Joseph Gonzalez

Layer-dependent properties of SnS 2 and SnSe 2 two-dimensional materials

The layer dependent structural, electronic and vibrational properties of SnS2 and SnSe2 are investigated using first-principles density functional theory (DFT). The in-plane lattice constants, interlayer distances and binding energies are found to be layer-independent. Bulk SnS2 and SnSe2 are both indirect band gap semiconductors with Eg = 2.18 eV and 1.07 eV, respectively. Few-layer and monolayer 2D systems also possess an indirect band gap, which is increased to 2.41 eV and 1.69 eV for single layers of SnS2 and SnSe2. The effective mass theory of 2D excitons, which takes into account the combined effect of the anisotropy, non-local 2D screening and layer-dependent 3D screening, predicts strong excitonic effects. The binding energy of indirect excitons in monolayer samples, Ex~0.9 eV, is substantially reduced to Ex = 0.14 eV in bulk SnS2 and Ex = 0.09 eV in bulk SnSe2. The layer-dependent Raman spectra display a strong decrease of intensities of the Raman active A1g mode upon decreasing the number of layers down to a monolayer, by a factor of 7 in the case of SnS2 and a factor of 20 in the case of SnSe2 which can be used to identify number of layers in a 2D sample.

Novel phases and superconductivity of tin sulfide compounds

Tin sulfides, Sn x S y , are an important class of materials that are actively investigated as novel photovoltaic and water splitting materials. A first-principles evolutionary crystal structure search is performed with the goal of constructing the complete phase diagram of Sn x S y and discovering new phases as well as new compounds of varying stoichiometry at ambient conditions and pressures up to 100 GPa. The ambient phase of SnS2 with P 3 ¯ m 1 symmetry remains stable up to 28 GPa. Another ambient phase, SnS, experiences a series of phase transformations including α-SnS to β-SnS at 9 GPa, followed by β-SnS to γ-SnS at 40 GPa. γ-SnS is a new high-pressure metallic phase with P m 3 ¯ m space group symmetry stable up to 100 GPa, which becomes a superconductor with a maximum T c = 9.74 K at 40 GPa. Another new metallic compound, Sn3S4 with I 4 ¯ 3 d space group symmetry, is predicted to be stable at pressures above 15 GPa, which also becomes a superconductor with relatively high T c = 21.9 K at 30 GPa.

Layer-dependent properties ofSnS2andSnSe2two-dimensional materials

The layer dependent structural, electronic and vibrational properties of SnS2 and SnSe2 are investigated using first-principles density functional theory (DFT). The in-plane lattice constants, interlayer distances and binding energies are found to be layer-independent. Bulk SnS2 and SnSe2 are both indirect band gap semiconductors with Eg = 2.18 eV and 1.07 eV, respectively. Few-layer and monolayer 2D systems also possess an indirect band gap, which is increased to 2.41 eV and 1.69 eV for single layers of SnS2 and SnSe2. The effective mass theory of 2D excitons, which takes into account the combined effect of the anisotropy, non-local 2D screening and layer-dependent 3D screening, predicts strong excitonic effects. The binding energy of indirect excitons in monolayer samples, Ex~0.9 eV, is substantially reduced to Ex = 0.14 eV in bulk SnS2 and Ex = 0.09 eV in bulk SnSe2. The layer-dependent Raman spectra display a strong decrease of intensities of the Raman active A1g mode upon decreasing the number of layers down to a monolayer, by a factor of 7 in the case of SnS2 and a factor of 20 in the case of SnSe2 which can be used to identify number of layers in a 2D sample.