Showing papers by "Kwang S. Kim published in 2015"
TL;DR: In this article, the CO2-interactions with various functional molecules including multi-N-containing superbases and heteroaromatic ring systems are investigated using density functional theory (DFT) with dispersion correction and high level wave function theory (resolution-of-identity (RI) spin-component-scaling (scs) Moller-Plesset second-order perturbation theory (MP2) and coupled cluster with single, double and perturbative triple excitations (CCSD(T))).
Abstract: The CO2 capturing and sequestration are of importance in environmental science. Understanding of the CO2-interactions with various functional molecules including multi-N-containing superbases and heteroaromatic ring systems is essential for designing novel materials to effectively capture the CO2 gas. These interactions are investigated using density functional theory (DFT) with dispersion correction and high level wave function theory (resolution-of-identity (RI) spin-component-scaling (scs) Moller-Plesset second-order perturbation theory (MP2) and coupled cluster with single, double and perturbative triple excitations (CCSD(T))). We found intriguing molecular systems of melamine, 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), 7-azaindole and guanidine, which show much stronger CO2 interactions than the well-known functional systems such as amines. In particular, melamine could be exploited to design novel materials to capture the CO2 gas, since one CO2 molecule can be coordinated by four melamine molecules, which gives a binding energy (BE) of ∼85 kJ mol(-1), much larger than in other cases.
100 citations
TL;DR: Structural, dynamical, and response properties of liquid water calculated by ab initio molecular dynamics using the embedded-fragment spin-component-scaled second-order many-body perturbation method with the aug-cc-pVDZ basis set are presented.
Abstract: A direct, simultaneous calculation of properties of a liquid using an ab initio electron-correlated theory has long been unthinkable. Here we present structural, dynamical, and response properties of liquid water calculated by ab initio molecular dynamics using the embedded-fragment spin-component-scaled second-order many-body perturbation method with the aug-cc-pVDZ basis set. This level of theory is chosen as it accurately and inexpensively reproduces the water dimer potential energy surface from the coupled-cluster singles, doubles, and noniterative triples with the aug-cc-pVQZ basis set, which is nearly exact. The calculated radial distribution function, self-diffusion coefficient, coordinate number, and dipole moment, as well as the infrared and Raman spectra are in excellent agreement with experimental results. The shapes and widths of the OH stretching bands in the infrared and Raman spectra and their isotropic-anisotropic Raman noncoincidence, which reflect the diverse local hydrogen-bond environment, are also reproduced computationally. The simulation also reveals intriguing dynamic features of the environment, which are difficult to probe experimentally, such as a surprisingly large fluctuation in the coordination number and the detailed mechanism by which the hydrogen donating water molecules move across the first and second shells, thereby causing this fluctuation.
90 citations
TL;DR: In this paper, a propeller-shaped triazine was used to synthesize microporous polycarbazole materials through an inexpensive FeCl3-catalyzed reaction using direct oxidative coupling and extensive cross-linking (PCBZL) polymerization routes.
Abstract: Propeller-shaped triazine was used to synthesize microporous polycarbazole materials through an inexpensive FeCl3-catalyzed reaction using direct oxidative coupling (PCBZ) and extensive cross-linking (PCBZL) polymerization routes. PCBZL has a Brunauer–Emmett–Teller specific surface area of 424 m2 g–1 and shows larger CO2 uptake (64.1 mg g–1 at 273 K, 1 atm). Selective adsorption of CO2 over N2 calculated using the ideal adsorbed solution theory shows that both PCBZ (125) and PCBZL (148) exhibit selectivity at 298 K, which is significantly higher than PCBZ (110) and PCBZL (82) at 273 K. These values of selectivity are among the highest reported for any triazine-based microporous material. By introducing the electron-rich carbazole structure into the nitrogen fertile triazine-based system, the adsorption enthalpy is increased drastically, which in turn contributes to high selective adsorption values. The larger existing binding energy between CO2 and propeller specifies more stable and favorable interaction...
74 citations
TL;DR: Pentacene (C22H14), a polycyclic aromatic hydrocarbon, was used as both supporting and sacrificing layers for the clean and doping-free graphene transfer and exhibited extremely homogeneous surface potential profiles over a large area.
