The Typical Applications Of CALYPSO In Surefcae Structure Design

With the development of miniaturized semiconductor devices,the influence of surface effect on the performance of devices is not ignored and the surface effect is widely existed.There are many chemical reactions occur on the surface,such as catalysis,defects,adsorption,corrosion,and oxidation.Therefore,it is the basic premise to determine surface atomic structues for studying the properties and reaction process of materials.However,the surface structures of some semiconductors are very complex.Although a variety of atomic resolution detection technologies have been develpoed,such as atomic force microscopy,scanning tunneling microscopy,personal experience plays a decisive role in these experimental methods.Thus,there are some limitations in determining the surface atomic structures.In recent years,with the improvement of computing power,the theoretical prediction method has been gradually mature,and a series of materials with excellent performance have been predicted and verified by experiments.In this paper,we explore three typitcal systems of TiO₂,FeSe/SrTiO₃,and MoS₂ using the self-developed CALYPSO method in combination with first-principles total energy calculations.We obtained the following innovative results:1.Titanium dioxide has possessed the honor as an efficient transition metal oxide photocatalyst.However,its band gap is too large beyong³ eV to exhibiting high photocatalytic activity.A metastable rutile-type TiO₂?011?–2?1 reconstructed surface has been obtained in experiment.This new surface structure has a narrow bandgap of².1 eV,showing high photocatalytic reactivity for an excellent photocatalyst.It is the key factor to determining the surface structure for explaining the reason of the decrease of band gap and exploreing the macroscopic properties.In this paper,we systematically studied the rutile-type TiO₂?011?–2?1 reconstructed surface structure by CALYPSO method and revealed the experimental structure.The surface structure is similar to the anatase?101?face,the topmost two coordinated O atoms form a quasi-hexagonal characters and bond with the five coordinated and four coordinated Ti atoms of the next layer.The surface model can be a good explanation for the experimental STM images,the electronic bandgap,and the oxidation state of Ti⁴⁺.Dangling bonds of the surface play an important role in photocatalyst.First-principles molecular dynamics simulations have supported it.This work provides a theoretical basis for further design of better photocatalysts.2.In nature,TiO₂ has three kinds of crystical structures:rutile,anatase and brookite.Among them,the photocatalytic activity of rutile and anatase TiO₂ are high,especially anatase TiO₂.The?001?face is the most stable one of anatase.In face,the anatase?001?surface has a 1?4 reconstructed pattern,but the fundamental atomic structure is still under debate,which greatly hindered analysis and exploration of its chemical activity.In this paper,we predicted two surface models?ADM and OR model?with lower energies and their simulated STM images are in good agreement with experimental results.The further investigation show they can stable under different temperature and pressure conditions.According to the computed results of stability and electronic properties,we found that the photocatalytic reactivity of ADM model is better than that of OR model.The molecular dynamics simulation for water molecules on the surface of the two models proved our conclusion.3.The monolayer FeSe grown on SrTiO₃?001?exhibits high superconducting transition temperature,which inspired many excellect scientists to devote themselves to exploring the icon based superconducting mechanism.The simplest model is that the monolayer FeSe on the SrTiO₃?001?substrate.In fact,the SrTiO₃?001?surface occurs in 2?1 reconstructed pattern in experiment.In order to invetagate the effect of SrTiO₃ surface on the superconducting temperatue,in our paper we firstly explore the atomic SrTiO₃?001?-2?1 surface reconstructions using the CALYPSO method.In addition to known two surface structures?the Ti₂O₃ and double TiO₂ layer?,we predict two new Ti₂O₂ and Ti₂O reconstructed surface structures.Electrnic structures calculation show that the Fermi-surface topology of FeSe monolayer is related to reconstructed surface.The established monolayer FeSe/Ti₂O₂ model can explain the electroninc band structure observed in experiment and the electrons counting per Fe atoms.Besides,we discussed the effect of band bending induced by surface reconstruction on the transfer of electrons from SrTiO₃ substrate to FeSe films.4.Monolayer of MoS₂ in 1H phase?denoted as 1H-MoS₂?shows a large absorption coefficient of 10⁷ m^-1.As a result,monolayer 1H-MoS₂ can only absorb incident sunlight up to 5-10%in the visible wavelength region.However,the band gap is about 0.56 eV larger than the optimal optical band gap,thus its conversion efficiency is low.A method to enhance light absorption is increasing the number of layers.Unfortunately,the major drawback of the MoS₂ thin film is the indirect nature of the bandgap where phonons are required to mediate electron excitations from light.We here report a theoretical discovery of the hitherto hidden symmetry-broken phase?denoted as 1T_d?of monolayer MoS₂ through a swarm structure search.The 1T_d phase has a distorted octahedral coordinated pattern of Mo leading to a direct band gap of1.27 eV.The theoretical maximum solar cell efficiency reaches³3.3%at 30 nm thickness,giving the highest conversion efficiency among all thin-film solar cell absorbers known thus far.Importantly,the thin film with multilayers remains as a direct nature of band gap that can be engineered up to¹.34 eV by properly controlling the number of the layers.Our results demonstrate that proper electron injection is a practical route for experimental synthesis of the 1T_d phase.Once synthesized,the superior photovoltaic property of the 1T_d phase may lead to the development of entirely new line of research for transition metal dichalcogenides solar cells.