3D Ordered Gold Strings by Coating Nanoparticles with Mesogens By Xiangbing Zeng, Feng Liu, Alan G. Fowler, Goran Ungar,* Liliana Cseh, Georg H. Mehl, and J. Emyr Macdonald The first 3D metamaterial with a negative refractive index in the near infrared has been announced recently. [1] Like previous examples of negative refraction at microwave frequencies, [2,3] the material was fabricated by a top-down technique using focused-ion-beam etching, in this case. There would be considerable advantages in being able to generate bulk 3D arrays of ‘‘meta-atoms’’, that is, metal nanoparticles, with prescribed geometrical layouts using a self-assembly approach. As recently theoretically shown, [4] devices with interesting optical properties, such as band-pass filters, could be generated from well-ordered 3D arrays of nanoparticles much smaller than the wavelength of light and spaced at a controlled distance from each other. Such systems would benefit from a Lorentz-type resonance in the permittivity at the collective plasmon frequency. Ordered clusters of such lattices, also producing resonances in permeability, could then lead to negative refractive indexes in a wide spectral range, including the visible-light range. [5] Here, we report on ordered bulk arrays of gold nanoparticles whose spatial arrangement is governed by their coating with a laterally attached nematic liquid-crystal-forming ligand. Grazing-incidence small-angle diffraction (GISAXS) of aligned thin films reveals highly ordered rhombohedral and two-columnar (hexagonal and rectangular) structures, with the gap between nanoparticles adjustable between 16 and 4A ˚ . Furthermore, the presence of the incorporated nanoparticles helps to provide evidence of long- range biaxial order in the surrounding nematic in one of the phases observed. Interest in adding colloids and nanoparticles to liquid crystals (LCs) has been largely based on their ability to significantly alter the dielectric behavior of LCs, which could potentially result in faster switching devices. [6–10] The striking ability of nematic LCs to induce 1D and 2D order in dispersed particles on the colloidal scale has been the subject of numerous experimental and theoretical studies. [11,12] Chains, braids, and rafts of microme- ter-sized colloidal spheres were assembled using laser tweezers and held together by dipolar and quadrupolar defects in the director field. [13] Assembling metallic nanoparticles is of great interest for electronic, [14] optical, [15] and photonic [16] applications. Metal nanoparticles derivatized with alkane thiols, acids, or amines could only be made to form bulk lattices with long-range order if they were highly monodisperse. [17,18] The lattices were those expected from packing of spheres, that is, body-centered cubic (bcc), hexagonal close-packed, or face-centered cubic (fcc). Derivatization of gold nanoparticles with bulkier monodendron thiols, [19] as well as with DNA sequences, [20,21] also produced bcc or fcc lattices. Using a bimodal particle-size distribution, interesting planar structures have been achieved on surfaces. [22] Self-assembly of nanoparticles of two different materials has yielded a considerable diversity of binary nanoparticle super- lattices, [23] the structure being determined primarily by the close-packing criteria, but also modified by dipolar or magnetic forces. Gold nanorods coated with surfactants were also found to self-organize, but forming only short-range positional order. [24,25] Attempts to order nanoparticles by functionalizing them with LC-forming moieties are relatively recent. By end-attaching cyanobiphenyl mesogens to gold nanoparticles via a thioalkyl spacer, In et al. [26] observed a nematic phase in which the particles formed wormlike chains. Bent core [27] and discotic [28] mesogens have also been attached to gold nanoparticles. In both cases, the functionalized particles were studied neat as well as dispersed as dopants in a host LC. Only one pure bent-core derivative formed a metastable LC phase of unknown nature. [27] The discotic-capped nanoparticles were thought to cluster at the grain boundaries of the columnar phase. Gold nanoparticles covered by rod-like mesogens attached laterally via a thioalkyl spacer were reported to form a nematic phase, without long-range nanoparticle order. [29,30] In a recent work, the attachment of mesogenic dendrons to Au naparticles resulted in loss of LC behavior, though interestingly, when deposited on surfaces, organization in layer-like structures was detected. [31] These results on mesogen- functionalized nanoparticles highlight the unique character of the materials described here, bearing laterally attached mesogens. We show that the nanoparticles can form more-complex anisotropic periodic lattices with 2D and 3D long-range order. The two systems studied here are defined in Figure 1a. Gold nanoparticles covered with hexyl- or dodecylthiol were prepared first. In the second stage, two fifths of the alkylthiol were replaced by the mesogen ligand in a solvent-mediated exchange reaction. COMMUNICATION www.advmat.de [*] Prof. G. Ungar, Dr. X. B. Zeng, F. Liu, Dr. A. G. Fowler Department of Engineering Materials University of Sheffield Mappin Street, Sheffield S1 3JD (UK) E-mail: g.ungar@shef.ac.uk Dr. L. Cseh, [+] Dr. G. H. Mehl Department of Chemistry University of Hull Hull HU6 7RX (UK) Dr. J. E. Macdonald School of Physics and Astronomy Cardiff University Cardiff CF24 3AA (UK) [ + ] Present address: Romanian Academy, Institute for Chemistry, Timi- soara, Romania DOI: 10.1002/adma.200803403 1746 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2009, 21, 1746–1750