STM Tip Manipulation-induced Controllable Constructions Of Folded Graphene And Pseudo-magnetic Fields

Graphene is a two-dimensional material with a thickness of a single atomic layer,and has excellent mechanical and electrical properties.Among them,due to the excellent flexibility and in-plane stiffness of graphene,it is possible to use a variety of methods to assist in the construction of different graphene structures,and to change its energy band structure and to adjust its electrical properties.This thesis mainly uses scanning tunneling microscope(STM)tip to manipulate graphene to prepare a unique folded graphene structure,and build an adjustable pseudo-magnetic field in it.The main results obtained in this thesis are as follows:(1)The experimental technology of controlled tearing and folding of graphene has been developed through STM,and a wealth of folded graphene nanostructures have been prepared.By deliberately increasing the STM tip-graphene interaction,the graphene sheet can be controlled to be torn and folded to the edge of the step,thereby preparing a wealth of folded graphene structures,such as nanoscale graphene islands and nanoribbon-like structures on graphite.At the same time,we did a detailed study on the morphology and structure of the prepared folded graphene.(2)An experimental method to prepare graphene nanostructures with zigzag edge termination was developed.This study found that the tearing direction of graphene nanostructures prepared by STM tearing and folding graphene is mainly along the zigzag orientation of graphene honeycomb lattice,leading to the generated graphene nanostructures possessing the same specified edges with well zigzag terminations.Theoretical modelling demonstrates that the enhanced tip-graphene repulsive force can account for the tearing and folding processes,whereas the preferential zigzag tearing is attributed to the predicted lowest bond-breaking energy of the zigzag orientation in graphene.Our experiment provides a simple and controllable method for fabrication of graphene nanostructures with precise zigzag edge terminations.(3)In the folded graphene,an adjustable pseudo-magnetic field and pseudo-Landau levels are constructed.The folded and curved boundary of the folded graphene structure provides a good platform for constructing strain-induced novel physical properties.We found that the graphene folding boundary has a non-uniform strain,which is manifested as the vertical folding boundary lattice is stretched,and the parallel folding boundary lattice is slightly compressed.At the same time,it is found that the strain size decreases with the increase of the width of the folding boundary.Through the measurement of the scanning tunneling spectroscopy,The experiment detected the pseudo-Landau levels structure caused by the strained pseudo-magnetic field at the strained graphene folding boundary,and the pseudo-magnetic field size was ～10 – 60 T.This thesis found that the intensity of the pseudo-magnetic fields can be tuned by changing the width of the curved folding edges,and the folding edge width is experimentally controlled by the folding area per unit length,leading to the adjustability of the pseudo-magnetic fields and the flat bands in the folded graphene system.Our experiment provides a new pathway to engineer the system with tunable flat-band structures and to investigate strain and interaction induced quantum phenomena.