Atomic Adsorption on Nitrogenated Holey Graphene - The Journal of

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Atomic Adsorption on Nitrogenated Holey Graphene Raphael Matozo Tromer, Marcos Gomes Eleuterio da Luz, Mauro S. Ferreira, and Luiz Felipe Cavalcanti Pereira J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10058 • Publication Date (Web): 17 Jan 2017 Downloaded from http://pubs.acs.org on January 20, 2017

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The Journal of Physical Chemistry

Atomic Adsorption on Nitrogenated Holey Graphene Raphael M. Tromer,† Marcos G. E. da Luz,‡ Mauro S. Ferreira,¶ and Luiz Felipe C. Pereira∗,† †Departamento de Física Teórica e Experimental, Universidade Federal do Rio Grande do Norte, Natal, 59078-970, Brazil ‡Departamento de Física, Universidade Federal do Paraná, Curitiba, 81531-980, Brazil ¶School of Physics and CRANN, Trinity College Dublin, Dublin 2, Ireland E-mail: [email protected] Phone: +55 84 3215 3793. Fax: +55 84 3211 9217

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Abstract Two-dimensional crystals with C2 N stoichiometry have recently been synthesized. This novel material, dubbed Nitrogenated Holey Graphene (NHG), is a semiconductor unlike pristine graphene. For any novel material it is fundamental to understand the action of different adatoms on its surface, a process responsible for a rich phenomenology. We employ first principles calculations and a hybrid Quantum Mechanics:Molecular Mechanics method to investigate the adsorption of H, B, and O on NHG sheets. The adsorption of H atoms may prove important for applications in hydrogen storage and gas sensors, while the adsorption of O in any new material is important to understand its oxidation process. Both N and B are common dopants in carbon-based systems, such as in BNC structures. We find that H and B prefer to adsorb on top of a nitrogen atom, while O prefers to adsorb on top of a carbon-carbon bond. The electronic structure of NHG also changes due to the presence of adatoms, with the appearance of midgap states, close to the Fermi level. In the case of NHG+H and NHG+B we observe the appearance of a finite magnetic moment, related to the midgap states, which could give rise to a magnetoresistance effect. Our results provide insight into the adsorption of impurities on this novel 2D carbon-based material, with potential for application in novel electronic devices.

Introduction In the last decade, graphene has attracted much interest due to its unique physical properties and promising applications. Among possible uses, graphene is an excellent candidate for gas sensors since its electronic transport properties are very sensitive to the presence of certain gas molecules. 1,2 In addition, ultra high-speed transistors, integrated circuits and other spintronic-based devices are some of the potential applications envisaged for this material. In fact, graphene is believed to be the basis of the next-generation of electronic devices. Despite its enormous potential, in its “usual" form graphene is a zero-gap semiconductor, which

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makes it unsuitable for electronic devices such as field effect transistors and logic gates, to name but a few. To overcome the gapless problem of graphene, different possibilities have been considered. It has been shown that modifications in the structure of graphene, e.g., with the addition of substitutional or adsorbed impurities, can open an energy gap. 3 Recently, the influence of adatoms and small molecules adsorbed on graphene has been investigated. It has been shown that non-local electronic correlations are crucial for a proper description of small molecule adsorption on graphene at the density functional theory (DFT) level. 4,5 The adsorption of molecules on graphene and graphene nanoribbons has also been studied, motivated by its potential as a gas sensor. The charge transfer between adsorbates and graphene has been shown to be very weakly dependent on the adsorption site, but it is strongly dependent on the orientation of the adsorbate with respect to the graphene surface. 6 Finally, it has been shown that adsorbed impurities on doped graphene may give rise to the formation of an energy gap. 7 Whilst these findings are certainly interesting, the obstacle of opening a gap through doping remains challenging and the search for alternatives is still on going. Credible alternatives have been suggested recently when a series of new graphene-like materials have been theoretically proposed, some of them already synthesized. 8–13 In particular, the so-called nitrogenated holey graphene (NHG) – derived via chemical routes – displays semiconducting characteristics and a 2D crystal structure with a C2 N stoichiometry and evenly distributed holes. 14–16 The structure of NHG is shown in figure 1, where we also indicate an orthorhombic unit cell composed of 36 atoms, 24 being carbon atoms and 12 being nitrogen atoms. The lattice vectors for this unit cell are ~a1 = (8.33 Å)î and ~a2 = (14.33 Å)ˆj. The atomic structure of NHG features two types of hexagonal rings one has only carbon atoms (CC) while the other contains four C atoms and two N atoms (NC). Motivated by the possibility of a graphene-like material that possesses a spontaneously formed energy gap, we must now investigate whether the NHG features are comparable to

