Insertion of Line Defect in Nanoribbons of Graphene, Boron Nitride

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Insertion of Line Defect in Nanoribbons of Graphene, Boron Nitride and Hybrid of them: An AIMD study Dibyajyoti Ghosh, Prakash Parida, and Swapan K. Pati J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5039128 • Publication Date (Web): 09 Jun 2014 Downloaded from http://pubs.acs.org on June 19, 2014

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The Journal of Physical Chemistry

Insertion of Line Defect in Nanoribbons of Graphene, Boron Nitride and Hybrid of them: An AIMD study Dibyajyoti Ghosh1, Prakash Parida2 and Swapan K Pati2,3* Chemistry and Physics of Materials Unit1, Theoretical Sciences Unit2 and New Chemistry Unit3, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore, 560064, India

ACS Paragon Plus Environment

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Abstract Using constant temperature ab-initio molecular dynamics simulation, we have demonstrated a way to insert extended line defects (ELDs) at the grain boundary in hybrid graphene and boron nitride nanoribbons (BNCNRs) as well as in pure graphene nanoribbons (GNRs) and in pure boron nitride nanoribbons (BNNRs). Our systematic studies have shown that, 5−8−5 and 8−8−8 extended line defects can be installed and stabilized by depositing different adatoms such as carbon, boron, nitrogen at the grain boundaries of graphene-graphene, boron nitride-boron nitride and graphene-boron nitride junctions. The electronic and magnetic structures of these nanoribbons are highly modulated in the presence of these ELDs. KEYWORDS: Hybrid boron nitride-graphene nanoribbon, structural reconstruction, line defect, ab-initio molecular dynamics, electronic structure.

Introduction In past few years, researches on two-dimensional (2D) and quasi one-dimensional (1D) materials have been accelerated because of their fascinating and extraordinary physical, chemical and mechanical properties.

1-5

Although these materials have a long history, isolation of single

layer graphene in 2004 has gotten tremendous impetus into these low dimensional materials.

6

This one-atom-thick carbon allotrope, graphene, has already shown its several unique electronic, magnetic, mechanical, spintronics properties.

1, 7-12

Apart from graphene, monolayer hexagonal

boron nitride (h-BN) and its hybrid with graphene (h-BNC) have also been demonstrated as potential candidates for different type of technological applications. 13-19 Particularly, the zigzag nanoribbons (ZNRs) of these 2D materials already have shown promising performance towards nanoscale electronics and spintronics application.

20-32

Appearance of the peculiar edge states


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makes the difference in various properties of ZNRs with respect to their corresponding 2D sheets. 33, 34 Most of the experimental procedures to form these 1D and 2D materials unavoidably include different kinds of defects in them.

35

Theoretical investigations have demonstrated that,

the presence of point as well as line defects can largely modify the electronic, magnetic and mechanical properties of these low dimensional materials.

36-42

However, insertion of defects

(particularly line defects) in these covalently bonded hexagonal lattices in a controlled way is a challenge for experimentalists. Very recently, Lahiri et al. experimentally have synthesized extended line defect (ELD), containing two pentagon and one octagon (5−8−5) periodically in a graphene sheet.43 They have introduced structural mismatch during the growth of graphene on Ni(111) to create this ELD and found it to be metallic in nature. However, this particular approach is highly sensitive to the experimental conditions, purity of the substrate etc. There are a few theoretical studies such as deposition of dimer44, reconstruction of vacancies or special type of ELDs,

41

adsorption of C/N atoms45 etc. which all demonstrate possible ways to form

5−8−5 ELD in the graphene/h-BN/h-BNC as well as in nanoribbons of them. In this work, performing ab-initio molecular dynamics (AIMD) simulations, we demonstrate a much easier and controlled way to insert an ELD in zigzag graphene nanoribbons (ZGNRs), zigzag boron nitride nanoribbons (ZBNNRs) and also hybrid of them (ZBNCNRs). By depositing C/B/N atoms at the grain boundaries, we have predicted different kinds of line defects during the reconstruction process. While a lattice translation between two halves was considered in the previous work,41 we don’t consider such lattice translation in our present work. Different to the previous studies,41 we find different kinds of stable line defects at the grain boundary of ZGNR-ZGNR, ZBNNR-ZBNNR and ZGNR-ZBNNR. Density functional theory (DFT) based


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study of these reconstructed ELD structures show a wide range of electronic and magnetic properties. Most importantly, the insertion of ELD at middle of ZBNNR and at the heterojunction of ZBNCNR shows huge tunability of above mentioned properties. Our findings certainly open up a new door to tune various properties of NRs by easy and controlled inclusion of ELD in them.

