(100g) Synthesis of Tin(II) Monosulfide Nanoplates: A Potential 2D Material

Two dimensional (2D) layered materials such as graphene, metal dichalcogenides, and black phosphorus are of increasing interest because of their unique electronic properties. Tin(II) monosulfide (SnS) has the same crystal structure as black phosphorus and is a semiconductor with a 1.3 eV direct band gap, a 1.1 eV indirect band gap, and a high absorption coefficient (>10⁴ cm^-1) in the visible range of the electromagnetic spectrum. SnS has potential applications in photovoltaics, photocatalysis, thermoelectrics, and lithium ion batteries. We present a facile chemical synthesis of SnS nanoplates via thermal decomposition of a single precursor in a coordinating solvent. Specifically, tin(IV) diethyldithiocarbamate (Sn(dedtc)₄) dissolved in oleic acid is injected into hot (300-340 °C) oleylamine to initiate rapid nucleation and subsequent growth of SnS after decomposition. Oleylamine complexes with Sn(dedtc)₄ and induces its decomposition to form SnS. Examination of the reaction product after 60 minutes at 340 °C with scanning electron microscopy shows thin plates that are 1 to 10 µm wide. Atomic force microscopy of these plates dispersed onto silicon show that the plates are 3-60 nm thick. Energy dispersive X-ray spectroscopy (EDS) indicates that the Sn-to-S ratio is one within the accuracy of EDS quantification. The crystal structure of the plates is confirmed to be orthorhombic (a=0.433 nm, b=1.119 nm, c=0.398 nm) Herzenbergite α-SnS phase with X-ray diffraction. When the plates are >1 µm wide, X-ray diffraction (XRD) from SnS plates cast onto substrates shows only the (040) diffraction as the plates settle on the substrate with their [010] axis aligned with the substrate normal. This textured XRD from plates cast on flat substrates and transmission electron microscopy indicate that SnS plates grow preferentially in the [100] and [001] directions with the [010] axis direction. Raman spectroscopy and confocal imaging show Raman scattering peaks at 96, 161, 186 and 217 cm^-1, all consistent with SnS. At low temperatures (300 °C) and short synthesis times (3 minutes) we also observe the presence of SnS₂ with both XRD and Raman spectroscopy. Raman spectroscopy shows a strong peak at 312 cm^-1. SnS₂ disappears at later times. This suggests that decomposition of Sn(dedtc)₄ first produces SnS₂ which is subsequently reduced, likely by oleylamine, to SnS. The SnS plate sizes depend on the oleylamine concentration, time, and temperature. Plate sizes increase with decreasing oleylamine concentration and increasing temperature. Plates also become larger and thicker with time. SnS nanoplate dispersions in toluene exhibit an optical absorption feature in the visible range of the electromagnetic spectrum, which we surmise to be of plasmonic origin. The nanoplate dispersions in toluene respond to electric fields.