Orientation Control And Mechanism Study Of Van Der Waals Epitaxial Perovskite Thin Films

Van der Waals epitaxy is a growth technique that utilizes Van der Waals interactions to prepare thin film（2D）materials.Compared to traditional epitaxy,Van der Waals epitaxy is not limited by lattice matching conditions,thus enabling more flexible structures and material combinations.As an emerging thin film fabrication technology,Van der Waals epitaxy offers advantages such as flexible material selection,high-quality crystal growth,and controllable interface properties（e.g.,charge transport and band engineering）.However,it also faces challenges and limitations,such as difficulties in achieving large-area film deposition due to low surface energy and nucleation barriers,as well as limitations in achieving high-precision crystal orientation and atomic arrangement control due to weaker Van der Waals interactions.This study focuses on researching the growth orientation control and magnetic manipulation of Fe4N with a perovskite structure.Two-dimensional（2D）layered materials such as mica,highly-oriented pyrolytic graphite（HOPG）,and molybdenum disulfide（Mo S₂）are used as substrates."Domain matching engineering"and"photo-assisted epitaxy"are introduced to explore the control of Fe₄N film orientation and its magnetic properties.In addition,first-principles calculations are employed to delve into the underlying mechanisms.The main work is as follows:（1）Orientation control of Fe₄N thin films on a two-dimensional mica substrate was achieved through"domain matching engineering".By employing domain matching strategies and designing appropriate transition structures,precise and selective control over the epitaxial orientation of Fe₄N films was achieved.Fe₄N thin films with（001）,（110）,and（111）orientations were simultaneously epitaxially grown on the mica substrate.（2）The concept of"photo-assisted Van der Waals epitaxy"is proposed.By introducing light excitation（white light）,the epitaxial growth orientation of Fe₄N on mica can be changed from（001）to（111）.The effects of parameters such as light properties（intensity,wavelength）,timing,and duration on the epitaxial growth of the thin film were systematically studied.First-principles calculations revealed that Fe₄N（001）/mica and Fe₄N（111）/mica have similar binding energies.The additional energy input from light during the initial stage of film growth enables sufficient diffusion of the adsorbed atoms on the mica surface,resulting in the formation of Fe₄N（111）crystal plane with higher surface energy.Photo-assisted van der Waals epitaxy provides a new pathway for achieving controlled orientation and high-quality epitaxial growth of thin films on 2D substrates.（3）A comprehensive study was conducted on the epitaxial growth of Fe₄N on various 2D substrates and the mechanisms behind the growth orientations.Factors such as lattice mismatch,surface potential and surface energy of the 2D substrate,and van der Waals interactions contribute to the epitaxial growth orientations of Fe₄N on HOPG and SnSe₂ substrates,favoring（001）orientation.On the other hand,on Mo S₂ and Ta S₂substrates,Fe₄N epitaxially grows with a（111）orientation.（4）The influence of the interface interaction between Fe₄N epitaxial films and 2D substrates on their magnetic properties was explored.By combining theoretical calculations and experimental results,it was discovered that the strong interface orbital hybridization between Fe₄N and Mo S₂ heterostructures is the reason behind the loss of room temperature ferromagnetism in Fe₄N films.Moreover,the magnetic properties of Fe₄N are influenced by atomic stacking.