Lecture Notes May 2nd 2007

Answers to quiz

Surface analysis

Surfaces are important. Much of the chemistry that we depend on for quality of life issues is based on chemical reactions at surfaces. Examples include, printing, dyeing, painting, adhesion, catalysis, washing, photography, semiconductor function, and combustion of solids; almost all aspects of the coloration of manufactured goods are based on surface chemistry. In addition corrosion is surface chemistry, as is electrochemistry and a lot of chromatography. All gas-solid and solution-solid reactions are reactions on/at surfaces.

We probe surfaces by bouncing "things" off them. The things are typically photons of one sort or another. To get chemical information, we examine the change in photon energy as a result of the interaction with the surface molecules or atoms. This gives various sorts of reflection spectroscopy, which is a probe of absorption of the light by molecules in the surface. We can also image the surface by bouncing photons of it and collecting the light with suitable optical elements. High magnification images can be obtained by bouncing electrons of the surface. This often requires the sample to be conducting and thus the material has to be coated in some way before the images can be obtained. Surface can be probed by bouncing ions off them. The recoil energy will relate to the mass of the ion/atom in the surface. If the probe beam consisted of ping-pong balls you would easily see the difference in behavior between a collision with a tennis ball and a collision with a canon ball. This technique is called ion scattering spectrometry (ISS).

In addition to bombarding the surface with photons or particles (ions or electrons) and analyzing what bounces off, it is also possible to get information about a surface by examining the photoelectrons that are ejected without undergoing further collisions. These electrons would be generated by irradiation with X-rays or even an electron beam. Although X-rays penetrate through the surface into the bulk of the material causing ionization as they go, and the fluorescent X-rays can emerge from within the bulk, the electrons ejected in the first stage can only escape with the discrete energy equal to the photon energy minus the binding energy if they come from close to the surface. The exact energy is a function of the oxidation state and chemical environment and so the chemical shift of the peak provides information about the bonding. The peaks are unique for a particular element and so the technique provides both qualitative and quantitative information. The technique is called X-ray photoelectron spectroscopy (XPS, though it is also known as electron spectrometry for chemical analysis (ESCA).

"XPS [The Surface Analysis Forum, http://www.uksaf.org/tech/xps.html (accessed April 2005)] was developed in the mid 1960s by K. Siegbahn and his research group. K. Siegbahn was awarded the Nobel Prize for Physics in 1981 for his work in XPS. The phenomenon is based on the photoelectric effect outlined by Einstein in 1905 where the concept of the photon was used to describe the ejection of electrons from a surface when photons impinge upon it. For XPS, Al Kalpha (1486.6eV) or Mg Kalpha (1253.6eV) are often the photon energies of choice. Other X-ray lines can also be chosen such as Ti Kalpha (2040eV). The XPS technique is highly surface specific due to the short range of the photoelectrons that are excited from the solid. The energy of the photoelectrons leaving the sample are determined using a hemispherical analyzer and this gives a spectrum with a series of photoelectron peaks. The binding energy of the peaks are characteristic of each element. The peak areas can be used (with appropriate sensitivity factors) to determine the composition of the materials surface. The shape of each peak and the binding energy can be slightly altered by the chemical state of the emitting atom. Hence XPS can provide chemical bonding information as well. XPS is not sensitive to hydrogen or helium, but can detect all other elements. XPS must be carried out in ultra high vacuum conditions."

A closely related technique is based on measurement of the secondary electrons emitted once a core vacancy has been created This process is known as the Auger pronounced the French way (O-zhay) and it parallels the X-ray emission process. It fact it is more probably for light elements (X-ray fluorescence is an not very sensitive for light elements). These secondary electrons have characteristic energies as they take away the excess energy once the core electron vacancy has been filled with an electron from a less tightly bound orbital. Thus there are three energy levels involved (those of the original "out"electron, the "down" electron and the second, Auger, "out" electron). Auger electron energies are even more influenced by the chemical environment, and Auger chemical shifts are a good diagnostic of the oxidation state and bonding of a particular element.

The Surface Analysis Forum, http://www.uksaf.org/tech/aes.html (accessed April 2005)

"Electrons of energy 3-20keV are incident upon a conducting sample. These electrons cause core electrons from atoms contained in the sample to be ejected resulting in a photoelectron and an atom with a core hole. The atom then relaxes via electrons with a lower binding energy dropping into the core hole. The energy thus released can be converted into an X-ray (see EDX) or emit an electron. This electron is called an Auger electron after Pierre Auger who discovered this relaxation process. After the emission of the Auger electron, the atom is left in a doubly ionized state. The energy of the Auger electron is characteristic of the element that emitted it, and can thus be used to identify the element. The short inelastic mean free path (IMFP) of Auger electrons in solids ensures the surface sensitivity of AES.

AES is a popular technique for determining the composition of the top few layers of a surface. It cannot detect hydrogen or helium, but is sensitive to all other elements, being most sensitive to the low atomic number elements.

AES must be carried out in UHV conditions. A popular method of looking at buried layers with AES is to use the technique in combination with sputter cleaning. Normally, when a sample is brought into the UHV environment from air, it will be coated with carbon and oxygen. This material has to be removed (usually by sputtering) before the clean surface can be investigated. Sputtering involves directing a beam of ions (usually Ar ions) at between 500 eV and 5 keV at the sample. This process cleans the surface, but can also be used to erode away the sample to reveal structure beneath the surface. Obviously this is a destructive technique."

A electron beam can be scanned across a surface and thus two dimensional information can be obtained about the surface coverage. When combined with the controlled erosion of the surface with an ion beam, a full three dimensional picture of the surface composition can be obtained.

Although a detailed interpretation of the XPS or AES spectrum is complicated, often the approach is to obtain the spectrum before and after some diagnostic experiment is performed. This could be breaking the material to find the surface composition related to regions of weakness, or it could be the destruction of a film of lubricant, or it could just be the controlled removal of material to observe concentration changes as one goes from the surface to the bulk compositions.