Mixed Valence of Bismuth in Hexagonal Chalcohalide Nanocrystals
Danila Quarta, Stefano Toso, Gabriele Saleh, Rocco Caliandro, Anna Moliterni, Andrea Griesi, Giorgio Divitini, Ivan Infante, Giuseppe Gigli, Cinzia Giannini, Liberato Manna, Carlo Giansante
Abstract
High Resolution Image Download MS PowerPoint Slide We report the colloidal synthesis of bismuth chalcohalide nanocrystals, which adopt a hexagonal phase that we describe with the Bi 13 S 18 X 2 (in which X = Br and I) structure. Such a model structure displays columns of Bi atoms that form dimers of subvalent Bi (formally Bi 2 4+ ), which we here ascribe to Peierls-type distortions. We suggest that the Bi 2 4+ dimers are at the origin of the anomalously low band gap of this material, with the lowest energy electronic transition showing analogies with an intervalence charge transfer. Our synthetic approach and insights into the structural and electronic features of the hexagonal bismuth chalcohalides are fundamental to sustain the rapidly increasing use of this class of (nano)materials for diverse applications, such as photocatalysis and thermoelectrics.