Block copolymer lithography has emerged as an alternative lithographic method to achieve large-area, high-density patterns at resolutions near or beyond the limit of conventional lithographic techniques for the formation of bit patterned media and discrete track media. In one embodiment, a structure includes a plurality of nanostructures extending upwardly from a substrate and a porous membrane extending across upper ends of the plurality of nanostructures. Other systems and methods are disclosed as well.