab and pure G layer, respectively. NC is the number of C atoms in the simulation cell. For the doped systems the increase of the adsorption energy Eads due to the substitutional single dopant (or BN pair) is defined as: Eads = [EXG@Cu(111) - EG@Cu(111)] − [EXG - EG] which is independent of the cell size for a large enough cell. The values of Eads and Eads obtained with vdW-DF2C09x and PBE-D2, for unstrained and strained graphene, are reported in Table 1 and in Table 2, respectively, together with the charge transfer per carbon atom q (and the doping induced charge transfer q) and equilibrium distances heq (in Å).


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We will first describe the effect that a chemical doping of G has on the adsorption properties (section 3.1.a) and then the effect of the lattice parameter strain (section 3.1.b). For each section, we will comment on the differences between vdW-DF2C09x and PBE-D2 methods. 3.1.a Doping effect With the vdW-DF2C09x functional, the doping effect on the adsorption energies of unstrained G on Cu(111) is significant and depends on the nature and relative position of the dopants with respect to the substrate (see first column of Table 1). The adsorption of BG on Cu(111) is much stronger compared to pure G, with a preference for B in hollow position and an energy gain of -0.76 eV per dopant. This is easily explained looking at the equilibrium distances between BG and the substrate, which are dramatically reduced due to the presence of B that has a partially empty pz orbital and thus creating a strong interaction with the underlying Cu. The shortening of the BG-Cu distance is particularly noticeable when B is in hollow position (see Figure 1 or Figure 2), then the B atom moves towards Cu and creates a corrugation in the BG sheet, with a large displacement along the z direction: B is 2.14 Å from the plane created by the three Cu atoms below, while C atoms away from the dopant are about 2.98 Å from Cu, see Table 1. For B in top position the energy gain is slightly smaller, but the overall effect is to increase the interaction with the metal by reducing the BG-metal separation. This kind of interaction is clearly more than just a physisorption, as indicated also by the charge transfer from the metal that increases by 0.47e for BhG or 0.45e for BtG. For NG, which is already electron rich due to the extra electron of N with respect to C, the binding energy is slightly larger compared to G, while the equilibrium distances are longer, and the NG sheet remains mainly planar. In this case, the electron transfer is from NG to Cu, in agreement with a lower work function of NG with respect to both G and BG, although the amount of charge transfer is rather small. When simultaneously doping G with both B and N atoms, the presence of N quenches completely the enhanced interaction due to the B atom: the binding energy resemble that of pure G, while the equilibrium distances are slightly shorter, in line with the estimated charge transfer.

Table 1: Adsorption energies per carbon atom Eads (in eV), charge transfer per carbon atom q (in e) and equilibrium distances heq (in Å) for pure G on Cu(111). Additional adsorption energies Eads per dopant (in eV), charge transfer difference q (in e) for doped and co-doped graphene on Cu(111), with respect to pure G. For doped G the equilibrium binding distances heq (in Å) are reported for B, N and for C far away from the dopant. The results are reported at the vdW-DF2C09x level of theory for unstrained and strained lattice parameters.


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vdW-DF2C09x Lattice

2.47 Å

2.52 Å

G Strain

0%

2%

q

heq

Eads

q

heq

2 x10-3

3.24

-0.065

6 x10-3

3.13

Eads

q

heq

Eads

q

heq

BtG

-0.55

0.45

2.46(B)/3.02(C)

-0.79

0.56

2.52(B)/2.84(C)

BhG

-0.76

0.47

2.14(B)/2.98(C)

-0.97

0.49

2.23(B)/2.78(C)

NtG

-0.14

-0.17

3.35(N)/3.32(C)

-0.02

-0.22

3.25(N)/3.21(C)

NhG

-0.14

-0.17

3.36(N)/3.34(C)

-0.02

-0.22

3.24(N)/3.21(C)

Bt NhG

-0.04

0.01

3.12(B)/3.21(N)

-0.06

0.04

3.02(B)/3.10(N)

Bh NtG

-0.07

0.05

3.09(B)/3.19(N)

-0.09

0.01

3.01(B)/3.10(N)

Eads G

-0.061

We now compare, for some selected cases, the performance of the dispersion corrected PBE-D2 (see first column of Table 2) with the corresponding vdW-DF2C09x results presented above. As expected, the PBE-D2 tends to overestimate the adsorption energies, giving a value for Eads which is 1.5 times larger than that for vdW-DF2C09x, and equilibrium distances generally shorter. On the other hand, the doping effect is comparable for the two methods, with energy gains for BG that are just slightly enhanced with respect to those for vdW-DF2C09x. Table 2: Adsorption energies Eads (in eV) and heq (in Å) for pure G on Cu(111). Additional adsorption energies Eads per dopant (in eV) for doped graphene on Cu(111), with respect to pure G. For doped G the equilibrium binding distances are reported for B, N and for C far away from the dopant. The results are reported at the PBE-D2 level of theory for unstrained and strained lattice parameters. PBE-D2 Lattice

