Tuning the electronic and magnetic properties of graphene/h-BN hetero nanoribbon: A first-principles investigation Tisita Das, Soubhik Chakrabarty, Y. Kawazoe, and G. P. Das
Citation: AIP Advances 8, 065111 (2018); doi: 10.1063/1.5030374 View online: https://doi.org/10.1063/1.5030374 View Table of Contents: http://aip.scitation.org/toc/adv/8/6 Published by the American Institute of Physics
AIP ADVANCES 8, 065111 (2018)
Tuning the electronic and magnetic properties of graphene/h-BN hetero nanoribbon: A first-principles investigation Tisita Das,1,a Soubhik Chakrabarty,1,2,a Y. Kawazoe,3 and G. P. Das1,b 1 Department
of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India 2 Department of Physics, School of Science, Adamas University, Barasat, Kolkata 700126, India 3 NICHe, Tohoku University, Sendai 980-8579, Japan (Received 21 March 2018; accepted 27 May 2018; published online 15 June 2018)
Inspired by the successful synthesis of phase separated in-plane graphene/h-BN heterostructures, we have explored the design of one dimensional graphene/h-BN hetero nanoribbon (G/BNNR). Using first-principles density functional based approach, the electronic and magnetic properties of the hybrid nanoribbons with mono-hydrogenated edges have been investigated for different configurations with alternative composition of C-C and B-N units in a ribbon of fixed width. Our results suggest that the electronic as well as magnetic properties of the ribbons can be regulated by varying the number of C-C (or B-N) units present in the structure. Both the hetero nanoribbons, either with N or B terminated edges, undergo a semiconductor-to-semimetal-to-metal transition with the increase in the number of C-C units for a fixed ribbon width. The spin density distribution indicates significant localization of the magnetic moments at the edge carbon atoms, that gets manifested when the number of C-C units is greater than 2 for most of the structures. © 2018 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/1.5030374
I. INTRODUCTION
Tunability of electronic, magnetic, mechanical and optical properties is a hallmark of low dimensional systems, making them ideally suited for various applications ranging from catalysis to energy storage to opto-electronic and other semiconducting devices. In particular, the avalanche of research on 2D materials during the past decade started with the advent of graphene having unique combination of superlative electronic and other properties such as linear spectrum, quantum hall effect, high electron mobility and high thermal conductivity etc.1,2 But the major drawback to deal with graphene sheet in many potential device applications, is its zero-gap semimetallic nature.3 Another 2D material that has drawn considerable attention of the scientific community, is hexagonal boron nitride (h-BN) sheet which is an iso-structural analogue of graphene, albeit with a large band gap of ∼5.9 eV.4,5 Due to its inherent properties such as chemical inertness and high thermal conductivity, h-BN is widely used as a defect free dielectric material.6,7 Various composite structures of graphene and h-BN have been attempted to be grown with the aim of tailoring the band gap to the range of ∼1 eV, which is ideal from the point of view of device applications. Vertically stacked graphene-h-BN heterostructure has been used for band gap engineering.8 Giovannetti et al. has reported the possibility of opening a small gap of ∼53 meV when graphene is placed on top of a h-BN monolayer.9 The other approaches suggested for tuning this band gap, are fabrication of laterally grown graphene-boron nitride (G/h-BN) hybrid structures.10–13 The nearly equal lattice parameters of a b
Tisita Das and Soubhik Chakrabarty contributed equally to this work Corresponding Author E-mail:
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graphene and h-BN has also stimulated the modeling of hybrid 2D ternary system (composed of B, C and N atoms) which has semiconducting property intermediate between semimetallic graphene and insulating h-BN nanosheets.14 Mixing C, B and N atoms with different stoichiometries have led to different layered binary composites BC3 , C3 N4 as well as ternary composites BCN, BC2 N, BC4 N etc.15–18 However the C-C and B-N bonds in BCN structure have a tendency to phase segregate and these structures are energetically less costly and thermodynamically more favourable to synthesize under certain experimental condition.10 