Computational Details Ab-initio molecular dynamic (AIMD) simulations are carried out using the QUICKSTEP module in the CP2K package

46, 47

with a reasonable energy cut off of 280 Ry. It uses a mixed

basis set where the Kohn-Sham orbitals are expanded in an atom centred Gaussian basis set while the electronic charge density is described using an auxiliary plane wave basis set.

48

The

valence electrons are treated with double-ζ valence basis set with one set of polarization functions i.e. DZVP basis set.49 Whereas, the core electrons and nuclei are represented using analytical duel-space pseudo-potential recommended by Goedecker, Teter and Hutter (GTH) 50. Largely used exchange-correlation, Perdew-Burke-Ernzerhof (PBE) functional within the Generalized Gradient Approximation

51

(GGA) is used for all the calculations. We have carried

out most of the simulations at constant temperature of 300 K, using Nose-Hoover thermostat 53

52,

. For some of the NRs, to check the stability of ELD at higher temperature, we also performed

AIMD at 1000K. The time step of 1 femtosecond is used to integrate the equations of motion. The 5 or 10 picosecond trajectory is generated and used for analysis. For electronic and magnetic structure calculations for all the NRs, we have used spinunrestricted density functional theory (DFT) as implemented in the SIESTA54 package. We have used a double-ζ polarized (DZP) basis set for all atoms and a mesh cut-off of 400 Ry. Moreover, same exchange-correlation functional as mentioned earlier i.e. GGA (PBE) is used. To avoid


4

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spurious interaction in non-periodic directions, a cell of 30×40×2c Å, where c is the length of unit cell in periodic direction, has been used. We have used Monkhorst-Pack 1×1×250 (total number of k-points is 126) to sample the 1D Brillouin zone for all the electronic and magnetic structure calculations. The formation energy of these NRs has been calculated using the following formula, Eform = Etot − [nC × EC + nB × EB + nN × EN ] where Etot is the total energy of the system and EC, EB and CN are the total energies per atom of graphene, α-boron and N2 molecule, respectively. nC, nB and nN represent the number of carbon, boron and nitrogen atoms present in the system, respectively.

Results and Discussion In Figure 1, we present an example of grain boundary (GB) at the heterojunction of ZGNR and ZBNNR. The two nanoribbons are grown close to each other and the adatoms are deposited at the grain boundary of the heterojunction. The width of two NRs on both sides of the grain boundary is considered to be the same in this work. As a proposed bottom-up approach, initially each side of GB can be grown separately on top of the same metal (e.g. Cu19, 55, Ru(0001) 56 or Ni(111) 43) surface. In the previous studies, 43 a mutual translation between the two halves of the lattice has been included to form an extended line defect which constitutes a 4-membered ring (4-ELD). On the contrary, in our present study, the initial geometry for the MD simulation does not include such lattice translation between the two halves. We bring in the two NRs reasonably close to each other without any mutual lattice translation and then deposit the adatoms in the same plane of the NRs. Very recently, similar kind of approach has been considered to fabricate planar h-BNC sheet with atomically separate domains of graphene and h-BN. 19


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We nomenclature the NRs with the line defect as X-ZNR-AB where X indicates the type of adatom, NR indicates the type of the ribbon (GNR/BNNR/BNCNR) and AB denotes the nature of atoms at two edges of the NR (atoms of kind A at one of the edges and atoms of kind B at the other edge of the NR). As ZGNRs always have C at the edge, we have not mentioned AB for them. The atoms deposited at the GB, are referred as XLD whereas atoms nearest to these XLD are denoted as Mzig (i.e. CLD and Bzig/ Czig at Figure 1c). As an example, the structure shown in Figure 1b can be denoted as C-ZBNCNR-CN. All the initial structures of our study can be found in Figure S1, S2 and S3. To explore the reconstruction processes in these systems, we perform finite temperature (300K/1000K) AIMD simulations. The equilibrated geometries of these NRs are considered further to investigate the magnetic and electronic properties using first principle methods. Unless it is mentioned, we have considered 4 zigzag chains (n = 4) of C/BN on both sides of the grain boundary.


