Conformal and Atomic Characterization of Ultrathin CdSe Platelets with a Helical Shape

• Published in: Nano letters Vol. 14; no. 11; pp. 6257 - 6262
• Main Authors: Hutter, Eline M, Bladt, Eva, Goris, Bart, Pietra, Francesca, van der Bok, Johanna C, Boneschanscher, Mark P, de Mello Donegá, Celso, Bals, Sara, Vanmaekelbergh, Daniël
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 12.11.2014
• Subjects: Cadmium selenides; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Condensed matter: structure, mechanical and thermal properties; Exact sciences and technology; Helical; Intermetallics; Letter; Low-dimensional structures (superlattices, quantum well structures, multilayers): structure, and nonelectronic properties; Nanostructure; Optical properties and condensed-matter spectroscopy and other interactions of matter with particles and radiation; Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures; Physics; Preserves; Semiconductors; Silicon dioxide; Surfaces and interfaces; thin films and whiskers (structure and nonelectronic properties); Two dimensional; CdSe nanoplatelets; nanohelices; silica coating; 2D semiconductor nanocrystals; Helicidae; Helical shape; Nanoplate; Semiconductor materials; Gastropoda; Precursor; Silica; Transmission electron microscopy; Pulmonata; Blood platelets; Atomic arrangement; Morphology; Optical properties; Helix; Ligands; Invertebrata; Mollusca; Scanning transmission electron microscopy
• ISSN: 1530-6984; 1530-6992; 1530-6992
• DOI: 10.1021/nl5025744

Currently, ultrathin colloidal CdSe semiconductor nanoplatelets (NPLs) with a uniform thickness that is controllable up to the atomic scale can be prepared. The optical properties of these 2D semiconductor systems are the subject of extensive research. Here, we reveal their natural morphology and atomic arrangement. Using cryo-TEM (cryo-transmission electron microscopy), we show that the shape of rectangular NPLs in solution resembles a helix. Fast incorporation of these NPLs in silica preserves and immobilizes their helical shape, which allowed us to perform an in-depth study by high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). Electron tomography measurements confirm and detail the helical shape of these systems. Additionally, high-resolution HAADF-STEM shows the thickness of the NPLs on the atomic scale and furthermore that these are consistently folded along a ⟨110⟩ direction. The presence of a silica shell on both the top and bottom surfaces shows that Cd atoms must be accessible for silica precursor (and ligand) molecules on both sides.