## 3 Results and Discussion  
The initial state of the investigated copper selenide films has a nontrivial structure. A typical initial film surface and the cross-section of the film shown in Figure 1. The films consist of nanocrystallites with dimensions of 50-120 nm connected in large clusters. There are large sinuous pores between the clusters. This structure of the copper selenide films is common and occurs in various growth methods [17; 18; 19; 20; 21]. In the cross-sectional image (Figure 1c), two layers are clearly distinguishable by the film thicknesses. The lower layer has a thickness of approximately 630-650 nm, larger crystallites, and larger pores, while the upper layer has a thickness of approximately 220 nm, smaller crystallites, and smaller pores. The XRD patterns of the initial state within the 20 range of 5-90\({}^{\circ}\) are shown in Figure 2. The characteristic peaks correspond to the (111), (200), (220), (311), (400), (331), (422), and (511) lattice planes of cubic phase (Fm-3m) Cu\({}_{1.75}\)Se to with high accuracy according to the values in ICDD card 01-075-2714. No other phases were observed in the studied layers. Notably, the highest intensity (Figure 2) is characteristic of the (111) plane, i.e., the growth of the crystallites occurs mainly in the [111] direction of the cubic structure, which is typical for Cu\({}_{2}\),Se films [20; 21; 22]. The EDX data indicate that the ratio of Cu:Se atoms in the copper selenide films after growing and annealing is 1.75:1.00; hence, the chemical composition of the films deposited by this method can be identified as Cu\({}_{1.75}\)Se. Ion sputtering in argon plasma led to significant changes in the surface morphology of the studied films. The dynamics of these changes during sputtering by 25 eV argon ions over a time interval of 30-90 s are shown in Figure 3. We consider an image taken normal to the surface, an image of a tilted surface and a cross-sectional image of the film/substrate. As shown in Figure 3b, when the process time was 30 s, vertical cone-shaped structure arrays were formed on the surface. The density of these cones is approximately \(7\times 10^{9}\) cm\({}^{-2}\), and the angle of the cone at the top varies between 25-50\({}^{\circ}\). Thus, a short duration of exposure to argon ions leads to the restructuring of previously formed copper selenide clusters and appearance of high (180-250 nm) cones. The height of these cones corresponds approximately to the thickness of the top layer in Figure 1c. Several explanations for the appearance of micro- and nanocones have been proposed in experimental and theoretical ion sputtering studies [23,24]. However, these explanations  
Figure 3: SEM images of the copper selenide film surface after plasma treatment at ion energy of 25 eV and durations of 30 s (**a**,**b**), 60 s (**c**,**d**), and 90 s (**e**,**f**). The sample tilts during imaging were 0\({}^{\circ}\) (**a**) and 70\({}^{\circ}\) (**b**,**d**,**f**). (**c**,**e**) Cross-sections of film, where the black lines are the borders of the lower and upper layers. are all based on the compact materials. In our case, it is necessary to consider both the complex porous structure of the surface and re-deposition processes of the chemical elements that lead to the formation of local masks. The ions in a dense homogeneous ion stream perform different functions when they interact with a porous surface. Some of the ions result in the sputtering of clusters on the surface. The other ions, which fall into the pore areas, participate in the etching of the material in the film bulk, and remove the thin partitions in the porous structure. Sputtering processes are known to be accompanied by the re-deposition of sputtered chemical elements on the surface, self-organization of local masks, plasma-stimulated coalescence of nanoparticles, and densification of the porous material [25,26]. In [27], it was shown that, during the initial stage of ion sputtering of complex compounds containing copper and selenium, self-formation of copper-rich nanoparticles occurs.