Gunn Kim

Bandgap modulation of carbon nanotubes by encapsulated metallofullerenes

Motivated by the technical and economic difficulties in further miniaturizing silicon-based transistors with the present fabrication technologies, there is a strong effort to develop alternative electronic devices, based, for example, on single molecules. Recently, carbon nanotubes have been successfully used for nanometre-sized devices such as diodes, transistors, and random access memory cells. Such nanotube devices are usually very long compared to silicon-based transistors. Here we report a method for dividing a semiconductor nanotube into multiple quantum dots with lengths of about 10 nm by inserting Gd@C82 endohedral fullerenes. The spatial modulation of the nanotube electronic bandgap is observed with a low-temperature scanning tunnelling microscope. We find that a bandgap of ∼0.5 eV is narrowed down to ∼0.1 eV at sites where endohedral metallofullerenes are inserted. This change in bandgap can be explained by local elastic strain and charge transfer at metallofullerene sites. This technique for fabricating an array of quantum dots could be used for nano-electronics and nano-optoelectronics.

Reduction-Controlled Viologen in Bisolvent as an Environmentally Stable n-Type Dopant for Carbon Nanotubes

Various viologens have been used to control the doping of single-walled carbon nanotubes (SWCNTs) via direct redox reactions. A new method of extracting neutral viologen (V(0)) was introduced using a biphase of toluene and viologen-dissolved water. A reductant of sodium borohydride transferred positively charged viologen (V(2+)) into V(0), where the reduced V(0) was separated into toluene with high separation yield. This separated V(0) solution was dropped on carbon nanotube transistors to investigate the doping effect of CNTs. With a viologen concentration of 3 mM, all the p-type CNT transistors were converted to n-type with improved on/off ratios. This was achieved by donating electrons spontaneously to CNTs from neutral V(0), leaving energetically stable V(2+) on the nanotube surface again. The doped CNTs were stable in water due to the presence of hydrophobic V(0) at the outermost CNT transistors, which may act as a protecting layer to prevent further oxidation from water.

Impact of the Topology of Global Macroeconomic Network on the Spreading of Economic Crises

Throughout economic history, the global economy has experienced recurring crises. The persistent recurrence of such economic crises calls for an understanding of their generic features rather than treating them as singular events. The global economic system is a highly complex system and can best be viewed in terms of a network of interacting macroeconomic agents. In this regard, from the perspective of collective network dynamics, here we explore how the topology of the global macroeconomic network affects the patterns of spreading of economic crises. Using a simple toy model of crisis spreading, we demonstrate that an individual countrys role in crisis spreading is not only dependent on its gross macroeconomic capacities, but also on its local and global connectivity profile in the context of the world economic network. We find that on one hand clustering of weak links at the regional scale can significantly aggravate the spread of crises, but on the other hand the current network structure at the global scale harbors higher tolerance of extreme crises compared to more “globalized” random networks. These results suggest that there can be a potential hidden cost in the ongoing globalization movement towards establishing less-constrained, trans-regional economic links between countries, by increasing vulnerability of the global economic system to extreme crises.

Reversible Metal-Semiconductor Transition of ssDNA-Decorated Single-Walled Carbon Nanotubes

A field effect transistor (FET) measurement of a single-walled carbon nanotube (SWNT) shows a transition from a metallic one to a p-type semiconductor after helical wrapping of DNA. Water is found to be critical to activate this metal-semiconductor transition in the ssDNA-SWNT hybrid. Raman spectroscopy confirms the same change in electrical behaviors. According to our ab initio calculations, a band gap can open up in a metallic SWNT with wrapped ssDNA in the presence of water molecules due to charge transfer.

Deep levels in the band gap of the carbon nanotube with vacancy-related defects

We study the modification in the electronic structure of the carbon nanotube induced by vacancy-related defects using the first-principles calculation. Three defect configurations which are likely to occur in semiconducting carbon nanotubes are considered. A vacancy-adatom complex is found to bring about a pair of localized states deep inside the energy gap. A pentagon-octagon-pentagon topological defect produced by the divacancy is structurally stable and gives rise to an unoccupied localized state in the gap. We also discuss the character of partially occupied localized state produced by a substitutional impurity plus a monovacancy.

Selective Oxidation on Metallic Carbon Nanotubes by Halogen Oxoanions

Chlorine oxoanions with the chlorine atom at different oxidation states were introduced in an attempt to systematically tailor the electronic structures of single-walled carbon nanotubes (SWCNTs). The degree of selective oxidation was controlled systematically by the different oxidation state of the chlorine oxoanion. Selective suppression of the metallic SWCNTs with a minimal effect on the semiconducting SWCNTs was observed at a high oxidation state. The adsorption behavior and charge transfer at a low oxidation state were in contrast to that observed at a high oxidation state. Density functional calculations demonstrated the chemisorption of chloro oxoanions at the low oxidation state and their physisorption at high oxidation states. These results concurred with the experimental observations from X-ray photoelectron spectroscopy. The sheet resistance of the SWCNT film decreased significantly at high oxidation states, which was explained in terms of a p-doping phenomenon that is controlled by the oxidation state.

