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First-Principles Study Of Zigzag Graphene Nanoribbons

Posted on:2011-06-24Degree:MasterType:Thesis
Country:ChinaCandidate:Y N ZhangFull Text:PDF
GTID:2121360308470756Subject:Condensed matter physics
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Due to its novel properties, graphene has been widely studied in recent years. Progress has been achieved on electronic characteristics and magnetic instability manipulated by doping, size confinement, electric field, chemical adsorption.This paper is divided into five chapters. The first chapter gives a simple introduction of the discovery, experimental preparation and novel properties of graphene. The second chapter presents the Density Function Theory. The third chapter discusses two methods to manipulate the electronic properties of zigzag graphene nanoribbon. The fourth chapter studies the structural and magnetic instability in layered zigzag graphene nanoribbon.In chapter 1, first we introduce the discovery of graphene, mainly introduce the novel electronic properties and transport characteristics, such as anomalous quantum hall effect, unusual weak localization. Then we present the experimental techniques to produce graphene. Last we describe their characteristics of the nanoribbons with different edge-shapes.In chapter 2, we present the Density Function Theory (DFT), base of the first-principle, and two important approximations:LDA and GGA. The basic idea of DFT is an ansatz, which assumes that any properties of many-body system can be determined by the ground state density. Many-body problem can be replaced by an auxiliary independent-particle problem, while many-body interaction can be included in the exchange-correlation energy. Finding good approximation of exchange-correlation functional is one of the main targets in DFT. At the end of this chapter, we introduce some packages used in this work.In chapter 3, we introduce two methods to control the electronic properties: applying transverse electric field and modifying by chemical group. In zigzag graphene nanoribbon, two localized states occur the both edges, which are ferromagnetically ordered, and antiferromagnetically coupled each other. Our calculated results with LDA show that the zigzag graphene nanoribbon can be converted to half metal. Then we alter the electronic property through modifying different chemical groups. The ribbon maintain a spin-polarized ground state, when modified by H and CH3, one of the spin appears near the Fermi level, the other far from the Fermi level.In chapter 4, we investigate both structural and magnetic instability of bilayer-zigzag graphene nanoribbons (B-ZGNRs) and trilayer-zigzag graphene nanoribbons (T-ZGNRs) with different edge alignments and get the following conclusions:(1) the edge states around the Fermi energy form flat bands in flat ZGNRs, they lead to structural instability or magnetic instability, or both, in layered ZGNRs; structural instability results in structural bending of edges; while magnetic instability results in magnetic ground state; (2) structural instability only happens in layered ZGNRs with a-alignment edges; (3) layered ZGNRs with P-alignment edges are always flat and have magnetic ground states; (4) the inlayer magnetic order in a layered ZGNR is always antiferromegnetic regardless of edge, alignment and number of layers, as long as its ground state is magnetic; (5) with the increasing of the number of layers, the magnetic order of the ground states can be complex and non-collinear. Though our conclusions are only based on the studies of B-ZGNRs and T-ZGNRs, they should be still valid when studying layered ZGNRs with more than four layers.Conclusions and outlook are given in chapter 5.
Keywords/Search Tags:Density function theory, Zigzag graphene nanoribbon, Electronic structure
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