Keun Hwa Chae

Carbon dioxide mediated, reversible chemical hydrogen storage using a Pd nanocatalyst supported on mesoporous graphitic carbon nitride

Reversible, carbon dioxide mediated chemical hydrogen storage was first demonstrated using a heterogeneous Pd catalyst supported on mesoporous graphitic carbon nitride (Pd/mpg-C3N4). The Pd nanoparticles were found to be uniformly dispersed onto mpg-C3N4 with an average size of 1.7 nm without any agglomeration and further exhibit superior activity for the dehydrogenation of formic acid with a turnover frequency of 144 h−1 even in the absence of external bases at room temperature. Initial DFT studies suggest that basic sites located at the mpg-C3N4 support play synergetic roles in stabilizing reduced Pd nanoparticles without any surfactant as well as in initiating H2-release by deprotonation of formic acid, and these potential interactions were further confirmed by X-ray absorption near edge structure (XANES). Along with dehydrogenation, Pd/mpg-C3N4 also proves to catalyze the regeneration of formic acid via CO2 hydrogenation. The governing factors of CO2 hydrogenation are further elucidated to increase the quantity of the desired formic acid with high selectivity.

Room-temperature ferromagnetism of Cu-implanted GaN

1MeV Cu2+ ion was implanted into GaN with a dose of 1×1017cm−2 at room temperature. After implantation, the samples were subsequently performed by rapid thermal annealing at 700, 800, and 900°C for 5min. Both nonmagnetic Cu ion implanted samples annealed at 700 and 800°C exhibit the ferromagnetism at room temperature, and the saturation magnetization of these samples is estimated to be 0.057μB and 0.27μB per Cu atom from M-H curve, respectively. However, the sample annealed at 900°C does not show ferromagnetism due to clustering of Cu during the annealing process.

X-ray absorption and magnetic circular dichroism characterizations of Mn doped ZnO

We report the near-edge x-ray absorption fine structure (NEXAFS) and x-ray magnetic circular dichroism (XMCD) experiments on well characterized Mn doped ZnO thin films that show ferromagnetism at room temperature. The NEXAFS measurements at O K edge clearly exhibit a preedge spectral feature which evolves with Mn doping, similar to one observed in hole-doped cuprates and manganites. The Mn L3,2 edge NEXAFS spectra exhibit divalent Mn apart from mixed valent Mn3+∕Mn4+ states. The spectral features of XMCD at Mn L3,2 edge demonstrate that ferromagnetism comes from Mn2+ ions and its dichroism shape is independent of Mn concentration.

Investigations on the structural, optical and electronic properties of Nd doped ZnO thin films

We report the synthesis and characterization of Nd doped ZnO thin films grown on Si (1 0 0) substrates by the spray pyrolysis method. The surface morphology of these thin films was investigated by scanning electron microscopy and shows the presence of randomly distributed structures of nanorods. Grazing angle x-ray diffraction studies confirm that the doped Nd ions occupied Zn sites and these samples exhibited a wurtzite hexagonal-like crystal structure similar to that of the parent compound, ZnO. The micro-photoluminescence measurement shows a decrease in the near band edge position with Nd doping in the ZnO matrix due to the impurity levels. The near-edge x-ray absorption fine structure (NEXAFS) measurements at the O K edge clearly exhibit a pre-edge spectral feature which evolves with Nd doping, suggesting incorporation of more charge carriers in the ZnO system and the presence of strong hybridization between O 2p‐Nd 5d orbitals. The Nd M5 edge NEXAFS spectra reveal that the Nd ions are in the trivalent state. (Some figures in this article are in colour only in the electronic version)

Ferromagnetism and metal-semiconducting transition in Fe-doped ZnO thin films

We report the room temperature ferromagnetism and a metal–semiconductor transition at 227 K in 200 MeV Ag15+-ion irradiated thin films of Fe-implanted ZnO. The single phase nature of Fe-doped ZnO after ion irradiation is confirmed by x-ray diffraction. Magneto-resistance measurements show spin polarization below 150 K. X-ray absorption spectroscopy and x-ray magnetic circular dichroism studies at room temperature reveal that Fe is oxidized in a mixed valence (Fe2+ and Fe3+) state and the magnetic signal is due to the Fe2+ state. The observations are explained on the basis of the combined effect of carrier doping by Fe3+ and oxygen vacancies.

Structural, electrical, magnetic, and electronic structure studies of PrFe1−xNixO3 (x⩽0.5)

We report the x-ray absorption studies on O K, Fe L3,2, Ni L3,2, and Pr M5,4 edges in PrFe1−xNixO3 along with their structural, electrical transport, and magnetization characterizations. All the samples are in single phase having orthorhombic structure with space group Pnma for x⩽0.4. Ni doping at Fe site brings the system in the conducting regime, resistivity decreases from GΩcmto260mΩcm at room temperature, and the magnetic ordering is stabilized. The temperature dependent resistivity follows the semiconducting behavior and fits well with Greaves’ variable range hopping model. The gap parameter is reduced from 2to0.118eV. The materials are in weak ferromagnetic state and magnetization is gradually decreasing with the enhancement of Ni substitution, whereas magnetic anisotropy is reduced substantially. A new feature about 2.0eV lower than the pre-edge of PrFeO3 in O K edge is observed with Ni substitution at Fe site due to the 3d contraction effect and is growing with the increase of Ni substitution. Fro...

