Mecanismos de la reacción de hidroaminación

This section introduces the field of self-assembled monolayers, what they are, and why they are useful as samples to test this new variant of laser driven scanning tunnelling microscopy. As described in the introduction, when developing a new technique such as tuneable infrared LDSTM, it is important to have a system that

58 can be used as a control sample. Self-assembled monolayers were suggested as suitable candidates for this.

A self-assembled monolayer is a single layer film of a chemical that forms automatically on a surface when the surface is exposed to the chemical. The conditions under which this occurred determine the extent of the ordering of these molecules. Typically these layers will always be ordered in the direction perpendicular to the surface but in directions parallel to the surface the extent of the ordering can vary.

This type of system has been available in one form or another for many years. Originally, the interest in organic monolayers was due to their ability to control the wetting behaviour of vertical metal plates in the condensers of steam engines[50-52]. Coating these metal plates with such substances replaced the hydrophilic metal surface with a hydrophobic organic one. As the steam condensed into water on these coated plates, it formed droplets that fell down and came off the plate by gravity, thus freeing up an area on the plate to continue the condensation process. In contrast, when steam condensed on uncoated plates it formed a film of water that insulates the steam from the cold plate, hence hindering the condensation process. More recently, these coatings are being investigated as a way of constructing nanoscale structures as part of a ‘bottom-up’ approach. There is an ongoing trend amongst the disciplines of chemistry and biochemistry to conduct experiments on a smaller and smaller scale.[53] This requires structures in which, and with which, to do this. They can be made in two ways, a ‘top-down’ and a ‘bottom-up’[54] approach. The ‘top-down’ approach takes a larger structure and uses fabrication techniques to modify it into a desired structure. Conversely the ‘bottom-up’ approach takes small components such as organic molecules and connects them together in such a way to form the desired structure.

Of the different types of chemicals used to make monolayers those investigated most extensively have been alkanethiols. SAMs of alkanethiols have been investigated predominately on surfaces of gold, specifically Au(111), due to the ease of their manufacture and characterisation. Initially these monolayers were made by immersing gold (Au) surfaces into solutions of alkane thiol. As STM technology developed in UHV, it became possible to make these SAMs in situ,

59 which allows for a greater control over the amount of coverage of the sample. Many papers have been published over the years on the different ways that the molecules pack on the surface as the density of molecules increases[55-58]. The packing structure of the molecules on the surface progresses through a series of ‘phases’ as their surface density increases. Some of the phases are ordered; others are not. The transition between two ordered phases is usually via a disordered phase and occurs by the formation of ‘islands’ of the more dense structure in the ‘sea’ of the less dense phase. When the surface concentration of the thiols is low, the molecules are thought to lie horizontally on the surface[56, 59] (see Figure 2.25). As the surface density increases the angle between the carbon backbones of the molecules increases and the structure becomes a bit like a carpet (see Figure 5.1).

S S S S S

Figure 2.25: A stylised representation of octanethiol molecules lying flat on an Au surface at low surface coverage. The diagonal zigzag lines are the carbon chains, the yellow S’s are the sulphur atoms and the horizontal wavy line is the metal surface.

For a variety of reasons, it was decided to use alkanethiols on gold as a test sample for this new type of LDSTM experiment:

1. It is relatively easy to make this Au(111) surface form of gold in air. (For more details see §3.3)

2. These Au(111) samples are air stable enough to survive the transit from their manufacture into the UHV chamber. For most metals when a clean metal surface is produced on a sample it will quickly react with constituents of the atmosphere (principally O2 and H2O) before it can be loaded into the chamber. Consequently the preparation of a surface of most metals has to be conducted inside the UHV chamber itself. This requires a special preparation chamber which was not available for the work in this thesis.

3. These thiol molecules react with the surface to made strong covalent S-Au bonds with the surface. This means that the sample can be made at room

60 temperature and that the adsorbed molecules are less likely to move about on the surface.

4. The chemistry of the molecules is very simple making it easier to assign recorded phenomena with molecular structure.

5. The wavelength of the OPO can be tuned to the C-H stretching vibrational transition.

6. It is possible to create partially covered surfaces so that the response to the laser of the covered areas can be compared with the uncovered areas.

In summary, SAMs provide a suitable system that can be investigated with this new technique given the constraints of the equipment available.

2.5 Laser-Driven Scanning Tunnelling Microscopy