Two Factors Bat Structure Prediction Method And Its Applications In Molybdenum Dichalcogenide

The advent of high-performance computing has revolutionized the method for excavation of the material,realizing the reverse material design and development model of theoretical guidance experiments.Rational optimization of prediction algorithms to short prediction time,reduce prediction costs,and improve prediction accuracy is the focus of current research.The essence of theoretical structure prediction is to find the global energy minimum on the potential energy surface.Compared to genetic algorithms,particle swarm optimization algorithm is simpler and more intelligent,and thus better suited for solving these problems.Different from the structure prediction method using the classical particle swarm algorithm,this thesis updates the search and judgment method for the global energy minimum point in particle swarm optimization algorithm and proposes a two factors bat structure prediction method,given the complexity of the potential energy surface.Based on this structure prediction method,we conduct a detailed study on the scientific problems that urgently needed to be solved in molybdenum dichalcogenide.The high-pressure phase diagram of the MoSe₂was established,and the reasons for the controversy over its experimental measurements were investigated.A design idea for improving the thermoelectric performance of molybdenum dichalcogenide was proposed and two potential thermoelectric materials were predicted.We put a way towards further studying of molybdenum dichalcogenide as electrocatalyst materials in the future,and report two potential electrocatalytic materials.The main contents and innovations in this thesis are as follows:1.A two factors bat algorithm based on the classical particle swarm algorithm was developed independently.On this ground,the two factors bat structure prediction software was designed.Traditional particle swarm optimization algorithm may sometimes get trapped in local minimum,resultinng in the local lowest point of the potential surface.The two factors bat structure prediction approach developed through two factors bat algorithm abandons the stochastic weighting factor and replaces the constant factor and individual optimal positions with an online learning factor and particle swarm suboptimal positions,respectively,making its“individual memory”more intelligent,and its“social cognition”further emphasized.Furthermore,the search status of the group is reflected more realistically and accurately,thus avoiding the technical difficulty that particle swarm optimization algorithms sometimes fall into local optimality.The results of the simulation experiments showed that the two factors bat algorithm can effectively optimize the search time and further improve the accuracy of the prediction results,which will provide some technical support for the development of future structure prediction methods.At the same time,the two factors bat structure prediction method used feature vectors to describe the crystal structure,which is simpler than the traditional feature vectors and can be used as the input file directly to reduce the possibility of information loss,thus further ensuring the reliability of the calculation results.2.The structural phase transition and electronic properties of MoSe₂ under high pressure based on two factors bat structure prediction method have been investigated.The resulting transition sequence of MoSe₂ is P6₃/mmc（0 GPa）→R3m（20 GPa）→P6₃/mmc（80 GPa）→R-3m（491 GPa）.Meanwhile,the superconductivity of MoSe₂ under high pressure have been explored to provide a theoretical guidance for exploring the evolution of the structure and electronic properties of transition metal sulfides under high pressure and for future experimental synthesis.The high-pressure phase diagram of MoSe₂ in the pressure range of 0～500 GPa was established theoretically by t the two factors bat structure prediction method in combination with density functional theory.Three new high-pressure phases of MoSe₂ are identified,namely,P6₃/mmc,R3m,and R-3m.A comparative analysis showed that the phase transition of MoSe₂was mainly attributed to the slip transformation of its local MoSe₆ units.The local MoSe₆ units is very sensitive to external condition changes due to the weak van der Waals forces between them.This may be the reason for the divergence in the experimental measurement of the phase transition pressure of MoSe₂.The calculations on electronic properties showed that MoSe₂ will transform from a semiconductor to a metal with pressure increasing.Based on the Mc Millan equations,we have concluded the superconducting transition temperatures of two metallic phases of of MoSe₂ at 80 GPa and 500 GPa,with the values of 0.81 K and 1.24 K,respectively.These results will provide theoretical reference for the application of such materials in the field of superconductivity.3.Based on the two factors bat structure prediction method,the ground state structures of layered MoSe₂ and two-dimensional MoSeX were obtained.Two-dimensional P3m1 phase of MoSe S and P3m1 phase of MoSe Te with excellent thermoelectric properties wre discovered through exploring their thermoelectric properties.Our results provide a theoretical basis for finding the new type transition-metal sulfide thermoelectric materials and further improving their thermoelectric performance.The electronic and thermoelectric properties of the system were systematically studied by substituting the misaligned layers of bulk materials and predicting the crystal structure of monolayer MoSeX（X=S,Te）,using first-principles calculation and Boltzmann transport theory.The results howed that two-dimensional P3m1 phase of MoSe S and P3m1 phase of MoSe Te have exhibiting excellent thermoelectric materials and had a wide temperature interval.The corresponding ZT values were as high as 2.4 and 3.2 at a temperature of1500 K.Comparing the properties between these bulk materials,we found that the introduction of S or Te elements into three-dimensional MoSe₂weakens the degeneracy of the Se impurity band at the Fermi level,thereby reducing the electronic localization density.It was detrimental to the enhancement of the native MoSe₂ conductivity and the improvement of the thermoelectric properties.However,introducing S and Te elements into the two-dimensional structure shows the opposite trend.Our findings provide a valuable theoretical reference for the exploration of transition-metal sulfide thermoelectric materials and the improvement of their thermoelectric performance.4.The two-dimensional Mo_xSe_y（x=1～4,y=1～4）structures at ambient pressure were systematically searched by combining the two factors bat structure prediction method and first-principles calculations,and the electrocatalytic properties of the systems were explored.The two-dimensional materials of Mo₃S₂（Pmmn phase）,Mo₂Se₃（P4bm phase）,and Mo₃Se₂（Pmmn phase）with excellent electrocatalytic properties were discovered.The results provided a theoretical basis for the development of transition metal dichalcogenide materials in the field of electrocatalysis.Compared with the bulk structure,two-dimensional structure was more conducive to increasing and exposing active sites.The structure of two-dimensional Mo_xSe_y at ambient pressure was predicted using a two factors bat structure prediction method,while the system was further extended to the Mo-S system with the help of substitution doping.The results of first-principles calculations indicated that both the P4bm phase of Mo₂Se₃and Pmmn phase of Mo₃Se₂structures were stable ground-state structures.Further calculations on electronic and electrocatalytic properties showed the Janus structure features and the charge transfer from Mo toS（Se）effectively solve the bottleneck of poor exposure rate and limited active sites resulting from the bulk structure.The existence of the V-H reaction channel ensured the barrier-free production of hydrogen.These results indicated that Mo₃S₂,Mo₂Se₃,and Mo₃Se₂ monolayers had good HER activity,and were ideal electrocatalytic materials.Our results provide a theoretical guidance towards the future industrial application of molybdenum dichalcogenide as electrocatalysts.