Graphene-like titanium carbides and nitrides Tin+1Cn, Tin+1Nn (n=1, 2, and 3) from de-intercalated MAX phases: First-principles probing of their structural, electronic properties and relative stability

Very recently [32], an elegant exfoliation approach was proposed to prepare a new family of 2D-like transition metal carbides, when selective etching of aluminum layers from some MAX phases yielded 2D materials Ti2C, Ti3C2. Moreover, according to the newest data [47,48], more complex 2D carbides (TiNbC) or carbonitrides (Ti3CNx) can be prepared. Here, employing first-principle band structure calculations, we have examined systematically the trends in structural, electronic properties and relative stability of a representative group of 2D (graphene-like, GL) materials: titanium carbides and nitrides Tin+1Cn, Tin+1Nn (n = 1, 2, and 3). The peculiarities of atomic relaxation effects for GL Tin+1Cn and GL Tin+1Nn were established and discussed in terms of socalled distortion indexes of basic polyhedrons. Our analysis of stability of GL Tin+1Cn and GL Ti(n+1)Nn ( n terms of cohesive energies and formation energies) has shown relative stabilization of these GL systems with the growth of their thickness (i.e. with the growth of index n). The most interesting feature of the electronic structure for GL Tin+1Cn and GL Tin+1Nn is a considerable growth of the density of near-Fermi states which becomes 2.5 to 4.5 times higher than for the parent MAX phases. The origin of this effect was explained by redistribution of Ti 3d states from broken Ti-Al bonds into delocalized Ti-Ti metallic-like bonding states placed in the window around the Fermi level. We also found that for ideal'' atomic-clean free-standing GL Tin+1Xn magnetization is likely to take place, when the ground state is AFM, with ferromagnetic ordering of the spin moments on Ti1 atoms within each external Ti sheet, and these opposite external titanium sheets of GL are coupled antiferromagnetically, whereas the internal Ti sheets remain non-magnetic. The possibility to design in future experiments a rich variety of new GL materials with variable electronic and magnetic properties (semiconductors, non-magnetic and magnetic metals) by modulating the type and degree of GL termination by various adatoms or molecules was proposed. (c) 2012 Elsevier B.V. All rights reserved.