Enhancement of the surface reactivity of zigzag boron ...

Journal of Physics and Chemistry of Solids 120 (2018) 34–43

Contents lists available at ScienceDirect

Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs

Enhancement of the surface reactivity of zigzag boron nitride nanoribbons by chlorine gas decoration: A computational study

T

Kiran Kumar Surthia, Bibekananda Deb, J. Ramkumarb, Kamal K. Kara,b,∗ a b

Advanced Nanoengineering Materials Laboratory, Materials Science Programme, Indian Institute of Technology Kanpur, Kanpur, 208016, India Advanced Nanoengineering Materials Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur, 208016, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Charge density Electronegativity First principles calculation Negative differential resistance Transmission

In this study, we explored the field emission and chlorine sensing mechanisms of zigzag boron nitride nanoribbons (ZBNNRs). First principle calculations were used to investigate the surface reactivity of ZBNNRs within the framework of the density functional theory based on the local density approximation and generalized gradient approximation. Six different possibilities were considered for chlorine-gas termination and adsorption on ZBNNRs. The band gap of the ZBNNRs decreased monotonically as the ribbon width increased. The charges on the functional groups and corresponding bond lengths, i.e., B(N)-X and X-X (Cl-Cl), were dependent on the adsorption sites (B or N edge) and the types of functional groups, but independent of the ribbon width. Chlorine gas (Cl2) induced electrons into the conduction band and their consequent electronic states over the Fermi level enhanced the electrical conductivity. Transmission spectra demonstrated the spin-dependent electron tunneling from the valence band to the conduction band. Charge density analysis indicated the charge trapping behavior of the ZBNNRs. Cl2 introduced excess negative charges onto the active surfaces of the ZBNNRs to enhance their surface reactivity. Therefore, Cl2-decorated ZBNNRs can be employed as Cl2 sensors and they may also be useful in field emission applications.

1. Introduction The latest development of graphene is attracting much attention because of its excellent characteristics in terms of the electronic, electrochemical, thermal, and mechanical properties, which make it a promising candidate for use in various technological applications [1–4]. Boron nitride is also a well-known ceramic material with excellent properties, such as low density, a high melting point, excellent thermal conductivity, high dielectric strength, and good electrochemical resistance in both hazardous and moist environments [5,6]. Boron (2p1) and nitrogen (2p3) are neighboring elements to carbon, and the electronic structure of boron nitride is identical to that of carbon (2s22p2) [6], which also demonstrates why their structural polymorphs are analogous to carbon polymorphs [7]. Hexagonal boron nitride (h-BN) comprises boron and nitrogen atoms in a honeycomb arrangement with an sp2 hybridized two-dimensional structure [8]. h-BN has interesting characteristics including a large surface area, lubricating nature, wide band gap, very low dimensional porosity, high anisotropy, excellent thermal and optical properties, chemical inertness, light emission in the deep ultraviolet region, and negative electron affinity (NEA) [8–11]. Boron nitride nanoribbons (BNNRs) have been synthesized successfully

∗

with several methods and employed in different technological applications [12–14]. A one-dimensional thick strip is carved out of twodimensional h-BN to form BNNRs [15]. According to the earlier convention used for graphene nanoribbons, BNNRs are classified based on the number of zigzag chains or lines over the ribbon width, which are represented as Nz-ZBNNR. In contrast to armchair structures, the number of dimer lines over the width of the BNNRs is denoted as NAABNNR [15,16]. In general, nanoribbons comprise vacancies, unsaturated bonds along both edges of the ribbon, and dangling bonds, where their linear combinations lead to eigenstates in the vicinity of the Fermi level. Both edges are the only active sites for the chemical modification of nanoribbons [16,17]. Recently, several theoretical studies have investigated BN structures with different morphologies. Mirzaei et al. studied the effects of oxygen termination and sulfur doping on the electronic structure, quadrupole coupling constants, and bond lengths of zigzag BN nanotubes [18–22]. Loh et al. reported the enhancement of NEA when a boron nitride nanofilm was exposed to hydrogen [23]. Wu et al. described the effects of chemical decoration using various functional groups on the band gap of ZBNNRs [24]. The chlorine (Cl) sensing properties of ZBNNRs and negative differential resistance in bare-

Corresponding author. Advanced Nanoengineering Materials Laboratory, Materials Science Programme, Indian Institute of Technology Kanpur, Kanpur, 208016, India. E-mail address: [email protected] (K.K. Kar).

https://doi.org/10.1016/j.jpcs.2018.04.021 Received 24 September 2017; Received in revised form 14 April 2018; Accepted 16 April 2018 Available online 21 April 2018 0022-3697/ © 2018 Elsevier Ltd. All rights reserved.

Journal of Physics and Chemistry of Solids 120 (2018) 34–43

K.K. Surthi et al.

Fig. 1. Schematic of various ZBNNR configurations: (a) ZBNNR2H, (b) ZBNNRCl2B, (c) ZBNNRCl2N, (d) ZBNNRCl2B-Cl2N, (e) ZBNNRAdCl2B, (f) ZBNNRAdCl2N, and (g) ZBNNRAdCl2B-AdCl2N (blue, brown, white, and green spheres are N, B, H, and Cl, respectively). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

solver for density measurement, the limited Broyden–Fletcher–Goldfarb–Shanno method for optimization based on 200 iterations, and the Pulay mixer algorithm. Using the Monkhorst–Pack grid, k-points sampling was set to 1 × 1 × 100 in the Brillouin zone along the x, y, and z directions, respectively. The periodicity of the ribbon was maintained along the z-axis. To calculate the band structure (BS), we selected 20 points per segment and considered all of the bands above the Fermi level. For the density of state (DOS) calculations, the spin up and down components were calculated with respect to 8 × 8 grid points and an energy range of −10 to +10 eV. The transport properties were investigated according to the Wolfsberg weighting scheme using the extended Hückel calculator without biasing. In the extended Hückel calculator, we selected the Hückel basis set with self-consistent calculations and an energy range of −8 to +8 eV. Recursion was used for the self-energy calculator. The spin up and down components were calculated with respect to 8 × 8 k-points sampled in the x and y directions, respectively, which were periodic in nature. The ZBNNR along the z-axis was considered as the central region, which was finite and non-periodic along the transport direction. All of the configurations were optimized without any constraints until the force on each atom was less than 0.01 eV/Å. After complete optimization under both LDA and GGA, the bond lengths of B-N, B-H, and N-H were 1.46, 1.06, and 1.1 Å, respectively, which are highly consistent with the previously reported values (1.43, 1.02, and 1.20 Å) [15,24]. We calculated the total energies with respect to the equilibrium (optimized) states of the configurations considered. The binding energies were calculated using equation (1):

