Showing papers by "Kwang S. Kim published in 2012"
TL;DR: Approaches, Derivatives and Applications Vasilios Georgakilas,† Michal Otyepka,‡ Athanasios B. Bourlinos,† Vimlesh Chandra, Namdong Kim, K. Kim,§,⊥ Radek Zboril,*,‡ and Kwang S. Kim.
Abstract: Approaches, Derivatives and Applications Vasilios Georgakilas,† Michal Otyepka,‡ Athanasios B. Bourlinos,‡ Vimlesh Chandra, Namdong Kim, K. Christian Kemp, Pavel Hobza,‡,§,⊥ Radek Zboril,*,‡ and Kwang S. Kim* †Institute of Materials Science, NCSR “Demokritos”, Ag. Paraskevi Attikis, 15310 Athens, Greece ‡Regional Centre of Advanced Technologies and Materials, Department of Physical Chemistry, Faculty of Science, Palacky University Olomouc, 17. listopadu 12, 771 46 Olomouc, Czech Republic Center for Superfunctional Materials, Department of Chemistry, Pohang University of Science and Technology, San 31, Hyojadong, Namgu, Pohang 790-784, Korea Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, v.v.i., Flemingovo naḿ. 2, 166 10 Prague 6, Czech Republic
3,460 citations
TL;DR: In this paper, the authors focus on recent progress in advanced nanostructured materials (NSMs) as building blocks for EEDs (such as fuel cells, supercapacitors, and Li-ion batteries) based on investigations at the 0D, 1D, 2D and 3D NSMs.
845 citations
TL;DR: N-doped porous carbon produced via chemical activation of polypyrrole functionalized graphene sheets shows selective adsorption of CO over N(2) at 298 K, which has the potential for large scale production and facile regeneration.
302 citations
TL;DR: The RGO-SnO(2) composite showed an enhanced photocatalytic degradation activity for the organic dye methylene blue under sunlight compared to bare SnO( 2) nanoparticles, leading us to believe that the RGO/SnO-2 composite could be used in catalytic photodegradation of other organic dyes.
Abstract: Graphene sheets decorated with SnO(2) nanoparticles (RGO-SnO(2)) were prepared via a redox reaction between graphene oxide (GO) and SnCl(2). Graphene oxide (GO) was reduced to graphene (RGO) and Sn(2+) was oxidized to SnO(2) during the redox reaction, leading to a homogeneous distribution of SnO(2) nanoparticles on RGO sheets. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images show uniform distribution of the nanoparticles on the RGO surface and high-resolution transmission electron microscopy (HRTEM) shows an average particle size of 3-5 nm. The RGO-SnO(2) composite showed an enhanced photocatalytic degradation activity for the organic dye methylene blue under sunlight compared to bare SnO(2) nanoparticles. This result leads us to believe that the RGO-SnO(2) composite could be used in catalytic photodegradation of other organic dyes.
242 citations
TL;DR: A method to selectively fluorinate graphene by irradiating fluoropolymer-covered graphene with a laser, which leads to a dramatic increase in the resistance of the graphene while the basic skeletal structure of the carbon bonding network is maintained.
Abstract: We have devised a method to selectively fluorinate graphene by irradiating fluoropolymer-covered graphene with a laser. This fluoropolymer produces active fluorine radicals under laser irradiation that react with graphene but only in the laser-irradiated region. The kinetics of C–F bond formation is dependent on both the laser power and fluoropolymer thickness, proving that fluorination occurs by the decomposition of the fluoropolymer. Fluorination leads to a dramatic increase in the resistance of the graphene while the basic skeletal structure of the carbon bonding network is maintained. Considering the simplicity of the fluorination process and that it allows patterning with a nontoxic fluoropolymer as a solid source, this method could find application to generate fluorinated graphene in graphene-based electronic devices such as for the electrical isolation of graphene.
229 citations
TL;DR: This work unambiguously proves that it is possible to open a bandgap with two molecular dopants (F4-TCNQ and NH2 -functionalized self-assembled monolayers (SAMs) even in a single-gate device structure.
Abstract: Dual doping-driven perpendicular electric field with opposite directions remarkably increase the on/off current ratio of bilayer graphene field-effect transistors. This unambiguously proves that it is possible to open a bandgap with two molecular dopants (F4-TCNQ and NH2 -functionalized self-assembled monolayers (SAMs)) even in a single-gate device structure.
