Structure and Electronic Properties of Edge-Functionalized Armchair

Jun 27, 2012 - Department of Physics, The Pennsylvania State University, University Park, ... Jia Zhou , Jingsong Huang , Bobby G. Sumpter , Paul R. C...
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Structure and Electronic Properties of Edge-Functionalized Armchair Boron Nitride Nanoribbons Alejandro Lopez-Bezanilla,†,* Jingsong Huang,† Humberto Terrones,‡,§ and Bobby G. Sumpter† †

Oak Ridge National Laboratory, Bethel Valley Road, Oak Ridge, Tennessee 37831-6493, United States Department of Physics, The Pennsylvania State University, University Park, Pennsylvania 16802-6300, United States § Departamento de Física, Universidade Federal do Ceará, P.O. Box 6030, Fortaleza, CEP 60455-900, Brazil ‡

ABSTRACT: We report a quantum mechanical description based on the density functional theory of the structures and electronic properties of armchair boron nitride nanoribbons (BNNRs) edge-terminated with O atoms and OH groups. The O edge termination was found to give a peroxide-like structure that is nonmagnetic and semiconducting with a bandgap of Eg = 2.8 eV. The O-terminated BNNR ribbon was stabilized by the reduction of the peroxide groups with H atoms leading to a polyol-like structure. The two chains of hydrogen bonds created along the edges lead to alternating 5- and 7-membered rings and cause the ribbon to become nonplanar with rippled edges. Three configurations of different ripple periods and amplitudes were found with energy differences up to 2 eV per unit cell but with virtually the same bandgap of Eg = 4.2 eV. The hydrogen bond mediated ripples are characterized through the lone pair orbitals showing a local σ−π separation and a pair of “rabbit-ears” on the acceptor O atoms in 5- and 7-membered rings respectively dictated by the hydrogen bond lengths. Energy bands and total and projected density of states are discussed for both functionalizations to show their effects on altering the electronic properties of armchair BNNRs. large area films of a variable number of layers of h-BN on Ni substrates and derived by optical measurements a bandgap of 5.92 eV. Jin et al.10 have synthesized free-standing h-BN single layers whose edges have been atomically resolved showing that vacancies with nitrogen-terminated zigzag edges are energetically favorable. Additionally, the quasi 1D strips of h-BN, namely boron nitride nanoribbons (BNNRs), have also received considerable experimental and theoretical attention. Electrical conductance has been experimentally measured for BNNRs produced by unzipping BN nanotubes.9 In addition to bare-edged11 and hydrogen-terminated ribbons,12 recent theoretical work also explored the functionalizations using chalcogen13 or halogen elements.14 The chemistry associated with the edge functionalized ribbons and specifically edge-oxidation can clearly alter the magnetic and/or electronic properties of the ribbons, thus introducing new functionalities.5,13 In this article, we present DFT-based studies of armchair BNNR edge-terminated with O atoms and OH groups. Detailed results on the effects of such functionalization on the structure and electronic properties of armchair BNNRs are provided. Unlike in the O-terminated zigzag BNNR counterpart where the edges are characterized by atom wires consisting

1. INTRODUCTION The fundamental experiments on few layered carbon films performed in 2004 have provoked a revolution in materials science.1 This surge of interest progresses in tandem with the design of novel nanoelectronic devices using these layered materials and derivatives.2 Extensive experimental and theoretical researches on graphene-based materials have been devoted to graphene nanoribbons (GNRs),3 quasi 1D strips of the 2D graphene, which can be synthesized by cutting a graphene sheet or by longitudinally unzipping carbon nanotubes.4 Main research foci are concentrated on the effects of ribbon width, edge orientation, and edge functionalization on the structures, electronic, and magnetic properties of GNRs,5 which are closely associated with their potential applications in nanoelectronic devices. The fruitful investigations of graphene and GNRs have stimulated the research of many other inorganic layered materials such as SiC,6 NbSe2, MoS2,1 and various other transition metal dichalcogenides.7 Particular attention has been devoted to hexagonal boron nitride (h-BN),8 a 2D, one atomthick layered material, isoelectronic and isomorphic to the graphene honeycomb lattice. h-BN consists of an equal number of boron and nitrogen atoms in sp2 hybridization bound together by strong covalent bonds and yet with a remarkable ionic character. Unlike graphene, h-BN exhibits a large bandgap of ∼6 eV, which accounts for the name white graphene dubbed for h-BN.9 Recently, Shi et al.8 have reported the synthesis of © 2012 American Chemical Society

