Seminars Archive

XPS for Charge-Sensitive Analyses of Nanostrucures

Abstract
Unlike what is generally thought in freshman textbooks, most materials carry uncompensated electrical charges, as a result of electrification or electret formation. This phenomena is well-known to men for more than 2500 years, and has also been intelligently utilized for that long. An “electret” is defined as a material that develops a permanent electrostatic potential or a permanent dipole moment. In complete contrast to metallic and semiconducting systems, the atomic/molecular-level nature of electrets is poorly understood in most cases. Therefore, better understanding at the atomic and molecular level, i.e. the chemistry, is a must for more intelligent utilization of these materials. Kelvin Probe Atomic Force Microscopy (KP-AFM) has been the most advanced analytical tool for probing and quantifying the charge developed and mapping in the submicron dimensions. However, as in most electrical based measurements it lacks chemical specificity. Spectroscopic techniques like IR, NMR or variants of them coupled with scanning probe techniques like NSOM, etc. have excellent chemical specificity, but they are not sensitive to charge and/or electrostatic potentials developed. In this respect, ESR and EPR techniques have been quite successful for analysis of trapped charges on polymer and/or oxide surfaces, especially in combination with other surface specific techniques. However, use of these techniques are also limited since they can probe only radicals and paramagnetic species. Unlike common optical spectroscopic techniques, XPS is a charged particle based chemical analysis technique, extremely sensitive to the electrical potentials developed due to uncompensated charges. In the early days of XPS, charging was considered as a nuisance for characterization of insulating (electrets) materials. Hence elaborate compensation methods were developed using low energy electron and/or ions to minimize it. However, complete removal is only an ideal, and besides one can learn a lot by controlled use of the charging/discharging phenomena (a blessing). The generated photoelectrons’ energy is influenced by the local electrical potential(s) (Vloc) resulting from charge accumulation, in addition to the chemical identity of the atoms, which is the mainstay of the technique. There are a myriad of chemical, physical, thermal, optical, mechanical phenomena contributing to charge accumulation on materials and/or surface structures, all amenable to charge sensitive XPS analysis. It is equally surprising that, although XPS has been utilized for more than 5 decades, this capability is almost completely left-out, untapped, and not utilized, except by few groups around the globe. We will describe its power by giving a variety of applications from our recent work, particularly related to ultra-thin coatings.