Low-temperature Scanning Tunneling Microscopy Study Of Single Magnetic Molecules Adsorbed On Metal Surfaces

The miniaturization of silicon-based field-effect transistor will reach the ultimate limitation in the near future. Molecular electronics is regarded as one of the most promising research fields for the further miniaturization of electronic devices. Its main goal is to use individual molecules as active electronic components to build the electronic circuits. Nowadays, molecular electronics has become a hot research field in physical sciences. It can provide the possibility to investigate electronic transport mechanisms at the single-molecule scale, where quantum mechanical effects are pronounced. The small size of molecules together with the great variety of electrical, magnetic, optical and mechanical properties of molecules can give rise to many exotic quantum effects. Here we investigate the adsorption behaviors and electronic properties of single magnetic molecules on metal surfaces using an ultrahigh vacuum low-temperature scanning tunneling microscope (STM).In chapter 1, a general introduction to the molecular electronics is given. We first present the concept of nanoelectronics, and then describe the development and the latest progresses of molecular electronics in detail. We further introduce the STM and its applications in molecular electronics and spin detection. In the end, the works in this dissertation are briefly summarized.In chapter 2, the self-assembly behaviors and electronic transport properties of REPc2 (RE= Tb, Dy and Y; Pc= phthalocyanine) molecules on Au(111) surface have been investigated with a low-temperature STM. First, we describe the adsorption behaviors of REPc2 molecules on Au(111) surface, analyze the molecular islands with various organizational chirality and provide the corresponding adsorption models. Second, we demonstrate the gate-modulating single-molecule transport in a STM setup without a gate electrode. Here the electrostatic potential provided by the reconstructed surface of Au(111) and intermolecular interaction is used as an equivalent gating potential to tune the energy levels of REPc2 molecules in the STM tunneling junction. By analyzing the dI/dV spectra obtained on the REPc2 molecules, we observed a strong suppression of the molecular conductance at low bias which can not be lifted by the local electrostatic potential on the molecules. This enables us to unambiguously confirm that it is an experimental evidence for the Franck-Condon blockade in STM single-molecule transport. We further theoretically demonstrate that the tunneling conductance suppression can be attributed to the strong electron-phonon coupling in our STM single-molecule transport supported by solving the rate equations based on Anderson-Holstein model. It is worth noting that, compared to the poor reproducibility for the Franck-Condon blockade effect in the traditional three-terminal nano-devices based on the metal-electrode broken technique, this effect can be found in every REPC2 molecules on Au(111) in our STM transport, demonstrating the high reproducibility for electronic transport properties of single-molecule using STM tunneling junction.In chapter 3, we perform inelastic electron tunneling spectroscopy and microscopy (IETS and IETM) studies for REPC2 molecules adsorbed on Au(111) and Cu(111) surfaces. For REPc2 on Au(111), we find five obvious molecular vibrational modes (30meV,52meV,77meV,96meV and 136meV), among which the modes at 30 meV,52 meV and 77 meV possess much larger change in conductance (△σ/σ> 50%). We perform STM-IETM for these vibrational modes and obtain the d2I/dV2 maps with high spatial resolution. In addition, our theoretical simulated IETS and IETM results based on first-principles calculations agree well with the experimental results. For REPC2 on Cu(111), only two molecular vibrational modes at 54meV and 78meV can be found, and their △σ/σ are only about 10%. The d2I/dV2 maps for these two modes possess poor spatial resolution in contrast with those of the same vibrational modes at 52meV and 77meV for REPC2 on Au(111). The difference in △σ/σ and d2I/dV2 maps for the same vibrational mode between REPc2-Au(111) system and REPc2-Cu(111) system can be attributed to the molecular orbital gating effect in the former, which can resonantly enhance the vibrational excitations and induce the large value of △σ/σ when the energy level of molecular orbital is tuned to the near-resonance region of the vibrational states, i.e. resonant-enhanced IETS. Hence the d2I/dV2 maps for the former possess high spatial resolution. However, the lack of these enhanced vibrational excitations in the latter and the strong hybridization between molecules and Cu substrate lead to the small value of Aa/a and the poor resolved d2I/dV2 maps.In chapter 4, by using a low-temperature STM, we show the in situ engineering of single magnetic molecules with intramolecular spin coupling. Single picene (Pc for short) molecules adsorbed on Au(111) can be manipulated to accommodate individual Co atoms one by one, forming artificial magnetic molecule CoxPc (x=1,2, and 3) with magnetism introduced by the Co atoms. Different from the previous reports of artificial magnetic molecules, the CoxPc structures are highly stable during the STM manipulation, and the magnetic properties are robust as well. By monitoring the evolution of the Kondo effect at each site of Co atom, we find that the magnetic properties of the CoxPc structures can be tailored by controlling the number of doping atoms. The Kondo resonances observed in CoPc and Co2Pc are identical, indicating negligible spin coupling between the two magnetic centers in Co2Pc. However, the dI/dV spectra measured on Co3Pc show a broader resonance near the Fermi level consisting of a Kondo resonance and two side shoulders, suggesting strong spin coupling between the magnetic Co atoms in Co3Pc which is confirmed by our theoretical calculations based on the density-functional theory. This finding indicates that single magnetic molecules with intramolecular spin coupling on surfaces can be artificially constructed in a controlled manner.