arXiv subjects

Kyeongjae Cho

Publications and source records attributed to Kyeongjae Cho.

31 records · Page 2Linked to original sources

arXiv2012

Reversible Superconductivity in Electrochromic Indium-Tin Oxide Films

Transparent conductive indium tin oxide (ITO) thin films, electrochemically intercalated with sodium or other cations, show tunable superconducting transitions with a maximum $T_c$ at 5 K. The transition temperature and the density of states, $D(E_F)$ (extracted from the measured Pauli susceptibility $χ_p$ exhibit the same dome shaped behavior as a function of electron density. Optimally intercalated samples have an upper critical field $\approx 4$ T and $Δ/{k_BT_c} \approx 2.0$. Accompanying the development of superconductivity, the films show a reversible electrochromic change from transparent to colored and are partially transparent (orange) at the peak of the superconducting dome. This reversible intercalation of alkali and alkali earth ions into thin ITO films opens diverse opportunities for tunable, optically transparent superconductors.

cond-mat.supr-con↗

arXiv2011

Thermal Properties of Isotopically Engineered Graphene

In addition to its exotic electronic properties graphene exhibits unusually high intrinsic thermal conductivity. The physics of phonons - the main heat carriers in graphene - was shown to be substantially different in two-dimensional (2D) crystals, such as graphene, than in three-dimensional (3D) graphite. Here, we report our experimental study of the isotope effects on the thermal properties of graphene. Isotopically modified graphene containing various percentages of 13C were synthesized by chemical vapor deposition (CVD). The regions of different isotopic composition were parts of the same graphene sheet to ensure uniformity in material parameters. The thermal conductivity, K, of isotopically pure 12C (0.01% 13C) graphene determined by the optothermal Raman technique, was higher than 4000 W/mK at the measured temperature Tm~320 K, and more than a factor of two higher than the value of K in a graphene sheets composed of a 50%-50% mixture of 12C and 13C. The experimental data agree well with our molecular dynamics (MD) simulations, corrected for the long-wavelength phonon contributions via the Klemens model. The experimental results are expected to stimulate further studies aimed at better understanding of thermal phenomena in 2D crystals.

cond-mat.mes-hall↗

arXiv2011

Computational studies for reduced graphene oxide in hydrogen-rich environment

We employ molecular dynamic simulations to study the reduction process of graphene-oxide (GO) in a chemically active environment enriched with hydrogen. We examine the concentration and pressure of hydrogen gas as a function of temperature in which abstraction of oxygen is possible with minimum damage to C-sp$^2$ bonds hence preserving the integrity of the graphene sheet. Through these studies we find chemical pathways that demonstrate beneficiary mechanisms for the quality of graphene including formation of water as well as suppression of carbonyl pair holes in favor of hydroxyl and epoxy formation facilitated by hydrogen gas in the environment.

cond-mat.mtrl-sci↗

arXiv2011

Field emission from atomically thin edges of reduced graphene oxide

Point sources exhibit low threshold electron emission due to local field enhancement at the tip. The development and implementation of tip emitters have been hampered by the need to position them sufficiently apart to achieve field enhancement, limiting the number of emission sites and therefore the overall current. Here we report low threshold field (< 0.1V/um) emission of multiple electron beams from atomically thin edges of reduced graphene oxide (rGO). Field emission microscopy (FEM) measurements show evidence for interference from emission sites that are separated by a few nanometers, suggesting that the emitted electron beams may be coherent. Based on our high-resolution transmission electron microscopy, infrared spectroscopy and simulation results, field emission from the rGO edge is attributed to a stable and unique aggregation of oxygen groups in the form of cyclic edge ethers. Such closely spaced electron beams from rGO offer prospects for novel applications and understanding the physics of linear electron sources.

cond-mat.mtrl-sci↗

arXiv2010

Isotope Effect on the Thermal Conductivity of Graphene

The thermal conductivity (TC) of isolated graphene with different concentrations of isotopes (C13) is studied with equilibrium molecular dynamics method at 300K. In the limit of pure C12 or C13 graphene, TC of graphene in zigzag and armchair directions are ~630 W/mK and ~1000W/mK, respectively. We find that the TC of graphene can be maximally reduced by ~80%, in both armchair and zigzag directions, when a random distribution of C12 and C13 is assumed at different doping concentrations. Therefore, our simulation results suggest an effective way to tune the TC of graphene without changing its atomic and electronic structure, thus yielding a promising application for nanoelectronics and thermoelectricity of graphene based nano-device.

cond-mat.mtrl-sci↗

arXiv2009

Ozone adsorption on graphene: ab initio study and experimental validation

We have investigated ozone adsorption on graphene using the ab initio density functional theory method. Ozone molecules adsorb on graphene basal plane with binding energy of 0.25 eV, and the physisorbed molecule can chemically react with graphene to form an epoxide group and an oxygen molecule. The activation energy barrier from physisorption to chemisorption is 0.72 eV, and the chemisorbed state has the binding energy of 0.33 eV. These binding energies and energy barrier indicate that the ozone adsorption on graphene is gentle and reversible. Atomic layer deposition experiment on ozone treated graphite has confirmed the presence of uniform hydrophilic groups on graphene basal plane. This finding can be applied to diverse chemical functionalization of graphene basal planes.

