Persistent Topological Surface State at the Interface of Bi₂Se₃ Film Grown on Patterned Graphene

Kim, Namdong,Lee, Paengro,Kim, Youngwook,Kim, Jun Sung,Kim, Yongsam,Noh, Do Young,Yu, Seong Uk,Chung, Jinwook,Kim, Kwang S. American Chemical Society 2014 ACS NANO Vol.8 No.2

<P>We employed graphene as a patternable template to protect the intrinsic surface states of thin films of topological insulators (TIs) from environment. Here we find that the graphene provides high-quality interface so that the Shubnikov de Haas (SdH) oscillation associated with a topological surface state could be observed at the interface of a metallic Bi<SUB>2</SUB>Se<SUB>3</SUB> film with a carrier density higher than ∼10<SUP>19</SUP> cm<SUP>–3</SUP>. Our <I>in situ</I> X-ray diffraction study shows that the Bi<SUB>2</SUB>Se<SUB>3</SUB> film grows epitaxially in a quintuple layer-by-layer fashion from the bottom layer without any structural distortion by interfacial strain. The magnetotransport measurements including SdH oscillations stemming from multiple conductance channels reveal that the topological surface state, with the mobility as high as ∼0.5 m<SUP>2</SUP>/(V s), remains intact from the graphene underneath without degradation. Given that the graphene was prepatterned on arbitrary insulating substrates, the TI-based microelectronic design could be exploited. Our study thus provides a step forward to observe the topological surface states at the interface without degradation by tuning the interface between TI and graphene into a measurable current for device application.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/ancac3/2014/ancac3.2014.8.issue-2/nn405503k/production/images/medium/nn-2013-05503k_0006.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nn405503k'>ACS Electronic Supporting Info</A></P>

Synthesis and Electrical Characterization of Magnetic Bilayer Graphene Intercalate

Kim, Namdong,Kim, Kwang S.,Jung, Naeyoung,Brus, Louis,Kim, Philip American Chemical Society 2011 Nano letters Vol.11 No.2

<P>We report synthesis and transport properties of the minimal graphite intercalation compound, a ferric chloride (FeCl<SUB>3</SUB>)<SUB><I>n</I></SUB> island monolayer inside bilayer graphene. Chemical doping by the intercalant is simultaneously probed by micro-Raman spectroscopy and Hall measurements. Quantum oscillations of conductivity originate from microscopic domains of intercalated and unintercalated regions. A slight upturn in resistance related to magnetic transition is observed. Two-dimensional intercalation in bilayer graphene opens new possibilities to engineer two-dimensional properties of intercalates.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/nalefd/2011/nalefd.2011.11.issue-2/nl104228f/production/images/medium/nl-2010-04228f_0006.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nl104228f'>ACS Electronic Supporting Info</A></P>

Friction and conductance imaging of sp2- and sp3-hybridized subdomains on single-layer graphene oxide

Lee, Hyunsoo,Son, Narae,Jeong, Hu Young,Kim, Tae Gun,Bang, Gyeong Sook,Kim, Jong Yun,Shim, Gi Woong,Goddeti, Kalyan C.,Kim, Jong Hun,Kim, Namdong,Shin, Hyun-Joon,Kim, Wondong,Kim, Sehun,Choi, Sung-Yoo The Royal Society of Chemistry 2016 Nanoscale Vol.8 No.7

<P>We investigated the subdomain structures of single-layer graphene oxide (GO) by characterizing local friction and conductance using conductive atomic force microscopy. Friction and conductance mapping showed that a single-layer GO flake has subdomains several tens to a few hundreds of nanometers in lateral size. The GO subdomains exhibited low friction (high conductance) in the sp(2)-rich phase and high friction (low conductance) in the sp(3)-rich phase. Current-voltage spectroscopy revealed that the local current flow in single-layer GO depends on the quantity of hydroxyl and carboxyl groups, and epoxy bridges within the 2-dimensional carbon layer. The presence of subdomains with different sp(2)/sp(3) carbon ratios on a GO flake was also confirmed by chemical mapping using scanning transmission X-ray microscopy. These results suggest that spatial mapping of the friction and conductance can be used to rapidly identify the composition of heterogeneous single-layer GO at nanometer scale, which is essential for understanding charge transport in nanoelectronic devices.</P>

