# Photocatalytic Lithography
## 4 Applications of Direct and Remote Photocatalytic Lithography
### Patterning of Polymer Brushes  
When polymer chains are tethered on a surface by one end with a high enough grafting density, the resulting interchain repulsion makes the chains highly stretched, to minimize the interaction between neighbors. The resulting system resembles a brush, hence the name \"polymer brushes\". Polymer brushes are one of the most versatile tools for the engineering of surfaces and interfaces and, as such, they represent a key topic in state-of-the-art polymer nanotechnology [30; 31; 32; 33; 34; 35]. The elective method for the generation of polymer brushes is by grafting-from, i.e., by means of surface-initiated polymerization. In this approach, the chains are grown directly from a surface which has been previously functionalized with a suitable initiator. The opposite approach, grafting-to, where pre-formed chains are attached on a surface, is usually less advantageous because it leads to lower grafting densities. Progress in the field of polymer brushes has been mostly of a theoretical nature for a long time because of the lack of suitable or sufficiently practical polymerization methods. Conventional free radical polymerization is easy to perform, but leads to grafted chains with uncontrolled, variable length and does not allow the formation of block copolymers. On the other hand, more sophisticated techniques, such as living polymerization (anionic or cationic), allow excellent control over the chains' length as well as the formation of block copolymers, but require a high level of expertise and are strongly sensitive to even traces of moisture, oxygen, and certain functional groups. The introduction of controlled radical polymerization approaches, such as nitroxide-mediated radical polymerization (NMP), reversible addition-fragmentation transfer (RAFT), and especially atom-transfer radical polymerization (ATRP), generated a major breakthrough in the field by combining the precision of living polymerization with the user-friendliness of free radical polymerization, thus making the synthesis of polymer brushes possible almost for everyone [36; 37] (Figure 3). The field of polymer brushes was not left behind by the dramatic progress achieved in the fields of micro- and nanofabrication. With an increasing focus on hybrid techniques, many patterning strategies (top-down as well as bottom-up) have been proposed. Direct patterning is a top-down approach, in which chains of a pre-formed polymer film or brush are selectively removed or degraded by means of a locally applied mechanical force (e.g., shaving), or irradiation with light or particle beams. In the bottom-up approach--called \"indirect patterning\"--on the other hand, patterns of surface-bound initiators are first prepared and subsequently amplified into polymer brushes by means of surface-initiated polymerization techniques. This latter approach is the most popular, thanks to its versatility. Patterns of initiators can be generated with a variety of strategies, based on irradiation (photo- and interference lithography, electron beam lithography), mechanical contact (scanning probe lithography, soft lithography, nanoimprinting lithography) and on surface forces (colloidal lithography). However, there is still a significant demand for affordable and easily implemented lithographic techniques, which should update the existing technologies to reliably pattern polymer brushes at the wafer scale with high resolution micro- and nanometric features [38]. Such challenges could be successfully tackled by photocatalytic lithography. Photocatalytic lithography has the potential to perform at the same level (if not better) as more conventional techniques can do for the surface patterning of polymerization initiators. The use of photocatalysts to obtain patterns of polymer brushes with micrometric resolution is recent, reported first in 2015 by [PERSON] et al. [39; 40].