# New Insights in the Ion Beam Sputtering Deposition of ZnO-Fluoropolymer Nanocomposites
## 1 Introduction  
Organic-inorganic composite materials are of primary interest for basic and technological applications, as inorganic nanophases' inclusion in polymeric matrices is an efficient method to produce materials with unique chemical and physical properties, gathering together the characteristics of polymers and nanostructures [1]. Poly-tetrafluoroethylene (PTFE) represents a common polymer choice in a variety of technological applications because of its advanced physicochemical properties, such as a low chemical reactivity, a high melting point (327 \({}^{\circ}\)C), a low friction coefficient, high hydrophobicity, and high surface resistivity [2; 3]. PTFE-based composite materials, containing inorganic nanophases, are well-known tools for the development of protective coatings [4], flexible electronic devices [5], super-hydrophobic self-cleaning surfaces [6; 7; 8; 9; 10], high thermal and electric conductive devices [11; 12], sensors [13], and antimicrobials [14; 15; 16; 17; 18]. Many such composites are nowadays prepared using deposition techniques, such as r.f. magnetron sputtering [19], ion beam sputtering [20], and plasma andvapor-phase deposition techniques [21; 22]. Few reports are available on solution-processed composite thin films [6; 23; 24]. One of the first examples of a co-deposited PTFE-inorganic composite was reported in 1983 by [PERSON] et al. [25], who described the preparation of metal- PTFE films (containing Au, Pt, or Cu) by the r.f. sputtering of a single, composite target; however, fluoride cross-contamination occurred during the process. [PERSON] et al. [26] prepared Ag-containing fluoropolymer nanocomposites by means of the so-called infusion-process technique, which consists in diffusing an organometallic precursor gas into the free volume of a fluoropolymer matrix, and decomposing it by thermal treatments that allow the nanoparticles' nucleation and growth. A similar composite was also prepared by [PERSON] et al. [27], with good nanoparticle size control, using Chemical Vapor Deposition (CVD) in a synthesis chamber holding an electron beam equipped with individually controlled pockets, which allowed for the contemporaneous deposition of different materials. Ag-, Cu-, or Mo-containing PTFE coatings were prepared by [PERSON] et al. by means of the same technique [28]. The co-deposition of titanium-PTFE films by unbalanced magnetron sputtering was also reported in 2002 [29] using a rotating substrate holder and two distinct targets. A similar approach was used later on to produce SiO\({}_{x}\)-fluoropolymer protective coatings [30]. Zinc oxide (ZnO) is widely used as inorganic filler in polymeric composites, and it is technologically important because of its wide range of electrical, optical, nontoxic, and antimicrobial properties [31]. Most of the publications about ZnO and fluoropolymer deal with the deposition of nanostructured ZnO onto PTFE surfaces, using many different techniques. Starting from a seeded PTFE sheet obtained by the thermal oxidation of a Zn thin film on the polymeric substrate, [PERSON] et al. prepared nanorod arrays onto PTFE, by a low-temperature hydrothermal reaction, for application in advanced electronics. This material possessed high flexibility and chemical stability combined with the outstanding electrical properties of nanostructured ZnO [32]. [PERSON] et al. [33] prepared the same composite material by the in-situ reduction of a zinc precursor, in PTFE solution, in order to improve the hydrophobicity of ZnO layers. [PERSON] et al. found that ZnO nanoparticles (NPs) were able to improve the friction and wear characteristics of PTFE; the material was prepared by the sintering of a mixed ZnONPs-PTFE nanometric powder [34]. Many articles report on the physical deposition of a ZnO thin layer on PTFE substrates for the development of flexible electronics [35], antistain, superhydrophobic, and omniphobic surfaces [36; 37; 38], low-friction surfaces [39], and components for photovoltaic cells [40]. The subsequent r.f.