Surface Science: Foundations of Catalysis and Nanoscience

Chapter 2. Experimental Probes of Surface & Adsorbate Structure: Supplemental Material

The scanning tunnelling microscope or STM has lead to a revolution in surface science. Indeed, it has ushered in the age of nanoscience and nanotechnology. Nonetheless this remarkable instrument is amazingly simple and you can even find instructions on how to build your own. However, it probably will be difficult to achieve atomic resolution on your kitchen table.

The STM was invented by Gerd Binnig and Heinrich Rohrer who won the Physics Nobel Prize in 1986 for this achievement. You can learn more about Binning and Rohrer by visiting the Nobel Prize Archive. They shared the Nobel Prize with Ernst Ruscka, who designed the first electron microscope. Two important types of electron microscope are the transmission electron microscope (TEM) and the scanning electron microscope (SEM). I don't talk much about TEM & SEM in the book but they are two of the most important tools for imaging on the micro- and nano-scales. You can find a great deal of information on TEM & SEM here as well as at the website of the LEXPIAC at West Chester University. For an online tutorial on SEM try this link.

The Nobel Prize in Chemistry 2014 was awarded jointly to Eric Betzig, Stefan W. Hell and William E. Moerner "for the development of super-resolved fluorescence microscopy". Their achievemetns open up the field of nanoscopy - imaging with visible light below the diffraction limit to < 200 nm. Their work has made possible the imaging of single molecules, proteins and living cells.

Lots of STM image galleries exist. For instance, here's one from the Technische Universität Wien.

Here's another from RHK Technologies.

And, of course, IBM since they invented it and also scientists at IBM (Manoharan, Lutz & Eigler) produced one of the images that was on the cover of the first edition of this textbook. IBM has also produced A Boy and His Atom, and the more interesting Moving Atoms: Making A Boy and His Atom. These are YouTube videos made by moving CO molecules with an STM .

A more venerable technique of surface structure determination is low energy electron diffraction (LEED). Clinton J Davidson shared the 1937 Physics Nobel Prize with George P Thomson for demonstrating that the wavelike characteristics of the electron. Thomson was the son of Sir JJ Thomson, who won his Nobel Prize for showing that the electron was a particle. Davidson and Germer are credited with discovering LEED.

Karl Manne Georg Siegbahn won the 1924 Nobel Prize in Physics for his discoveries and research in the field of X-ray spectroscopy. His son Kai developed X-ray spectroscopy further to create electron spectroscopy for chemical analysis (ESCA), which is also known as the surface sensitive spectroscopy XPS. For this he won a share of the 1981 Nobel Prize in Physics. Afterward he visited the University of Pittsburgh and became the first Nobel Prize winner I ever met. It didn't make a big impression on him but it was a big deal to me at the time.

NIST provides a number of databases with information pertinent to electron spectroscopy.

The NIST X-ray Photoelectron Spectroscopy online database has been a valuable source of binding-energy and related data for the surface analysis of a wide range of materials by x-ray photoelectron spectroscopy.

The NIST Electron Elastic-Scattering Cross-Section Database provides values of differential elastic-scattering cross sections, total elastic-scattering cross sections, phase shifts, and transport cross sections for elements with atomic numbers from 1 to 96 and for electron energies between 50 eV and 300 keV (in steps of 1 eV). These data can be used in simulations of electron transport in Auger-electron spectroscopy, x-ray photoelectron spectroscopy, electron-probe microanalysis, and analytical electron microscopy.

The NIST Surface Structure Database provides 3-dimensional graphics to allow researchers to visualize the structures of crystal surfaces on the atomic scale.

The NIST Electron Inelastic-Mean-Free-Path Database provides values of electron inelastic mean free paths (IMFPs) for use in quantitative surface analyses by AES and XPS.

The NIST Electron Effective-Attenuation-Length Database provides values of electron effective attenuation lengths (EALs) for applications in AES and XPS.

The NIST Database for the Simulation of Electron Spectra for Surface Analysis (SESSA) provides data for many parameters needed in quantitative Auger electron spectroscopy and X-ray photoelectron spectroscopy (differential inverse inelastic mean free paths, total inelastic mean free paths, differential and total elastic-scattering cross sections, transport cross sections, photoionization cross sections, photoionization asymmetry parameters, electron-impact ionization cross sections, photoelectron lineshapes, Auger-electron lineshapes, fluorescence yields, and Auger-electron backscattering factors). SESSA can also simulate Auger-electron and X-ray-photoelectron spectra of mulitlayered samples with compositions and thicknesses specified by the user. The simulated spectra can then be compared with measured spectra (for specified measurement conditions), and the layer compositions and thicknesses adjusted to find maximum consistency between simulated and measured spectra.

Questions and Exercises

  1. (a) Calculate the ultimate pressure that can be attained by a 1000 l s-1 turbomolecular pump in a chamber that has a leak rate of 0.001 mbar l s-1. (b) What would the leak rate have to be for the pump to maintain UHV in the chamber?
  2. How do we know if we are imaging the occupied or unoccupied states of the surface in an STM experiment?
  3. Why does the inelastic mean free path of an electron in a solid vary with electron energy?
  4. Why is low energy electron diffraction sensitive to surface structure?
  5. Does the symmetry of a LEED pattern uniquely define the symmetry and coverage of the adsorbate layer?
  6. How is the specular LEED reflex identified?
  7. The intensity of the LEED spots from an unreconstructed clean surface drops more rapidly with increasing temperature for electrons incident at 100 eV than at 1500 eV. Explain.
  8. In the literature, one will sometimes encounter a term along the lines of "the Koopmans' energy of the orbital is ..." What is meant by this?
  9. Why does the emission of electrons originating from one initial state often lead to the observation of more than one peak in XPS?
  10. Describe how a shake off event differs from Auger electron formation.
  11. If a photoelectron peak has an intrinsic width of 0.75 eV, approximate the associated hole lifetime.
  12. What types of electrons are probed in UPS and XPS?
  13. Why are AES spectra commonly rendered as derivative spectra? The tabulated energy of an Auger transition corresponds to which feature in the spectrum?
  14. Describe the difference between spectromicroscopy and microspectroscopy.
  15. Why are both parallel and perpendicular vibrational modes observed for adsorbates on semiconductor surfaces, but only perpendicular modes are observed on metals?
  16. When is it most likely that IR absorbance is linearly proportional to adsorbate coverage?

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