Received: February 22, 2012 Revised: May 29, 2012 Published: June 27, 2012 15675

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Figure 1. (a) Energy band diagram for the nonmagnetic 19-aBNNR-H for which all states are spin degenerate. (b) Energy band diagram of the nonmagnetic ground state of the 19-aBNNR-O where two dispersionless states in the ribbon gap can be observed. (c) and (d) are the projected DOS of the O atoms linked to the N and B atoms at the ribbon edges (denoted as O@N and O@B, respectively) revealing that the major contribution to those dispersionless states comes from the p-orbitals of the O atoms. The gap of the O edge-functionalized ribbon is thus reduced down to 2.8 eV from the 4.5 eV of the hydrogenated ribbon. (e) shows the total DOS of the system. For (a) through (e), horizontal dashed lines indicate the Fermi energy level. (f) and (g) show the fully relaxed geometries of 19-aBNNR-H and 19-aBNNR-O, respectively. (h) is the side view of 19-aBNNR-O showing the peroxide O atoms tilted above and below the ribbon plane. (i) The charge density in silver color localized on the O atoms at the energy corresponding to the dispersionless states right below the Fermi level.

of equidistant O atoms,13 we found that in the armchair ribbon, the neighboring O atoms form a dimerized peroxide-like structure. The peroxide structure is further reduced with H atoms giving a polyol-like structure with fringes of OH groups terminating the two ribbon edges. Hydrogen bonds between neighboring OH groups cause locally rippled edges with various ripple periods and amplitudes. Both O and OH functionalizations on the armchair BNNR yield semiconducting ribbons with reduced electronic bandgaps relative to the hydrogenated analogue. Energy bands, total density of states (TDOS), and projected density of states (PDOS) are presented for all of the O and OH edge-terminated ribbons, and the hydrogen bond mediated edges are characterized through the lone pair (LP) orbitals on OH groups.

0.02 GPa. The numerical integrals are performed on a real space grid with an equivalent cutoff of 300 Ry. To sample the irreducible Brillouin zone (BZ), a 1 × 1 × 24 k-point mesh is used for small unit cells and a 1 × 1 × 8 k-point mesh is used for the ∼1.7 nm long unit cells that are quadrupled based on the small cells. The hydrogen bond mediated ripples are characterized using one-center LP orbitals that correspond to the conventional Lewis pictures. Although the canonical orbitals obtained from SCF calculations cannot be used to visualize such localized orbitals, they can be transformed into local block eigenfunctions of the one-electron density matrix using for example the natural bond orbital (NBO) analysis.16 To this end, model compounds are built by using the atomic coordinates in the quadrupled unit cells obtained from the SIESTA relaxations and by H saturation of the dangling bonds of B and N atoms exposed due to the termination of periodicity. Using the Gaussian 09 program,17 the H atom coordinates were optimized at the level of LDA/6-31G*, whereas the other atoms were frozen. An NBO analysis of the optimized structure was performed using the NBO 3.1 program interfaced with Gaussian 09.