Abstract: Pentacene (C22H14), a polycyclic aromatic hydrocarbon, was used as both supporting and sacrificing layers for the clean and doping-free graphene transfer. After successful transfer of graphene to a target substrate, the pentacene layer was physically removed from the graphene surface by using intercalating organic solvent. This solvent-mediated removal of pentacene from graphene surface was investigated by both theoretical calculation and experimental studies with various solvents. The uses of pentacene and appropriate intercalation solvent enabled graphene transfer without forming a residue from the supporting layer. Such residues tend to cause charged impurity scattering and unintentional graphene doping effects. As a result, this clean graphene exhibited extremely homogeneous surface potential profiles over a large area. A field-effect transistor fabricated using this graphene displayed a high hole (electron) mobility of 8050 cm2/V·s (9940 cm2/V·s) with a nearly zero Dirac point voltage.
60 citations
TL;DR: At low temperatures, the material has two distinct types with low and high surface areas; however, activation at elevated temperatures drives off the low surface area carbon, leaving behind the porous high surface area activated carbon.
Abstract: An activated carbon material derived from waste coffee grounds is shown to be an effective and stable medium for methane storage. The sample activated at 900 °C displays a surface area of 1040.3 m(2) g(-1) and a micropore volume of 0.574 cm(3) g(-1) and exhibits a stable CH4 adsorption capacity of ∼4.2 mmol g(-1) at 3.0 MPa and a temperature range of 298 ± 10 K. The same material exhibits an impressive hydrogen storage capacity of 1.75 wt% as well at 77 K and 100 kPa. Here, we also propose a mechanism for the formation of activated carbon from spent coffee grounds. At low temperatures, the material has two distinct types with low and high surface areas; however, activation at elevated temperatures drives off the low surface area carbon, leaving behind the porous high surface area activated carbon.
47 citations
TL;DR: N-Type doping of mixed single- and double-layer graphene grown by chemical vapor deposition (CVD) using decamethyl-cobaltocene reveals a local-quasilinear relationship between the work function and the logarithm of the dopant solution concentration.
Abstract: n-Type doping of mixed single- and double-layer graphene grown by chemical vapor deposition (CVD) using decamethyl-cobaltocene reveals a local-quasilinear relationship between the work function and the logarithm of the dopant solution concentration. The relationship that arises from bandgap opening is deduced by comparing the relationship between the two factors for single- or double-layer graphene. This work has extensive applicability and practical significance in doping CVD-grown graphene.
44 citations
TL;DR: In this article, the electronic and magnetic structures of MoS2 nanotubes were studied by using a first-principles method and various kinds of defects such as substitution and vacancy were examined for triggering spin magnetic moments toward one-dimensional diluted magnetic semiconductors.
Abstract: We have studied the electronic and magnetic structures of MoS2 nanotubes by using a first-principles method. Various kinds of defects such as substitution and vacancy are examined for triggering spin magnetic moments toward one-dimensional diluted magnetic semiconductors. Our results suggest that the presence of impurity states within the energy gap and its large contribution to the density of states at the Fermi level are the key factors in inducing a magnetic moment. In particular, the nanotube curvature turns out to affect the energy level of impurity states, which can be exploited for tailoring magnetic properties. Also, 3d transition metal impurities (V, Mn, Fe, and Co atoms) on a Mo site can create large magnetic moments.
39 citations
TL;DR: In this paper, the response function of graphene is calculated in the presence of a constant current across the sample, for small drift velocities and finite chemical potential, analytic expressions are obtained and consequences on the plasmonic excitations are discussed.
Abstract: The response function of graphene is calculated in the presence of a constant current across the sample. For small drift velocities and finite chemical potential, analytic expressions are obtained and consequences on the plasmonic excitations are discussed. For general drift velocities and zero chemical potential, numerical results are presented and a plasmon gain region is identified that is related to interband transitions.
38 citations
TL;DR: It is reported that the π-electrons of graphene can be spin-polarized to create a phase with a significant spin-orbit gap at the Dirac point (DP) using a graphene-interfaced topological insulator hybrid material.
