Graphene Nanoribbons: Geometric, Electronic, and Magnetic Properties

Abstract

IntroductionGraphene nanoribbons (GNRs) have one-dimensional structures with hexagonal twodimensional carbon lattices, which are stripes of graphene.Their structures and their electronic and magnetic properties have been intensively studied both experimentally and theoretically.Due to their various edge structures, GNRs present different electronic properties ranging from normal semiconductors to spin-polarized half metals, which opens the possibility of GNRs as electric devices.In this chapter, the geometric, electronic, and magnetic properties of GNRs are discussed.First, the electronic and magnetic properties of pristine GNRs are understood with their special structures.We emphasize the importance of one-dimensional quantum confinement effect and edge states.Secondly, since GNRs have large surface-volume ratio and special edge states, their properties can be modifie d b y m a n y m e t h ods , s u c h a s dopi n g a n d adsorption.The electronic property and its response to modulation are described in detail.Finally, the experimental realizations of GNRs are introduced, which provide substantial bases to theoretical prediction of GNRs' electronic and magnetic properties.Possible future research directions are also discussed. Magnetic and electronic properties of pristine GNRsAfter the successful isolation of graphene, its amazing properties make it become a rising star of current materials research.However, as we know, graphene is a zero-gap semiconductor.To extend the real applications, an energy gap is needed, which enables the basic electric logic states: on and off.Besides, carbon-based magnetic materials are very important, which have small spin scattering and large potential to be immigrated into future electric devices.Due to the modern technology, etching or patterning graphene in some special direction has been realized.When graphene is etched or patterned along one specific direction, a novel quasi one-dimensional (1D) structure is obtained, which is a strip of graphene, referred as graphene nanoribbon (GNR).There are some critical questions about such GNRs: How do they organize the structures?Do they have energy gaps?And is there any magnetic state in these GNRs? Geometric structures of pristine GNRsThe typical width of GNR is of nanometers.Different with 2D graphene, the termination in one direction introduces important quantum confinement effect (QCE), which endows GNR www.intechopen.comPhysics and Applications of Graphene -Theory 332 various attractive properties.Depends on the termination style, normally, nonchiral GNR can be divided into two kinds: Armchair and Zigzag (Fig. 1 shows the structures of Armchair and Zigzag GNRs).Adopting the standard convention, the width of armchair GNRs is classified by the number of dimer lines (N a ) across the ribbons.Likewise, the one of zigzag GNRs is classified by the number of zigzag chains (N z ) across the ribbons.Perpendicular to the direction of defined width, GNRs repeat their geometric structures, and form one-dimensional periodic structures.