Dongwook Kim

We present an atomic resolution structural study of covalently bonded dopant pairs in the lattice of monolayer graphene. Two iron (Fe) metal atoms that are covalently bonded within the graphene lattice are observed and their interaction with each other is investigated. The two metal atom dopants can form small paired clusters of varied geometry within graphene vacancy defects. The two Fe atoms are created within a 10 nm diameter predefined location in graphene by manipulating a focused electron

We study the band topology and the associated linking structure of topological semimetals with nodal lines carrying Z_{2} monopole charges, which can be realized in three-dimensional systems invariant under the combination of inversion P and time reversal T when spin-orbit coupling is negligible. In contrast to the well-known PT-symmetric nodal lines protected only by the π Berry phase, in which a single nodal line can exist, the nodal lines with Z_{2} monopole charges should always exist in pai

-hybridized honeycomb ZnSb layers. Using structural analysis combined with theoretical calculation, it is found that the 2D-ZnSb has a stable and robust layered structure. The bidimensional polymorphism is a previously unobserved phenomenon at ambient pressure in Zintl families and can be a common feature of transition metal pnictides. This dimensional manipulation of a crystal structure thus provides a rational design strategy to search for new 2D layered materials in various compounds, enablin

We investigated the admittance spectra of resistive switching Pt/Nb-doped SrTiO3 single-crystal junctions at different resistance states in air and vacuum. The analyses showed that the carrier lifetime at the traps was largely varied depending on the resistance state, indicating the surface potential modification. The ambient dependence suggested that the charges at the trap states were affected by the oxygen adsorption/desorption at the surface. The conductance spectroscopy method clearly revea

We investigated the electrical properties of Cu/TaOx/Pt structures, which were prepared by RF-sputtering at room temperature. Spectroscopic ellipsometry (SE) measurements revealed that the refractive indices of the TaOx thin films decreased with increasing oxygen-to-argon gas ratio during the growth. The transport analyses based on the Poole-Frenkel emission model yielded the dielectric constant of the TaOx layers; the results were consistent with the SE results. The Cu/TaOx/Pt structures exhibi

In this study, we identify the local structures of ex-solved nanoparticles using machine-learned potentials (MLPs). We develop a method for training machine-learned potentials by sampling local structures of heterointerface configurations as a training set with its efficacy tested on the Ni/MgO system, illustrating that the error in interface energy is only 0.004 eV/Å 2 . Using the developed scheme, we train an MLP for the Ni/La 0.5 Ca 0.5 TiO 3 ex-solution system and identify the local structur

We investigated the resistive switching behaviors of metal/oxide junctions consisting of Pt electrodes and Nb-doped SrTiO3(001) single crystals. The doping level affected the resistive switching ratio and the transport mechanism (thermionic emission for low doping and thermionic field emission for high doping). Pulse-mode switching experiments showed that an increase in the interface electric field by several times could enhance the switching speed by hundreds of times. The dependence of the ret

We investigated the electronic properties of thermally-oxidized Ni films, which have recently attracted much attention due to their good resistive switching behaviors. We found that the XRD patterns and the optical responses of the oxidized Ni films exhibited systematic changes with oxidation degree. The optical response of the NiO films appeared to be consistent with those characteristic of doped Mott insulators. We also discuss the possibility of phase coexistence.

We report on the resistance switching behavior of NiO thin films grown on Pt bottom electrodes, with top electrodes of Pt, Au and Ni. NiO/Pt films with all the top electrodes show reversible switching from high-resistance state (HRS) to low-resistance state (LRS) and {\it vice versa} during unipolar current-voltage ($I-V$) measurements. The resistance switching ratio of the Au/NiO/Pt structure is much smaller than those of others. The HRS $I-V$ curve of the Au/NiO/Pt structure is linear, while t

We investigated the transport and photovoltaic properties of Cu(In1-xGax)Se2 (CIGS) thin-film solar cells. The shunt-current-eliminated diode current could be obtained from the currentevoltage characteristics by subtracting the parasitic shunt leakage current from the total current. The temperature dependence of the open-circuit voltage, extracted from the shunt-eliminated (total) current, suggested that the recombination activation energy is comparable to (much less than) the CIGS bandgap. The