# Nano- and Micropatterning on Optical Fibers by Bottom-Up Approach: The Importance of Being Ordered
## 3 Bottom-Up Fabrication of Periodic Structures on OFs
### Other Self-Assembly Methods
#### 3.2.2 Directed Self-Assembly  
Nanofabrication methods based on directed self-assembly (DSA) utilize a template to guide the self-assembly process with the aim of minimizing the defect formation and promoting a long-range order organization. This technology has reached a considerable advancement level in the semiconductor and data-storage field, where DSA is one of the leading candidates for next-generation lithography. In this case, in fact, the spontaneous orientation of block-copolymers (BCPs) occurring by phase separation (giving rise to lamellae or cylinder arrays) is directed by a pre-patterned substrate [76; 77]. As a result, BCP long-range organization is promoted (Figure 4a) and the pitch of the final pattern can be reduced towards the sub-10 nm limit [78; 79]. Since the pre-patterning is often fabricated by conventional lithography methods, we can consider DSA as a combination of top-down and bottom-up approaches. Similar to NSL and BF approaches, the polymeric pattern can be transformed by further fabrication steps like selective etching or metal deposition into metallo-dielectric periodic patterns suitable for a variety of photonic applications. DSA of BCPs can be also adapted to produce patterning geometries that would be impossible to obtain by natural BCP self-organization. For example, [PERSON] et al. employed a circular template as a guiding pattern for a poly(styrene-block-dimethylsiloxane) (PS-b-PDMS) [80]. Depending on the presence of a notch in the circular template, either concentric rings or spiral nanostructures can be obtained from the cylindrical organization of the BCP (Figure 4b). Moreover, the authors demonstrated that they could control the 2D chirality of the spirals by adjusting the position of the notch. Considering that 2D and 3D chiral nanostructures have many potential applications in the sensing of molecular chirality or as chiral metamaterials, adapting this DSA process to the OF tip would be particularly advantageous. Besides BCPs, DSA can also be applied to nanoparticles [81]. As for BCPs, nanoparticle organization can be guided by a substrate bearing a pre-formed pattern, so that uniform and defect-free monolayers of colloidal particles can be obtained. In the example of Figure 4c, the authors study the assembly of PS nanoparticles in confined geometries fabricated by direct laser writing lithography. By simple spin-coating of a colloidal solution over substrates patterned with linear, hexagonal and circular structures, single-grain crystals with uniform orientation and regular shape over a large area were achieved [82]. Another option is to employ the periodic nanopattern of BCPs as a template, to guide the nanoparticle assembly. This approach was followed by [PERSON] et al. to decorate the OF tip with cluster arrays of Au nanoparticles [83]. Poly(styrene-block-2-vinylpyridine) (PS-b-P2 VP) was spin-coated on the chosen substrate (either a piece of silicon wafer or the OF tip) to obtain an array of reverse micelles (Figure 4(d-ii)). Then, the external surface of this film was plasma etched, to expose the positively charged part of the micelles to the outside. By immersion in a colloidal solution of Au nanoparticles, the micelles were selectively coated by the electrostatic assembly. The resulting OFs decorated by periodic  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Main Methods** & **Structures** & **Feature Sizes** & **Symmetry** & **Applications** \\ \hline Colloidal Self-assembly (SA)—basic method [43; 44; 45; 46; 50; 51] & 2D and 3D, opal and inverse opal, photonic crystals & from hundreds of nm up to a few \(\mu\)m & hexagonal & Refractive index and RH sensors [51], label-free biosensors [16] \\ \hline \multirow{4}{*}{Nanosphere lithography (NSL) [52; 53; 54; 55; 56; 57; 58; 59; 60; 61; 62; 63; 64; 65; 66; 67; 68; 69; 70; 71; 72; 73; 74; 75; 76; 77; 78; 79; 80; 81; 82; 83; 84; 85; 86; 87; 88; 89; 90; 91; 92; 93; 94; 95; 96  
patterns of clusters composed of randomly aggregated metal nanoparticles were used as SERS probes (Figure 4(d-iv)).