Scanning Tunneling Microscopy/Spectroscopy Study Of The Physical Properties Of Low-Dimensional Electronic Systems

There are a variety of two-dimensional(2D)materials,collectively covering a very broad range of properties.For example,the metallic graphene,insulating hexagonal boron nitride(hBN)and semiconducting transition metal dichalcogenides(TMDs).These materials offer a platform that allows us to tune their physical properties.Tailoring the crystal structure of a 2D material and building heterostructures are two effective ways to tune the physical properties of these 2D materials.In our work,we successfully fabricated hBN/Cu(111),MoSe2/hBN/Ru(0001)heterostructures and the narrowest graphene nanoribbon using the molecular precursor.Using STM,we first study the morphology of the low-dimensional system to check the quality of the sample.Then combining with various STS,we systemically investigate the electronic structures of the systems,to check whether the electronic structures are effectively tuned.In chapter one,the basic theory of STM,operation modes and STS are present.In chapter two,we fabricated the hBN/Cu(111)heterostructure.In this work,we systemically investigated the effect of moire pattern on the electronic properties of hBN on Cu(111)surface.Our scanning tunneling microscopy/spectroscopy(STM/S)show that the work function shift of hBN exhibits strong dependence on the moire pattern size,which increases as the wave length of the moire pattern increases.Analysis on the conduction and valence band edge shifts demonstrate that the work function change originates solely from the shift of the conduction band,indicating that the electron affinity of hBN on Cu(111)is independent of the spatial position within the moire pattern.Generally,the work function modulation is due to the coulomb interaction between hBN and Cu(111)surface.Our assumption is further confirmed by density functional calculations.Basically,when Cu(111)surface is covered by hBN,due to the interlayer interaction,the work function of the system is reduced compared to bare Cu(111)surface.Herein,interlayer distance plays an important role in tuning the electronic properties of the system.Within the hill region of the moire pattern,N atom which sits right above top Cu atom has shortest interlayer distance and shows remarkable charge transfer.While in the valley region,B and N atom sit in the hollow center of the top Cu atom and have much weaker interaction with the substrate.It is also found that the interlayer distance increases in larger moire patterns,and the work function shift can be further tuned by changing the moire pattern size as observed in our experimental results.On the other hand,the conduction band minima(CBM)and valence band maximum(VBM)of the system are mainly contributed by the pz orbitals of boron and nitrogen atoms,respectively,and the band gap of hBN can be modulated by the relative orientation between hBN and Cu(111)as well.In general,we mimic the mechanism of work function shift of hBN on Cu(111)surface and study the influence of moire pattern to the atomic structure and electronic properties of this heterostructure.Our results are essential for the understanding of the moire patterns emerged from the heterostructure in general,and show that the electronic properties such work function of thin film can be turned locally by changing its relative orientation to the substrate.In chapter three,we report the successful growth of MoSe2 on single-layer hexagonal boron nitride(hBN)on the Ru(0001)substrate using molecular beam epitaxy.Using scanning tunneling microscopy and spectroscopy,we found that the quasi-particle bandgap of MoSe2 on hBN/Ru is about 0.25 eV smaller than those on graphene or graphite substrates.We attribute this result to the strong interaction between hBN/Ru,which causes residual metallic screening from the substrate.In addition,the electronic structure and the work function of MoSe2 are modulated electrostatically with an amplitude of 0.13 eV.Most interestingly,this electrostatic modulation is spatially in phase with the moire pattern of hBN on Ru(0001)whose surface also exhibits a work function modulation of the same amplitude.Thus,our study convincingly demonstrates the concept of band structure renormalization in TMDs.Nevertheless,the actual manifestation of renormalization is probably more complex than an intuitive interpretation of the substrate electrostatic screening and call for further theoretical and experimental investigations.In chapter four,by using the one-dimensional reconstructed Au(110)surface,the linear pentacene molecules can only react in the direction along the gold atomic chain,finally form the narrowest graphene nanoribbon superlattices.Combining the atomic resolution STM image with the DFT calculations,we confirm the neighboring pentacenes are connected by a carbon tetragon.Furthermore,we use first-principles calculations to show that when narrow zigzag graphene nanoribbons are connected to form junctions or superlattices,properly placed square-shaped carbon tetragons not only serve as effective bundles of the two incoming spin edge channels,but also act as definitive topological spin switches for the two outgoing channels.We further show that such spin switches can lift the degeneracy between the two spin propagation channels,which enables tunability of different magnetic states upon charge doping.This work provides a basic logic unit for the development of spintronics,which is expected to have wide application prospects.