# Photocatalytic Lithography
###### Abstract  
Patterning, the controlled formation of ordered surface features with different physico-chemical properties, is a cornerstone of contemporary micro- and nanofabrication. In this context, lithographic approaches owe their wide success to their versatility and their relative ease of implementation and scalability. Conventional photolithographic methods require several steps and the use of polymeric photoresists for the development of the desired pattern, all factors which can be deleterious, especially for sensitive substrates. Efficient patterning of surfaces, with resolution down to the nanometer scale, can be achieved by means of photocatalytic lithography. This approach is based on the use of photocatalysts to achieve the selective chemical modification or degradation of self-assembled monolayers, polymers, and metals. A wide range of photoactive compounds, from semiconducting oxides to porphyrins, have been demonstrated to be suitable photocatalysts. The goal of the present review is to provide a comprehensive state-of-the-art photocatalytic lithography, ranging from approaches based on semiconducting oxides to singlet oxygen-based lithography. Special attention will be dedicated to the results obtained for the patterning of polymer brushes, the sculpturing of metal nanoparticle arrays, and the patterning of graphene-based structures. photocatalysis; lithography; titanium dioxide; patterning; sol-gel; self-assembled monolayers; polymer brushes; grafting-from; surface-initiated atom transfer radical polymerization; noble metal nanoparticles; graphene; porphyrin; reactive oxygen species; singlet oxygen +
Footnote †: journal: Applied Physics Letters  
_Applied Physics Letters_  
refers to the use of photographic images in lithographic printing, whether these images are printed directly from a stone or from a metal plate, as in offset printing. This technique and its terminology were introduced in Europe in the 1850s. Starting from the 1960s, photolithography has played an important role in the fabrication and mass production of integrated circuits in the microelectronics industry. The reason for such success is due to the enormous benefits associated with smaller electrical circuits, notably higher speed and much less energy consumed per computing function. Together with the search for pattern strategies, a race to achieve increasingly smaller features with high resolution was started. In general, the development of inexpensive, fast, and robust micro- and nanomanufacturing processes represents a major challenge. The fabrication of microprocessors, biomedical assays, lab- and organ-on-chip structures, actuators, and sensors of different kinds, are plagued by problems such as high costs, low throughput, and low overall process efficiency. Present chemical patterning methodologies include photolithography (PL), microcontact printing (\(\upmu\)CP), selective molecular assembly patterning (SMAP), dip-pen nanolithography (DPN), colloidal patterning or lithography (CP, CL), and nanoimprint lithography (NIL) [2; 3]. Limitations include contamination from resists, lack of pattern versatility and resolution, and time and temperature constraints. For these reasons, new lithographic techniques able to significantly reduce the cost and time associated with the fabrication of nanometer-sized features are required for pushing the boundaries of technological advance. Recently, a patterning strategy which exploits the peculiar physico-chemical properties of photoactive semiconductors (especially titanium dioxide) was found to be able to overcome many of these limitations. This approach is called \"photocatalytic lithography\" and is the subject of the present review. Photocatalytic lithography is an inexpensive, fast, and robust method of oxidizing (or reducing) chemical species on a target surface to produce patterns. Applications for this technique have been demonstrated not only on a laboratory scale, for the fabrication of functional surfaces and sensors, but also in industrial settings (offset printing).