arXiv subjects

Young-Kyun Kwon

Publications and source records attributed to Young-Kyun Kwon.

At least 37 records · Page 2Linked to original sources

arXiv2020

Simple linear response model for predicting energy band alignment of two-dimensional vertical heterostructure

The Anderson and midgap models are often used in the study of semiconductor heterojunctions, but for van der Waals (vdW) vertical heterostructures they have shown only very limited success. Using the group-IV monochalcogenide vertical heterostructures as a prototypical system, we propose a linear response model and compare the effectiveness of these models in predicting density functional theory (DFT) band alignments, band types and bandgaps. We show that the DFT band alignment is best predicted by the linear response model, which falls in between the Anderson and midgap models. Our proposed model can be characterized by an interface dipole $α\times(E_{m2}-E_{m1})$, where the linear response coefficient $α$ = 0 and 1 corresponds to the Anderson and midgap model respectively, and $E_{m}$ is the midgap energy of the monolayer, which can be viewed as an effective electronegativity. For group-IV monochalcogenides, we show that $α$ = 0.34 best captures the DFT band alignment of the vdW heterostructure, and we discuss the viability of the linear response model considering other effects such as strains and band hybridization, and conclude with an application of the model to predict experimental band alignments.

cond-mat.mtrl-sci↗

arXiv2020

Effects of paramagnetic fluctuations on the thermochemistry of MnO (100) surfaces in the oxygen evolution reaction

We investigated the effects of paramagnetic (PM) fluctuations on the thermochemistry of the MnO(100) surface in the oxygen evolution reaction (OER) using the "noncollinear magnetic sampling method \textit{plus} $U$" (NCMSM$+U$). Various physical properties, such as the electronic structure, free energy, and charge occupation, of the MnO (100) surface in the PM state with several OER intermediates, were reckoned and compared to those in the antiferromagnetic (AFM) state. We found that PM fluctuation enhances charge transfer from a surface Mn ion to each of the intermediates and strengthens the chemical bond between them, while not altering the overall features, such as the rate determining step and resting state, in reaction pathways. The enhanced charge transfer can be attributed to the delocalized nature of valence bands observed in the PM surface. In addition, it was observed that chemical-bond enhancement depends on the intermediates, resulting in significant deviations in reaction energy barriers. Our study suggests that PM fluctuations play a significant role in the thermochemistry of chemical reactions occurring on correlated oxide surfaces.

cond-mat.mtrl-sci↗

arXiv2020

Versatile Physical Properties of a Novel Two-Dimensional Materials Composed of Group IV-V Elements

Owing to the fascinating physical characteristics of two-dimensional (2D) materials and their heterostructure, much effort has been devoted to exploring their basic physical properties as well as discovering other novel 2D materials. Herein, based on first-principles calculations, we propose novel 2D material groups with the form A$_2$B$_2$, composed of group IV (A = C, Si, Ge, or Sn) and V (B = N, P, As, Sb, or Bi) elements; the group forms two stable phases with the $P\bar{6}m2$ ($\mathcal{M}$ phase) and $P\bar{3}m1$ ($\mathcal{I}$ phase) crystal symmetries. We found that a total of 40 different freestanding A$_2$B$_2$ compounds were dynamically stable and displayed versatile physical properties, such as insulating, semiconducting, or metallic properties, depending on their elemental compositions. Our calculation results further revealed that the newly proposed 2D materials expressed high electrical and thermal transport properties. Additionally, some of the compounds that contained heavy elements exhibited non-trivial topological properties due to strong spin-orbit interactions.

cond-mat.mtrl-sci↗

arXiv2019

Symmetry-Protected Degeneracies in the Electronic Band Structure of Oxidized Black Phosphorous

We explore the oxidation of a single layer of black phosphorous using ab initio density functional theory calculation. We search for the equilibrium structures of phosphorene oxides, PO$_x$ with various oxygen concentrations $x$ ($0{\le}x{\le}1$). By evaluating the formation energies with diverse configurations and their vibrational properties for each of various $x$ values, we identify a series of stable oxidized structures with $x$ and confirm that the oxidation occurs naturally. We also find that oxidation makes some modes from the P-O bonds P-P bonds IR-active implying that the infrared spectra can be used to determine the degree of oxidation of phosphorene. Our electronic structure calculation reveals that the fully oxidized phosphorene (PO) has a direct band gap of 0.83 eV similar to the pristine phosphorene. Intriguingly, the PO possesses two nonsymmorphic symmetries with the inversion symmetry broken, guaranteeing a symmetry-protected band structures including the band degeneracy and four-fold degenerate Dirac points. Our results may provide a significant insight into the intriguing relations between symmetry of lattice and band topology of electrons.

