Hosik Lee

Graphdiyne as a high-capacity lithium ion battery anode material

Using the first-principles calculations, we explored the feasibility of using graphdiyne, a 2D layer of sp and sp2 hybrid carbon networks, as lithium ion battery anodes. We found that the composite of the Li-intercalated multilayer α-graphdiyne was C6Li7.31 and that the calculated voltage was suitable for the anode. The practical specific/volumetric capacities can reach up to 2719 mAh g−1/2032 mAh cm−3, much greater than the values of ∼372 mAh g−1/∼818 mAh cm−3, ∼1117 mAh g−1/∼1589 mAh cm−3, and ∼744 mAh g−1 for graphite, graphynes, and γ-graphdiyne, respectively. Our calculations suggest that multilayer α-graphdiyne can serve as a promising high-capacity lithium ion battery anode.

Excited state carbene formation from UV irradiated diazomethane.

The laser flash photolysis process of diazomethane has been studied by using a real time propagation time-dependent density functional theory (RTP-TDDFT) combined with molecular dynamics. The activation energy barrier for disintegrating diazomethane into nitrogen (N(2)) and carbene (CH(2)) molecules significantly decreases in the electronic excited S(1) state compared to that in the S(0) ground state. Furthermore, the produced carbene molecule can be in the electronic excited state of (1)CH(2) ((1)B(1)) instead of the lowest state among singlet states (1)CH(2) ((1)A(1)), which is evident in the wave function characteristics of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) throughout the disintegration. This is regarded as the initial stage of the rearrangement in the excited state (RIES), the evidence of which has been given by experiments in the past decade. In the RIES mechanism scheme, we suggest that the photoreaction in the S(1) state contributes considerably to the photochemistry of carbene formation. The passing near the S(1)/S(0) conical intersection, which allows the transition to ground state diazomethane producing the lowest singlet state carbene molecule, is considered a rare event from our molecular dynamics, although this has been regarded as the dominant mechanism in previous theoretical studies.

High-throughput screening of metal-porphyrin-like graphenes for selective capture of carbon dioxide

Nanostructured materials, such as zeolites and metal-organic frameworks, have been considered to capture CO2. However, their application has been limited largely because they exhibit poor selectivity for flue gases and low capture capacity under low pressures. We perform a high-throughput screening for selective CO2 capture from flue gases by using first principles thermodynamics. We find that elements with empty d orbitals selectively attract CO2 from gaseous mixtures under low CO2 pressures (~10−3 bar) at 300 K and release it at ~450 K. CO2 binding to elements involves hybridization of the metal d orbitals with the CO2 π orbitals and CO2-transition metal complexes were observed in experiments. This result allows us to perform high-throughput screening to discover novel promising CO2 capture materials with empty d orbitals (e.g., Sc– or V–porphyrin-like graphene) and predict their capture performance under various conditions. Moreover, these findings provide physical insights into selective CO2 capture and open a new path to explore CO2 capture materials.

Infrared phonon study of charge ordering in La1/2Sr3/2MnO4

We investigated the temperature- and polarization-dependent optic phonon modes of La1/2Sr3/2MnO4 using optical conductivity analyses. The phonon modes in the ab-plane are significantly changed, while those along the c-axis remain nearly constant with decreasing temperature. Below the charge-ordering temperature, the strength of the stretching phonon mode increases and its frequency becomes higher. These far-infrared phonon spectrum changes are closely related to the increase of the optical gap in the near-infrared region. Our experimental results suggest the importance of lattice distortion and strong electron-phonon interaction for the charge ordering in La1/2Sr3/2MnO4.

A comprehensive study of piezomagnetic response in CrPS4 monolayer: mechanical, electronic properties and magnetic ordering under strains

We performed first-principles calculations to investigate the magnetic, mechanical and electronic properties of the tetrachalcogenide CrPS4. Although bulk CrPS4 has been shown to exhibit a low-dimensional antiferromagnetic (AFM) ground state where ferromagnetic (FM) Cr-chains are coupled antiferromagnetically, our calculations indicated that the monolayer can be transformed to an FM material by applying a uniaxial tensile strain of  ⩾4% along the FM Cr-chain direction. The AFM-to-FM transition is explained to be driven by an increase of the exchange interaction induced by a decrease in the distance between the FM Cr-chains. A huge nonlinear piezomagnetism was predicted at the strain-induced magnetic phase boundary. Our study provides insight about rational design of single-layer magnetic materials for a wide range of spintronic devices and energy applications.

p-type conductivity generated by ferromagnetic ordering via percolative anionic H chain formation in ZnCoO

We report on the simultaneous realization of p-type conductivity and strong ferromagnetism in heavily Co-doped ZnO thin films in the presence of a high concentration of hydrogen impurities. Through ab initio calculations, we find that the microscopic origin of hole carrier generations and ferromagnetic ordering is due to the partially occupied band of the percolative structures wherein the carrier-induced magnetic interactions can stabilize the strong spin-parallel state.

Surface reconstruction and charge modulation in BaFe2As2 superconducting film

Whether or not epitaxially grown superconducting films have the same bulk-like superconducting properties is an important concern. We report the structure and the electronic properties of epitaxially grown Ba(Fe1-x Co x )2As2 films using scanning tunneling microscopy and scanning tunneling spectroscopy (STS). This film showed a different surface structure, [Formula: see text]R45° reconstruction, from those of as-cleaved surfaces from bulk crystals. The electronic structure of the grown film is different from that in bulk, and it is notable that the film exhibits the same superconducting transport properties. We found that the superconducting gap at the surface is screened at the Ba layer surface in STS measurements, and the charge density wave was observed at the surface in sample in the superconducting state.

Magnetic-field-dependent optical studies of Pr0.69Ca0.31MnO3

Abstract We investigated magnetic-field-dependent optical conductivity spectra σ ( ω ) of Pr 0.69 Ca 0.31 MnO 3 at 4.2 K, which was in a charge-ordered insulating state at 0 T. With increasing the magnetic field up to 12 T, we observed an insulator–metal transition around 4 T and large spectral weight changes up to 4 eV. Especially, we found that the spectral weight above 0.5 eV moved to a lower energy region and that a Drude-like peak appeared clearly at 12 T. By analyzing dielectric response changes, we suggested that the field-dependent insulator–metal transition might occur through a percolation process of ferromagnetic metal domains. On the other hand, with decreasing the magnetic field from 12 to 0 T, σ ( ω ) showed little changes, which showed that the ferromagnetic metallic state did not return to the insulating state.