Folded Growth Of Graphene And Quantum Control Of Graphene Plasmon

Graphene is a monolayer of carbon atoms packed into a dense honeycomb crystal structure that can be viewed as an individual atomic plane extracted from graphite.As an ideal two dimensional material possessing many intriguing properties,graphene have been a perennial hot topic of the condensed matter physics ever since its discovery in 2004.One of the most interesting aspects of the graphene problem is that its low-energy excitations are massless,chiral,Dirac fermions,due to which many novel physical phenomena and electronic properties emerge in graphene,i.e.finite minimum conductivity,anomalous half-integer quantum Hall effect,Klein paradox,etc.Recently,it has also been found that the linear energy-momentum dispersion of carriers in graphene could also lead to unusual plasmonic behaviors distinct from conventional plasmons.Furthermore,the as-observed high tunability,strong energy confinement,and long lifetime of plasmon make graphene an ideal platform for the fabrication of infrared devices.In this thesis,we mainly focus on the possible quantum behaviors emerging in the excitation and propagation process of plasmon in graphene.The single-atom thickness of graphene make its plasmon quite sensitive to the local environment.Therefore,many approaches have been attempted towards the modulation of the plasmon behavior in graphene,including periodic potential field,periodic potential steps and interlayer coupling.Besides,growth of novel structures of graphene for the plasmon study using chemical vapor deposition(CVD)was also performed,and an unexpected folded growth mode was found.The details are as follows:In the first chapter,we briefly introduce the crystal structure and extraordinary linear dispersion of graphene.Then we give a detailed description about its novel electronic and optical properties.The distinctive plasmon behaviors of graphene are discussed at last.In the second chapter,a novel CVD growth mode of graphene,called folded growth,is demonstrated in detail.During the growth of grahene on hexagonal boron nitride(h-BN)via CVD,some of them would fold themselves spontaneously and then grown along the opposite direction,due to which a closed bilayer edge forms at the folding position.The weak interaction between graphene and h-BN substrate is attributed to be the key issue for the occurrence of considerable out-of-plane motion of graphene at elevated temperatures.Benefit from such novel growth mode,a special structure named "hexagram bilayer graphene" with all edge closed is designed and successfully achieved via CVD method.The DFT calculations were performed and the stability of the bending edges are confirmed.This work demonstrates the potential to fabricate desiring 3D graphene structures.In the third chapter,graphene periodic potential steps were performed by partly doping using molecular treatment.Plasmon was used as a probe to confirm the quantum mechanical effect of single particles propagating at potential steps.The plasmon reflection is revealed to be tunable within a broad range by varying the ratio y between the carrier energy and potential height.It is expected that the collective plasmon behaviors were highly influenced by the single particles behaviors.Moreover,the plasmon excitation by free space photos can be regulated from fully suppressed to fully launched in graphene potential wells also through adjusting γ,which defines the degrees of the carrier confinement in the potential wells.These discovered quantum plasmon effects offer a unified quantum-mechanical solution toward ultimate control of both plasmon launching and propagating,which are indispensable processes in building plasmon circuitry.In the fourth chapter,double-layer graphene coupling system was fabricated by layering the graphene/h-BN/graphene heterostructure.The Coulomb interaction between the two layers lead the plasmon mode to form two branches called optical plasmon and acoustic plasmon.We mainly focus on the near field imaging of the optical plasmon.The relation of "coupling intensity" and carrier density was demonstrated by varying the back gate to the under layer graphene.By comparing the theoretical and experimental results,we found an unexpected saltus when the distance falls to zero.We attributed this to quantum tunneling between the double layer graphene when the distance is too small.So we will make more studies on the relation between "coupling intensity" and distance d to confirm the quantum tunneling plasmon mode between the two graphene layers.