First-principles Study On The Structure And Phase Transition Of Two-dimensional Ferroelectric Data-storage Material In₂Se₃

With the rapid development of electronic technology and the comprehensive popularization of the Internet and the internet of things,the 21st century has developed into a society of human beings and digital information.The development of smartwatches,mobile phones,laptops,new energy vehicles,and quantum computers are all inseparable from data storage.The data-storage devices that are commonly used in our daily life are also gradually changing.For example,the early memory devices were mainly magnetic tapes and CDs,but in recent years the mainstream data-storage devices have evolved into magnetic disks and solid-state drives.In addition,due to the high degree of information sharing,the way of data storage has also changed from offline devices to online software.These changes also reflect the increasing demand for data storage.Furthermore,the artificial intelligence industry is also developing at a rapid pace.For example,the Open AI company from the United States launched an artificial intelligence model called“Chat GPT”at the end of 2022,and Microsoft also embedded it into the Microsoft Bing search engine in February this year.So far,artificial intelligence has fully integrated into our daily life.The work of artificial intelligence software needs to continuously access massive amounts of data,which brings huge challenges to the existing data-storage technologies.Therefore,it is urgent to develop a new type of non-volatile memory device with high speed,low-power consumption,small size,and high data-storage density.Ferroelectric memory has become one of the most promising candidates for next-generation nonvolatile memory due to its advantages of low-power consumption,high speed,and strong endurance.After decades of development and optimization,ferroelectric memories have been commercially available in the early 21st century.However,the miniaturization of ferroelectric memory devices is hindered by the depolarization field.In 2017,In₂Se₃ has been proven to be able to maintain robust ferroelectricity at the limit of the monolayer,and it is expected to break through the bottleneck of the miniaturization of ferroelectric devices.So far,ferroelectric devices based on two-dimensional（2D）In₂Se₃,such as ferroelectric field-effect transistors,ferroelectric channel transistors,ferroelectric synaptic semiconductor junctions,ferroelectric memristors,and ferroelectric field-effect transistors integrating memory and computing functions have been successfully fabricated.Although the performance of these devices has met most of the requirements for memory devices,there are still some shortcomings in the 2D In₂Se₃ ferroelectric devices,such as slow write/erase speed.The main reason is the lack of a comprehensive understanding of the dynamic atomic structure and ferroelectric switching mechanism of 2D In₂Se₃.In this paper,through first-principles calculations,we study the diverse phases of2D In₂Se₃ and the transition among them.On the one hand,we find the real atomic structure of the 2D In₂Se₃ paraelectric phase,which not only explains the divergence between previous experiments and theories but also helps to deepen the understanding of the paraelectric-to-ferroelectric transition.Furthermore,we also predict a series of charge density wave（CDW）phases of 2DβIn₂Se₃.These phases have different optoelectronic properties,which lays the foundation for their application in the field of data storage.On the other hand,we investigate the ferroelectric transition mechanism of the 2D In₂Se₃ and propose a feasible scheme to accelerate the paraelectric-to-ferroelectric transition.The specific details and conclusions are as follows:1.Ultrafast ferroelectric（α）to paraelectric（β）phase transition driven by a shear phonon mode of 2D In₂Se₃.As the working basis of ferroelectric memory devices,the ferroelectric transition has a crucial impact on their performance.However,due to the lack of atomic-scale dynamics studies,the mechanism of the ferroelectric transition is still a mystery.In this study,through molecular dynamics（MD）simulations and phonon analysis,we find that the high temperature will induce the activation of an in-plane shear phonon mode of 2D ferroelectric In₂Se₃.With the assistance of this phonon mode,the ferroelectric-to-paraelectric transition of 2D In₂Se₃ can be completed within 4 ps via coherent slip between atomic layers.The discovery of this mechanism inspires the