First-Principles Study Of Strain Engineering And Hydrogen Effect For 2D Ferroelectricity In Monolayer Group-Ⅳ Monochalcogenides

When looking back on the long history of human beings,it is not difficult to find that every major breakthrough in science and technology is achieved with the progress of new advanced materials.The rapid development of social civilization has been closely associated with the emergence of new materials.Ferroic materials,which are referred to as a class of materials with spontaneous polarization states,including traditional ferromagnetism and ferroelectricity,play an important role in the modern information industry and data storage applications.To meet the increasing demand for electronic products,it is highly desired to realize the non-volatile devices with the advantages of lower powered consumption,higher data density,and faster-operating speed.However,traditional ferroic materials are not suitable for the applications of the next-generation information storage devices at the nanoscale.In the last decade,the rise of graphene has been leading to an explosion of research on two-dimensional(2D)materials.In particular,the emerging of intrinsic ferromagnetism and ferroelectricity in low-dimensional systems have attracted immense attraction,providing a potential platform for the realization of electronic devices with superior performance.In this regard,this thesis takes the intrinsic ferroelectricity in 2D materials via strain-tunable and their effect on contaminated hydrogen(H)impurities as the research objects.Combining the first principle calculations and theoretical analysis methods,we focus on the strain engineering ferroelectric properties of 2D materials and modulates the resulting ferroelectric properties.In addition to the above studies,by using the ferroelectric properties and suitable band gap,some 2D ferroelectric materials have been successfully identified to be promising ferroelectric photovoltaics materials.Moreover,we explored the effect of H impurities on 2D materials and study the most stable configuration,absorption energy,electronic properties,and influence on spontaneous polarizationIn the first chapter,we briefly review the theoretical limitations and restrictions for the pursuit of ferroelectricity,which can exist at the nanoscale.This chapter puts emphasis on the recent process of the discovery of robust ferroelectricity in 2D systems.At the end of this chapter,the research objective and contents are summarized as well.In the second chapter,the theoretical research methods are adopted in this thesis.Topics include first-principles computing fundamentals,density functional theory,Berry phase approach,and computing package VASP.In chapter three,we discuss the in-plane ferroelectricity in monolayer group-Ⅳ monochalcogenides MX materials.We have confirmed that these monolayers have greater polarization compared to other 2D ferroelectrics materials and find that GeS has the highest in-plane polarization value among the monolayer MX,thus can achieve a wider memory window and enhanced the retention time of a FeFET based memory and may facilitate the application of nanoscale ferroelectric devices while SnSe has the smallest in-plane polarization due to the least electronic affinity different between the type of atoms and more significantly,the smallest displacement between Sn and Se atoms with respect to the paraelectric state.Additionally,GeSe,GeTe and GeS do not undergo transition up to the compressive strain of-5% consequently having great potential for device applications.In chapter four,we discuss the effect of hydrogen impurities in monolayers SnTe and SnS simulated in VASP are investigated by using the first-principle methods based on density functional theory(DFT).Firstly,we study the different positions of and molecule atomic H in the SnS and SnTe monolayers.Interestingly,our results reveal,that the most stable hydrogen configuration is in the molecular rather than in the atomic hydrogen system of monolayer SnTe and SnS.In addition,hydrogen molecule absorption characteristics in the monolayer system are revealed by studying the geometry,stability,electronic properties and effect on polarization of hydrogen molecule absorbed on monolayer SnTe and SnS before and after modification.The study demonstrates hydrogen molecules are physisorbed on pristine monolayer SnTe and SnS with small formation energy and long absorption distance.Also,the result shows that H molecules have little effect on the polarization and electronic structure of monolayer SnS and SnTe.In the last chapter,we have summarized the main conclusion of this thesis,and the potential research directions that can be further explored are prospected,aiming to promote the practical applications of functional devices based on 2D ferroelectric materials and providing new ideas for materials development.