Centro de Nanociencias y Nanotecnología

First principles total energy calculations have been applied to describe the ReCN bulk structure and the formation of ReCN monolayers and bilayers. Results demonstrate a strong structural rearrangement in the monolayer due to a reduced dimension effect: an increase in the lattice parameter, accompanied with the contraction of the distance between the C and N planes. On the other hand, a ReCN bilayer has structural parameters similar to those of the bulk. Surface formation energies show that the monolayer is more stable than bilayer geometries. Although bulk ReCN shows a semiconductor behavior, the monolayer ReCN presents a metallic behavior. This metallic character of the ReCN monolayer is mainly due to the d-orbitals of Re atoms. Graphene is a well-known 2D material with outstanding properties, making it one of the most studied materials. It can be obtained by exfoliation 1 , or by epitaxial growth on different substrates 2, 3. It presents physical properties such as substrate-induced band gap opening 3 , a half-metallic behavior 4 , a tunable gap 5 , remarkable electronic mobility 6 , and a topological insulator behavior 7. These properties make graphene suitable for many technological applications 8. For example, it may be used as photodetector 9 , optical modulator 10 , as key constituent in solar cells, in light emitting devices, and in ultrafast lasers 11 , as well as in flexible optoelectronic devices 12 , among other specific applications 13, 14. Since graphene gained such attention, many research groups started to look for new 2D materials with equally interesting properties. As consequence, several 2D materials have been found: silicene 15 , germanene 16 , and tri-layer transition metal dichalcogenides 17–19 are some examples. These 2D materials have excellent properties , which may be exploited to construct new generation devices, or to form heterostructures with graphene to expand its well established applications as well as to tune its unprecedented properties 20–22. Recently, a new two dimensional semiconductor has been obtained: black phosphorene. This novel material has been proposed as a strong competitor to graphene since it has a semiconductor behavior, with a band gap that can be modulated by increasing the number of layers 23. It also can be highly strained without losing its semiconductor character, making it suitable for applications in flexible electronic devices 24. It can change from direct to indirect gap by tensile strain 25. Furthermore, its high carrier mobility makes it a potential material for the channel in the construction of electronic and optoelectronic devices 26–28. Keeping in mind the interest that 2D materials have generated in the past years, we have turned our attention to the ReCN compound. This is a super hard material with an orthogonal bulk structure with two ReCN tri-layers in the unit cell 29. Since two consecutive tri-layers are not bonded by weak van der Waals forces, it is not possible to obtain 2D ReCN by exfoliation. However, it surely may be grown by techniques such as molecular beam epitaxy, chemical vapor deposition, spray pyrolysis or some other chemical or physical growth techniques. To explore this material deeper, we have carried out first principles calculations to characterize the structural and electronic properties of ReCN in bulk and as a 2D material. Method Calculations have been carried out using the density functional theory as implemented in the PWscf code of the Quantum ESPRESSO package 30. The Kohn-Sham states have been expanded in plane waves with a kinetic energy cutoff of 30 Ry, while the charge density cutoff was set to 240 Ry. The generalized gradient approximation with the Perdew-Burke-Ernzerhof parametrization 31 has been used to treat the non-classical exchange and correlation