2. COMPUTATIONAL METHODOLOGY Geometry optimization and electronic structure calculations are performed using the SIESTA DFT-based code15 with a doubleζ basis set and additional polarization orbitals. The spindependent local density approximation (LSDA) is adopted for the exchange correlation functional and the Troullier-Martins scheme is used for the norm-conserving pseudopotentials. Generalized gradient approximation (GGA) was also employed to compare with LSDA results. All armchair BNNRs are modeled within a supercell with at least 10 Å of vacuum between noninteracting neighboring cells. All atoms are relaxed with a force tolerance of 0.01 eV/Å and the unit cell vectors are relaxed with the maximum stress component being smaller than

3. RESULTS AND DISCUSSION All edge-functionalized BNNRs are defined by the notation naBNNR-X, where n refers to the number of BN dimers per unit cell across the ribbon width, “a” denotes the armchair orientation of the BNNR, and X is the edge terminating 15676

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Figure 2. (a) Energy band diagram, (b) total DOS for the nonmagnetic ground state of the 19-aBNNR-OH, in the ab configuration as shown in (f), where a and b denote one O above and the other O below the ribbon plane, respectively. As described in the text, two other configurations were found, as shown in (g) and (h), which are similarly denoted as aabb and aaaabbbb, respectively. All three configurations display locally rippled edges but with different ripple periods. (c) Total DOS normalized by a factor of 4, (d) and (e) projected DOS of the O atoms bonded to the N and B atoms at the ribbon edge (denoted as O@N and O@B, respectively) for the most stable aaaabbbb configuration (energies in Table 1). Horizontal dashed lines indicate the Fermi energy level. (i) Regardless of the ripple period, the OH groups on each edge point in the same direction forming a chain of hydrogen bonds and leading to alternating 5- and 7-membered rings along the edge.

electronic behaviors can be obtained as the two edges are terminated with these different atoms or chemical groups. The fully relaxed geometries of the ribbons are illustrated in parts f− h of Figure 1 and parts f−i of Figure 2. Note that the ribbon axis is along z. Part a of Figure 1 shows the energy band diagram for the ∼2.5 nm wide hydrogenated ribbon 19-aBNNR-H (part f of Figure 1) exhibiting a direct energy bandgap of Eg = 4.5 eV at the Γ point of the BZ.19 Both the conduction and the valence bands possess dispersive electronic states. The full band spectrum (not shown) reveals that the valence band structure consists of four groups of bands with 19 bands in each group, which describe the sigma-bonds corresponding to the atomic

atom or group. We restricted our studies to a relatively wide ribbons, such as 19-aBNNR, to minimize the interactions across the ribbon between the edge functional groups. Additionally, we do not systematically address the effect of ribbon width because the trends in the geometrical deformation and relative energies are expected to be independent of the ribbon width. Indeed, it has been predicted that the bandgap of armchair BN nanoribbons reaches a constant value when the number of BN dimers across the ribbon width is more than 19.18 We examine the electronic properties of hydrogenated, oxidized, and OH edge-functionalized structures of BNNRs in the armchair edge orientation, that is, 19-aBNNR-H, 19-aBNNR-O, and 19aBNNR-OH. We will show that different geometrical and 15677