Abstract: We report that the π-electrons of graphene can be spin-polarized to create a phase with a significant spin-orbit gap at the Dirac point (DP) using a graphene-interfaced topological insulator hybrid material. We have grown epitaxial Bi2Te2Se (BTS) films on a chemical vapor deposition (CVD) graphene. We observe two linear surface bands from both the CVD graphene notably flattened and BTS coexisting with their DPs separated by 0.53 eV in the photoemission data measured with synchrotron photons. We further demonstrate that the separation between the two DPs, Δ(D-D), can be artificially fine-tuned by adjusting the amount of Cs atoms adsorbed on the graphene to a value as small as Δ(D-D) = 0.12 eV to find any proximity effect induced by the DPs. Our density functional theory calculation shows the opening of a spin-orbit gap of ∼20 meV in the π-band, enhanced by 3 orders of magnitude from that of a pristine graphene, and a concomitant phase transition from a semimetallic to a quantum spin Hall phase when Δ(D-D) ≤ 0.20 eV. We thus present a practical means of spin-polarizing the π-band of graphene, which can be pivotal to advance graphene-based spintronics.
37 citations
TL;DR: The aim of this article is to offer high level QM reference data based on coupled-cluster singles and doubles calculations with perturbative triple excitations, CCSD(T), and a complete basis set limit estimate that can be used to assess the accuracy of various DFT-based predictions.
Abstract: Hydrogen storage in carbonaceous materials and their derivatives is currently a widely investigated topic. The rational design of novel adsorptive materials is often attempted with the help of computational chemistry tools, in particular density functional theory (DFT). However, different exchange–correlation functionals provide a very wide range of hydrogen binding energies. The aim of this article is to offer high level QM reference data based on coupled-cluster singles and doubles calculations with perturbative triple excitations, CCSD(T), and a complete basis set limit estimate that can be used to assess the accuracy of various DFT-based predictions. For one complex, the CCSD(T) result is verified against diffusion quantum Monte Carlo calculations. Reference binding curves are calculated for two model compounds representing weak and strong hydrogen adsorption: coronene (−4.7 kJ mol−1 per H2), and coronene modified with boron and lithium (−14.3 kJ mol−1). The reference data are compared to results obtained with widely used density functionals including pure DFT, M06, DFT-D3, PBE-TS, PBE + MBD, optB88-vdW, vdW-DF, vdW-DF2 and VV10. We find that whereas DFT-D3 shows excellent results for weak hydrogen adsorption on coronene, most of the less empirical density based dispersion functionals except VV10 overestimate this interaction. On the other hand, some of the less empirical density based dispersion functionals better describe stronger binding in the more polar coroB2Li2⋯2H2 complex which is one of realistic models for high-capacity hydrogen storage materials. Our results may serve as a guide for choosing suitable DFT methods for quickly evaluating hydrogen binding potential and as a reference for assessing the accuracy of the previously published DFT results.
36 citations
TL;DR: In this issue of ACS Nano, He et al. report an in situ heating experiment in aberration-corrected transmission electron microscopy to elucidate the temperature dependence of graphene edge termination at the atomic scale, and reveal that graphene edges predominantly have zigzag terminations below 400 °C, while above 600 ° C, the edges are dominated by armchair and reconstructedZigzag edges.
Abstract: The atomic configuration of graphene edges significantly influences the various properties of graphene nanostructures, and realistic device fabrication requires precise engineering of graphene edges. However, the imaging and analysis of the intrinsic nature of graphene edges can be illusive due to contamination problems and measurement-induced structural changes to graphene edges. In this issue of ACS Nano, He et al. report an in situ heating experiment in aberration-corrected transmission electron microscopy to elucidate the temperature dependence of graphene edge termination at the atomic scale. They revealed that graphene edges predominantly have zigzag terminations below 400 °C, while above 600 °C, the edges are dominated by armchair and reconstructed zigzag edges. This report brings us one step closer to the true nature of graphene edges. In this Perspective, we outline the present understanding, issues, and future challenges faced in the field of graphene-edge-based nanodevices.
