Ambient pressure photoelectron spectroscopy: a new tool for surface science and nanotechnology

Journal Article · Wed Mar 12 00:00:00 EDT 2008 · Surface Science Reports

DOI:https://doi.org/10.1016/j.surfrep.2008.01.001· OSTI ID:927873

Salmeron, Miquel; Salmeron, Miquel; Schlogl, Robert

Progress in science often follows or parallels the development of new techniques. The optical microscope helped convert medicine and biology from a speculative activity in old times to today's sophisticated scientific disciplines. The telescope changed the study and interpretation of heavens from mythology to science. X-ray diffraction enabled the flourishing of solid state physics and materials science. The technique object of this review, Ambient Pressure Photoelectron Spectroscopy or APPES for short, has also the potential of producing dramatic changes in the study of liquid and solid surfaces, particularly in areas such as atmospheric, environment and catalysis sciences. APPES adds an important missing element to the host of techniques that give fundamental information, i.e., spectroscopy and microscopy, about surfaces in the presence of gases and vapors, as encountered in industrial catalysis and atmospheric environments. APPES brings electron spectroscopy into the realm of techniques that can be used in practical environments. Decades of surface science in ultra high vacuum (UHV) has shown the power of electron spectroscopy in its various manifestations. Their unique property is the extremely short elastic mean free path of electrons as they travel through condensed matter, of the order of a few atomic distances in the energy range from a few eV to a few thousand eV. As a consequence of this the information obtained by analyzing electrons emitted or scattered from a surface refers to the top first few atomic layers, which is what surface science is all about. Low energy electron diffraction (LEED), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), Ultraviolet photoelectron spectroscopy (UPS), and other such techniques have been used for decades and provided some of the most fundamental knowledge about surface crystallography, composition and electronic structure available today. Unfortunately the high interaction cross section of electrons with matter also prevents them from traveling long distances unscattered in gas environments. Above the millibar pressure range this distance is reduced to less that a millimeter, effectively preventing its use in the most relevant environments, usually between millibars and atmospheric pressures. There is therefore a large gap of several orders of magnitude where information about surfaces is scarce because these powerful electron spectroscopies cannot operate. One characteristic of surfaces in ambient pressure environments is that they are covered by dense layers of molecules, even when their binding energy is weak. Water for example is known to form layers several molecules thick at room temperature in humid environments. Metals readily form oxide films several layers thick in oxygen atmospheres. Dense layers of adsorbed molecules can also be produced in ultra high vacuum, often by the simple and expedient method of cooling the sample to cryogenic temperatures. A large amount of data has been obtained in the past in UHV by surface scientists using this method. While this has provided valuable information it begs the question of whether the structures formed in this manner represent equilibrium structures or metastable ones, kinetically trapped due to high activation energies that cannot be overcome at low temperature. From a thermodynamic point of view is interesting to consider the entropic contribution to the Gibbs free energy, which we can call 'the pressure factor', equal to kT.logP. This factor amounts to a sizeable 0.3 eV difference at room temperature between UHV (<10{sup -8} Pascal) and atmospheric pressures. Such change if free energy can definitely result in changes in surface structure and stability. Entire areas of the phase diagram are out of reach due to the pressure gap. Even when cooling is not necessary, many surface treatments and most chemical reactions necessitate the presence of gases at pressures ranging from millibar to bars. What is the structure and chemical nature of the species formed on the surface in equilibrium with such gases? As we shall illustrate in this review, APPES provides a much needed electron spectroscopy to analyze surface electronic structure and composition in equilibrium with gases.

• Research Organization: Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
• Sponsoring Organization: Materials Sciences Division
• DOE Contract Number: DE-AC02-05CH11231
• OSTI ID: 927873
• Report Number(s): LBNL-183E; SSREDI; TRN: US200816%%1106
• Journal Information: Surface Science Reports, Vol. 63; ISSN 0167-5729
• Country of Publication: United States
• Language: English

Related Subjects

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ATMOSPHERIC PRESSURE
AUGER ELECTRON SPECTROSCOPY
BINDING ENERGY
CHEMICAL REACTIONS
CROSS SECTIONS
ELECTRON DIFFRACTION
ELECTRON SPECTROSCOPY
ELECTRONIC STRUCTURE
ELECTRONS
ENERGY RANGE
FREE ENERGY
FREE ENTHALPY
GASES
MEAN FREE PATH
OPTICAL MICROSCOPES
PHASE DIAGRAMS
PHOTOELECTRON SPECTROSCOPY
PRESSURE RANGE
SOLID STATE PHYSICS
SPECTROSCOPY
SURFACE TREATMENTS
X-RAY DIFFRACTION
X-RAY PHOTOELECTRON SPECTROSCOPY
photoelectron spectroscopy nanotechnology