Advances in the Application of X-Ray Photoelectron Spectroscopy (ESCA) Part II. New Methods

Abstract

Charging occurs in ESCA because a non-conductive sample does not have sufficient de-localized, conduction band electrons available to neutralize the charged centers that build from clustering of the positive holes created with the photoelectron and/or Auger electron ejection. As a result, a positive outer (or Volta) potential builds near the materials surface, producing a retardation or “drag” on the outgoing electrons. This retardation appears in the electron spectrum as an additional positive shift, either subtracting from the uncharged kinetic energy or, correspondingly, adding to the “normal” binding energies of the outgoing electrons. Compared to most of the features in these experiments, charging shifts are often relatively slow in their establishment; thus, in the time frame of a normal experiment, charging has both dynamic and static component. Charging also depends upon the macrosurface and bulk morphology of the measured system, as well as its chemistry, and microstructre. As indicated, charging does not arise for “good” conductors (or relatively narrow band gapped semiconductors, see PART 1.ll.C) because these systems possess, at room temperature, sufficient (internally and externally provided) conduction band electrons to neutralize the aforementioned “excess” charge. 1.6.7 In the ESCA process itself, when direct X-ray impingement onto the sample surface is utilized, there is often sufficient stray, low energy electrons from Bremsstrahlung and other processes to partially neutralize these charge centers. On the other hand, when a monochromator is employed, indirect, crystal scattered X-rays are focused onto the sample, and these, along with other features, provide a relatively electronclean environment around the sample. Thus, most of the aforementioned stray, neutralizing electrons are eliminated and, as a result, a substantial, nearly complete, charging effect emerges. The latter includes the various differential charging features that may result from different components and unique morphological variation. Hidden in this dichotomy are a variety of additional major problems that may arise, particularly pertaining to the lack of establishment of a valid Fermi edge and differential charging. These difficulties tend to make energy referencing for insulators a persistent and quite formidable problem, particularly when the system being examined is a doped catalyst with poor dispersion. For this reason, it is common for some researchers to avoid all use of absolute binding energies for insulators and wide band gapped semiconductors. This we find to be an extreme measure that may be avoided in many (but not all) circumstances. In fact, at times this problem may be favorably overshadowed since, in some cases, the presence of Fermi edge decoupling and differential charging may be employed as a useful auxiliary tool.6 Although this “tool” has obvious limitations, it has exhibited a surprising versatility in practical, as well as basic, problems.