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the well-publicized properties of graphene. One one way of doing that is by asking how the electronic structure of NHG is impacted by the presence of dopants, which is the goal of our manuscript. In this communication we employ first principles calculations and a hybrid Quantum Mechanics:Molecular Mechanics (QM:MM) approach to investigate the adsorption of H, B, and O on NHG sheets. Our particular choices for adsorbed impurities are motivated by the following. (i) Understanding the interaction between H atoms and NHG can be useful for applications in hydrogen storage. (ii) The adsorption of O in any material has important implications for the oxidation process, which can change important properties of the material. (iii) Both N and B are common dopants in carbon-based systems. (iv) Finally, the stability of BNC systems opens-up the possibility for further modifications in the structure of NHG. 17 We believe our results about adsorption of impurities on NHG will shed some light on the potential applications for this new graphene-like material, such as its viability in hydrogen storage, fabrication of gas sensors and development of next-generation electronic devices.

Methods We perform first principles calculations based on DFT and classical molecular mechanics force fields to investigate atomic adsorption on NHG. In order to avoid interaction of adsorbed impurities with their periodic images in standard DFT calculations, 18 we employ the hybrid QM:MM method known as ONIOM, 19,20 implemented in the GAUSSIAN03 package. The ONIOM method has been used in a large range of applications, such as enzyme reactions and adsorption effects. 21 For instance, a recent study performed a comparison between ONIOM and standard periodic calculations for the adsorption of NH3 and H2 O on Acidic Chabazite. 22 They showed that ONIOM results are in excellent agreement with periodic calculation, at a fraction of the computational time. Therefore, the ONIOM scheme has been chosen for our calculations and drastically reduced the computational cost relative to a fully first principles treatment.

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For the QM:MM calculations, we divide the system into two regions: a quantum region (QR) where the atomic adsorption takes place, and a classical region (CR) which models the rest of the NHG sheet. The QR was treated with DFT using two different exchangecorrelation functionals and cc-pVDZ basis set. First we employed the B3LPY 23,24 hybrid functional, and later we checked the calculated results with the PBE functional. 25 Meanwhile, the CR was modeled by the Universal Force Field (UFF), 26 where we also employed the electronic embedding scheme to take into account charge polarization effects in the CR. For the sake of clarity, we shall refer to the QM:MM calculations as B3LYP+UFF and PBE+UFF, depending on the exchange-correlation functional adopted for the QR. Within this hybrid QM:MM scheme we are able to investigate the adsorption of impurities on large sheets of NHG, composed of up to 756 atoms. In the ONIOM scheme the total energy of the system is obtained from three independent MM calculations. First, the energy of the whole system EQR+CR (the subscript represents the

region and superscript represents the method used) is calculated with the UFF. Next, we MM DF T calculate the energy of the QR with UFF: EQR . Finally, the energy of QR, EQR , is calcu-

lated with DFT. The Hamiltonian operator of the QR, redefined to include the interaction between electrons and nuclei from the QR with partial charges from the CR, is written as

ˆ =H ˆ0 − H

NQR NCR XX i=1 j=1

CR XX qj Z i qj + . rij Rij i=1 j=1

NQR N

(1)

ˆ 0 is the Hamiltonian of the QR in the absence of the CR, NQR is In the equation above, H the number of electrons and nuclei in the QR, and NCR is the number of partial charges in the CR, qj are the partial charges of the CR, Zi is the nuclear charge of the atoms in the QR. Rij and rij are distances between the respective charges. The total energy expression is given by: MM MM DF T Etot = EQR+CR − EQR + EQR .

(2)

It is important to keep in mind that total energies obtained in the ONIOM method are 6


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designed to be used in a relative manner only. In order to calculate the adsorption distances and energies, we choose the ONIOM scheme shown in figure 2. The QR is composed of 30 atoms, being 20 C and 10 N atoms, while the CR contains 726 atoms (not all shown). The adsorption equilibrium distances and energies are calculated considering three possible sites on a NC ring and four sites on a CC ring, since the bridge sites are not equivalent for the latter. Adsorption distances and energies were verified with a larger QR, composed of 79 atoms, to check for size effects.

