Kwangwoo Hong

Composition-Controlled PtCo Alloy Nanocubes with Tuned Electrocatalytic Activity for Oxygen Reduction

Modification of the electronic structure and lattice contraction of Pt alloy nanocatalysts through control over their morphology and composition has been a crucial issue for improving their electrocatalytic oxygen reduction reaction (ORR) activity. In the present work, we synthesized PtCo alloy nanocubes with controlled compositions (Pt(x)Co NCs, x = 2, 3, 5, 7, and 9) by regulating the ratio of surfactants and the amount of Co precursor to elucidate the effect of the composition of nanocatalysts on their ORR activity. Pt(x)Co NCs had a Pt-skin structure after electrochemical treatment. The electrocatalysis experiments revealed a strong correlation between ORR activity and Co composition. Pt₃Co NCs exhibited the best ORR performance among the various Pt(x)Co NCs. From density functional theory calculations, a typical volcano-type relationship was established between ORR activity and oxygen binding energy (E(OB)) on NC surfaces, which showed that Pt₃Co NCs had the optimal E(OB) to achieve the maximum ORR activity. X-ray photoelectron spectroscopy and X-ray diffraction measurements demonstrated that the electronic structure and lattice contraction of the Pt(x)Co NCs could be tuned by controlling the composition of NCs, which are highly correlated with the trends of E(OB) change.

Accuracy of Lagrange-sinc functions as a basis set for electronic structure calculations of atoms and molecules

We developed a self-consistent field program based on Kohn-Sham density functional theory using Lagrange-sinc functions as a basis set and examined its numerical accuracy for atoms and molecules through comparison with the results of Gaussian basis sets. The result of the Kohn-Sham inversion formula from the Lagrange-sinc basis set manifests that the pseudopotential method is essential for cost-effective calculations. The Lagrange-sinc basis set shows faster convergence of the kinetic and correlation energies of benzene as its size increases than the finite difference method does, though both share the same uniform grid. Using a scaling factor smaller than or equal to 0.226 bohr and pseudopotentials with nonlinear core correction, its accuracy for the atomization energies of the G2-1 set is comparable to all-electron complete basis set limits (mean absolute deviation ≤1 kcal/mol). The same basis set also shows small mean absolute deviations in the ionization energies, electron affinities, and static polarizabilities of atoms in the G2-1 set. In particular, the Lagrange-sinc basis set shows high accuracy with rapid convergence in describing density or orbital changes by an external electric field. Moreover, the Lagrange-sinc basis set can readily improve its accuracy toward a complete basis set limit by simply decreasing the scaling factor regardless of systems.

Fano-Resonance-Driven Spin-Valve Effect Using Single-Molecule Magnets†

One of the distinctive features of waves is the interference phenomenon between waves that are propagating along different paths. Fano resonance, which occurs by the quantum mechanical interference between discrete and continuum states, is a manifestation of the wave-nature of electrons, which has been observed in various systems from atomic spectrum to conductance in nanoscale devices. Recently its fundamental importance in nanoscience has been discussed in an extensive Review article. For example, its usefulness for novel applications was addressed through theoretical studies showing that ultrafast DNA sequencing can be accomplished by deciphering the unique Fanoresonance patterns of each nucleotide. On the other hand, single-molecule magnetism, which originates from unpaired electron spins with anisotropy, has attracted great attention because of its potential applications to molecular-scale spintronics, namely molecular spintronics. As a result, diverse magnetic molecules have been synthesized and characterized, stimulating new ideas for the development of novel spintronic devices. Typically a singlemolecule magnet (SMM) is chemically attached to two metallic electrodes to form a metal-molecule-metal junction, and spin-dependent currents are driven across the junction. Though the small size of these molecules provides a great advance toward the miniaturization of electronic devices, it also gives rise to a lack of reproducibility owing to highly sensitive electronic coupling in the atomic-level contact. To circumvent such a contact problem, we propose using spin-dependent Fano resonance. When discrete spin states of an SMM are coupled to the continuum bands of a nanoconductor, the conductance of the device exhibits a strong spin-dependent Fano-resonance pattern. As a proof of concept, we studied a carbon nanotube (CNT) decorated with magnetic molecules through p–p stacking, whose spin-dependent conductance can be controlled by means of an external magnetic field. We believe that this is the origin of the large magnetoresistance observed in recent experiments. Because such a device would not require direct metal– molecule contacts as well as ferromagnetic electrodes, it offers a practical way to realize true molecular spintronic devices, as demonstrated by Ruben, Wernsdorfer, and coworkers in Ref. [10]. Figure 1 shows a schematic drawing of the spin-valve device we studied, consisting of two SMMs and an armchair

Supersampling method for efficient grid-based electronic structure calculations

The egg-box effect, the spurious variation of energy and force due to the discretization of continuous space, is an inherent vexing problem in grid-based electronic structure calculations. Its effective suppression allowing for large grid spacing is thus crucial for accurate and efficient computations. We here report that the supersampling method drastically alleviates it by eliminating the rapidly varying part of a target function along both radial and angular directions. In particular, the use of the sinc filtering function performs best because as an ideal low pass filter it clearly cuts out the high frequency region beyond allowed by a given grid spacing.

Effects of the locality of a potential derived from hybrid density functionals on Kohn–Sham orbitals and excited states

Density functional theory has been an essential analysis tool for both theoretical and experimental chemists since accurate hybrid functionals were developed. Here we propose a local hybrid method derived from the optimized effective potential (OEP) method and compare its distinct features with conventional nonlocal ones from the Hartree-Fock (HF) exchange operator. Both are formally exact for ground states and thus show similar accuracy for atomization energies and reaction barrier heights. For excited states, the local version yields virtual orbitals with N-electron character, while those of the nonlocal version have mixed characters between N- and (N+1)-electron orbitals. As a result, the orbital energy gaps from the former well approximate excitation energies with a small mean absolute error (MAE = 0.40 eV) for the Caricato benchmark set. The correction from time-dependent density functional theory with a simple local density approximation kernel further improves its accuracy by incorporating multi-configurational effects, resulting in the total MAE of 0.27 eV that outperforms conventional functionals except for MN15.