Beryllium decorated armchair BC₂N nanoribbons: coexistence of planar tetracoordinate carbon and nitrogen moieties

Abstract

Planar tetracoordinate carbon and nitrogen (ptC and ptN) are the most important members in planar tetracoordinate chemistry. On the other hand, the isostructure of graphene, hexagonal monolayer boron–carbon–nitrogen (i.e., h-BC₂N) has been viewed as one of the most potential materials in many fields. In this paper, we make the first attempt to design a structure with the coexistence of ptC and ptN moieties by using edge-decoration of armchair BC₂N nanoribbons (aBC₂NNR) using various atomic types. Among them, one new structure with alternate ptC and ptN moieties via Be-decorated aBC₂NNR has been obtained, and its excellent thermal stability is confirmed by using the global minimization method and molecular dynamic simulations. The electronic conductivity of aBC₂NNR undergoes a change from semiconductor to conductor before and after Be atom decoration. Especially, large spin-splitting behavior is induced in aBC₂NNR via partially Be atom decoration. Our work will be helpful not only for enriching ptC and ptN chemistry, but also providing a promising candidate to act as a spintronic device.

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1. Introduction

Planar tetracoordinate carbon (ptC) was proposed by Hoffmann and co-workers,¹ and has received considerable attention due to its novel bonding mode of carbon atoms. So far, various kinds of ptC systems have been identified or proposed, such as CAl₄²⁻, CAl₄⁻, CAl₃Si⁻, CAl₃Ge⁻, NaCAl₄⁻, CAl₄Si, CAl₄Ge, CAl₄Be, and CAl₃Be₂⁻ etc.² In addition, planar tetracoordinate nitrogen (ptN) is another important member in the planar tetracoordinate family. The ptN structure was first successfully designed in NSiAl₃/NAl₄⁻,³ where the N atom is coordinated with Si and Al atoms. Since then, both experimental and theoretical studies have identified or proposed various ptN structures, such as NM₄⁻ (M = (BH), Al, Ga, In), Cu₄H₄N, and Ni₄H₄N et al.⁴

It should be noted that all the ptC and ptN structures mentioned above were designed based on small molecules with nonperiodic structures, which will be not suitable for the experimental synthesis and their future applications. Thus, it is desirable to design the material with periodic ptC or/and ptN moieties. One possible way to realize it is to increase the coordination number of C or/and N atoms in the two dimensional materials via atomic decoration. Simulated by this ideal, Wu et al.⁵ and our group⁶ have recently designed the periodic ptC moieties based on the Cu-decorated zigzag graphene nanoribbon (GNR) and B/Be-decorated armchair GNR. Meanwhile, periodic ptN moieties also have been constructed via Be-decorated armchair boron nitride nanoribbon (BNNR).⁷ Based on these results, the planar C or/and N-containing materials, i.e., boron–carbon–nitrogen nanoribbons,⁸ are the strong candidates to construct the ptC and ptN structures.

Hexagonal monolayer boron–carbon–nitrogen is the isostructure of graphene and hexagonal boron nitride sheet (h-BN), and has been successfully obtained in experiments.⁸ Hexagonal BC₂N sheet is one of the most stable stoichiometry in a group of compound B_xC_yN_z.⁹ Unlike graphene and h-BN, the electronic property of BC₂N sheet indicates that the material is a semiconductor with small band gap.⁹ Moreover, it has been proposed that the BC₂N sheet possesses potential applications in nanoelectronics, opto-electronics and hydrogen storage.^9–11 In this paper, we study the edge decoration of armchair BC₂N nanoribbon (aBC₂NNR) by using 31 types of atoms from second, third and fourth row of periodic table through first principle simulation. Our results reveal a new structure with the coexistence of ptC and ptN moieties, i.e., Be-decorated aBC₂NNR, and its stability and electronic property have been discussed.

