Development and Applications of Ambient Pressure X-ray Photoemission Spectroscopy Beamline

Abstract

Synchrotron-based ambient pressure x-ray photoemission spectroscopy has become the prominent technique to unveil physicochemical phenomena at the surface or interface of materials. In this thesis, the development and applications of synchrotron-based ambient pressure X-ray photoelectron spectroscopy (AP-XPS) beamline are discussed, which include basic principles of beamline components and AP-XPS endstation. First, the design and construction of beamline for AP-XPS in Pohang Accelerator Laboratory, are discussed. For the purpose of providing soft x-ray with high flux and resolving power for AP-XPS and scanning photoemission electron microscope (SPEM) endstations, the major optical components of beamline are designed with rigorous calculations and tested. The beamline performance tests showed nearly ~10,000 resolving power of photon energy at 400 eV with flux 1011 ~ 1012 ph/s/0.1%B.W., and the size of focused beam is measured as 100 μm (horizontal) x 10 μm (vertical) at the endstation. Windowless differential pumping systems for both beamline and AP-XP spectrometer worked effectively without any loss of flux or signal. Both total electron or Auger electron yield mode of the AP-XP spectrometer showed the expected features of Ar L-edge photoabsorption spectrum. The AP-XPS endstation was successfully tested under gas pressure of nitrogen, up to 10 mbar. A case study for graphene/Ge(110) under photon irradiation and 10 mbar N2 gas pressurization showed photo-induced effect of N-doped graphene layer. As of application of AP-XPS endstation, in situ experiment of oxidation states of vanadium thin film was carried out. Using synchrotron-based AP-XPS system at beamline 9.3.2 in Advanced Light Source, the redox processes of vanadium metal thin film was monitored. The results showed stable surface oxide states with abundant VO2 and V2O5 after 523K annealing under 500 mtorr of O2 gas pressure. Followed measurements of Raman spectroscopy and temperature-dependent resistivity showed typical MIT characteristics of VO2 state. The newly found fabrication condition of vanadium oxide thin film, which temperature is much lower than the previously reported values, shows possibility for utilization of VO2 for temperature-sensitive electronic devices.