TL;DR: In this article, it was shown that the hydrogenation of a single crystal $2H\text{\ensuremath{-}}{\mathrm{MoS}}_{2}$ induces a novel intermediate phase between 2H and 1T phases on its surface, i.e., the large area, uniform, robust, and surface array of atomic stripes through the intralayer atomic-plane gliding.
Abstract: We report that the hydrogenation of a single crystal $2H\text{\ensuremath{-}}{\mathrm{MoS}}_{2}$ induces a novel-intermediate phase between 2H and 1T phases on its surface, i.e., the large-area, uniform, robust, and surface array of atomic stripes through the intralayer atomic-plane gliding. The total energy calculations confirm that the hydrogenation-induced atomic stripes are energetically most stable on the ${\mathrm{MoS}}_{2}$ surface between the semiconducting 2H and metallic 1T phase. Furthermore, the electronic states associated with the hydrogen ions, which is bonded to sulfur anions on both sides of the ${\mathrm{MoS}}_{2}$ surface layer, appear in the vicinity of the Fermi level $({E}_{\mathrm{F}})$ and reduces the band gap. This is promising in developing the monolayer-based field-effect transistor or vanishing the Schottky barrier for practical applications.
TL;DR: It is illustrated that both C and O atom-ends show negative EP (where the C end gives more negative EP), favoring positively charged species, whereas the cylindrical surface of the CO bond shows positive EP, favoring negatively charged ones.
Abstract: The strong electronegativity of O dictates that the ground state of singlet CO has positively charged C and negatively charged O, in agreement with ab initio charge analysis, but in disagreement with the dipole direction. Though this unusual phenomenon has been fairly studied, the study of electrostatic potential (EP) for noncovalent interactions of CO is essential for better understanding. Here we illustrate that both C and O atom-ends show negative EP (where the C end gives more negative EP), favoring positively charged species, whereas the cylindrical surface of the CO bond shows positive EP, favoring negatively charged ones. This is demonstrated from the interactions of CO with Na(+), Cl(-), H2O, CO and benzene. It can be explained by the quadrupole driven electrostatic nature of CO (like N2) with very weak dipole moment. The EP is properly described by the tripole model taking into account the electrostatic multipole moments, which has a large negative charge at a certain distance protruded from C, a large positive charge on C, and a small negative charge on O. We also discuss the EP of the first excited triplet CO.
TL;DR: It is shown that the flexible nature of the transient pathways leads to the temperature-driven reversible CO2 sorption, understanding of which can contribute to the design of a system with controlled capture/release of gas molecules.
Abstract: Crystallographic observation of adsorbed gas molecules is a highly difficult task due to their rapid motion. Here, we report the in situ single-crystal and synchrotron powder X-ray observations of reversible CO2 sorption processes in an apparently nonporous organic crystal under varying pressures at high temperatures. The host material is formed by hydrogen bond network between 1,3,5-tris-(4-carboxyphenyl)benzene (H3BTB) and N,N-dimethylformamide (DMF) and by π–π stacking between the H3BTB moieties. The material can be viewed as a well-ordered array of cages, which are tight packed with each other so that the cages are inaccessible from outside. Thus, the host is practically nonporous. Despite the absence of permanent pathways connecting the empty cages, they are permeable to CO2 at high temperatures due to thermally activated molecular gating, and the weakly confined CO2 molecules in the cages allow direct detection by in situ single-crystal X-ray diffraction at 323 K. Variable-temperature in situ synchrotron powder X-ray diffraction studies also show that the CO2 sorption is reversible and driven by temperature increase. Solid-state magic angle spinning NMR defines the interactions of CO2 with the organic framework and dynamic motion of CO2 in cages. The reversible sorption is attributed to the dynamic motion of the DMF molecules combined with the axial motions/angular fluctuations of CO2 (a series of transient opening/closing of compartments enabling CO2 molecule passage), as revealed from NMR and simulations. This temperature-driven transient molecular gating can store gaseous molecules in ordered arrays toward unique collective properties and release them for ready use.
TL;DR: It is found that the conduction band edge is mainly dominated by surface atoms, and that a larger number of surface atoms for the tube is likely to increase the bandwidth, thus reducing the optical bandgap.