2.47 Å

2.52 Å

2.57 Å

G strain

0%

2%

4%

Eads

heq

Eads

heq

Eads

heq

-0.090

3.03

-0.095

2.98

-0.098

2.87

Eads

heq

Eads

heq

Eads

heq

BtG

-0.83

2.51(B)/2.95(C)

-0.98

2.57(B)/2.84(C)

-1.11

2.52(B)/2.73(C)

BhG

-0.99

2.15(B)/2.98(C)

-1.09

2.33(B)/2.82(C)

-1.27

2.26(B)/2.67(C)

Nt G

-0.02

3.13(N)/3.08(C)

0.05

3.07(N)/2.99(C)

0.08

2.97(N)/2.88(C)

G


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3.1.b Lattice parameter strain effect We now describe the effect that a small strain on the G lattice parameter has on the adsorption energies and distances. For vdW-DF2C09x (see Table 1), the general trend when expanding G is to increase the adsorption energies and to damp the corrugation. For example, for BhG, which has the largest corrugation, the possibility for the lattice parameter to expand tends to restore the planarity of BG: for the unstrained lattice parameter, the B atom is displaced from the average BG plane by -0.55 Å (towards the Cu) and the C farthest away from the dopant is displaced by +0.17 Å. When applying a 2% strain the corresponding values are -0.33 Å and +0.10 Å. As a consequence, for the strained lattice parameter, the BG equilibrium distances are longer for the B atom and shorter for the C atoms. The adsorption energies of BG are considerably increased as well as the charge transfer from the metal. Comparable effects are found when using PBE-D2, namely an increase in the binding (see Table 2) and a reduction in the corrugation (see Figure 2 for side view of BhG with different lattice parameter strains). In this case, the G lattice parameter has been further expanded to match a value of 2.57 Å (close to the experimental Cu lattice parameter), with a strain of 4% on G. The weakening of the carbon network always works in favor of the (doped) G-Cu interaction, increasing the adsorption energy of G and decreasing the equilibrium distances. The energy gain due to the boron dopant is also enhanced when increasing the lattice parameter, passing from -0.99 eV per B atom for the 0% strain to -1.27 eV per B atom for the 4% strain.


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Figure 2 Side view of the BhG structure on Cu(111) for different lattice parameter strains: the planarity of the BG sheet increases when increasing the lattice parameter.

3.2 Electronic Properties 3.2.1 Free standing G and doped G We report in Figure 3 the effect of the chemical doping on the G density of states (DOS) for the unstrained G case only. The effect of the lattice parameter strain on the DOS of pure G and (co-)doped G is a small change on the main features, namely a shift of 0.2 eV for the pz peak. In BG we observe the lowering of the Fermi level below the top of the valence band, due to the depletion of one electron comparing with pure G (p-type doping), whereas for NG, which contains one more electron than the G model, we observe the Fermi level above the bottom of the conduction band (n-type doping), as expected.15,36,37 In both cases the Fermi level shift, at this doping level, is just below 1 eV and a band gap (of about 0.2 eV) opens, due to the lower symmetry of the systems, in agreement with previously reported DOS for various doping concentrations.15,16,18,34,38 For BNG, which is iso-electronic to G, the band gap comprises the Fermi level.18,34,38 In Figure 4, we report the band structure plots for pure, doped and co-doped free-standing graphene, where the Fermi level shift and band gap opening for the (co-)doped cases is clearly evident at the K point. We also note that for BNG, due to the lower symmetry of the system, the irreducible Brillouin zone is larger compared to G, as highlighted on the right side of Figure 4, with the M and M´ points no longer equivalent.

Figure 3: Density of states (DOS) of free-standing G (gray area), BG (blue, dotted line), NG (red, dashed line) and BNG (green, dash-dotted line) for the unstrained G lattice parameter of 2.47 Å.


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Figure 4: Band structure of G, BG, NG along the KM path and band structure of BNG along the (MK, K´K, K´M´) path. On the right, the striped portion defines the irreducible Brillouin zone for G, BG and NG and the green portion defines the irreducible Brillouin zone for BNG.

3.2.2 Metal supported pure, doped and co-doped G We now compare the DOS of free standing, doped and co-doped G with those of the corresponding systems supported on Cu (111). In Figure 5 we report the DOS of free standing G and supported G for the unstrained (a = 2.47 Å) and strained (a = 2.52 Å) lattice parameter, as obtained with vdWDF2C09x. The Fermi level of supported G has moved upwards, showing a flow of electrons from the metal to the conduction band of G, with a Fermi level shift which depends on the lattice parameter strain, and increases when stretching G.19,20,21,22,23 For supported G, the electron donation from the substrate moves the Fermi level by -0.1 eV for the unstrained G (see top-left panel of Figure 5), and -0.4 eV for a 2% strain on G (see top-right panel of Figure 5). For the most stable configuration of BG on Cu (BhG), the strong electron donation from the metal to BG is able to compensate the electron hole of BG (compared to pure G) and partially fill the conduction band of BG (see second row of Figure 5). This explains the highest binding energy and shortest equilibrium distance found for this system. At this concentration of boron atoms the n-type doping of the metal overcomes the p-type chemical doping of B and the graphene sheet presents an overall excess of electrons. The deformation of the DOS for the metal supported BG compared to the free standing case (in Figure 4) is striking, and it confirms the strong interaction between the BG sheet and the Cu substrate. On the other hand, the lattice parameter strain effect on supported BhG is rather small compared to the substrate effect (see black arrows in Figure 5). For NtG, the influence of the metal is almost negligible, as evident


