Transforming One-Dimensional Nanowalls to Long- Range Ordered Two- Dimensional Nanowaves: Exploiting Buckling Instability and Nanofi bers Effect in Holographic Lithography

Abstract

Two-dimensional nanowaves with long-range order are fabricated by exploiting swelling-induced buckling of one-dimensional (1D) nanowalls with nanofibers formed in-between during holographic lithography of the negative-tone photoresist SU-8. The 1D film goes through a constrained swelling in the development stage, and becomes buckled above the critical threshold. The degree of lateral undulation can be controlled by tuning the pattern aspect ratio (height/width) and exposure dosage. At a high aspect ratio (e.g., 6) and a high exposure dosage, nanofibers (30-50 nm in diameter) are formed between the nanowalls as a result of overlapping of low crosslinking density regions. By comparing experimental results with finite-element analysis, the buckling mechanism is investigated, which confirms that the nanofibers prevent the deformed nanowalls from recovery to their original state, thus, leading to long-range ordered two-dimensional (2D) wavy structures. The film with nanowaves show weaker reflecting color under an ambient light and lower transmittance compared to the straight nanowalls. Using double exposure through a photomask, patterns consisting of both nanowaves and nanowalls for optical display are created.