arXiv subjects

Gunn Kim

Publications and source records attributed to Gunn Kim.

At least 37 records · Page 2Linked to original sources

arXiv2020

Equilibrium structures of water molecules confined within a multiply connected carbon nanotube: a molecular dynamics study

Water confinement inside a carbon nanotube (CNT) has been one of the most exciting subjects of both experimental and theoretical interest. Most of the previous studies, however, considered CNT structures with simple cylindrical shapes. In this paper, we report a classical molecular dynamics study of the equilibrium structural arrangement of water molecules confined in a multiply connected carbon nanotube (MCCNT) containing two Y-junctions. We investigate the structural arrangement of the water molecules in the MCCNT in terms of the density of water molecules and the average number of hydrogen bonds per water molecule. Our results show that the structural rearrangement of the H2O molecules takes place several angstroms ahead of the Y-junction, rather than only at the CNT junction itself. This phenomenon arises because it is difficult to match the boundary condition for hydrogen bonding in the region where two different hydrogen-bonded structures are interconnected with each other.

cond-mat.mes-hall↗

arXiv2020

Edge-functionalization of armchair graphene nanoribbons with pentagonal-hexagonal edge structures

Using density functional theory calculations, we have studied the edge-functionalization of armchair graphene nanoribbons (AGNRs) with pentagonal-hexagonal edge structures. While the AGNRs with pentagonal-hexagonal edge structures (labeled (5,6)-AGNRs) are metallic, the edge-functionalized (5,6)-AGNRs with substitutional atoms opens a band gap. We find that the band structures of edge-functionalized (5,6)-N-AGNRs by substitution resemble those of defect-free (N-1)-AGNR at the {\Gamma} point, whereas those at the X point show the original ones of the defect-free N-AGNR. The overall electronic structures of edge-functionalized (5,6)-AGNRs depend on the number of electrons, supplied by substitutional atoms, at the edges of functionalized (5,6)-AGNRs.

cond-mat.mes-hall↗

arXiv2018

Dynamics of liquid crystal on hexagonal lattice

Nematic liquid crystal (LC) molecules adsorbed on two dimensional materials are aligned along the crystal directions of the hexagonal lattice. It was demonstrated that short electric pulses can reorient the aligned LC molecules in the preferred armchair direction of hexagonal boron nitride (h-BN). Several states with a variety of colors were obtained by changing the direction and strength of the electric pulses. The ab initio calculations based on density functional theory was carried out to determine the favorable adsorption configurations of the LC molecules on the h-BN surface. A non-volatile display, in which pixel resolution can be determined by grains of hexagonal surface, is proposed, which can offer a pathway towards dynamic high-quality pixels with low power consumption, and could define a new paradigm for all non-volatile display applications.

cond-mat.soft↗

arXiv2016

Zero-line modes at stacking faulted domain walls in multilayer graphene

Rhombohedral multilayer graphene is a physical realization of the chiral two-dimensional electron gas that can host zero-line modes (ZLMs), also known as kink states, when the local ap opened by inversion symmetry breaking potential changes sign in real space. Here we study how the variations in the local stacking coordination of multilayer graphene affects the formation of the ZLMs. Our analysis indicates that the valley Hall effect develops whenever an interlayer potential difference is able to open up a band gap in stacking faulted multilayer graphene, and that ZLMs can appear at the domain walls separating two distinct regions with imperfect rhombohedral stacking configurations. Based on a tight-binding formulation with distant hopping terms between carbon atoms, we first show that topologically distinct domains characterized by the valley Chern number are separated by a metallic region connecting AA and AA$'$ stacking line in the layer translation vector space. We find that gapless states appear at the interface between the two stacking faulted domains with different layer translation or with opposite perpendicular electric field if their valley Chern numbers are different.

cond-mat.mes-hall↗

arXiv2016

Ab initio Study of Aspirin Adsorption on Single-walled Carbon and Carbon Nitride Nanotubes

We use ab intio density functional theory to investigate the adsorption properties of acetylsalicylic acid or aspirin on a (10, 0) carbon nanotube (CNT) and a (8, 0) triazine-based graphitic carbon nitride nanotube (CNNT). It is found that an aspirin molecule binds stronger to the CNNT with its adsorption energy of 0.67 eV than to the CNT with 0.51 eV. The stronger adsorption energy on the CNNT is ascribed to the high reactivity of its N atoms with high electron affinity. The CNNT exhibits local electric dipole moments, which cause strong charge redistribution in the aspirin molecule adsorbed on the CNNT than on the CNT. We also explore the influence of an external electric field on the adsorption properties of aspirin on these nanotubes by examining the modifications in their electronic band structures, partial densities of states, and charge distributions. It is found that an electric field applied along a particular direction induces aspirin molecular states in the in-gap region of the CNNT implying a potential application of aspirin detection.

