Spontaneous ordering of bimodal ensembles of nanoscopic gold clusters

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 nanocrystals1,2,3,4,5,6,7. 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). Figure 1 An ordered raft comprising Au nanoparticles of two distinct sizes with RB/RA ≈ 0.58. Shown are electron micrographs at low (a) and higher (b) magnification. c, The low-angle superlattice electron diffraction pattern obtained from this bimodal raft structure. Figure 2 Electron micrograph of a phase-separated A+B mixture of Au nanoparticles obtained when the RB/R A ratio is ∼0.47. In this case, RA = 4.5 ± 0.7 nm and RB = 9.6 ± 1.5 nm. Figure 3 Electron micrograph of a ‘random alloy’ of Au nanoparticles obtained for an RB/RA ratio greater than 0.85.