CHEM3033

1. CHEM3033 Surface and Bulk characterisation of solids Prof. D.G. Billing

2. Course outline and content • Introduction • Importance of surface chemistry • Surface vs Bulk • Analytical Chem of Surfaces • Surface processes / a clean surface • Types of techniques and results • UHV • Surface techniques. • ESCA/XPS • AES • SIMS • SPM / AFM • Bulk techniques. • DSC, DTA, TGA • XRF • XRD

3. Course assessment. • Coursework component • Assignment • Describe the basic principles of operation and at least one application of one of the more common surface or bulk characterisation techniques • 50% principles & 50% application • 10-15 pages. • Wikipedia warning – don’t just copy. At least use own words, diagrams and examples. • No practicals • Use time to read background material and work on assignment.

4. References (Intro & Surface) • JC Vickerman (ed) ‘Surface Analysis’, Johan Wiley and Sons, New York 1997. • J. Chem. Ed, 61 (1984), 402-409, 483-489, 593-599 • JF Watts & J Wolstenholme, ‘An Introduction to Surface Analysis by XPS and AES’, Wiley,2003 • EM McCash, ‘Surface Chemistry’, OUP, 2001

5. Introduction • Interfaces between states of matter has interested “scientists” for thousands of years. • Solid surfaces are particularly important to everyday life. 90% of the world’s industrial output of materials such as fertilizers and plastics are produced via het. cat.

7. Consequences of lower coordination no. • Unbalanced forces at the surface means that the surface area is at a relative high energy compared to the bulk. • The electronic structure of the surface is different from the bulk • Crystallographic structure of surface is different. • The binding or ‘adsorption’ of gases is strongly favoured at the surface.

8. Surface vs Bulk

9. Operational definition • The surface region is that volume of the solid that the measurement technique or probe samples. • Therefore different techniques could sample different surfaces.

10. How many atoms in a surface? • Assume a cube of materials • 1cm3 (Density = 1 mol in 1 cm3) • Ratio of surface to bulk is : • 1015/1023 = 10-8 • Generally only probe 1 mm2 ie 1013 atoms, and about 10 layers deep, ie 1014 atoms.

11. Information required: • To understand the properties and reactivity of a surface, the following info is required: • The physical topography • The chemical composition • The chemical structure • The atomic structure • The electronic state • A detailed description of bonding of molecules at the surface. • No one technique can provide all this

12. Techniques for studying surfaces

15. Note: Issues of measurement relate to both penetration and emission. In XPS X-rays penetrate to 10000 A, but it is still a surface sensitive technique!

16. Surface Sensitivity

17. Sample Charging • Build up of charge can occur on a surface. (esp. if it is non-conducting) • Leads to spectral distortion and peak shifts. • Insulators > semiconductors > conductors • Remove the problem: • Surface conduction • Stray electrons • Calibration • Flood-gun

18. Contamination • Contamination & surface layer normally samples! • Usually composed of species absorbed onto the surface. • From atmosphere, • Handling • Vacuum systems

19. Quantification aspects • Extremely sensitive to the distribution of the measured species on the surface.

20. Angle of take off

21. Depth profiling via etching

22. Resolution

24. Why is UHV required for surface studies ? • Ultra high vacuum is required for most surface science experiments for two principal reasons : • To enable atomically clean surfaces to be prepared for study, and such surfaces to be maintained in a contamination-free state for the duration of the experiment. • To permit the use of low energy electron and ion-based experimental techniques without undue interference from gas phase scattering. • To put these points in context we shall now look at the variation of various parameters with pressure

25. 1. Gas Density

26. 2. Mean Free Path of Particles in the Gas Phase

27. 3. Incident Molecular Flux on Surfaces

28. 4. Gas Exposure - the "Langmuir"

29. 5. Sticking Coefficient & Surface Coverage

30. How long will it take for a clean surface to become covered with a complete monolayer of adsorbate ? • This is dependent upon the flux of gas phase molecules incident upon the surface, the actual coverage corresponding to the monolayer and the coverage-dependent sticking probability…. • however , it is possible to get a minimum estimate of the time required by assuming a unit sticking probability (i.e. S = 1) and noting that monolayer coverages are generally of the order of 1015 per cm2 or 1019 per m2 . Then        Time / ML ~ ( 1019 / F )        [ s ]

31. Summary - Variation of Parameters with Pressure

32. For most surface science experiments there are a number of factors necessitating a high vacuum environment : • For surface spectroscopy, the mean free path of probe and detected particles (ions, atoms, electrons) in the vacuum environment must be significantly greater than the dimensions of the apparatus in order that these particles may travel to the surface and from the surface to detector without undergoing any interaction with residual gas phase molecules. This requires pressures better than 10-4 Torr. There are, however, some techniques, such as IR spectroscopy, which are "photon-in/photon-out" techniques and do not suffer from this requirement.(On a practical level, it is also the case that the lifetime of channeltron and multiplier detectors used to detect charged particles is substantially reduced by operation at pressures above 10-6 Torr).

33. Most spectroscopic techniques are also capable of detecting molecules in the gas phase; in these cases it is preferable that the number of species present on the surface substantially exceeds those present in the gas phase immediately above the surface - to achieve a surface/gas phase discrimination of better than 10:1 when analysing ca. 1% of a monolayer on a flat surface this requires that the gas phase concentration is less than ca. 1012 molecules cm-3 ( = 1018 molecules m-3), i.e. that the (partial) pressure is of the order of 10-4 Torr or lower.

34. In order to begin experiments with a reproducibly clean surface, and to ensure that significant contamination by background gases does not occur during an experiment, the background pressure must be such that the time required for contaminant build-up is substantially greater than that required to conduct the experiment i.e. of the order of hours. The implication with regard to the required pressure depends upon the nature of the surface, but for the more reactive surfaces this necessitates the use of UHV (i.e. < 1 x 10-9 Torr).