cond-mat.mtrl-sci↗

arXiv2015

Electronic properties of bilayer graphenes strongly coupled to interlayer stacking and an external electric field

Bilayer graphene (BLG) with a tunable bandgap appears interesting as an alternative to graphene for practical applications, thus its transport properties are being actively pursued. Using density functional theory and perturbation analysis, we investigated, under an external electric field, the electronic properties of BLGs in various stackings relevant to recently observed complex structures. We established the first phase diagram summarizing the stacking-dependent gap openings of BLGs for a given field. We further identified high-density midgap states, localized on grain boundaries, even under a strong field, which can considerably reduce overall transport gap.

cond-mat.mtrl-sci↗

arXiv2014

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

Among two-dimensional atomic crystals, hexagonal boron nitride (hBN) is one of the most remarkable materials to fabricate heterostructures revealing 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) 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 act as a magnetic dopant for graphene whereas a nitrogen monovacancy in hBN serves as a nonmagnetic dopant for graphene. In contrast, smallest triangular vacancy defects in hBN are unlikely to result in significant changes in the electronic transport of graphene. Our findings reveal that the hBN layer with some vacancies or metal impurities enhance the interlayer coupling in the graphene/hBN heterostructure with respect to charge doping and electron scattering.

cond-mat.mes-hall↗

arXiv2013

Decay behavior of localized states at reconstructed armchair graphene edges

Density functional theory calculations are used to investigate the electronic structures of localized states at reconstructed armchair graphene edges. We consider graphene nanoribbons with two different edge types and obtain the energy band structures and charge densities of the edge states. By examining the imaginary part of the wavevector in the forbidden energy region, we reveal the decay behavior of the wavefunctions in graphene. The complex band structures of graphene in the armchair and zigzag directions are presented in both tight-binding and first-principles frameworks.

cond-mat.mes-hall↗

arXiv2013

Carbon nanotubes with atomic impurities on boron nitride sheets under applied electric fields

We perform first-principles calculations to investigate the structural and electronic properties of metal-doped (10, 0) carbon nanotubes (CNTs) on a single hexagonal boron nitride (hBN) sheet in the presence of an external electric field. We consider K, Cl and Ni atoms as dopants to study the dependence of the electronic properties of the CNT on doping polarity and concentration. The electric field strength is varied from -0.2 V/\AA to +0.2 V/\AA to explore the effects of an external electric field on the electronic structures. Although the electronic energy bands of the hBN sheet are modified in accordance with the field strength, its electronic state in the valence or conduction band does not touch the Fermi level under the field strength considered. We conclude that the hBN as a substrate does not modify the electronic structure of the CNT, thereby leading to improvements in the device performance, compared with that of devices based on conventional substrate materials such as SiO$_2$.

cond-mat.mes-hall↗

arXiv2012

Molecular adsorption study of nicotine and caffeine on the single-walled carbon nanotube from first principles

Using first-principles calculations, we investigate the electronic structures and binding properties of nicotine and caffeine adsorbed on single-walled carbon nanotubes to determine whether CNTs are appropriate for filtering or sensing nicotine and caffeine molecules. We find that caffeine adsorbs more strongly than nicotine. The different binding characteristics are discussed by analyzing the modification of the electronic structure of the molecule-adsorbed CNTs. We also calculate the quantum conductance of the CNTs in the presence of nicotine or caffeine adsorbates and demonstrate that the influence of caffeine is stronger than nicotine on the conductance of the host CNT.

cond-mat.mes-hall↗

arXiv2012

Adsorption properties of a nitrogen atom on the anionic golden fullerene Au$_{16}^{-}$

Using density functional theory, we examine a nitrogen-doped anionic golden cage (NAu$_{16}^-$). For the exohedral adsorption that is more stable than the endohedral doping, the bridge and hollow sites have larger binding energies than the atop sites by ~1 eV. When the N atom is adsorbed on the cage, electrons are transferred from Au$_{16}^-$ to the N atom. The transition between the exohedral and endohedral adsorption may occur thermally through a bridge site. In the infrared active vibrational spectra, exohedral doping causes greater intensities at higher frequencies than endohedral doping.

cond-mat.mes-hall↗

arXiv2011

First-principles study of substitutional carbon pair and Stone-Wales defect complexes in boron nitride nanotubes

