Method To Incorporate Anisotropic Semiconductor Nanocrystals of All Shapes in an Ultrathin and Uniform Silica Shell Eline M. Hutter, †,§ Francesca Pietra, †,§ Relinde J. A. van Dijk - Moes, † Dariusz Mitoraj, + Johannes D. Meeldijk, # Celso de Mello Donega ́ ,* ,† and Danië l Vanmaekelbergh* ,† † Condensed Matter and Interfaces, Debye Institute for Nanomaterials Science, Utrecht University, Princetonplein 1, 3584 CC Utrecht, The Netherlands + Faculty of Chemistry and Biochemistry, Ruhr University Bochum, Universitä tstrasse 150, 44780 Bochum, Germany # Electron Microscopy Utrecht, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands * S Supporting Information ABSTRACT: In this work, we present a method for the incorporation of anisotropic colloidal nanocrystals of many different shapes in silica in a highly controlled way. This method yields a uniform silica shell, with thickness tunable from 3 to 17 nm. The silica shell perfectly adapts to the shape of the nanocrystals, preserving their anisotropy, a crucial requisite for shape-dependent applications. Our method is based on an adaptation of the reverse microemulsion method. High control over the nucleation and growth of the shell is obtained by slowing down the hydrolysis and condensation rates of the silica precursor by lowering the ammonia concentration. This is shown to be essential for the formation of a uniform silica shell in the case of CdSe/CdS core/shell nanorods. Additionally, the general applicability of this method is demonstrated by coating different anisotropic semiconductor nanocrystals such as nanostars and 2D nanoplatelets. These results thus represent a crucial step toward the fabrication of highly processable and functionalized anisotropic nanoparticles. ■ INTRODUCTION Colloidal semiconductor nanocrystals (NCs) show unique size- and shape-dependent physical properties due to quantum and dielectric confinement. 1-3 In the past years, new synthetic procedures have opened the possibility to design colloidal NCs with anisotropic shapes, such as one-dimensional colloidal semiconductor NCs, i.e. nanorods (NRs) 4-6 and two-dimen- sional nanosheets and nanoplatelets (NPLs), 7-12 characterized by a remarkable uniformity both in size and shape. 13 A major drawback of colloidal semiconductors in applications such as fluorescent biolabels 14 or light-emitting devices 15 is their poor stability in water- and oxygen-rich environments. A possible strategy to deal with this problem is encapsulation in an inert shell that shields the materials both chemically and physically from the direct environment. In this respect, the incorporation of colloidal NCs in silica is highly interesting, because it increases their photochemical stability while the optical properties are preserved. 16 Furthermore, the silica shell can easily be functionalized with organic ligands. 17-21 This allows the NCs to be dispersed in both polar and nonpolar solvents, which largely increases their processability. In the past decades, extensive work has been done on the incorporation of spherical NCs (i.e., quantum dots (QDs)) in silica. 16,17,19,22-31 The two main approaches to coat nanoparticles with a silica shell are the traditional Stö ber method 32,33 and the so-called reverse microemulsion method. 34-37 Although the Stö ber method is highly effective in growing silica shells around micrometer-sized colloids 33,38 or metal nanoparticles, 20 it does not yield uniform silica shells on single semiconductor NCs. 39 The reverse microemulsion method however allows for the incorporation of individual QDs located exactly in the middle of silica spheres. 17,34 In this approach, the silica shell grows around single QDs that are individually trapped inside an aqueous micelle of a water-in-oil (w/o) microemulsion, together with silica precursor molecules. The silica nucleation and growth are catalyzed by the basic environment resulting from the addition of an ammonia solution. The incorporation mechanism is explained in terms of a ligand exchange process, through which the hydrophobic capping molecules on the surface of the QDs are replaced by the silica precursor tetraethoxysilane (TEOS), prior to silica growth. 17,36 Only recently, silica-coating with the reverse microemulsion method was successfully applied to anisotropic NCs like NRs 40-42 and tetrapods. 43,44 However, the incorporation in silica causes a drastic decrease in the particles’ aspect ratio as the shape changes into dumbbells and ellipsoids. 40-42 More- over, uniform shells thinner than 10 nm cannot be obtained by the currently available methods. Hence, a method to coat highly Received: December 16, 2013 Revised: February 12, 2014 Published: February 20, 2014 Article pubs.acs.org/cm © 2014 American Chemical Society 1905 dx.doi.org/10.1021/cm404122f | Chem. Mater. 2014, 26, 1905-1911