Research On CVD Growth Of Ferromagnetic Two-dimensional Material Fe₃GeTe₂

The discovery of graphene has sparked people to deeply think and explore the two-dimensional material.Two-dimensional materials have displayed excellent optical and electrical properties,and have attracted great attention from a widely range of growth,preparation and devices.However,ferromagnetic two-dimensional(2D)van der Waals(vd W)materials have always been great challenge owing to the limits of Mermin-Wagner theorem.Until 2017,ferromagnetic two-dimensional materials Cr₂Ge₂Te₆,Cr I₃ at the limits of single atomic layer have been discovered.So far,the 2D vd W ferromagnetic materials have been obtained through an arduous mechanical exfoliation,typically with the limited yield and reproducibility,which really hinder the development and technological application of 2D materials.Chemical vapor deposition(CVD)approach has been explored to directly synthesize diverse 2D vd W materials and associated vd W heterostructures from a wide range of layered crystals,with good crystallinity,high thermal stability and controllable size,shape and thickness.However,to date,the growth and preparation of 2D vd W ferromagnetic materials with single or few atomic layers through a simple CVD strategy remain a challenge and has not achieved yet.Here,we study the CVD growth of 2D vd W ferromagnetic Fe₃GeTe₂The main contents are as follows:(1)We investigate the influence of experimental factors such as reaction precursor,reaction substrate,reaction gas flow rate,maximum reaction temperature and reaction time on the preparation 2D Fe₃GeTe₂ material.etc.The study has found that choosing elementary reaction precursors with higher melting points facilitate the growth of high-quality samples free of impurities.The Si O2/Si substrates with drop-casted PTCDA solution favor product deposition and epitaxial growth.The gas flow of 50sccm can accurately transport the reactants to the substrate position for deposition and growth.And the maximum reaction temperature of 900°C and the holding time of 1min ensure that the sample will not be thermally decomposed,therefore it can prevent the regular shape of the sample from decomposing into a round or"beveled"triangle.Through continuous adjustment of experimental parameters,the optimal reaction conditions for preparing Fe₃GeTe₂ by CVD method were determined,which provide a rich platform for further fundamental studies.(2)This thesis also explores the effect of pressure on the preparation of two-dimensional materials by CVD.During the experiment,the pressure in the quartz tube is controlled by tuning the balance of injecting gas and exhaust gas to enable a positive pressure higher than the atmospheric pressure.This study show that more nucleation sites will be generated on the substrate after applying pressure.This method make it easier to grow a thin layer of material.Although the size of the material is smaller than that during normal pressure growth,it will increase with the increase of pressure after pressurization.Therefore,it is determined that the pressure during the growth process will affect the thickness and size of the final product.(3)In this study,optical microscopy characterization,scanning electron microscopy characterization,EDS element analysis and Raman spectroscopy are used to investigate the properties of Fe₃GeTe₂ flakes.The study found that the samples prepared by the CVD method have a regular triangular or hexagonal shape,with a smooth surface and good quality.The prepared sample contains the elements Fe,Ge,Te,which are consistent with the element types of the target material Fe₃GeTe₂.The Raman peak position is nearly the same as described in the literature but there is a slight deviation,which is due to the difference in thickness and material preparation technology.We can determine that the sample prepared in the experiment is Fe₃GeTe₂.