Tin disulfide with bright near-IR luminescence centers obtained at high pressures

In this work, the high-pressure and high-temperature (HPHT) method was applied to obtain crystalline tin disulfide (${\mathrm{SnS}}_2$). The synthesis was performed in a sulfur-tin-carbon growth system by placing the reaction mixture of sulfur and tin in a graphite capsule. The synthesized ${\mathrm{SnS}}_2$ demonstrated to be a highly crystalline $2H$-polytypic form suitable for mechanical exfoliation of atomically thin films. The crystals reveal strong near-IR photoluminescence attributed to a new luminescence center in ${\mathrm{SnS}}_2$. At 5 K, this center is characterized by a narrow zero-phonon line at 885 nm and phonon sideband whose structure is governed by ${\mathrm{SnS}}_2$ phonon density of states. Based on $\mathit{ab}$ initio calculations, the center is attributed to carbon in the tin sublattice. The side product of the HPHT synthesis is the diamond phase containing optically active tin-vacancy centers (${\mathrm{SnV}}^{-}$).

Figure 1

(a) X-ray diffractogram of the synthesized ${\mathrm{SnS}}_2$ powder with the designation of the main reflexes. The reflexes of diamond and graphite are marked with blue and red arrows, respectively. The inset shows the crystal structure of the $2H$-polytypic ${\mathrm{SnS}}_2$. (b) Raman spectrum for the synthesized powder. The $E_g$ and $A_{1g}$ peaks correspond to the first-order scattering processes in ${\mathrm{SnS}}_2$; D is the well-known diamond peak at 1333 ${\mathrm{cm}}^{-1}$. The inset shows a fragment of the Raman spectrum recorded at 5 K.

Figure 2

Examples of the flakes obtained by the mechanical exfoliation of the synthesized ${\mathrm{SnS}}_2$ crystals. (a),(b) AFM surface morphology. (c) Thickness profiles for the flakes from (a) and (b). The directions corresponding to the profile are marked with the white lines in (a) and (b). (d) Raman spectra for the flakes (a) (bottom) and (b) (top). “subst” indicates the peaks corresponding to the Si/${\mathrm{SiO}}_2$ substrate.

Figure 3

(a) 5 K luminescence spectra of the initial powder (lower curve), the diamond phase extracted from it (middle curve), and the ${\mathrm{SnS}}_2$ flake obtained by mechanical exfoliation from a large ${\mathrm{SnS}}_2$ microcrystal selected from the synthesized powder (upper curve). The excitation wavelength equals 532 nm. (b) 5-K micro-photoluminescence spectra of the ${\mathrm{SnS}}_2$ flake in the near IR. The inset shows a fragment of the spectrum including a zero phonon line (ZPL) and phonon replicas ($P_i$) corresponding to the processes with a single-phonon emission. The density of phonon states in $2H\text{−}{\mathrm{SnS}}_2$ is shown in the lower part of the inset. Zero energy corresponds to ZPL maximum.

Figure 4

(a) Formation energy of the substitutional carbon defect center in ${\mathrm{SnS}}_2$. Dashed lines correspond to the formation energy of individual charge states [for charged states it linearly depends on the position of the Fermi level; see Eq. (1)]. The thick red broken line designates the global minimum of formation energy. The kinks on the red broken line are charge transition levels of carbon defect. (b) Density of states in pristine ${\mathrm{SnS}}_2$ in the vicinity of the band gap. (c) Density of states in doped ${\mathrm{SnS}}_2$ (solid black curve). Projections of the density of states on the sulfur and tin atoms adjacent to the impurity carbon atoms are shown by the dashed curves (green - S, red - Sn).

Figure 5

(a) Geometry of the carbon substitutional center in ${\mathrm{SnS}}_2$. Carbon atom is shown in gray. The thick red line is the symmetry second-order axis. The distance between the carbon atom and the adjacent two-coordinated sulfur atoms is designated by thin dashed lines. The translucent bubbles depict the Khon-Sham orbitals. Green stands for HOMO and blue for LUMO. (b) Geometry of the carbon substitutional center in ${\mathrm{SnS}}_2$ in the excited electron state. The magenta arrows designate relative displacement of corresponding atoms with respect to their positions in the ground electron state.