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

Abstract

We report the colloidal synthesis of a series of surfactant-stabilized lead chalcohalide nanocrystals. Our work is mainly focused on Pb<sub>4</sub>S<sub>3</sub>Br<sub>2</sub>, a chalco-halide phase unknown to date that does not belong to the ambient-pressure PbS – PbBr<sub>2</sub> phase diagram. The Pb<sub>4</sub>S<sub>3</sub>Br<sub>2</sub> 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 Pb<sub>4</sub>S<sub>3</sub>I<sub>2</sub> phase, and indeed we were able to extend our synthesis scheme to Pb<sub>4</sub>S<sub>3</sub>I<sub>2</sub> colloidal nanocrystals, whose structure matches the one that has been published for the bulk. Finally, we could also prepare nanocrystals of Pb<sub>3</sub>S<sub>2</sub>Cl<sub>2</sub>, which proved to be a structural analogue of the recently reported bulk Pb<sub>3</sub>Se<sub>2</sub>Br<sub>2</sub> phase. It is remarkable that one high-pressure structure (for Pb<sub>4</sub>S<sub>3</sub>I<sub>2</sub>) and two metastable structures that had not yet been reported (for Pb<sub>4</sub>S<sub>3</sub>Br<sub>2</sub> and Pb<sub>3</sub>S<sub>2</sub>Cl<sub>2</sub>) 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.

Multilayer Diffraction Reveals That Colloidal Superlattices Approach the Structural Perfection of Single Crystals

“…This is demonstrated on assemblies of recently synthesized ultrathin PbS orthorhombic nanoplatelets (Figure 6a-d) 35 and spheroidal rock-salt PbS nanocrystals (Figure 6e-g). 36 For both the morphologies, particles assemble with the same crystal planes parallel to the substrate [cubic (200) = orthorhombic (400)], a circumstance convenient for the comparison. Table 1 summarizes the result of the fits.…”

Multilayer Diffraction Reveals That Colloidal Superlattices Approach the Structural Perfection of Single Crystals

“…PbS nanocrystals were grown via a modification of the method we originally published for the synthesis of lead chalcohalide nanocrystals. 36 Briefly, 111 mg (0.4 mmol) of PbCl2 were dissolved in a 20 mL flask containing 10 mL ODE, 750 μL OLAM and 750 μL OA at 120°C under continuous stirring. The solubilization of PbCl2 is slow and might be incomplete.…”

Halide Perovskites as Disposable Epitaxial Templates for the Phase-Selective Synthesis of Lead Sulfochloride Nanocrystals

“…30,31 Our group pioneered the investigation of lead chalcohalides at the nanoscale, discovering two additional compounds (Pb4S3Br2 and a yet not identified Pb-S-Cl phase) in the form of NCs. 25 More recently, we found that Pb4S3Br2 can match epitaxially the CsPbX3 perovskite to form colloidal heterostructures, thus demonstrating a remarkable synthetic and structural compatibility. 26 Here we focus our attention on the lead sulfochlorides, demonstrating that the yet unknown Pb-S-Cl phase obtained through direct synthesis is Pb3S2Cl2, whose structure, here identified as monoclinic and solved for the first time, is a distorted version of the cubic Pb3Se2Br2 prototype (Figure 1, left panels).…”

“…6,7,12,13,19 There, the combination of as little as four elements (Cs, Pb, X and E, where X = F, Cl, Br or I and E = S, Se or Te) can yield NCs of a variety of phases: the binaries CsX, PbX2 and PbE, 6,7,[20][21][22] the well-known cesium lead halides (Cs4PbX6, CsPbX3, and CsPb2X5), 12,19,23,24 and the still little explored lead chalcohalides (Pb4S3Br2 and Pb4S3I2). 25,26 All these compounds are often obtained under similar reaction conditions, and can therefore compete during the synthesis, requiring a careful tuning of the synthetic protocols to achieve the impurity-free synthesis of the desired product. This condition is not dissimilar from that of many organic chemistry reactions, where two molecules can react following different pathways and resulting in different products.…”

“…The reason why CsPbCl3 induces such phase selectivity lies in the fact that the structures of Pb3S2Cl2 and Pb4S3Cl2 are remarkably different, 25,30 to the point that only Pb4S3Cl2 can match epitaxially with the CsPbCl3 perovskite. Indeed, Pb4S3Cl2 meets two strict structural constraints: i) the presence of a perovskite-like atomic plane, 26 and ii) the continuity of the cationic subnetwork across the chalcohalide/perovskite interface.…”