Study Of Optical Absorption,Charge Transport And Ac Response Properties In Graphene Nanoribbons And Molecular Junctions

Since Novoselov et al discovered graphene experimentally in 2004,this two-dimensional material has attracted a lot of experimental and theoretical studies.Due to the absence of the bandgap in bulk graphene,researchers manufacture quasi-one-dimensional nanoribbons by etching graphene samples,resulting in successfully opening a bandgap,therefore people expect that graphene nanoribbon can behave as the next generation of semiconductor nanometer electronic devices.Except for graphene,organic molecule materials are also considered as the strong candidate for future nanometer electron devices,because of their rich optical,magnetic,and electronic properties.As a result,associated molecular electronics has become a significant branch of current nanoelectronics.Thanks to the invention of the scanning tunnelling microscope(STM)and later the atomic force microscope(AFM),and the adoption of the Landauer-B(?)ttiker transport theory and the non-equilibrium Green’s function(NEGF)technique,there are important experimental and theoretical advances in molecular electronics.For further exploration of the application of graphene and organic molecule materials in the nanotechnology field,in this dissertation,based on the tight-binding model and NEGF approaches,we will study properties of optical absorption,charge transport and ac response in zigzag graphene nanoribbons(ZGNRs)and molecular junctions.By investigating optical absorption selection rules for the incident light polarized along different directions,we argue that excitations from edge states are critical for the optical properties in the neutral ZGNRs.When the chemical potential shifts away from the Dirac point,the optical conductivity in the low frequency region will be drastically enhanced for the transversepolarized incident light.Different from the single-layer case,we investigate the phase transitions in bilayer ZGNRs(BL-ZGNRs)due to the external electric fields.It is noted that excitations from edge states are important for optical properties of BL-ZGNRs in the antiferromagnetic phase,nevertheless,in the charge-polarized state,there exists a broad band absorption enhancement in the low frequency region for the transverse-polarized incident light.In the angle-resolved photoabsorption spectrum of single-layer ZGNRs,we find that,with the transferred momentum increasing,the energy of the absorption peaks of ZGNRs shows the dispersion and splitting effects.So we wish that these findings about optical absorption properties in ZGNRs will provide some useful information for the subsequent optical spectroscopy experiments on ZGNRs and also provide new possibility for employing ZGNRs in future optoelectronic applications.In addition,with analyzing the charge transport property in the ZGNR p-n junction,we show that the magnetization of the ZGNR edges exists at the center part of the p-n junction,but is quenched in the p and n regions.It is found that electrons in ZGNRs can tunnel through the p-n junction with the localized modes existing at the interface of the p and n regions.By applying the external transverse electric field,in the ZGNR with a half-metal state,the electron current through the p-n junction is spin-polarized,however,in the ZGNR with a charge-polarized state,the current is switched off.As a result,we believe that the magnetic property in ZGNR can largely influence the electron transport of p-n junction,therefore ZGNR p-n junction can be utilized in the future spintronic device application.Based on the NEGF approach,we calculate the I-V characteristics in benzene molecular junction(BMJ),observing obvious negative differential conductance behavior,which is from charge redistribution in this BMJ and a Coulomb blockade effect at the interface of moleculeelectrode contacts.With a transverse electric field,we obtain hysteretic switching behavior and large spin-polarization of current through BMJ.So our calculation suggests that the BMJ affected by the transverse electric fields can be used as the current switching and spintronic devices.Except for the I-V characteristic,we also study the current noise and ac response properties in single molecular junction(SMJ).We find that the applied bias voltage does not affect the noise power density of this SMJ in the high frequency region,but at the frequencies where electron transfers from the occupied molecular states to the unoccupied ones,the corresponding noise will be drastically suppressed with dip structures.In the low frequency region,current noise spectra and ac conductances can be largely affected by the bias voltage and back-gate voltage.It is expected that these results can provide some bases for future experimental and theoretical explorations about current noise and ac response in SMJs.The specific chapters of this dissertation are arranged as follows:Chapter One is our Introduction section.In this chapter,we briefly introduce the background knowledge of graphene,including graphene nanoribbon band structure,graphene optical absorption,and graphene nanoribbon charge transport properties.Besides,we also briefly introduce the background knowledge of molecular electronics,mainly regarding molecular junction charge transport property.In Chapter Two,we mainly introduce the optical absorption property in single-layer and bilayer ZGNRs,and the angle-resolved photoabsorption spectrum property in single-layer ZGNRs.In Chapter Three,we mainly introduce the charge transport property in ZGNR p-n junction and BMJ,and the current noise and ac response properties in SMJ.Finally,we will summarize the important conclusions of the dissertations and give some foresight for the future work in detail in Chapter Four.