Applications of Scanning Tunneling Microscopy for the Study of Atomic, Molecular, and Surface Properties

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.

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本数据集采用低温扫描隧道显微镜（scanning tunneling microscopy, STM），在超高真空（ultra-high vacuum）条件下，探究小分子与氨基酸在金属表面的自组装行为。本系列实验旨在解析分子间相互作用与表面效应在体系组装为不同晶体多晶型过程中的作用机制。实验制备并研究非平衡体系，以获取势能面（potential energy surface）上各局部能量极小值以及对应全局能量极小值的构型相关信息。实验辅以经典力场（classical force fields，Amber）、密度泛函理论（density functional theory）以及VASP等计算方法。在分子于表面结晶前，还通过电喷雾电离质谱（electrospray ionization mass spectrometry, ESI-MS）数据对溶液中的团簇行为进行了探究。

针对S-吲哚啉-2-羧酸与S-脯氨酸组装体的研究发现，其在表面形成了亚稳五聚体结构；尽管所用样品为对映纯样品，表面却表观上形成了由风车结构组成的外消旋混合物。经多种计算方法分析后，研究团队认定这些结构实为两种在表面形成的不同结晶多晶型。

此前针对Au(111)晶面上FcCOOH的研究发现了准晶单层膜的形成。为评估模型中忽略分子-表面相互作用这一设定的合理性，我们在Ag(111)晶面上开展了相同实验。结果显示未观测到准晶结构，反而发现了四种从未在任何表面上观测到的单层膜结构。研究通过VASP计算辅助完成了分子结构指认，而ESI-MS实验则揭示了表面观测到的二聚体基结构的潜在起源。

进一步工作采用了扫描隧道谱（scanning tunneling spectroscopy, STS）——STM的一项重要应用方向。当前锕系元素研究，尤其是钚相关研究，实验数据极度匮乏，而通过STS表征含钚化合物的电子结构可有效填补这一研究空白。本文报道了针对钚天然氧化物的室温STS扫描研究，该氧化物通常被理论计算为半导体材料。实验结果表明，该表面不存在能带隙，仅表现出态密度的凹陷，并未在价带与导带之间形成断裂，其特性与半金属类似。

此外，本文还报道了钚基金属间化合物PuCoGa5的电子结构初步研究工作，该化合物是首个含钚的超导体。初始实验中观测到的特征与此前的光电子能谱结果吻合，同时成功观测到了超导能隙。研究还发现了一处反向微分电阻区域，并报道了可能为全球首例的钚原子分辨率STM图像。

本系列STM研究可为结晶初期过程以及自组装与多晶型形成的驱动力提供新的研究视角；而STS研究则填补了锕系领域内稀有材料可靠实验电子结构数据的重大空白。

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