Al2O3 nanotube History of engineering & technology Pd nanoparticles-assisted chemical etching X-ray diffractive optics X-ray grating interferometry anisotropic dry etching antireflection atomic layer deposition black GaAs bulk Si etching catalyst catalyst encapsulation curved Si structure electroless deposition etching rate ethanol electrooxidation gold (Au) metal assisted chemical etching gold electroplating high aspect ratio nanostructures magnetically guided metal-assisted chemical etching metal assisted chemical etching metal-assisted chemical etching photon recycling porous silicon silicon silicon cones transversal pores ultra-high aspect ratio zone plate