A structural analysis of solids at an atomic level is necessary to develop advanced materials, such as semiconductors, superconductors, and catalysis. Transmission electron microscopy and scanning probe microscopy, which are widespread in scientific and engineering fields, are recognized as general atomic visualization techniques. Structural analysis techniques using X-rays, such as X-ray diffraction, have made it possible to evaluate crystal structures for over 50 years. However, the determination of atomic arrangements is not straightforward. It needs an accurate data fitting procedure for experimental and theoretical diffraction profiles, and this requires sufficient knowledge and experience of X-ray diffraction analysis. Therefore, a direct three-dimensional (3D) atomic imaging technique, which will help determine crystal structure, has long been desired. X-ray fluorescence holography (XFH) is one solution. Holography, which is a way of recording and then reconstructing waves, was invented by Dennis Gabor in 1948. The waves may be of any kind-light, sound, X-ray, corpuscular, etc. The word "holography" originates from the Greek "holos" meaning "the wholes." By using the word holography the inventor of the technique wanted to stress that it records complete information about a wave. In conventional photography, only the distribution of the amplitude is recorded in a two-dimensional projection of an object onto the plane of the photograph. However, a hologram can regenerate the field of the wave scattered by an object, and therefore it can reconstruct the object. Holography's unique ability makes it a valuable tool for industry, science, business, and education. It is commonly used on labels and covers of magazines. Recently, we have seen holograms on credit cards and paper currencies, which prevents copies because the hologram is difficult to reproduce. For advanced scientific fields, holography using photons, electrons, and neutrons with 2 ∼sub-Ångstrom wavelength has attracted attention as an atomic-order microscopic tool. Gabor (1948) proposed the principle of holography and demonstrated that it improves the power of electron microscopes. His idea was very simple. He used the interference between the wave scattered by the object (object wave) and one passing through the object (reference wave), and recorded the wavefront of the object wave. Although many researchers expected to visualize atoms in solids using Gabor's method, it could not be realized at that time. Holography became widely used in many areas of science and technology with the introduction of the laser (Leith and Upatnieks, 1965). Szöke (1986) pointed out that photoelectrons and fluorescent X-rays from ionized atoms in a single crystal formed atom-resolved holograms. His idea was first proved by Harp et al. (1990) as X-ray photoelectron holography. Photoelectron holography is a powerful tool for studying surface structures. Much theoretical and experimental work on photoelectron holography has been conducted. However, the atomic image obtained from the hologram is not clear due to a phase shift resulting from electron scattering and multiple scattering. Since the effect of phase shift and the multiple scattering of X-rays are negligible in data analysis, X-ray scattering is much better than electron scattering. Feasibility studies of XFH by computer simulations started in 1991 (Tegze and Faigel, 1991). However, the experimental application of X-rays for holographic imaging has been delayed in comparison to photoelectron holography. The primary reason for this is the weakness of the holographic oscillation, which is 0.1-0.01% in the angular distribution of X-ray fluorescence intensity. Moreover, the weak oscillations are masked by strong and sharp Kossel or X-ray standing wave lines due to X-ray diffraction. An XFH experiment was first performed by Tegze and Faigel (1996) as a demonstration of the structural analysis of strontium titanate (SrTiO3). Similar to photoelectron holography, XFH measures the spherically distributed fluorescence intensity varying the detector position. The number of measurable holograms is limited by number of X-ray emission lines, such as Kα and Kβ. Shortly after the XFH experiment by Tegze and Faigel, Gog et al. (1996) demonstrated multiple energy X-ray holography, which was a time-reversed version of normal XFH. Here, we call this method "inverse XFH." In inverse XFH, a holographic pattern can be obtained by detecting the fluorescence by varying the sample orientation relative to the incident beam. In contrast to normal XFH, inverse XFH allows holograms to be recorded at an arbitrary energy using an energy tunable X-ray source, such as synchrotron radiation. This is the reason why Gog et al. named this method "multiple energy X-ray holography.". The first experiment by Tegze and Faigel (1991) using a conventional X-ray generator and solid-state X-ray detector needed a few months to record one hologram due to the weakness of the holographic signals. Today, we can measure the hologram within a few hours using a strong incident X-ray beam and advanced X-ray detecting system (Tegze et al., 1999; Hayashi et al., 2001a). The XFH setup at the European Synchrotron Radiation Facility (ESRF) makes it possible to take a hologram within 10 min using a pink beam, which is a fundamental undulator radiation (Marchesini et al., 2001). The spatial resolution of the atomic images was 0.5 Å with a 4π full extension technique using crystal symmetry (Tegze et al., 1999). Moreover, light atoms such as oxygen can be visualized due to a data set with extremely high statistical accuracy (Tegze et al., 2000). Since the sample of XFH needs a regular orientation of atomic arrangement around a specific element, the amorphous or powder samples cannot be measured. However, it is not limited to systems with a long-range order but can also be applied to cluster, surface adsorbates, and impurities. Several attractive applications have been demonstrated. Holograms of Zn doped in a GaAs wafer were measured and a dominant site was clarified by visualizing the environment around Zn (Hayashi et al., 2001b). Takahashi et al. (2003c) measured the holograms of FePt thin films and successfully reconstructed atomic images of a Pt layer up to 15 Å in radius. Marchesini et al. (2000) applied XFH to understanding the icosahedral atomic arrangement of quasicrystal AlPdMn. In this article we describe the principle of X-ray holography, reconstruction techniques, experimental systems, and some applications. Furthermore, other related holographic techniques, such as πXAFS, γ-ray holography, and neutron holography, are reviewed. Concerning photoelectron holography, there is so much work in this field that a review is outside the scope of this chapter. Finally, we would like to mention the perspective of atom-resolved holography.
ASJC Scopus subject areas
- Nuclear and High Energy Physics
- Condensed Matter Physics
- Electrical and Electronic Engineering