Unusual features of nitrogen substitutions in silicene

Abstract

The quasiparticle properties resulting from charge and spin are clearly identified in nitrogen-substituted silicenes, for which a theoretical framework is successfully developed from first-principles calculations. Such systems create extremely non-uniform chemical and physical environments through the distribution of the guest atoms. They present unusual geometric, electronic, and magnetic properties, which can be identified from the optimal honeycomb lattices, the atom- and spin-dominated energy spectra, the spatial charge density distributions, and the atom-, orbital- and spin-projected van Hove singularities [the net magnetic moments]. The complicated relations between the highly hybridized sp²-N-Si bonds and the ferromagnetic/non-magnetic configurations are responsible for the p-type or semiconducting behavior, the significant modifications to the Dirac cone structures, the difficulty in identifying the π and σ bands, and the vanishing or finite magnetic moments. The theoretical predictions could be verified by high-resolution experimental measurements.

Fig. 1. The top and side views of the optimal geometric structures for nitrogen-substituted silicene systems in (a) pristine, (b) full, (c) 50%-meta, (d) 50%-ortho/para, (e) 33.3%-meta, (f) 33.3%-ortho, (g) 33.3%-para, (h) 12.5%-meta, (i) 12.5%-ortho, (j) 12.5%-para, (k) 20%-single and (l) 14.3%-single substitution cases.

Fig. 2. Electronic band structures of the nitrogen-substituted silicene systems, showing the dominance of N and Si atoms [the green triangles and blue circles, respectively], for the substitution cases: (a) no substitution, (b) [1 : 1], (c) [2 : 4]-meta, (d) [2 : 4]-ortho/para, (e) [2 : 6]-meta, (f) [2 : 6]-ortho, (g) [2 : 6]-para, (h) [2 : 16]-meta, (i) [2 : 16]-ortho, (j) [2 : 16]-para, (k) [1 : 5]-single, (l) [1 : 7]-single.

Fig. 3. The spatial charge distributions of nitrogen-substituted silicene materials for various concentrations and configurations: (a) without guest atoms, (b) [1 : 1], (c) [2 : 4]-meta, (d) [2 : 4]-ortho/para, (e) [2 : 6]-meta, (f) [2 : 6]-para, (g) [1 : 5]-single, and (h) [1 : 7] conditions. The two projections along the [x, z] and [x, y] planes clearly indicate the multi-orbital chemical bonding.

Fig. 4. The spin-charge distributions in nitrogen-substituted silicene systems for certain cases: (a) [1 : 1], (b) [2 : 4]-meta, (c) [1 : 5] and (d) [1 : 7], showing top and side views. Red and yellow correspond to spin-up and spin-down, respectively.

Fig. 5. The atom- and orbital-decomposed density of states of nitrogen-substituted silicenes for various cases: (a) pristine, (b) [1 : 1], (c) [2 : 4]-meta, (d) [2 : 4]-ortho/para, (e) [2 : 6]-meta, (f) [2 : 6]-ortho, (g) [2 : 6]-para, (h) [2 : 16]-meta, (i) [2 : 16]-ortho, (j) [2 : 16]-para, (k) [1 : 5], and (l) [1 : 7]. For ferromagnetic systems, negative DOS corresponds to the spin-down component.