cond-mat.mtrl-sci↗

arXiv2006

First Principles Study of Work Functions of Double Wall Carbon Nanotubes

Using first-principles density functional calculations, we investigated work functions (WFs) of thin double-walled nanotubes (DWNTs) with outer tube diameters ranging from 1nm to 1.5nm. The results indicate that work function change within this diameter range can be up to 0.5 eV, even for DWNTs with same outer diameter. This is in contrast with single-walled nanotubes (SWNTs) which show negligible WF change for diameters larger than 1nm. We explain the WF change and related charge redistribution in DWNTs using charge equilibration model (CEM). The predicted work function variation of DWNTs indicates a potential difficulty in their nanoelectronic device applications.

cond-mat.mtrl-sci↗

arXiv2005

First Principles Study of Work Functions of Single Wall Carbon Nanotubes

We performed first principles calculations on work functions of single wall carbon nanotubes (SWNTs), which can be divided into two classes according to tube diameter (D). For class I tubes (D > 1 nm), work functions lie within a narrow distribution (~0.1 eV) and show no significant chirality or diameter dependence. For class II tubes (D < 1 nm), work functions show substantial changes, with armchair tubes decrease monotonously with diameter, while zigzag tubes show opposite trend. Surface dipoles and hybridization effects are shown to be responsible for the observed work function change.

cond-mat.mtrl-sci↗

arXiv2003

Point defect energetics in silicon using the LDA+U method

We present the first principles results of point defect energetics in silicon calculated using the LDA+U method: a Hubbard type on-site interaction added to the local density approximation (LDA). The on-site Coulomb and exchange parameters were tuned to match the experimental band gap in Si. The relaxed configuration was obtained using the LDA; LDA+U was used only to calculate the energies. Our values of point defect energetics are in very good agreement with both recent experimental results and quantum Monte Carlo (QMC) calculations.

cond-mat.mtrl-sci↗

arXiv2002

Patterned nanostructure in AgCo/Pt/MgO(001) thin film

The formation of patterned nanostructure in AgCo/Pt/MgO(001) thin film is simulated by a technique of combining molecular dynamics and phase-field theory. The dislocation (strain) network existing in Pt/MgO is used as a template whose pattern is transferred to AgCo phase in spinodal decomposition, resulting in regular arrays of Co islands that are attracted by the dislocations. The influence of various factors, such as component concentration and film thickness, is studied. It is found that the spinodal decomposition of AgCo in this system is mainly characterized by a competition between a surface-directed layer structure and the strain-induced patterned structure, where the patterned Ag-Co structure only dominates in a small range near the interface (less than 10 atomic layers). However, if the interlayer diffusion can be minimized by controlling film growth conditions, it is shown that the patterned structure can be formed throughout the entire film.

cond-mat.mtrl-sci↗

arXiv2002

Effect of biaxial strain and composition on vacancy mediated diffusion in random binary alloys: A first principles study of the SiGe system

We present the results of a systematic study using the density functional theory (within the local density approximation) of the effects of biaxial strain and composition on the self-diffusion of Si and Ge in SiGe alloys diffusing by a vacancy mechanism. The biaxial strain dependence of the vacancy formation energy was reconfirmed with previous calculations. The effect of biaxial strain on the interaction potential energy between a substitutional Ge atom and a vacancy was calculated. These calculations were used to estimate the change in the activation energy (due to biaxial strain) for the self-diffusion of Si and Ge in Si by a vacancy mechanism. The composition dependence of the vacancy formation energy was calculated. A database of ab initio migration energy barriers for vacancy migration in different local environments was systematically developed by considering the effect of the first nearest neighbor sites explicitly and the effect of the other sites by a mean field approximation. A kinetic Monte Carlo simulation based on the migration energy barrier database was performed to determine the dependence (on the composition)of the activation energy for the diffusion of Si and Ge in SiGe. A detailed study of the variation of the correlation factor with composition and temperature in SiGe was performed using the results of the KMC simulation. These analyses constitute essential building blocks to understand the mechanism of vacancy mediated diffusion processes at the microscopic level.

cond-mat.mtrl-sci↗

arXiv2002

Thermal Expansion and Diffusion Coefficients of Carbon Nanotube-Polymer Composites

Classical molecular dynamics (MD) simulations employing Brenner potential for intra-nanotube interactions and Van der Waals forces for polymer-nanotube interfaces are used to invetigate the thermal expansion and diffusion characteristics of carbon nanotube-polyethylene composites. Additions of carbon nanotubes to polymer matrix are found to increase the glass transition temperature Tg, and thermal expansion and diffusion coefficients in the composite above Tg. These findings could have implications in CNT composite processing, coating and painting applications.

cond-mat.mtrl-sci↗

arXiv2002

Tensile Strength of Carbon Nanotubes under Realistic Temperature and Strain Rate

Strain rate and temperature dependence of the tensile strength of single-walled carbon nanotubes has been investigated with molecular dynamics simulations. The tensile failure or yield strain is found to be strongly dependent on the temperature and strain rate. A transition state theory based predictive model is developed for the tensile failure of nanotubes. Based on the parameters fitted from high-strain rate and temperature dependent molecular dynamics simulations, the model predicts that a defect free $μm$ long single-wall nanotub at 300K, stretched with a strain rate of $1%/hour$, fails at about $9 \pm 1%$ tensile strain. This is in good agreement with recent experimental findings.

cond-mat↗