Yongsup Park

We used ultraviolet and x-ray photoelectron spectroscopy (XPS) and (UPS) techniques to directly measure absolute values of vacuum work function of indium tin oxide (ITO) thin films. We obtained a work function of 4.4–4.5 eV which is lower than the commonly cited value. These values do not change substantially by heating and Ar ion sputtering. The atomic concentrations of each element in ITO, measured with XPS, are also quite stable under heat treatment and ion sputtering.

We demonstrate that the gap states at the interface of Ca and a phenylene vinylene oligomer thin film are responsible for the dramatic quenching of its photoluminescence (PL). Upon oxidation of the Ca layer, the midgap states are removed, and the PL intensity recovers. From the cumulative Ca deposition and oxidation study, a 30 \AA{} Ca oxide layer between the oligomer and the Ca metal prevents PL quenching due to metal induced midgap states. The implications of these results in the design and o

In-plane uniaxial magnetic anisotropy is observed in the magnetic hysteresis loops of ultrathin Fe films grown on MgO(001) by means of oblique-incidence molecular beam epitaxy. The films are deposited from an angle of 45° from the surface normal in the (100) azimuth toward the [100]. The easy axis of magnetization is perpendicular to the incident Fe-beam direction. A strong structural asymmetry is observed in x-ray photoelectron diffraction and attributed to an oriented morphology that drives th

We have studied the interface formation of a vinylene phenylene oligomer with a Ca substrate using photoemission spectroscopy. The evolution of core and valence spectra during the deposition of the oligomer on Ca has indicated the molecular energy level bending. The total energy level bending was 0.5 eV and the thickness of the level bending region was about 100 Å. We propose an energy level diagram of the oligomer-Ca interface based on the information obtained from the photoemission spectra.

Using x-ray and ultraviolet photoelectron spectroscopy (XPS and UPS), we have studied the formation of metal/organic interfaces in organic electroluminescent devices. Oligo(p-phenylenevinylenes) (OPV) and tris-(8-hydroxyquinoline)aluminum (Alq3) were used as the organic materials and Ca was used as the metallic layer. Interfaces are formed differently by depositing organic layer on Ca and Ca on organic substrate. For Ca/OPV, UPS revealed a clear evidence for interface state formation upon Ca dep

Although rapid progress has been made recently in bulk heterojunction organic solar cells, systematic studies on an ultrathin interfacial layer at the electron extraction contact have not been conducted in detail, which is important to improve both the device efficiency and the lifetime. We find that an ultrathin BaF2 layer at the electron extraction contact strongly influences the open-circuit voltage (Voc ) as the nanomorphology evolves with increasing BaF2 thickness. A vacuum-deposited ultrat

Superparamagnetic behavior is investigated for Fe grown at 700 K onto MgO(001) to a thickness equivalent to that of a ten monolayer film. Two such Fe deposits separated by a 200-\AA{} deposit of MgO exhibit a ferromagnetic response with no hysteresis at either 300 or 150 K, but with identical reduced magnetization curves M(H/T) which confirms the existence of superparamagnetism. M(H) data at 300 K were fitted to a Langevin function to yield an average particle size of 100 \AA{} diameter. M(T) fo

We have studied the electronic structures of Al/MgF2/tris-(8-hydroxyquinoline)aluminum (Alq3) interface using UV and x-ray photoelectron spectroscopy [(UPS) and (XPS), respectively]. The UPS revealed that the valence peak shift occurred with MgF2 deposition before Al was deposited and was independent of the gap state formation. The XPS core level peaks indicated that the MgF2 strongly interacted with Al and O atoms in Alq3 even before Al was deposited, and the deposition of Al caused a slight ch

Abstract The effect of thermal annealing on interfacial mixing of solution‐processed organic light‐emitting diodes (OLEDs) using direct sputter‐depth profiling techniques is investigated. X‐ray and ultraviolet photoelectron spectroscopy and argon gas cluster ion beam sputtering are used to investigate the distribution of chemical species near the interface. Extensive interfacial mixing is found in solution‐processed OLEDs after the thermal annealing at temperatures below the glass transition tem

Abstract The impact of anode buffer layers (ABLs) on the performance of CdSe quantum-dot light-emitting diodes (QLED) with a ZnO nanoparticle (NP) electron-transport layer and 4,4′-cyclohexylidenebis[ N , N -bis(4-methylphenyl)benzenamine] (TAPC) hole-transport layer was studied. Either MoO 3 or 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN) was used as the ABL. The QLED with a HAT-CN ABL exhibited better luminance performance, while the ultraviolet photoelectron spectroscopy and hol

Highly efficient green, blue, and white phosphorescent inverted organic light-emitting diodes were demonstrated by improving charge injection and balance.

Atomically thin two-dimensional (2D) hexagonal boron nitride (hBN) has emerged as an essential material for the encapsulation layer in van der Waals heterostructures and efficient deep ultraviolet optoelectronics. This is primarily due to its remarkable physical properties and ultrawide bandgap (close to 6 eV, and even larger in some cases) properties. Color centers in hBN refer to intrinsic vacancies and extrinsic impurities within the 2D crystal lattice, which result in distinct optical proper