Folding 2D Structures into 3D Configurations at the Micro/Nanoscale: Principles, Techniques, and Applications

Abstract

configurations are no longer sufficient, either in terms of realizing certain crucial functionalities or in performance improvement. Compared to 2D structures, 3D structures have one more dimension, which enables more diversity in structure/ device design and is crucial in realizing richer physical interactions, better performance, and advanced functionalities. For example, to obtain a negative permeability from split ring resonators (SRRs) to construct negative index metamaterial, a vertical configuration of SRRs is needed to couple with the magnetic field, while planar SRRs can only couple with the electric field to obtain a negative permittivity. [4] With respect to device performance, 3D gold helixes [5] have larger polarization contrast than 2D chiral structures, [6] and dynamic 3D microcontainers [7] can realize drug delivery in a much more controllable way than absorption on 2D structures. [8] Therefore, 3D micro/nanostructures are of significant importance in such scenarios, acting as an indispensable supplement to the present 2D micro/nanostructures. However, the fabrication of 3D micro/nanostructures is a formidable challenge with the state-of-the-art equipment, because the traditional planar process cannot be used directly due to its 2D projection nature.Much effort has been devoted in past decades, and significant progress has been demonstrated with 3D micro/nanofabrication, which can be divided into two types of strategies. The first one is brand new technologies/equipment development, including 3D laser direct writing (LDW) [9] and focused ion beam (FIB) [10] processing using two-photon polymerization and ion beam milling/deposition to construct 3D structures. Great scientific advantages have been demonstrated in the fields of mechanics, optics, and biology using these techniques. [11] However, due to their intrinsic point-by-point writing style, the efficiency of LDW and FIB is limited. Moreover, the materials that can be proceeded by the two techniques are usually photoresists (two-photon absorption) and metals (FIB-assisted deposition), and transferring to other materials can be quite challenging in most cases. [5,12] The other strategy is a combination or modification of the technologies in planar processes (including lithography, deposition, and etching) and is referred to as "planar technology" in this review. In this strategy, a variety of 3D fabrication methods have been developed, such as multilayer stacking, [13] oblique angle deposition, [14] and self-aligned Compared to their 2D counterparts, 3D micro/nanostructures show larger degrees of freedom and richer functionalities; thus, they have attracted increasing attention in the past decades. Moreover, extensive applications of 3D micro/nanostructures are demonstrated in the fields of mechanics, biomedicine, optics, etc., with great advantages. However, the mainstream micro/nanofabrication technologies are planar ones; therefore, they cannot be used directly for the construction of 3D micro/nanostructures, making 3D fabrication at the m...

Symmetry-Breaking Actuation Mechanism for Soft Robotics and Active Metamaterials

“…shows the normalized deflections as functions of the thickness ratio for bending (solid lines) and folding (dashed lines), respectively, under 4 6 10 B − = . As the thickness ratio Tc/Tf increases, both bending and folding deflections increase, but the bending deflection reaches a plateau.…”

A Programmable Nanofabrication Method for Complex 3D Meta-Atom Array Based on Focused-Ion-Beam Stress-Induced Deformation Effect

“…Micromachines 2020, 11, x FOR PEER REVIEW 3 of 15 capability of bending suspended thin films of various materials in both directions (−70-90°) precisely by controlling the ion dose and irradiation area of FIB and, the deformation might be caused by the stress change in the films under FIB irradiation [28]. The FIB-SID technique is a newly developed method to fabricate complex 3D micro/nanostructures [29][30][31][32][33], which can be used in optical modulation, controllable drug delivery, cell capsulation, and many other areas [34]. Compared to other 3D nanostructure array fabrication methods, the FIB-SID technique can process, in real-time, complex 3D structures with nanoscale precision and is versatile for diverse materials [28,35].…”

Self‐Folding Using Capillary Forces

“…In contrast, while the integration and assembly of inherently 2D and stacked devices are relatively facile at small size scales, i.e., below about 1 mm, using very large scale integration (VLSI) processes, it is extremely challenging to fabricate and assemble nonplanar, curved, and angled 3D structures with dissimilar materials. The challenge is rooted in several fundamental material and engineering constraints that are encountered at small size scales . For example, due to large surface area‐to‐volume ratios, surface roughness, trapped charges, and local condensation, stiction increases significantly.…”