ZBNNRs were investigated by Srivastava et al. [25]. However, to the best of our knowledge, there have been no previous studies of the effects of Cl2 decoration on the surface reactivity of ZBNNRs. Therefore in the present study, we investigated whether Cl2 decoration can changes the electronic properties, transport properties, and surface reactivity of single edge Cl2-terminated ZBNNRs (ZBNNRCl2B and ZBNNRCl2N), both edge Cl2-terminated ZBNNRs (ZBNNRCl2B-Cl2N), single edge Cl2-adsorbed on ZBNNRs (ZBNNRAdCl2B and ZBNNRAdCl2N), and both edge Cl2-adsorbed on ZBNNRs (ZBNNRAdCl2B-AdCl2N). The various ZBNNR configurations are shown in Fig. 1. It should be noted that two layers of Cl2 adsorption are possible on ribbons, where adsorption may occur on the upper surface of the ribbon or on the opposite surface of the ribbon (Fig. S1). However, in the case of termination, two layers cover the two edges of the ribbon (Fig. 1(d)). We conducted calculations to determine the potential of band gap engineering for ZBNNRs with Cl2. Electronegativity (EN) has a considerable impact on charge distribution analysis so it is useful to know the relevant value. The EN values for chlorine (Cl), nitrogen (N), hydrogen (H), and boron (B) are 3.16, 3.04, 2.2, and 2.04, respectively [24]. 2. Computational details To investigate the electronic, transport, and charge density properties of Cl2-decorated ZBNNRs, we conducted first principles calculations based on the two exchange-correlations comprising the local density approximation (LDA) and generalized gradient approximation (GGA) within the framework of density functional theory (DFT), as implemented in the atomistic tool kit–virtual nano lab (ATK-VNL) computational package [26]. LDA was applied as proposed by Perdew and Zunger [27]. GGA in the Perdew–Burke–Ernzerhof form was employed as the exchange-correlation [28]. All the configurations of the ZBNNR model were considered within the supercell and they were separated by 10 Å to avoid inter-ribbon interactions. To optimize the geometry, we used LDA and GGA as exchange-correlations, with the double –ξ basis set to solve the Kohn–Sham equations under these exchange-correlations, 100 Rydberg as the density mesh cut-off energy of the expanded wave function, a force tolerance of 0.05 eV/Å, stress of 0.01 GPa, step size of 0.2 Å, the fast Fourier transform as the Poisson

EB = E T − Ebare − n Ei,

(1)

where ET is the total energy of the ZBNNR with impurity atoms (H and Cl2), Ebare is the total energy of the bare ZBNNR, n is the number of impurity atoms, and Ei is the total energy of an isolated impurity atom [17]. The termination/adsorption energies of impurities (H and Cl2) were calculated using equation (2):

35

Journal of Physics and Chemistry of Solids 120 (2018) 34–43

K.K. Surthi et al.

Table 1 Total energies, binding energies, adsorption/termination energies, charge analysis results, and bond lengths of chemical functional groups calculated using DFT-LDA. Functional group

Total energy (eV)

Binding energy (eV/ group)

Adsorption/ termination energy (eV/group)

Charge (e–) (Coulomb)

Bond length (Å)

H(N)-H(B) Cl2(N)-Cl2(B) t Cl2(N)-H(B) H (N)-tCl2(B) a Cl2(N)-H(B) H(N)-aCl2(B) A Cl2(N)-Cl2(B)

−28.5397 −46.5180 −37.5193 −37.5384 −37.6828 −37.6823 −46.8255

−11.07 −6.56 −7.86 −9.77 −24.26 −24.61 −37.30

−11.04 −6.52 −17.86 −9.74 −0.39 −0.74 −26.23

0.22*, −0.086** 0.126*, −0.02** 0.136*, −0.084** 0.222*, −0.054** 0.042*, −0.086** 0.22*, −0.02** 0.28*, −0.092** /0.218*, −0.016**

1.06b, 1.1d 1.84b, 1.81d 1.83b, 1.1d 1.07b, 1.8d 1.04b, 1.2d 1.07b, 1.4d 1.02b, 1.8d /1.07b, 1.4d

T

*Charge analysis for the chemical group connected to the N-edge of ZBNNR. **Charge analysis for the chemical group connected to the B-edge of ZBNNR. b and d are the lengths of N-X and B-X (X = H, Cl). A and a are both edges and single edge adsorption, respectively. T and t are both edges and single edge termination, respectively.

termination (ZBNNRCl2B, ZBNNRCl2N, and ZBNNRCl2B-Cl2N), and three other dissimilar configurations for adsorption (ZBNNRAdCl2B, ZBNNRAdCl2N, and ZBNNRAdCl2B-AdCl2N), where the width of Nz varied from 2 to 8 with a step size of one using the LDA and GGA method. The results showed that the electronic profiles near the Fermi layer depended on both the edge structure of the ribbon and the type of functional group attached. The charge on the functional groups and corresponding bond lengths (B(N)–X and X-X (Cl-Cl)) were dependent on the adsorption sites (B or N edge) and the type of functional group. However, they were independent of the ribbon width because the charge occupation of the atomic orbitals could be enhanced along the ribbon width, but there was no effect on the bond length. Therefore, there was no effect of the ribbon width on the bond lengths for B(N)–X and X-X (Cl-Cl) [24]. For ZBNNR2H, the band gap decreased as the ribbon width increased and there could be an increment in the atomic orbital charge occupation, which was due to the additional atoms as the ribbon width increased. This charge could explain the extra energy state near the Fermi level and the decrease in the band gap. However, it was independent of the ribbon width for the six different configurations with terminations and adsorptions because Cl2 induced energy states over the Fermi level. The lower width (Nz = 2–6) configurations with both terminations and adsorptions were unstable due to the insufficient equilibrium energy and high electrostatic potential, respectively, which could lead to the spontaneous movement of atoms from the edges of the ribbon. We only show the results with a width (Nz) of eight in Fig. 1 among all the

E= [Etotal (BNNR − X) + n Etotal (H) − Etotal (BNNR − H) − n Etotal (X)] (2)

/n,

where Etotal is the total energy of the system, X = H and Cl2, and n is the number of impurity atoms [24]. The transport properties depend mainly on the chemical potential (V) and number of charge carriers with energy (E), as calculated using equation (3):

T(E, V) = Tr [TR (E, V) GC (E, V)],

(3)

where τR is the coupling matrix, GC is the Green function, and the transmission coefficient is expressed as follows.

