Nanolithographic processes

Currently, the fabrication of well-defined nanostructures for both scientific and industrial purposes mainly relies on top-down nanolithographic processes, such as nanoimprint, X-ray, extreme ultraviolet, and electron-beam lithography [1,2], Most of these methods require time consuming, complex, and expensive beam sources for exposure as well as harmful corrosive chemicals for the involved etching steps. However, the resolution of the structures, the quantities of nanostmctures produced, and the fabrication of complex three-dimensional structures are usually rather limited for these techniques. 

However, it is worth to stress that, because of the nanolithographic process itself, quite often, the nanoelectrodes obtained are slightly recessed, so that theoretical model for such geometries must be taken into account. ... 

As a tool for nanotechnology, the STM has been well publicized in its ability to push or manipulate atoms into specific locations to create nanopattems. However, this is a relatively slow process and would not be commercially viable. A more feasible approach has been undertaken using the AFM to create nanolithographic patterns by using the tip to remove material from a siuface and create the desired pattern. 

Recently, many scientists and engineers have looked for methods to control the sizes of QDs and make possible the formation of ordered lateral two-dimensional superlattices or vertical superlattices for heterojunction thin films. One approach is the top-down method, molecular beam epitaxy nanolithographic technology, which has been developed with the development of microelectronics and processing techniques for traditional inorganic semiconductors. This technique of nanoscale manipulation can reach only the upper limits of sizes defined by nanostructure physics, and has successfully manipulated artificial atoms and molecules [10-12). Bottom-up method is based on molecular and supramolecular assembly techniques that have been proposed by chemists in recent years. With this method, it is possible to prepare monodispersed defect-free nanocrystal QDs 1-10 nm in size and to control easily QDs coupling to form nanocrystal molecules, even quantum dot superlattices in two or three dimensions. 

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