Chapter 6
Discussion & Conclusions

Although the primary technique used during the course of my studies was STM, it is clear from the preceding chapters that STM alone does not give a complete picture of the complexity of the organic molecule-substrate system. A combination of theory, modelling and DFT calculations, and ensemble-techniques such as LEED and spectroscopy complement the ultimate precision of STM.

In this work, the self-assembly and properties of three distinct organic molecules on a diverse range of surfaces, such as a conducting oxide (Chapter 3), a semiconductor (Chapter 4), and a noble metal (Chapters 4 and 5) have been demonstrated.

C60 fullerenes on the WO2/W(110) surface exhibit rich dynamic behaviours. It could instinctively be assumed that the time-resolution of STM would not be sufficient to observe the individual molecular switching seen, but with a careful choice of experimental parameters, single molecules gaining and losing charge have been observed. The rotational and kinetic transitions observed also show the power of STM in measuring phase changes in the reduced-symmetry 2-dimensional monolayer. Phase transitions are vital for many biological and industrial reactions and the results obtained by STM reveal the nanoscale mechanisms driving them.

The molecule-substrate interface governs charge injection in molecular devices, so in order to study this, NiDPP has been used as a probe molecule to show the difference between its interaction with the noble surface of Ag(111) and the more-reactive Ag/Si(111)-√ --
  3 ×√ --
  3 R30 surface. Large single-domain monolayers are observed on Ag(111), indicating that the substrate plays only a minor role in its self-assembly. In contrast, assembly on the Ag/Si(111)-√ --
  3 ×√ --
  3 R30 surface results in the formation of randomly-oriented domains due to the higher strength of the molecule-substrate interaction.

In the design of a molecular device, control over the self-assembly of the molecular components will be key. If pristine long-range order is required it is likely that noble metals such as Ag(111) would represent suitable candidates for supporting substrates, however for more complex architecture it is likely that a template would be necessary. In this way, the Ag/Si(111)-√ --
  3 ×√ --
  3 R30 surface with its strong molecular affinity directs the monolayer order by forcing the molecules to adopt one of three orientations, dictated by the substrate symmetry. Control over the balance between these two extremes by carefully choosing the adsorbate and substrate will be a key step in the development of molecular electronics and future devices.

The chemistry of industrially-relevant organic molecules such as the epoxidation-catalysing MnClTPP is of great importance to current and future technologies. By transforming the axial ligand attached to the metal centre of the MnClTPP molecule, it is possible to control the oxidation state of the metal centre. The chemical and electronic structure information revealed by XPS and XAS and the structural details calculated using DFT allow complex STM images to be interpreted with high confidence.

Although recent studies [211, 228] make the case that liquid-STM is superior for the study of biologically relevant reactions, STM in UHV can provide more detail and allows for greater control over the condition of the molecules. Molecules in solution are inherently unstable due to interactions with dissolved substances; a property which is exploited in every biological and solution-phase chemical reaction, but which makes accurate characterisation of the molecular state difficult.

By performing a single reaction step at a time – cleaving the axial bond or adding a new axial ligand – the different stages of the reaction can be easily observed in UHV, as shown in this work. However, as with most aspects of surface science, there is no single technique which can provide all the information required, and so by combining detailed structural and electronic data measured in UHV with the dynamic behaviour measured in situ, a complete picture can be built up.

In summary, although organic molecules supported by surfaces have been widely studied for decades and show a lot of promise, there is still much unknown about their behaviour. In order to develop future devices, a clear picture of the kinetics, chemistry, charge injection and molecule-substrate interactions governing these interesting materials must first be built up through fundamental studies and quality surface science.

 6.1 Future work