2, University of Minnesota, Twin Cities, Minneapolis, Minnesota, United States
Tin sulfides constitute a diverse group of compounds containing tin (Sn) and sulfur (S) elements and can exist in a variety of phases and polytypes. A subset of these phases and polytypes take the form of layered two-dimensional (2D) structures that give rise to a wide host of electronic and optical properties. Hence, achieving control over the phase and polytype is necessary to utilize this wide range of properties exhibited by the compound. Herein, we demonstrated the phase-controlled growth of 2D tin sulfides, SnS2 and SnS, on silicon dioxide (SiO2) substrates by vapor-phase method. While the structural, chemical, optical and electronic properties of both the SnS2 and SnS phases were studied by using state-of-art experimental and theoretical techniques, special attention was given to the SnS2 phase. High-resolution annular dark-field (ADF) scanning transmission electron microscope (STEM) analysis indicate that the SnS2 crystals crystallize in 1T phase, which is in consistent with the ab-initio density functional theory (DFT) calculations predicting that SnS2 stabilizes the 1T phase at ground state. Photoluminescence (PL) and ultraviolet-visible (UV-vis) spectroscopy measurements suggest that the SnS2 crystals have an indirect band gap of 2.20 eV and 2.35 eV, respectively, which is in good agreement with the DFT-calculated band gap of 2.31 eV. The electrical transport measurements performed on back-gated field-effect transistors (FETs) exhibit n-type semiconductor characteristics of the SnS2 crystals. High-angle annular dark-field (HAADF) STEM imaging and STEM energy dispersive X-ray (EDX) chemical analysis demonstrate that the SnS2 crystals are chemically homogeneous with a stoichiometric S/Sn atomic ratio of ~ 2. Electron energy loss spectroscopy (EELS) and X-ray photoelectron spectroscopy (XPS) analysis present the characteristic Sn and S peaks of SnS2, confirming the phase purity of the SnS2 crystals. Ultraviolet photoelectron spectroscopy (UPS) measurements of the SnS2 crystals provide an ionization potential of 7.51 eV, which is in a perfect agreement with the DFT calculations. Raman spectroscopy in conjunction with the ab-initio DFT calculations reveal the characteristic first-order and second-order Raman modes of the 1T phase of the SnS2 crystals. Angle-resolved polarized Raman spectroscopy (ARPRS) mappings with different polarization angles show unique edge features of the SnS2 crystals. Furthermore, we found that the SnS2 can occasionally crystallize in the 4H phase in our growths. The 4H-SnS2 was identified by both the measured and calculated Raman spectra. Finally, we discussed possible strategies for the polytype-engineering in 2D SnS2.