Abstract: We investigate nontrivial surface effects on the optical properties of self-assembled crystalline GaN nanotubes grown on Si substrates. The excitonic emission is observed to redshift by ∼100 meV with respect to that of bulk GaN. We find that the conduction band edge is mainly dominated by surface atoms, and that a larger number of surface atoms for the tube is likely to increase the bandwidth, thus reducing the optical bandgap. The experimental findings can have important impacts in the understanding of the role of surfaces in nanostructured semiconductors with an enhanced surface/volume ratio.
TL;DR: Imaging of HeLa cells observed by confocal fluorescence microscopy reveals that probe 1 can be used to monitor Cr(3+) in live cells to map its subcellular distribution, andexceptional selectivity of probe 1 with Cr( 3+) over Fe(3-) and other metals has been confirmed by theoretical studies in addition to experimental results.
Abstract: Pyrene-based turn-on ratiometric fluorescent probe 1 demonstrates high sensitivity and exceptional selectivity toward Cr(3+) in the presence of other metals, including Fe(3+) in aqueous media. Interaction of Cr(3+) with probe 1 brings pyrene moieties close enough to have better aligned π-π stacking, thus enhancing the excimer peak many fold. On the other hand, the interaction of Fe(3+) with probe 1 brings forth a negligible difference in stacking, resulting in an insignificant change in fluorescence intensity. Exceptional selectivity of probe 1 with Cr(3+) over Fe(3+) and other metals has been confirmed by theoretical studies in addition to experimental results. Imaging of HeLa cells observed by confocal fluorescence microscopy reveals that probe 1 can be used to monitor Cr(3+) in live cells to map its subcellular distribution.
TL;DR: In this paper, the second-order Moller-Plesset theory was used to describe the properties of small even-numbered AunO2-clusters and showed that Au5− is a chemically O2-adsorbed singlet state at 0 K, against a commonly accepted physisorbed triplet state.
Abstract: Most density functionals do not properly describe the characteristics of superoxide (O2–) (i.e., first two vertical electron detachment energies and the excitation energies of neutralized singlet state) of small even-numbered AunO2– clusters. However, the second-order Moller–Plesset theory (MP2) shows significant charge transfer from Au cluster anions to oxygen molecule and so provides proper electronic characteristics of superoxide of small even-numbered AunO2– clusters. This has allowed us to properly describe the properties of even-numbered AunO2– clusters. Even in the case of odd-numbered AunO2– clusters, we find that Au5– is a chemically O2-adsorbed singlet state at 0 K, against a commonly accepted physisorbed triplet state. This is further evidenced by our extensive coupled cluster with single, double, and perturbative triple excitations [CCSD(T)] calculations, including the relativistic effect. However, the entropy effect makes the physisorbed triplet state more stable than the chemisorbed singlet ...
TL;DR: In this paper, non-coplanar shaped carbazole based monomers were used to synthesize microporous polycarbazole materials utilizing an inexpensive FeCl3 catalyzed reaction.
Abstract: Non-coplanar shaped carbazole based monomers were used to synthesize microporous polycarbazole materials utilizing an inexpensive FeCl3 catalyzed reaction. The reactions proceed through direct oxidative coupling and extensive crosslinking polymerization routes. The obtained porous networks exhibit a maximum Brunauer–Emmett–Teller specific surface area of 946 m2 g−1 with a total pore volume of 0.941 cm3 g−1, and display a high carbon dioxide uptake capacity (up to 13.6 wt%) at 273 K and 1 atm. Selective adsorption of CO2 over N2 calculated using the ideal adsorbed solution theory (IAST) shows that these networks display enhanced selectivity with a maximum value of 155 at 298 K. Remarkably, in contrast to other materials, this value is significantly higher than the selectivity values (102–107) obtained at 273 K. Introduction of the electron rich carbazole structure into the aromatic system and pore geometry contribute to higher adsorption enthalpy which in turn leads to high selective adsorption values. These polymeric networks also show a high working capacity with reasonably high regenerability factors. The combination of a simple inexpensive synthesis approach and high selective adsorption make these materials potential candidates for CO2 storage, selective gas adsorption, and other environmental applications.
