surface analysis
surface analysis, in analytical chemistry, the study of that part of a solid that is in contact with a gas or a vacuum. When two phases of matter are in contact, they form an interface. The term surface is usually reserved for the interface between a solid and a gas or between a solid and a vacuum; the surface is considered to be that part of the solid that interacts with its environment. Other interfaces—those between two solids, two liquids, a solid and a liquid, or a liquid and a gas—are studied separately.
In surface chemistry the most important solids are of two types. The first is a nominally pure solid on which a surface layer has been produced by interaction with the layer’s environment. An example of this is kitchen aluminum foil, which is pure aluminum with a layer of oxides produced by interaction with oxygen in the air. The second is a solid on which a separate layer has been intentionally created. An example of such a solid is a heterogeneous catalyst, which is created when a layer of a reactive species is deposited on a solid support made of a different material.
For any nominally “pure” solid (with very few exceptions, such as extremely nonreactive alloys and gold), the atoms or molecules at the surface are different from those in the bulk. This difference arises from the reaction of the surface layer with the environment, and the depth to which this reaction extends differs from solid to solid. Therefore, how far into the solid what is considered the “surface” extends must be defined. Operationally, the surface is defined as that region of a solid that differs from the bulk. For solids consisting of a support and a deposited thin layer, the entire deposited layer and its bonding layer with the support can be thought of as the “surface.”
Consider some everyday surface phenomena that are largely taken for granted. Contact lenses are compatible with the eye because the lens material is surface treated. Cloth raincoats shed water and clothing is stain resistant because the cloth has been subjected to surface-modification reactions. Stainless steel does not corrode because the alloying process produces a noncorrosive surface layer. One can colour fibreglass fabric with dyes because the glass surface has been modified with an organic covering layer. Eggs will not stick to one side of a Teflon layer in a frying pan, but the other side of the layer is bonded to the metal. Why? Because the one side of the Teflon has been chemically modified. The important surface layers on the above materials vary in thickness from 1 to nearly 1,000 molecular layers. How can these layers be monitored or studied to determine their composition and to understand their efficacy? How can research be carried out to improve these materials? The answer is surface analysis.
Methods for studying surfaces
Methods for characterizing surfaces have undergone a tremendous evolution since the mid-20th century. Classical methods, which were developed during the first half of the 20th century, provide descriptive information about the physical characteristics of the surface. These methods, which are still used with great effectiveness, include adsorption isotherms (which give surface areas and pore-size distributions), measurements of surface roughness, ellipsometry (for thickness measurements), reflectivity, and microscopy to obtain surface topography.
Modern spectroscopic methods of surface analysis, which began to appear around 1960, are able to provide elemental analysis, chemical state information, quantitative analysis, and horizontal and vertical distributions of species. With these techniques it is possible to investigate oxidation states of elements, specific compounds present as minute crystallites, and organic functional groups or compounds. Quantitative analysis can provide elemental ratios or oxidation states ratios of the same element. The distribution of species across a surface, called surface mapping, or into the bulk from the surface plane, called depth profiling, can be done with remarkable clarity in many cases.
Spectroscopic techniques
Spectroscopic techniques function through a “beam in, beam out” mechanism. A beam of photons, electrons, or ions impinges on a material and penetrates to a depth that is dependent on the beam characteristics. A second beam, resulting from the interaction of the first beam with the solid, exits from the surface and is analyzed by a spectrometer. The exiting beam carries with it information regarding the composition of the material with which the beam interacted. By varying both the type of particle and the energy of the entering beam, one can generate a large number of surface analytical techniques.
For a surface analytical technique, the information obtained by the spectrometer from the exiting beam should be characteristic of that region of the solid that is defined as the surface. Either the penetration depth of the incident beam, the escape depth of the exiting beam, or both therefore must be limited to the thickness of the surface. This “sampling depth” differs with the type of particle or the energy of the incident beam. For photons, electrons, and ions with an energy of 1,000 electron volts (1 keV), the sampling depths in the energy range used for surface analysis are 1,000, 2, and 1 nm (nanometre; 1 nm is 10−9 metre), respectively.
The thickness of a surface is not the same in all materials. The depth into the solid that is of interest therefore depends on the solid being measured. It also depends on the specific application for which the analysis is being carried out. For catalysts, for example, the surface thickness of interest is less than 1 nm, or five atomic layers, which is the thickness of the deposited reactive species. For corrosion studies, on the other hand, the depths of interest are in the 100-nm, or 500-atomic-layer, range.
Each technique has a specific sampling profile, and not all techniques are appropriate for all applications. However, different techniques can provide complementary information. Usually no single technique provides sufficient information to solve a complex real-world problem.
Many of these techniques are familiar to the analytical chemist, and each has its own unique capabilities. During the 1970s and ’80s, however, four techniques emerged as being most useful for real-world surface analysis because of their general applicability and ease of use. The use of photons in and electrons out provides X-ray photoelectron spectroscopy (XPS, or electron spectroscopy for chemical analysis [ESCA]). Electrons in and out gives Auger electron spectroscopy (AES). The use of ions in and out yields two techniques: secondary ion mass spectroscopy (SIMS) and ion scattering spectroscopy (ISS); the latter includes both low-energy (keV) ISS and high-energy (MeV; 1 million electron volts) ISS, normally called Rutherford back-scattering spectroscopy (RBS).