cond-mat.mtrl-sci↗

arXiv2019

Symmetry dictated grain boundary state in a two-dimensional topological insulator

Structural imperfections such as grain boundaries (GBs) and dislocations are ubiquitous in solids and have been of central importance in understanding nature of polycrystals. In addition to their classical roles, advent of topological insulators (TIs) offers a chance to realize distinct topological states bound to them. Although dislocation inside three-dimensional TIs is one of the prime candidates to look for, its direct detection and characterization are challenging. Instead, in two-dimensional (2D) TIs, their creations and measurements are easier and, moreover, topological states at the GBs or dislocations intimately connect to their lattice symmetry. However, such roles of crystalline symmetries of GBs in 2D TIs have not been clearly measured yet. Here, we present the first direct evidence of a symmetry enforced Dirac type metallic state along a GB in 1T'-MoTe$_2$, a prototypical 2D TI. Using scanning tunneling microscope, we show a metallic state along a grain boundary with non-symmorphic lattice symmetry and its absence along the other boundary with symmorphic one. Our large scale atomistic simulations demonstrate hourglass like nodal-line semimetallic in-gap states for the former while the gap-opening for the latter, explaining our observation very well. The protected metallic state tightly linked to its crystal symmetry demonstrated here can be used to create stable metallic nanowire inside an insulator.

cond-mat.mes-hall↗

arXiv2019

Noncollinear magnetic sampling method for paramagnetic Mott insulator MnO

We present a new approach based on the static density functional theory (DFT) to describe paramagentic MnO, which is a representative paramagnetic Mott insulator. We appended the spin noncollinearity and the canonical ensemble to the magnetic sampling method (MSM), which is one of the supercell approaches based on disordered local moment model. The combination of the noncollinear MSM (NCMSM) with DFT$+U$ represents a highly favorable computational method called NCMSM$+U$ to accurately determine the paramagnetic properties of MnO with moderate numerical cost. The effects of electron correlations and spin noncollinearity on the properties of MnO were also investigated. We revealed that the spin noncollinearity plays an important role in determining the detailed electronic profile and precise energetics of paramagnetic MnO. Our results illustrate that the NCMSM$+U$ approach may be used as an alternative to the $\textit{ab initio}$ framework of dynamic mean field theory based on DFT in the simulation of the high-temperature properties of Mott insulators.

cond-mat.str-el↗

arXiv2019

Strain Effects on Phase Transitions in Transition Metal Dichalcogenides

We perform density functional theory calculation to investigate the structural and electronic properties of various two-dimensional transition metal dichalcogenides, MX$_2$ (M$=$Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, or W, and X$=$S or Se), and their strain-induced phase transitions. We evaluate the relative stability and the activation barrier between the octahedral-$T$ and the trigonal-$H$ phases of each MX$_2$. It is found that the equilibrium and phase transition characteristics of MX$_2$ can be classified by the group to which its metal element M belongs in the periodic table. MX$_2$ with M in the group 4 (Ti, Zr, or Hf), forms an octahedral-$T$ phase, while that with an M in the group 6 (Cr, Mo, or W) does a trigonal-\textit{H} phase. On the other hand, MX$_2$ with M in the group 5 (V, Nb, Ta), which is in-between the groups 4 and 6, may form either phase with a similar stability. It is also found that their electronic structures are strongly correlated to the structural configurations: mostly metallic in the $T$ phase, while semiconducting in the $H$ phase, although there are some exceptions. We also explore the effects of an applied stress and find for some MX$_2$ materials that the resultant strain, either tensile or compressive, may induce a structural phase transition by reducing the transition energy barrier, which is, in some cases, accompanied by its metal-insulator transition.

cond-mat.mtrl-sci↗

arXiv2019

Low Lattice Thermal Conductivity of a Two-Dimensional Phosphorene Oxide

A fundamental understanding of the phonon transport mechanism is important for optimizing the efficiency of thermoelectric devices. In this study, we investigate the thermal transport properties of the oxidized form of phosphorene called phosphorene oxide (PO) by solving phonon Boltzmann transport equation based on first-principles density functional theory. We reveal that PO exhibits a much lower thermal conductivity (2.42-7.08 W/mK, at 300 K) than its pristine counterpart as well as other two-dimensional materials. To comprehend the physical origin of such low thermal conductivity, we scrutinize the contribution of each phonon branch to the thermal conductivity by evaluating various mode-dependent quantities including Gruneisen parameters, anharmonic threep-honon scattering rate, phase space of three-phonon scattering processes. Our results show that its exible puckered structure of PO leads to smaller sound velocities; its broken-mirror symmetry allows more ZA phonon scattering; and the relatively-free vibration of dangling oxygen atoms in PO gives rise to additional scattering resulting in further reduction in the phonon lifetime. These results can be verified by the fact that PO has larger phase space for three-phonon processes than phosphorene. Furthermore we show that the thermal conductivity of PO can be optimized by controlling its size or its phonon mean free path, indicating that PO can be a promising candidate for low-dimensional thermoelectric devices.

