Block copolymer templated chemistry for the formation of metallic nanoparticle arrays on semiconductor surfaces

Precise positioning of metallic nanostructures on semiconductor surfaces is important for applications such as photovoltaics, photoelectrocatalysis, metal interconnects, sensing platforms, and many others. In this paper, we demonstrate the utilization of self-assembling diblock copolymer monolayer films, made up of polystyrene-block-poly(2- or 4-vinylpyridine) (PS-b-P2VP or PS-b-P4VP), to spatially direct an aqueous metal reduction reaction on semiconductor surfaces, a process we call galvanic displacement. The diblock copolymer forms hexagonal arrays of spherical micelles consisting of a P2VP or P4VP core surrounded by a PS corona. Two approaches were developed, termed method I and method 2, to deliver metal ions to the semiconductor interface in a spatially defined manner utilizing the diblock template. In method 1, a metal complex preloaded into the P4VP cores is spontaneously reduced on the surface to form hexagonally ordered metallic nanoparticles whose structures mirror the parent polymer templates. This approach was employed to produce ordered Ag nanoparticles on Ge(100), InP(100), and GaAs(100) surfaces. Method 2, on the other hand, involves coating the semiconductor surface with an unloaded self-assembled block copolymer monolayer, followed by immersion in a solution of the metal ions and additional reagents, if required. Method 2 is particularly useful to pattern semiconductor surfaces that require the presence of hydrofluoric acid (HF) as an etchant to initiate the galvanic displacement, including Si(100). Using method 2, Cu, Au, Pt, and Pd nanoparticles were patterned on the semiconductor surfaces. In addition, the apparent order of the self-assembled monolayers is better as compared to that of the preloaded block copolymers (prepared via method 1). Since the self-assembling nanostructures of the PS-b-P2VP or PS-b-P4P diblock copolymers can be inverted to a PS core surrounded by a P2VP or P4VP corona (the so-called core-corona inversion) in the presence of HF, patterns of the resulting metallic structures are influenced by this morphological shift. The effects of polymer morphology on the galvanic displacement is described, and as an alternative approach, metal ion reduction and polymer removal with hydrogen/argon plasma is outlined.