Time-shared Control System Designed For 4-Probe STM And Electronic Properties Of Graphene

Graphene consists of a layer of carbon atoms forming a honeycomb pattern,where a unit cell contains a pair of sublattices,denoted as A and B.Particularly,all the sites on the honeycomb lattice are occupied by the same atoms,which introduces sublattice inversion symmetry.The low energy electronic excitations are described by the equation which would normally be used to describe a massless particle of spin 1/2.And the corresponding Pauli matrices operate on the pseudospin presenting sublattice degrees of freedom instead of spin.Thus,the low energy quasiparticles in graphene are often recognized as‘massless Dirac fermions’.The direction of motion coupled to this pseudospin orientation endows the carriers in graphene a novel property,i.e.,chirality,which has important consequences for transport.Manipulating the pseudospin in graphene with a non-chemical method has aroused a great research interest both experimentally and theoretically in order to harness those Dirac fermions.An intriguing proposal to control the pseudospin in graphene is to distort carbon-carbon（C-C）bonds by either the strain or the curvature.This bond deformation will introduce an effective gauge field that have shown its potential to control the pseudospin polarization,and inspire the study of quantum valley Hall effect,valleytronics or even the magnetic confinement states.The main work of this thesis is based on an upgraded commercial four-probe scanning tunneling microscope（STM）system,which consists of three parts.1. An upgraded commercial four-probe STM system was completely optimized on its imaging performance.Firstly,the electrical noise is reduced to the base noise of preamplifier by taking appropriate grounding and shielding manner,reducing the exposed area of critical signal cables,and reducing crosstalk between signal lines.Secondly,via STM imaging,the upgraded damping system is proved to play significant role on the isolation of mechanical noise.However,the cooling copper braid subsequently connected to the sample stage and the cryostat cold finger disable the spring of the damping system,which induces large mechanical noise.After qualitative theoretical analysis,additional thermally-insulated fixed points were added to obtain a clear atomic-resolution image again.Then,compared to the upgrading,the tunneling current power spectrum is reduced by an order of magnitude in the range of 0-3k Hz and almost all noise peaks disappear.The remaining 2.7 Hz noise is further reduced by nearly an order of magnitude by replacing the pneumatic legs with the active isolation system（avoiding the natural frequency of the spring system）.In addition,the technology of in-situ field emission tip treatment on Au（111）surface with almost 100%success rate was also explored.Finally,the deeply optimized four-probe STM system from several aspects is able to obtain clear atomic-resolution in various material systems stably and controllably,which provides important reference to other similar STM systems.2. A unique time-shared solution was designed and the defects of the four-probe STM system on circuit were overcome.Compared with the commercial solution,the time-shared solution has the advantages of high cost to performance ratio and compatibility.Subsequently,the self-developed and designed time-shared control system,combined with the existing circuit system and ultra-high vacuum four-probe STM system,has made a series of new progress:a)Switching between large and small scanner of the same STM probe and continuous spatial resolution from centimeter to atomic scale;b)switching between multiple STM probes;c)Relative distance of the lowest point between the tips can be obtained by distinguishing the featured topography in the STM image.Then,two or more tips can be moved and positioned closer within1μm x 1μm,breaking through the limit resolution of optical microscope;d)Surface potential distribution of graphite is measured and the effect of wrinkles on the potential distribution of graphite surface is studied.The success of the solution can provide a viable reference to other similar commercial systems.3. The pseudospin of graphene tuned by the morphology of substrate was studied.The sublattice symmetry breaking in graphene on GaSe substrate is observed by the optimized four-probe STM system.Topographic images of graphene reveal a triangular lattice with three-fold symmetry rather than intrinsic honeycomb lattice with six-fold symmetry when it is placed on a GaSe substrate.It is further confirmed by AFM that the corrugation of graphene on GaSe is caused by the large fluctuation and corrugation of topography in GaSe substrate（～3 nm）,which is much larger than that of SiO₂substrate（～1 nm）.Experimental LDOS contrast between two sublattices of graphene according to height measured by STM is consistent with theoretical simulations.And the results provide a way to engineer the pseudospin or related electronic structure through a mechanical knob.In summary,the upgraded and optimized four-probe STM system is able to simultaneously characterize the structure and study the transport properties and can be further applied to study the relationship between structure and transport properties of materials in nanoscale,e.g.,the influence of pseudospin polarization induced by local stress in graphene on its local conductance.In addition,the software and hardware supporting automatical STP（scanning tunneling potentiometry）characterization can be further integrated into the time-shared control system.Besides,the function of automatically tip pisitioning on the insulating substrate can be realized by the piezoelectric effect of the scanning tube.Or upgrade single STM probe with qPlus function to compensate for the limitations of STM required conductive substrates.