224 citations
TL;DR: A new method to simultaneously transfer and dope chemical vapor deposition grown graphene onto a target substrate using a fluoropolymer as both the supporting and doping layer is reported.
Abstract: Chemical doping can decrease sheet resistance of graphene while maintaining its high transparency. We report a new method to simultaneously transfer and dope chemical vapor deposition grown graphene onto a target substrate using a fluoropolymer as both the supporting and doping layer. Solvent was used to remove a significant fraction of the supporting fluoropolymer, but residual polymer remained that doped the graphene significantly. This contrasts with a more widely used supporting layer, polymethylmethacrylate, which does not induce significant doping during transfer. The fluoropolymer doping mechanism can be explained by the rearrangement of fluorine atoms on the graphene basal plane caused by either thermal annealing or soaking in solvent, which induces ordered dipole moments near the graphene surface. This simultaneous transfer and doping of the graphene with a fluoropolymer increases the carrier density significantly, and the resulting monolayer graphene film exhibits a sheet resistance of ∼320 Ω/sq...
117 citations
TL;DR: An assessment of a different supramolecular system - glycine anhydride interacting with an amide macrocycle - demonstrates that both the dynamical screening and the many-body dispersion energy are complex contributions that are very sensitive to the underlying molecular geometry and type of bonding.
Abstract: Supramolecular host–guest systems play an important role for a wide range of applications in chemistry and biology. The prediction of the stability of host–guest complexes represents a great challenge to first-principles calculations due to an interplay of a wide variety of covalent and noncovalent interactions in these systems. In particular, van der Waals (vdW) dispersion interactions frequently play a prominent role in determining the structure, stability, and function of supramolecular systems. On the basis of the widely used benchmark case of the buckyball catcher complex (C60@C60H28), we assess the feasibility of computing the binding energy of supramolecular host–guest complexes from first principles. Large-scale diffusion Monte Carlo (DMC) calculations are carried out to accurately determine the binding energy for the C60@C60H28 complex (26 ± 2 kcal/mol). On the basis of the DMC reference, we assess the accuracy of widely used and efficient density-functional theory (DFT) methods with dispersion i...
105 citations
TL;DR: This algorithm does away with the integral transformation as the hotspot of the usual algorithms, has a far superior size dependence of cost, does not suffer from the sign problem of some quantum Monte Carlo methods, and is potentially easily parallelizable and extensible to other more complex electron-correlation theories.
Abstract: With the aid of the Laplace transform, the canonical expression of the second-order many-body perturbation correction to an electronic energy is converted into the sum of two 13-dimensional integrals, the 12-dimensional parts of which are evaluated by Monte Carlo integration. Weight functions are identified that are analytically normalizable, are finite and non-negative everywhere, and share the same singularities as the integrands. They thus generate appropriate distributions of four-electron walkers via the Metropolis algorithm, yielding correlation energies of small molecules within a few mEh of the correct values after 108 Monte Carlo steps. This algorithm does away with the integral transformation as the hotspot of the usual algorithms, has a far superior size dependence of cost, does not suffer from the sign problem of some quantum Monte Carlo methods, and potentially easily parallelizable and extensible to other more complex electron-correlation theories.
76 citations
TL;DR: It is shown that marginal breakdown of the FS nesting is an essential condition to the spin-fluctuation mediated superconductivity, while the good FS nesting in NaFeAs induces a spin density wave ground state.
Abstract: Based on the dynamical mean field theory and angle resolved photoemission spectroscopy, we have investigated the mechanism of high ${T}_{c}$ superconductivity in stoichiometric LiFeAs. The calculated spectrum is in excellent agreement with the measured angle resolved photoemission spectroscopy. The Fermi surface (FS) nesting, which is predicted in the conventional density functional theory method, is suppressed due to the orbital-dependent correlation effect within the dynamical mean field theory method. We have shown that such marginal breakdown of the FS nesting is an essential condition to the spin-fluctuation mediated superconductivity, while the good FS nesting in NaFeAs induces a spin density wave ground state. Our results indicate that a fully charge self-consistent description of the correlation effect is crucial in the description of the FS nesting-driven instabilities.
63 citations
TL;DR: Fluorescence, (1)H-NMR spectra and ab initio calculations demonstrate that excimer formation and fluorescence enhancement occur upon GTP and ATP binding, respectively, through (C-H)(+)···A(-) hydrogen bond interactions.
TL;DR: Graphene-CdS nanowire (NW) hybrid structures with high-speed photoconductivity were developed in this article, which was comprised of CdS NWs which were selectively grown in specific regions on a single-layer graphene sheet.
