Nanocrystals of Lead Chalcohalides: A Series of Kinetically Trapped Metastable Nanostructures

We report the colloidal synthesis of a series of surfactant-stabilized lead chalcohalide nanocrystals. Our work is mainly focused on Pb₄S₃Br₂, a chalco-halide phase unknown to date that does not belong to the ambient-pressure PbS – PbBr₂ phase diagram. The Pb₄S₃Br₂ nanocrystals herein feature a remarkably narrow size distribution (with a size dispersion as low as 5%) a good size tunability (from 7 to ∼30 nm), an indirect bandgap, photoconductivity (responsivity = 4 ± 1 mA/W) and stability for months under air. A crystal structure is proposed for this new material by combining the information from 3D electron diffraction and electron tomography of a single nanocrystal, X-Ray powder diffraction and density functional theory calculations. Such a structure is closely related to that of the recently discovered high-pressure chalcohalide Pb₄S₃I₂ phase, and indeed we were able to extend our synthesis scheme to Pb₄S₃I₂ colloidal nanocrystals, whose structure matches the one that has been published for the bulk. Finally, we could also prepare nanocrystals of Pb₃S₂Cl₂, which proved to be a structural analogue of the recently reported bulk Pb₃Se₂Br₂ phase. It is remarkable that one high-pressure structure (for Pb₄S₃I₂) and two metastable structures that had not yet been reported (for Pb₄S₃Br₂ and Pb₃S₂Cl₂) can be prepared on the nanoscale by wet-chemical approaches. This highlights the important role of colloidal chemistry in the discovery of new materials and motivates further exploration into metal chalcohalides nanocrystals.