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Figure 1. (a-b) A proposed model for stepwise formation of initial heterojunction of ZBNCNR. Here, we represent C-ZBNCNR-CN as an example. (a) Two separate ZGNR and ZBNNR segments are formed; (b) C atoms are deposited at the grain boundary of ZGNR and ZBNNR. Highlighted portion is interface. (c) A zoomed view of interface. The white, light pink, grey and blue balls represent H, B, C and N, respectively.

We begin our study investigating the reconstruction processes of ZBNCNRs. In Figure S1(a) and S1(b), we present the initial structures for the reconstruction process of ZBNCNR, where C atoms are deposited (C-ZBNCNR-CB and C-ZBNCNR-CN). By investigating the MD trajectories very cautiously, we have a clear idea about the atomic processes happening at the GB during the reconstruction of these NRs. In Figure 2, we have shown the snapshots of the reconstruction process of C-ZBNCNR-CB at about 300K. As the simulation begins, C5 and C6 atoms move closer to C1 and N2, respectively, and form very short bonds shown in Figure 2b. However, these bonds are not stable and CLD (C5 and C6) atoms remain mobile at the GB before they could form a dimer between them as can be seen from Figure 2c. This dimer formation stabilizes the equilibrated structure of ZBNCNR (see Figure 2d, 2e). Along with the dimer formation, C5 forms covalent bonds with C1 and N1 and C6 forms covalent bonds with C3 and N2. Thus, CLD atoms become sp2 hybridized, forming three covalent bonds. The reconstruction finally leads to the creation of 5−8−5 ELD at the GB. Within 4.0 ps, the reconstructed ZBNCNR attains the equilibrium state, where the fluctuation in the total energy becomes negligible as shown in Figure 3. Therefore, without any further structural reconstruction, the 5−8−5 ELD at the GB in the ZBNCNR appears to be very stable. Similarly, the other structure, C-ZBNCNRCN, also gets reconstructed to form 5−8−5 ELD at the GB.


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Figure 2. Snapshot of top and side view of C-ZBNCNR-CB during AIMD simulations at constant temperature (300 K) and pressure (1 atmosphere) after (a) 0.0ps (b) 0.008ps (c) 0.01ps (d) 0.05ps and (e) 5ps.


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Figure 3. Total energy during AIMD simulation of reconstruction of C-ZBNCNR-CB at 300 K and 1 atm pressure.

Next we study N-deposited ZBNCNR (see Figure S1(c)). Interestingly, N-ZBNCNR-CN in particular, gets reconstructed into two different geometries at two different temperatures. It forms a new kind of 8−8−8 ELD (see Figure 4) at room temperature while a planar 5−8−5 ELD is formed at higher temperature (1000K) (see Figure S5). Initially, for both temperatures, deposited N atoms remain quite mobile at the GB without forming a stable covalent bond (see Figure 4b, c). Later on, at 300K, NLD forms stable linear covalent bonds with Czig and Bzig, giving rise to almost planar 8−8−8 ELD at the GB (See Figure 4d, e). In this particular low-temperature structure, each NLD remains under co-ordinated and electron rich as it has only two C−N covalent bonds. Plotting total energy vs. simulation time in Figure S4, it is evident that within 2.5 ps, this reconstructed ZBNCNR achieve the equilibrium geometry. To check the stability of this 8−8−8 ELD at larger simulation time, we have extended the simulation up to 10 ps and we find that, this


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structure is well-equilibrated within that time period (see Figure 4e). On the other hand, at higher temperature i.e. at 1000K, NLD form dimers among them and give rise to 5−8−5 ELD at heterojunction as equilibrated structure. The details of reconstruction processes are described in supporting information (Figure S5). This temperature dependent reconstruction phenomenon can be understood from the electron density at the ELD. The formation of dimer among electron rich NLD atoms at the GB faces an energy barrier due to strong electron lone-pair repulsion. As a result, at low temperature, the GB cannot get reformed to 5−8−5 ELD as formation of NLD dimer is primary step for it. Thus, at 300K, it forms 8−8−8 ELD, where NLD atoms remain relatively far from each other (~ 2.5Å), reducing the electron lone-pair repulsion between them. However, at higher temperature, NLD are much more mobile and can overcome the energy barrier quite easily giving rise to 5−8−5 ELD.


