A Research Of STM Related Problems Based On The First Principles Calculations

With rapid progress in related theories and development of computer technology, the first-principles calculations based on the density functional theory (DFT) have been widely used in researches of condensed matter physics, quantum chemistry and material science. As an important tool, scanning tunneling microscope (STM) has the powerful ability in researches of physics, chemistry, material science and life science. Employing the first-principles calculations in the research of STM can help people understand the experimental results from the basic physic concepts vividly. In this dissertation, the first-principles calculations have been employed to explore the origin of various experimental results in physics and make predictions on the further researches.In chapter 1, the histories and basic concepts of DFT and STM have been introduced. Then some theoretical methods ofen used in STM related problems and the density functional packages used in the works presented in this dissertation have been also introduced briefly.In chapter 2, a theoretical study on the electronic and magnetic properties of metal phthalocyanines (Mpcs, including MnPc, FePc, NiPc, and CuPci) absorbed on Au(111) surface has been carried out. The electronic structures of MnPc and FePc change obviously when they are absorbed onto the Au(111) surface, while those of NiPc and CuPc change slightly near the Fermi level due to the week molecule-substrate interactions. By analyzing the distributions of d orbitals in the spatial and energy spaces, the possible Kondo effect related to these absorbed molecules has been discussed.In chapter 3, a series of dI/dV maps obtained from different MPcs have been explained through a first-principles calculation, including CoPc, FePc, CuPc, and H₂Pc. The dI/dV maps at +1.20 V represent the electronic states of N atoms and central C atoms in the ligand. The dI/dV maps at -0.6～-0.8 V as well as those at -0.44V represent the electronic states of fringe C atoms in the ligand. Specially, the dI/dV maps at -0.44V are not observed in the cases of all four molecules, and the possible reason has been discussed.In chapter 4, a theoretical study combined with experimental results on the influence of different noble metal surface (Au, Ag and Cu) on electronic structure of CoPc has been carried out. It is found that Ag(111) and Au(111) surfaces do not change the electronic structure of absorbed molecule very much, whereas Cu(111) surface change the electronic structure of absorbed molecule obviously and there should be formation of new bonds between the molecule and the substrate. Further more the electronic structures of d-CoPc (CoPc whit 8 Hydrogen atoms cut) absorbed on the Ag(111) and Cu(111) surfaces have been calculated. Unlike d-CoPc on Au(111), the d-CoPc molecules on the Ag(111) and Cu(111) surfaces remain spin non polarized electronic structures as same as those of the CoPc on these two surface.In chapter 5, an explanation in good agreement with differential experimental observation of STS when using a tungsten tip and an iron coated tip has been given. It demonstrated a molecule-surface hybrid state can be detected and enhanced in dI/dV spectra at -0.4 V, when using an iron coated tip, via high matching of the orbitals in the iron coated tip and sample molecules.In chapter 6, a new mechanism of negative differential resistance (NDR) phenomenon has been given, which is based on the matching of orbitals in the tip and sample in spatial and energy spaces. A model based on this mechanism explains very well the NDR phenomenon observed in the experiments using a nickel tip. The simulated results are in good agreement with experimental data. Additionally, even for a nickel plate, same NDR phenomenon should still be observed. It is conceptually important that this mechanism provides practical guideline for design of molecule based nano-devices.