Transforming One‐Dimensional Nanowalls to Long‐Range Ordered Two‐Dimensional Nanowaves: Exploiting Buckling Instability and Nanofibers 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.

Directional elastic wave propagation in high-aspect-ratio photoresist gratings: liquid infiltration and aging

“…These nanowalls are amenable to buckling by boundary forces resulting from dissimilar expansion of the sample to the substrate. 13 Also, capillary forces during drying of patterned lithographic films from developer lead to pattern collapse if the AR is high or the material is mechanically weak. 14 The stability and mechanical properties of nanostructures is of paramount importance to guarantee their optimal performance.…”

“…Through finite element method (FEM) modeling we access to the complete characterization of the 1D grating consisting of an array of nanowalls. Because the susceptibility of nanowall collapsing is dependent on the aspect ratio, 13,24 we evaluate the size-dependent fidelity on high AR and low AR nanowalls. The direction-dependence is assessed in both, pristine and buckled-defect gratings.…”

Polymeric Flaky Nanostructures from Cellulose Stearoyl Esters for Functional Surfaces

“…The formation of organized hierarchical structures from polymers on surfaces showing uniform structures and morphologies is still demanding, although they will provide novel aspects for the preparation of diverse functional surfaces . In particular, the surfaces with porous hierarchical structures will be of great interest for a broad range of applications, such as superhydrophobic materials,[6,10c,23] catch and release system, and sensors …”

Designing unit cell in three-dimensional periodic nanostructures using colloidal lithography

“…The process optimization of the ratio of photo-initiator to SU-8 solution is currently underway to avoid these defects due to the incomplete cross-linking. Another limitation of our work came from the nature of SU-8, which requires a postexposure bake step to crosslink the polymer and swells during development [33]. Although the baking process was optimized experimentally to obtain structures with high quality, the swelling issue cannot be resolved thoroughly and resulted in mechanical instability of 3D nanostructure.…”