Study On The Growth And Physical Properties Of Environmentally Stable Transition Metal Selenide Films

Two-dimensional(2D)materials possess excellent optical,electrical,mechanical,thermal and magnetic properties.Among them,2D transition metal selenides(TMSs)hold great promise for future applications.As a compatible technique with the semiconductor industry,chemical vapour deposition(CVD)has been utilized to grow a variety of 2D materials,such as graphene,h-BN and MoS2.However,the TMSs show poor environmental stability and their intrinsic properties vanish rapidly even after exposure to air for a short time,which limits their potential application.Currently,the preparation of selenide films with high environmental stability remains challenging.In addition,as the rich physical phenomenons in unconventional 2D materials,the growth of unconventional two-dimensional materials is also an important topic.Taking FeSe as an example,the high-temperature superconductivity found in FeSe has attracted tremendous experimental and theoretical interest.In this thesis,we have systematically study the two step vapor deposition for growing 2D TMSs,explore the origin of stability in 2D TMSs,and grow unconventional 2D materials.The main results are shown as follows:1)The growth of TMSs and exploration of stability in TMSsWe develop a two-step vapour deposition method to grow stable TMS films,including NbSe2 and TiSe2 films.To obtain TMS films with high crystallinity,we conduct the magnetron sputtering in a H2O/O2 free condition by deep degassing,and deposit the metal films on sapphire with heating substrate.Subsequently,a typical CVD process is applied to transform the crystalline metal films into 2D TMSs with different source materials.The stability of TMSs is explored.And we take NbSe2 films,a typical 2D superconductor,as an example to investigate the environmental stability of 2D TMSs grown by the two-step chemical vapour method.We study the environmental stability of the bilayer NbSe2 films by tracing their Tc changes after different treatments,including exposure in the atmosphere(without or with heating),immersion in different aqueous solutions,heating in high vacuum.And the above tests unambiguously confirm the excellent environmental and thermal stability of NbSe2 films,which holds significant implication for the future applications of TMSs in various electrical devices.In comparison,it shows that the Tc of as-grown NbSe2 films from pre-oxidization Nb films has poor environmental stability.After exposure in the atmosphere for one day,no superconductivity can be detected.Moreover,their structural stability can be largely impaired after Ar ion irradiations,which can easily introduce atomic vacancies into most 2D materials.Since oxygen is completely excluded from the growth processes and the precursors in our two-step vapour deposition,we propose that the superior structural stability of our NbSe2 films should originate from the absence of atomic vacancy and oxygen bonds,which often causes the activated reactive sites in the grown NbSe2 films and thereby results in a poor structural stability of TMSs.This hypothesis is further confirmed by the theoretical simulation of oxidization of few-layer 2H stacked NbSe2.The oxygen can be easily absorbed to the pre-existed oxygen bonds on the NbSe2 surface,which could then diffuse easily into the interlayers,as facilitated by the vacancies,resulting in a poor structural stability of TMSs.2)The growth of unconventional 2D materials——superconductor FeSe filmsBased on the previous one chapter,we grow FeSe2 by two-step vapor deposition method firstly,and further implement the structural transformation by heating treatment,and complete the growth of superconducting FeSe films.The systematic research on the structure conversion process from orthorhombic FeSe2 films to tetragonal FeSe films is performed by characterization methods such as AFM,Raman,EDS and STEM.Further electrical measurements show that FeSe2 films gradually transform to FeSe films with the structural transformation process,and a transition from insulator phase to superconducting phase at low temperature is found.A temperature-dependent phase transition is observed from a metallic tetragonal phase to a superconducting orthorhombic phase in FeSe films.The method could also be one alternative growth strategy for the preparation of other unconventional two-dimensional materialsTo conclude,we develop a general two-step vapour deposition method and grow different 2D TMSs with superior structural stability.These TMS films can grow up to wafer size and possess a homogenous structure,as well as high quality.The grown high quality 2D TMSs allows the easy stacking with other materials into a superlattice,thereby greatly simplifying the fabrication of TMSs-based devices.Therefore,the growth of environmentally stable TMSs films is of both fundamental and technological significance to the development of TMSs-based devices with complicated integration structure.And unconventional FeSe films are grow by modified two-step vapour deposition method.The structural transformation process from orthorhombic FeSe2 films to tetragonal FeSe films is studied at macroscopic and microcosmic level.Our growth strategy is not only supply a way to research the phase transformation engineering in 2D materials,but also provide one alternative method to grow various unconventional functional materials in two dimensions,such as 2D high-temperature superconductor cuprates with more complex structure.