2. Computational methods and models

The density functional theory (DFT) calculation was performed using the Vienna Ab-Initio Simulation Package.¹² The electron–ion interaction was described by projector augmented-wave (PAW)¹³ pseudopotentials. For the exchange and correlation functionals, we use the Perdew–Burke–Ernzerhof (PBE) version of the generalized gradient approximation (GGA).¹⁴ We choose aBC₂NNR in a periodic structure with lattice constants of a = 15 Å, b = 25 Å and c taken to be the one-dimensional lattice parameter (8.82 Å) as the studied systems. Ten k points are employed for sampling the one-dimensional brillouin zone in the calculations. All the configurations are studied by the spin-polarized calculations. The energy cutoff of 520 eV was used for the wave functions expansion. The geometries were fully optimized until the forces on each atom is less than 0.01 eV Å⁻¹.

The adsorption energy (E_ads) per atom is defined as

E_ads = [E_{(aBC2NNR-mAtom)} − E_aBC2NNR − mE_Atom]/m,

where E_{(aBC2NNR-mAtom)}, E_aBC2NNR, and E_Atom denote the total energy of the studied systems and m is the number of adsorbed atoms on the aBC₂NNR. According to this definition, E_ads < 0 corresponds to a stable configuration.

The first-principle molecular dynamics (MD) simulation is carried out considering a canonical ensemble (NVT). To control the temperature during MD simulation, the standard Nosé thermostat¹⁵ is used as implemented in VASP with the default Nosé mass set by the package. The time step is set to be 2 fs with total time 10 ps. Meanwhile, we also calculated several finite molecular model systems using the Gaussian 09 program¹⁶ with the B3LYP/6-31+G(d) method.

In addition, isomeric sampling is examined by using CALYPSO (Crystal structure Analysis by Particle Swarm Optimization) code¹⁷ based on a global minimization of free energy surfaces merging first-principle total-energy calculations via the Particle Swarm Optimization technique.

3. Results and discussion

So far, there is still controversy regarding the most stable structure of hexagonal monolayer BC₂N in the literature. Using the pseudopotential local orbital method and experimental data of structure parameters, Liu et al. predicted that the structure with the optimally matched C–C and B–N bonds (the chain-like model) is the most stable one.¹⁸ However, the strip-like model (Fig. 1) was recently shown to be the most stable structure based on the global optimization method in conjunction with the density functional theory method.⁹ In this work, hexagonal monolayer BC₂N with strip-like structure is verified to be the most stable one as shown in Fig. 1. We note that only C, B, or N atoms locate at the zigzag edge of BC₂N nanoribbon (Fig. 1), which is quite similar to the case in the zigzag graphene nanoribbon (GNR) or boron nitride nanoribbon (BNNR). Previously, the edge-decoration of zigzag GNR and BNNR by using various atomic types has been studied,^5,7 and it is expected that similar results could be obtained in the case of zigzag BC₂N nanoribbon. Thus, only armchair BC₂N nanoribbon (aBC₂NNR) is considered in this work. Previously, the armchair GNR have been synthesized by direct oriented growth on germanium¹⁹ or unzipping of carbon nanotube,²⁰ and the BNNR with armchair orientation were obtained by longitudinal splitting of boron nitride nanotube.²¹ In view of the structure similarity between GNR/BNNR and BC₂NNR, we expected that aBC₂NNR could also be made using similar or other techniques.

Fig. 1 The most stable structure of hexagonal monolayer BC₂N.

As shown in Fig. 1, two types of atomic structures exist at the edge of aBC₂NNR: (i) aBC₂NNR with alternate C–C and B–N edges (denoted as type-I); (ii) aBC₂NNR with alternate C–B and C–N edges (denoted as type-II). It is found that the former case is energetically more stable than the later one, and thus only the aBC₂NNR with alternate C–C and B–N edges (type-I) is considered for the subsequent atomic decoration.

