## 4 Preparation of Plasmonic Thin Film Sensors
### Low-Pressure Plasma Treatment for Surface Etching  
To partially expose the nanoparticles embedded in the metal oxide thin film, a low-pressure capacitively coupled plasma (CCP) treatment is a meaningful alternative to chemical etching. Low-pressure plasma treatment is recognised as a versatile, environmental friendly and effective technology for surface modification, cleaning and etching of surfaces at the micro- and nanolevels [174; 175].  
Figure 13: Sequence of SEM micrographs of nanocomposite Au–TiO\({}_{2}\) thin films during an in situ thermal treatment experiment, from room temperature (thin film after deposition by reactive magnetron sputtering) up to 400 \({}^{\circ}\)C. The labels I, II, and III show different nanoparticles at different moments that appeared due to the migration of dissolved Au atoms. Micrographs were taken and processed by [PERSON] et. al. using a FEI QUANTA 650 FEG scanning electron microscope.  
A low-pressure plasma system runs under (primary) vacuum in a controlled atmosphere and by using an alternating voltage source (Figure 14a). The characteristics of this low temperature, nonequilibrium plasma depend mainly on the gas species introduced, on the electrode configuration, and on the voltage source [176]. After a vacuum below 50 Pa is reached, the desired gas is introduced into the vacuum chamber up to a maximum pressure of 100 Pa, followed by the excitation using radiofrequency (40 kHz or 13.56 MHz) or microwaves (2.45 GHz) voltage sources (see simplified schematics in Figure 14b). The energy provided to the gas produces free electrons, ions, and free radicals. The species created in the plasma interact with the sample, thus functionalizing its surface. After the alternating voltage source is turned off, the particles recombine, and the plasma is extinguished.  
Since the appearance of low-pressure plasma systems in the 1960s, the main usage has been in the electronic industry, and given that it is a low-temperature process, in the 1980s this technique was also applied to metallic and polymeric materials [177]. Currently, the variety of materials that are treated with this technology includes polymers, textiles, resins, paper, metals, ceramics, and inorganic and biologic materials. The modified surface properties include adhesion, wettability, protection, biocompatibility, and chemical affinity, among others, depending on the materials treated [178].  
When compared to wet chemistry techniques, plasma treatment has several advantages, namely: (i) only the top surface layers are modified by the plasma treatment, hence maintaining the bulk material characteristics; (ii) it allows for achieving higher quality surface characteristics; (iii) several delicate materials can be processed, beyond those in wet chemistry; (iv) plasma processing does not involve a vast water supply, heating, or drying; and (v) only a very small amount of chemicals are needed to produce the required functionalities [179]. Low-pressure plasma systems also have several benefits when compared to other plasma techniques (e.g., atmospheric plasma): (i) low breakdown voltages; (ii) stable operation; (iii) electron temperature capable of dissociating molecules (1-5 eV), but a low ion temperature (0.03-0.05 eV); (iv) relatively high concentrations of ions and radicals to drive etching; and (v) a uniform glow over a large gas volume [179; 180].  
Figure 14: (**a**) Low-pressure plasma treatment simplified schematics. (**b**) Simplified schematics of a commercial cylindrical low-pressure capacitively coupled plasma system with optimized electrode shape for uniform surface.