Formation of Hierarchical Nanoparticle Pattern Arrays Using Colloidal Lithography and Two-Step Self-Assembly: Microspheres atop Nanospheres

We report a simple approach to the fabrication of hierarchical nanoparticle arrays and film patterns using a novel combination of colloidal lithography (CL), two-step self-assembly, and reactive-ion etching (RIE). In this approach, a uniform nanoparticle film (∼15-50 nm particle diameter) is first deposited on a substrate. Then, larger (several hundred to thousands of nanometers diameter) microparticles with a different composition are self-assembled into well-ordered patterns atop the nanoparticle film. Next, reactive-ion etching is used to remove parts of the initial nanoparticle film using the upper layer of large particles as a mask. After selective removal of the remaining upper layer of large particles, hierarchical nanoparticle patterns are obtained on flat surfaces. Hexagonal patterns of small nanoparticle film arrays were easily fabricated with a monolayer of large spheres using this approach. Moreover, the shape and diameter of nanoparticle film disks depend on the etching duration while the periodicity of the selfassembled upper layer is preserved during the etching process. The profiles of the nanoparticle film patterns (pitch, thickness of film, etc.) are adjustable with the size of the large particles, thickness of nanoparticle film, and nanoparticle size. Furthermore, additional nanoparticle film patterns are possible with the use of additional layers of large particles. We have demonstrated the feasibility of this approach with both polystyrene (PS) spheres on silica nanoparticles and silica spheres on PS nanoparticles. This bottom-up approach offers a novel lithography-free method for the fabrication of patterned nanoparticle films that will be useful for material growth, biosensing, and catalysis as well as serving as a unit operation for further fabrication.