Abstract: Graphene–CdS nanowire (NW) hybrid structures with high-speed photoconductivity were developed. The hybrid structure was comprised of CdS NWs which were selectively grown in specific regions on a single-layer graphene sheet. The photoconductive channels based on graphene–CdS NW hybrid structures exhibited much larger photocurrents than graphene-based channels and much faster recovery speed than CdS NW network-based ones. Our graphene–CdS NW structures can be useful because they were much faster than commercial CdS film-based photodetectors and had photocurrents large enough for practical applications.
TL;DR: The results indicate that the novel graphene-NP hybrid biosensor has the ability to detect concentrations of targeted enzyme on the micromolar scale and is the first demonstration of a graphene-based biosensor that utilizes a hysteresis change resulting from metallic NPs assembled on a graphene surface.
Abstract: Over the last few decades, the carbon nanomaterial-based electrical detection of chemical and biological molecules has gained much attention with its extensive applications to genomic, proteomic and environmental analysis as well as for clinical diagnosis.[1–12] Typical carbon nanomaterial-based electrical detection systems are composed of two components: a biological recognition element and a signal transduction element. These devices can detect minute concentrations of target molecules by measuring the change in electrical conductance that results from the binding of target molecules to the surface of the carbon nanomaterial.[13–17] However, the sensitivity as well as selectivity of these devices depends heavily on the electrical properties of the nanomaterial used in the system and on the interfacial chemistry that exists between the nanomaterial and the receptor molecules. Carbon nanotube field-effect transistors (CNT-FETs) currently lead the field and are one of the most extensively developed systems.[24–29] This is due to the excellent electrical properties and the relatively well-developed surface chemistry of CNTs. However, certain limitations do exist with CNT-based electrical detection devices. These include inconsistencies in the assembly of CNTs resulting in device unreliability, limits in its surface area, and a high-cost of fabrication.
TL;DR: The present results could provide a possible explanation of why on the Ni surface graphene tends to grow in a few layers, while on the Cu surface the weak interaction between the copper surface and graphene allows for the formation of a single layer of graphene, in agreement with chemical vapor deposition experiments.
Abstract: Structures of neutral metal–dibenzene complexes, M(C6H6)2 (M = Sc–Zn), are investigated by using Moller–Plesset second order perturbation theory (MP2). The benzene molecules change their conformation and shape upon complexation with the transition metals. We find two types of structures: (i) stacked forms for early transition metal complexes and (ii) distorted forms for late transition metal ones. The benzene molecules and the metal atom are bound together by δ bonds which originate from the interaction of π-MOs and d orbitals. The binding energy shows a maximum for Cr(C6H6)2, which obeys the 18-electron rule. It is noticeable that Mn(C6H6)2, a 19-electron complex, manages to have a stacked structure with an excess electron delocalized. For other late transition metal complexes having more than 19 electrons, the benzene molecules are bent or stray away from each other to reduce the electron density around a metal atom. For the early transition metals, the M(C6H6) complexes are found to be more weakly boun...
TL;DR: A new imidazolium-based fluorescent cyclophane 1 was designed and synthesized that was quenched selectively in the presence of iodide but not other anions, as assessed by fluorimetry.
Abstract: For your I's only: A new imidazolium-based fluorescent cyclophane 1 was designed and synthesized that was quenched selectively in the presence of iodide but not other anions, as assessed by fluorimetry. In addition, fluorescence titration experiments, 1H NMR spectroscopic data, and theoretical calculations provide evidence that 1 encapsulates two iodides inside its cavity.
TL;DR: The binding mechanism involves a conformational change of the two urea receptors to a trans orientation after binding of the first anion, which facilitates the second interaction.
Abstract: Cyclo-bis-(urea-3,6-dichlorocarbazole) (1) forms a 1 : 2 complex with CH3CO2− and H2PO4− through hydrogen bonding with the two urea moieties, resulting in fluorescence enhancement via a combined photoinduced electron transfer (PET) and energy transfer mechanism. The binding mechanism involves a conformational change of the two urea receptors to a trans orientation after binding of the first anion, which facilitates the second interaction.
TL;DR: This accurate structure prediction of small gold clusters corresponding to experimental photoelectron spectral peaks is valuable in the field of atom-scale materials science including nanocatalysts.
