# Nano- and Micropatterning on Optical Fibers by Bottom-Up Approach: The Importance of Being Ordered
## 3 Bottom-Up Fabrication of Periodic Structures on OFs
### Post-Processing Methods to Increase the Order  
Nanofabrication methods based on self-assembly processes are naturally inclined to include a certain number of defects in the assembled structure. Sometimes the presence of defects may be a benefit for the function of the nanostructure. For example, the controlled introduction of defects into the PC structure allows for additional tailoring of light propagation (Figure 5a) [84]. Most of the time, however, the material scientist is called to minimize the number of defects produced by the chosen manufacturing process. This can be achieved by accurate control of the key parameters of the process, so that the self-assembly phenomenon is piloted to produce always the same periodic structure. These may include environmental parameters (T, relative humidity (RH), light exposition) purity of materials used, homogeneity of size and shape in case of nanoparticle assembly, process speed-rate, and extension of the patterning surface. Besides using rigorous operational precautions, it is sometimes possible to increase the order degree of a self-assembled periodic pattern by applying a post-processing treatment. Figure 4: (a) Representation of the directed self-assembly (DSA) approach: the combination of block-copolymer self-assembled into organized nanoscale structures with lithographically defined structures acting as guiding patterns, produces highly regular patterns of lines/spaces. Reproduced from Ref. [79] (b) Schematic (top) and experimental (bottom) examples of concentric rings (i) and spirals of right-handed (ii) or left-handed (iii) chirality, formed by a block copolymer self-assembled in circular (i) and notched (ii,iii) templates. The template diameters in (i–iii) are 350 nm. Reproduced from Ref. [80] with permission from 2017 IOP Publishing Ltd. (c) Schematic representation of the lithographic process to obtain a hexagonal pattern for DSA of colloidal particles (i) and SEM micrograph of the assembled particles in the hexagonal pattern. Reproduced from Ref. [82]. (d) Schematic representation of gold nanoparticle clusters grown around PS-b-P2 VP inverse micelles assembled on a substrate (i); AFM image of the array of PS-b-P2 VP micelles (ii); SEM micrograph of the array of Au nanoclusters resulting from the decoration of the BCP micelles (iii); schematic representation of the SERS analysis setup including the patterned optical fiber probe (iv). Reproduced from Ref. [83] with permission from the 2012 American Chemical Society. Thermal treatments, for example, can be effective in increasing the order when self-assembling elements organize themselves to reach their thermodynamic equilibrium. As illustrated in the previous paragraph, DSA is a clear example in this contest. BCP thin films, generally produced by spin or blade coating, are partially organized in short-range order domains. A post-processing treatment consisting in heating the BCP thin film to temperatures above the glass transition temperatures of all blocks induces the long-range order guided by the pre-pattern present on the substrate [85]. This happens because temperature increases the mobility of the crystallizing elements (the polymer blocks) facilitating reorganization into thermodynamically favored structures. Besides temperature, solvent vapor annealing is frequently used to modulate the orientation and increase the order of BCP films. When the thin film is exposed to solvent vapors, the solvent adsorption promotes the formation of a swollen and mobile polymer layer at temperatures well below the highest glass transition temperature of the system. The choice of solvent and its optimal concentration in the ambient surrounding the film are key factors for the success of the annealing. For example, in a detailed study on the self-assembly of poly(styrene)-block-poly(lactide) thin films, [PERSON] et al.