Versatile van der Waals epitaxy-like growth of crystal films using two-dimensional nanosheets as a seed layer: orientation tuning of SrTiO3 films along three important axes on glass substrates

Abstract

One of the basic requirements for attaining a good epitaxy is a close structural matching between a substrate and a growing crystal epilayer. This restrictive requirement causes a major obstacle for its wide application to a range of functional crystal films in electronic, magnetic or optical devices. One approach for overcoming this problem is the so-called van der Waals epitaxy (VDWE) method, which can effectively implement the epitaxy of various crystals on cleaved faces of layered materials having no dangling bonds. The weak adatom–substrate interaction without directional covalent bonding plays a crucial role in the initial stage of VDWE, which drastically relaxes the lattice matching limitation. However, the method requires special materials for use as a substrate, thereby meaning that its applicability is limited. In this study, the concept is extended to the two-dimensional (2D) lattice of inorganic nanosheets, which are molecularly thin 2D crystals produced via artificial exfoliation of layered metal oxides. The nanosheets can neatly cover the surface of conventional substrates such as glass via a facile solution-based process. Similar to the above-mentioned cleaved faces of layered materials, such substrates can promote VDWE-like crystal growth because of their dangling bond-free nature. Based on this principle, we have demonstrated a selective deposition of highly textured (100), (110) and (111) SrTiO₃ films, a fundamentally important archetype of functional crystals, on glass substrates covered with single-layer nanosheets with suitable 2D periodicities as a trigger for VDWE-like film growth. The rich varieties of nanosheet structures and their facile deposition onto almost any kinds of substrates provide a significant advantage, expanding potential applications for a range of devices based on functional crystal films.