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Figure 4. Snapshot of top and side view of N-ZBNCNR-CN during AIMD simulations at constant temperature (300 K) and pressure (1 atmosphere) after (a) 0.0ps (b) 0.0035ps (c) 0.0085ps (d) 1ps and (e) 10ps.

Alike C-ZBNCNRs, B-ZBNCNR-CB (see Figure S1(d)) also gets reconstructed to form 5−8−5 ELD where BLD atoms form B−B dimer at the GB. We note that, when deposited adatoms at the GB are C or B, structures always form thermodynamically preferred 5−8−5 ELD, irrespective of


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temperature. In fact, as strong lone pair-lone pair repulsion during dimerization of BLD or CLD is absent for these NRs, the energy barrier to form 5−8−5 ELD is negligibly small. In the following section, we briefly describe the reconstruction process in ZBNNRs and ZGNRs. A GB is formed between two half-lattices of ZBNNR and B/N/C atoms get deposited at the GB following the same procedure as in the case of ZBNCNR (see Figure S2). When C atoms are deposited at the GB, we have considered three types of edges: BB-edge, NN-edge and BNedge (i.e. C-ZBNNR-BB, C-ZBNNR-NN and C-ZBNNR-BN) (see Figure S2 (a-c)). When deposited atoms at GB are B or N, we have considered BB-edge or NN-edge ribbons, (i.e. BZBNNR-BB and N-ZBNNR-NN) respectively, to avoid the formation of “wrong bonds” i.e. unfavourable B−B or N−N homonuclear bonds there (see Figure S2(d, e)). For all of these NRs, our simulation predicts the formation of planar 5−8−5 ELD at the GB for 300K as well as for 1000K. These reconstructed structures can be found in SI, Figure S6. As the lone pairs of NLD get involved in back bonding with Bzig atoms, the repulsion between the lone pairs of NLD is much weaker. As a result, the GB gets reconstructed to form a stable 5−8−5 ELD even with a short N−N dimer bond. During reconstruction of ZGNRs, deposited C or B atoms form dimers and NRs get reformed to 5−8−5 ELD at the GB (see Figure S7). In case of deposition of N atoms, NLD atoms are accommodated between the two half-lattices of ZGNR by forming either 8−8−8 ELD (at 300K) (Figure S7(c)) or 5−8−5 ELD (at 1000K) (Figure S7(d)) as seen for N-ZBNCNR-CN. The negative formation energies for all these structures as tabulated in Table 1, proves their thermodynamic stability. Doubling the width of NRs (i.e. 8 zigzag chains on either side of the GB), we have performed AIMD simulations of reconstruction processes. We have found that, the nature of each reconstruction does not depend upon the width of NRs. Furthermore, the


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formation energies tabulated in Table 1 show that width of NRs has a little influence over the thermodynamic stability of them. To investigate the magnetic ground state of ZBNCNRs, we have considered several magnetic configurations of edge C (i.e. Ced) atoms and Czig atoms as can be seen in Figure S8. Note that, B/N atoms with hydrogen passivation at the edge contribute negligible to the total spin polarization. Our band structure calculations show that, C-ZBNCNR-CB and C-ZBNCNR-CN are spin-polarized metals. Each Ced atom of C-ZBNCNR-CB (C-ZBNCNR-CN) contributes a magnetic moment of 0.27 µB/C atoms (0.26 µB/C atoms). On the other hand, the magnetic ordering of the Czig gets destroyed because of presence of 5−8−5 ELD. Thus, these Czig atoms in C-ZBNCNR-CB (C-ZBNCNR-CN) contribute much less moment of -0.07 µB/C atom (-0.01 µB/C atom) to the total magnetic moment of 0.82 µB/supercell (0.74 µB/supercell). The relative energies among different magnetic states for both C-ZBNCNR-CB and C-ZBNCNR-CN are summarized in Table 1. The electronic band structures for both systems are plotted in Figure 5. Both NRs are spin polarized metal as band 2 (majority spin band) and band 3 (minority spin band) cross the Fermi level. The wavefunction analyses of these electronic bands demonstrate that the band 1and band 3 are localized at the defect region whereas other two bands arise from carbon edges, as shown in Figure 5d and 5e. We note that, the low-energy electronic bands are mainly derived from ZGNR portion of C-ZBNCNR-CN as bands from ZBNNR are situated far from the Fermi level. Due to the Lewis acidic nature of boron, the CLD−CLD dimer in CZBNCNR-CN (where it is covalently attached to B atoms) becomes electron deficient after donating electrons to adjacent BN chain. Throughout the ribbon, a potential gradient is developed due to this charge transfer. The Mulliken population analysis reveals that each CLD


