Explore The Planar And Nonplanar Siligraphenes:A First-Principles Study

Graphene is a prototype of 2D honeycomb material,based on which similar 2D materials consisting of other group IV elements,such as silicene,germanene and stanene,can be conceived of.Recently,a new trend is developed to partially replace the C atoms in graphene by Si and Ge atoms to obtain siligraphenes and germagraphenes,which are expected to have excellent properties.As known,graphene is flat and silicene is buckled.For siligraphenes,it is generally assumed that the transition from planar to nonplanar occurs when the concentration of Si atoms exceeds that of C atoms.It is also believed that the buckling,if has developed,will be silicene-like.However,the above assumptions,based on which many predictions have been made in recent literatures,have not been validated.In this work,we scrutinize the planarity and nonplanarity of siligraphenes g-SiCn and g-SinC by exploring various nonplanar configurations and compare their stability with respect to the planar one.Our major findings are as follows:1)Although g-Si5C has silicene-like buckled structure,it is very energetically close to its planar counterpart,which suggests that its planar and nonplanar structures may coexist at room temperature.2)We find a new high buckled g-Si7C structure,which is much more complex and stable than the silicene-like buckled g-Si₇C,whereas there is no similar structure found for g-Si₅C.Our results are at variance with the normal expectation that g-Si₇C should be more silicene like since it is closer to silicene stoichiometrically.This indicates that the development from graphene to silicene by Si doping,complex buckled structure may occur in addition to the silicene-like structure.3)g-SiC3 and SiC5 are verified to be planar by our calculations.g-SiC7,with stoichiometry closer to graphene than g-SiC3 and SiC5,is accepted as planar in previous studies.Contrary to the usual expectation,we find a nonplanar g-SiC₇ with Si atoms alternately buckled up and down,which is much more energetically favorable than the planar g-SiC₇.This signifies that whether the system is planar or nonplanar does not solely depend on the stoichiometry.4)We further investigated the widely studied g-SiC7 in more detail to show the effect of planarity and nonplanarity by comparing the total energy,electronic structure,and thermal properties.Our calculations indicate the importance of examination of the planarity and nonplanarity in the study of siligraphenes.The new insights into siligraphenes regarding the planarity and nonplanarity will be helpful for future study of the physical properties of group IV atom doped graphene,silicene and germanene.