Nb-Doped VO₂-Based Coatings on Glass: Substrate Effects, Thermochromic Performance, and an Effective Transition...

Niobium-doped VO₂ thin films were deposited on soda-lime and borosilicate glass substrates to investigate the coupled effects of dopant concentration, substrate chemistry, and thermal processing on microstructure, thermochromic performance, and metal-insulator transition (MIT) kinetics. Coatings with nominal Nb contents of 3, 4.5, and 6 at. % were fabricated using controlled mono- and multilayer codeposition strategies followed by rapid thermal annealing. Structural and morphological analyses confirm VO₂(M) as the dominant phase on both substrates, while revealing a strong substrate-dependent stability of secondary phases driven by Na diffusion, especially in soda-lime glass. Advanced scanning-transmission electron microscopy image and spectroscopic-related techniques demonstrate that Nb exhibits a less homogeneous spatial distribution than other dopants (such as W) under comparable processing conditions, leading to compositional modulations and multistep MIT behavior, particularly on soda-lime substrates. In contrast, borosilicate glass enables enhanced grain coalescence, reduced Na-induced phase instability, and a more uniform Nb incorporation. These microstructural differences directly impact the optical response. Borosilicate-supported coatings exhibit a more favorable balance between luminous transmittance (T_lum) and solar modulation efficiency (ΔT_sol), while films on soda-lime glass show a progressive degradation of ΔT_sol at higher Nb contents. Despite a lower T_c reduction efficiency compared to W, Nb doping preserves or even improves T_lum at elevated concentrations. Finally, an application-oriented effective transition temperature, T_eff, is proposed as a more realistic metric for assessing the suitability of VO₂-based coatings for smart-glazing applications.