Two-dimensional nitrogen-rich transition metal compounds: The case of TiN₂

In search of novel two-dimensional materials, here we present a density functional theory study of TiN₂ as a representative of layered transition metal nitrides isotypic to MoS₂. Owing to the difficulty of synthesizing binary nitrogen-rich transition metal compounds at ambient temperature and pressure conditions, we first study the properties of ternary compounds of the form MTiN₂ where M is an early alkali metal (Li, Na, and K). We show that the binary TiN₂ can be obtained by chemical or electrochemical deintercalation of M in MTiN₂ only for Na and K. In the case of Li, and at low Li content, the interlayer separation of intercalated as well as empty galleries decreases significantly thus inhibiting Li migration. We find that this problem, that would pose serious challenges for full alkali metal deintercalation, is not present in Na and K intercalated TiN₂. Additionally, we studied the electronic properties of bulk, bilayer, and monolayer TiN₂. We found that TiN₂ is a direct band gap semiconductor in both the monolayer and bulk limits, with no significant influence of interlayer interaction on the electronic structure. Furthermore, we show that the interlayer binding energy in TiN₂ is smaller than in other layered materials such as MoS₂ and MoSe₂, indicating that layered TiN₂ can be easily exfoliated into two-dimensional sheets.