Structural Prediction And Anomalous Mechanical Properties Of Two-dimensional Carbon Networks

Carbon is one of the most abundant elements in nature having strong and rich atomic bonding characteristics.Carbon atoms can form,~2and~3hybridized bonds,enabling carbon to have a variety of allotropes,such as zero-dimensional fullerene,one-dimensional carbon nanotubes,two-dimensional graphene,and three-dimensional diamond.In 2004,graphene was successfully exploited by mechanical exfoliation.Its excellent mechanical,electrical and thermal properties greatly stimulated researchers’enthusiasm for the study of two-dimensional carbon materials.Over time,more and more two-dimensional carbon allotropes have been discovered and studied.However,how to efficiently find two-dimensional carbon allotropes that are both stable and easy to synthesize remains a research hotspot in the field of condensed matter physics.Currently,researchers can use structure prediction methods to predict new structures,but each structure prediction method or software has its own applicable scope.There are currently few structure prediction methods focusing on two-dimensional carbon materials.This study proposes a crystal structure search method that combines plane groups and primitive cell reconstruction based on the primitive cell enumeration method to predict two-dimensional planar carbon structures.In the actual operation process,according to the plane group classification,many two-dimensional carbon structures can be systematically searched.Through systematic structure search,we have obtained many newly discovered carbon structures and studied their stability,mechanical and electrical properties in depth.Some new carbon structures are superior to graphene in terms of physical properties,such as,they have abundant hybrid bonds,Poisson’s ratio effects,and semi-metallic properties.The main conclusions of this study are summarized as follows:(1)This study uses the concept of diagrams in group theory and the theory of plane groups to study the characteristics of two-dimensional carbon structures,such as the mapping relationship between diagrams and crystal structures,adjacent relationships and symmetry transformations.On this basis,we propose a two-dimensional carbon structure search method based on primitive cell enumeration.This method consists of 8 steps and generates a total of 648two-dimensional carbon structures.We used the first-principles density functional theory calculation method to study the stability,computational complexity and feasibility of these structures,and selected some structures with interesting geometric configurations for further study of their electrical and mechanical properties.Among them,some ultra-hard materials,semi-metallic materials and high-speed electronic device materials with high symmetry,low energy and good stability were found.We classified these two-dimensional carbon structures and generated a data set to provide feasibility support for subsequent research using machine learning to predict carbon material properties.Our structure search operator successfully predicted a series of new complex two-dimensional carbon networks,showing certain advantages over traditional structure search algorithms.However,this method is only suitable for searching the structure of two-dimensional carbon materials,and there is still room for improvement in the algorithm iteration.(2)To study the negative Poisson’s ratio(NPR)effect in two-dimensional carbon materials,we performed high-throughput calculations and molecular dynamics simulations on the new structures obtained by the structure search method to study their mechanical properties.It was found that the negative Poisson’s ratio phenomenon in two-dimensional carbon materials may originate from its sawtooth carbon chain.We boldly speculate that the existence of sawtooth carbon chains may be directly related to the generation of negative Poisson’s ratio effects.By studying the relationship between bond length,bond angle and binding energy after the sawtooth carbon chain is stretched,we found that the sawtooth carbon chain structure has a negative Poisson’s ratio property.Based on the carbon structures obtained in the first part,according to geometry,plane groups and symmetries,we constructed a series of new two-dimensional planar carbon structures containing sawtooth carbon chains.When strain is applied perpendicular to the sawtooth carbon chain in these structures,we observed the negative Poisson’s ratio effect.In addition,under external strain,the critical position of negative Poisson’s ratio in these new carbon materials is much smaller than that of two-dimensional graphene,and the critical position is positively correlated with the proportion of sawtooth carbon chains.As a control,we also constructed some two-dimensional carbon structures without sawtooth carbon chains,and did not observe the negative Poisson’s ratio effect in these structures.For the first time,our study pointed out that sawtooth carbon chains are closely related to the origin of negative Poisson’s ratio in two-dimensional pure planar carbon networks,and can be generalized to explain the phenomenon of negative Poisson’s ratio in all two-dimensional honeycomb structures or other two-dimensional structures containing sawtooth chains.This paper proposes a two-dimensional carbon structure search method based on primitive cell enumeration.Based on diagrams,546 two-dimensional carbon structures were obtained according to the plane group classification,and their electrical and mechanical properties were further studied.According to the successfully predicted series of new two-dimensional carbon networks,it is found that the negative Poisson’s ratio phenomenon originates from the sawtooth carbon chain.By studying the relationship between bond length,bond angle and binding energy after stretching the sawtooth carbon chain,it is pointed out for the first time that the sawtooth carbon chain is the origin of negative Poisson’s ratio in two-dimensional pure planar carbon net-works,and is extended to explain the generation of negative Poisson’s ratio effects in all two-dimensional honeycomb structures or other two-dimensional structures containing sawtooth chains.On this basis,several new two-dimensional planar carbon structures containing sawtooth carbon chains were designed.Under external strain,the critical point of negative Poisson’s ratio of these new carbon materials is much lower than that of two-dimensional graphene.