Flexible Single-crystal Oxide Films Based On Van Der Waals Epitaxy

Transition metal oxides have many excellent physical and chemical properties,such as ferroelectricity,ferromagnetism and semiconductor,which may play an important role in the next generation of flexible electronic technology revolution.As the cornerstone of the flexible electronic technology,there are still many challenges in the preparation of flexible oxide films.Traditional method is to epitaxy the target film on a rigid substrate and then etch the sacrificial layer,but the strict lattice matching limits it`s application.Van der Waals epitaxy does not require strict lattice matching,films grow on the substrate surface through weak Van der Waals force,which provides a new idea for the preparation of materials.Muscovite has good thermal and chemical stability.It has a atomic flat surface and almost no hanging bonds on the surface,which is very suitable for fabricating many kinds of two-dimensional semiconductor and complex heterojunction materials by Van der Waals epitaxy.As a layered crystal,the muscovite`s layers are combined by van der Waals force,this special crystal structure enables mica to have a set of extremely complete cleavage planes,which can be dissociated into flat sheets of arbitrary thickness.When the thickness is down to tens of microns,the mica sheets can be bent to 180 degrees and remain intact,showing good flexibility.It is one of the ideal substrates for preparation of flexible materials and device research.Based on the above,this paper chooses mica as the flexible substrate to fabricate a variety of single crystal oxide films and composite heterojunctions by Van der Waals epitaxy.The film and substrate are successfully separated by solution intercalation process,which is universal for mica substrate.Subsequently,the prototype flexible memory device were prepared with 0.7%_wtt Nb-doped strontium titanate and the characteristics of the devices was analyzed.The main work includes the following parts:High quality flexible single crystal films like CoFe₂O₄,0.7%_wt Ni-doped SrTiO₃ thin films and CoFe₂O₄-BaTiO₃ heterojunction thin films were successfully grown on mica using pulsed laser deposition system.The crystalline quality and surface morphology were characterized by XRD,AFM,SEM and TEM.In the experiment,we found that insert a CFO seed layer in the growth of NSTO and BaTiO₃ can greatly improve the crystallization quality of the films.Films were successfully separated from mica substrates by solution intercalation process.Flexible self-supporting two-dimensional films were obtained which could be transferred to any substrates.The transferred materials has very smooth surface and matained the good crystallization quality.This method has universal significance for Van der Waals epitaxy materials by mica.Flexible resistive prototype devices were prepared based on NSTO films epitaxied on mica.The relationship between properties of SRO/NSTO interface and resistive properties of devices was analyzed.Bipolar non-volatile resistive devices based on SRO/NSTO Schottky were observed directly.The effects of electric field and current on the resistance of devices are studied.The effect of bending on the electrical properties of the device is studied by applying different stresses to the film in different curvature states.We find that the device has a sensitive response to bending stress and has potential application in the field of flexible memory and flexible sensor.In this paper,a variety of high-quality flexible single-crystal oxide films have been grown by Van der Waals epitaxy on mica substrates,which enables us to study the effect of bending stress on devices by bending mica directly.For the first time,the film transfer process on mica substrate has been achieved by solution intercalation technology.Those works are of great value to the preparation of flexible materials and the study of flexible devices.