About Active-Sites

The term “active-site” is the keyword used to explore mechanisms for the functional properties of materials from a new perspective. People employ various methods to realize a useful functional materials. For example, p-type and n-type semiconductors are created by placing a small quantity of impurities in a semiconductor crystal to control the flow of electrons. Moreover, various impurities are added to iron to harden it or make it rust-resistant like stainless steel.  These atoms that are added in small quantities greatly affect the characteristics of the material.

In a catalyst, the chemical reaction is considered to progress at a specific atom. Similarly, in living organisms too, it is considered that in the proteins that conduct photosynthesis, the reaction progresses at the place where the Mn atoms are arrayed. Furthermore, it is well known that when a surface is covered with thin films, new functionalities (magnetism and catalytic properties) manifest because of the atoms present at the interface. Additionally, it has been discovered that when the particle size becomes nanoscale, a special atomic arrangement occurs, and new physical properties manifest.

Thus, “active-sites” in a substance refer to atoms that have special structures and give rise to the realization of functions. Therefore, the research on these “active-sites” is very important.

The findings regarding the atomic arrangement of the base material prior to the addition of impurities have enabled considerable scientific developments. However, no research has been performed concerning the question “Where should the added atoms enter the material so that the function appear?” This is because X-ray diffraction techniques cannot be applied; therefore, no science has been established thus far that explores the active-sites — which do not have a translational symmetry structure—with certainty from experimental data.

For example, there is hardly any research toward the observation of the local structures around boron in boron-doped silicon, which is indispensable to the aforementioned semiconductor industry. Materials are being developed by searching in the dark, without an accurate understanding of the “3D active-site” structure. To survive the fierce global competition in the leading fields of materials science, which employ advanced doping techniques, interface architectonics, and quantum-dot manufacturing technology, a material design based on the precise evaluation of “3D active-sites” in dopant/interface structures and nanostructures is indispensable.

Fortunately, in Japan, 3D atomic-imaging technologies have been actively performed, whereby the 3D atomic arrangement around the local structure that governs the physical properties can be selectively determined.
Three of these technologies are the following:
(1)    Doped-atom holography using photoelectrons and fluorescent X-rays
(2)    Surface and interface holography based on surface X-ray diffraction
(3)    Nanostructure imaging, which applies the phase retrieval technique in the electron diffraction pattern
Using these technologies, it is possible to measure and analyze the “3D active-sites” in several different points of view. It is mandatory to transcend the respective research areas and systematize/integrate them to realize a new science.