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valence orbitals in sp2 hybridization of the BN-network and the px-derived π-bonding states. There are also two dispersionless bands (below the presented energy window close to the Fermi level) that are associated with the atomic orbitals of the terminating H atoms. Spin-resolved calculations yield the same results indicating that the electronic structure is spin degenerate and nonmagnetic. Edge termination of the ribbon by six-valence-electron O atoms produces the 19-aBNNR-O ribbon (part g of Figure 1). Unlike the zigzag BNNRs studied previously where the edges are characterized by atom wires consisting of equidistant O atoms,13 the neighboring O atoms of the 19-aBNNR-O form a dimerized peroxide-like structure on the armchair edges. The optimized lattice does not change the planar structure across the ribbon width except for the edges where the O atoms are tilted above and below the ribbon plane (as shown in part h of Figure 1). This is partly due to a longer O−O bond length of 1.52 Å than the B−N bond length of 1.43 Å. This O−O bond length is close to the upper bound value of 1.49 Å for typical inorganic peroxides.20 The O atoms are bonded to the B or N atoms on the ribbon edges forming O−B and O−N single bonds with a length of 1.38 Å. In addition, each of the O atoms accommodates two lone pairs. The stabilization energy of a 19aBNNR-O relative to a bare-edged 19-aBNNR and two triplet O2 molecules is found to be −8.3 eV. In comparison, the stabilization energy of a 19-aBNNR-H relative to a bare-edged 19-aBNNR and two H2 molecules is −6.8 eV. These negative stabilization energies should be mainly ascribed to the passivation of the dangling bonds at the B and N edge atoms. The O functionalization stabilizes the 19-aBNNR by slightly a greater amount than the H functionalization because of the formation of the dimerized O−O peroxide bonds which may further lower the energies. Similar to the 19-aBNNR-H ribbon, the 19-aBNNR-O ribbon has a spin-degenerate electronic structure and a nonmagnetic ground state, which is different from that of the zigzag counterpart, which becomes metallic and magnetic upon edge-oxidation.13 A bandgap reduction to Eg = 2.8 eV with respect to the hydrogenated ribbon is observed, which is due to the appearance of two groups of new bands close to the Fermi level, each with double degeneracy (part b of Figure 1). The origin of these bands is the p-states localized on the ribbon edges, especially the p-states of the O atoms, which can be confirmed by analyzing the contribution of each O atomic orbital to the TDOS. Comparing the PDOS shown in parts c and d of Figure 1 for the two O atoms bonded to N and B (denoted as O@N and O@B, respectively) and their contributions to the TDOS shown in part e of Figure 1, it can be seen that the dispersionless states within the ribbon gap are mainly localized on the functionalizing O atoms. On the basis of peroxide chemistry, it can be expected that the stability of 19-aBNNR-O might be compromised by the relatively high reactivity of the oxygen atoms in a peroxide-like geometry. To lower the reactivity of the O edge-functionalized ribbon, we reduce the peroxide with H to study an OH edgefunctionalized ribbon 19-aBNNR-OH that has a polyol-like structure (part f of Figure 2). By comparison of the unit cell energy for 19-aBNNR-OH and the sum of those for 19aBNNR-O and two H2 molecules, we found that the OH edgefunctionalized ribbon lowers its energy by ∼9.5 eV with respect to the oxidized ribbon, clearly indicating that the reduction of the oxygen atoms on the edges greatly stabilizes the edgeoxidized ribbon.

