Chang-Ki Min

Effects of UV/ozone treatment of a polymer dielectric surface on the properties of pentacene thin films for organic transistors

Ultraviolet photoelectron spectroscopy and atomic force microscopy (AFM) were used to investigate the energy level alignment and growth morphology of pentacene (Pn) films deposited on a PMMA derivative-based dielectric surface with and without ultraviolet/ozone treatment. The treated surface exhibited higher offset values for the highest occupied molecular orbital levels between Pn and the polymer, which would result in higher threshold voltages for the device. However, aligned vacuum levels of the treated surface and the Pn at the interface were observed, suggesting that the dipole field would be reduced in the Pn film on the treated surface. The hydrophilic nature of the treated surface, observed by water contact angle measurement, allowed for a larger grain size of the Pn film, as confirmed by the AFM measurements, which will also favorably contribute to device mobility.

Soft X-ray harmonic lasing self-seeded free electron laser at Pohang Accelerator Laboratory X-ray free electron laser

The demonstration of a harmonic lasing self-seeded free-electron laser (HLSS FEL) scheme in the soft X-ray range at the Pohang Accelerator Laboratory X-ray Free Electron Laser is presented. We report the experimental results of HLSS FEL radiation with the shortest wavelength of 1 nm by using the optimized phase shift of 2/3π. The key feature of the HLSS scheme is that the mode number is decreased (the longitudinal coherence length is enhanced) which is directly observed using a single-shot spectrometer. The spectral brightness is enhanced by a factor of 1.7 compared to the self-amplified spontaneous emission FEL because of the narrowed bandwidth. Our results show a good agreement with the theoretical expectation and simulations. The HLSS mode is a promising standard operation mode to generate a stable and high-brightness X-ray FEL that will provide more benefits to users for various applications.The demonstration of a harmonic lasing self-seeded free-electron laser (HLSS FEL) scheme in the soft X-ray range at the Pohang Accelerator Laboratory X-ray Free Electron Laser is presented. We report the experimental results of HLSS FEL radiation with the shortest wavelength of 1 nm by using the optimized phase shift of 2/3π. The key feature of the HLSS scheme is that the mode number is decreased (the longitudinal coherence length is enhanced) which is directly observed using a single-shot spectrometer. The spectral brightness is enhanced by a factor of 1.7 compared to the self-amplified spontaneous emission FEL because of the narrowed bandwidth. Our results show a good agreement with the theoretical expectation and simulations. The HLSS mode is a promising standard operation mode to generate a stable and high-brightness X-ray FEL that will provide more benefits to users for various applications.

Interface electronic structure and dynamics of tris-(8-hydroxyquinoline) aluminum on Cu(100)

We have investigated the evolution of occupied and unoccupied electronic states of tris-(8-hydroxyquinoline) aluminum (Alq3) molecules adsorbed on Cu(100) using ultraviolet photoelectron spectroscopy and interferometric time-resolved two-photon photoemission (ITR-2PPE). The adsorption of Alq3 lowers the work function of the surface, which allows an unambiguous observation of Cu(100) surface resonance coupled with d-band state and molecular interband transition in our ITR-2PPE setup. No evidence of an anion state, which used to be formed by dynamic charge transfer from metal to a molecular unoccupied state, is identified. Analysis of relaxation dynamics of the transiently occupied electronic states shows that the lifetimes for those excited states at this surface are independent of the Alq3 thickness. These results indicate that the Alq3–Cu(100) interface is described as a weak van der Waals interaction.

Femtosecond Surface Plasmon Dynamics of Single Ag Nanostructures

Femtosecond plasmon dynamics of single Ag nanostructures was investigated by the combination of a femtosecond laser system and a time-of-flight photoemission electron microscope (TOF-PEEM). Time-resolved two-photon photoemission spectra of the nanostructures showed that the higher photoelectron yield is originated from the slower relaxation time of surface plasmon.