Two-dimensional Materials Design Based On Structure Evolution Algorithm And Electronic Structure Calculations

The properties and applications of nano-materials were determined by their micro-atomic structures.Benefiting from the algorithmic developments as well as increases of computer power,nowadays,people can use computer simulations to design new materials.That is to say,by using large scale first-principles simulations,people can get all possible micro-atomic structures of new materials under any definite atomic ratio.Through analzing their electronic structures,materials scientists can further have knowledge about their application prospects.If some of those materials present great application values,which might motivate the experimenters to realize them.More importantly,design materials by using computer simulations is much cheaper in cost and time than the traditional way.During the computer simulations,one of most important parts is to confirm their atomic stacking style just by atomic ratio,which was called“structure evolution algorithm”.Until now,it's a hot topic to design two-dimensional(2D)materials by using structure evolution algorithms.In this paper,based on structure evolution algorithms and first principle calculations,we first report a series of planar boron allotropes with honeycomb topology and demonstrate that their band structures exhibit Dirac cones at the K point,similar with graphene.Then,we systematically study the effect of microporosity to the stability and electronic properties of graphene like materials,including the 2D C,B and B-C allotropes by using a modified crystal structure-search algorithm.Finally,we report a new 2D material with the specific anisotropic mobilities of both electrons and holes by using first principles calculations.The major results of our thesis are as follows:1.Graphene possesses exceptional carrier mobility and ballistic transport properties,which originate from its unique electronic band structure,the Dirac cone structure.Boron,as the neighbor of carbon in period table,has also received considerable attention for its chemical and structural complexity.Thus,the search for the other 2D boron materials that present Dirac cones has attracted considerable interest.Although several 2D Dirac boron allotropes have been reported,their structures are either not strictly planar or their Dirac point not at the Fermi level.Here,we report a series of planar boron allotropes with honeycomb topology and demonstrate that their band structures exhibit Dirac cones at the K point,the same as graphene.In particular,the Dirac point of one honeycomb boron sheet locates precisely on the Fermi level,rendering it as a topologically equivalent material to graphene.Its Fermi velocity(v_f)is 6.05×10~5 m/s,close to that of graphene.Although the freestanding honeycomb boron allotropes are higher in energy thanasheet,our calculations show that metal substrate can greatly stabilize these new allotropes.They are actually more stable thanasheet on Ag(111)surface.Furthermore,we find that the honeycomb borons form low energy nanoribbons that may open gaps or exhibit strong ferromagnetism at the two edges in contrast to the antiferromagnetic coupling of the graphene nanoribbon edges.From those comparisons,we have reported a true boron analogue to grapheme and hope it have the similar applications like graphene.2.The importance of the microporosity to the stabilities and properties of graphene and related materials has been noticed.Increasing the mircoporosity can also increase the surface of materials,which may drastically change their electronic properties or their catalytic capability.However,there are no systematic studies of such structural features nor a comprehensive understanding of their effects on the properties of different 2D materials.Here,we have performed a systematic study on the effect of microporosity to the stability and electronic properties of graphene-like materials,including the 2D C,B and B-C allotropes.We used a modified crystal structure-search method to yield planar structures with systematically varying microporosity.Our results show that the stability of allotropes is strongly correlated with the microporosity,and that the trends for C and B allotropes are qualitatively different.The formation energies of the most stable C allotropes with the same microporosity increases quickly with the microporosity,which shows a parabolic relation.In contrast,B allotropes show a much weaker and linear dependence on microporosity.Our calculations also reveal that the basic electronic properties such as metallicity and band gaps also depend strongly on the microporosity.The allotropes with low microporosity and low formation energies tend to be metallic,and the semiconducting allotropes appear more often for high microporosity.In contrast to the general trend,our study identified a C allotrope with low microporosity that is quite stable and has a large gap of 0.58 eV.3.The two-dimensional materials have received significant attention due to their superior transport and optical properties and their potential roles in future nanoscale devices.Compared to three-dimensional materials,there is still a lack of variety of 2D materials,especially with desired band gap.The number of wide gap 2D materials is quite limited.In this study,using first principles calculations,we proposed a new 2D material(?-CNH)consisting of C,N,and H.It consists of array of polyethylene chains connected by N atoms in the perpendicular direction.Because of its framework formed by C–C and C–N bonds,?-CNH shows excellent stability and mechanical properties.It is a direct gap semiconductor with a band gap of 3.03 eV,as calculated by the hybrid functional,and exhibits interesting electronic and optical properties that are very anisotropic,as determined via its structure.The mobilities of both electrons and holes in this material are very anisotropic.The mobility along easy direction is 5to 10 times higher than that along the hard direction.Interestingly,the high mobility directions of electrons and holes are different;this allows to design novel devices in which the high conducting directions can be altered by changing the carriers by applying gate voltage.