Metal-assisted chemical etching uses two steps. First, a thin layer of gold is patterned on top of a semiconductor wafer with soft lithography (top-left). The gold catalyzes a chemical reaction that etches the semiconductor form the top down, creating three-dimensional structures for optoelectronic applications (bottom-left). A scanning electron microscope image of “nanopillars” etched in gallium arsenide (right). | Graphic & Image by Xiuling Li