## 3 Results and Discussion
The nanoparticles later serve as local masks in the formation of cones. A similar situation occurs in our case. The rapid heating of the surface during plasma treatment and the influence of argon plasma ultraviolet radiation play an additional role in the ongoing processes. Consequently, arrays of vertical structures are formed. In a previous study, we described the formation of vertical nanostructures with the petal-shaped plate nanocrystallites on porous surfaces during the plasma sputtering of SnS films [28]. However, in the case of SnS, there is an important additional vapor-liquid-crystal mechanism during the vertical structures formation. Similar experimental results with the formation of vertical structures have been reported in porous boron and tungsten powders [29]. From the analysis of Figure 3, it can be seen that, when the exposure times were increased to 60 and 90 s (Figure 3c-f), the cone-shaped structures arrays on the surface were preserved; however, the height of the cones decreased, and the angle at the top of the cone increased. The latter is due to the prevalence of the process of sputtering the cones in this time interval. The analysis of the cross-section images shows that the ensemble of cones is still located in the upper layer of the film. If we consider the tops of the cones, the overall decrease in the film thickness is extremely small. This indicates that the sputtering processes in Cu\({}_{1.75}\)Se films at the ion energy of 25 eV are weakly expressed, and that the local masks on the tops of cones are practically not etched. We observe a different situation when the energy of the argon ions increased to 200 eV. A set of SEM images for the sputtering duration of 30-90 s is shown in Figure 4. The physical processes described above for the ion energy of 25 eV were repeated for ions with an energy of 200 eV and a sputtering duration of only 30 s. Within such a short duration, the sputtering still affected only the top layer of the film. At the processing time of 60 s, the top layer of the film was completely etched (Figure 4c), and at a processing time of 90 s, the lower layer of the film is etched (Figure 4e). The etching rate became uniform over the sample area, and was approximately 3 nm/s. The changes in the surface morphology of Cu\({}_{1.75}\)Se films during plasma treatment are accompanied by changes in the composition and crystal structure of the layers. EDX studies have shown that if the ratio of Cu/Se atoms was initially 1.75, then after sputtering by 25 eV ions for 60-90 s, the ratio became 1.87-1.88. Further, after plasma treatment by 200 eV ions for 60-90 s, the ratio increases to 1.98-2.01. Thus, the process of sputtering Cu\({}_{1.75}\)Se nanocrystalline porous films by argon ions decreases the number of chalcogen atoms. From a physical perspective, this decrease is due to both the peculiarities of the copper selenide sputtering process and the effect of heating the sample during plasma treatment. Within the framework of linear cascade theory [30] the ratio of partial sputter yields \(Y_{Se}/Y_{Cu}\) is determined by the surface concentrations \(N_{Se}^{S}\), \(N_{Cu}^{S}\), surface binding energies \(U_{Se}\), \(U_{Cu}\), and masses \(M_{Se}\) and \(M_{Cu}\):  
\[\frac{Y_{Se}}{Y_{Cu}}\simeq\frac{N_{Se}^{S}}{N_{Cu}^{S}}\bigg{(}\frac{M_{Cu}}{M _{Se}}\bigg{)}^{2m}\bigg{(}\frac{U_{Cu}}{U_{Se}}\bigg{)}^{1-2m} \tag{1}\]  
where \(m\) can be assumed to be zero at low ion energies of 25 and 200 eV of our experiment [30]. Unfortunately, no information exists on the values of the surface binding energies of Cu and Se atoms in Cu\({}_{1.75}\)Se in the literature for accurate calculations. Hence, we used the corresponding values for the single-component materials as a rough estimate. Consequently, sublimation heat values of 102.2 kJ/mol for Se and 337 kJ/mol for Cu from [31] were used. Thus, at \(Y_{Se}/Y_{Cu}\gtrsim 1.8\), preferential sputtering of selenium occurs for the Cu\({}_{1.75}\)Se films.