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from comparing the DOS of free standing and supported NG, and also as suggested by the long equilibrium distance, in agreement with previously reported results.39 Last, for the co-doped BhNtG system the effect of the supporting metal is very similar to that on G, the charge transfer fills the BNG sheet conduction band for about 0.1 eV above the Fermi level for the unstrained G and 0.4 eV for strained G. For both NG and BNG, the shape of the free standing and supported DOS is very similar.

Figure 5: Projected density of states (PDOS) of free standing (dotted line) and supported (full line) G (in black), BG (in blue), NG (in red) and BNG (in green) on Cu (111) for the unstrained G lattice parameter (left) and a 2% strain (right). The total DOS for the supported layers on Cu(111) is reported as well (brown full line). To investigate in details the mixing between the metal and the pure and (co-)doped G layers, we have reported the projected band structures for G, BhG, NtG and BhNtG in Figure 6, together with the projection on the B and N atoms for the BhNtG case.


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Figure 6: Projected (on the G or doped G layer) band structure of supported G (left) , BhG (middle) and NtG (right) on the top panel. Projected band structure of supported BhNtG on the bottom panel: on the BNG layer (left), on the B atom (middle) and on the N atom (right). The color code, from white to the corresponding color, is a function of the DOS value. For supported G, a small energy gap opens at the Dirac point (so tiny that is not visible in Figure 6 top left because of the thick line resulting from the projection), just below the Fermi level, but the conical shape of the bands of the free-standing G is mostly kept (compare Figure 4 and Figure 6). The hybridization with the metal bands takes place at lower energies, at about -2 eV.22 The projected bands on the C atoms show that a small Cu component is present, which is responsible of the nonzero density of states at the Dirac point, as reported in Figure 5. The role of the metal on the BG bands (top middle panel of Figure 6) is evident: there is a large metal component on the BG bands and, due to the strong interaction between BG and Cu, there is a noticeable perturbation on the shape of the bands compared to the free standing case. On the other hands, for NG (top-right panel of Figure 6) the mixing between NG and Cu is very similar to that of G, with the Dirac point far below the Fermi level, and the shape and the band gap of the free-standing NG are preserved. For supported BhNtG the overall band structure is slightly lowered in energy compared to the free standing case, yet keeping a small band gap around the Fermi level. Interestingly, the bottom of the conduction band has its main component on the B atom while the top of the valence band has its main component on ACS Paragon Plus Environment

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the N atom. This peculiar feature, due to the simultaneous presence of the B and N dopants, could have great relevance on the chemical and catalytic properties of co-doped G because the supported BNG could behave both as an electron poor system, easily accepting electrons through the presence of B, and as an electron rich system, easily loosing electrons through the presence of N. A proper underlying substrate could selectively enhance the reduction or oxidation properties of BN, depending on the desired type of charge transfer, thus making the properties of BNG highly tunable. Conclusions Doping, co-doping and/or supporting G on metal surfaces are hot topics of experimental research for various fundamental and technological applications. However, in the literature, the interplay between these effects is not fully understood and, thus, cannot be yet controlled or tuned for a more efficient use. The purpose of the present density functional study is to provide the details of some model systems, namely B or N doped G and BN co-doped G on Cu (111), in order to unravel the underlying factors which determine the modified electronic properties, related to the active dispersion forces, the charge transfers, the orbital mixing, the lattice parameter mismatch and local or extended structural distortions. We find that B-doped graphene is the only doped layer which strongly interacts with the Cu substrate, due to chemical bonds between the B atom and the underlying metal. The binding energy and charge transfer from Cu are also highly enhanced, compared to both pristine and N-doped graphene. The BN co-doped system behaves similarly to pristine G with a weakly physisorbed state and a small charge transfer from Cu. However, the presence of the two non-metal dopants makes the co-doped sheet extremely tunable for redox purposes, with the B site acting as electron acceptor and the N site as an electron donor. In conclusion, our detailed DFT study on B, N doped and co-doped G/Cu (111) sheds light on the specific structure-function or structure-properties relations. Most of the general considerations made on the basis of the theoretical analysis reported here can be transferred to other analogous systems in the broader family of doped G on metal surfaces. We expect that the same or similar factors will be at place and, therefore, our findings could be a useful guide to the experimental community for their explorative work. Acknowledgments We thank Lorenzo Ferraro for his technical help. This work has been supported by the Italian MIUR through the national grant Futuro in ricerca 2012 RBFR128BEC "Beyond graphene: tailored C-layers


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for novel catalytic materials and green chemistry" and by CINECA supercomputing center through the LISA grant 2014 " LI03p_CBC4FC".

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Boron-Doped, Nitrogen-Doped, and Codoped Graphene on Cu(111

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