First-principle Simulation And Design Of Non-metallic Elements' Allotropic Materials

The material structure consists of chemical elements;and the elementary structure consists of a single chemical element.Under normal pressure,due to the structures of metal elements are basically metallic,their application range is extremely limited.Compared to metal elements,the structures of non-metallic elements have more diverse,novel physical and chemical properties,which results in wider potential of applications.From the element periodic table,considering the bonding ability under normal pressure,only the non-metallic elements of B,? group and V group elements with a total number of 11 species are able to form continuous two-dimensionally,three-dimensionally periodic structure.Considering the similarity and stability of these 11 kinds of elements,boron(B),carbon(C),silicon(Si)and phosphorus(P)are almost representative of other non-metallic element.B element possesses multi-coordination bonding capability to form a variety of structures.Since there is a significant difference between C and other elements from same IV group,there is strong conjugation in the carbon structure to form double bond and triple bond,while the other elements of IV group are basically in single bond form in structure.As the representative of elements of V group,P is able to to form three single bonds.There are many reports on the progress of theoretical computation of new materials for non-metallic elements as well as those of experimental investigation.Still,there is a lot of space to explore and develop new structures and properties.It is worth pursuing a broader,more in-depth study of non-metallic elemental structures.With the rise of the global search of structure,it is development trend to obtain the new material with specific structure,specific property and specific function from non-metallic elements.The theoretical simulation of density functional theory based on the first principle is a powerful tool for new material search and research.This thesis is divided into five chapters,including:chapter I,brief introduction of first-principle method,density functional theory&search and design of structure:chapter ?.new structure investigation of boron(B)element allotropes;chapter ?,new structure investigation of carbon(C)element allotropes;chapter IV,new structure investigation of silicon(Si)element allotropes;chapter V,new structure investigation of phosphorus(P)element allotropes.In Chapter ?,the theoretical framework,development and application of density functional theory as one kind of first-principle are briefly introduced.The density functional theory is used to solve the ground state electronic density by solving the Kohn-Sham equation with self-consistent iteration,to obtain the energy of the ground state,then to obtain other properties of system.In the structure search and design part,we introduce some methods and basic requirements of the new structure search,and discuss the issues of structural stability.In chapter ?,we designed and studied a series of three-dimensional boron structure,through the layered structure of self-reconstruction and structural unit combination approach,including dense layered structure C2/m-B8,reconstructed structures from layered ?3-B monolayer(The newly discovered stable structures are P2/m-B8,P21/C-B64 and Pnnm-B16),the three-dimensional structure of Cmcm-B16 and porous tunnel structure h-B24.Energy.they are higher than the common phase of ?-B12 by 0.157-0.314 eV/atom.Structurally,these structures are layered or porous tunnel structures without icosahedra cage B12 but with semi-cage or coplanar cage as structural unit.Interestingly,these are ionic structures,such as P2/m-B8,P21/C-B64 and h-B24.Among them,the most obvious one is P21/C-B64 with a largest difference of charge of 0.74 |e-| between the most positive and the most negative ion,so far.These systems are basically metallic,while P21/C-B64 is a semiconductor.As anode material for lithium ion battery,h-B24 is with relatively large specific capacity,small volume change,and smooth diffusion path for lithium ion transport.We also studied the layered structures of P2/m-B8(100)plane and Cmcm-B16(010)plane in different layer number.Energy of many of the newly obtained two-dimensionally layered structure energy is lower than Pmmm-B8,which is lowest in energy within double-layered atom thickness.These layered structures are also metallic.In chapter ?,we designed and studied a series of two-dimensional cabon materials,including Cmima-C12,Cmma-C12-B.P-4m2-C13,P-4m2-C21,P4/nmm-C18 and P6/mmm-C22.In contrast to the previous two-dimensional carbon material,the structures here are with both sp2-coordinated and sp'-coordinated C atom.In particular,the P-4m2-C21 is a super structure with sp-sp2-sp3 multi-hybridization,and 3-,4-coordinated carbon atom.These structures contain different electronic properties including metal,quasi-metal,and semiconductor.Particularly,four of these structures have negative-Poisson's-ratio properties.Many cases indicate that the "sp3-like 4-coordinated"+ "five-membered-ring structure" is the key to the negative Poisson's ratio for such structures.As new progress in the field of 2D C materials,we obtain large direct band gap materials within visible light range,two-dimensional superhard materials,strong anisotropic Poisson's ratio structure,material with high carrier mobility,versatile structure with variety of many properties,super structure with sp-sp2-sp3 multi-hybridization and 2-,3-,4-multi-coordination.In chapter IV,we design and study a series of quasi-two-atom-thick layered silicon structures.Structurally,they contain a variety of multi-membered rings and four-fold coordinated Si atom.The energy of these structures is lower than that of silicene,except P-6m2-Si5.All of them are semiconductors with band gap range of 0.71-1.67 eV except Cmma-Si12.The band gap of two-dimensional nano-silicon material is effectively opened.Many semiconductor two-dimensional silicon materials possess good optical absorption in wide energy zone to be applied as excellent donor material for solar cells.Among them,OP-D-A-Si16 and OP-D-B-S116 are direct band gap materials,and A-B-Si10 and Cz-A-Si12 are quasi-direct band gap semiconductor to be used as infrared detector material.They are mechanically soft.CZ-B-Si-2 is an anisotropic material with a negative Poisson's ratio property in the 28.9%angle region with a maximum negative Poisson's ratio of-0.055.In addition,Cmma-Si12 is a structure with full four-fold coordinated Si atom,with relatively hard with Young's stiffness of 90N/m.Most importantly,the carrier mobility calculation results show that these structures have high or super high carrier mobility,with a minimum of 3.7×103 cm2s-1V-1 and a maximum of 1.7×105 cm2s-1V-1.Thus these materials can be potential applications in the electronic devices.In chapter V,we designed and studied a large number of two-dimensional porous structure and five three-dimensional covalently connected phosphorus structures.We summed up some rules and found some special properties.There are a large number of allotropes in the two-dimensional structure with low energy,and their energy distribution zone is relatively narrow.The zone is mainly with 5-and 6-membered rings and low concentration of other types of rings to be able to find many new structures in this low energy area.Secondly,two-dimensional,three-dimensional structures of phosphorus are mechanically soft.Third,two-dimensional layered structures of phosphorus are basically semiconductor with rare direct band gap.In properties,the band gap distribution of various structures is wide,and the maximum Mono-(P4)is up to 3.42 eV.It is found that many two-dimensional porous structures may be used for photo-driven water splitting.(P7)n-(P7)n-A,(P7)n-(P7)n-B,t-(P8)2-B,P21/c-(P11)4 and ?-P36 possess negative Poisson's ratio.The relationship of band gap and single axis strain is nearly linear in ?-P36 and the corresponding band gap range is 2.49-1.98 eV,which corresponds to the energy range of visible light.Thus it can be used as strain sensor by measuring its band gap and seeing its color,or a material can be changed its color by controlling strain.The three-dimensional covalent linked phosphorus structures expand the new possibilities of phosphorus allotropic structure and their structure can survive agaist tensile strain insead of cleavage like previous vdw staking phases.Through the study of multiple structures of the four kinds of elements,we have found a series of interesting and special structures,and learned some of their basic properties and found many desirable properties.Combined with previous studies,it can be seen that the study of non-metallic elementary two-dimensional,three-dimensional allotropic structure is vigorous with discovery of many interesting properties.The research of non-metallic elementary structures is at the right time period with promised future.