## 2 Materials and Methods
### Sample Preparation
The electrolyte was obtained in three steps: a Te-based stock solution was first prepared by dissolving the TeO2 powder in concentrated H2 SO4 under vigorous stirring; then, an aqueous Cd-based stock solution was made; and subsequently, the electrolytic bath was finalized by mixing out the two stock solutions. Notably, the Te and Cd species were used in a 1:10\({}^{3}\) ratio in the electrolyte  
Figure 2: (**a**) Current curve acquired for the anodization of \(\sim\)1 μm Al layer sputtered on the Au-coated Si substrate, leading to the formation of a supported AAO nanoporous template with a thickness of \(\sim\)1.4 μm; (**b**) typical cathodic reduction current obtained during the electrochemical deposition of Cu NWs within the nanopores of the AAO template via application of \(-\)0.2 V versus an Ag/AgCl reference electrode; (**c**) chronoamperometric electrodeposition curve corresponding to CdTe synthesis on top of the Cu NW array (after the complete removal of the AAO nanoporous host) while applying \(-\)0.5 V versus an Ag/AgCl reference electrode. due to previous reports that the concentration ratio between the two active species can have a major impact upon the elemental composition of the obtained CdTe thin film, as a Te-rich electrolyte could determine the occurrence of separated Cd and Te crystalline phases [30]. The ECD process was then carried out in a three-electrode potentiostatic configuration using a Voltalab PGZ-100 Potentiostat (Radiometer Analytical Inc., Hegnau, Switzerland). In this setup, the array of Cu NWs vertically aligned to the Au-coated Si substrate represented the working electrode and a Pt strip was used as auxiliary electrode. The CdTe film was grown by applying \(-0.5\) V versus an Ag/AgCl reference electrode (KCl-saturated, E = 0.197 V against NHE), while the process temperature was maintained at 60 \({}^{\circ}\)C. As is well known, ECD is typically a low-temperature process when compared to the VTE or RF-MS alternatives, allowing CdTe thin films with good structural properties to be obtained at temperatures even below 60 \({}^{\circ}\)C [28]. The overall electrochemical growth of CdTe has been commonly reported to occur in two steps, with the cathodic reduction of \(\text{HTeO}_{2}^{+}\) to Te (Equation (1)) subsequently accompanied by the temperature-mediated reaction of Te with Cd\({}^{2+}\) (Equation (2)), ultimately favoring CdTe formation [32; 33]:  
\[\text{HTeO}_{2}^{+}+3\text{H}^{+}+4\text{e}^{-}\rightarrow\text{Te}+2\text{ H}_{2}\text{O} \tag{1}\]  
\[\text{Cd}^{2+}+\text{Te}+2\text{e}^{-}\rightarrow\text{CdTe} \tag{2}\]  
Notably, the overall process of CdTe electrochemical synthesis is diffusion-controlled, with ECD potential playing a crucial role in establishing the Cd/Te atomic ratio within electrodeposited CdTe films. In this context, it has been previously demonstrated that a more negative potential leads to the formation of Cd-rich n-type deposits, while in the opposite conditions Te-rich p-type deposits are obtained. Only within a narrow potential electrodeposition range is the Cd/Te atomic ratio equal to unity, favoring the formation of stoichiometric CdTe thin films [29; 34]. Figure 2c presents the cathodic chronoamperometric current curve obtained during the electrochemical growth of CdTe on the Cu NWs for about 15 min; the higher current on the application of the constant potential is attributed to the fast diffusion of anion species through the Cu NWs and subsequent formation of double-layer charges between the electrode's surface and the electrolytic solution, while the sudden change in the current marks the beginning of the CdTe nucleation process.