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atom donates 0.22 e−. However, for C-ZBNCNR-CB, where CLD−CLD dimer is attached to electron rich N atom, such kind of charge transfer is absent at the GB.

Figure 5. (a) Spin polarized band structure of (a) C-ZBNCNR-CB and (b) C-ZBNCNR-CN. Black and red solid lines correspond to majority and minority spin band, respectively. (c) Spin density plot and wave function plot of C-ZBNCNR-CN for (d) band 1 and (e) band 2 at Γ-point. Grey and magenta colored isosurface represent the positive and negative values of the moment, respectively. The isosurface for spin density plot is 0.025 a.u. This isosurface remains same for all the cases.

Interestingly, both B-ZBNCNR-CB and the high-temperature (1000K) structure of NZBNCNR-CN (i.e. 5−8−5 ELD) show UUDD spin configuration as their ground state (see Figure S8 for the definition of UUDD configuration). The Ced atoms at the edge couple ferromagnetically among themselves while they couple antiferromagnetically to Czig atoms, giving rise to zero magnetic moment. The formation of covalent bonds among Czig and dimer of B/N at the heterojunction, allows Czig to retain their spin ordering. From Table 1, it can be seen


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that UUDD spin configuration is energetically more stable than that of UUUU by 38 meV and 24 meV for B-ZBNCNR-CB and N-ZBNCNR-CN, respectively. These two NRs show semiconducting behaviours with different band gaps for two different spin channels (see Figure 6a and 6b).

Figure 6. Spin polarized band structures for (a) B-ZBNCNR-CB and (b) N-ZBNCNR-CN which contain 5−8−5 ELD.

In Figure 7 (a), we plot the band structure for the low-temperature structure of N-ZBNCNRCN (i.e., 8−8−8 ELD). This NR is predicted to be a spin-polarized metal with a magnetic moment of 0.7 µB/unit cell. Spin density plot confirms that, the spin moment is mostly localized on C atoms at the ZGNR edge (see Figure 7(b)). Wave function analysis demonstrates that, both majority band 1 and minority band 3 originates mainly from the edge states (Figure 7(c) and


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7(d)). As shown in Figure 7e, other two highly dispersive and degenerate bands, band 2 and band 4, which cross the Fermi level, are delocalized over the line defect regions at the GB. The edge states are mainly contributed from 2pz orbitals of Ced atoms, whereas the states at defect region are mainly appear from 2px/2py orbitals of B, N and C atoms.

Figure 7. (a) Spin polarized band structure of (a) N-ZBNCNR-CN containing 8−8−8 ELD. (b) Spin density plot and wave function plot of N-ZBNCNR-CN for (c) band 1 (d) band 3 and (e) band 2 at Γ-point.