A narrower 13-aBNNR-OH ribbon has been studied by Wu et al.,21 who found that the OH termination induced a local distortion at the edges mainly due to the short distance between two neighboring groups. A careful scrutiny, however, reveals that the edge distortions of the 19-aBNNR-OH studied here give rise to two locally rippled structures along the two edges where one OH is displaced above and the next OH below the ribbon plane (part f of Figure 2). The nonplanarity is mainly manifested at the rippled edges and also includes the B/ N atoms connected immediately to the O atoms. The presence of periodic local ripples instead of random distortion justifies a more detailed examination of the origin for the local distortion than simply ascribing it to the repulsion caused by short distances. Relaxation leads to the H atom of each OH group pointing in the same direction toward the O atoms on the neighboring OH group. Such directional orientation of the OH groups is indicative of hydrogen bonds, which is mainly an electrostatic dipole−dipole interaction in addition to some partial covalent bond features.22 For extended nanoribbons, the hydrogen bonds repeat along the two edges, forming two chains of alternating out-of-plane 5- and 7-membered rings as can be seen from a top view in part I of Figure 2. Note that changing the hydrogen bond donors/acceptors gives an opposite direction for the hydrogen-bonded chains but it does not change the electronic structure significantly. By doubling the unit cell and varying the orientations of the OH groups followed by full geometry relaxations, we found three configurations with different ripple periods. The abovedescribed periodically rippled structure is denoted as an ab configuration (part f of Figure 2), where a and b indicate one O above and one O below the ribbon plane, respectively. By the same token, two other configurations shown in Figures (g) and (h) are denoted as aabb and aaaabbbb respectively meaning that one group of two (four) O atoms are displaced above and another group of two (four) O atoms below the ribbon plane. Going from ab, aabb, to aaaabbbb, all three configurations display an increasing number of ribbon units in the supercell based on the ripple period of the hydrogen-bonded chains, with the period of the latter configuration doubling that of the former one consecutively. It is worth pointing out that these notations only apply to local distortions on one edge. Since all ribbons under investigation have a 2-fold axis located in the center of the ribbon and pointing along the ribbon direction, the ab, aabb, and aaaabbbb notations on one ribbon edge would become ba, bbaa, and bbbbaaaa on the other edge. Ribbons with a bisecting mirror perpendicular to the ribbon plane would establish another set of geometries but we did not pursue any further calculations along that line because of the large ribbon width. As can be observed in parts f−h of Figure 2, along with the increasing of the ripple periods, the ripple amplitudes increase as well. For the configuration with the longest period, a sinusoidal wave oscillation can be clearly noticed at both edges. The structures and energetics of the three configurations are compared in Table 1. The bond lengths between the O atom of a hydroxyl group and the N and B atoms on the ribbon edges are dN−O = 1.36 Å and dB−O= 1.41 Å. These bond lengths remain nearly constant as the ripple period doubles and quadruples. This is also the case with the O−H bonds in the length of dO−H = 1.03 Å. There is a small bond length difference of 0.4−0.8 pm depending on whether the hydroxyl group is anchored on B or N atoms but this difference, smaller than the significant figures, is not shown in Table 1. However, the hydrogen bond lengths (dO···H)23 between neighboring 15678

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membered rings is quite different from that in the range of 1.55−1.63 Å for 5-membered rings. In addition, these hydrogen bond lengths elongate by 0.08−0.09 Å as the ripple period doubles from ab to aabb and then remain nearly unaffected as the ripple period further doubles from aabb to aaaabbbb. These variations reflect a dependence of hydrogen bond lengths on the bond angles between OH groups, as the average ∠H− O···H bond angle changes by a larger degree from ab to aabb and by a smaller degree from aabb to aaaabbbb. Energy-wise, we found that the aabb and aaaabbbb configurations are more stable than the ab configuration by ca. 2 eV and the most stable geometry corresponds to a sinusoidal edge-terminated structure with four hydroxyl groups displaced above and four below the ribbon plane. We note that a similar trend can be reported for the same calculations as performed within the GGA approach. GGA results compare quite favorably with the LDA values and do not show significant variations on relevant quantities such as ribbon lattice parameters and relative binding energies. OHfunctionalized GNRs have been recently reported by Wagner et al.,24 where they found that the strain induced at the ribbon edges by functionalization with bulkier OH groups is relieved as the ribbons form rippled geometries. In their study, they mentioned that the aabb configuration is more stable than the aaaabbbb configuration but no explicit energy difference was given. This is corroborated in our calculations for OH edgeterminated GNRs indicating that the former is 30 meV more

Table 1. Comparison of Bond Lengths, Bond Angles, and Energetics of the Three Configurations of 19-aBNNR-OH Ribbon with Different Ripple Periods configurations

dB−O (Å)

dN−O (Å)

dO−H (Å)

dO···H (Å)