T(E) =

∑ Tij (E ) (4)

ij

The binding and termination/adsorption energies, including the total energy of impurities (H and Cl2), were calculated with respect to equilibrium states of the bare ZBNNR configurations, as shown in Tables 1 and 2. The detailed calculations for the termination energy, adsorption energy, and total energy are given in the supplementary information. 3. Results and discussion We calculated various properties based on both edges of the Hterminated ZBNNR (ZBNNR2H), three dissimilar configurations for

Table 2 Total energies, binding energies, adsorption/termination energies of chemical functional groups, charge analysis results, and bond lengths (Å) calculated using DFTGGA. Functional group

Total energy (eV)

Binding energy (eV/ group)

Adsorption/ Termination energy (eV/group)

Charge (e−) (Coulomb)

Bond length (Å)

H(N)-H(B) Cl2(N)-Cl2(B) t Cl2(N)-H(B) H(N)- tCl2(B) a Cl2(N)-H(B) H(N)-aCl2(B) A Cl2(N)-Cl2(B)

−28.4901 −46.4655 −37.6355 −37.4870 −37.6328 −37.6358 −46.782

−11.06 −1.31 −7.64 −9.58 −24.16 −24.46 −32.97

−11 −1.31 −13.37 1.48 −0.3 −0.6 −21.9

0.23*, −0.11** 0.126*, −0.092** 0.172*, −0.11** 0.232*, −0.052 ** 0.002*, −0.112** 0.23*, −0.02** 0.50*, −0.092** /0.218*, −0.018**

1.06b, 1.1d 1.84b, 1.81d 1.83b, 1.1d 1.07b, 1.8d 1.2b, 1.04d 1.07b, 1.4d 1.2b, 1.04d /1.07b, 1.4d

T

*Charge analysis results for the chemical group connected to the N-edge of ZBNNR. **Charge analysis results for the chemical group connected to the B-edge of ZBNNR. b and d are lengths of N-X and B-X (X = H, Cl). A and a are both edges and single edge adsorption, respectively. T and t are both edges and single edge termination, respectively. 36

Journal of Physics and Chemistry of Solids 120 (2018) 34–43

K.K. Surthi et al.

Fig. 2. BS results for various configurations: (a) ZBNNR2H, (b) ZBNNRCl2B, (c) ZBNNRCl2N, (d) ZBNNRCl2B-Cl2N, (e) ZBNNRAdCl2B, (f) ZBNNRAdCl2N, and (g) ZBNNRAdCl2B-AdCl2N.

configurations considered, and the others are shown in Figs. S2–S8. 3.1. Electronic properties The BS results for various configurations are shown in Fig. 2. Fig. 2(a) shows the BS for ZBNNR2H with a band gap of 4.0 eV. The results showed that the hydrogenation of ZBNNRs was an endothermic process (−11.06 eV). Our calculations showed that the band gap decreased monotonically from 4.56 to 4.0 eV as the ribbon width increased (Nz = 2 to 8), and the corresponding values were highly consistent with the previously reported values given in Table 3. The decrease in the band gap depended on both the N and B edges. Therefore, as the ribbon width increased, the charge occupation of the atomic orbital increased to result in the formation of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), which led to shifts in the energy states upward or downward near to the Fermi level. This may explain the decrease in the band gap as the ribbon width increased. However, the BS determined for bare ZBNNR, as shown in Fig. 3, demonstrated that the electronic states exhibited good degeneracy, where the vacancy and dangling bonds at both edges of the bare ZBNNRs disappeared when they were saturated with σ-bonding of the H-atom. The H-1s electron bonded with the unpaired electrons in the N-2p and B-2p orbitals. The strong B-H and N-H bonds yielded a semi-metallic to semiconductor transition.

Fig. 3. BS results for bare ZBNNR.

The BS results for ZBNNRCl2B are shown in Fig. 2(b). Cl2-termination was an endothermic process for all of the configurations considered. The electronic states of the valence band (VB) and conduction band (CB) were fully degenerate. The N-edge was hydrogenated and Cl2 was attached at the B-edge. The H-1s electron was stable due to σ-bonding with N. The Cl-4s orbital with an unpaired electron was bonded chemically via π-bonding with the B-2s (sp2-hybridized)-orbital. Therefore, the empty B-pz orbital induced an unoccupied state over CB, which led to LUMO and the single electron in the Cl-2py orbital was available for conduction. This may explain the origin of the electronic states over the Fermi level and the initiation of the HOMO in VB. Fig. 2(b) shows that there were two energy bands near the г-point, where one had flat energy dispersion and other had a curved nature. The BS results for ZBNNRCl2N shown in Fig. 2(c) demonstrate that the electronic states near the г-point and over the Fermi level were completely degenerate and they exhibited curve dispersion. The subbands of the BS are also shown, which provided channels for electron movement from the CB to VB. The liberal movement of electrons from the VB to CB resulted in the disappearance of negative differential

Table 3 Energy band gap Calculated for ZBNNR2H using the LDA and GGA methods for various ribbon widths, and comparisons with their reported values. Ribbon width

2 4 6 8

Calculated energy gap (eV) (LDA)

(GGA)

4.56 4.34 4.22 4.0

4.66 4.46 4.35 4.27

Reported theoretical values (eV) [24]

4.42 4.25 4.12 4.0

37

Journal of Physics and Chemistry of Solids 120 (2018) 34–43

K.K. Surthi et al.