X-ray photoelectron spectroscopy and Auger electron spectroscopy
For XPS and AES the primary process is an ionization caused by either a photon or an electron, m + hν → m+* + e−, or m + e− → m+*+ 2e−, where m is an atom in the material. In photoionization an incident photon causes the ejection of an electron with a discrete kinetic energy, which is measured in XPS, leaving an excited ion (m+*). The kinetic energy of the ejected electron is essentially the difference between the energy of the incident photon and the binding energy of the electron.
Electron ionization of an atom also produces both an excited ion and a second electron. Unlike in XPS, because of electron-electron interactions, these primary electrons emitted from the atom do not have discrete energies.
Excited ions, produced by either photon or electron ionization, can relax through two mechanisms, or secondary processes. One path involves emission of photons in the form of X-rays, the same process observed in X-ray fluorescence. These photons are usually of such an energy that they have large escape depths (that is, the photons come from deep within the sample) and therefore do not usually contain information relevant to surfaces. The excited ion can also relax through emission of a secondary electron, called an Auger electron, to form a doubly ionized atom. It is important to note that XPS can produce electrons with discrete energies through both photoionization and the Auger process, while electron ionization produces electrons with discrete energies only through the Auger process. The kinetic energy of an Auger electron does not depend on the energy of the ionizing photon or electron, whereas that of the XPS electron does.
X-ray photoelectron spectroscopy
Since the binding energies of the electrons emitted through XPS are discrete and atoms of different elements have different characteristic electron-binding energies, the emitted electron beam can provide a simple method of elemental analysis. The specificity of XPS is very good, since there is little systematic overlap of spectral lines between elements.
Swedish physicist Kai Siegbahn, who won the Nobel Prize for Physics in 1981 for the development of XPS, found that the chemical environment of an element has small but measurable effects on electron binding energies as measured by XPS. This discovery dramatically increased the value of XPS to surface analysis. For example, the binding energy for the 3d3/2 electrons in elemental molybdenum on the surface of a catalyst is at 227.6 eV, while that for the same electrons in molybdenum trioxide (+6 oxidation state) on the same surface is at 232.7 eV. Similarly, the binding energy for the same electrons in molybdenum in the +4 oxidation state is at 229.6 eV. This makes it possible to measure different oxidation states of molybdenum on the same sample. Through the use of such “chemical shift” information, the XPS spectrum can therefore provide not only elemental analysis but also analysis of the chemical state of the elements on the surface.
XPS is very effective as a technique for qualitative analysis of elements on a surface and can detect all elements except hydrogen and helium. The detection limit of an element varies from as high as 1 percent of a surface layer for some light elements to less than 0.1 percent of a surface layer for some heavy elements. Overlap of XPS lines is very rare, and, when it occurs, secondary lines are able to yield a definitive analysis.
XPS is also a quantitative technique. It is possible to estimate the composition of a surface without extensive calibration, using published sensitivity factors, to within ±30 percent of the real value. In well-calibrated systems the relative standard deviation of measurements is ±5 percent or better.
XPS is limited in its use for mapping the composition of a surface. A main difficulty with XPS relative to other techniques is that it has little lateral resolution across a surface because the diameter of the X-ray beam is usually several millimetres, which is large relative to the distances of importance in surface mapping. It is also not very effective in obtaining depth profiles because it must be combined with ion sputtering to remove layers of the material and data acquisition is too slow.
Auger electron spectroscopy
Energies of Auger electrons (named after French physicist Pierre Auger), like energies of XPS photoelectrons, are characteristic of the individual chemical elements. Thus, it is possible to use AES to analyze surfaces in much the same way as XPS is used. However, because of the differences in the characteristics and limitations of the primary beams for the two techniques, photons versus electrons, AES has developed differently and represents a complementary technique.
An electron beam is much easier to focus than X-rays. The resulting high-intensity, small-diameter beam renders AES ideal for surface mapping, a capability that XPS lacks. To be practical for scanning surfaces, a fast measurement speed is necessary, since individual analyses must be done as the beam crosses the surface. The Auger instrumentation therefore concentrates on rapid analysis. Increased speed, however, sacrifices spectral resolution, and, although Auger electrons show chemical shifts in much the same way as XPS electrons, the Auger instrumentation normally cannot measure them.
Used in combination with ion sputtering to expose successive layers into the surface, the finely focused electron beam and the rapid analytical capabilities of AES make it an ideal technique for depth profiling. Thus, XPS and AES, which both operate on similar principles, are useful in different ways and have complementary capabilities.
AES is somewhat limited in the materials that it can study, a limitation not shared by XPS. Because the AES primary beam is composed of electrons, it can be used only in the analysis of conductors and semiconductors, which conduct away charge. On insulators, a charge buildup occurs that affects the energy of the emerging Auger electron and renders its signal useless for analysis.
Sensitivity for most elements is better with AES than with XPS because a highly intense focused beam can be obtained with electrons but not with photons. Sensitivity is increased with a more intense beam.
