X-ray standing waves imaging

Abstract

Modern disciplines in materials science such as coating technology, microelectronics, photovoltaics, sensors, catalysis, and nano-technology, and the increasing complexity of the systems studied have created an insatiable need for more and more powerful analytical tools in surface and interface analysis. An excellent review about current problems and solutions in surface science is presented in the special edition of the 500th issue of Surface Science [1]. The prime information demanded from any system under study is its chemical composition and atomic structure. Without this knowledge, all other properties cannot be fully understood. The paper by Zhang et al. [2] in this issue shows how this information can be obtained from crystalline surface adsorbate systems with the help of the X-ray standing wave (XSW) technique, in a new and very direct way that completely removes the guesswork. Equally exciting is the fact that this has been done to reveal the structure of adsorbates on a solid surface under aqueous solution. Indeed, the XSW technique is one of the few that can be applied to probe such ‘‘buried interfaces’’. The last decades have seen the advent of very powerful techniques, such as scanning tunneling microscopy (STM) [3], transmission electron microscopy (TEM) [4], electron spectroscopy for chemical analysis (ESCA) [5], grazing incidence X-ray diffraction (GID) [6], and more, which all quickly had a strong impact on surface/interface analysis. Hardly noticed went the birth of another method already 40 years ago: the X-ray standing wave technique. The dynamical theory of X-ray diffraction [7] predicts that during Bragg reflection from a crystal, due to the interference of the incoming and reflected X-ray wave, an X-ray standing wave is created, which can be shifted through the unit cell of the diffracting crystal by scanning through the Bragg reflection condition in angle (or energy). Boris Batterman reported in a landmark paper that the X-ray fluorescence of the atoms of the germanium crystal he used, if recorded as a function of the angle of the incident radiation while passing the range of strong reflection, bears the signature of the movement of the exciting X-ray standing wave [8]. In a subsequent paper, he demonstrated five years later that on the other hand the characteristic X-ray fluorescence radiation of foreign, in this case As dopant atoms in silicon, monitored as a function of Bragg-angle while traversing the range of strong Bragg reflection bears the signature of the position of the As atoms within the unit cell of the host crystal [9].