Complex Chiral Colloids and Surfaces via High-Index Off-Cut Silicon Kevin M. McPeak, † Christian D. van Engers, † Mark Blome, ‡,§ Jong Hyuk Park, †,∥ Sven Burger, ‡,§ Miguel A. Gosa ́ lvez, ⊥,¶,□ Ava Faridi, † Yasmina R. Ries, † Ayaskanta Sahu, † and David J. Norris* ,† † Optical Materials Engineering Laboratory, Department of Mechanical and Process Engineering, ETH Zurich, 8092 Zurich, Switzerland ‡ Zuse Institute Berlin, 14195 Berlin, Germany § JCMwave GmbH, 14050 Berlin, Germany ∥ Photo-Electronic Hybrids Research Center, Korea Institute of Science and Technology, Seoul 136-791, South Korea ⊥ Department of Material Physics, University of the Basque Country UPV/EHU, Donostia-San Sebastian 20018, Spain ¶ Donostia International Physics Center (DIPC), Donostia-San Sebastian 20018, Spain □ Centro de Física de Materiales CFM − Materials Physics Center MPC, centro mixto CSIC − UPV/EHU, Donostia-San Sebastian 20018, Spain *S Supporting Information ABSTRACT: Silicon wafers are commonly etched in potassium hydroxide solutions to form highly symmetric surface structures. These arise when slow-etching {111} atomic planes are exposed on standard low-index surfaces. However, the ability of nonstandard high-index wafers to provide more complex structures by tilting the {111} planes has not been fully appreciated. We demonstrate the power of this approach by creating chiral surface structures and nanoparticles of a specific handedness from gold. When the nanoparticles are dispersed in liquids, gold colloids exhibiting record molar circular dichroism (>5 × 10 9 M −1 cm −1 ) at red wavelengths are obtained. The nanoparticles also present chiral pockets for binding. KEYWORDS: Chiral nanoparticles, anisotropic etching, localized surface plasmons, gold colloids, circular dichroism I f a planar material is patterned with a specific surface structure, its optical properties can be modified through scattering, diffraction, and absorption. To exploit this effect, various lithographic techniques have been advanced to create specific two-dimensional (2D) surface structures at micrometer to nanometer length scales. 1 In this same size regime, the optical properties of colloidal particles (solids dispersed in a liquid) can also be strongly influenced by their shape. 2 Consequently, chemical methods have been developed to synthesize particles with complex geometries. 3−6 However, while both surface structures and colloidal particles that are highly anisotropic can now be made, they are typically achiral (i.e., contain at least one plane of symmetry). Chiral shapes, those that are not superimposable on their mirror image, remain challenging. Such structures can lead to interesting optical effects such as superchiral electromagnetic fields. 7−10 More generally, chirality is a fundamental asymmetry found in nature that should be available for the design of artificial optical surfaces and colloids. To address this challenge, several strategies to obtain chiral structures have been developed. Conventional electron-beam lithography can create flat surface patterns with 2D chirality. 11,12 This can be extended to multiple layers that together exhibit true three-dimensional (3D) chirality. 13 For continuous structures attached to a surface, such as micro- meter-scale helices, direct laser writing in a photopolymer can also be employed. 14,15 To go beyond surface structures, the preparation of chiral colloids has been pursued. If solid particles with a well-defined size and chiral shape could be dispersed in a liquid, many new effects could be explored, including interactions with large chiral molecules for biological or pharmaceutical applications. However, while nanocrystals have recently been synthesized from HgS, a crystalline solid that has a chiral unit cell, 16 particles with a controllable chiral shape have not been possible through colloidal chemistry. Rather, more sophisticated approaches have been used to assemble achiral particles into specific chiral aggregates through magnetic fields, 17 twisted fibers, 18 or DNA. 19−21 Colloidal helices have also been grown at cryogenic temperatures from surfaces using glancing angle deposition and then released as a liquid dispersion. 22 Similarly, colloidal chiral nanoshells have been released after multiple depositions at tilted angles onto achiral ZnO pillars. 23 Despite this progress, important chiral shapes beyond aggregates and helices remain unavailable. Received: March 19, 2014 Revised: April 18, 2014 Published: April 19, 2014 Letter pubs.acs.org/NanoLett © 2014 American Chemical Society 2934 dx.doi.org/10.1021/nl501032j | Nano Lett. 2014, 14, 2934−2940