Chung-Jong Yu

Demonstration of Feasibility of X-Ray Free Electron Laser Studies of Dynamics of Nanoparticles in Entangled Polymer Melts

The recent advent of hard x-ray free electron lasers (XFELs) opens new areas of science due to their exceptional brightness, coherence, and time structure. In principle, such sources enable studies of dynamics of condensed matter systems over times ranging from femtoseconds to seconds. However, the studies of “slow” dynamics in polymeric materials still remain in question due to the characteristics of the XFEL beam and concerns about sample damage. Here we demonstrate the feasibility of measuring the relaxation dynamics of gold nanoparticles suspended in polymer melts using X-ray photon correlation spectroscopy (XPCS), while also monitoring eventual X-ray induced damage. In spite of inherently large pulse-to-pulse intensity and position variations of the XFEL beam, measurements can be realized at slow time scales. The X-ray induced damage and heating are less than initially expected for soft matter materials.

Morphological and structural characterization of epitaxial α-Fe2O3 (0001) deposited on Al2O3 (0001) by dc sputter deposition

We investigated α-Fe2O3 (0001) film growth on α-Al2O3 (0001) using a laboratory-built dc faced magnetron sputtering system with x-ray diffraction, x-ray reflectivity, and atomic force microscopy (AFM). The films were deposited from iron targets at a growth rate of ∼6.3nm∕min in an ambient argon and oxygen mixture. The structural properties and quality of the α-Fe2O3 thin films were characterized using high-resolution synchrotron x-ray scattering. Films thinner than 16nm were to be found highly strained due to the large lattice mismatch between film and substrate of 5.8%. This strain was found to decrease as the film thickness increased, and most of the strain was released through surface modulation once the film thickness reached 20nm. The epitaxial relationships were found to be α-Fe2O3 [0001]‖α-Al2O3 [0001] in the out-of-plane direction. That the film and substrate display the in-plane alignment anticipated for the epitaxy of isomorphous materials was established by verifying that the [112¯0] directions of film and substrate were coincident. X-ray reflectivity and AFM were used to show that layer-by-layer growth of α-Fe2O3 (0001) occurs up to a thickness of 11nm despite the large lattice mismatch.We investigated α-Fe2O3 (0001) film growth on α-Al2O3 (0001) using a laboratory-built dc faced magnetron sputtering system with x-ray diffraction, x-ray reflectivity, and atomic force microscopy (AFM). The films were deposited from iron targets at a growth rate of ∼6.3nm∕min in an ambient argon and oxygen mixture. The structural properties and quality of the α-Fe2O3 thin films were characterized using high-resolution synchrotron x-ray scattering. Films thinner than 16nm were to be found highly strained due to the large lattice mismatch between film and substrate of 5.8%. This strain was found to decrease as the film thickness increased, and most of the strain was released through surface modulation once the film thickness reached 20nm. The epitaxial relationships were found to be α-Fe2O3 [0001]‖α-Al2O3 [0001] in the out-of-plane direction. That the film and substrate display the in-plane alignment anticipated for the epitaxy of isomorphous materials was established by verifying that the [112¯0] directions...

Structure of multi-wall carbon nanotubes: AA′ stacked graphene helices

The structure of multi-wall carbon nanotubes has been attributed previously to disordered stacking of the graphene planes. Evidence is presented that the nanotubes analyzed in this paper occur with stacked graphene layers in the sequence of AA′, where alternate graphene planes are translated by half the hexagon width. We further present proof that the crystalline materials comprise graphene helices (∼5 nm in width), rather than in the form of a perfect tube. We also show that the structural model proposed here may be a common structure for multi-wall carbon nanotubes.

Note: Construction of x-ray scattering and x-ray absorption fine structure beamline at the Pohang Light Source

A new hard x-ray beamline, 10B KIST-PAL beamline (BL10B), has been designed and constructed at the Pohang Light Source (PLS) in Korea. The beamline, operated by Pohang Accelerator Laboratory-Korean Institute of Science and Technology consortium, is dedicated to x-ray scattering (XRS) and x-ray absorption fine structure (XAFS) experiments. X rays with photon energies from 4.0 to 16.0 keV are delivered to the experimental station passing a collimating mirror, a fixed-exit double-crystal Si(111) monochromator, and a toroidal mirror. Basic experimental equipment for XAFS measurement, a high resolution diffractometry, an image plate detector system, and a hot stage have been prepared for the station. From our initial commissioning and performance testing of the beamline, it is observed that BL10B beamline can perform XRS and XAFS measurements successfully.

Coherent X-ray scattering beamline at port 9C of Pohang Light Source II

The coherent X-ray scattering beamline at the 9C port of the upgraded Pohang Light Source (PLS-II) at Pohang Accelerator Laboratory in Korea is introduced. This beamline provides X-rays of 5-20 keV, and targets coherent X-ray experiments such as coherent diffraction imaging and X-ray photon correlation spectroscopy. The main parameters of the beamline are summarized, and some preliminary experimental results are described.

Direct-write X-ray lithography using a hard X-ray Fresnel zone plate.

Results are reported of direct-write X-ray lithography using a hard X-ray beam focused by a Fresnel zone plate with an outermost zone width of 40 nm. An X-ray beam at 7.5 keV focused to a nano-spot was employed to write arbitrary patterns on a photoresist thin film with a resolution better than 25 nm. The resulting pattern dimension depended significantly on the kind of underlying substrate, which was attributed to the lateral spread of electrons generated during X-ray irradiation. The proximity effect originated from the diffuse scattering near the focus and electron blur was also observed, which led to an increase in pattern dimension. Since focusing hard X-rays to below a 10 nm spot is currently available, the direct-write hard X-ray lithography developed in this work has the potential to be a promising future lithographic method.