Figure 2: Adsorption positions considered with the ONIOM method. The QR is represented by balls and sticks, while the CR is represented by wireframes. We consider three positions in the NC ring and four in the CC ring. The CC ring has two non-equivalent bridge configurations, one with bond length RCC = 1.43 Å (white circle) and the other with RCC = 1.47 Å (yellow circle). First principles calculations were performed with the QUANTUM-ESPRESSO package 27 and the projector augmented-wave (PAW) method 28 to verify adsorption distances and energies, as well as to elucidate the change in electronic states due to the adsorption of impurities. The ground state of the electronic structure is described with spin-polarized DFT using the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional, 25 and compared with the results obtained with the non-local van der Waals density functional (vdW-DF). 29 The cutoff energy for the plane wave basis set is 408 eV, and the distance between two neighboring sheets was set at 30 Å. A 5 × 5 × 1 equivalent Monkhorst-Pack k-point grid was taken for reciprocal space sampling, 30 and we use the same unitcell shown in figure 1. In the next 7



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section we present results for equilibrium distances, adsorption energies and charge transfer obtained within the QM:MM scheme compared to first principles calculations, followed by a brief analysis of the electronic structure and magnetic moments for NHG with adsorbed impurities.

Results and Discussion Adsorption Equilibrium Distance We begin by determining the preferred single-atom adsorption sites on NHG within the ONIOM scheme with the B3LYP+UFF combination. Equilibrium adsorption distances are summarized in table 1. H and B adatoms present minimum adsorption distances on top of the N atom in a NC ring. Meanwhile, the O adatom has a minimum distance on the bridge site between two carbon atoms with distance RCC = 1.43 Å, marked by a white circle in figure 2. In the case of O, the calculated equilibrium position (bridge) is consistent with the one found for pristine graphene. 31 In order to check for possible size-effects of the QR, we also perform the calculations with B3LYP+UFF and a larger QR, containing 79 atoms in total. The calculated distances with the larger QR are equal to the ones obtained with the smaller QR. Therefore, we can rule out finite size effects and assume the 30-atom QR is large enough for our calculations. We have also compared our calculations with a different exchange-correlation functional for the QR, which we indicate by PBE+UFF in table 1. The calculated adsorption distances are in excellent agreement, and show a maximum difference of 3%. Finally, we also compared our QM:MM results with pure first principles calculations based on two different exchangecorrelation functionals: PBE and vdW-DF. Both predict adsorption distances in remarkable agreement with each other and the ONIOM-based calculations, as shown in the bottom rows of table 1. We expect that shorter adsorption distances relate to larger adsorption energies, which 8


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Table 1: Equilibrium distances of H, B and O adatoms on top, bridge and hollow sites of NHG. For the CC rings there are two nonequivalent bridge sites, indicated by white and yellow dots in figure 2. R(N C)[Å] Method Atom Top (N) Bridge Hollow B3LYP+UFF H 1.06 2.94 2.93 B3LYP+UFF B 1.53 1.66 1.72 B3LYP+UFF O 1.56 1.38 2.34 PBE+UFF H 1.05 2.95 2.89 PBE+UFF B 1.52 1.63 1.66 PBE+UFF O 1.52 1.34 2.29 PBE H 1.06 1.08 2.89 PBE B 1.52 1.64 1.68 PBE O 1.53 1.39 2.59 vdW-DF H 1.06 1.10 2.81 vdW-DF B 1.55 1.69 1.76 vdW-DF O 1.59 1.44 2.71

R(CC)[Å] Top (C) Bridge Bridge (yellow) Hollow 1.20 3.03 2.98 3.02 1.93 3.08 3.03 3.65 1.57 1.33 1.35 2.16 1.19 3.04 1.14 2.97 1.83 1.78 1.80 3.37 1.54 1.29 1.30 2.11 1.18 1.13 1.12 1.68 1.82 1.78 1.78 1.75 1.55 1.34 1.35 1.85 1.20 1.20 1.14 3.14 1.89 1.87 1.86 1.92 1.60 1.39 1.42 1.85

correspond to more stable configurations. The adsorption energies will be analyzed in the next section.

