Metal films prepared by stepwise assembly. 2. Construction and characterization of colloidal Au and Ag multilayers

This manuscript describes the stepwise, ligand-directed assembly, character ization, and prospective applications of three-dimensional Au and Ag nanopa rticle, multlilayered films. Films were prepared by successive treatments o f a Au nanoparticle monolayer with a bifunctional cross-linker and colloida l Au or Ag solutions. Changes in film electrical and optical properties are reported for a series of bifunctional cross-linkers of varying molecular l engths. Interestingly, these films exhibit Beer's law behavior despite the presence of strong interparticle optical coupling. Multilayer films with gr eater than six exposures to 2-mercaptoethylamine and Au colloid were highly conductive and resembled bulk Au in appearance. In contrast, films of simi lar particle coverage generated using a longer cross-linker (1,6-hexanedith iol) exhibited higher transmission in the near-infrared region and exhibite d a reduced conductivity. Measurement of the multilayer morphology with ato mic force microscopy, electrostatic force microscopy, and field emission sc anning electron microscopy revealed a porous, discontinuous morphology comp osed of large, continuous regions of aggregated nanoparticles. This, in tur n, results in a surface roughness contribution to surface plasmon scatterin g and surface-enhanced Raman scattering observed for Au, Au/Ag, and Ag coll oid multilayers. Particulate multilayer films made using horseradish peroxi dase as a cross-linker remained enzymatically active, even beneath three la yers of colloidal Au. Multilayers could also be prepared on surfaces patter ned by microcontact printing. These data show how Au colloid multilayers gr own in solution are a viable alternative to evaporated metal films for a nu mber of applications.