Study On Electronic And Optical Properties Of Novel Monolayer Lattice Systems

Since 2004,when graphene was successfully exfoliated from graphite through micromechanical exfoliation by Geim and Novoselov,two renowned physicists from the University of Manchester,monolayer lattice systems with graphene as a typical one have gained unprecedented attention.Both experimental and theoretical researches on two-dimensional monolayer materials have developed rapidly.These two-dimensional materials exhibit inherent properties very different from their three-dimensional counterparts,but with unique optical,electronic and magnetic properties as well as new quantum properties.As well,they have a wide range of applications in high-performance electronics devices,nano-optoelectronic devices,spintronic devices,etc.Therefore,the study of the physical mechanism of the monolayer lattice system is particularly necessary.because they can provide theoretical guidance for the experimental preparation of quantum devices.This paper presents a theoretical study of the electronic and optical properties of zero-,one-and two-dimensional monolayer lattice systems based on the single-orbit tight-binding approximation,linear optical absorption and the non-equilibrium Green function method,covering different bandgap cases such as conductors,semimetals,semiconductors and insulators.The main research contents and innovations of the paper are as follows.Ⅰ.We constructed a theoretical model of the helical graphene and systematically studied its electronic properties.It includes:(1)We constructed a theoretical model of helical graphene by introducing the momentum quantum number in the helical direction.Based on the model,we can turn helical system into a one-dimensional superlattice one.Under the tight-binding approximation,we study the electronic properties of helical graphene with different edge-types,different widths and different internal radii.It is found that the helical graphene exhibits sensitive but periodic regular energy band structures as the bandwidth increasing.(2)We discussed the influence of the absence of atoms at the boundary isolated points on the energy band structrure and the edge states.(3)We calculated the microscopic conductance and the current distribution of the helical graphene with the help of the non-equilibrium Green function method.It is found that the conductance at different external excitation energies corresponds to completely different current channels.Meanwhile,the helical graphene with an edge state near the Fermi energy level has one more inner edge current channel than the helical graphene without an edge state.The research is helpful for the inductance devices in the graphene-based integrated microcircuits.Ⅱ.We studied the electronic and optical properties of the phosphorene and its nanoribbons,and more importantly,propose a method that could detect defects at the edges of phosphorene nanoribbons by detecting the optical absorption.It includes:(1)We studied the electronic properties of phosphorene and the phosphorene nanoribbons with different edges,and discussed the correspondence between different edge-types and edge states in detail.(2)We studied the optical properties of phosphorene nanoribbons,specifically calculated the linear optical absorption spectra at different edge-types,and obtained the evolution of the optical absorption sp ectra with different percentages of defects by exhaustive method.It is found that a significant blue shift in the first linear optical absorption peak position occurs and the absorption peak intensity increases during the transition from the perfect beard-edge-type phosphorene nanoribbon to the perfect zigzag-edge-type one.Based on the fact,we proposed that the optical absorption can be used to detect the edge roughness of the phosphorene nanoribbons.(3)We preliminarily calculated the electronic properties of the phosphorene quantum dots.It is found that each zigzag-edge-type atom contributes one edge state whereas the beard-edge-type atoms do not contribute edge states,and the edge states between the conduction bands and valence bands gradually disappear with the number of beard-edge-type atoms increasing.Ⅲ.We refined the single-orbit tight-binding approximation model for the insulating monolayer lattices with a wide band gapsuch as the hexagonal boron nitride lattice system.And it allows this paper of monolayer lattice systems to be linked with different band gap from semimetals/conductors to semiconductors/conductors to insulators.