Abstract: Since gold clusters have mostly been studied theoretically by using DFT calculations, more accurate studies are of importance. Thus, small neutral and anionic gold clusters (Au(n) and Au(n)(-) , n = 4-7) were investigated by means of coupled cluster with singles, doubles, and perturbative triple excitations [CCSD(T)] calculations with large basis sets, and some differences between DFT and CCSD(T) results are discussed. Interesting isomeric structures that have dangling atoms were obtained. Structures having dangling atoms appear to be stable up to n = 4 for neutral gold clusters and up to n = 7 for anionic clusters. The relative stabilities and electronic properties of some isomers and major structures are discussed on the basis of the CCSD(T) calculations. This accurate structure prediction of small gold clusters corresponding to experimental photoelectron spectral peaks is valuable in the field of atom-scale materials science including nanocatalysts.
TL;DR: The designed organocatalysts for nitrogen fixation using density functional theory (DFT) and ab initio theory and revealed that the electron donating ability of the central carbene carbon is the most important factor for trapping N2.
Abstract: Nitrogen fixation is a great challenge in solving the food supply of mankind. However N2 activation is extremely hard. Up until recently, the investigated catalysts for N2 fixation were based on metallic reducing agents. They are generally not environment friendly. We designed organocatalysts (carbenes) for nitrogen fixation using density functional theory (DFT) and ab initio theory. The reactivity of the carbene catalysts is mainly related to the electrostatic properties of the side chains. We compared the binding affinity to N2 with various carbenes (:CF2, :CCl2, :CBr2, and :CI2). We revealed that the electron donating ability of the central carbene carbon is the most important factor for trapping N2. Among heterocyclic carbenes, the cyclic diphospinocarbenes (PHC) represented a good candidate moiety for an efficient catalyst. We further designed the carbene based catalyst which has two carbene moieties to chelate N2 and investigated the whole catalytic mechanism. The highest energy barrier of the entire catalytic cycle is 28.5 kcal/mol, which is comparable to the previously reported metallic catalysts. This demonstrates the possibility of novel organic catalysts for nitrogen fixation.
TL;DR: The extended electron-deficient arenes investigated as potential neutral receptors for polyanions exhibit strong binding and the best geometrical fit between guest and host, reminiscent of Lego blocks.
Abstract: Extended electron-deficient arenes are investigated as potential neutral receptors for polyanions. Anion binds via σ interaction with extended arenes, which are composed solely of C and N ring atoms and CN substituents. As a result, the positive charge on the aromatic C is enhanced, consequently maximizing binding strength. Selectivity is achieved because different charge distributions can be obtained for target anions of a particular geometry. The halides F(-) and Cl(-) form the most stable complex with 6, while the linear N3(-) interacts most favorably with 7. The trigonal NO3(-) and tetrahedral ClO4(-) fit the 3-fold rotational axis of 6 but do not form stable complexes with 5 and 7. The Y-shaped HCOO(-) forms complexes with 4, 5, and 7, with the latter being the most stable. Thus, the anion complexes exhibit strong binding and the best geometrical fit between guest and host, reminiscent of Lego blocks.
TL;DR: The proposed probe combines the advantages of the diffraction-limited focusing due to annular propagation of the plasmon with its nanofocusing by a tapered metal wedge (i.e. a metal film with reducing local thickness) to demonstrate strongly subwavelength resolution of the described structure.
Abstract: We propose and analyze a new type of mechanically robust optical nanofocusing probe with minimized external environmental interference. The probe consists of a dielectric optical fiber terminated by a dielectric hemisphere – both covered in thin gold film whose thickness is reduced (tapered) along the surface of the hemisphere toward its tip. Thus the proposed probe combines the advantages of the diffraction-limited focusing due to annular propagation of the plasmon with its nanofocusing by a tapered metal wedge (i.e. a metal film with reducing local thickness). The numerical finite-element analysis demonstrates strongly subwavelength resolution of the described structure with the achievable size of the focal spot of ~20 nm with up to ~150 times enhancement of the local electric field intensity. Detailed physical interpretations of the obtained results are presented and possible application as a new type of SNOM probe for subwavelength imaging, spectroscopy and sensing are also discussed.
TL;DR: In this paper, the authors employ a cavity QED approach to describe Raman lasing in microscopic spherical dielectric resonators with third-order nonlinearity to derive explicit expressions for the Raman gain and lasing threshold in terms of mode-overlapping coefficients determined from full analytic expressions of the electromagnetic eigenmodes of a dielectral sphere.
