Nanofabrication approaches for functional three-dimensional architectures

Three-dimensional (3D) functional systems with sizes in the micro- and nanometer regime are of growing importance across a wide range of electrical, optical, and biological contexts. The unique functionalities of such platforms follow from engineering the shapes, geometries, and architectures of materials into 3D layouts. The main challenges are in developing techniques/methods with sufficient levels of resolution and with broad materials compatibility. The most successful routes include (1) top-down schemes that involve masking and selective material removal, often in repetitive sequences, (2) bottom-up strategies that rely on component assembly or materials deposition, (3) two-dimensional (2D) to 3D shape transformations triggered by internal forces or external stimuli, and more recently, (4) mechanically guided assembly driven by substrate deformations. This review highlights recent progress in these areas, with a focus on techniques with demonstrated capabilities in constructing functional 3D structures and/or devices with key dimensions in the nanoscopic regime, and on their demonstrated or potential applications.