Exploring Structural, Electronic, and Switching Properties of Functionalized Platform and Fe(III) Spin-Crossover Molecules with a Scanning Tunneling Microscope
In this thesis, molecules on metal surfaces are explored using low-temperature scanning tunneling microscopy in ultra-high vacuum. Several organic molecules and an organometallic complex are addressed on the single-molecule scale and results are presented concerning (i) electron transport through metal-molecule contacts, (ii) switching of molecules in response to external stimuli, (iii) self-assembly, (iv) stability of molecules on metal substrates and (v) the deposition of molecules on substrates. First, flat organic platforms, namely trioxatriangulenium (TOTA) molecules, are used to place functional moieties in a freestanding and upright position on metal substrates. After deposition of such platforms on an atomically flat Au(111) substrate, their electric conductance is probed with the tip of a scanning tunneling microscope (STM) to maximize the control of the junction geometry. Inert hydrogen, methyl, and reactive propynyl subunits that stand in an upright position on the platforms are contacted. Despite the simplicity of these single-molecule junctions the conductances vary in an intriguing manner with the electrode separation. It is shown that the platform molecule is suitable as high-conductance anchor and the current flow is not directly correlated with the chemical interaction between the electrode and the molecule. Atomistic density functional theory (DFT) and Green's functions calculations for the propynyl and methyl case reproduce the respective conductance evolution and reveal the role of the junction geometry, forces, orbital symmetries, and formation of chemical bonds at the tip-molecule interface. Next, two closely-related three-states rotors, namely ethoxy and ethyl moieties, mounted on the TOTA platform on Au(111) were addressed individually with an STM tip. In contrast to other anchors, the platforms retain important gas-phase properties of the rotor. This simplifies a detailed analysis, and permits, e. g., the identification of the vibrational modes involved in the rotation process with the help of action spectroscopy and gas-phase DFT calculations. The symmetry provided by the platform enables active control of the rotation direction through electrostatic interactions with the tip and charged neighboring adsorbates. Another chapter of the thesis addresses the self-assembly of molecules on substrates. A competition between honeycomb and hexagonal self-assembly of molecules can lead to large honeycomb superstructures on surfaces. Such honeycomb structures are presented for the C3 symmetric platform methyl-TOTA on Au(111) and Ag(111) surfaces. The structures cover nearly mesoscopic areas with unit cells containing up to 3000 molecules, more than an order of magnitude larger than previously reported. The unit cell size can be controlled via the coverage. A fairly general model is developed that is consistent with the present experimental results and also reproduces various published results. The model identifies the relevant driving force, mostly related to geometric aspects, of the pattern formation. Next, the stability of TOTA derivatives sublimated onto Au(111) substrates were studied. STM data reveal that > 99% of ethyl-TOTA and methyl-TOTA remain intact whereas 60% of H-TOTA and > 99% of propynyl-TOTA and ethynyl-TOTA decompose. The observed tendency towards fragmentation on Au(111) is opposite to the sequence of gas-phase stabilities of the molecules. Although Au(111) is the noblest of all metal surfaces, the binding energies of the possible decomposition products to Au(111) destabilize the bond between the platform and the above mentioned functional groups by 2 to 3.9 eV and even render some of the molecules unstable on Au(111) as revealed by DFT calculations. Finally, an organometallic complex was studied. STM data reveal the first successful deposition of a charged Fe(3) spin-crossover complex, [Fe(pap)2]+ (pap=N-2-pyridyl-methylidene-2-hydroxyphenylaminato), on Au(111). Furthermore, the bulk form of the molecules is stabilized by a perchlorate counter ion, which depending on the deposition technique, may affect the quality of the deposition and the measurements. The final part reports on [Fe(pap)2]+ molecules on a Cu2N/Cu(100) surface. The adsorbed compound is controllably switched between three different states, each of them exhibiting a characteristic tunneling conductance. The conductance is therefore employed to readily read the state of the molecules. A comparison of the experimental data with the results of gas-phase DFT calculations suggests that all Fe(pap)2 molecules are initially in their high-spin state. The two other states are compatible with the low-spin state of the molecule but differ with respect to their coupling to the substrate.