Microstructure tuning facilitated photo-efficiency enhancement and environmental benign nature of HfO2/Mo/HfO2 multilayer films

Abstract Nano-layered, multilayered films consisting of HfO 2 (dielectric)/Mo(metal)/HfO 2 (dielectric), (D/M/D architecture) with controlled layer thickness, chemistry and microstructure were demonstrated for efficient utilization of solar energy with well-designated spectral control. The effect of Mo metal interlayer on the energy performance and efficiency of HfO 2 /Mo/HfO 2 multilayer films made by electron-beam deposition onto silicon and glass substrates has been evaluated. The Mo interlayer thickness [Mo( t )] was varied in the range of 5–25 nm while the top/bottom HfO 2 layer thickness was kept constant at ∼50 nm. Structural, morphological and optical characterization was performed to understand the effect of Mo( t ) induced microstructure on mechanical durability and optical behavior of the D/M/D multilayer films. Structural studies revealed that D/M/D multilayer films with relatively low Mo( t ) were amorphous, while higher Mo( t ) induces the amorphous-to-nanocrystalline microstructure transformation, which also induces surface roughening effects. The D/M/D multilayer film with Mo( t ) = 20 nm, where the nano-columnar morphology of the D-M-D layers are aligned perpendicular to the substrate surface and exhibits the optimum spectral selectivity. Such microstructure and morphological tailoring also facilitates the environmentally friendly (self-cleaning/stain repellent) nature of these multilayer films by the high contact angle (∼102°) achieved. Molybdenum interlayer facilitated higher contact angle leads to super-hydrophobic nature of the HfO 2 /Mo/HfO 2 multilayered film surfaces. The low surface roughness values (0.2–0.8 nm), which are primarily due to Mo at the interface, of the HfO 2 /Mo/HfO 2 multilayered films satisfy the general requirements of heat mirror applications. In addition, the microstructure of intermediate Mo layer and HfO 2 -Mo interfaces significantly control the mechanical properties of D/M/D multilayer films. Optimal mechanical properties (hardness = 30 GPa, elastic modulus = 312 GPa, wear resistance = 0.094, adhesion strength = 2050 µN) have been obtained for Mo(t) = 20 nm D/M/D multilayer film. The mechanical response strongly supports flexible and wear resistive nature of HfO 2 /Mo/HfO 2 multilayer films.