Al2O3 barrier layer for the synthesis of high-quality thermochromic SmNiO3 thin films annealed in air

This study presents a soft synthesis method for producing thin films of SmNiO${}_3$${}_3$ with enhanced thermochromic properties after annealing in air. The films were fabricated using magnetron co-sputtering under reactive atmosphere, followed by a lowtemperature annealing process at 550 $°\mathrm{C}$$°\mathrm{C}$ for 30 min to crystallize the thermochromic perovskite phase. Structural and chemical analyses, including X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), confirmed the successful formation of the orthorhombic SmNiO${}_3$${}_3$ phase with lattice parameters consistent with previous studies. The thermochromic properties of the films were assessed by electrical measurements displaying an important decrease in resistivity of several decades from −263 to 250 $°\mathrm{C}$$°\mathrm{C}$, and through temperature-dependent transmittance and reflectance measurements, revealing significant optical changes in the infrared range (2.5–25 $\mathrm{\mu }$$\mathrm{\mu }$m) between 30 $°\mathrm{C}$$°\mathrm{C}$ and 190 $°\mathrm{C}$$°\mathrm{C}$. As anticipated, when deposited atop the amorphous SmNiO${}_3$${}_3$ thin film, a thin Al₂O₃ layer effectively acted as a diffusion barrier. This approach stabilizes Ni${}^{3+}$${}^{3+}$ and enhances thermochromic properties compared to uncapped SmNiO${}_3$${}_3$ films annealed in air. Electron energy loss spectroscopy (EELS) confirmed a high Ni${}^{3+}$${}^{3+}$ concentration of $\sim$$\sim$ 85%, enabling an emissivity modulation of $>$$>$20% in the 8–14 $\mathrm{\mu }$$\mathrm{\mu }$m range. This performance is comparable to that of samples annealed under extreme conditions (oxygen pressure of 200 bars at 950 $°\mathrm{C}$$°\mathrm{C}$). Moreover, the atmospheric-pressure process offers a scalable, industrially viable route for smart applications such as solar thermal coatings and passive radiative cooling devices.