Ultrasmall Designed Plasmon Resonators by Fused Colloidal Nanopatterning.

This work presents a procedure for large-area patterning of designed plasmon resonators that are much smaller than possible with conventional lithography techniques. Fused Colloidal Nanopatterning combines directed self-assembly and controlled fusing of spherical colloidal nanoparticles. The two-step approach first patterns a surface covered with hydrogen silsesquioxane (HSQ), an electron beam resist, forming traps into which the colloidal gold nanoparticles self-assemble. Secondly, the patterned nanoparticles are controllably fused to form plasmon resonators of any 2D designed shape. The heights and widths of the plasmon resonators are determined by the diameter of the nanoparticle building blocks, which can be well below 10 nm. By performing the fusing-step with UV-ozone and heat exposure, we demonstrate that the process is easily scalable to cover large areas on silicon wafers with designed gold nanostructures. The procedure neither requires adhesion layers, nor a lift-off process, making it ideally-suited for plasmonics, in comparison with regular electron beam lithography. We use monochromated electron energy-loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM) as well as boundary-element method simulations to demonstrate that the designed plasmon resonators are directly tunable via the pattern design. We foresee future expansion of this approach for applications such as plasmon-enhanced photocatalysis, and for large-scale patterning where chemical, optical, or confinement properties require sub-10 nm metal lines.