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

Nano-layered, multilayered films consisting of HfO2 (dielectric)/Mo(metal)/HfO2 (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 HfO2/Mo/HfO2 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 HfO2 layer thickness was kept constant at similar to 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 (similar to 102 degrees) achieved. Molybdenum interlayer facilitated higher contact angle leads to superhydrophobic nature of the HfO2/Mo/HfO2 multilayered film surfaces. The low surface roughness values (0.2-0.8 nm), which are primarily due to Mo at the interface, of the HfO2/Mo/HfO2 multilayered films satisfy the general requirements of heat mirror applications. In addition, the microstructure of intermediate Mo layer and HfO2-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 IAN) have been obtained for Mo(t) = 20 nm D/M/D multilayer film. The mechanical response strongly supports flexible and wear resistive nature of HfO2/Mo/HfO2 multilayer films.