Surface-Bound Molecular Film Structure Effects on Electronic and Magnetic Properties.

摘要

This thesis dissertation will discuss the importance of understanding the driving forces of molecular assembly on surfaces and the need to characterize the electronic and magnetic properties of the resulting organic films. Furthermore, experimental results on model organic molecular assemblies, benzoate on Cu(110) and Fe[(H2BPz2)2bpy] ("Fe-bpy") on Au(111), and their novel film properties will be presented. The primary experimental techniques used in this work are scanning tunneling microscopy and spectroscopy (STM, STS), and so a theoretical characterization of constant current distance-voltage STS (z(V)-STS) will also be developed.;Deposition of benzoic acid (C6H5COOH) on to Cu(110) will be used to create a diverse molecular environment of benzoate molecules (C6H5COO+). In this film we will utilize structural phases consisting of co-existing orientation (alpha-phase) and uniform molecular orientation (c(8x2) phase) to probe electric potential variation across the surface of the film. Using z( V)-STS find that the electron affinity level of a molecule's near-neighbor will exert a substrate-mediated influence on the energy of the molecule's image potential state; which we describe using a 1-D dielectric continuum model.;Motivated by the unique utility of z(V)-STS for gentle probing of molecular electronic structure and electric potential we perform a thorough theoretical characterize of z( V)-STS. We derive a differential equation for simulating z(V)-STS spectra under the standard approximation of a square tunneling barrier. Moreover, we derive an equation for sample density of states (DOS) that is applicable for all modes of STS. The central result of this work for interpretation of z(V)-STS results is a characterization of systematic error between state energy and z(V)-STS peak location, as well we show that empirical normalization procedure for removing background distortion from constant height current-voltage STS, (V/I)dI/dV, is also applicable to z(V)-STS is a similar form, (V/z)dz/dV. .;Lastly, we present a STM study of how the well-know molecular spin crossover properties can be modified in the ultra-thin molecular film regime. As a model system for studying molecular spin crossover we create bilayer films of Fe-bpy on a Au(111) surface. Topographic imaging from STM shows defect sites in the 2nd molecular layer that suggest an internal stress in the film. We perform conductance mapping of the 2nd an find significant amounts of conductance variation across the top of the bilayer, with low conductance domains coinciding with the 2nd layer defects. Based on domain-specific I(V)-STS and density functional theory calculated DOS we assign the domains as co-existing high-spin and low-spin molecules. We found that this co-existing spin-state domain pattern persisted at temperatures spanning the bulk crystal spin transition, leading us to conclude that the bulk crystal spin transition is drastically changed by the differences in the crystal structure and the bilayer structure packing.