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First-principles Studies Of Low Dimensional Materials

Posted on:2010-08-09Degree:DoctorType:Dissertation
Country:ChinaCandidate:S L HuFull Text:PDF
GTID:1101360275955467Subject:Chemical Physics
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The scale of nano materials is at the nanometer magnitude at one or more dimensions. In these dimensions,there are new quantum effects and peculiar properties that can not be found in bulk systems.The new properties brought by low dimensions deepen human being's understanding of the nature world,and drive people to develop new applications.Studies on the various fields of low dimensional materials,such as electric,magnetic,optical,mechanic,and catalysis,have been launched for many years,and new quantum phenomenon due to low dimensionality and symmetry also attracted much attentions.On the other hand,first-principles calculations were used widely and successfully in condensed matter,quantum chemistry,and materials science. This method is also a powerful tool to study low dimensional materials.In this dissertation,we mainly apply the first principles method to low dimensional nanotubes and graphene nanoribbons.In the first chapter,we introduce the basic background and development of quantum chemistry,the idea of density functional theory and review its recent progress,and introduce some related parts in practical calculations.The density functional theory (DFT) bases on ground state electron density.It tells that any properties of a many body system can be determined by the the electron density of a system at ground state. To describe some special systems more precisely or some large systems faster,there are many branches of developing DFT.The development of DFT on some specific problems have approached some certain degree of success.Besides,some more complicated many-body theory have been developed and applied.At the end of this chapter, we briefly introduce some commonly used DFT based simulation packages.In the second chapter,we give some brief introduction of nanotubes and graphene. The discovery and studies of carbon nanotubes(CNTs)and boron nitride nanotubes (BNNTs) opened up a new field of nano materials and low dimensional physics.The preparation,geometry configurations,electronic,optical,mechanical properties,and applications on hydrogen storage and catalysis are reviewed concisely.Graphene,a new type of two-dimensional materials,are attracted enormous interests due to its peculiar physical properties.In recent years,there are many reports on its geometry, electronic,optical,and magnetic properties and its derivatives,such as graphane and graphene oxide.Graphene nanoribbons,as easy controlled one-dimensional nano materials, are also attracted many attentions.From the third chapter,we begin with the defective BNNTs with divacancies. The divacancy defects are proved stable under electron illumination by an experimental group.Based on this discovery,we study the hydrogen storage properties of the defective BNNTs.We found that by saturation the defects with hydrogen atoms,hydrogen molecules can go into and out BNNTs through the defects.It facilitates room-temperature hydrogen storage to utilize the spaces in BNNTs.The hydrogen saturation reduces the potential around the defects,thus reduce the repulsive interaction of the hydrogen molecules and defects.This idea can be used to the other similar systems.In the fourth chapter,we investigate more defects in BNNTs.The different defects bring different electronic structures,we try to use external electric field to modify the properties of these systems.The static transverse electric field at some moderate strength can move the defect states to near the valance band or conduction band edge,when reducing the gap of BNNTs,thus the electric or optical properties of these systems can be adjusted.The direction and strength of electric field are all useful controlling parameters.We can also explain the observed phenomenon with change of electrostatic potential.The gap closure of BNNTs under transverse electric field may be related to the nearly free electron(NFE) state in conduction band.There are also delocalized states with atomic orbital character in C60 found in experiments,they can be related to NFE states in graphite.In the fifth chapter,we study the NFE states in CNTs and BNNTs. We found that there are also NFE states with atomic orbital character in nanotubes, and the NFE state in literature are only NFE state with s symmetry.Further,we study the relations of NFE states in 1D nanotubes and corresponding 2D molecular planes. The NFE states in 2D molecular planes,1D nanotubes,and 0D fullerenes are from the same potential origin.Besides,the NFE states in multi-walled nanotubes and nanotube dimer are also investigated,and the way to downshift the NFE states to the Fermi energy,such as applying transverse electric field and alkali metal intercalation are also explored.In the sixth chapter,we continue to study NFE states in another type of 1D nano structures,the graphene nanoribbons.The NFE states adapt to the new geometry and symmetry.But in the superlattice of 1D nanoribbons,there are two type of NFE states. One is bound weakly over the ribbon plane,should be the same origin as in graphene, similar to nanotubes mentioned in the last chapter.The other is results of superlattice, confined between ribbons.After electron doping,the NFE states can be moved to the Fermi energy,especially,the second type of NFE states even can be occupied,to be utilized as a new ballistic transport channel.In the appendix,we cooperate with the STM experimental group to investigate the origin of the rectification effect on Ag/Au cluster with more than 3 atoms on Si(111)-(7×7) surface system.Combining with the density of states analysis,we proposed the "wave function selective" mechanism,and obtainedⅠ/Ⅴcurves with qualitative agreement with experiments.
Keywords/Search Tags:Low dimensional materials, Density functional theory, Nanotubes, Graphene, Defects, Electric field, Nearly free electron states
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