## 4 Preparation of Plasmonic Thin Film Sensors
### Thermal Annealing to Promote Gold Nanoparticle Growth  
Once the nanocomposite thin films have been deposited, a post-deposition thermal treatment may be required to promote the growth of nanoparticles up to sizes that originate a LSPR band, measurable and observable in the transmittance spectrum. The growth of the nanoparticles from atoms initially dissolved in the matrix and distributed uniformly occurs through three main stages: (i) nucleation, when some new nanoparticles, formed by fluctuations, eventually reach a critical size and form stable nuclei; (ii) \"normal growth\" of the nuclei at the expense of dissolved matter; and (iii) Ostwald ripening (when small nanoparticles are dissolved and the atoms are transferred to larger nanoparticles) or coalescence (when two NPs are in contact and merge to form one larger NP) [154,155].  
Figure 12: Schematics of a custom-made reactive DC magnetron sputtering vacuum chamber showing the placement of noble metal “pellets” (adapted from [PERSON] et al. [153], copyright 2019 Elsevier).  
Magnetron sputtering deposition typically originates gold clusters/nanoparticles with sizes below 10 nm dispersed throughout the matrix, meaning that kinetic conditions are only favourable for the early stages of growth [156].  
As an example, in Figure 13, one can visualize how the Au nanoparticles, in a nanocomposite Au-TiO\({}_{2}\) thin film, underwent a growing process when thermal treatment was applied up to 400 \({}^{\circ}\)C. While the nanoparticles in I (yellow circles) and III (green circles) seem to grow at the expense of dissolved Au atoms, the two nanoparticles in II (blue circles) seem to be involved in a coalescence process (shown by the formation of a transient \"neck\") that resulted in a larger nanoparticle.  
The thin film preparation methodology described, magnetron sputtering followed by thermal annealing, was proposed about a decade ago as a versatile approach for the development of a new generation of decorative coatings [157]. In the following years, the optimization of these kinds of nanocomposite thin films led to the development of different systems such as Au and/or Ag nanoparticles embedded in TiO\({}_{2}\)[158; 159; 160], Al\({}_{2}\)O\({}_{3}\)[161], CuO [162; 163], ZnO [164], and AlN [165; 166] matrices.  
Envisaging their application in optical LSPR sensing [166], there must be a compromise between the annealing temperature and size distribution of nanoparticles, taking into account economic reasons and considering important parameters for LSPR sensing (e.g., signal-to-noise-ratios and surface area of Au nanoparticles). In fact, annealing temperatures lower than 500 \({}^{\circ}\)C are preferable since transparent glass substrates are inexpensive. Yet, since the size distribution that can be obtained are in the range up to 40 nm, and the nanoparticles are mostly spherical, this might hinder the signal and sensitivity of the plasmonic sensor. On the other hand, this effect can be mitigated, for example, (i) by partially exposing the nanoparticles that are initially embedded in the matrix, using low vacuum plasma etching [166; 167; 168; 169], or (ii) by developing nanostructured thin films deposited by glancing angle deposition (GLAD) [170; 171], which may allow for increasing the availability of adsorption sites for analyte molecules and to produce nanoparticles with higher aspect ratios, and thus to be more sensitive [172; 173].