level in the CB. This also indicates that the bare ZBNNR was half metallic for spin down states and a semiconductor for spin up states. The electronic states of the bare ZBNNR vanished after hydrogen termination and the DOS results are shown in Fig. 5. All of the spin up and down electronic states of the VB and CB were completely degenerate and mirror images of each other (Fig. 4(a)). At the B-edge, the empty 2pz orbital of the B-atom contributed to the electronic states near the Fermi level in the CB as well as the energy levels in the VB due to the 2pz orbital of the edge N-atom. Projected DOS (PDOS) analysis was conducted with respect to the equilibrium state of the ribbon for selected impurities (H and Cl2). Fig. 6(a) shows the PDOS results for ZBNNR2H, which indicates the equal availability of the spin up and down states, and the band gap of 4 eV. These findings also support the BS and DOS results. The DOS results for ZBNNRCl2B are shown in Fig. 4(b). The electronic state preferentially localized near the Fermi level in the CB due to the empty 2pz orbital of the B-atom and the single electron of the Cl-2py orbital, which were responsible for both the conduction and electronic states in the VB. Moreover, the energy window was completely shifted into the CB and this led to a metallic nature as well as a symmetric nature, which persisted in the spin up and down electronic states. As shown in Fig. 6(b), the PDOS results for ZBNNRCl2B were analogous and they agreed well with the BS studies. Fig. 4(c) shows the DOS profile for ZBNNRCl2N. The HOMO preferentially localized over the Cl2 terminated edge, where Cl2 contributed electrons as a donor and N accepted electrons because its electron accepting probability is much higher compared with B. The Nedge sites were more favorable for chlorine gas termination compared with the B-edges. The electronic states over the Fermi level were due to the single electron from the Cl-3py orbital. Therefore, similar results were obtained based on the PDOS analysis of ZBNNRCl2N (Fig. 6(c)) and they agreed well with the corresponding DOS and BS results (Fig. 4(c)). The spin up and down states with symmetry indicated the spin-dependent magnetic behavior of ZBNNRCl2N, which is useful for volatile memory device applications [29]. The DOS profile for ZBNNRCl2B-Cl2N is illustrated in Fig. 4(d), which shows that the spin up and down states were symmetric in nature. According to Fig. 4(d), the opening of the band gap in the CB indicates that the pathway of the electron wave function as an energy state over the Fermi level provided a channel for the unrestricted movement of electrons and the NDR vanished as a result. In addition, as the number of electronic states over the Fermi level increased, the corresponding conduction channels also increased. Therefore, the density of the conduction channels was inversely proportional to NDR, and the corresponding equation (S8) is given in the supplementary information. The energy states over the Fermi level were due to the single electron in the Cl-3py orbital, which was available on both sides of the ribbon. The PDOS results in Fig. 6(d) are also similar and they demonstrate the good agreement with the DOS results and BS findings. The DOS profile for ZBNNRAdCl2B is illustrated in Fig. 4(e), which shows clearly that the energy window shifted toward the CB. The fully degenerate spin up and down states were symmetrical on both sides of the Fermi level. Hydrogen was present on both sides of ribbon, which may explain the NEA characteristics [23,29]. Similar results were found according the PDOS analysis (Fig. 6(e)) and they agreed well with the BS findings. The DOS profile for ZBNNRAdCl2N is illustrated in Fig. 4(f), which shows that all of the spin up and down states were symmetric in nature. The single electron in Cl-2py and the unpaired electron in the N-2s (sp2hybridized) orbital were responsible for the electronic states over the Fermi level. Similar results were suggested by PDOS analysis, i.e., the single electron in Cl-2py and unpaired electron in the N-2s (sp2-hybridized) orbital were responsible for the electronic states over the Fermi level (Fig. 6(f)) and this agreed well with the BS findings. The DOS profile for the ZBNNRAdCl2B-AdCl2N configuration is shown in Fig. 4(g), which demonstrates that the Cl2 on both sides induced

resistance (NDR). As the number of sub-bands increased, the corresponding conduction channels also increased and NDR decreased. Therefore, NDR was inversely proportional to the density of sub-bands in the BS, and the corresponding equations (S8–S10) are given in the supplementary information. Hence, ZBNNRCl2N was shown to be metallic in nature. The unpaired N-2px-orbital electron was bonded chemically via π-bonding with the Cl-4s orbital electron. Therefore, the Cl3py orbital with a single electron was available for conduction. In addition, at the B-edge, the unpaired electron in the H-1s orbital was saturated by the σ-bond with a single electron in the B-2s orbital. Fig. 2(d) shows the BS results obtained for ZBNNRCl2B-Cl2N, where the unrestricted electron movement over the Fermi level is clearly illustrated. Four electronic states were present near the г-point, with two from the B-edge and two others from the N-edge. At the B-edge, the Cl4s orbital with unpaired electrons was bonded via π-bonding with the B-2s (sp2-hybridized) orbital. Similarly, at the N-end, an unpaired N-2px orbital was chemically bonded via π-bonding with the Cl-4s orbital. The BS results for the ZBNNRAdCl2B configuration are shown in Fig. 2(e). Cl2-adsorption was also endothermic in nature for all of the possibilities considered. Fig. 2(e) shows that the quadratic energy bands due to the H-atom were away from the Fermi level. The unoccupied B1s orbital provided electronic states to the LUMO and its single electron was available for conduction. The B-edges were more favorable for adsorption than the N-edges due to the greater electrostatic potential among N, Cl2, and H compared with that among B, Cl2, and H, which was also responsible for the higher stability of the B-edge structure than the N-edge. The H-atom bonded with the B-atom via σ-bonding. The Cl4s1 orbital with a single electron was chemically bonded via π-bonding with the B-2s1 orbital. Unpaired electrons on the Cl (2Py) and B(1s1) atoms were responsible for conduction over the Fermi level, which made ZBNNRadCl2B metallic in nature; therefore, NDR became zero. The electronic states over the Fermi level led to the formation of the HOMO in the VB, as indicated clearly in Fig. 2(e), and the empty B-2Pz orbital led to the formation of the LUMO. At the N-edge, the H-atom was bonded with the N-pz orbital via σ-bonding. The BS results for ZBNNRAdCl2N are illustrated in Fig. 2(f). With adsorption, an extra H-atom was present on both edges of the ribbon as well as Cl2 at the N-edge. We found that two energy levels exhibited curve dispersions near the г-point and Z-point. The other energy level near the г-point was due to the unoccupied 2pz-orbital of the edge Batom, as shown clearly in Fig. 2(f). The Cl-3s (sp2-hybridized) orbital with a single electron was bonded via π-bonding with the N-4s electron. The single (2py) electron of Cl and the unpaired electron of the 2s (sp2hybridized) orbital of N were available for conduction. Therefore, the electronic states over the Fermi level improved the electrical conductivity. The instability of the configuration was due to the electrostatic potential among N, H, and Cl2. Therefore, the Cl atoms of Cl2 tended to escape from the edges spontaneously. Fig. 2(g) shows the BS results for ZBNNRAdCl2B-AdCl2N. We observed two electronic states near the г-point with curved dispersion via the Fermi level, which made the ribbon conducting in nature. In addition to the Cl2 molecule, H-atoms were present on both sides of ribbon. At the N-edge, the Cl-3s (sp2-hybridized) orbital with a single electron was bonded with the 4s1 electron of N via π-bonding. Thus, the single (2py) electron of Cl and the unpaired electron of the 2s (sp2-hybridized) orbital of N were available for conduction. At the B-edge, the H-atom bonded with B via σ-bonding, the Cl-4s1 orbital bonded with the 2s1orbital of the B-atom via π-bonding, and the unpaired electrons of Cl (2Py) and B (1s1) atoms were also available for conduction. The DOS results for various configurations are shown in Fig. 4. Fig. 4(a) illustrates the DOS for ZBNNR2H, which clearly indicates that no energy bands were found within the energy band gap of −1.0–3.03 eV. Similar results were also obtained based on the BS. The electronic states associated with the vacancy and dangling bonds produced the HOMO, which localized near to the Fermi level in the VB, whereas the LUMO was preferentially localized away from the Fermi 38