Adsorption Energies In order to obtain the adsorption energies, we consider the same ONIOM scheme shown in figure 2, and define the adsorption energy as

Ead = EN HG+adatom − EN HG − Eadatom ,

(3)

where EN HG+adatom is the energy of the adatom interacting with NHG and considering the equilibrium distance shown in table 1, EN HG is the energy of the isolated NHG and Eadatom is the energy of the isolated atom. In the case of hybrid QM:MM calculations, the two first terms in Eq. (3) are calculated with the ONIOM energy expression Eq. (2), and the last term is obtained with DFT using the same exchange-correlation potential. In the case of

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pure DFT calculations, all energies are obtained directly from the calculations. Therefore, in our definition, larger negative values indicate stronger adsorption. Table 2: Adsorption energies on different sites of the NHG sheet with QM:MM scheme and pure DFT. Ead (N C)[eV] Method Atom Top (N) Bridge Hollow B3LYP+UFF H -0.5 -0.0 -0.0 B3LYP+UFF B -1.0 -0.4 0.5 B3LYP+UFF O -1.9 -2.6 -0.4 PBE+UFF H -0.7 -0.1 -0.1 PBE+UFF B -1.4 -0.8 0.0 PBE+UFF O -2.0 -2.9 -0.3 PBE H -1.4 1.04 0.00 PBE B -3.4 -2.4 -1.7 PBE O -0.9 -2.0 -0.7 vdW-DF H -0.6 0.7 -0.1 vdW-DF B -1.4 -0.9 -0.2 vdW-DF O -0.2 -0.5 -0.5

Ead (CC)[eV] Top (C) Bridge Bridge (yellow) Hollow 0.3 -0.0 -0.0 -0.0 0.1 -0.0 -0.0 -0.0 -2.1 -3.4 -2.1 -0.4 0.2 -0.1 0.27 -0.1 -0.3 -0.2 -0.3 0.3 -2.3 -3.8 -2.5 -0.4 0.02 0.55 0.06 0.7 -1.5 -1.4 -1.7 -1.3 -1.5 -3.5 -1.8 -0.44 0.1 0.4 0.2 -0.1 -0.5 -0.4 -0.6 -0.3 -0.4 -1.2 -0.4 -0.3

As in the case of adsorption distances, the adsorption energies are initially calculated with the ONIOM scheme with two DFT flavors, and then compared with pure first principles calculations. Calculated adsorption energies are shown in table 2. With the B3LYP+UFF calculation, the results show that H and B adatoms prefer the top site of the NC ring (on top of a N atom), with adsorption energies of −0.5 eV and −1.0 eV, respectively. Meanwhile, the O adatom prefers the bridge site on the CC ring with shorter C-C distance (1.43 Å ) with an adsorption energy equal −3.4 eV. Again, in order to check for possible size-effects of the QR, we check B3LYP+UFF energies with a larger QR, containing 79 atoms in total. The adsorption energies for B and O calculated with the larger QR are equal to the ones obtained with the smaller QR, while for H the adsorption energy was smaller (−0.3 eV), indicating a less stable configuration. The QM:MM adsorption energies were also compared with pure first principles calculations employing the PBE functional and the vdW-DF functional. In both cases the most 10


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stable configurations coincide with the QM:MM predictions, and we observe an excellent qualitative agreement between the ONIOM scheme and full first principles methods, as shown in table 2. Furthermore, in all cases considered, the most stable configurations coincide with the smallest adsorption distances predicted in the previous section and presented in table 1. The adsorption energy for the O adatoms in the QM:MM scheme is considerably larger than what was found for the other two species considered, but it is similar to the B adsorption energy in the pure first principles methods. Furthermore, we notice that H is inert in all bridges and hollow sites, presenting approximately zero or positive adsorption energies. Meanwhile, O presents a strong interaction in most configurations, which is an indication that NHG might oxidize easily on an O-rich environment, as recently observed in black phosphorus (phosphorene), which is yet another novel 2D material. 32

NHG-adatom Charge Transfer We investigate the transfer of charge between NHG and the adatoms, by calculating the variation in electronic charges. A negative variation indicates charge is transferred from the adatoms to NHG, while a positive variation implies a transfer of charge from NHG to the adatoms. Our results are summarized in table 3. Considering the ONIOM method, where we compare Mulliken charges, the B3LYP+UFF calculations show that H adsorbed on the top site of the NC ring donates electrons to NHG, with a charge transfer of ∆q = −0.0045e, which is expected due to the donor character of H. On the other hand, both B and O behave as receptors and accept charge from NHG. In the case of B the charge transfer is ∆q = 0.0029e, and for O it is ∆q = 0.0095e. Meanwhile, PBE+UFF calculations show different values for the charge transferred but agree in terms of direction. Therefore, our QM:MM calculations indicate that in all cases with large adsorption energies electronic charge is transferred between the adatom and NHG. Nonetheless, the charge variations are very small, and thus it is not possible to assert if the bonds are predominantly ionic in character. 11