cond-mat.mtrl-sci↗

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

Rigid Unit Modes in $sp$-$sp^2$ Hybridized Carbon Systems: Origin of Negative Thermal Expansion

Using density functional theory combined with quasi-harmonic approximation, we investigate the thermal expansion behaviors of three different types (α, β, and γ) of graphyne which is a two-dimensional carbon allotrope composed of sp and $sp^2$ bonds. For each type of graphyne, we obtain the temperature dependent area variation by minimizing its free energy as a function of temperature, which is calculated by considering all the phonon modes in the whole Brillouin zone. We find that all three types of graphyne exhibit negative in-plane thermal expansion up to $T\lesssim1000$ K. The observed in-plane thermal contraction can be attributed partially to the ripple effect, similarly in graphene. The ripple effect itself, however, is not sufficient to explain anomalously larger thermal contraction found in graphyne than in graphene. Our deliberate analysis on the phonon modes observed in graphyne enables us to discover another source causing such thermal expansion anomaly. We find that there are particular phonon modes with frequencies around a few hundreds of cm$^{-1}$ existing exclusively in graphyne that may fill empty spaces resulting in area reduction. These modes are identified as "rigid unit modes" corresponding to the libration of each rigid unit composed of $sp^2$ bonds.

cond-mat.mes-hall↗

arXiv2013

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↗

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/Åto +0.2 V/Åto 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

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↗

arXiv2010

Tunable charge donation and spin polarization of metal adsorbates on graphene using applied electric field

Metal atoms on graphene, when ionized, can act as a point charge impurity to probe a charge response of graphene with the Dirac cone band structure. To understand the microscopic physics of the metal-atom-induced charge and spin polarization in graphene, we present scanning tunneling spectroscopy (STS) simulations based on density functional theory calculations. We find that a Cs atom on graphene are fully ionized with a significant band bending feature in the STS, whereas the charge and magnetic states of Ba and La atoms on graphene appear to be complicated due to orbital hybridization and Coulomb interaction. By applying an external electric field, we observe changes in charge donations and spin magnetic moments of the metal adsorbates on graphene.

cond-mat.mtrl-sci↗

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

Band gap control of small bundles of carbon nanotubes using applied electric fields: A density functional theory study

Electrostatic screening between carbon nanotubes (CNTs) in a small CNT bundle leads to a switching behavior induced by electric field perpendicular to the bundle axis. Using a first-principles method, we investigate the electronic structures of bundles consisting of two or three CNTs and the effects of the electric field applied perpendicular to the bundle axis. The applied field causes band gap closure in semiconducting bundles, while a gap opening occurs in metallic ones, which enables considerable modulation of bundle conductivity. The modulation effect originates from symmetry breaking due to electrostatic screening between the adjacent tube walls.

cond-mat.mes-hall↗

arXiv2009

Designing rigid carbon foams

We use ab initio density functional calculations to study the stability, elastic properties and electronic structure of sp2 carbon minimal surfaces with negative Gaussian curvature, called schwarzites. We focus on two systems with cubic unit cells containing 152 and 200 carbon atoms, which are metallic and very rigid. The porous schwarzite structure allows for efficient and reversible doping by electron donors and acceptors, making it a promising candidate for the next generation of alkali ion batteries. We identify schwarzite structures that act as arrays of interconnected quantum spin dots or become magnetic when doped. We introduce two interpenetrating schwarzite structures that may find their use as the ultimate super-capacitor.

cond-mat.mes-hall↗

arXiv2002

Giant magneto-conductance in twisted carbon nanotubes

Using the Landauer-Buttiker formalism, we calculate the effect of structural twist on electron transport in conducting carbon nanotubes. We demonstrate that even a localized region of twist scatters the propagating pi electrons and induces the opening of a (pseudo-) gap near the Fermi level. The subsequent conductance reduction may be compensated by an applied axial magnetic field, leading to a twist-induced, giant positive magneto-conductance in clean armchair nanotubes.

cond-mat.mes-hall↗