Such phase separated h-BN and graphene islands result in the realization of a reasonable band gap.12,13 Several reported theoretical and experimental studies claim that such in-plane G/h-BN heterostructures can be used as promising candidates for device fabrication due to their relatively high carrier mobility, anti-ferromagnetic nature and half-metallic behaviour.19–22 It is worth mentioning here that this approach of band gap engineering by linking sp2 hybridized graphene and h-BN has also been extended to mixed sp-sp2 hybridized graphyne sheet and its BN analog.23 Another viable approach through which energy band structures of these two iso-structural sheets can be modified is via control of edge states of the 2D sheet of finite width. For example, truncating the graphene sheet in any one direction leads to quasi 1D graphene nanoribbon (GNR) which has a finite band gap.24 Depending upon shape and edge geometry, the nanoribbons can be categorized into two groups: (i) Armchair graphene nanoribbon (AGNR) and (ii) Zigzag graphene nanoribbon (ZGNR). Both the ribbons with edge carbon atoms passivated by atomic hydrogen exhibit semiconducting nature (with band gap inversely proportional to the width of the ribbon), whereas the magnetic nature of the ribbons depend on the edge geometry.1,25 In ZGNR, the localized electrons give rise to magnetic moment. The pseudospin symmetry in ZGNR is broken at the edges due to the presence of atoms from different sublattices, that results in antiparallel spin alignment between the edges.24,26,27 Conversely in case of AGNR, since the edges contain atoms from both the sublattices, the paired pseudospin is responsible for the cancellation of their magnetic moment.27 Due to the unpaired electrons at the edges, ZGNRs are found to possess magneto-resistance28 and spin transport behaviour 29 which make them a potential candidate for spintronic device application in contrast to AGNR. Similarly, Armchair boron nitride nanoribbon (ABNNR) shows nonmagnetic and semiconducting behaviour.30 On the contrary, Zigzag boron nitride nanoribbon (ZBNNR) can have either magnetic or nonmagnetic nature depending on the edge passivation.31,32 The motivation of the present work emanated from the successful synthesis of BCN monolayer with separately existing in-plane graphene and h-BN domains.33 This prompted us exploring a uniform graphene/h-BN hybrid phase separated nanoribbon (G/BNNR) by altering the C, B and N atoms in the ribbon systematically, while keeping the ribbon width fixed. Earlier research shows that band gap can be regulated by adjusting the ratio of C atoms to B and N atoms in graphene/h-BN composite superlattice structure.34 However, what remains unexplored is how the electronic and magnetic properties of G/BNNR of fixed ribbon width depends on the systematic variation of numbers of B and N atoms; and also how the properties of the hybrid ribbons get modified when the positions of the B and N atoms are altered. Hence, here we have reported a first-principles study of the G/BNNR structure, designed by tailoring the width of the phase separated hybrid monolayer sheet. Keeping in mind the fascinating properties of zigzag edges of GNR and BNNR we have focused only on the zigzag G/BNNR (Z-G/BNBNR). We systematically study the stability, electronic and magnetic properties of Z-G/BNNRs by varying the numbers of C, B and N atoms in the ribbons as well as altering the position of the B and N atoms and have found an approach for monitoring band gap in this structure. In order to investigate width dependencies on the properties of Z-G/BNNR ribbons of four different widths viz. 8, 9, 10, 11 have been considered. Result shows that electronic as well as magnetic behaviour depends upon the number of C-C (or B-N) units present in the structure as well as on the nature of edge terminations. II. COMPUTATIONAL METHODOLOGY AND STRUCTURAL MODEL
The model structure has been shown in Fig. 1. The hetero nanoribbon has been constructed by cutting the 2D hybrid G/h-BN monolayer sheet along zigzag direction. The atoms at the edges
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FIG. 1. Model structure of graphene/h-BN hetero nanoribbon of width 10 (10-Z-G/BNNR); (a) shows the geometry of a B terminated structure viz. 5C-C 5N-B and (b) shows the geometry of a N terminated structure viz. 5C-C 5B-N. Brown, green, silver and light pink balls represent carbon, boron, nitrogen and hydrogen atoms respectively.