∠H−O···H (deg)

ab (×4)a,b

1.36

1.41

1.03

1.37,c 1.55d 1.46,c 1.63d 1.45,c 1.62d

125.7e

aabb (×2) aaaabbbb (×1)a,b

a,b

1.36

1.40

1.03

1.35

1.41

1.03

132.4

e

135.9e

energy (eV) 0 −1.94f, −2.32f,g −2.00f, −2.40f,g

a

The three configurations are denoted by a series of a and b symbols depending on the arrangement of the O atoms above or below the nanoribbon plane, respectively. The ab notation represents an aBNNR unit cell with alternating OH groups in a lateral zigzag arrangement. The aabb notation represents an aBNNR unit cell with a ripple period doubling that of ab configuration. Likewise, the aaaabbbb notation represents an aBNNR unit cell with ripple period doubling that of aabb configuration or quadrupling that of ab configuration. bThe factors ×4, ×2, and ×1 are employed to ensure the same number of atoms in the supercells to facilitate the direct comparison of relative energies. cAverage in the 7-membered rings. dAverage in the 5membered rings. eAverage in the entire unit cell. fRelative to that of the ab configuration. gResults from the optimizations at the level of GGA.

hydroxyl groups vary as a function of the group location, as the average dO···H value in the range of 1.37−1.46 Å for 7-

Figure 3. Visualization of lone pair (LP) orbitals on O atoms of OH groups for (a) ab, (b) aabb, and (c) aaaabbbb configurations. The hydrogen bond acceptor O atoms show common features along the hydrogen bonded chains, regardless of the ripple configurations. However, a sharp difference exists between 5-membered rings and 7-membered rings in terms of local σ−π separations on acceptor O atoms in 5-membered rings (green and orange) and pairs of rabbit-ears on acceptor O atoms in 7-membered rings (blue and red). 15679

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neighboring O atoms form O−O peroxide single bonds with the O atoms alternatively tilting above and below the ribbon plane. In comparison, OH-termination gives rise to a polyol-like structure with fringes of OH groups terminating the two ribbon edges. From the comparison of the total energy of OH group terminations with that of simple edge oxidations, it is observed that the edge-oxidized ribbons stabilize by OH group functionalization. The hydrogen bond interactions between neighboring OH functional groups push these functional groups above and below the ribbon plane forming alternating 5- and 7-membered rings along the two edges that yield locally sinusoidal shaped rippled geometries. Three configurations of different ripple periods and amplitudes were found with energy difference of up to 2 eV per unit cell but with virtually the same electronic structures. The underlying contribution of the oxygen p-orbitals to the new states introduced in the ribbon’s electronic bandgap upon functionalization was rationalized. The wide bandgap insulating behavior of hydrogenated armchair BNNRs is conserved upon edge oxidation or termination with hydroxyl groups but a reduction of bandgap by 0.3−1.7 eV is observed. The deformation induced by the functional group could represent a pathway to control the growth and shape of aBNNRs while introducing minor changes in their electronic properties which could potentially find nanoelectronic device oriented applications.

