Yun Chang Park

Photoelectron spectroscopy study of the electronic structures of Al/MgF2/tris-(8-hydroxyquinoline) aluminum interfaces

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 change to the N 1s core level peak. These results indicate that the interaction mechanism in Al/MgF2/Alq3 is different from those found in Al/LiF/Alq3 and other metal/Alq3.

Single-crystal apatite nanowires sheathed in graphitic shells: synthesis, characterization, and application.

Vertically aligned one-dimensional hybrid structures, which are composed of apatite and graphitic structures, can be beneficial for orthopedic applications. However, they are difficult to generate using the current method. Here, we report the first synthesis of a single-crystal apatite nanowire encapsulated in graphitic shells by a one-step chemical vapor deposition. Incipient nucleation of apatite and its subsequent transformation to an oriented crystal are directed by derived gaseous phosphorine. Longitudinal growth of the oriented apatite crystal is achieved by a vapor-solid growth mechanism, whereas lateral growth is suppressed by the graphitic layers formed through arrangement of the derived aromatic hydrocarbon molecules. We show that this unusual combination of the apatite crystal and the graphitic shells can lead to an excellent osteogenic differentiation and bony fusion through a programmed smart behavior. For instance, the graphitic shells are degraded after the initial cell growth promoted by the graphitic nanostructures, and the cells continue proliferation on the bare apatite nanowires. Furthermore, a bending experiment indicates that such core-shell nanowires exhibited a superior bending stiffness compared to single-crystal apatite nanowires without graphitic shells. The results suggest a new strategy and direction for bone grafting materials with a highly controllable morphology and material conditions that can best stimulate bone cell differentiation and growth.

Photoelectron spectroscopy studies on the electronic structures of Al/RbF and Al/CaF2 cathodes for 8-hydroxyquinoline aluminium-based organic light-emitting devices

The electronic structures of Al/RbF/8-hydroxyquinoline aluminium (Alq3) and Al/CaF2/Alq3 interfaces were investigated using x-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). For both systems, the UPS showed that a significant valence band shift occurred by evaporating the thin fluoride layer on Alq3. However, the gap state formation and the evolution of N 1s core level spectra showed rather different trends, suggesting that the alkali fluoride and alkali–earth fluoride interlayer have different reaction mechanisms at the interface with the Al cathode. In addition, the deposition of Al has considerably less effect on the valence band shift compared to the deposition of both RbF and CaF2. These results suggest that the complex charge transfer mechanism across the interface has considerably less effect on the enhancement of organic light-emitting device performance than the significant valence band shift as a result of deposition of interlayer materials.

Transmission electron microscopy study of damage layer formed through ion beam induced deposition of platinum on silicon substrate

Ion beam induced deposition is used for nanofabrication operations (e.g., transmission electron microscopy sample preparation) but is still not perfectly understood. Here, the authors describe the use of a wide variety of analytical instruments to develop such an understanding. Pt-films were deposited on native oxide films on silicon (Si) by decomposing trimethyl platinum C5H4CH3Pt(CH3)3 with focused electron or gallium ion beams. Unlike electron beam induced deposition, ion beam induced deposition produces a deep and complex damage layer below the deposition. It is revealed that the damaged area can be divided into three layers: (1) a “white-band” with a lower Pt-concentration and concurrent higher carbon (C)-concentration; (2) a-Pt–Si region where amorphized silicon crystal is mixed with Pt, and Pt-particle formation starts somewhere in the middle area; and (3) a-Si, which is silicon amorphized by Ga ions. The change in the diffusion rate of platinum into the amorphized silicon layer plays a crucial role in the formation of the distinct layers. The diffusions are affected by interatomic chemical affinity as well as by concentration, so that the Pt–Si combination initially dominates Pt agglomeration and the combination of C–Si, resulting in the creation of the novel white-band.Ion beam induced deposition is used for nanofabrication operations (e.g., transmission electron microscopy sample preparation) but is still not perfectly understood. Here, the authors describe the use of a wide variety of analytical instruments to develop such an understanding. Pt-films were deposited on native oxide films on silicon (Si) by decomposing trimethyl platinum C5H4CH3Pt(CH3)3 with focused electron or gallium ion beams. Unlike electron beam induced deposition, ion beam induced deposition produces a deep and complex damage layer below the deposition. It is revealed that the damaged area can be divided into three layers: (1) a “white-band” with a lower Pt-concentration and concurrent higher carbon (C)-concentration; (2) a-Pt–Si region where amorphized silicon crystal is mixed with Pt, and Pt-particle formation starts somewhere in the middle area; and (3) a-Si, which is silicon amorphized by Ga ions. The change in the diffusion rate of platinum into the amorphized silicon layer plays a crucial rol...

