Hangil Lee

Quantum confinement-induced tunable exciton states in graphene oxide.

Graphene oxide has recently been considered to be a potential replacement for cadmium-based quantum dots due to its expected high fluorescence. Although previously reported, the origin of the luminescence in graphene oxide is still controversial. Here, we report the presence of core/valence excitons in graphene-based materials, a basic ingredient for optical devices, induced by quantum confinement. Electron confinement in the unreacted graphitic regions of graphene oxide was probed by high resolution X-ray absorption near edge structure spectroscopy and first-principles calculations. Using experiments and simulations, we were able to tune the core/valence exciton energy by manipulating the size of graphitic regions through the degree of oxidation. The binding energy of an exciton in highly oxidized graphene oxide is similar to that in organic electroluminescent materials. These results open the possibility of graphene oxide-based optoelectronic device technology.

Surface property change of graphene using nitrogen ion

We introduced nitrogen ions to modify the graphene surface and its property changes were investigated. A graphene layer grown on 6H-SiC(0001) was irradiated with 100 eV nitrogen ions. Surface property changes were studied using photoemission spectroscopy (PES), near edge x-ray adsorption spectroscopy (NEXAFS), and atomic force microscopy(AFM). N 1s core level spectra show that three kinds of nitrogen species, nitrogen gas, graphite-like and pyridine-like nitrogen were induced on the nitrogen ion implanted graphene surface.

The adsorption configuration of valine on Ge(100).

The interaction between organic (bio) molecules and semiconductor surfaces has been studied by several research groups for the development of applications in a variety of fields. In particular, recent studies on the interaction between semiconductor surfaces and amino acids have laid the foundations for understanding protein adsorption on semiconductor surfaces. Understanding the behavior of amino acids on semiconductor surfaces, therefore, assists in the development of biosensors, bioartificial organs, biochips, and medical implants. Amino acids contain several groups attached to the acarbon: a carboxyl group, an amino group, a hydrogen atom, and a variable side chain (the R-group). Although the adsorption structures of glycine and alanine (in which the R-groups are hydrogen and methyl, respectively) on Group IV semiconductor surfaces have been wellcharacterized, no systematic examinations of the adsorption of valine (in which the R-group is isopropyl group, Figure 1) have been carried out. Because saturated alkyl groups (such as methyl, ethyl, and propyl groups) in various organic molecules are generally unreactive toward Group IV semiconductor surfaces, we expected that the isopropyl group of valine would also be inert on semiconductor surfaces. However, even if the isopropyl group itself is unreactive, its size may influence the adsorption structure of valine molecules on surfaces, similar to observations of the adsorption of amino acids with large saturated R-groups onto conductive surfaces. For example, the adsorption structures of glycine on Cu(111) surfaces fall into two classes (flat-lying and unidentate conformations), while the adsorption geometry of leucine (with an isobutyl R-group) converges to only the flat-lying configuration on same surface. Moreover, Wang et al. reported that two types of adsorption structures were found for valine adsorbed onto a Cu(111) electrode in aqueous solution, whereas only one adsorption geometry is observed for leucine and phenylalanine (with a benzyl R-group) adsorbed onto a Cu(111) electrode in same condition. In these studies, the authors posit that the inert side chains of leucine and phenylalanine have different interaction energies with the substrate than glycine and valine do, leading to the stabilization of the flat-lying configuration. Although these investigations examined amino acid adsorption on the Cu(111) surface, the results suggest that the inert R-groups of amino acids play a critical role in determining amino acid adsorption geometries on metal surfaces. Recently, our group reported that the adsorption geometry of glycine molecules on a Ge(100) semiconductor surface is an “intrarow O H dissociated and N dative bonded structure” based on scanning tunneling microscopy, density functional theory calculations, and high-resolution core-level photoemission spectroscopy (HRCLPES). Furthermore, as shown recently by HRCLPES experiments, the adsorption structure of alanine on the Ge(100) surface is the “intrarow O H dissociated and N dative bonded structure” at low initial coverage, while an “O H dissociation structure” also appears with this adsorption structure at higher coverage [over 0.10 monolayer (ML)] . Despite the analogous molecular structures of glycine and alanine, these dissimilar phenomena can be induced by either the side chain or the molecule’s own character. Therefore, a study of the adsorption configuration of valine on the Ge(100) surface is the next step in assessing the effects of the side chain on the adsorption geometry. Without a side chain effect in the system of valine, the adsorption structures of alanine on the Ge(100) surface are expected to result from its molecular nature. Herein, the adsorption geometry of valine molecules on the Ge(100) surface is investigated by measuring four core-level spectra (Ge3d, C1s, N1s, and O1s) using HRCLPES. To our knowledge, the adsorption structure of this system has not previously been characterized systematically. Figure 2 displays the Ge3d, C1s, N1s, and O1s core-level spectra of 0.20 ML valine on a Ge(100) surface at 300 K. After confirming the clean Ge(100) surface, which contains three well-defined features that were assigned to the bulk Ge atoms (B), the subsurface Ge atoms (S’), the up-atoms of asymmetric Ge dimers (S), as the valine molecules were deposited. As shown in Figure 2a, the adsorption of valine on the Ge(100) surface resulted in the emergence of two new peaks (marked Ge1=29.7 eV and Ge2=30.0 eV). In the HRCLPES spectrum, the spectral peaks of atoms that are more positive than other atoms of the same element are shifted to higher binding energies. The Pauling electronegativities (PEs) for nitrogen (PE= 3.1), oxygen (PE=3.6), and germanium (PE=2.0) atoms imply that Ge in the Ge OOC unit is more positive than Ge in the Ge N unit. Therefore, we assigned the two new peaks that [a] Y.-S. Youn, Prof. Dr. S. Kim Department of Chemistry Molecular-Level Interface Research Center, KAIST 373-1 Guseong-dong, Yuseong-gu, Daejeon, 305-701 (Korea) Fax: (+82)42-350-2810 E-mail : sehun-kim@kaist.ac.kr [b] Prof. Dr. H. Lee Department of Chemistry Sookmyung Women’s University 52 Hyochangwon-gil, Yongsan-gu, Seoul, 140-742 (Korea) Fax: (+82)2-2077-7321 E-mail : easyscan@sookmyung.ac.kr Figure 1. Molecular structure of valine. Italics indicate the peak assignment labels shown in Figure 2.

