Optimizing CuFeS\({}_{2}\) Chalcopyrite Thin Film Synthesis: A Comprehensive Three-Step Approach Using Ball-Milling, Thermal Evaporation, and Sulfurization Applied for Thermoelectric Generation  
[PERSON], [PERSON]  
For bulk CFS materials, the reduction in the thermal conductivity (\(\kappa\)) relies on strategies such as nanostructuring [7; 13], doping [14; 15; 16; 17], the use of a sulfur-deficient stoichiometry [12], and/or a combination of them. Nanostructuring induces so-called phonon-glass electron-crystal behavior [18], where the grain boundaries of the crystallites scatter the phonon waves transporting heat without a significant decrease in \(\sigma\). This strategy reduced CFS thermal conductivity by 47%, although the electron-crystal behavior was compromised [7]. Other than that, lattice engineering via doping and vacancy induction can promote more obstacles to phonon propagation within the lattice. Even though intrinsic doping can significantly enhance the power factor of CFS due to the introduction of more charge carriers into the system, the drop of lattice \(\kappa\) is still not satisfactory. According to [PERSON] et al. [7], the reason is that the phonon waves responsible for heat transportation have a wavelength of about 100-1000 nm. To block the propagation of these phonons, they suggest transforming the material into a thin film form, having a thickness around the value of the phonon wavelengths.  
Practical ways to employ thin film materials as Thermoelectric Generators (TEGs) employ an in-plane \(\pi\) geometry [19; 20]. For these TEGs, \(p\)- and/or \(n\)-type legs are deposited in a flat substrate, displaced electrically in series and thermally in parallel, conveniently joined by metallic contacts. Upon heating one of its extremities, electrical energy is produced. To the best of our knowledge, no CFS in-plane generator has been fabricated in this manner. Nonetheless, various techniques are employed to convert bulk CFS into thin films. They include sputtering [11] from high-density spark-plasma sintered (SPS) materials [7; 12; 21], flash methods [22; 23], spray pyrolysis [24; 25; 26], and thermal evaporation [27]. Evaporation is considered the cheapest and least complex method due to its higher deposition rate and the use of readily available powders. However, the different melting points of Cu and Fe mean that additional treatments are needed to promote the intermixing between the two. A strategy to solve this issue is proposed by [PERSON] et al. [27], where the targeted substrates are heated while multiple Cu/Fe layers are being deposited. The substrates are later exposed to a sulfur-rich atmosphere, a process called sulfurization, in order to introduce sulfur into the structure.  
Thus, the current study introduces a novel three-step method for CFS fabrication using a previous ball-milling step prior to the thermal evaporation and sulfurization processes. The optimization of every synthesis process is thoroughly discussed, with the intention of providing a clear and instructive roadmap for future fabrication of this material in thermoelectricity (TE) and other potential fields. This optimization comprises tests on the evaporation of the non-milled powders (Section 3.1.1) and the milling parameters (Section 3.1.2). Characterization of the optimized film uncovers its structural properties, phase purity, surface morphology, and stoichiometry balance through X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Electron Dispersive X-ray Spectroscopy (EDXS) analysis, and Hall-effect measurements (Section 3.2). To gain further insights into secondary phase formations within the phase diagram, Density Functional Theory (DFT) calculations are also employed. The viability of this synthesis approach is substantiated by the fabrication of a proof-of-concept Thermoelectric Generator (TEG), having its specific power output measured using I-V curves. In Section 4, we provide commentary and comparisons regarding this synthesis route, along with an exploration of potential applications.