Formation of unconventional standing waves at graphene edges by valley mixing and pseudospin rotation

We investigate the roles of the pseudospin and the valley degeneracy in electron scattering at graphene edges. It is found that they are strongly correlated with charge density modulations of short-wavelength oscillations and slowly decaying beat patterns in the electronic density profile. Theoretical analyses using nearest-neighbor tight-binding methods and first-principles density-functional theory calculations agree well with our experimental data from scanning tunneling microscopy. The armchair edge shows almost perfect intervalley scattering with pseudospin invariance regardless of the presence of the hydrogen atom at the edge, whereas the zigzag edge only allows for intravalley scattering with the change in the pseudospin orientation. The effect of structural defects at the graphene edges is also discussed.Changwon Park, Heejun Yang, Andrew J. Mayne, Gérald Dujardin, Sunae Seo, Young Kuk, Jisoon Ihm, and Gunn Kim ∗ Department of Physics and Astronomy, Seoul National University, Seoul 151-747, Korea Semiconductor Devices Lab, Samsung Advanced Institute of Technology, Yongin, Gyeonggi-Do 449-712, Korea Laboratoire de Photophysique Moléculaire, CNRS, Bât. 210, Univ Paris Sud, 91405 Orsay, France Department of Physics, Sejong University, Seoul 143-747, Korea (Dated: January 11, 2013)

Interlayer coupling enhancement in graphene/hexagonal boron nitride heterostructures by intercalated defects or vacancies

Hexagonal boron nitride (hBN), a remarkable material with a two-dimensional atomic crystal structure, has the potential to fabricate heterostructures with unusual properties. We perform first-principles calculations to determine whether intercalated metal atoms and vacancies can mediate interfacial coupling and influence the structural and electronic properties of the graphene/hBN heterostructure. Metal impurity atoms (Li, K, Cr, Mn, Co, and Cu), acting as extrinsic defects between the graphene and hBN sheets, produce n-doped graphene. We also consider intrinsic vacancy defects and find that a boron monovacancy in hBN acts as a magnetic dopant for graphene, whereas a nitrogen monovacancy in hBN serves as a nonmagnetic dopant for graphene. In contrast, the smallest triangular vacancy defects in hBN are unlikely to result in significant changes in the electronic transport of graphene. Our findings reveal that a hBN layer with some vacancies or metal impurities enhances the interlayer coupling in the graphene/hBN heterostructure with respect to charge doping and electron scattering.

Hydrolysis-induced immobilization of Pt(acac)2 on polyimide-based carbon nanofiber mat and formation of Pt nanoparticles

Electrospun polyimide (PI)-based carbon nanofibers have recently garnered much interest due to their high conductivity and high mechanical strength. Promising applications include electrodes for supercapacitors, filters, sensors, and fuel cells. Here, we demonstrate that Pt nanoparticles can be loaded on the surface of PI nanofibers via an immobilization process induced by hydrolysis. The uniform distribution and sizes of Pt nanoparticles were controlled further by carbonization. Pt(acac)2 dissolved in acetone was impregnated on the hydrolyzed PI nanofibers. Pt ions were localized exclusively on the surface of PI nanofibers by precise control of the hydrolysis process. Our X-ray photoelectron spectroscopy results show that Pt ions in Pt(acac)2 molecules (40%) are immobilized on the hydrolyzed PI surface while some of them (60%) bind to O in the carboxylic group to form a PtO structure, and then are fully decomposed into Pt nanoparticles during carbonization. Using density functional calculations, we show that the binding of Pt(acac)2 on hydrolyzed PI is strong with a binding energy of −4.3 eV, which originates mostly from Pt–O binding and π-stacking between (acac) and PAA, confirming experimental observations of robust formation of Pt nanoparticles on hydrolyzed PI. The cyclic voltammetric test demonstrates that our robust carbon nanofiber mat can be utilized for fuel cell electrodes.

Enhancement of electrocatalytic activity of gold nanoparticles by sonochemical treatment

We demonstrate that gold nanoparticles can become catalytically active for the electrochemical hydrogen oxidation reaction by a sonication treatment. Experimental data and theoretical calculations indicate that the activity arises from the supercooled molten state of gold nanoparticles which are enriched with coordinatively unsaturated gold atoms.