1. Materials Science and Engineering, Department of Engineering, The University of Liverpool, Liverpool L69 3BX, UK
2. Department of Chemistry, The University of Liverpool, Liverpool L69 7ZD, UK

Correspondence to: C. J. Kiely¹ Correspondence and requests for materials should be addressed to C.J.K. (e-mail: Email: kiely@liv.ac.uk).

The controlled fabrication of very small structures at scales beyond the current limits of lithographic techniques is a technological goal of great practical and fundamental interest. Important progress has been made over the past few years in the preparation of ordered ensembles of metal and semiconductor nanocrystals^{1, 2, 3, 4, 5, 6, 7}. For example, monodisperse fractions of thiol-stabilized gold nanoparticles⁸ have been crystallized into two- and three-dimensional superlattices⁵. Metal particles stabilized by quaternary ammonium salts can also self-assemble into superlattice structures⁹,¹⁰. Gold particle preparations with quite broad (polydisperse) size distributions also show some tendency to form ordered structures by a process involving spontaneous size segregation¹¹,¹². Here we report that alkanethiol-derivatized gold nanocrystals of different, well defined sizes organize themselves spontaneously into complex, ordered two-dimensional arrays that are structurally related to both colloidal crystals and alloys between metals of different atomic radii. We observe three types of organization: first, different-sized particles intimately mixed, forming an ordered bimodal array (Fig. 1); second, size-segregated regions, each containing hexagonal-close-packed monodisperse particles (Fig. 2); and third, a structure in which particles of several different sizes occupy random positions in a pseudo-hexagonal lattice (Fig. 3).