regulation of other phase transitions in 2D In₂Se₃,such as the acceleration of the reverse paraelectric-to-ferroelectric transition.2.Discovery of a new pseudo-centrosymmetric paraelectric phase of 2DβIn₂Se₃and the entropy barrier for the paraelectric-to-ferroelectric transition.Ferroelectric memory devices based on 2D In₂Se₃ have the problem of low write/erase speed,which is caused by the different understanding of the atomic structure of 2D paraelectric In₂Se₃.In this study,through first-principles calculations,we reveal that the middle layer Se atom of 2D paraelectric In₂Se₃ has a Mexican-hat potential energy surface（PES）.At a finite temperature,the unique PES will induce the middle layer Se atoms to move randomly at the basin of the Mexican-hat PES,and the symmetry of the material will be broken.However,these Se atoms follow a random walk within a region centered at the centrosymmetric site.Therefore,the time-averaged structure of 2D In₂Se₃ during MD simulations can be equivalently centrosymmetric.The random motions of pseudo-centrosymmetricβ_pc In₂Se₃ break the coherence between atoms and set a huge entropy barrier for theβ-to-αtransition.By ordering the positions of middle layer Se atoms,theβ-to-αtransition can occur within 20 ps.This study provides a theoretical scheme for accelerating the reversible ferroelectric transition of 2D In₂Se₃.3.Prediction of various CDW orders in 2DβIn₂Se₃.As one of the most important properties in condensed matter physics,CDW has been proven to exist in one-dimensional（1D）,quasi-1D,and some 2D materials.The appearance of CDW is usually accompanied by other interesting physical properties.In this study,using the first-principles calculations,we predict the CDW in 2DβIn₂Se₃ for the first time.Compared with traditional CDW materials such as 1T-Nb Se₂,the electronic structure of 2DβIn₂Se₃ is much simpler,but the CDW order is more complex,including the2×√3 superstructure,√7×√7 reconfiguration,√13×√13 star of David（SOD）reconfiguration,etc.Finally,we find that the energy and structure differences between these CDW orders are very small,but they have different optoelectronic properties which can provide a signal contrast for data storage.This study lays the foundation for the application of 2D In₂Se₃ in the field of low-power consumption memories.4.Electronic excitation induces the transitions of 2D In₂Se₃ among its variousβphases.Previous studies have shown that the tuning of the 2D In₂Se₃ structure at the atomic scale has a significant impact on the performance of related memory devices.Besides the electric fields,ultrafast lasers are also used as excitation sources for device regulation.In this work,through density functional theory（DFT）and time-dependent density functional theory（TDDFT）,the“order-to-order”transition and the“local disorder-to-order”transition in 2Dβ-In₂Se₃ under the ultrafast laser excitation are simulated.These analyses reveal that the excitation of electrons will change the shape of the PES,which in turn triggers the atomic forces that drive the phase transition.Our study provides a theoretical solution for realizing precise atomic-scale regulation in 2D In₂Se₃,which is expected to realize the precise regulation of 2D In₂Se₃-based memories.In summary,through first-principles calculations,we discover a pseudo-centrosymmetric paraelectricβ_pc phase caused by the Mexican-hat PES of 2D In₂Se₃for the first time.Furthermore,we point out that theβ_pc phase brings a huge“entropy barrier”that hinders the paraelectric-to-ferroelectric transition.Inspired by the ultrafast coherent ferroelectric-to-paraelectric transition of 2D In₂Se₃,we propose a scheme to increase the speed of reverse transition by ordering the atomic positions ofβ_pc In₂Se₃and verifying it by MD simulations.Besides,we find that the Mexican-hat PES induces a series of CDW orders in 2DβIn₂Se₃.The slight structural differences and obvious signal contrast between different CDW orders lay the foundation for the application of2DβIn₂Se₃ in the field of low-power consumption memory devices.Finally,we propose that the Mexican-hat PES of 2DβIn₂Se₃ can be effectively converted into the single-well PES by electronic excitation,which is expected to achieve finer control of atomic positions.Overall,this study provides an in-depth theoretical understanding and technical route for the application of 2D In₂Se₃ in the field of nano-scale,low-power consumption,ultrahigh-speed,nonvolatile data-storage devices,which will help to speed up the process of its industrial application.