Assembly of Metal Nanoparticle Arrays Using Molecular Bridges

organizing metal nanoparticles into symmetrically and spatially well defined architectures. The impetus for this work is two-fold. First, metal nanoparticle aggregates display rich optical behaviors that are distinctly different from a corresponding collection of individual particles or the extended solid. A range of fundamentally interesting new materials can thus be constructed from a single inorganic building block (gold, silver nanoparticles) simply by controlling such parameters as aggregate size, shape, and interparticle distance. Indeed, nanoparticle linear and nonlinear optical properties, hyperpolarizabilities, and electric field enhancement factors have been found to depend strongly on these parameters.1 In addition to generating significant fundamental interest, applications of nanoparticle-based materials are emerging, in which collective nanoparticle optical properties are exploited for colorimetric, surface-enhanced Raman (SERS) and surface plasmon resonance (SPR) bioassays.2 A number of home pregnancy and drug test kits based on the optical properties of gold nanoparticles are already available commercially.3 A second theme that has emerged in nanoparticle research more recently pertains to their electrical characteristics. Individual 5 nm diameter particles have been wired up and shown to have properties useful in fabricating nanoscale analogues of traditional electronic device components such as transistors and tunnel diodes.4 Less conventional forms of computing based on assemblies of nanoparticles have also been proposed. These schemes, coined quantum cellular automata (QCA), rely on electrostatic coupling between square planar assemblies of metal dots.5 However, the implementation of QCA at room temperature will require dots smaller than 10 nm in diameter. Arranging such small structures into well defined “integrated” systems is difficult using most forms of lithography (e.g., photo, electron beam, scanning probe lithographies). Chemical self-assembly likely will become an important tool for the realization of integrated nanoscale electronics. In this respect, metal nanoparticles are potentially powerful minimum device components because their surfaces can be modified using a number of well developed and relatively routine chemical attachment strategies. With these fundamentally interesting and technologically important goals in mind, we sought new protocols for assembling nanoparticles into covalently linked arrays. We chose to begin our studies by linking gold and silver particles together to form relatively small aggregates—dimers, trimers, and Assembly of Metal Nanoparticle Arrays Using Molecular Bridges