Enhancement of Photo-Oxidation Activities Depending on Structural Distortion of Fe-Doped TiO ₂ Nanoparticles

Kim, Yeonwoo,Yang, Sena,Jeon, Eun Hee,Baik, Jaeyoon,Kim, Namdong,Kim, Hyun Sung,Lee, Hangil Springer US 2016 NANOSCALE RESEARCH LETTERS Vol.11 No.1

<P>To design a high-performance photocatalytic system with TiO<SUB>2</SUB>, it is necessary to reduce the bandgap and enhance the absorption efficiency. The reduction of the bandgap to the visible range was investigated with reference to the surface distortion of anatase TiO<SUB>2</SUB> nanoparticles induced by varying Fe doping concentrations. Fe-doped TiO<SUB>2</SUB> nanoparticles (Fe@TiO<SUB>2</SUB>) were synthesized by a hydrothermal method and analyzed by various surface analysis techniques such as transmission electron microscopy, Raman spectroscopy, X-ray diffraction, scanning transmission X-ray microscopy, and high-resolution photoemission spectroscopy. We observed that Fe doping over 5 wt.% gave rise to a distorted structure, i.e., Fe<SUB>2</SUB>Ti<SUB>3</SUB>O<SUB>9</SUB>, indicating numerous Ti<SUP>3+</SUP> and oxygen-vacancy sites. The Ti<SUP>3+</SUP> sites act as electron trap sites to deliver the electron to O<SUB>2</SUB> as well as introduce the dopant level inside the bandgap, resulting in a significant increase in the photocatalytic oxidation reaction of thiol (–SH) of 2-aminothiophenol to sulfonic acid (–SO<SUB>3</SUB>H) under ultraviolet and visible light illumination.</P><P><B>Electronic supplementary material</B></P><P>The online version of this article (doi:10.1186/s11671-016-1263-6) contains supplementary material, which is available to authorized users.</P>

In-Situ High-Temperature Heating Setup for Spectronanoscopy at Pohang Light Source

Mikang Kim,Namdong Kim,Hooman Hosseinkhannazer,Ki-Jeong Kim,Hyun-Joon Shin 한국진공학회(ASCT) 2022 Applied Science and Convergence Technology Vol.31 No.1

The scanning transmission X-ray microscope (STXM) beamline at the Pohang Light Source was recently setup for in-situ heating. In this setup, the STXM is operated normally with ~30 nm diameter focused X-rays, and the heating temperature is manipulated using control software. The upper limit and maximum heating rate of the heating temperature are 900℃ and 900℃/s, respectively, and the time taken to reach the target temperature is less than ~10 s. The heating setup was used in the following examples: step-like heating of soldering-material particles of up to 380℃, step-like heating of Au thin films of up to 900℃, and pulse-like heating of submicron FeOx particles at 900℃ for 20 s.

나노바이오와 에너지 소재를 위한 연 X-선 나노스코피 연구

김남동(Namdong Kim) 한국진공학회 2022 진공 이야기 Vol.9 No.1

Soft X-ray nanoscopy employing the scanning transmission X-ray microscope (STXM) in the Pohang Light Source (PLS-II), which can provide chemical structural information of various functional materials at tens of nanometer scale, has become a powerful study in analytical microscopic research. The nanoscopy beamline is operating currently at the optimum condition in its focused beam size ~30 nm and photon energy resolution < 0.1 eV in the soft X-ray energy range (200-1650 eV). This article presents a short review of the STXM and its applications of functional materials including Li-ion batteries. A brief discussion on the opportunities in the STXM applications is presented at the end.

Topological modification of the electronic structure by Bi-bilayers lying deep inside bulk Bi₂Se₃

Lee, Paengro,Kim, Jinwoong,Kim, Jin Gul,Ryu, Min-tae,Park, Hee-min,Kim, Namdong,Kim, Yongsam,Lee, Nam-Suk,Kioussis, Nicholas,Jhi, Seung-Hoon,Chung, Jinwook IOP 2016 Journal of physics, an Institute of Physics journa Vol.28 No.8

