What are the Limitations in the Characterization of Self-Assembled Metamaterials using Advanced Microscopy Techniques? C.J. Kiely,* M. Watanabe,* A. Burrows,* P. Clasen,* M.P. Harmer,* B.Rodríguez-González,** L. Liz-Marzán,** I. Hussain,*** J. Fink*** and M. Brust *** * Center for Advanced Materials and Nanotechnology, Lehigh University, 5 East Packer Avenue, Bethlehem, PA 18015-3195, USA. ** Departmento de Quimica Fisica, Universidad de Vigo, 36200, Vigo, Spain. *** Center for Nanoscale Science, University of Liverpool, Liverpool, Merseyside, L69 7ZD, UK. The controlled manipulation of materials on the nanometer scale is an important task for the production and precision positioning of ever more compact electronic, optical and magnetic components. It is now possible to create thin films, nanowires and 3-D supercrystals by exploiting the order inducing chemical interactions that are inherent to a particular system. For instance, ordered structures comprised of monosized ligand stabilized nanoparticles (of most materials) can be ‘self-assembled’ from a solution of their components simply by evaporating a drop of such solution onto a suitable substrate. The current state-of-the-art is the production of metamaterials whereby nanoparticles of different chemical identities and sizes are self-assembled to form nanocomposite materials. Alkanethiol capped gold and silver nanoparticles were the first to be assembled in regular arrays [1] followed by metamaterials composed of ordered magnetic (iron oxide) and semiconducting (lead selenide) nanoparticles [2] . In principle there an infinite number of combinations of different nanoparticle types that could be self-assembled into new metamaterials, provided that in each case, the ligand shell and solvent chemistry of both species can be made compatible. This opens up a whole new vista of designer nanomaterial combinations (e.g. metals with semiconductors, non- magnetic with magnetic metals, ceramics with polymers, etc). Each new metamaterial will display physical properties determined by the nature of the nanoparticle components and the linker ligand molecules. Characterizing the structure, composition and physical properties of such metamaterials poses significant new challenges for the materials scientist. Consider the Au-Ag nanoalloy raft shown in Figure 1(a). Particle size and morphology, as well as the quality of the raft ordering, can to some extent be assessed by simple visible inspection of BF and ADF images. In the future however, HAADF tomography offers the possibility of directly visualizing the 3D facet structure of individual nanoparticles [3] . Particle and superlattice crystallography can currently be evaluated by analysis of polycrystalline ring patterns and low angle diffraction patterns respectively (Figures 1(b,c)). A future goal is to be able to perform automated CBED pattern analysis to map the relative orientations of all the particles in the array (analogous to the automation now available in EBSD). The chemical identity of the individual particles in the array can be established by EDS or EFTEM analysis (Figure 1(d)). However if the array were composed of chemically more complicated core-shell or alloy nanoparticles, then a much higher spatial resolution spectroscopic analysis would be required. For instance, Figure 2 shows STEM- EDS maps of multi core-shell Au@Ag@Au nanoparticles. These maps have been enhanced using multivariate statistical analysis (MSA): a statistical method ideally suited to spectral analysis of small particles [4] . Protocols for quantitatively mapping the composition of such complex entities are only just being developed. Aberration corrected STEM affords the new possibility of performing atomic-column-by-atomic-column XEDS and EELS spectroscopy of nanomaterials [5] , allowing in DOI: 10.1017/S1431927605507153 Copyright 2005 Microscopy Society of America Microsc Microanal 11(Suppl 2), 2005 204