are passivated by single hydrogen (H) atom in order to nullify the effect of the dangling bond. The dotted line shown in the figure represents the unit cell. The structures are periodic along Y direction and confined along X and Z directions. Depending on edge termination, the hetero nanoribbons are divided into two categories viz. boron (B) terminated structure and nitrogen (N) terminated structure. Fig. 1(a) represents B terminated structure where the BNNR unit is attached to GNR unit with carbon-nitrogen bond at the heterojunction region whereas Fig. 1(b) represents N terminated structure where the two regions are connected via carbon-boron bond at the junction. The nomenclature mC-C nN-B (mC-C nB-N) signifies that the unit cell structure of the ribbon contains ‘m’ number of zigzag units of GNR and ‘n’ number of zigzag units of BNNR respectively with B (N) terminated edge where (W = m + n) denotes the width of the ribbon. We have carried out a systematic study of the hetero Z-G/BNNR by varying the number of C-C (or B-N) units while keeping the ribbon width fixed, viz. varying ‘m’ from 1 to 9 (consequently ‘n’ varies from 9 to 1) in case of 10-Z-G/BNNR structures as shown in Fig. 2. The calculations reported here are not only restricted to the ribbon of width 10, but also those of width 8, 9 and 11 have been considered. All the calculations are carried out using first-principles density functional theory (DFT)35,36 as implemented in Vienna Ab Initio Simulation Package (VASP).37,38 To approximate the exchangecorrelation functional, generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE)39 form has been used. Projector augmented wave (PAW)40 potential was used to describe the ion-electron interaction. The energy cut off for the plane wave basis was set to 500 eV. Full ionic relaxations have been performed using conjugate gradient (CG) algorithm until the HellmanFeynman forces between two constituent atoms become smaller than 0.005 eV/Å and the energy convergence threshold for SCF loop was set to 1×10-6 eV. In our model supercells we have incorporated vacuum of length greater than 15 Å along X and Z directions, in order to eliminate the interaction between the images of two adjacent ribbons. Brillouin zone sampling has been done by using 1×9×1 and 1×45×1 Monkhorst-Pack 41 k-point grids during geometry optimization and electronic structure calculation. The band structure is calculated using 40 special k-points along the high symmetry direction Γ→Y.
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FIG. 2. Systematic variation of C-C and B-N units for 10-Z-G/BNNR.
III. RESULTS AND DISCUSSIONS A. Stability analysis
The binding energies per atom are calculated using the following formula EB =
E(mC-C nB-N)-2m × E(C)-n × E(B)-n × E(N) -2 × E(H) 2n + 2m + 2
Where E(mC − C nB − N), E(C), E(B), E(N) and E(H) represent the total energy of hybrid nanoribbons, isolated C, B, N and H atoms respectively as obtained from VASP calculation and ‘m’ and ‘n’ be the width of the graphene and B-N units in the structure as already discussed above. Fig. 3 shows the variation of binding energy with number of C-C units for 10-Z-G/BNNR for both B and N terminated configurations. Binding energies of the hetero nanoribbon increase linearly with the number of C-C units as is clearly evident from Fig. 3. This is a clear indication that the structures become more stable as we increase the carbon contents in the ribbon. This can be justified from the binding energies of their parent structures. The binding energies of graphene and h-BN monolayer are reported to be 7.66 eV/atom and 6.84 eV/atom respectively.42 Therefore, it is expected that stability
FIG. 3. Variation of binding energy with the number of C-C units for both N and B terminated 10-Z-G/BNNR. The data has been least square fitted to a straight line y = m ∗ x + c, with slope ‘m’ and y-intercept value given by ‘c’.
