Properties Of Low-Dimensional Group-Ⅳ Monochalcogenides

The development of ferroelectrics plays an important role in modern electric and information techniques.Overwhelmed by the trend of miniaturization and integration,some stable low-dimensional ferroelectric materials walk on to the stage which overcome limitations of traditional ferroelectrics like large leaking electric current and dielectric loss and accelerate applications of low-dimensional ferroelectrics.In this thesis,we use first-principles calculations to systematically investigate lowdimensional group Ⅳ chalcogenides ferroelectrics,consisting of nanotubes rolling from MX(M=Ge,Sn;S,Se)monolayer and one-dimensional MX chains.In different types of MX nanotubes,spontaneous polarization in armchair tubes goes around the tube and cancels itself.Zigzag nanotubes have polarization along tube axis and chiral nanotubes have polarizations both long tube axis and around tube surface.We compare energies of different types of nanotubes and finds that armchair nanotubes are more stable with close radius.On the one hand,we also find spontaneous polarization and switching barrier in zigzag nanotubes decrease with tube radius.On the other hand,employing an accurate deep-learning interatomic potential of first-principles precision,we uncover a general stepwise mechanism for polarization switching in zigzag and chiral GeS nanotubes,which has an energy barrier that is substantially lower than the one associated with the conventional one-step switching mechanism.The switching barrier gradually decreases as increasing the number of intermediate steps and converges to a value that is almost independent of the tube diameter.In the chiral GeS nanotubes,there exists a threshold number of steps,below which chirality coupling is preferred.This study unveils novel ferroelectric switching behaviors in one-dimensional nanotubes,which is critical to coupling ferroelectricity and chirality.In one-dimensional MX chains we demonstrate the stability of freestanding MX chains by calculating phonon bands.Looking through electronic structures of MX chains,we find characters like positions of VBM/CBM and dispersion around band gap are very close.Also,spontaneous polarization in chains is close to that in zigzag nanotubes and switching barrier is close to that of switching 180°in MX monolayer.In 2D arrangement of freestanding GeS chains,system energy is lowest when adjacent chains are ’head to head’ and have relative sliding of c/2.Considering the growth of MX chains,we calculate the formation energy of GeS chain in single-walled carbon nanotubes and find that mismatch along c direction and formation energy are lowest when outer carbon nanotube has index(10,5)and radius around 10?.Combined with high-precision interatomic deep-learning potential,we study the influence of domain effects on the switching path of polarization in MX nanotubes and elucidate the general stepwise polarization switching mechanism in ferroelectric systems which provides the insight of the possibility of coupling between ferroelectricity and chirality.On the other hand,the research progress of ferroelectric materials in different dimensions is comprehensively reviewed and introduced.The research results enrich the spectrum of low-dimensional ferroelectric materials and have positive significance for promoting the development of related materials and devices.