Next, we briefly discuss the electronic and magnetic structures of ZBNNRs and ZGNRs with 5−8−5 ELD at the GB (see Figure S6). As can be seen from Figure S9, all the ZBNNRs are semiconductors with no spin polarization. The wavefunction analysis shows that ELD region produces highly dispersive impurity bands, which appear in the interior of the large band gap and effectively reduces the band gap in these NRs compared to pure ZBNNRs (see Figure S10). For


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ZGNRs with 5−8−5 ELD ( see Figure S7(a, b, d)), the ground state shows ferromagnetic coupling among the Ced atoms and antiferromagnetic coupling across two edges (i.e. UUDD), giving rise to nonmagnetic semiconducting nature (see Figure S11(a-c)). Thus, the presence of 5−8−5 ELD at the middle of ZGNR does not affect the magnetic ground state of this ribbon. However, the ZGNR with 8−8−8 ELD (i.e. room temperature structure of N-ZGNR) is spin polarized metal as the bands, localized at ELD, are dispersive in nature and cross the Fermi level (see S11 (d)).


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Table 1. Relative energies (with respect to the ground state) of different spin configurations of the studied nanoribbons. The formation energies of their ground state are given. The values in the bracket correspond to wider NRs. All the energies are given in meV. The spin configuration of the ground state remains same for both the widths. Structures

UUUU

UUDD

UDUD

UDUU

UUUD

(meV)

(meV)

(meV)

(meV)

(meV)

Formation Energy(meV/atom)

C-ZBNCNR-CB

0(0)

-

84(110)

-

-

-720(-710)

C-ZBNCNR-CN

0(0)

-

82(99)

-

-

-750(-730)

N-ZBNCNR-

0(0)

-

90(106)

-

-

-780(-750)

24(13)

0(0)

100(147)

76(51)

76(51)

-820(-780)

38(15)

0(0)

137(170)

71(53)

71(53)

-710(-710)

C-ZGNR

12(10)

0(0)

147(189)

79(50)

79(50)

-0.160(-100)

B-ZGNR

16(31)

0(0)

162(205)

70(89)

70(89)

-110(-80)

N-ZGNR(8-8-8)

0(0)

5(7)

143(93)

65(100) 65(100)

-130(-80)

N-ZGNR(5-8-5)

22(3)

0(0)

186(100)

75(97)

75(97)

-190(-100)

C-ZBNNR-BB

-

-

-

-

-

-1280(-1350)

C-ZBNNR-NN

-

-

-

-

-

-1340(-1380)

C-ZBNNR-BN

-

-

-

-

-

-1320(-1370)

B-ZBNNR-BB

-

-

-

-

-

-1330(-1370)

N-ZBNNR-NN

-

-

-

-

-

-1470(-1450)

CB(8-8-8) N-ZBNCNRCB(5-8-5) B-ZBNCNR-CB


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Conclusions In conclusion, we have described an easier way to create an extended line defect at the grain boundary

of

ZGNR−ZGNR,

ZBNNR−ZBNNR

and

ZGNR−ZBNNR

heterojunctions.

Performing AIMD, we have shown that, when foreign atoms are deposited at the grain boundaries, the nanoribbons are reconstructed to 5−8−5 or 8−8−8 ELD. Nitrogen deposited heterojunction of ZBNCNR−ZGNR and ZGNR−ZGNR form 8−8−8 ELD at room temperature (300K), while 5−8−5 ELD at a higher temperature (1000K). All other nanoribbons form 5−8−5 ELD irrespective of temperature. The lone pairs of N atoms repel each other very strongly at low temperatures avoiding the formation of N-N dimer which in turn hinders the formation of 5−8−5 ELD during the reconstruction process. Among ZBNCNRs, C-ZBNCNR-CB, C-ZBNCNR-CN and low temperature structure of N-ZBNCNR-CN are spin-polarized metal, whereas BZBNCNR-CB and high temperature structure of N-ZBNCNR-CN show semiconducting nature. Inclusion of 5−8−5 ELD in ZBNNRs only introduces defect-states at the band gap of perfect ZBNNR. Among defective ZGNRs, only N-ZGNR containing 8−8−8 ELD shows spin polarized metallic nature whereas others are semiconductors. We believe that, these kinds of line defects may be realized during the growth process of studied nanoribbons in near future. ASSOCIATED CONTENT Supporting Information Initial configurations of all nanoribbons, AIMD snapshots of different simulation time, energy vs. time plots, equilibrated structures and band structures of BNNRs and GNRs containing different types of ELD, various spin configurations. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Acknowledgments SKP acknowledges research support from the CSIR and DST, Government of India.

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Insertion of Line Defect in Nanoribbons of Graphene, Boron Nitride

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