SPring-8, the large synchrotron radiation facility

Release Date

13 Mar, 2007

Background and purpose of research
     Holography is a method of recording and reconstructing a three-dimensional image, and the recorded three-dimensional image is called a hologram. The light-based hologram has been used in various applications. For instance, a three-dimensional image can be seen when light is applied to the hologram printed on a credit card. The hologram records a three-dimensional image in a plane by using light, which has the character of a wave. Similarly, the electron has the character of a wave on the nano scale. An electron based hologram can be formed by using the character of a wave. Recording the three-dimensional atomic arrangement in the electron hologram becomes possible, because the size of the wave of the electron (wavelength) is smaller than the atomic radius. 
     A three-dimensional atomic arrangement is very important information for material science. Roughly speaking, there are two types; atomic arrangement of a bulk and atomic arrangement on the surface. For the same object, the configuration of an atom differs markedly between the bulk and the surface. It is more important to obtain the information of the atomic arrangement on the surface. For instance, consider the case in which a molecule is adsorbed by the surface of a catalyst, and a reaction proceeds as atoms move on the catalyst surface. The information regarding the atomic position on the surface may provide a clue to understanding catalyst function improvement. 
     There are other methods of measuring atomic position besides electron holography; for instance, X-ray diffraction analysis and scanning tunneling microscopy. In the case of the X-ray diffraction method, a considerable amount of work for visualizing the surface is needed, since hard X-rays penetrate the material. In the case of scanning tunneling microscopy, which is mainly used in the laboratory, we can measure the surface by tracing it with a sharp needle. However, only the topmost layer of the surface can be visualized; the underlying layers cannot be seen. 
     Using the electron hologram, it is possible to measure the three-dimensional atomic arrangement of the surface with high sensitivity and high accuracy, since electrons with energies below 1000 eV hardly penetrate the material, and the signal mainly comes from an area of 1nm scale. The measurement method is simple. Synchrotron radiation is irradiated onto the sample and we analyze the number, speed, and direction of the electron that comes out. 
     Let us explain this in more detail. A photoelectron or Auger electron is ejected when the synchrotron radiation is irradiated onto the material (Fig. 1). The electron behaves like a wave in nano space. The wave of the electron is scattered by a surrounding atom in the material, thereby causing interference. Afterwards, the electron is ejected from the material. The number of electrons at the target speed is measured by using an electron energy analyzer in various directions. The distribution of the obtained electron number becomes an electron hologram that reflects a three-dimensional atomic arrangement. It is possible to select the electron that is ejected from the target element by selecting the photon energy and speed of the observed electron. Therefore, the atomic arrangement around the target element can be observed. Research on this ultimate metrology started in 1986. It took several hours to measure one hologram by the present measurement method, because it was necessary to measure the hologram while rotating the sample since the electron energy analyzer was able to take measurement only at a specific direction (Fig. 2). Moreover, it was necessary to measure tens of holograms while changing the speed of the observed electron, and this measurement took a few days. Only about ten atoms at most were visualized even if tens of measured holograms were utilized (Fig. 3). This explains the fact that the past theories that enable us to calculate the atomic arrangement are not good. Therefore, the electron hologram could not be used as a practical method of determining atomic arrangement.

Research and Result
     To overcome the above-mentioned problem, Dr. T. Matsushita of JASRI/SPring-8 developed a new theory. Among earlier methods, the Fourier transform method was used to calculate the atomic arrangement from the electron hologram. However, because the approximate expression of the process of scattering brought about by the atom was not good, it was difficult to reconstruct the atomic arrangement. Dr. T. Matsushita developed the scattering pattern extraction algorithm, which does not use the Fourier transform, as a more accurate atomic arrangement calculation method. This calculation method is based on the scattering theory of quantum mechanics and the information-theory –based maximum entropy method. This algorithm would enable the reconstruction of the atomic arrangement from only one hologram. The verification experiment of this theory was carried out in cooperation with Prof. Daimon’s team at the Nara Institute of Science and Technology. Because a highly accurate and high-luminance soft X-ray whose wavelength can be changed was needed for the experiment, the experiment was performed at SPring-8’s BL25SU, and a two-dimensional display-type electron energy analyzer was jointly developed. The jointly developed device has a unique structure that makes it is possible to take one picture of an electron hologram at a time (Fig. 4). An electron hologram can be obtained at an exposure time of 0.1 seconds, because of the high luminance of the synchrotron radiation of SPring-8. This device is about as much as 100,000 times faster than earlier devices. We took a very accurate picture of an electron hologram with this device (left of Fig. 5). The sample used for verification is a copper crystal. The Auger electron whose kinetic energy was 914 eV was used. An atomic arrangement of 102 atoms was reconstructed as a result of applying the theory to this electron hologram (Fig. 5). The spatial resolution was confirmed to be high, 0.05-0.02 nm. As a result of the verification of this method, the number of holograms is about 1/10, and the number of obtained atoms is about ten times. In addition, the measurement time can be shortened up to 1/1,000,000 that of the past method, and the measurement is completed at only 0.1 seconds if the developed device is used in combination with the new theory. The new method enables us to visualize the change of a specific element at the an atomic level in real time. 
     This development has opened the way for the practical use of electron holography in the real-time metrology of the three-dimensional arrangement on the surface of a material.

Future prospects
     Electron holography technology can be applied to various samples. In particular, visualization of the surrounding atoms around a target element has become an important feature that earlier methods did not have. For instance, consider the case in which a molecule is adsorbed into the surface of a catalyst, and a reaction proceeds on the catalyst surface. We can determine the situation of the catalyst and the reactive molecule by visualizing the atomic structure that contains the reactive molecule by using an electron hologram when a certain specific element is related to the catalysis reaction. As a result, designing a surface with a more efficient catalysis reaction becomes possible. 
     Moreover, it is difficult to measure the atomic arrangement of impurities by using the X-ray diffraction method, which needs a pure crystal. The character of a semiconductor is controlled by impurities, and the performance of the semiconductor is controlled by the impurity atomic sites. Even in this case, the electron hologram can measure the atomic arrangement of impurities. Therefore, the application to semiconductor performance enhancement can be expected. Moreover, this measurement technique can be combined with a variety of methods. For instance, observing the structure of a micro device established on a semiconductor surface while specifying the element and atomic arrangement becomes possible if this technique is combined with micro focused synchrotron radiation. Therefore, it is projected that this technique will be able to contribute to the improvement of the atomic level control of nano technology. It seems that new theories and devices from Japan will encourage similar experiments all over the world in the near future, and contribute to the development of material science.

Figure 1: Measurement method of electron hologram.

Figure 2: Previously developed electron energy analyzer.

Figure 3: The past method for reconstructing the atomic arrangement.

Figure 4: Developed electron energy analyzer.

Figure 5: An electron hologram of Cu crystal measured using developed electron analyzer, and atomic arrangements reconstructed from one electron hologram on the basis of the new theory.

Animation 1: Animation of the electron motion in the developed two-dimensional display analyzer.　(MPEG 702KB)

Animation 2: Animation for the relation between the copper crystal and the electron hologram.

Animation 3: Result of the atomic arrangement obtained on the basis of previous theory.

Animation 4: Result of the atomic arrangement obtained on the basis of the new theory.

For more information, please contact Dr. T. Matsushita (JASRI/SPring-8):
matusita@spring8.or.jp