Study On The Interfacial Charge Transfer Of Graphene

As a fundamental chemical element of all organisms on Earth,carbon behaves so importantly that in 1964,the famous academic journal Carbon,named after carbon element,was established.Among many well-known materials of the carbon family,graphene appears closest to us in terms of time.Literature reporting graphene can be traced back to the 1950s.This single-atomic carbon layer is used as a physical model to study the physical properties of graphite,e.g.,massless Dirac Fermions near each Dirac point in the first Brillouin Zone.Theoretically,ideal graphene is expected to be unstable due to the inevitable thermal fluctuation that causes distortion to reduce system energy.Therefore,when Science reported the first successfully prepared graphene in 2004,it attracted widespread attention.The researchers verified some theoretical predictions like quantum Hall effects in a very short period.Another milestone is the first achievement of the synthesis of large-area high-quality graphene by using chemical vapor deposition(CVD).The success of utilizing CVD techniques has greatly promoted the industrial production and practical application of graphene.Graphene exhibits several excellent properties,such as high charge-carrier mobility,high thermal conductivity,high mechanical strength,and chemical stability.The two-dimensional nature with these excellent properties makes graphene an outstanding material.Graphene seems to be regarded as a versatile material,which can be used in various applications and then results in thousands of papers.This exaggerated phenomenon in academia is worth pondering.Graphene is a promising candidate in the application of radio-frequency devices as well as flexible touch screens.However,the zero bandgap structure of graphene makes it an uncompetitive material in FET-based logic circuits.Since the density of states(DOS)in the vicinity of the Dirac point is in a low level,interfacial interactions or the transfer of a small number of charge carriers can lead to a large change of electrical signals,thus enabling the sensing capability of graphene-embedded electronic devices.The wavefunction overlap between two materials serves as the path for electron exchange,which is driven by Fermi-level differences.Electrons in stacking materials have the possibility to travel through van der Waals interfaces to reduce the system energy.Graphene is a zero-bandgap semi-metal material,in which each carbon atom is bonded to three adjacent carbon atoms through ? bonds.A remaining electron occupies the ? orbital,whereas the anti-bond ?*band is empty for un-doped graphene.In general,the metal-metal contact allows electrons redistribute to balance the workfunction difference.Naturally,the electron redistribution may also occur in when different materials are in contact with graphene.In this dissertation,assembles of graphene-copper(metal),graphene-fluorescent molecule(organics),and graphene-MoS2(2D semiconductor)are used as prototypes to study the charge transfer occurred in the van der Waals interfaces.For the graphene-copper systems,we discover a two-process cycle containing ion migration and electron transfer.In this cycle,electrons in copper transfer to graphene based on Fermi-level variations,and ion migrate vice versa due to Coulomb force.To validate this proposed cycle,we monitor the copper-oxide formation,variation of Raman-active mode,and the morphological evolution of copper oxides via Raman spectroscopy analyses.Interestingly,the charge transfer process between graphene and copper could be regarded as an extension of galvanic corrosion in 2D system.Accordingly,graphene is not suitable as an anti-oxidation coating for metals in practice(Weiyi Lin et al,Carbon,116,201 7,15-19).Similar with the graphene-copper assembly,the intermolecular ?-orbital overlap between graphene and fluorescent molecules with aromatic structure can also serves as the path for electron exchange.We observe that the fluorescence intensity of fluorescein isothiocyanate(FITC)is quenched by graphene by using Raman mapping.This quenching phenomenon can be governed by photon-induced electron transfer.This mechanism is then validated by conducting analyses on Dirac-point shifts of FITC-graphene back-gate FETs.Based on electrical measurements,the charge transfer between FITC and graphene is demonstrated.Accordingly,we fabricated graphene-embedded biosensors showing the sensing capability of measuring labeled-biomolecule concentrations(Weiyi Lin et al.,Nano Letters,2016,16,5737-5741).We also used the graphene-MoS2 van der Waals heterostructure as an experimental prototype to study the interaction between 2D)semiconductors and semimetals.By using the combination of electrical measurements,Raman,and PL analyses,we demonstrated that both charge transfer and energy transfer occur in graphene/MoS2 van der Waals heterostructures.The experimental results show that for 2D semiconductors with in-plane excitons,energy transfer,instead of charge transfer,is the dominant interaction between 2D semiconductors and graphene(Weiyi Lin et al.,Applied Physics Letters,2019,114,113103).In summary,we have studied the charge transfer in three types of van der Waals interfaces containing graphene.We point out the drawback of graphene to be a protective coating.Moreover,we fabricated graphene-embedded sensors and 2D van der Waals heterostructure based FETs to show the potential applications in nanoelectronics.