Tunable Magnetic Interaction at the Atomic Scale in Oxide Heterostructures

We report on a systematic study of a number of structurally identical but chemically distinct transition metal oxides in order to determine how the material-specific properties such as the composition and the strain affect the properties at the interface of heterostructures. Our study considers a series of structures containing two layers of ferromagnetic SrRuO₃, with antiferromagnetic insulating manganites sandwiched in between. The results demonstrate how to control the strength and relative orientation of interfacial ferromagnetism in correlated electron materials by means of valence state variation and substrate-induced strain, respectively.

Fruit and vegetable consumption and hypertriglyceridemia: Korean National Health and Nutrition Examination Surveys (KNHANES) 2007-2009.

Background:Limited research has been conducted on the association between intake of fruits and vegetables and hypertriglyceridemia, especially in Asian populations. This study aimed to investigate the association between total fruit and vegetable intake, as well as subgroups of fruit and vegetable intake, with hypertriglyceridemia among Korean adults.Methods:We conducted a cross-sectional study of 7934 adults aged 19–64 years from the fourth Korean Health and Nutrition Examination Survey. Fruit and vegetable intake was estimated from a food frequency questionnaire. Subgroups of fruits and vegetables included citrus, non-citrus and carotene-rich fruits and cruciferous, green leafy and carotene-rich vegetables. Hypertriglyceridemia (plasma triglyceride ⩾150 mg/dl) was diagnosed using a blood sample drawn after 12+ hours of fasting.Results:There were 2001 (25.2%) cases of hypertriglyceridemia among the participants. Total fruit intake was significantly inversely associated with the prevalence of hypertriglyceridemia; the multivariate odds ratios (95% confidence intervals) of hypertriglyceridemia across increasing quintiles were 1.00 (ref), 0.76 (0.62, 0.92), 0.72 (0.58, 0.90), 0.68 (0.54, 0.85) and 0.64 (0.49, 0.82; Ptrend=0.001) after controlling for survey year, body mass index, waist circumference, smoking, alcohol drinking, physical activity, education and income. Similar inverse associations were found for all fruit subgroups. However, we found no significant association between intakes of total or subgroups of vegetable and hypertriglyceridemia; the odds ratio for top vs bottom quintile was 1.00 (0.81–1.24) for total vegetable intake.Conclusions:Our findings support a potential beneficial role of fruit consumption to reduce blood triglyceride levels in Asian populations.

Determining the Catalytic Activity of Transition Metal-Doped TiO 2 Nanoparticles Using Surface Spectroscopic Analysis

The modified TiO2 nanoparticles (NPs) to enhance their catalytic activities by doping them with the five transition metals (Cr, Mn, Fe, Co, and Ni) have been investigated using various surface analysis techniques such as scanning electron microscopy (SEM), Raman spectroscopy, scanning transmission X-ray microscopy (STXM), and high-resolution photoemission spectroscopy (HRPES). To compare catalytic activities of these transition metal-doped TiO2 nanoparticles (TM-TiO2) with those of TiO2 NPs, we monitored their performances in the catalytic oxidation of 2-aminothiophenol (2-ATP) by using HRPES and on the oxidation of 2-ATP in aqueous solution by taking electrochemistry (EC) measurements. As a result, we clearly investigate that the increased defect structures induced by the doped transition metal are closely correlated with the enhancement of catalytic activities of TiO2 NPs and confirm that Fe- and Co-doped TiO2 NPs can act as efficient catalysts.