Using density functional theory, we study physical properties of boron nitride nanotubes (BNNTs) with the substitutional carbon pair defect. We also consider the Stone-Wales (SW) rearrangement of the C-C pair defect in the BNNT. The formation energy of an SW defect of the carbon dimer is approximately 3.1 eV lower than that of the SW-transformed B-N pair in the undoped BNNT. The activation energies show that the SW defect in the C-doped BNNT may be experimentally observed with a higher probability than in the undoped BNNT. Finally, we discuss the localized states originating from the carbon pair impurities.

cond-mat.mes-hall↗

arXiv2011

Origins of dihydrogen binding to metal-inserted porphyrins: electric polarization and Kubas interaction

Using density functional theory calculations, we have investigated the interactions between hydrogen molecules and metalloporphyrins. A metal atom such as Ca or Ti is introduced for incorporation in the central N$_4$ cavity. Within local density approximation (generalized gradient approximation), we find that the average binding energy of H$_2$ to the Ca atom is about 0.25 (0.1) eV/H$_2$ up to four H$_2$ molecules, whereas that to the Ti atom is about 0.6 (0.3) eV per H$_2$ up to two H$_2$. Our analysis of orbital hybridization between the inserted metal atom and molecular hydrogen shows that H$_2$ binds weakly to Ca-porphyrin through a weak electric polarization in dihydrogen, but is strongly hybridized with Ti-porphyrin through the Kubas interaction. The presence of $d$ orbitals in Ti may explain the difference in the interaction types.

cond-mat.mes-hall↗

arXiv2011

Pseudospin rotation and valley mixing in electron scattering at graphene edges

In graphene, the pseudospin and the valley flavor arise as new types of quantum degrees of freedom due to the honeycomb lattice comprising two sublattices (A and B) and two inequivalent Dirac points (K and K') in the Brillouin zone, respectively. Unique electronic properties of graphene result in striking phenomena such as Klein tunnelling, Veselago lens, and valley-polarized currents. Here, we investigate the roles of the pseudospin and the valley in electron scattering at graphene edges and show that they are strongly correlated with charge density modulations of short-wavelength oscillations and slowly-decaying beat patterns. Theoretical analyses using nearest-neighbor tight-binding methods and first-principles density-functional theory calculations agree well with our experimental data from the scanning tunneling microscopy. We believe that this study will lead to useful application of graphene to "valleytronics" and "pseudospintronics".

cond-mat.mes-hall↗

arXiv2010

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 global macroeconomic network affects the patterns of spreading of economic crises. Using a simple toy model of crisis spreading, we demonstrate that an individual country's 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 a 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 the vulnerability of global economic system to extreme crises.

q-fin.ST↗

arXiv2010

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

A field effect transistor (FET) measurement of a 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 SWNT-ssDNA hybrid. Raman spectroscopy confirms the same change in electrical behavior. 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.

cond-mat.mes-hall↗

arXiv2010

Electrical transport in small bundles of single-walled carbon nanotubes: intertube interaction and effects of tube deformation

We report a combined electronic transport and structural characterization study of small carbon nanotube bundles in field-effect transistors (FET). The atomic structures of the bundles are determined by electron diffraction using an observation window built in the FET. The single-walled nanotube bundles exhibit electrical transport characteristics sensitively dependent on the structure of individual tubes, their arrangements in the bundle, deformation due to intertube interaction, and the orientation with respect to the gate electric field. Our ab-initio simulation shows that tube deformation in the bundle induces a bandgap opening in a metallic tube. These results show the importance of intertube interaction in electrical transport of bundled nanotubes.

cond-mat.mes-hall↗

arXiv2010

Dissociation of ssDNA - Single-Walled Carbon Nanotube Hybrids by Watson-Crick Base Pairing

The unwrapping event of ssDNA from the SWNT during the Watson-Crick base paring is investigated through electrical and optical methods, and binding energy calculations. While the ssDNA-metallic SWNT hybrid shows the p-type semiconducting property, the hybridization product recovered metallic properties. The gel electrophoresis directly verifies the result of wrapping and unwrapping events which was also reflected to the Raman shifts. Our molecular dynamics simulations and binding energy calculations provide atomistic description for the pathway to this phenomenon. This nano-physical phenomenon will open up a new approach for nano-bio sensing of specific sequences with the advantages of efficient particle-based recognition, no labeling, and direct electrical detection which can be easily realized into a microfluidic chip format.

cond-mat.mes-hall↗