Journal of Physics and Chemistry of Solids 120 (2018) 34–43

K.K. Surthi et al.

Fig. 4. DOS results for various configurations: (a) ZBNNR2H, (b) ZBNNRCl2B, (c) ZBNNRCl2N, (d) ZBNNRCl2B-Cl2N, (e) ZBNNRAdCl2B, (f) ZBNNRAdCl2N, and (g) ZBNNRAdCl2B-AdCl2N.

respect to the band energies, which are explained very well by the BS results, as discussed above. The derivative of the band energies provides the group velocities using the Fourier expansion. The transmission spectrum (TS) indicates the probability of an electron with incident energy E moving from one extremity of the channel to the other extremity. Transmission pathway analysis is characterized by the transmission coefficient for the local bond contribution. The transmission over the ribbon at a particular electron energy is equal to the transmission probability time, the number of channels, and the pathway across the atoms in the ribbon, where their summation gives the transmission coefficient (TC) [30]. The TS for ZBNNR2H is depicted in Fig. 7(a), which indicates the tunneling of electrons. The corresponding energy level gives the spindependent TS over the Fermi level. The spin up and down states with a symmetric nature confirmed the equal contribution of electrons to the TS. The peaks due to the degeneracy of the HOMO and LUMO disappeared around the Fermi level. No transmission was observed over the Fermi level within the energy range of −1.04–3.01 eV, which was equivalent to the energy band gap, and this demonstrated the semiconducting nature. In the TS (Fig. 7(a)), the positions of the transmission peaks depend mainly on the energy levels of the atomic orbitals and self-consistent eigenvalues. We found two sharp transmission peaks on either side of the Fermi level, which showed that the electron could readily overcome an energy barrier if external energy was supplied. Two sharp transmission peaks were found at different energy levels in the VB and CB, and the corresponding TC value is given in Table 4. The TS profile for ZBNNRCl2B is shown in Fig. 7(b). An energy window shifted into the CB and this indicated Fowler–Nordheim tunneling (FNT) through various potential surface barriers, where this dominated the field emission (FE) characteristics after Cl2 decoration

Fig. 5. DOS results for bare ZBNNR.

electronic states in the vicinity of the Fermi level and acted as a donor. In addition, the movement of electrons from the VB to CB is shown. The DOS and BS results supported the PDOS findings (Fig. 6(g)).

3.2. Transport properties In general, the transport properties depend mainly on the number of charge carriers and their corresponding electronic state profile with 39

Journal of Physics and Chemistry of Solids 120 (2018) 34–43

K.K. Surthi et al.

Fig. 6. PDOS results for various configurations: (a) ZBNNR2H, (b) ZBNNRCl2B, (c) ZBNNRCl2N, (d) ZBNNRCl2B-Cl2N, (e) ZBNNRAdCl2B, (f) ZBNNRAdCl2N, and (g) ZBNNRAdCl2B-AdCl2N.

CB. The presence of hydrogen over both edges of the ribbon further enhanced the NEA characteristics of ZBNNRadCl2B. Combinations of these properties are advantageous for predicting better FE properties [34,35]. The TS profile for ZBNNRadCl2N is shown in Fig. 7(f), which indicates that the nature was symmetric between the spin up and down states. The larger TC value allowed electrons to tunnel better over different potential barriers and improved conduction through the ribbon. The energy range (2.9–3.2 eV) in the CB is shown clearly in Fig. 7(f). Fig. 7(g) shows the TS profile for ZBNNRadCl2B-adCl2N, which demonstrates that the energy window was shifted into the CB and the NDR also vanished. Cl2 acted as an electron donor and its electronic wave functions crossed the Fermi level as transmission peaks. The transmission value is larger when more bands are involved in the electron transmission phenomenon. Therefore, transmission over a ribbon at a particular electron energy is equal to the transmission probability time and the number of channels [36].

even in the absence of bias. The possible FE characteristics can be predicted over the surface of ZBNNRCl2B, which may be useful for FE panel displays and vacuum microelectronic applications [31,32]. The TS profile for ZBNNRCl2N (Fig. 7(c)) demonstrates the opening of the energy window in the CB, and the symmetry between the spin up and down states. These states provided channels for electron tunneling from the VB to CB, and the NDR vanished as a result. As the number of channels increased, the transfer matrix (Tij (E ) ) value increased, thereby decreasing the suppression of the conduction channels. The number of transmitted electrons and the transfer matrix value increased as the number of channels increased. Therefore, NDR is inversely proportional to Tij(E) and the corresponding equations (S8 and S10) are given in the supplementary information. Cl2 termination provided excess negative charge and boosted electrons to overcome various potential barriers, thereby suppressing hopping conduction. The remarkable reductions of potential surface barriers and the free movement of negative charge carriers over the surface of ZBNNRCl2N are highly beneficial for various applications. Fig. 7(d) shows the TS profile for ZBNNRCl2N-Cl2B, which demonstrates that the energy window opened. Cl2 released electrons on both sides of the ribbon, which were transported to the surface by reducing the effective surface potential barriers and scattering loss, and they eventually boosted the FNT phenomenon over the surface of the ribbon. Thus, ZBNNRCl2N-Cl2B could be useful as a film for FE applications [32,33]. The TS profile for ZBNNRadCl2B is illustrated in Fig. 7(e). Even with adsorption, the nature was symmetric between the spin up and down states. However, the lowest transmission energy window shifted into the CB, and thus the conduction of electrons occurred from the VB to

