STM/STS Study Of Low Dimensional Materials Modified By Atoms Or Molecules

Scanning tunneling microscopy(STM)and spectroscopy(STS)enable the direct imaging of both the structural and electronic properties of the surfaces and adsorbates with atomic resolution,showing its powerful capability in determining of the change of the electronic properties of nanostructures due to the introduction of exotic atoms or molecules.In the past decades,STM has been widely used in many fields such as physics,chemistry and biology,and has become a key technology in nanomaterials.In this thesis,we use an ultra-high vacuum low temperature STM to study the electronic properties of various materials under different modifications.In chapter 1,the principle of STM and its applications are briefly introduced.The details of the ultra-high vacuum low-temperature STM system used in our experiments are also shown.In chapter 2,we investigate the high quality nitrogen-doped graphene grown by chemical vapor deposition(CVD),by using photoelectron spectroscopy,Raman spectroscopy and Hall effect.Our results show that both the n-and p-type graphene sheets can be realized by doping nitrogen monoelement.The atomic structures and the electronic properties of the N-dopants are determined by STM/STS.The Graphitic and pyridinic N respectively contribute to the n-type and p-type doping to the graphene sheet,which tend to form different domains.In chapter 3,the electro-phonon coupling(EPC)in Highly Oriented Pyrolytic Graphite(POPG)is studied by STM/STS,in combination with angle-resolved photoemission spectroscopy(ARPES).We find that the gap-like feature in the STS spectra is an intrinsic property because of the EPC in graphite.Under our control experiment using W,Pt/Ir,and Ag tips,we find that the presence of hydrogen at the tip apex or in the tip-substrate gap is responsible for smearing out the gaps in the STS spectra by providing additional tunneling channels.The use of Ag tip,because of its relatively inert nature to hydrogen,largely prevents the additional tunneling channels,and can be more suitable for detecting the intrinsic properties of the EPC in carbon-based materials.In chapter 4,we focus on the electronic properties of different adsorption configurations of H atoms on HOPG.The H adsorption shows dominant dimmers,compared to monomers.We observe the local spin states between these different configurations with clean gapped STM tips.We also explore the effect of H-adsorbed tip on the electronic propterties for H on graphite.In chapter 5,we show a new type of catalyst,formed by depositing ferrocene molecules(FeCp2)on the surface of SiO2-supported Pt metal nanoparticles by the ALD technology.This system enables 100 percent selective removal of CO through the PROX reaction over a broad temperature range of 198 to 380 K,with a mass-specific activity of about 30 times higher than those of conventional catalysts consisting of Pt on iron oxide supports.In order to determine the mechanism of this high catalytic activity,we deposite FeCp2 on the surface of a Pt(111)single crystal.Combined with theoretical simulations,we find that the ferrocene deposits on the surface of Pt(111)undergo a dissociative adsorption,forming discrete structures of FeCp*and Cp*on the surface.After annealing to 200℃ in an oxygen atmosphere,atomically dispersed iron hydroxide is formed,which is responsible for the high catalytic efficiency of this new PtFe catalyst.In chapter 6,we investigate the structural and electronic properties of a iron-based superconductor(Li,Fe)OHFeSe on lithium glass substrates using STM/STS under a real FET geometry.With the injection or extraction of lithium ions into(Li,Fe)OHFeSe by applying the gate voltage,we observe a clear size change of the superconducting gap,accompanying with a phase transition between the superconductor and insulator in(Li,Fe)OHFeSe.Importantly,the atomically resolved STM images allow us to identify the critical role of the injected Li ions played in the tuning process.Our results not only provide clear evidence of the microscopic mechanism of the tunable superconductivity in the SIC-based(Li,Fe)OHFeSe devices,but also establish the SIC-gated STM as a powerful tool for investigating the complicated phase diagram of the correlated electrons with the field-effect approach.