The Fabrication,characterization,and Application Of Twisted Graphene

The study focus of the graphene has shifted from studying graphene itself to the use of the material in advanced devices and applications.The synthesis,fabrication and transfer of graphene are quite important for the development of the graphene-based devices.In this paper,we studied the transfer techniques for the chemical vapor deposition(CVD)grown graphene,and compared the advantages and disadvantages of the graphene transfer techniques using poly(methyl methacrylate)(PMMA),polydimethylsiloxane(PDMS),and thermal release tape(TRT).A new transfer method based on polyethylene terephthalate(PET)/silicone was developed.The graphene transferred using this method has a clean surface,due to the van der Waals attachment between the silicone layer and graphene.And larger-scale transfer can be achieved with the PET as the support layer.With this method,graphene can be transferred onto arbitrary substrates in a very short time,and the PET/silicone can be used again after the transfer process.This new technique satisfies the requirements of laboratory and factory,and will be widely used in applications.We also studied the highest-quality graphene fabricated by mechanical exfoliation.However,the critical challenge of the mechanical exfoliation for application is the massive disordered graphene flakes around the useful graphene.These unwanted graphene flakes will inevitably contaminate the substrates and damage the structures of the devices.These unwanted flakes also make it impossible to develop integrated graphene devices on one substrate.Therefore,it's quite necessary to develop a new method that only the chosen graphene can be transferred to new substrate.Here we develop a selective transfer technique based on the positive photoresist and PMMA.The photoresist is used to separate the chosen graphene from the unwanted flakes,and the PMMA is used to transfer the chosen graphene to the target position.Furthermore,femtosecond laser is used to pattern the graphene before the transfer process,and the patterned graphene is transferred successfully using this selective transfer method.The femtosecond laser fabrication can avoid the contamination to the graphene surface.The unique electrical and optical properties of twisted graphene can be tuned by changing the twist angle between layers.Currently,twisted bilayer graphene can be obtained by(1)thermal deposition of SiC,(2)folding mechanical exfoliation graphene,(3)CVD,and(4)stacking monolayer graphene one atop the other.The twist angles fabricated using these methods are random,lacking the twist-control capability.And it's hard to fabricate double-twisted trilayer graphene using these methods.Here we developed a method to fabricate twisted multilayer graphene with controllable twist angles.By cutting one graphene monolayer into several pieces and then stacking them sequentially,the designated twist angles were achieved with the deviation less than 0.1°,which was impossible for previous methods.For the whole process,the fabrication of the artificial orientations is the key step: the twist angle of the two graphene equals to the relative angle between the two artificial orientations.The twisted bilayer graphene fabricated using this method also has a quite good quality.In addition,we fabricate the double-twisted trilayer graphene with specitic angles for the first time,and investigate their interlayer coupling via Raman spectroscopy.The results demonstrate that the Raman spectrum of the double-twisted trilayer graphene can be considered as the superposition of the Raman spectra of two twisted bilayer graphene with corresponding twist angles.This is consistent with the theory studies on the electronic band,density of states,and optical conductivity of the double-twisted trilayer graphene.We investigated the applications of twisted bilayer graphene on optoelectronic devices.Under total internal reflection(TIR),the absorption of graphene is enhanced significantly,and the photocurrent is also enhanced.We investigated the responses of the twisted bilayer graphene under TIR.The photocurrent of the graphene changes with the polarization of the light in a sine curve,and exhibits a linear dependence on the incident laser intensity under TIR.The photocurrent of the bilayer graphene with twist angle of ? is enhanced by their corresponding laser energy,which are well consistent with the absorption enhancement of the bilayer graphene.The enhancement of the optical absorption and photocurrent can be achieved by adjusting the twist angle of the twised bilayer graphene.