Revealing the structural dynamics using ultrafast time-resolved diffraction instruments

Abstract

which I-I bond, if two I-I bonds are unequal, dissociates depending on the electronic state. Here, femtosecond anisotropic X-ray solution scattering allows us to provide the following answers in addition to the overall rich structural dynamics. The analysis unravels that the negative charge of I3- is highly localized on the terminal iodine atom forming the longer bond with the central iodine atom, and the shorter I-I bond dissociates in the excited state, whereas the longer one in the ground state. We anticipate that this work may open a new avenue for studying the atomic charge distribution of molecules in solution and taking advantage of orientational information in anisotropic scattering data for solution-phase structural dynamics. Metal-semiconductor bilayer is frequently used in optoelectronic devices and electrical devices. The flow of charge carriers and structural changes at the metal-semiconductor interface determine the performance and efficiency of the device Therefore, many studies have been conducted on the metal-semiconductor interface. However, it has been a challenge to observe the charge transfer dynamics and structural change at the interface due to its structural complexity. Since the structural change at the interface accompanies the structural change in each layer, it has been difficult to perfectly separate the signals even using spectroscopy or diffraction methods. In this study, to remedy these difficulties and investigate the metal-semiconductor interface, we introduce the Au/TiO2 bilayer system. Au/TiO2 is a representative system used to investigate the metal-semiconductor interface, and has the advantage of being able to separate the structural motion from each layer through diffraction method. In the experiment, we observe the thermal dynamics and lattice changes of Au/TiO2 bilayer through the ultrafast electron diffraction (UED). From the results, structural evidence for interlayer charge transfer from Au to TiO2 was observed. In addition, we found that this charge transfer induces coherent oscillations at the Au/TiO2 interface. This coherent vibration propagates mechanical stress through the interface and act as a new pathway to transfer the force and heat of Au to the TiO2. As a result, it was possible to investigate the effect of electron transfer at the metal-semiconductor interface which modulate the force and heat transfer at the interface.

Energy, structure, and charge are fundamental quantities characterizing matters around us. Therefore, numerous efforts have been made to elucidate the properties of matter through spectroscopy and scattering techniques over the past decades, which has had a significant impact on basic science to overall engineering fields. Since most experimental techniques, such as spectroscopy and scattering techniques, are sensitive to only one specific property, such as structure or energy, these efforts have led to the integrated development of various experimental techniques. There have been numerous efforts to measure multiple quantities simultaneously. However, unfortunately, relatively small progress has been made in this field due to the experimental difficulties. For example, the charge distribution and structure in a solid crystal can be measured accurately by using diffraction techniques. But, although remarkable advances have been made in this field, it was impossible to measure the charge distribution of molecules in solution. In addition, there has been little progress in observing charge transfer and structural changes in heterogeneous systems such as metal-semiconductor bilayer systems even in solid samples. In this study, we focused on developing a methodology to measure multiple quantities simultaneously. For this purpose, a time-resolved diffraction (scattering) technique was used to solve these difficulties. In general, diffraction is known to have a high sensitivity to structural information and relatively low sensitivity to energy or charge. However, the high structural sensitivity of the scattering technique has the potential to provide small clues to observe such charge distribution and their movements. In this study, an analysis strategy was developed to interpret these clues, and through this, it was possible to unveil the dynamics that had not been observed before. Whereas the energy flow and structure change in chemical reactions are experimentally characterized, determining the atomic charges of a molecule in solution has been elusive, even for a triatomic molecule such as triiodide ion, I3-. Moreover, it remains to be answered how the charge distribution is coupled to the molecular geometry