3.3. Charge analysis Charge density and distribution analysis was conducted using Mulliken population analysis. The electron contribution from the atomic orbitals increased and the value of the charge density also increased, and the corresponding equations (S11, S13, and S14) are given in the supplementary information. EN has the greatest impact on charge analysis studies. The spin up and down states are localized on opposite edges of the ribbon, and thus there will be a difference in the total spin up and down electron charge densities. The difference in EN between B and N led to the redistribution of a charge of 0.286 e from B to the N 40

Journal of Physics and Chemistry of Solids 120 (2018) 34–43

K.K. Surthi et al.

Fig. 7. TS results for various configurations: (a) ZBNNR2H, (b) ZBNNRCl2B, (c) ZBNNRCl2N, (d) ZBNNRCl2B-Cl2N, (e) ZBNNRAdCl2B, (f) ZBNNRAdCl2N, and (g) ZBNNRAdCl2B-AdCl2N.

atoms. The charge transferred from Cl to B was −0.054 e at the B-edge, and 0.222 e from H to N at the N-edge. The valence charge density for the highest VB was 0.246 e/Å3 and it depended on the N-H bond. For the lowest CB, the charge density was assessed as 0.052 e/Å3 and it was mainly due to the B-Cl bond at the B-edge. Fig. 8(b) shows that the charge density of the B-Cl bond accumulated completely around the Clatom, whereas the charge density of the N-H bond was highly localized around the N-atom. The charge density isosurface for ZBNNRCl2N shown in Fig. 8(c) demonstrates the electrostatic potential between the N and Cl2 molecules, which indicates that the charge contribution for the highest VB depended mainly on the N-Cl and Cl-Cl bonds. The charge transferred from Cl to N was 0.136 e, which moved onto the active surface of the ZBNNRs and improved the charge mobility. The active surface of ZBNNRs could also effectively sense changes in the local Cl2 environment, which would be useful for chlorine gas sensing [25]. The valence charge density was 0.138 e/Å3 for the highest VB and 0.084 e/Å3 for the lowest CB, where it depended mainly on the B-H bond. The charge density of the B-H bond was surrounded by the H-atom, whereas the charge density of the N-Cl band accumulated equally between the N and Cl-atoms according to the approximately equal EN values. Thus, the surface reactivity of the ZBNNR was influenced more strongly by the Nedge compared with the B-edge. The charge density isosurface for ZBNNRCl2N-Cl2B is depicted in Fig. 8(d). The charge transferred from Cl to the N-atom was 0.126 e. Therefore, the valence charge density for the highest VB was 0.10 e/Å3 and it was due mainly to the N-Cl bond at the N-edge. The charge density assessed for CB was 0.164 e/Å3 and the charge transferred from Cl to B was −0.04 e. Fig. 8(e) shows the charge density isosurface for ZBNNRadCl2B,

Table 4 Calculated optimum transmission peak values and their TC in the VB and CB. Configurations

ZBNNR2H ZBNNRCl2N-Cl2B ZBNNRCl2N ZBNNRCl2B ZBNNRadCl2N ZBNNRadCl2B ZBNNRadCl2B-adCl2N

Optimum transmission peak value (eV)

TC

VB

VB

CB

13 12 15 15 15 15 12

7 6 7 6 7 5 6

−5.6 −6.7 −5.06 −7.06 −4.8 −5.4 −5.4

CB 4.2 3.2 4.8 4.8 4.4 4.8 3.2

atom, and this phenomenon was observed throughout the ribbon. The charge deficiency at B trapped the charge from Cl2 and this is known as charge trapping behavior (CTB), which may also explain the partially ionic character of N-B bonding. The charge density isosurface for ZBNNR2H shown in Fig. 8(a) demonstrates that a charge of 0.22 e was transferred from H to N at the N-edge. At the B-edge, the charge transferred from the B-atom to H-atom was about 0.086 e. However, the valence charge density for the highest VB was 0.24 e/Å3, which depended mainly on the N-H bond over the N-edge. The charge density assessed for the lowest CB was 0.089 e/Å3 and it depended on the B-H bond over the B-edge. Fig. 8(a) shows clearly that the charge density of the N-H bond accumulated greatly around the N-atom compared with the H-atom. The charge density of the B-H bond completely surrounded the H-atom, which confirmed that the surface of ZBNNR was active. Fig. 8(b) shows the charge density isosurface for ZBNNRCl2B, which demonstrates the bulk electrostatic potential between the two Cl-atoms of Cl2 compared with the electrostatic potential between the B and Cl41

Journal of Physics and Chemistry of Solids 120 (2018) 34–43

K.K. Surthi et al.

Fig. 8. Charge density isosurfaces for various configurations: (a) ZBNNR2H, (b) ZBNNRCl2B, (c) ZBNNRCl2N, (d) ZBNNRCl2B-Cl2N, (e) ZBNNRAdCl2B, (f) ZBNNRAdCl2N, and (g) ZBNNRAdCl2B-AdCl2N.

