The study of molecular physics has become increasingly important from both a scientific and technological viewpoint. The physical behavior of materials at nanometer length scales holds many surprises and the potential technological applications of molecular science are
vast. This dissertation focuses on the fundamental physics of molecules adsorbed to metallic and semiconducting surfaces.
Using a scanning tunneling microscope, four different molecular systems, C60, Gd@C82, tetramantane, and tetracyanoethylene (TCNE), were studied. The main effects investigated were (1) how can the properties of these molecules be atomically controlled, (2) how do metal surfaces affect molecular properties, (3) how do electron-electron and electron-vibration coupling influence molecular behavior, and (4) how do spins behave in molecule-scale
structures. For C60 we demonstrate a fine control of molecular properties such as energy levels, electron-electron interactions, and electron-vibration interactions via potassium
doping. We also find that metal surfaces strongly influence the electronic screening and ordering of adsorbed molecules. In Gd@C82 and tetramantane molecules, the spatial
distribution of the electron-vibration coupling is found to be very inhomogeneous at sub-nanometer (< 10-9 m) length scales. In titanocene, we find that Au(111) induces molecular dissociation, with titanocene fragments displaying a spin-induced Kondo effect. The final molecule, TCNE, displays variable surface coupling and also enables tunable magnetic exchange coupling between covalently bonded spin centers in Vx(TCNE)y complexes.