Self-assembly of charged, equally sized metal nanoparticles of two types (gold and silver) leads to the formation of large, sphalerite (diamond-like) crystals, in which each nanoparticle has four oppositely charged neighbors. Formation of these non-close-packed structures is a consequence of electrostatic effects specific to the nanoscale, where the thickness of the screening layer is commensurate with the dimensions of the assembling objects. Because of electrostatic stabilization of larger crystallizing particles by smaller ones, better-quality crystals can be obtained from more polydisperse nanoparticle solutions.C rystalline aggregates composed of one or more types of metallic and/or semiconductor nanoparticles (NPs) are of great interest for the development of new materials with potential applications in areas such as optoelectronics (1), high-density data storage (2), catalysis (3), and biological sensing (4). To date, methods for the crystallization of two-dimensional (2D) and 3D NP superlattices have relied on the differences in the sizes of component particles and on attractive van der Waals or hard-sphere interactions between them. This strategy has been successful in preparing several types of lattices Esuch as AB (5), AB 2 (6), AB 5 (7), and AB 13 (6)^, but the all-attractive nature of the interparticle potentials limits its applicability to relatively few and usually (8) close-packed structures.To overcome this limitation, we and others (8, 9) have focused on systems of NPs interacting via electrostatic forces; such forces provide a basis for ionic, colloidal (9), or even macroscopic (10) crystals, but, despite promising attempts (8, 11), have not been successfully exploited for controllable or predictable long-range organization of matter at the nanoscale. Here, we report electrostatic self-assembly (10) (ESA) of oppositely charged, nearly equally sized metallic NPs of different types into large 3D crystals characterized by sphalerite (diamond-like) (12) internal packing, and of overall morphologies identical to those of macroscopic diamond or sphalerite crystals (Figs. 1 to 4). Formation of these nonclose-packed structures results from the change in electrostatic interactions in the nanoscopic regime, where the thickness of the screening layer becomes commensurate with the dimensions of the assembling particles, and is facilitated by the presence of smaller, charged NPs in the crystallizing solutions that stabilize larger NPs by what can be termed a nanoscopic counterpart of Debye screening.We used Ag and Au NPs coated with w-functionalized alkane thiols (13): HS(CH 2 ) 10 COOH (MUA) and HS(CH 2 ) 11 NMe 3 þ Cl j (TMA) (Fig. 1A). These NPs were prepared according to a modified procedure (14) Esee Supporting Online Material (15)^and had average diameters of 5.1 nm (with dispersity s 0 20%) for Au and 4.8 nm (s 0 30%) for Ag (Fig. 1B). We chose this pair as a model system, because the average sizes of Au NPs passivated with MUA Eself-assembled monolayer (SAM) thickness 0 1.63 nm (16)^and Ag NPs cov...

Deformable, spherical aggregates of metal nanoparticles connected by long-chain dithiol ligands self-assemble into nanostructured materials of macroscopic dimensions. These materials are plastic and moldable against arbitrarily shaped masters and can be thermally hardened into polycrystalline metal structures of controllable porosity. In addition, in both plastic and hardened states, the assemblies are electrically conductive and exhibit Ohmic characteristics down to approximately 20 volts per meter. The self-assembly method leading to such materials is applicable both to pure metals and to bimetallic structures of various elemental compositions.

Electrostatic aggregation of oppositely charged silver and gold nanoparticles leads to the formation of core-shell clusters in which the shell is formed by the nanoparticles, which are in excess. Arguments based on Debye screening of interactions between like-charged particles help explain why these clusters are stable despite possessing net electric charge. The core-shell aggregates exhibit unusual optical properties with the resonance absorption of the shell particles enhanced by the particles in the core and that of the core suppressed by the shell. Experimental UV-vis absorption spectra are faithfully reproduced by Mie theory. The modeling allows for estimation of the numbers of particles forming the shell and of the shell's effective thickness. These theoretical predictions are substantiated by experiments using nanoparticles covered with different combinations of charged groups and performed at different values of pH.

For nanoparticles coated with binary SAMs (m-SAM NPs) containing charged or ionizable molecules, the relative equilibrium constants and the difference in the free energies of absorption of the m-SAM's components can be obtained in a straightforward way by titrating such NPs with oppositely charged nanoparticle “standards” until precipitation at the point of overall electroneutrality.

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