TL;DR: This work elucidate the unusual features and origin of the anisotropic noncovalent interactions in the ground and excited states of the 2nd and 3rd row elements belonging to groups IV–VII and provides an understanding of their unusual molecular configuration, binding and recognition modes involved in new types of molecular assembling and engineering.
Abstract: Anisotropic Charge Distribution and Anisotropic van der Waals Radius Leading to Intriguing Anisotropic Noncovalent Interactions
TL;DR: One-pot synthesis of graphene/amorphous carbon (a-C) heterostructures from a solid source of polystyrene via selective photo-cross-linking process is reported, confirming the reliable quality of graphene and well-defined graphene/ a-C interface.
Abstract: Precise graphene patterning is of critical importance for tailor-made and sophisticated two-dimensional nanoelectronic and optical devices. However, graphene-based heterostructures have been grown by delicate multistep chemical vapor deposition methods, limiting preparation of versatile heterostructures. Here, we report one-pot synthesis of graphene/amorphous carbon (a-C) heterostructures from a solid source of polystyrene via selective photo-cross-linking process. Graphene is successfully grown from neat polystyrene regions, while patterned cross-linked polystyrene regions turn into a-C because of a large difference in their thermal stability. Since the electrical resistance of a-C is at least 2 orders of magnitude higher than that for graphene, the charge transport in graphene/a-C heterostructure occurs through the graphene region. Measurement of the quantum Hall effect in graphene/a-C lateral heterostructures clearly confirms the reliable quality of graphene and well-defined graphene/a-C interface. The...
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TL;DR: In this article, the π-electrons of graphene were spin-polarized to create a phase with a significant spin-orbit gap at the Dirac point (DP) using a graphene-interfaced topological insulator hybrid material.
Abstract: We report that the {\pi}-electrons of graphene can be spin-polarized to create a phase with a significant spin-orbit gap at the Dirac point (DP) using a graphene-interfaced topological insulator hybrid material. We have grown epitaxial Bi2Te2Se (BTS) films on a chemical vapor deposition (CVD) graphene. We observe two linear surface bands both from the CVD graphene notably flattened and BTS coexisting with their DPs separated by 0.53 eV in the photoemission data measured with synchrotron photons. We further demonstrate that the separation between the two DPs, {\Delta}D-D, can be artificially fine-tuned by adjusting the amount of Cs atoms adsorbed on the graphene to a value as small as {\Delta}D-D = 0.12 eV to find any proximity effect induced by the DPs. Our density functional theory calculation shows a spin-orbit gap of ~20 meV in the {\pi}-band enhanced by three orders of magnitude from that of a pristine graphene, and a concomitant phase transition from a semi-metallic to a quantum spin Hall phase when {\Delta}D-D $\leq$ 0.20 eV. We thus present a practical means of spin-polarizing the {\pi}-band of graphene, which can be pivotal to advance the graphene-based spintronics.
TL;DR: H2S2O7 requires only two water molecules, the fewest number of water molecules for deprotonation among various hydrated monomeric acids reported so far, which implies that the decomposition leading to H2SO4 and SO3 hardly occurs prior to the 2nd depotonation at low temperatures.
Abstract: We have studied geometries, energies and vibrational spectra of disulfuric acid (H2S2O7) and its anion (HS2O7(-)) hydrated by a few water molecules, using density functional theory (M062X) and ab initio theory (SCS-MP2 and CCSD(T)). The most noteworthy result is found in H2S2O7(H2O)2 in which the lowest energy conformer shows deprotonated H2S2O7. Thus, H2S2O7 requires only two water molecules, the fewest number of water molecules for deprotonation among various hydrated monomeric acids reported so far. Even the second deprotonation of the first deprotonated species HS2O7(-) needs only four water molecules. The deprotonation is supported by vibration spectra, in which acid O-H stretching peaks disappear and specific three O-H stretching peaks for H3O(+) (eigen structure) appear. We have also kept track of variations in several geometrical parameters, atomic charges, and hybrid orbital characters upon addition of water. As the number of water molecules added increases, the S-O bond weakens in the case of H2S2O7, but strengthens in the case of HS2O7(-). It implies that the decomposition leading to H2SO4 and SO3 hardly occurs prior to the 2nd deprotonation at low temperatures.