Investigation of the surface electronic structure variation of a solid under X-ray irradiation with resonant photoemission spectroscopy and ambient pressure X-ray photoelectron spectroscopy

Abstract

While X-ray photoelectron spectroscopy (XPS) has been widely utilized to investigate the electronic structure of a material, the effect of X-ray irradiation on the surface electronic structure has not been well disclosed yet. The X-ray irradiation can result in inevitable photoexcitation and related phenomena on the material surface, which can directly affect the surface electronic structure of a material. Nevertheless, both the nature of X-ray-induced photoexcited states and the effect of X-ray photoexcitation on material properties, such as surface reactivity, are still in question. Therefore, the X-ray irradiation during XPS measurements can induce unintentional electronic structure variation in a sample, leading to the misinterpretation of XPS results. Such misinterpretation makes it very challenging to associate the measured electronic structure with the actual properties of a material in its practical applications. Then, understanding the effect of X-ray irradiation and consequent photoexcitation on the surface electronic structure is essential for the correct interpretation of XPS data and the application of XPS results to understand various material properties. Then, to identify the effect of X-ray irradiation on the surface electronic structure during XPS measurements, the resonant photoexcitation of electrons in both a polycrystalline Pt3Co alloy and MnO(001) single crystal has been investigated in this thesis, utilizing resonant photoemission spectroscopy (RPES) and ambient pressure XPS (AP-XPS).

In the case of the Pt3Co, the photon energy dependence of photoexcited final states is discussed, by observing a resonant Auger emission process in the Pt3Co. The different electron dynamics in each photoexcited state is also covered. When incident photon energy is tuned near Co L3 absorption edge of the Pt3Co, the resonant Co L3M4,5M4,5 Auger emission occurs during the de-excitation of a resonantly photoexcited state. With the relatively low photon energy, the resonant Co L3M4,5M4,5 Auger electron has constant binding energy, which means that the incident photon energy is directly reflected on Auger electron kinetic energy. In contrast, with the relatively high photon energy, the Auger electron has constant kinetic energy, recovering the characteristic of normal Auger emission. In this study, it is revealed that the photon energy threshold for the crossover between the resonant Auger to normal Auger emission indicates the energy difference between the resonant and normal Auger final state. Additionally, it is found that the delayed Auger crossover for the Pt3Co compared to the Auger crossover for pure 3d transition metals is related to the decrease in Co 3d valence electron correlation due to alloying with Pt.

In the case of the MnO, the effect of the X-ray photoexcitation on surface energy level alignment is discussed, by monitoring the variation of surface potential, i.e. core level binding energy shift, as a function of photon energy. At the same time, the influence of the surface potential on adjacent gas molecules is also estimated from the gas phase binding energy shift. Under resonant X-ray irradiation, photocarrier injection from a core to an unoccupied valence level is expected to occur. At the same time, there is intense photoemission due to high X-ray absorption strength near the resonance. The former phenomenon results in negative surface potential in the case of a p-type semiconductor like MnO, whereas the latter phenomenon always results in positive surface potential. Then, the resulting surface potential will change the surface band bending of MnO, which is the intrinsic surface property of the semiconductor surface. Moreover, in an AP-XPS setup, the surface potential propagates to a gas phase layer above the surface. As a result, the gas phase potential also emerges, and the magnitude of the gas phase potential is less than that of the surface potential due to the attenuation by the gas-solid distance. In this study, surprisingly, it is revealed that the surface band bending is independent of the magnitude of the surface potential, which indicates the formation of a strong inversion layer below the surface, as well as the maximization of the surface band bending due to a significant loss of electrons. In addition, it is found that the ratio between the surface and gas phase potential is not constant, depending on the surface potential itself. This indicates that the gas phase potential is not simply attenuated from the surface potential by the gas-solid distance only, but some other factors, e.g. secondary electron cloud, would intervene in the potential attenuation between the solid surface and gas phase layer, in an AP-XPS setup.

In conclusion, this thesis will advance knowledge on the electron dynamics in X-ray-induced photoexcited states, as well as the effect of X-ray irradiation on solid electronic structure. This contributes to the correct interpretation of XPS data for material analysis and then improves the utilization of XPS results for the understanding of various material properties in the practical applications of a material.