Abstract: Remarkable demonstrations of a cavity-induced giant reduction of the Raman lasing threshold in silica microspheres have been reported in Spillane et al. [Nature (London) 415, 621 (2002)], yet a complete quantum electrodynamics (QED) treatment of this process including the rigorous expressions for the electromagnetic modes of the sphere is lacking. In this paper, we employ a cavity QED approach to describe Raman lasing in microscopic spherical dielectric resonators with third-order nonlinearity to derive explicit expressions for the Raman gain and lasing threshold in terms of mode-overlapping coefficients determined from full analytic expressions of the electromagnetic eigenmodes of a dielectric sphere. We present dependencies of the Raman lasing threshold on the the order $n$ of overlapping whispering gallery modes for microspheres with diameter in the range 20--35 $\ensuremath{\mu}$m, demonstrating ultralow thresholds of between 70 and 90 $\ensuremath{\mu}$W, consistent with recent experimental results. We explain the reduction of the lasing threshold as due to an increase in the overlapping coefficients as the mode order is increased toward the regime where the modal energy is mostly confined to the surface region of the resonator. The presented theory can be easily generalized to any microcavity of regular morphology and for any combination of interacting whispering gallery modes.
TL;DR: The dependence between prime numbers and the real and imaginary parts of the zeros of the Riemann zeta function has been studied in this paper, where the Legendre polynomials and the partial derivatives have been used to investigate the above dependence.
Abstract: We study the dependence between prime numbers and the real and imaginary parts of the nontrivial zeros of the Riemann zeta function. The Legendre polynomials and the partial derivatives of the Riemann zeta function are used to investigate the above dependence along with the Riemann hypothesis with physical interpretations. A modified zeta function with finite terms is defined as a new implement for the study of the zeta function and its zeros.
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TL;DR: In this paper, the problem of analytic continuation of the Monte Carlo (MC) data to higher-order cumulants is considered in both classic and quantum domains, and the key result is the theorem that links the differential properties of thermal averages to the higher order cumulant.
Abstract: The Monte Carlo (MC) estimates of thermal averages are usually functions of system control parameters $\lambda $, such as temperature, volume, interaction couplings, etc. Given the MC average at a set of prescribed control parameters $\lambda_{0}$, the problem of analytic continuation of the MC data to $\lambda $-values in the neighborhood of $\lambda_{0}$ is considered in both classic and quantum domains. The key result is the theorem that links the differential properties of thermal averages to the higher-order cumulants. The theorem and analytic continuation formulas expressed via higher-order cumulants are numerically tested on the classical Lennard-Jones cluster system of N=13, 55, and 147 neon particles.
TL;DR: Observations of the uniform distribution and localization of these NPs show the possibility of further tuning their size and density by controlling periodically strained nanorod surfaces, and the formation of tensile strain in the NP.
Abstract: Semiconductor nanopyramids (NPs) provide advantages in the development of novel functional optoelectronic devices due to their unique size-dependent properties. Here we demonstrate a new method for the fabrication of selectively self-assembled single-crystalline GaN NPs on the m-plane of periodically strained GaN/InGaN multiquantum disks embedded in the middle of GaN nanorods. The GaN NPs, which have ~100 nm diameters and heights, are observed by scanning electron microscopy and their crystalline structure is confirmed by high-resolution transmission electron microscopy. Experimental analysis directly reveals the strain distribution along the growth direction of the NPs. Cathodoluminescence measurements on a single NP show that its emission energy redshifts compared with that of bulk GaN, corroborating the results showing the formation of tensile strain in the NP. Observations of the uniform distribution and localization of these NPs show the possibility of further tuning their size and density by controlling periodically strained nanorod surfaces.
TL;DR: The theorem that links the differential properties of thermal averages to the higher order cumulants and analytic continuation formulas expressed via higher ordercumulants are numerically tested on the classical Lennard-Jones cluster system of N=13, 55, and 147 neon particles.
Abstract: The Monte Carlo (MC) estimates of thermal averages are usually functions of system control parameters λ, such as temperature, volume, and interaction couplings. Given the MC average at a set of prescribed control parameters λ{0}, the problem of analytic continuation of the MC data to λ values in the neighborhood of λ{0} is considered in both classic and quantum domains. The key result is the theorem that links the differential properties of thermal averages to the higher order cumulants. The theorem and analytic continuation formulas expressed via higher order cumulants are numerically tested on the classical Lennard-Jones cluster system of N=13, 55, and 147 neon particles.