Communication
Electronic properties and stability of 4–8 BxCyNz monolayers

https://doi.org/10.1016/j.ssc.2020.114174Get rights and content

Highlights

  • DFT calculated 4–8 BCN2 present lower formation energy than hexagonal BCN2.

  • 4–8 allotropes of BCN might be an alternative to band gap tuning.

  • BCN2 present wider band gap on 4–8 geometry in comparison to hexagonal geometry.

Abstract

We have investigated the stability and electronic structure of several 4–8 BxCyNz compounds via first-principle calculations. We have done calculations for structures that vary in configuration and stoichiometry (BCN2 and BC2N). We have compared our results, for 4–8 structures, with that obtained for the equivalent hexagonal structures. We have found that 4–8 BCN2 structures present an overall wider band gap and higher stability than their hexagonal counterparts. Our results indicate that the 4–8 geometry might be an alternative form of stabilizing and band gap tuning some BxCyNz stoichiometries.

Introduction

The discovery of graphene [1] opened the doors to the area of research and development of two-dimensional materials. Graphene is a two-dimensional material that exhibits exceptional electrical, thermal, mechanical, and optical properties. The two-dimensional aspect allows for extra applications in the form of a rolled up nanotube [2]. Driven by the novel characteristics of graphene, research on two-dimensional materials has intensified and as a result many new two-dimensional materials were proposed and/or synthesized, ranging from other carbon based structures such as graphyne [3,4], graphdiyne [5], and graphane to structures based off of other elements such as germanene [6], silicene [7], and so on.

The result of a full replacement of the carbon atoms in the graphene structure by boron and nitrogen pairs is the hexagonal boron nitride (h-BN), which is an insulator material with a large band gap, a high chemical stability, excellent mechanical properties, and high thermal conductivity [8,9]. Furthermore, h-BN can be rolled into a boron nitride nanotube (BNNT) while mantaining its characteristics [10,11].

The partial replacement of carbon atoms achieves a desirable midpoint set of characteristics between graphene and h-BN. The products of the process of partial replacement form an interesting group of two-dimensional structures usually referred to as borocarbonitrides or simply BxCyNz. The characteristics of the material, such as band gap, will depend on the final configuration of the structure and the prevalent stoichiometry, making the material “tunable”, thus the interest in studying the characteristics of specific configurations and also how to form it. The borocarbonitride group of materials has been studied extensively [[12], [13], [14], [15], [16], [17]]. In this article we will focus on a 4–8 variation of BxCyNz inspired by the proposed 4–8 variation of graphene [18], and use calculations on hexagonal BxCyNz as a way to have a reference of comparison to the 4–8 variations of BxCyNz.

4–8 graphene is a proposed two-dimensional carbon based structure formed by octagons and tetragons. The 4–8 graphene structure can be rolled into two different variations of nanotubes, where both variations are metallic. 4–8 graphene can also be wrapped into many stable fullerenes. Its proposal study suggests that it is a semimetal and it can have its band gap opened via boron and nitrogen pair doping. 4–8 graphene presents a lower density when compared to graphene. These characteristics could translate into a difference of stability to certain stoichiometries of BxCyNz structures under this configuration in relation to its hexagonal counterpart.

In this manuscript, we will analyze two different BxCyNz stoichiometries that conforms into two structural geometries, 4–8 and hexagonal. We will check for their stability through formation energy comparison, and argue about the stability of 4–8 structures when compared to hexagonal ones. For each geometry we will analyze six structures sorted into two stoichiometries: BCN2 and BC2N. Theoretically, the structures will be “assembled” via stacking of copies of a predetermined armchair layers (shown in Fig. 1). The assembly is arbitrary and many different arrangements of atoms can come from stacking a single type of layer, thus an assembly rule is necessary. The assembly rules used in this work are as shown in Fig. 2.

Section snippets

Computational details

In this manuscript, the main source of data are calculations based on the density functional theory [19] as implemented in the SIESTA program [20]. We make use of norm-conserving Troullier-Martins pseudopotentials [21] in the Kleinman-Bylander factorized form [22], and a double-ζ basis set composed of numerical atomic orbitals of finite range. Polarization orbitals are used for all atoms, and we use the generalized gradient approximation [23] (GGA) for the exchange-correlation potential. All

Results and discussion

According to the results obtained for 4–8 compounds and shown in Table 2, in a nitrogen-rich environment, the BCN2 stoichiometry exhibits a lower formation energy than the BC2N stoichiometry, which was expected due to its higher nitrogen concentration. Furthermore, the structure with the lowest formation energy, (a), presents no nitrogen clusters with more than two nitrogen atoms bonded together in its configuration while we can find clusters of three nitrogen atoms present in both (b) and (c).

Conclusion

We have investigated the stability and the electronic structure of six 4–8 BxCyNz compounds within two stoichiometries: BCN2 and BC2N. The investigation took mainly a comparative character, with the 4–8 compounds being compared to their hexagonal equivalent. While it was not possible to observe general rules of behavior based solely off of the prevalent geometry, it was possible to observe that 4–8 BCN2 compounds tend to present overall lower formation energy and wider band gaps. Making the 4–8

CRediT author statement

Y. S. Miranda: Investigation, Data curation, Writing - Original draft preparation, Writing - Review and Editing. R de Paiva: Investigation, Writing - Review and Editing. S. Azevedo: Conceptualization, Methodology, Supervision, Writing - Review and Editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We would like to thank the financial support provided by the Brazilian Agencies CAPES, CNPq, INCT Nanomateriais de Carbono, PRONEX - FAPESQ/PB - MCT/CNPq (Grants No. 006/2018 and 151/2018) and FAPEMIG/MG.

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    This document is the results of the research project funded by the Brazilian Agencies CAPES and CNPq, INCT Nanomateriais de Carbono and PRONEX – FAPESQ/PB - MCT/CNPq (Grants No. 006/2018 and 151/2018) and FAPEMIG/MG.

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