DOI: 10.7274/28665869
title: Applications of Scanning Tunneling Microscopy for the Study of Atomic, Molecular, and Surface Properties

description: Low-temperature scanning tunneling microscopy (STM) is used to investigate the self-assembly behavior of small molecules and amino acids on metal surfaces at ultra-high vacuum conditions. These experiments are used to study the role in intermolecular interactions and surface effects in the process of a system assembling into different crystal polymorphs. Non-equilibrium systems are prepared and studied to gain access to information about local energy minima across a potential energy surface alongside the configuration that represents the global energy minimum. These experiments are supported by computational methods such as classical force fields (Amber), density functional theory, and VASP. Electrospray ionization mass spectrometry data was also used to investigate solution clustering before molecules crystallized on the surface. 

Studying S-indoline-2-carboxylic acid and S-proline assemblies found metastable pentamer structures on the surface that apparently formed a racimic mixture of pinwheel structures on the surface despite being formed of enantiopure samples. After a variety of computational methods were used, it was concluded that these were two different polymorphs forming on the surface.

Previous studies of FcCOOH on Au(111) revealed the formation of a quasicrystalline monolayer. To evaluate whether the absence of molecule-surface interactions in the model was reasonable, we performed the same experiment on Ag(111). We found that quasicrystals were not observed, Instead, we identified four monolayer structures distinct from any observed on any surface. VASP calculations are used to support our molecular assignments, and ESI-MS experiments show the potential origin of the dimer-based structures observed on the surface.

Further work is presented using scanning tunneling spectroscopy (STS), an application of STM. The paucity of experimental data surrounding actinide research, and plutonium specifically, would be well addressed with characterization of the electronic structure of plutonium-containing compounds by STS. A study is presented here on room-temperature STS scans of the plutonium native oxide, which is commonly calculated as being a semiconductor. The results here show that the surface does not have a band gap, instead showing a dip in the density of states that never creates a break between the valence and the conduction band, similar to a semimetal.

Additionally, preliminary work is presented on the electronic structure of the plutonium intermetallic \ce{PuCoGa5}, which is the first plutonium-containing superconductor. Features in the initial experiments match with previous photoemission spectra, and the superconducting gap is observed. A region of inverse differential resistance is observed, and images of what may be the first ever atomic-resolution STM images of plutonium are reported.

The STM work may shed light on the early stages of crystallization and the driving forces behind self-assembly and polymorphism. The STS work filled a significant need within the actinide community for reliable experimental electronic structure data of rare materials.

LA-UR-25-21873