<P>We observe the modified surface states of an epitaxial thin film of a homologous series of (Bi<SUB>2</SUB>)<SUB>m</SUB>(Bi<SUB>2</SUB>Se<SUB>3</SUB>)<SUB>n</SUB>, as a topological insulator (TI), by angle-resolved photoemission spectroscopy measurements. A thin film with <I>m</I> : <I>n</I>  =  1 : 3 (Bi<SUB>8</SUB>Se<SUB>9</SUB>) has been grown with Bi<SUB>2</SUB> bilayers embedded every other three quintuple layers (QLs) of Bi<SUB>2</SUB>Se<SUB>3</SUB>. Despite the reduced dimension of continuous QLs due to the Bi<SUB>2</SUB> heterolayers, we find that the topological surface states stem from the inverted Bi and Se states and the topologically nontrivial structures are mainly based on the prototype of 3D TI Bi<SUB>2</SUB>Se<SUB>3</SUB> without affecting the overall topological order.</P>

Spatially Resolved Ni L<SUB>3-</SUB>, Co L<SUB>3-</SUB>, Mn L<SUB>3-</SUB>, and O K-edge Spectra Obtained from Pristine and Charged-State Li<SUB>1.0</SUB>Ni<SUB>0.88</SUB>Co<SUB>0.08</SUB>Mn<SUB>0.04</SUB>O₂ Cathode Materials Prepared by Focused-Ion-Beam Method

HyunJoon Shin,Mikang Kim,Namdong Kim,Hyeong-Do Kim,Changhoon Jung,JaeGwan Chung,KiHong Kim,Woo Sung Jeon 한국진공학회(ASCT) 2020 Applied Science and Convergence Technology Vol.29 No.4

Pristine and 4.35 V charged-state particles of the Li1.0Ni0.88Co0.08Mn0.04O₂ lithium–ion battery (LIB) cathode material were cross-sectioned by the focused-ion-beam method to allow the acquisition of spatially resolved Ni L₃-edge, Co L₃-edge, Mn L₃-edge, and O K-edge X-ray absorption spectra by scanning transmission X-ray microscopy with a spatial resolution of ~30 ㎚. The Co L₃-edge and the Mn L₃-edge spectra, respectively, displayed almost the same features throughout the particles for both the pristine-state and charged-state samples. The average oxidation states of the Co and Mn ions were estimated to be ~3+ and ~4+, respectively. The Ni L₃-edge and O K-edge spectra included different features, depending on the charge state and intra-particle location. The estimated average oxidation state of the Ni ions ranged from ~2.7+ to ~3.0+ for the pristine-state sample (space-averaged value: ~2.9+) and from ~2.7+ to ~3.5+ for the charged-state sample (space-averaged value: ~3.3+). A correlation was observed between the changes in the features of the O K-edge and Ni L₃-edge. These results would provide important implications for the development of high-performance LIBs.

Fast benchtop visualization of graphene grain boundaries using adhesive properties of defects

Yu, Seong Uk,Cho, Yeonchoo,Park, Beomjin,Kim, Namdong,Youn, Il Seung,Son, Minhyeok,Kim, Jin Kon,Choi, Hee Cheul,Kim, Kwang S. The Royal Society of Chemistry 2013 Chemical communications Vol.49 No.48

<P>Highly adhesive properties of graphene grain boundaries to permanganate lead to a very quick, easy and convenient method to visualize the grain boundaries simply using an optical microscope, which would be vital to improve specific properties of graphene.</P> <P>Graphic Abstract</P><P>Adhesive properties of graphene grain boundaries to permanganate lead to a convenient method to visualize the grain boundaries using an optical microscope. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c3cc42464b'> </P>

Nanometer-scale local probing of X-ray absorption spectra of Co/Pt multilayer film

Quach, Duy-Truong,Pham, Duc-Thang,Handoko, Djati,Shim, Je-Ho,Eon Kim, Dong,Lee, Kyung-Min,Jeong, Jong-Ryul,Kim, Namdong,Shin, Hyun-Joon,Kim, Dong-Hyun Elsevier 2018 PHYSICA B-CONDENSED MATTER - Vol.532 No.-

<P><B>Abstract</B></P> <P>We report our local X-ray absorption spectra (XAS) measurement mapping for a Co/Pt multilayer using scanning transmission microscopy with 25-nm spatial resolution and 0.1-eV spectral resolution. We have systematically analyzed the two-dimensional XAS intensity variation over the corresponding magnetic domain patterns, revealing a XAS profile across the magnetic domain wall as well as the simultaneous high-throughput measurement of local XAS spectra.</P>