the charge availability over the surface of ZBNNRadCl2B-adCl2N was greater compared with ZBNNRCl2N-Cl2B. However, the high electrostatic potential on both sides of ZBNNRadCl2B-adCl2N made its edge structure highly unstable. Hence, the NEA, FNT, and CTB characteristics of ZBNNRadCl2B-adCl2N are useful for FE and Cl2 sensing applications. The DOS results for the systems considered are shown in Figs. 4 and 5, where they demonstrate that the spin up and down states mirrored each other. Thus, the spin density waves had spatially homogeneous charge densities in both the spin up and spin down states [37,38]. The charge density waves had spatially variable charge densities, but they were identical for both the spin up and down states according to the DOS and PDOS analyses (Figs. 4–6). Comparisons of the spin density for the bare ZBNNR and the other systems showed that it was independent of the absorption site, dangling bonds, and attached functional groups because no foreign atoms (impurities) were present at edges of the ribbon. Therefore, the charge and charge density of the ZBNNRs depended on the functional group and absorption site. The spin-dependent isosurfaces indicated the spatial variations in the spin density, which differed for the spin up and down states, as shown in Fig. S9. The spin density was homogeneous over the charge density isosurfaces and there were no variations in the contour values over the contour plot [38–40]. Charge accumulation and redistribution occurred between atoms throughout the ribbon's surface, and electrostatic potential was present among the atoms along the ribbon's edges according to the spin-dependent Bloch states of the charge density isosurfaces (Fig. S9). This is the main reason for selecting spin-independent exchange-correlations instead of spin-dependent correlations. The corresponding charge density isosurface plots for different configurations are shown in Fig. 8.

which indicates that the charge of the N-B bond was localized completely around the N-atom. The charge density of the VB depended on the N-H bond, whereas the B-Cl and B-H bonds had key effects for the CB. The charge transferred from B to Cl was 0.02 e and that from B to H was 0.084 e. Therefore, the charge deficiency in the B-atom on the Bedge was responsible for the lowest unoccupied states of the CB and the charge density was 0.02 e/Å3. For the highest VB, the charge density was 0.22 e/Å3 and the charge transferred from H to N was 0.22 e. The charge density of the B-Cl bond was distributed completely around the Cl-atom, which indicated a high carrier concentration along the edges of the ribbon. By contrast, the charge density of the N-H bond accumulated mainly around N rather than H. The charge density isosurface for ZBNNRadCl2N is illustrated in Fig. 8(f), which indicates that the charge density of the N-Cl bond was distributed equally between two atoms according to the almost identical EN values for Cl (3.16) and N (3.04). The charge density of the lowest CB was 0.086 e/Å3. The charge transferred from B to H was 0.086 e, which resulted in the charge deficiency in B, and the empty pzorbital of the B-atom led to the LUMO. The valence charge density for the highest VB was 0.042 e/Å3 and it depended mainly on the N-Cl and N-H bonds. The charge transferred from Cl to N was 0.042 e, and that from H to N was 0.22 e, which resulted in the HOMO. The charge density isosurface for ZBNNRadCl2B-adCl2N is shown by Fig. 8(g), which indicates that the effect of the electrostatic potential over both edges of the ribbon was higher compared with that in the ZBNNRCl2N-Cl2B configuration. The charge density for the lowest CB was 0.016 e/Å3 and that for the highest VB was 0.28 e/Å3. The charge density of the N-Cl bond accumulated equally between the two atoms. However, for the N-H bond, the charge density accumulated more strongly around the N-atom compared with the H-atom. The charge transferred from Cl to N was 0.28 e and the charge transferred from H to N was 0.218 e. However, the charge density of the B-Cl bond accumulated completely around the Cl-atom. Therefore, on both sides, Cl2 and H induced negative charges on the active surface of the ribbon and

4. Conclusions In this study, we investigated the effects of Cl2 decoration on the electron transport properties and surface reactivity of ZBNNRs using 42

Journal of Physics and Chemistry of Solids 120 (2018) 34–43

K.K. Surthi et al.

both the LDA and GGA within the DFT. We found that the band gap decreased monotonically as the ribbon width increased. The structural properties were independent of the ribbon width according to both approximations, and we concluded that the LDA always agreed with the GGA. Cl2 decoration saturated both edges of the ribbon and the corresponding electronic states over the Fermi level made the ribbon electrically conducting in nature. The transition spectrum suggested the complete vanishing of NDR. Free electron movement via the Fermi level into the CB indicated the effect of the FNT phenomenon with Cl2 decoration. Charge analysis showed that the active surface entrapped the charge from Cl2 and enhanced the charge mobility over the surface of the ZBNNRs to increase the surface reactivity. The electrostatic potential among N, B, Cl, and H was high with adsorption and this made the corresponding configurations unstable. The CTB and FNT characteristics of Cl2-decorated ZBNNRs may be useful for Cl2 sensors, FE applications, and other devices.

