The First-Principle Study Of New Two-Dimensional Carbon Or Nitrogen Based Functional Materials

The novel functional materials,especially two-dimensional layered materials,exhibit many fascinating properties,attracting tremendous attention and promoting the revolutions of related device.These materials naturally have larger surface area,carrier tunability,and unique electronic properties,which basically provide a platform to study new electronic,photon,and phonon properties in low dimensional materials.Meanwhile,these materials contribute to the development of flexible electronic devices.Graphene triggered the growth of two-dimensional materials that become cutting-edge research in materials engineering,condensed matter physics,and quantum chemistry.However,it is highly needed to design on-demand customized functional materials,which always a highlight for material,physical,and chemical scientists.First principle calculation based on computational technique has become a powerful tool in materials theory and design.It has achieved great success in studying basic properties and developing new functional materials.Particularly,the newly risen machine learning approach based on neural network in computer science,has combined closely with material science and played more and more important role in designing new functional materials.This thesis studies the fundamental properties of some new carbon-nitrogen-based functional materials from first principle calculation,provides theoretical instructions for corresponding applications.The main work and novelties are as bellow.Firstly,the structural features for nanoperforated graphene based on patterned method are studied.The investigated configurations have periodically pores with zigzag edges,which construct a triangular lattice.The dangling bonds are passivated with hydrogen atoms.Three integer indexes,representing the neck width between nanoholes along two directions and pore size,are used to classify different structures.There are no omissive structures in the definition.Secondly,the electronic properties for nanoperforated graphene are systematically studied.The results show the band gap opening and closing are both existed.The metallic and semiconducting states are periodically switching and have even-odd character.It is easy to identify the probability of gap opening is 1/3 for nanoperforated graphen from the periodical relationship.Meanwhile,an atomic preciously rule for gap opening is proposed,which depends on three integers.Based on the gap opening rule,the main optical properties of nanoperforated graphene with direct gap are studied.It has a very high absorption coefficient and direct light-emitting at（38）point.Thirdly,the band alignments among different configurations are investigated in order to build intrinsic heterostructures base on patterned graphene.It is found most become type-Ⅰand few are type-Ⅱheterostructures.This is mostly because the variations of work functions.The work functions become decreased then unchanged when reduce the pore densities.The type-Ⅰare easily formed between two structures with same pore size,whereas type-Ⅱneed the structures with different pore size.Besides,the gap scaling rule for nanoperforated graphene are investigated.Generally,the gap size is inversely proportional with the pore size and neck width.However,anomalous gap sizes are found in two unique types of structures with new scaling rules.The neck width in these two structures,treated as two dimensional large molecular structures,is equal to one along one direction;the gap scaling is solely depended on one parameter the pore size.Interestingly,the flat and Dirac bands coexist in its electronic band structures.The calculated results have significance for the applications of nanoperforated graphene in nanoelectronic devices and transparent conducting electrodes.It is also important for designing new two-dimensional covalent frameworks.Then,a new two-dimensional carbon nitride C₉N₄ with regular holes is predicted from first principle calculation.From the electronic structure,it is found the C₉N₄ is a two-dimensional nodal-line semimetal.The nodal line centered on（38）point in the momentum space forms a closed ring.It is revealed the nodal line is protected by the out-of-plane mirror or C₂ rotation symmetry.Due to the light mass of carbon and nitrogen,the spin-orbit coupling is ignored,which is very important to the formation of nodal line.Meanwhile,the intrinsic chemical potential difference plays an important role to the formation of nodal line as well based on the charge transfer and tight binding model analysis.The results for two-dimensional C₉N₄ enrich the understanding of nodal line in organic frameworks,which have great potential applications in high electron devices and functional flexible coatings.Finally,the surface functionalization for the transition-metal-nitride,Mo₂N is studied.The flat surface becomes buckled after modifying by O and H atoms due to their different electronegativity intrinsically.Based on the energy bands calculation,it is found the Mo₂N is a Dirac-type semimetal in the absence of spin-orbit coupling,in which the linear bands mainly stem from d orbitals of Mo atoms.The degenerate point in Mo₂N is very sensitive to the spatial inversion symmetry,where the degeneracy is lifted when it is broken.From first principle calculation based on wannier function,the results show that Mo₂N has nontrivial energy band when introducing spin-orbit coupling.The results are of great significance for designing functionalized transition metal nitrides with novel physical properties.