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TABLE I. The slope and the intercept values for N and B terminated Z-G/BNNR structures of different width under consideration. Structures 8-Z-G/BNNR: B terminated 8-Z-G/BNNR: N terminated 9-Z-G/BNNR: B terminated 9-Z-G/BNNR: N terminated 10-Z-G/BNNR: B terminated 10-Z-G/BNNR: N terminated 11-Z-G/BNNR: B terminated 11-Z-G/BNNR: N terminated
Slope in eV (m)
Intercept in eV (c)
-0.09 -0.09 -0.08 -0.08 -0.07 -0.07 -0.07 -0.07
-6.47 -6.50 -6.53 -6.56 -6.58 -6.61 -6.62 -6.65
also increases with the increase in graphene content (i.e. the number of C-C units) in this composite ribbon. Similar trend has been observed for ribbons of width 8, 9 and 11. The corresponding plots for other ribbons (w = 8, 9 and 11) have been given in supplementary material (Fig. S1 in supplementary material). (The binding energies of the hetero ribbons of width 8, 9, 10, 11 for both B and N terminated configurations are listed in Table SI in supplementary material.) The slope and the intercept values as obtained from the plots of ribbons of width 8, 9, 10 and 11 are tabulated in Table I. The intercept values for N terminated structures are greater than that of B terminated ones by 0.03 eV, irrespective of ribbon width, indicating higher stability of the N terminated ribbons over the B terminated ones by 30 meV. B. Electronic structure analysis
In order to study the effect of number of C-C or B-N units on the electronic properties of the hetero ribbons, we have analyzed their DOS and band dispersions. The electronic band structures of the selected hetero nanoribbons (of width 10) are plotted along high symmetry path of hexagonal Brillouin zone (Fig. 4). The band gaps are found to depend on the number of C-C units present in
FIG. 4. Band structure plot of different B terminated 10-Z-G/BNNR. (a) 1C-C 9N-B (b) 2C-C 8N-B (c) 3C-C 7N-B (d) 4CC 6N-B (e) 5C-C 5N-B (f) 9C-C 1N-B. Gradual transition from semiconductor to semimetal to metal has been observed. The Fermi level is marked at zero and denoted by black solid line. Blue and red lines represent the bands for up and down spin channel respectively. Inset pictures put in (c) and (d) show the zoomed view at Fermi level near Y point for a clear understanding of the semimetallic nature of the corresponding ribbons. [Note: Band structure plots for 6C-C 4N-B, 7C-C 3N-B and 8C-C 2N-B are not included as they are metallic and quite similar to that of 5C-C 5N-B and 9C-C 1N-B].
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the structure. For a lesser number of C-C units both B and N terminated ribbons are found to behave like non magnetic semiconductor irrespective of the ribbon width. As we increase the width of the carbon section (which in principle decreases the B-N portion for fixed ribbon width) in the structures, the band gap keeps on decreasing and vanishes after a certain number of C-C units. This result is in good agreement with previously reported theory by Kan et al. where they observed continuous reduction of band-gap of hetero graphene/h-BN sheet with increasing concentration of graphene units.13 As a consequence a semiconductor to semimetal to metal transition has been found to occur. Fig. 4(a) and 4(b) denote the band structure of 1C-C 9N-B and 2C-C 8N-B respectively. The former structure has a band gap value of 1.03 eV which decreased to 0.38 eV with the increase of one C-C unit in the later structure. On further increase of C-C unit the structures start to behave like a semimetal. 3C-C 7N-B and 4C-C 6N-B are semimetal as is clearly observed from Fig. 4(c) and 4(d). In order to get a clear outlook of these two structures, the zoomed view near the Fermi level has been put in the inset of Fig. 4(c) and 4(d). The up spin channel for these above two structures are semiconducting having a direct band gap of 0.61 eV and 0.65 eV respectively, whereas for the down spin channel both the conduction and valence bands touch the Fermi level, thereby revealing its semimetallic nature. Fig. 4(e) and 4(f) represent the band diagram of the ribbon with higher carbon concentration (5C-C 5N-B and 9C-C 1N-B respectively) which turn out to be metallic in nature.
TABLE II. Band gap values for the considered Z-G/BNNR structures of different width. N terminated structures Ribbon width (W)
8
9
10
11
B terminated structures
Width of C-C unit (m)
Band gap (eV)
Nature
Band gap (eV)
Nature
1 2 3 4 5 6 7
0.59 0.09 0 0 0 0
Semiconductor Semiconductor Semimetal Metal Metal Metal Metal
1.03 0.38 0 0 0
Semiconductor Semiconductor Semimetal Semimetal Metal Metal Metal
1 2 3 4 5 6 7 8
0.59 0.09 0 0 0 0 0
Semiconductor Semiconductor Semimetal Metal Metal Metal Metal Metal
1.03 0.38 0 0 0 0
Semiconductor Semiconductor Semimetal Semimetal Metal Metal Metal Metal
1 2 3 4 5 6 7 8 9
0.57 0.11 0 0 0 0 0 0 0
Semiconductor Semiconductor Metal Metal Metal Metal Metal Metal Metal
1.03 0.38 0 0 0 0 0
Semiconductor Semiconductor Semimetal Semimetal Metal Metal Metal Metal Metal
1 2 3 4 5 6 7 8 9 10
0.59 0.10 0 0 0 0 0 0 0