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Table 3: Electronic charge variation on NHG due to interaction with adsorbed impurities. Negative values indicate that charge is transferred from the adatom to NHG. ∆q(N C)[e] Method Atom Top (N) Bridge B3LYP+UFF H -0.0045 0.0000 B3LYP+UFF B 0.0029 0.0008 B3LYP+UFF O 0.0111 0.0084 PBE+UFF H -0.0014 0.0057 PBE+UFF B 0.0090 0.0068 PBE+UFF O -0.0005 0.0134 PBE H -0.3710 -0.3900 PBE B -0.0420 -0.0350 PBE O 0.3950 0.3230 vdW-DF H -0.3540 -0.3690 vdW-DF B -0.0600 -0.0550 vdW-DF O 0.4250 0.3580

Hollow 0.0000 -0.0043 0.0069 0.0056 0.0020 0.0120 -0.0100 -0.1420 0.2040 -0.0060 -0.1840 0.1270

Top (C) -0.0041 -0.0024 0.0119 0.0005 0.0030 0.0180 -0.2900 -0.0960 0.4690 -0.2570 -0.1020 0.4960

∆q(CC)[e] Bridge Bridge (yellow) 0.0000 0.0000 0.0002 0.0002 0.0095 0.0092 0.0056 0.0003 0.0042 0.0029 0.0149 0.0130 -0.2840 -0.3010 -0.1010 -0.0660 0.4050 0.4010 -0.2390 -0.2660 -0.1140 -0.0760 0.4400 0.4380

We then turn to pure first principles calculations, where we consider the variation in Löwdin electronic charges, employing the same two functionals as before. Charge transfer values with PBE and vdW-DF are at least two orders of magnitude larger than QM:MM methods, but it is important to stress out that they refer to two different types of charges: Muliken and Löwdin. For H and O adatoms, the direction of the charge transfer predicted by DFT is the same as in the QM:MM calculation, namely H is a donor and O is an acceptor. However, the sign of the charge variation is reversed in the case of B adsorption. Notice that NHG+B shows very small charge variations throughout table 3, regardless of the method employed. In these circumstances, small uncertainties inherent to the methods could certainly overcome the calculated values. Nonetheless, pure first principles calculations are certainly more precise than the QM:MM calculations, and are expected to provide better results. From our comparison we conclude that although ONIOM calculations predict adsorption distance and energies in good agreement with DFT calculations, the same cannot be said about charge variations.

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Hollow 0.0000 0.0000 0.0082 0.0056 0.0022 0.0136 -0.0380 -0.0850 0.3010 -0.0070 -0.1120 0.2560

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Band Structure Analysis We now analyze the electronic band structure of NHG in the presence of adatoms. The equilibrium configurations obtained from QM:MM calculations with the PBE exchangecorrelation functional are used as starting point, therefore we do not expect our results to be influenced by spurious image effects. 18 In figure 3(a), we show the band structure for isolated NHG which presents a bandgap of approximately 1.6 eV. Our results are consistent with the DFT calculations presented by Mahmood et. al., which predicted a bandgap of 1.70 eV and measured a bandgap of 1.96 eV. 14 In spite of the small charge transfer between adatoms and NHG, reported in the previous section, the presence of adatoms interferes with the electronic energy levels, as shown in the remaining panels of figure 3. b) E - EF (eV)

1 0 -1 -2 Γ

c)

NHG

2

A1

A2

A3

NHG+O

2

d)

1 0 -1 -2 Γ

A1

A2

A3

Γ

NHG+H

2 1 0 -1 -2 Γ

Γ

E - EF (eV)

E - EF (eV)

a)

E - EF (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


A1

A2

A3

Γ

A3

Γ

NHG+B

2 1 0 -1 -2 Γ

A1

A2

Figure 3: DFT electronic band structures for (a) NHG, (b) NHG+H, (c) NHG+O, (d) NHG+B. Notice the appearance of midgap states in (b) and (d). In the case of H adsorption, figure 3(b), we observe the appearance of a midgap state close to the Fermi level. Figure 3(c) shows the band structure in the presence of an O adatom. In this case there is no midgap state, and the system remains semiconducting with a bandgap of approximately 1 eV. However, we notice the appearance of a low-dispersion state in the 13



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conduction band, which could damage the electronic conductivity of this system because of the very large electronic effective mass. Finally, in figure 3(d) we consider the electronic states for NHG+B. As in the case of NHG+H, we notice the appearance of a midgap state close to the Fermi level, as well as a lowering of the first state in the conduction band. The adatom-induced changes observed in the band structure of NHG are consistent with our proposal of using NHG as a gas sensor. In particular, in the case of NHG+O the change in electrical current could be significant.

