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 comprises a plurality of nanostructures extending upwardly from a substrate and a porous membrane extending across upper ends of the plurality of nanostructures. A method, according to another embodiment, comprises forming a block copolymer layer on a substrate, inducing self assembly of the block copolymer layer, selectively degrading a block polymer from the block copolymer layer, forming a porous membrane over the block copolymer layer, and removing a portion of the block copolymer layer for defining a plurality of nanostructures extending upwardly from the substrate after forming the porous membrane over the block copolymer layer. Other systems and methods are disclosed as well.