Cubic form of Pb _2-xSn _xS ₂ stabilized through size reduction to the nanoscale

We demonstrate the synthesis of semiconductor Pb _2-xSn _xS ₂ nanocrystals with a cubic rock salt crystal structure in a composition range where this structure is unstable in the bulk. The cubic Pb _2-xSn _xS ₂ nanocrystals were prepared using a modified hot injection colloidal synthetic route. The x value is in the range 0.40 < x < 1. Even though these compositions lie in a region of the PbS-SnS phase diagram where no single phase exists, and despite the fact that PbSnS ₂ is a distorted orthorhombic phase, the Pb _2-xSn _xS ₂ nanocrystals are single phase solid solutions with cubic NaCl-type structure. Experimental evidence for this derives from powder X-ray diffraction (PXRD), electron diffraction, and pair distribution function (PDF) analysis. Elemental compositions determined using scanning transmission electron microscopy/energy dispersive spectroscopy (STEM/EDS), inductively coupled plasma-atomic emission spectroscopy (ICP-AES), and electron energy loss spectroscopy (EELS) reveal a composition close to the nominal ones. The band gaps of the Pb _2-xSn _xS ₂ nanocrystals (0.52-0.57 eV) are blue-shifted by quantum confinement relative to that of the hypothetical cubic PbSnS ₂ phase which density functional theory (DFT) calculations show to be much narrower (0.2 eV) than in the case of orthorhombic PbSnS ₂ (1.1 eV). The Pb _2-xSn _xS ₂ nanocrystals exhibit a well-defined band gap in the near-IR region and are stable up to ∼300 °C above which they phase separate into cubic PbS and orthorhombic α-SnS.