Atomic scale investigations of the Co/Pt(111) interface structure and magnetic properties

The interface structure of an ultrathin Co overlayer on a Pt(111) crystal was investigated with atomic-layer resolution medium-energy ion scattering spectroscopy and surface magneto-optical Kerr effect (SMOKE). For a 7 ML Co, interdiffusion begins at 673 K to form a heavily distorted Co–Pt surface alloy layer with little change in SMOKE intensity. However, annealing at 773 K formed a 30 atomic-layer-thick Co–Pt substitutional alloy with 3.7% maximum tensile strain, at which the SMOKE intensity increased more than 200%. The enhancement of the Kerr intensity is discussed with the interface alloy formation.

Microscopic and spectroscopic analyses of Pt-decorated carbon nanowires formed on carbon fiber paper.

We report the synthesis of carbon nanowires (CNWs) via chemical vapor deposition using catalytic decomposition of ethanol on nanosized transition metals such as Co, Fe, and Ni. Dip-coating process was used for the formation of catalytic nanoparticles, inducing the growth of CNWs on the surface of the carbon fiber paper (CFP). The liquid ethanol used as carbon source was atomized by an ultrasonic atomizer and subsequently flowed into the reactor that was heated up to a synthesis temperature of 600-700°C. Microscopic images show that CNWs of <50 nm were densely synthesized on the surface of the CFP. Raman spectra reveal that a higher synthesis temperature leads to the growth of higher crystalline CNWs. In addition, we demonstrate the successful decoration of platinum nanoparticles on the surface of the prepared CNWs/CFP using the electrochemical deposition technique.

Effect of graphitic layers encapsulating single-crystal apatite nanowire on the osteogenesis of human mesenchymal stem cells.

An ideally designed scaffold for tissue engineering must be able to provide an environment that recapitulates the physiological conditions to control stem cell function. Here, we compared vertically aligned single-crystal apatite nanowires sheathed in graphitic layers (SANGs) with single-crystal apatite nanowires (SANs), which had the same geometric properties as--but differing nanotopographic surface chemistry than--SANGs, in order to evaluate the effect of the graphitic layer on the behavior of human mesenchymal stem cells (hMSCs). The difference in nanotopographic surface chemistry did not affect hMSC adhesion, growth, or morphology. However, hMSCs were more effectively differentiated into bone cells on SANGs through interaction with graphitic layers, which later degraded and thereby allowed the cells to continue differentiation on the bare apatite nanowires. Thus, SANGs provide an excellent microenvironment for the osteogenic differentiation of hMCS.

Hexagonal and pentagonal shapes of self-catalyzed one-dimensional GaAs nanostructures: Shape dependence of the phase evolutions

Hexagonal and pentagonal shapes of one-dimensional (1-D) nanostructures were observed at the growth of GaAs by molecular beam epitaxy method without catalyst aid. We report on the phase evolution dependence on the shapes of 1-D GaAs nanostructures. The hexagonal-shaped nanostructures showed a transition region composed of twin boundaries and stacking faults during the phase transition from a zinc-blende (ZB) structure to a wurtzite (WZ) structure. On the other hand, the pentagonal-shaped nanostructures appeared to have an abrupt transition from a ZB structure to a WZ structure and to be elongated along 〈112¯〉 directions of a ZB structure.