We initially apply a periodic model of aBC₂NNR with the width n = 4. A total of 31 atoms from periods 2–4, i.e., Li, Be, B, C, N, O, F, Na, Mg, Al, Si, P, S, Cl, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Br are chosen for edge decoration. Two initial configurations have been considered for each atom-decorated aBC₂NNR, they are (i) each decorating atom is connected to only one edge B, N, or C atom, and (ii) each decorating atom links with two neighboring edge B, N or/and C atoms. After the structural relaxation with frequency confirmation, only Be-decorated aBC₂NNR edge has the alternate planar tetracoordinate carbon (ptC) and nitrogen (ptN) moieties as shown in Fig. 2a. The remaining edge-decorated aBC₂NNR are shown in Fig. S1 in the ESI,† the edge C or N atom either has the distorted structures (Li, N, O, F, Na, Mg, Al, Si, P, S, Cl, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Ga, Ge, As, Se, Au, Ag) or the planar tricoordinate structure (B, C, K, Zn, Br). Similar to the structure in Be-decorated armchair GNR or BNNR,^6,7 “zigzag-like” Be-chain could be found on the edge of Be-decorated aBC₂NNR. And all the C and N atoms at the edge of aBC₂NNR form the ptC and ptN structures, respectively. In addition, we examined the structure of Be decorated aBC₂NNR with type-II termination, and the results shown the distorted structure at the edge C or N atom. Thus, only Be decorated type-I aBC₂NNR structure was discussed in following section.

Fig. 2 (a) The most stable structure of Be-decorated armchair BC₂N nanoribbon (Be-〈aBC₂NNR〉), (b) the snapshot of the equilibrium structure of Be-〈aBC₂NNR〉 under 1000 K at the end of 10 ps MD simulation and (c) the energies of various structures generated by CALYPSO.

It should be noted that the sum of covalent radii (1.92 Å) of Be atom and Be–Be bond length (2.22 Å) matches well with a quarter of the one-dimensional lattice parameter (2.21 Å) of aBC₂NNR. Thus, little mechanical strain is induced upon the edge decoration of the aBC₂NNR by Be atoms, which reveals the steric stabilization for the maintenance of the ptC and ptN moieties. Furthermore, the thermal stability of the Be-decorated aBC₂NNR is examined by using molecular dynamics (MD) simulation. The results indicate that ptC and ptN structures in Be-decorated aBC₂NNR could be sustained for 10 ps even at 1000 K as shown in Fig. 2b, indicating the good thermal stability. Generally, MD simulation is sometimes insufficient to confirm the structural stability due to its short time scale compared to the time scale in the practical use. Thus, it is desirable to inspect the stability of Be-aBC₂NNR structure relative to the neighboring structures. Accordingly, hundreds of structures have been considered for the adsorption of Be atoms on the surface or the edge of aBC₂NNR by using the global minimization of free energy surfaces merging first-principle total-energy calculations.¹⁷ Fig. 2c shows the energies of the first two hundred stable structures, it is found that the structure with the coexistence of ptC and ptN moieties is energetically the most stable one among all the Be atoms decorated aBC₂NNR with the adsorption energy of −0.77 eV. In addition, an independent geometric optimization and frequency calculations for the Be decorated aBC₂NNR are carrier out by using the Gaussian 09 program. To do this, two finite molecular models (Fig. S2†) are constructed based on the 1 × 1 × 2 supercell structure of Be decorated aBC₂NNR, in which the B, C and N atoms at the edge of aBC₂NNR are saturated by H atoms. The results indicate that no imaginary frequencies are found, which further confirms the stability of ptC and ptN structures in Be decorated aBC₂NNR. Besides, we examine the width effect on the planar property of Be-〈n-aBC₂NNR〉 edge with n from 1 to 8 (the numbers refer the aBC₂NNR with n armchair chains contained in its unit cell), and the results reveal that the planarity of considered systems is independent on the width of aBC₂NNR.

Subsequently, we consider the possibility for decoration both of aBC₂NNR edges by Be atoms, as denoted by Be-〈aBC₂NNR〉-Be. After the optimization, frequency analysis and MD simulation at 1000 K, Be-〈aBC₂NNR〉-Be also possesses both good stability and interesting alternate ptC and ptN moieties as shown in Fig. 3a. The bader charge distribution (Fig. 3a) shown that the ptC and ptN atoms are the electronic capture center, indicating the strong σ-donation from ligands to ptC and ptN, which is due to the larger electronegativity of C and N atoms than Be and B atoms. Meanwhile, positive charges can be observed on the Be atoms, i.e., Be1 (1.21e), Be2 (1.05e), Be3 (1.23e), and Be4 (1.02e). In Fig. 3b, the electron charge density difference (i.e., the electron charge density of the Be decorated aBC₂NNR, minus the electron charge density of the Be2/Be4 decorated aBC₂NNR and of the isolated Be1/Be3 atoms, calculated with the same atomic positions of Be decorated aBC₂NNR) clearly shows the charge density piling between the Be1 and Be4, Be2 and Be3 atoms, indicating the covalent bonding property in Be1–Be4 and Be2–Be3 bonds. In addition, the weak bonds could be found in Be1–Be2 and Be3–Be4, which corresponds to the relatively large bond lengths in Be1–Be2 (2.24 Å) and Be3–Be4 (2.40 Å), as compared with that in Be1–Be4 (2.12 Å) and Be2–Be3 (2.11 Å). In the center region of Be-〈aBC₂NNR〉-Be, C–B, C–N, and B–N bonds show the ionic bonding property due to the large charge transfer within them.

