Fabrication and Growth Orientation Control of NaBH4 Epitaxial Thin Films Using Infrared Pulsed-Laser Deposition

Complex hydrides are well-known reducing agents and hydrogen storage materials and have recently attracted attention as ionic conductors. To expand the application scope of complex hydrides, physically well-defined epitaxial films that can investigate intrinsic properties and interfacial effects are essential. However, fabricating the epitaxial films of complex hydrides is difficult because of the difficulty in maintaining [BH4]- complex anions during deposition. Herein, we present the fabrication and growth orientation control of NaBH4 epitaxial thin films, whereby an infrared pulsed laser was used to evaporate [BH4]- complex anions. We obtained NaBH4(100) and (111) epitaxial thin films on MgO(100) and CaF2(100) substrates, respectively, with the assistance of H2 gas. This is the first example of the growth orientation control of complex hydride thin films. The influence of lattice mismatch at the interface and stacking sequence along the out-of-plane direction on the growth orientation control is comparatively discussed. This study provides a basis for the advanced investigation of complex hydrides and their surface and interfacial phenomena.

Automated summary

Complex hydrides are important reducing agents, hydrogen storage materials, and ionic conductors. However, it is challenging to fabricate epitaxial thin films of complex hydrides due to the difficulty in maintaining stable [BH4]- complex anions during deposition. This study successfully achieved the growth orientation control of NaBH4 thin films and discussed the influence of lattice mismatch and stacking sequence on the control. The findings provide a basis for advanced investigations of complex hydrides and their surface and interfacial phenomena.