G. Strack, A. Luce, A. Akyurtlu
University of Massachusetts Lowell,
Keywords: nanoparticle array, intense pulsed light, nanosphere lithography
Summary:Over the recent decades, the growth of nanotechnology research has resulted in innovative work; however, the implementation of the demonstrated technologies is only possible if the fabrication process is rendered scalable. One promising approach, additive manufacturing, relies on the application of multiple layers to build the desired structure instead of removing or fusing components. In this work, we combined nanofabrication and additive manufacturing techniques to fabricate ordered nanoparticle arrays on flexible substrates. Ordered nanoparticle arrays have several promising applications, for example, thin films with tailored light scattering signatures, sensors based on surface-enhanced Raman scattering, nanostructured electrode arrays, and ordered catalytic islands for nanostructure growth. Traditional fabrications techniques, such as e-beam lithography, require highly trained experts and yield small, laboratory-scale sample sizes. Moreover, these processes are typically applied to ideal surfaces, such as Si wafers and are not readily transitioned to flexible, low temperature substrates. In this work, we use a low thermal budget method, intense pulsed light (IPL), as an alternative to radiant heating. IPL is the application high energy density broadband light (200 to 1500 nm) to nanometric metallic films. The light pulse occurs on the micro or millisecond timescale and the resultant energy is absorbed by the thin metallic thin film causing brief local heating. This scalable technique allows the local temperature to elevate enough to induce changes in the metallic film without damaging the substrate. Fabrication of the ordered nanoparticle arrays was carried out using nanosphere lithography (NSL) and IPL. First, polystyrene (PS) nanospheres (d=200 or 500 nm) were assembled in a hexogonally-packed monolayer on flexible glass (Willow® glass) or polyimide (PI) sheets by spin coating. Next, a thin (5 to 50 nm) layer of gold was deposited onto of the packed PS beads using sputtering or e-beam evaporation. Finally, IPL was applied to the sample until the PS layer was degraded and the structure of the metallic film was altered. IPL parameters were optimized for each substrate: insufficient energy density would not cause enough local heating, while excess energy density would result in sample degradation. When IPL was applied to gold films on PS nanophere-coated Willow glass, the PS degraded and left behind a gold nanoparticle (AuNP) smaller than the original nanosphere while retaining the original spacing. Control experiments at elevated temperatures (T=650° C) revealed similar results, indicating that the elevated temperature caused the Au film to melt while the PS nanophere directed the particle formation. The resultant AuNP size and spacing can be controlled by the thickness of the Au film and the diameter of the PS nanospheres. When IPL was applied to gold films on PS nanophere-coated PI, the PS degraded and left behind ordered clusters of nanoparticles in the shape of the original nanosphere. These results demonstrate that the substrate material directs particle formation, in addition to the metallic layer thickness, PS nanopshere size, and IPL parameters.