large area periodic nanostructures exhibit unique optical and electronic properties and have been applied into many areas , such as photonic band - gap materials , high dense data storage , and photonic devices . to fabricate these periodic nanostructures , standard photolithography methods can not easily reach the resolution required . high - resolution methods such as e - beam lithography and focal ion beam milling are too slow to reach a large area because of their inherent serial property . nano - imprint methods are fast to be applied , but it needs to use the mold , which requires the same resolutions as the patterns . so , it also benefits from the development of fast , economic , and high throughput fabrication methods with a high resolution . we have developed a novel photolithography technique , the nanosphere photolithography ( nsp ) technique , which utilizes a self - assembled ordered monolayer of hexagonally close packed ( hcp ) micro - spheres as nano - jets   to generate sub - wavelength regular patterns over a large area on standard photoresist . here , we will fabricate periodic metallic nanoholes perforated in gold and aluminum films using nsp technique . we used nsp to produce a large area of hexagonally packed nanopillars of negative photoresist with a strong undercut . using these nanopillars , we produced large area uniform nanoholes perforated in different metal layers with controlled thickness by lift - off process . the diameter of the nanoholes is about 180 nm and the period of the hexagonal array is 1 m , which is depending on the size of the microspheres used . the exposure wavelength we used for nsp is a broadband source centered about 400 nm , which is far greater than the pattern size . figure 1 shows the 3d - fdtd simulations of light s intensity profile for silica microsphere with the diameter of 1 m for a conventional uv lithography i - line (  = 365 nm ) . the focused light s intensity is about 30 times as strong as the input light s and the full width at half - maximum ( fwhm ) of the focused light is about 150 nm , even smaller than half of the wavelength . figure 2 is the normalized light intensity cross section after being focused by silica microsphere of 1 m diameter with different wavelengths of uv light from 300 to 500 nm . it shows that the fwhm values of the focused light are getting smaller when the wavelength is scaling down . fwhm of the light intensity is a good measure of the photoresist exposure , so it is possible that the features we produced will scale down using the uv light of smaller wavelengths . one example of the focus process by the silica micro - sphere of 1 m the normalized intensity of the focused light versus the position with different wavelengths of uv light 3 . a standard commercial positive or negative photoresist such asshipley 1805was spun on the substrate . a large area of hcp monolayer of silica microspheres was formed by the convective self - assembly setup shown in fig . the high - resolution electrical motor made the movement of the stage with the sample . the method utilizes the convective force of water to push the spheres into a close monolayer along the meniscus as the stage moves . inthearea of the meniscus , the thin water layer is easily evaporated and the microspheres stay on the surface . the samples were exposed by conventional photolithography instrument ( quintel q-4000 ) under low exposure energy with a broad wavelength centered at 400 nm . before development , the spheres were removed by ultrasonication in d.i . the schematic process of nsp for fabrication nanostructures ; from ( b ) to ( c ) , it switches to two different processes with positive or negative photoresist used the setup used for forming a large area of hcp micro-/nano - spheres and one snap shot of the spheres forming hcp monolayer a large area of hcp monolayer of silica microspheres was formed as shown in fig . 5a . the scanning electron microscope ( sem ) image shows the top view of a typical monolayer of silica spheres with the diameter of about 1 m on top of photoresist . as shown in the figure , there seems an adhesion force between the spheres that makes the spheres close to each other . figure 6a shows a large area of hexagonally packed nanopillars of photoresist after development . in the array there are some defects including the pillars falling off and an empty region without pillar , but they have a very small ratio . the diameter of the nanopillars is about 180 nm and the periodicity of the hexagonal array is 1 m , identical to the diameter of the microspheres . the height of the photoresist pillars is about 500 nm , which is depending on the photoresist thickness . figure 7a and b shows the sem images of the nanoholes perforated in gold and aluminum films separately , which were formed by lift - off on the photoresist nanopillars . the diameter of the nanoholes is about 180 nm and they are very uniform . besides using 1-m microsphere , different sizes of microspheres can be applied in nsp , so the periods of the nanohole arrays produced can also be changed . ( a ) a typical sem image of a large area of hcp monolayer of silica microspheres forming on top of photoresist ; ( b ) the titled view of the spheres on photoresist ( a ) a typical sem image of a large area of uniform nanopillars of photoresist after development ; ( b ) the enlarged view of the nanopillars sem images of hexagonal distributed uniform nanoholes ( diameter of about 180 nm ) perforated in the gold ( a ) and aluminum ( b ) films fabricated by our lithography technique combined with lift - off process we have presented a large area of uniform metallic nanoholes of about 180 nm produced by nanosphere photolithography technique with a broadband wavelength centered at 400 nm . our simulation results show that even smaller nanoholes with tunable periods can be generated with a shorter wavelength .
large area periodic nanostructures exhibit unique optical and electronic properties and have found many applications , such as photonic band - gap materials , high dense data storage , and photonic devices . 
 we have developed a maskless photolithography method 
 nanosphere photolithography ( nsp)to produce a large area of uniform nanopatterns in the photoresist utilizing the silica micro - spheres to focus uv light . here , we will extend the idea to fabricate metallic nanostructures using the nsp method . 
 we produced large areas of periodic uniform nanohole array perforated in different metallic films , such as gold and aluminum . 
 the diameters of these nanoholes are much smaller than the wavelength of uv light used and they are very uniformly distributed . 
 the method introduced here inherently has both the advantages of photolithography and self - assembled methods . besides , it also generates very uniform repetitive nanopatterns because the focused beam waist is almost unchanged with different sphere sizes .