Lumin. 127 (2007) 595–600. [12] K.J. Erickson, A.L. Gibb, A. Sinitskii, M. Rousseas, N. Alem, J.M. Tour, A.K. Alex, K. Zettl, Longitudinal splitting of boron nitride nanotubes for the facile synthesis of high quality boron nitride nanoribbons, Nano Lett. 11 (2011) 3221–3226. [13] Q. Tang, Z. Zhou, P. Shen, Z. Chen, Band gap engineering of BN sheets by interlayer dihydrogen bonding and electric field control, ChemPhysChem 14 (2013) 1787–1792. [14] P.C. Mende, Q. Gao, A. Ismach, H. Chou, M. Widom, R. Ruoff, L. Colombo, R.M. Feenstra, Characterization of hexagonal boron nitride layers on nickel surfaces by low-energy electron microscopy, Surf. Sci. 659 (2017) 31–42. [15] A.L. Bezanilla, J. Huang, H. Terrones, B.G. Sumpter, Electronic structure calculations on edge-functionalized armchair boron nitride nanoribbons, J. Phys. Chem. C 116 (2012) 15675–15681. [16] J. Yu, Z. Zhang, W. Guo, Electronic properties of graphene nano ribbons stacked on boron nitride nanoribbons, J. Appl. Phys. 113 (2013) 133701 (4 pp). [17] Y. Ding, Y. Wang, J. Ni, The stabilities of boron nitride nanoribbons with different hydrogen-terminated edges, Appl. Phys. Lett. 94 (2009) 233107–133701. [18] M. Mirzaei, M. Mirzaei, An electronic structure study of O-terminated zigzag BN nanotubes: density functional calculations of the quadrupole coupling constants, Solid State Commun. 150 (2010) 1238–1240. [19] M. Mirzaei, M. Mirzaei, Sulfur doping at the tips of (6,0) boron nitride nanotube: a DFT study, Physica 42 (2010) 2147–2150. [20] M. Mirzaei, M. Giahi, Computational investigation of the electronic and structural properties of ultra small-diameter boron nitride nanotubes, Physica B 405 (2010) 2542–2544. [21] M. Mirzaei, N.L. Hadipour, A. Seif, M. Giahi, Density functional study of zigzag BN nanotubes with equivalent ends, Physica 40 (2008) 3060–3063. [22] M. Giahi, M. Mirzaei, M. Meskinfam, M. Yousefi, Density functional studies of the fluorine-terminated boron nitride nanotubes through computations of quadrupole coupling constants, Comput. Theor. Chem. 977 (2011) 29–33. [23] K.P. Loh, I. Sakaguchi, M.N. Gamo, S. Tagawa, T. Sugino, T. Ando, Surface conditioning of chemical vapor deposited hexagonal boron nitride film for negative electron affinity, Appl. Phys. Lett. 741 (1999) 28–30. [24] X.J. Wu, M.H. Wu, X.C. Zeng, Chemically decorated boron-nitride nanoribbons, Front. Phys. China 4 (2009) 367–372. [25] P. Srivastava, N.K. Jaiswal, V. Sharma, Chlorine sensing properties of zigzag boron nitride nanoribbons, Solid State Commun. 185 (2014) 41–46. [26] M. Brandbyge, J.L. Mozos, P. Ordejón, J. Taylor, K. Stokbro, Density-functional method for non-equilibrium electron transport, Phys. Rev. B 65 (2002) 165–401. [27] J.P. Perdew, A. Zunger, Self-interaction correction to density-functional approximations for many-electron systems, Phys. Rev. B 23 (1981) 5049–5079. [28] J.P. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation made simple, Phys. Rev. Lett. 77 (1996) 3865–3868. [29] Z. Shi, X. Zhao, X. Huang, First principles investigation on the stability, magnetic and electronic properties of the fully and partially hydrogenated BN nanoribbons in different conformers, J. Mater. Chem. C 1 (2013) 6890–6898. [30] http://www.quantumwise.com. [31] T. Sugino, C. Kimura, T. Yamamoto, Electron field emission from boron-nitride nanofilms, Appl. Phys. Lett. 80 (2002) 3602–3604. [32] H. Luo, S. Funakawa, W. Shen, T. Sugino, Field emission characteristics of BN nanofilms grown on GaN substrates, J. Vac. Sci. Technol. B 22 (4) (2004) 1958–1963. [33] H.Z. Zhang, Q. Zhao, J. Yu, D.P. Yu, Y. Chen, Field-emission characteristics of conical boron nitride nanorods, J. Phys. D Appl. Phys. 40 (2007) 144–147. [34] Z.F. Wang, Q. Li, Q.W. Shi, X. Wang, J. Yang, J.G. Hou, J. Chen, Chiral selective tunneling induced negative differential resistance in zigzag graphene nano ribbon: a theoretical study, Appl. Phys. Lett. 92 (2008) 133114–3. [35] G.J. Ferreira, M.N. Leuenberger, D. Loss, J.C. Egues, Low-bias negative differential resistance in graphene nano-ribbon superlattices, Phys. Rev. B 84 (2011) 12545–5. [36] V.N. Do, P. Dollfus, Negative differential resistance in zigzag-edge graphene nanoribbon junctions, J. Appl. Phys. 107 (2010) 063705. [37] S. Grimme, Density functional theory with London dispersion corrections, WIREs Comput. Mol. Sci. 1 (2011) 211–228. [38] L.P. Gofkov, G. Grüner, Charge Density Wave in Solids, first ed., (1989) Amsterdam, The Netherlands. [39] I.I. Fabrikant, T. Leininger, F.X. Gadea, Low-energy dissociative electron attachment to Cl2 molecules, J. Phys. B Atom. Mol. Opt. Phys. 33 (2000) 4575–4580. [40] M.V. Kurepa, D.S. Belic, Electron-chlorine molecule total ionization and electron attachment cross sections, J. Phys. B Atom. Mol. Phys. 21 (1978) 11.

Acknowledgments The authors thank the Computational NanoScience and Technology (CNST) lab, Atal Bihari Vajpaee-Indian Institute Information Technology and Management (ABV-IIITM), Gwalior for computational facilities. Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx. doi.org/10.1016/j.jpcs.2018.04.021. References [1] J.H. Chen, C. Jang, S. Xiao, M. Ishigami, M.S. Fuhrer, Intrinsic and extrinsic performance limits of graphene devices on SiO2, Nat. Nanotechnol. 3 (2008) 206–209. [2] S. Armaković, S.J. Armaković, Investigation of boron modified graphene nanostructures; optoelectronic properties of graphene nanoparticles and transport properties of graphene nanosheets, J. Phys. Chem. Solid. 98 (2016) 156–166. [3] A.A. Balandin, Thermal properties of graphene and nanostructured carbon materials, Nat. Mater. 10 (2011) 569–581. [4] K. Min, N.R. Aluru, Mechanical properties of graphene under shear deformation, Appl. Phys. Lett. 98 (2011) 013113 (3 pp). [5] Q. Tang, Z. Zhou, Z. Chen, Innovation and discovery of graphene-like materials via density-functional theory computations, WIREs. Comput. Mol. Sci. 5 (2015) 360–379. [6] J. Eichler, C. Lesniak, Boron nitride (BN) and BN composites for high-temperature applications, J. Eur. Ceram. Soc. 28 (2008) 1105–1109. [7] Q. Tang, J. Bao, Y. Li, Z. Zhou, Z. Chen, Tuning band gaps of BN nanosheets and nanoribbons via interfacial dihalogen bonding and external electric field, Nanoscale 6 (2014) 8624–8634. [8] W. Xie, T. Yanase, T. Nagahama, T. Shimada, Carbon-doped hexagonal boron nitride: Analysis as π-conjugate molecules embedded in two dimensional insulator, C 2 (2016) 2. [9] Q. Wang, L. Chen, L. Xiong, H.R. Gong, Mechanical and thermodynamic properties of cubic boron nitride from ab initio calculation, J. Phys. Chem. Solid. 104 (2017) 276–280. [10] Y. Kubota, K. Watanabe, O. Tsuda, T. Taniguchi, Deep ultraviolet light–emitting hexagonal boron nitride synthesized at atmospheric pressure, Science 317 (2007) 932–924. [11] L. Museur, D. Anglos, J.P. Petitet, J.P. Michel, A.V. Kanaev, Photoluminescence of hexagonal boron nitride: effect of surface oxidation under UV-laser irradiation, J.

43