stable than the latter. However, such a trend in GNRs is different from that in BNNRs. Parts a and b of Figure 2 show the energy band diagram and the TDOS of a 19-aBNNR-OH ribbon with an ab configuration where a doubly degenerate bonding state allows for a slight bandgap reduction down to 4.2 eV with respect to the hydrogenated counterpart. It is worthwhile noticing that despite the geometrical and energetic differences among the three configurations of 19-aBNNR-OH ribbon, the electronic structures remain similar. As can be seen from part c of Figure 2, the TDOS of the aaaabbbb configuration normalized by a factor of 4 is comparable to that of the ab configuration. We note that the different contributions of O atomic orbitals to the TDOS of these two states strongly depend on the atom to which the OH is bonded. The PDOS shown in part d of Figure 2 reveals that the contribution to the TDOS shown in part c of Figure 2 close to the Fermi energy primarily comes from the px orbital (perpendicular to the ribbon plane) of the O attached to the N atom. In comparison, as can be seen from the PDOS shown in part e of Figure 2, more significant and nearly equal contributions to the TDOS come from the px and pz orbitals for the O attached to the B atom. To further characterize the hydrogen bond mediated ripples, we conducted NBO analyses using the converged wave functions of the model compounds that correspond to the supercell structures of the three configurations but with the B and N edges terminated with H atoms to saturate the dangling bonds of B and N atoms. All LP orbitals on O atoms are superposed onto the same molecular frameworks and shown in Figure 3. We find that the LP orbitals show common features along the hydrogen bonded chains regardless of the ripple configurations. However, a sharp difference exists between the hydrogen bond acceptor O atoms in 5-membered rings and 7membered rings in terms of local symmetry and the (in)equivalency of the two LP orbitals on each O atom. The hydrogen bond acceptor O atoms in 5-membered rings show a local σ−π separation just like that observed in isolated water molecules and other divalent oxygen-containing molecules25,26 whereas those in 7-membered rings display a pair of rabbit-ears orbitals in line with an expectation based on the traditional valence shell electron-pair repulsion (VSEPR) theory. The inequivalency in terms of local σ−π separation of the two LP orbitals in an isolated water molecule can only be broken by chemical bonding interactions.26 Putting this into context, the difference of local σ−π separation and the rabbit-ears of the LP orbitals can be justified by the hydrogen bond lengths dO···H in the 5- and 7-membered rings. As we have noted previously in Table 1, the average dO···H value in the range of 1.37−1.46 Å for 7-membered rings is ca. 10 pm shorter than that in the range of 1.55−1.63 Å for 5-membered rings. Therefore, the local σ−π symmetry is broken in the 7-membered rings at the hydrogen bond acceptor O atoms, giving rise to the expected nearly equivalent “rabbit ears” LP orbitals. However, the local σ−π separation persists in the 5-membered rings on the hydrogen bond acceptor O atoms since the well-separated hydrogen bond donor H atoms did not break the symmetry on the acceptor O atoms.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research used computational resources of the Oak Ridge Leadership Computing Facility, located in the National Center for Computational Sciences at Oak Ridge National Laboratory, and the computational resources of the National Energy Research Scientific Computing Center, which are supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC05-00OR22750 and Contract No. DEAC02-05CH11231, respectively. The research was supported by the Center for Nanophase Materials Sciences (CNMS), sponsored at Oak Ridge National Laboratory by the Division of Scientific User Facilities, U.S. Department of Energy. H.T. acknowledges support from the Penn State University and CAPES, Brazil, as visiting scientist.

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REFERENCES

(1) (a) Novoselov, K.; Geim, A.; Morozov, S.; Jiang, D.; Zhang, Y.; Dubonos, S.; Grigorieva, I.; Firsov, A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666−669. (b) Novoselov, K.; Geim, A.; Morozov, S.; Jiang, D.; Katsnelson, M.; Grigorieva, I.; Dubonos, S.; Firsov, A. Two-dimensional gas of massless Dirac fermions in graphene. Nature 2005, 438, 197−200. (c) Berger, C.; Song, Z.; Li, T.; Li, X.; Ogbazghi, A.; Feng, R.; Dai, Z.; Marchenkov, A.; Conrad, E.; First, P.; de Heer, W. Ultrathin Epitaxial Graphite: 2D Electron Gas Properties and a Route toward Graphenebased Nanoelectronics. J. Phys. Chem. B 2004, 108, 19912−19916. (2) Yang, X.; Liu, G.; Balandin, A. A.; Mohanram, K. Triple-Mode Single-Transistor Graphene Amplifier and Its Applications. ACS Nano 2010, 4, 5532−5538. (3) Wang, X. R.; Ouyang, Y. J.; Li, X. L.; Wang, H. L.; Guo, J.; Dai, H. J. Room-Temperature All-Semiconducting Sub-10-nm Graphene

4. SUMMARY In summary, we have presented a first-principles DFT study of the geometrical and electronic structures of O and OH edgeterminated armchair BNNRs. The results show how BNNR edge-oxidation leads to peroxide-like structures where 15680

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp3036583 | J. Phys. Chem. C 2012, 116, 15675−15681