Facile and green cinchonidine-assisted synthesis of ultrafine and well-dispersed palladium nanoparticles supported on activated carbon with high catalytic performance

We report the facile and green synthesis of activated carbon-supported palladium (Pd/AC) containing homogeneously dispersed Pd nanoparticles (Pd NPs) by using eco-friendly and naturally available cinchonidine (CD) as the capping agent. The Pd NPs in the synthesized Pd/AC hybrid are uniform with sizes predominantly in the range 4 to 7 nm. The synthesized Pd/AC was characterized with various methods, such as TEM, XRD, and XPS, and the influence of the synthetic conditions on its properties were investigated. The advantages of CD over conventional capping agents include its easy depletion after the synthesis with a simple rinsing process. Owing to the ultrafine, well-dispersed and purified Pd NPs, the synthesized hybrid exhibits excellent catalytic activities in the reduction of 4-nitrophenol and methylene blue. These findings further the development of novel stabilizing agents from naturally available sources for the preparation of heterogeneous catalysts with enhanced performance.

Confirmation of the coexistence of two tautomers of 2-mercaptothiazoline on the Ge(100) surface†

We confirmed the coverage dependent variation of tautomers of 2-mercaptothiazoline (the thiolate and thione forms) adsorbed on the Ge(100) surface under UHV conditions by using HRXPS measurements in conjunction with the DFT calculation method, which was studied before only in aqueous systems. The C 1s, S 2p, and N 1s core-level spectra obtained using HRXPS revealed the simultaneous presence of two distinct adsorption structures in different proportions at both low (0.15 ML) and high (0.65 ML) coverages. Moreover, we modelled the adsorption structures and geometric configurations of the bond states of 2-mercaptothiazoline on the Ge(100) surface by using the DFT calculation method, and found that the S dative bonded structure is the most stable adsorption structure for the thione form of 2-mercaptothiazoline and that the S-H dissociated-N dative bonded structure is the most stable adsorption structure for the thiolate form.

Enhancement of Catalytic Activity of Reduced Graphene Oxide Via Transition Metal Doping Strategy

To compare the catalytic oxidation activities of reduced graphene oxide (rGO) and rGO samples doped with five different transition metals (TM-rGO), we determine their effects on the oxidation of L-cysteine (Cys) in aqueous solution by performing electrochemistry (EC) measurements and on the photocatalytic oxidation of Cys by using high-resolution photoemission spectroscopy (HRPES) under UV illumination. Our results show that Cr-, Fe-, and Co-doped rGO with 3+ charge states (stable oxide forms: Cr3+, Fe3+, and Co3+) exhibit enhanced catalytic activities that are due to the charge states of the doped metal ions as we compare them with Cr-, Fe-, and Co-doped rGO with 2+ charge states.Graphical AbstractThe SEM images and the corresponding EC measurements for (a) Cr3+, (b) Fe3+, and (c) Co3+ doped rGO

The aniline-to-azobenzene oxidation reaction on monolayer graphene or graphene oxide surfaces fabricated by benzoic acid

The oxidation of aniline to azobenzene was conducted in the presence of either monolayer graphene (EG) or graphene-oxide-like surface, such as GOx, under ultra-high vacuum conditions maintaining a 365-nm UV light exposure to enhance the oxidation reaction. The surface-bound products were investigated using micro Raman spectroscopy, high-resolution photoemission spectroscopy, and work function measurements. The oxygen carriers present on the GOx surfaces, but not on the EG surfaces, acted as reaction reagents to facilitate the oxidation reaction from aniline to azobenzene. Increasing the aniline concentration at 300 K confirmed that the exchange ratio from the aniline to the azobenzene was enhanced, as determined by the intensity ratio between the aniline- and azobenzene-induced N 1 s core-level spectra. The work function changed dramatically as the aniline concentration increased, indicating that the aniline on the GOx surface conveyed n-type doping characteristics at a low coverage level. A higher aniline concentration increased the p-type doping character by increasing the azobenzene concentration on the GOx surface. A comparison of the oxidation reactivity of aniline molecules on the EG or GOx surfaces revealed the role of the oxygen carriers on the GOx surfaces in the context of catalytic oxidation.