Spin-polarized Density of States One interesting aspect emerging from the band structure analysis is the appearance of midgap states close to the Fermi level, in the presence of H and B adatoms. Due to their lowdispersion, these states give rise to a large density of states (DOS) at the Fermi level, and NHG+H or NHG+B appear as suitable candidates to display some degree of spin polarization. In fact, according to the Stoner criterion, 33–35 the larger the DOS at the Fermi level, the more unstable the band structure is against a small spin splitting. As a result, localized magnetic moments may be spontaneously formed. This happens often in the case of weakly dispersive media, in the case of structures with midgap states or in the case of narrow energy bands. All these cases give rise to high values of density of states. With that in mind, we have also performed spin-polarized DOS calculations for all cases considered in figure 3. The spin-polarized DOS are presented in figure 4, both for spin-up and spin-down electrons. As expected there is no difference in DOS for pure NHG, and as such the material does not have an intrinsic magnetic moment. Hydrogen adatoms have been predicted to induce a finite magnetic moment on carbon nanotubes and graphene sheets 36 and we also observe this behavior on NHG+H, as shown in figure 4(b). In the case of NHG+O we do not observe the appearance of a finite magnetic moment, consistent with our observation of the electronic states in the previous section. Finally, in the presence of a B adatom, NHG also develops a magnetic moment, as shown in figure 4(d). The appearance of adatom-induced magnetic 14


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40

NHG (µ=0.00µB)

b) DOS (states/eV)

DOS (states/eV)

a)

0

c)

40

-1

0 1 E - EF (eV) NHG+O (µ=0.00µB)

-40 -2

-1

0 1 E - EF (eV)

2

NHG+H (µ=0.97µB) spin-up spin-down

0

d)

0

40

-40 -2

2

DOS (states/eV)

-40 -2

DOS (states/eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40

-1

0 1 E - EF (eV) NHG+B (µ=0.97µB)

2

0

-40 -2

-1

0 1 E - EF (eV)

2

Figure 4: Spin-polarized density of states for (a) NHG, (b) NHG+H, (c) NHG+O, (d) NHG+B. Notice the appearance of finite magnetic moments related to midgap states in (b) and (d). moments suggests that these materials can be used in spintronic applications. One possibility worth exploring is to study the magnetoresistive response of NHG+H and NHG+B. The spin-polarized densities of states shown in Figs. 4(b) and 4(d) indicate that an external magnetic field is likely to have an impact on the material conductance. Combined with the weak spin-orbit coupling characteristic of carbon-based materials, these structures are likely to transport spin currents with very little dissipation if the system is at moderately low impurity concentrations. 37

Conclusion In summary, we have performed simulations based on first principles and a hybrid QM:MM method to investigate the adsorption of H, B and O on 2D C2 N sheets, known as NHG. Our calculations show that NHG is not inert to the presence of these atomic species, and that H and B prefer to adsorb on top of a nitrogen atom, while O prefers to adsorb on 15



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top of a carbon-carbon bond. The electronic structure of NHG also changes due to the presence of adatoms, with the appearance of midgap states, close to the Fermi level. An analysis of the density of states for up- and down-spins showed that in the case of NHG+H and NHG+B a non-vanishing magnetic moment develops in the system, which could give rise to magnetoresistance effects in NHG with a low concentration of adsorbed impurities. Our results provide important insight into the adsorption of impurities on this novel 2D carbon-based structure, unveiling its basic physical and chemical properties. Certainly, first principles understanding is a fundamental step towards technological applications of new materials.

Acknowledgement The authors are grateful to C. G. Rocha for a critical reading of the manuscript. We acknowledge financial support from Brazilian government agency CAPES for project “Physical properties of nanostructured materials" (grant number 3195/2014) via its Science Without Borders program, and provision of computational resources by the High Performance Computing Center (NPAD) at UFRN and Laboratório Central de Processamento de Alto Desempenho (LCPAD) at UFPR. We thank Carlos de Carvalho for technical support at UFPR. MGEL also thanks CNPq for a research fellowship. MSF acknowledges financial support from Science Foundation Ireland (Grant No. SFI 11/ RFP.1/MTR/ 3083).

Supporting Information Available A detailed description of partial charge distribution in the QM:MM calculations is available as Supporting Information.

This material is available free of charge via the Internet at

http://pubs.acs.org/.

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