Fig. 3 (a) The Bader charge distribution, and (b) the isosurface (0.005e per au) of electron charge density difference of Be-〈aBC₂NNR〉-Be structure.

At last, we examined the magnetic and electronic properties of aBC₂NNR with fully or partially Be atoms decoration. In doing so, one edge of aBC₂NNR is fully decorated by Be atoms, and the other edge is decorated by Be atoms with the concentration (c) of 0, 1/4, 2/4, 3/4, and 1, respectively, as denoted by Be-〈aBC₂NNR〉-(c)Be. In each case, all the possible models have been considered in order to find the most stable structure of aBC₂NNR with Be atoms decoration, and the most stable ones are shown in Fig. S3 in the ESI.† The density of states (DOS) of the most stable structures are shown in Fig. 4, it is found that all the Be-decorated aBC₂NNRs show metallic properties. As a result, the electronic property of aBC₂NNR undergoes the change from semiconductor to conductor before and after the Be atoms decoration as shown in Fig. 4. Taking Be-〈aBC₂NNR〉-Be structure as the example, the local density of states around Fermi level E_F (ranging from E_F − 0.5 eV to E_F + 0.5 eV) is shown in Fig. 5a. It is found that the doping states around the Fermi level are mainly contributed from the Be atoms, and partly from the interface C, B and N atoms. Among all the Be-decorated aBC₂NNR, only Be-〈aBC₂NNR〉-(1/4)Be structure exhibits the magnetic property, which is due to the appearance of the delocalized electronic states on the Be atoms at one edge, and the localized unsaturated electronic states on B and C atoms at another edge of aBC₂NNR as shown in Fig. 5b. It is found that the four Be atoms at one edge of aBC₂NNR mainly contribute to the highest occupied electron states (HOES) of the majority spin, while the electronic state of the isolated Be atom at another edge of aBC₂NNR locates far away from the Fermi level. More interestingly, the large spin-splitting behavior could be found in Be-〈aBC₂NNR〉-(1/4)Be structure with the energy difference of ∼1.5 eV between the HOES of the majority spin and minority spin, which render it a promising candidate to act as the spintronic device.

Fig. 4 The density of states of aBC₂NNR after the edge decoration of Be atoms under different concentrations.

Fig. 5 (a) The local density of states around Fermi level E_F (ranging from E_F − 0.5 eV to E_F + 0.5 eV) for the structure of Be-〈aBC₂NNR〉-Be, and (b) the spin density of Be-〈aBC₂NNR〉-(1/4)Be structure.

4. Conclusion

In summary, we have designed a new structure with the coexistence of ptC and ptN moieties, i.e., Be-decorated aBC₂NNR. The excellent thermal stability of this structure has been confirmed by the global minimization method and molecular dynamic simulation. Thus, the incorporation of both the exotic ptC and ptN structures into one extended system like aBC₂NNR is very probable. The electronic conductivity of aBC₂NNR has significantly increased after the decoration of Be atoms. In particular, the large spin-splitting behavior is found in Be-〈aBC₂NNR〉-(1/4)Be structure, which render it a promising candidate to act as the spintronic device.

Acknowledgements

This work was partially supported by the Natural Science Foundation of Shandong Province (no. ZR2013BM016 and ZR2015BQ013). A part of the computation in this work has been done using the Supercomputing Environment of Chinese Academy of Sciences.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra12660f

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