Semiconductor Semiconductor Semimetal Metal Metal Metal Metal Metal Metal Metal
1.03 0.37 0 0 0 0 0 0
Semiconductor Semiconductor Semimetal Semimetal Metal Metal Metal Metal Metal Metal
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Values of the energy band gaps corresponding to different hetero ribbons of width 8, 9, 10 and 11 are tabulated in Table II for both N and B terminated configurations. The table shows that the hetero nanoribbons are semiconducting and independent of ribbon width when the number of C-C units n ≤ 2. For example, the N terminated configurations 1C-C 7B-N, 1C-C 8B-N, 1C-C 9B-N, 1C-C 10B-N have very similar values of the band gap. Similarly the B terminated structures 1CC 7N-B, 1C-C 8N-B, 1C-C 9N-B, 1C-C 10N-B have same band gap values. For n > 2 metallicity is induced in the hetero nanoribbons. Most of the ribbons are found to be semimetal for n=3/4. For n ≥ 5 all the hetero nanoribbons are found to be metallic. Thus the electronic properties of these hetero nanoribbons can be tailored from semiconductor to semimetal to metal by controlling the number of C-C or B-N units. C. Spin density analysis
In order to investigate the magnetic behaviour of the hetero nanoribbons we have analyzed their spin densities. The spin density distributions for the different configurations of N as well as B terminated Z-G/BNNR of width 10 have been shown in Fig. 5. The structures are non magnetic for lower number of C-C units. For the N terminated hetero nanoribbons, magnetic behaviour is observed with number of C-C units m ≥ 2. Whereas for the B terminated structures magnetism is introduced for m ≥ 3. The moment values are maximum at the C edges and decrease as we move away from the C edge; however enhancement of moments has been observed at the junction of C-C and B-N units, for most of the configurations. As already mentioned, graphene and BN units are coupled to each other via C-N bonds in case of B terminated nanoribbons, whereas for the N terminated ones the two units are connected via C-B bonds. The spin density plots clearly suggest that, the moments induced at the N atoms of C-N interface are much larger compared to that at B atoms of C-B interface. The magnetic moments at the C-N interface, are found to be aligned parallelly on C and N atoms. These findings match well with the earlier published reports.11,13 Moments on the edge B or N atoms are found to be vanishingly small. The moments at the C edges is attributed to the spin splitting of C-2pz orbital. The vanishingly small moments at the N edges are mainly due to the presence of paired pz electrons. On the other hand, due to the absence of pz electrons in B atoms, no localized magnetic
FIG. 5. Spin density distributions for different (a) N terminated (b) B terminated 10-Z-G/BNNR configurations. All the spin density plots are shown for the same iso-surface value of 0.002 e/Å3 .
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moments have been observed at the B edges. Ribbons of width 8, 9 and 11 also show similar magnetic features (see Fig. S9, S10, S11 of supplementary material). IV. CONCLUSIONS
In summary, we have designed a new model of 1D graphene/h-BN hybrid nano composite that is zigzag graphene/h-BN hetero nanoribbon with mono-hydrogenated edges and presented an approach for regulating the electronic properties of these materials using first principles method. According to edge termination the ribbons can be categorized into two classes viz N terminated and B terminated G/BNNR. The calculated binding energy suggested that the former structure is more stable than the later. The band gap size of the ribbons can be controlled effectively by changing the C-C or B-N proportions in the structure. As we increase the width of graphene portion a semiconductorto-semimetal-to-metal transition is found to occur. Also the structural stability of the ribbons for a fixed width, are found to increase proportionally with the number of C-C unit present in the structure. In addition, we found that, the width of the C-C units plays an important role for controlling the magnetic behaviour of the ribbons. Spin density distribution confirms that the moment at carbon edges are mainly responsible for magnetic nature of the ribbons and in most of the structures moment arises when number of C-C unit crosses m>2. SUPPLEMENTARY MATERIAL
See supplementary material for the plot of electronic structures and spin densities of other (of width 8, 9 and 11) hetero nanoribbons. ACKNOWLEDGMENTS
GPD gratefully acknowledges the financial support received from the Dept. of Atomic Energy, Govt. of India (DAE) for the IBIQUS project. TD acknowledges the financial support from DST INSPIRE (IF140751). GPD also thanks the staff of the Center of Computational Materials Science at IMR for the use of Hitachi SR 11000-K2 Supercomputing facility, where part of the computations has been carried out. YK would like to acknowledge support from JSPS KAKENHI Grant Number 17H03384. 1 K.
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