Nanoscale Patterning of Colloidal Nanocrystal Films for Nanophotonic Applications Using Direct Write Electron Beam Lithography.

The small size of colloidal nanocrystal quantum dots (QDs) leads to a variety of unique optical properties that are well-suited to nanophotonics, including bright, tunable photoluminescence (PL). However, exploring the properties of solid QD assemblies at the nanoscale has proven challenging because of the limitations in the nanoscale QD patterning methods. Generally, the precise placement of QD solids is difficult to achieve, especially for tall structures with multiple QD layers, and when it is achieved the patterns often cannot withstand the further processing steps required for final device construction. Direct electron beam lithography of QDs has emerged as a straightforward patterning process that does not require ligand exchange and results in structures that retain bright PL. Here, we demonstrate that direct patterning QD films on substrates treated with a self-assembled monolayer of octadecyltrichlorosilane allows us to create feature sizes as thin as 30 nm with heights of multiple layers and characterize the pattern resolution, robustness, and placement accuracy. These structures withstand sonication in a variety of solvents, and the structures are placed within 20 nm of their intended location nearly 100% of the time. We further show how this patterning method can be applied to nanophotonics by measuring the complex refractive index of the QD materials to model the absorption and scattering cross sections of QD structures of various sizes and shapes. These simulations reveal that edge effects arising from the finite shape of the QD nanostructure lead to substantial absorption enhancement when compared to an equivalent volume region taken from a continuous QD film. Finally, we explore more complex structures by patterning QD arrays, multilayer QD structures, and QD disks inside plasmonic resonators.