Direct observation of oxygen induced room temperature ferromagnetism in MoO2 thin films by x-ray magnetic circular dichroism characterizations

We report the element specific polarized near edge x-ray absorption fine structure (NEXAFS) and x-ray magnetic circular dichroism (XMCD) experiments on well characterized undoped MoO2 thin films that show ferromagnetism at room temperature. The polarization dependent of O K edge NEXAFS spectra indicate a strong hybridization of O 2p-4d Mo orbitals followed by a strong anisotropy in the electronic properties. An unquenched orbital magnetic moment within the O 2p shell is clearly evident from the XMCD O K edge, which is ferromagnetically coupled to the neighboring Mo moments as confirmed by Mo M3,2 edge XMCD experiment.

Nanoscopic Management of Molecular Packing and Orientation of Small Molecules by a Combination of Linear and Branched Alkyl Side Chains

We synthesized a series of acceptor-donor-acceptor-type small molecules (SIDPP-EE, SIDPP-EO, SIDPP-OE, and SIDPP-OO) consisting of a dithienosilole (SI) electron-donating moiety and two diketopyrrolopyrrole (DPP) electron-withdrawing moieties each bearing linear n-octyl (O) and/or branched 2-ethylhexyl (E) alkyl side chains. X-ray diffraction patterns revealed that SIDPP-EE and SIDPP-EO films were highly crystalline with pronounced edge-on orientation, whereas SIDPP-OE and SIDPP-OO films were less crystalline with a radial distribution of molecular orientations. Near-edge X-ray absorption fine structure spectroscopy disclosed an edge-on orientation with a molecular backbone tilt angle of ∼22° for both SIDPP-EE and SIDPP-EO. Our analysis of the molecular packing and orientation indicated that the shorter 2-ethylhexyl groups on the SI core promote tight π-π stacking of the molecular backbone, whereas n-octyl groups on the SI core hinder close π-π stacking to some degree. Conversely, the longer linear n-octyl groups on the DPP arms facilitate close intermolecular packing via octyl-octyl interdigitation. Quantum mechanics/molecular mechanics molecular dynamics simulations determined the optimal three-dimensional positions of the flexible alkyl side chains of the SI and DPP units, which elucidates the structural cause of the molecular packing and orientation explicitly. The alkyl-chain-dependent molecular stacking significantly affected the electrical properties of the molecular films. The edge-on oriented molecules showed high hole mobilities in organic field-effect transistors, while the radially oriented molecules exhibited high photovoltaic properties in organic photovoltaic cells. These results demonstrate that appropriate positioning of alkyl side chains can modulate crystallinity and molecular orientation in SIDPP films, which ultimately have a profound impact on carrier transport and photovoltaic performance.

Synthesis of Au nanoparticles at the surface and embedded in carbonaceous matrix by 150 keV Ar ion irradiation

We report on synthesis of spherical Au nanoparticles at the surface and embedded in carbonaceous matrix by 150 keV Ar ion irradiation of thin Au film on polyethyleneterepthlate (PET). The pristine and irradiated samples are characterized by Rutherford backscattering spectrometry (RBS), atomic force microscopy, scanning electron microscopy and transmission electron microscopy (TEM) techniques. RBS spectra reveal the sputtering of Au film and interface mixing, increasing with increasing fluence. Surface morphology shows that at the fluence of 5 × 1015 ions cm−2, dewetting of thin Au film begins and partially connected nanostructures are formed whereas, at the higher fluence of 5 × 1016 ions cm−2, isolated spherical Au nanoparticles (45 ± 20 nm) are formed at the surface. Cross-sectional TEM observations also evidence the Au nanoparticles at the surface and mixed metal–polymer region indicating the formation of nanocomposites with small Au nanoparticles. The results are explained by the crater formation, sputtering followed by dewetting of the thin Au film and interdiffusion at the interface, through molten zones due to thermal spike induced by Ar ions.

Effect of Ti substitution on multiferroic properties of BiMn2O5

We present here the magnetic, dielectric, and electronic structural properties of Ti doped BiMn2O5 multiferroic materials. The x-ray diffraction (XRD) studies of BiMn2−xTixO5 (0≤x≤0.5) indicate no structural change up to x=0.5, though the lattice parameters (a and c) increase with increasing value of x. Dielectric constant and magnetization data show the evolution of new dielectric anomalies at ∼120 K and weak magnetic feature at ∼86 K with the Ti substitution compared to that of undoped one. However, Ti replacement vanishes the ferroelectric transition of BiMn2O5 at ∼35 K and gradually suppresses the antiferromagnetic ordering at ∼39 K. Polarization of Bi 6s2 lone pair electrons is attributed to the reason behind new dielectric anomalies, whereas new weak magnetic feature at ∼86 K is attributed to strong spin-phonon coupling. X-ray absorption spectroscopy (XAS) studies on O K, Mn K, L3,2, and Ti L3,2 edges of BiMn2−xTixO5 samples along with the reference compounds have been performed and compared to best...