Thermally-induced chemical transformations and self-assembly of short peptides on metal surfaces

Peptides have been identified as having high potential for creating molecular building blocks for nanoscience, due to their intrinsic biocompatibility, wide availability, and chiral properties. In particular, these molecules are well suited to the fabrication of functional thin films via supramolecular self-assembly, because of their wide-ranging functionalities around a common core, in addition to chemical motifs that facilitate self-assembly, as observed abundantly in nature. However, relatively few studies of such molecules on surfaces can be found, leaving a gap in the exploration of the potential of the class. Therefore, the primary goal of this thesis is to investigate the self-assembly of short peptides, specifically on metal surfaces. However, in order to ensure high quality thin films are manufactured, peptides should ideally be deposited on a surface in vacuum via molecular beam epitaxy, a technique that can result in their decomposition. An understanding of the capabilities and limits of this technique, in particular with respect to peptides, is currently limited, and so part of this thesis focuses on tackling this shortfall in a methodical manner. Depending on the primary structure of the peptide studied, mass spectrometry data indicated a sublimation limit can be found at just a few residues (~4/5). Additionally, these investigations demonstrated the possibility for cyclisation by thermal action for aromatic dipeptides, forming diketopiperazines. Scanning tunnelling microscopy and X-ray photoelectron spectroscopy data for ultrathin films of aromatic dipeptides and diketopiperazines follow, and reveal interesting self-assembly behaviour that also sheds light on previous peptide studies in the literature. Furthermore, it proved possible to chemically modify the diketopiperazines by onsurface annealing, resulting in stark changes to their assembly, and offering a simple pathway to challenging synthetic products. Key results from the study of L-Tyr-L-Tyr are supported by computational data for a more complete picture of the nature of the observed assemblies.