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Developed in 1981, scanning tunnelling microscopy (STM)6 is a non-optical microscopy technique that allows atomic scale imaging and measurement of surfaces by specific probing of individual atoms and molecules in contrast to traditional surface science techniques that collect and average measurement across a large area. The underlying physical basis of STM operation is electron tunnelling. Electron tunnelling can occur between two conductors that are separated by a thin insulating barrier, which may be vacuum, liquid or gas.

In STM operation, an atomically sharp tip, commonly Pt-Ir, Pt-Rh or W, is brought close to a conducting surface and a bias voltage of between a few millivolts and a few volts is applied between them. As the tip approaches to within a few nanometres of the surface, its quantum mechanical wavefunctions overlap with those of the sample and electrons are shared between them. The movement of electrons across the barrier is known as ‘tunnelling’, this phenomenon violates the laws of classical dynamics which disallow such an interaction as the electrons possess insufficient kinetic energy to overcome a potential barrier of this magnitude.

A tunnelling current, It, is typically established at tip-surface separations of 3-7 Å, it can be described as:

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where k is a constant, is the barrier height and d is the tip-sample separation7. The exponential dependence of It on the tip-sample separation implies that STM exhibits high vertical resolution, an increase of 1 Å in d can result in a corresponding decrease of It by around an order of magnitude. This relationship also means that the atom at the apex of the tip is the one through which the majority of the electrons tunnel. Likewise, electrons will only tunnel into the closest sample atom, thus during scanning, the tunnelling current is confined to a filament between the apex of the tip and the surface; for a particularly pointed tip – where a single atom exists at the apex – the dimension of this filament is reduced to atomic dimensions and high resolution images can be acquired.

Figure 2.5 Schematic representation of STM apparatus6

The scanner consists of a piezoelectric actuator upon which the tip is mounted; piezoelectric materials change length upon a voltage being applied, the position of the tip can be controlled accurately in the x-, y- and z- directions6-8_{(figure 2.5).}

The actuator and the sample are both contained within a unit atop a vibrationally isolated stage.

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The tip first approaches the sample in ‘coarse’ steps by a control unit motor, before precision positioning by piezoelectrics. Figure 2.6 schematically shows the stages of electron tunnelling that takes place6. Initially, the tip and the sample are independent of each other and separated by a macroscopic distance; the wavefunctions of both the tip and the sample decay exponentially into the vacuum. A potential difference is applied between the tip and the sample and once brought within close proximity, electrons will tunnel between overlapping wavefunctions of the tip and the sample.

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The sign of the bias voltage determines the direction in which the current flows. If the sample is biased by a positive voltage, Vs, with respect to the tip, then the energy levels of the tip will increase by eVs. Electrons tunnel from the filled tip states into empty sample states and a dc tunnel current is established. By biasing the sample negatively with respect to the tip, the tunnelling current flows in the opposite direction.

Once a tunnelling current is established, the tip can be raster scanned across a chosen sample area to allow an image of the surface to be generated. Scanning may be carried out in two modes: constant current or constant height6,8. In constant current, the tunnelling current is fixed by the user. As scanning progresses changes to the chosen current are corrected by feedback signals controlling the z-piezodrive. A record of the height of the tip (the retraction and the extension of the piezodrive) at points in the xy plane is used to generate an image of a specific area of the surface. In constant height mode, the tip is scanned in the xy plane at a fixed height from the surface. The tunnelling current is recorded as a function of lateral position and used to create the STM image. The computer converts the data acquired into pictures or maps where apparently high areas are bright and low areas are dark, image processing software allows measurements or improvements such as contrast adjustment or noise removal. In general, constant current mode results in slower scanning of the surface but it also limits the risk of the tip crashing into the surface. Surface roughness is important in deciding which mode is most suitable; if scanning a large area of the sample, where many surface terraces may be expected, constant current mode offers more stable scanning whereas upon a small, atomically flat areas, constant height mode yields images of better resolution.

The tunnelling current depends not only upon the distance between the tip and the surface but upon the electronic structure of both the tip and the sample8. Careful analysis of STM images is required to distinguish between features that are due to changes in topography or to the local density of electronic states across the surface. STM images are therefore maps of the electronic structure of the surface

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and not merely steric representations. Interpretation may not be as simple as relating regions of brightness or darkness to high and low points on a surface, and gaining data on actual atomic positions or their chemical nature may also be difficult. With the knowledge of typical or anticipated chemical behaviour, computer modelling and possibly, in combination, with other surface science techniques, STM images may be interpreted to give an accurate understanding of the processes occurring on the surface. The unique visualisation of the surface offered by STM makes it an invaluable surface science tool.

2.6 References 

(1) Attard, G.; Barnes, C. Surfaces; Oxford University Press: Oxford, 1998. (2) Woodruff, D. P.; Delchar, T. A. Modern Techniques of Surface Science; Cambridge University Press: Cambridge, 1986.

(3) Sales, J. L.; Zgrablich, G. Physical Review B1987, 35, 9520. (4) Spinicci, R. Thermochimica Acta1997, 296, 87.

(5) Atkins, P.; de Paula, J. Physical Chemistry, 7th ed.; Oxford University Press: Oxford, 2002.

(6) Davies, P. R.; Roberts, M. W. Atom Resolved Surface Reactions: Nanocatalysis; RSC: Cambridge, 2007.

(7) Vickerman, J. C., Surface Analysis – The Principal Techniques; John Wiley & Sons: Chichester, 1997.

(8) Kirkland, A. I.; Hutchinson, J. L. Nanocharacterisation; RSC: Cambridge, 2007.

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3 Self­assembly on a Nickel Surface 

Controlling the assembly of adsorbate molecules on a reactive metal surface is fundamental to the design of effective enantioselective heterogeneous catalysts. The formation of chiral, functionalised and porous molecular networks upon a reactive metal surface is most desirable for applications in catalysis. Systems of molecules which reliably self-assemble into two-dimensional networks present themselves as ideal candidates to combine with surfaces of relevant catalytic properties. Once the ability to form supramolecular networks is established, organic synthesis may then be used to confer functionality into the component molecules and tailor the networks to specific chemical reactions.

The aim of the experiments carried out in this chapter is to assess the possibility of growing a hydrogen bonded network from PTCDI and melamine, which are known to reliably form regular supramolecular structures on Ag-Si(111)-

√3x√3R30o1 and Au(111)2 substrates, atop a nickel substrate. The surface

behaviour of PTCDI and melamine may be probed directly using scanning tunnelling microscopy (STM) to determine the viability of this approach whilst temperature-programmed desorption (TPD) is also used to further the understanding of the energetic interactions of each of these components with the substrate. The TPD experiments may be used to determine the stability of PTCDI and melamine on a reactive metal substrate and estimate quantitative values for comparison with those derived from experiments on Au(111).