title: Pyramid-Shaped Wurtzite CdSe Nanocrystals with Inverted Polarity.
abstract: We report on pyramid-shaped wurtzite cadmium selenide (CdSe) nanocrystals (NCs), synthesized by hot injection in the presence of chloride ions as shape-directing agents, exhibiting reversed crystal polarity compared to former reports. Advanced transmission electron microscopy (TEM) techniques (image-corrected high-resolution TEM with exit wave reconstruction and probe-corrected high-angle annular dark field-scanning TEM) unequivocally indicate that the triangular base of the pyramids is the polar (0001̅) facet and their apex points toward the [0001] direction. Density functional theory calculations, based on a simple model of binding of Cl(-) ions to surface Cd atoms, support the experimentally evident higher thermodynamic stability of the (0001̅) facet over the (0001) one conferred by Cl(-) ions. The relative stability of the two polar facets of wurtzite CdSe is reversed compared to previous experimental and computational studies on Cd chalcogenide NCs, in which no Cl-based chemicals were deliberately used in the synthesis or no Cl(-) ions were considered in the binding models. Self-assembly of these pyramids in a peculiar clover-like geometry, triggered by the addition of oleic acid, suggests that the basal (polar) facet has a density and perhaps type of ligands significantly different from the other three facets, since the pyramids interact with each other exclusively via their lateral facets. A superstructure, however with no long-range order, is observed for clovers with their (0001̅) facets roughly facing each other. The CdSe pyramids were also exploited as seeds for CdS pods growth, and the peculiar shape of the derived branched nanostructures clearly arises from the inverted polarity of the seeds.
authors.affiliation: Department of Nanochemistry, Istituto Italiano di Tecnologia (IIT) , via Morego 30, I-16163 Genova, Italy.
authors.forename: Sandeep
authors.initials: S
authors.lastname: Ghosh
authors.affiliation: CONCEPT Lab, Istituto Italiano di Tecnologia (IIT) , via Morego 30, I-16163 Genova, Italy.
authors.forename: Roberto
authors.initials: R
authors.lastname: Gaspari
authors.affiliation: Department of Nanochemistry, Istituto Italiano di Tecnologia (IIT) , via Morego 30, I-16163 Genova, Italy.|IMEM-CNR , Parco Area delle Scienze 37/A, I-43124 Parma, Italy.
authors.forename: Giovanni
authors.initials: G
authors.lastname: Bertoni
authors.affiliation: CNR-NANO, Centro di Ricerca S3 & Dipartimento di Fisica, Informatica e Matematica (FIM), Università di Modena e Reggio Emilia , via G. Campi 213/A, I-41125 Modena, Italy.
authors.forename: Maria Chiara
authors.initials: MC
authors.lastname: Spadaro
authors.affiliation: Department of Nanochemistry, Istituto Italiano di Tecnologia (IIT) , via Morego 30, I-16163 Genova, Italy.
authors.forename: Mirko
authors.initials: M
authors.lastname: Prato
authors.affiliation: EMAT, University of Antwerp , Groenenborgerlaan 171, B-2020 Antwerp, Belgium.
authors.forename: Stuart
authors.initials: S
authors.lastname: Turner
authors.affiliation: CONCEPT Lab, Istituto Italiano di Tecnologia (IIT) , via Morego 30, I-16163 Genova, Italy.|CompuNet, Istituto Italiano di Tecnologia (IIT) , via Morego 30, I-16163 Genova, Italy.|Department of Pharmacy and Biotechnology, University of Bologna , via Belmeloro 6, I-40126 Bologna, Italy.
authors.forename: Andrea
authors.initials: A
authors.lastname: Cavalli
authors.affiliation: Department of Nanochemistry, Istituto Italiano di Tecnologia (IIT) , via Morego 30, I-16163 Genova, Italy.
authors.forename: Liberato
authors.initials: L
authors.lastname: Manna
authors.affiliation: Department of Nanochemistry, Istituto Italiano di Tecnologia (IIT) , via Morego 30, I-16163 Genova, Italy.
authors.forename: Rosaria
authors.initials: R
authors.lastname: Brescia
keywords: CdSe nanocrystals; chloride assisted; colloidal synthesis; crystal polarity; self-assembly