at the tops owing to etching in fluoridecontaining electrolyte leads to thinning and collapse of tubes with formation of nanograss (a,b) or perforated tube walls in the upper tube parts (c). d) Bundling at tube tops owing to a lack of mechanical support. e) Highly ordered grass-free and open tubes prepared by photoresist masking. f) Tube-wall thinning and formation of needles (grass).