"Theoretical Insight into the Origin of Dielectric and Magnetoelectric FeCo Alloy–Metal Fluoride Insulators",
J. Phys Chem. C, 128, 1386-1392 (2024).
FeCo alloy–metal fluoride nanogranular films are known for their significant magnetoelectric responses, which are useful as several electromagnetic devices. In this study, we analyzed the interfaces between (110) FeCo and various clusters of metal fluorides (MgF2, AlF3, and YF3) to understand the origin of the large dielectric and magnetoelectric response caused by metal fluoride insulators through density functional molecular dynamics (DF-MD). Our DF-MD computations revealed that the metal fluoride clusters adhered to the (110) FeCo surface via Co/Fe–F bonds. The electronic structures of the (110) FeCo surfaces with the adsorbed metal fluoride clusters showed that FeCo was oxidized by the metal fluorides, resulting in polarization at the interface. Additionally because metal fluorides contain extra electrons, they can be magnetic, and the performance of electromagnetic devices depends on the electron affinity of fluoride insulators and the orbital magnetic moment of their acceptor levels.
"High reactivity of H2O vapor on GaN surfaces",
Science and Technology of Advanced Materials, 23, 189-198 (2022).
Understanding the process of oxidation on the surface of GaN is important for improving metal-oxide-semiconductor (MOS) devices. Real-time X-ray photoelectron spectroscopy was used to observe the dynamic adsorption behavior of GaN surfaces upon irradiation of H2O, O2, N2O, and NO gases. It was found that H2O vapor has the highest reactivity on the surface despite its lower oxidation power. The adsorption behavior of H2O was explained by the density functional molecular dynamic calculation including the spin state of the surfaces. Two types of adsorbed H2O molecules were present on the (0001) (+c) surface: non-dissociatively adsorbed H2O (physisorption), and dissociatively adsorbed H2O (chemisorption) molecules that were dissociated with OH and H adsorbed on Ga atoms. H2O molecules attacked the back side of three-fold Ga atoms on the (000-1) (-c) GaN surface, and the bond length between the Ga and N was broken. The chemisorption on the (10-10) m-plane of GaN, which is the channel of a trench-type GaN MOS power transistor, was dominant, and a stable Ga-O bond was formed due to the elongated bond length of Ga on the surface. In the atomic layer deposition process of the Al2O3 layer using H2O vapor, the reactions caused at the interface were more remarkable for p-GaN. If unintentional oxidation can be resulted in the generation of the defects at the MOS interface, these results suggest that oxidant gases other than H2O and O2 should be used to avoid uncontrollable oxidation on GaN surfaces.
"Dynamic Observation and Theoretical Analysis of Initial O2 Molecule Adsorption on Polar and m-Plane Surfaces of GaN",
The Journal of Physical Chemistry C, 124, 25282-25290 (2020).
The initial adsorption behavior of different GaN surfaces (the polar Ga-face (+c) and N-face (-c) and the nonpolar (1010) (m)-plane) under O2 molecule beam irradiation was studied by continuous real-time monitoring of the O 1s core X-ray photoelectron spectra generated under synchrotron radiation. The adsorption of O2 molecules on these crystal planes was also modeled by density functional molecular dynamics calculations. The results predict that triplet O2 either dissociates or chemisorbs at the bridge position on the +c GaN surface. On the other hand, the O2 molecule on the N-terminated -c GaN surface only undergoes dissociative chemisorption. On the m-GaN surface, although the dissociation of O2 is dominant, the bond length and angle were found to fluctuate from those of O2 molecules adsorbed on the polar surfaces. These theoretical results for the oxygen-binding states on GaN surfaces are consistent with the obtained O 1s spectra under O2 beam irradiation. The computational model including both the surface spin and polarity of GaN is useful for understanding the interface between GaN and oxide layers in metal-oxide electronic devices because it can successfully explain the adsorption behavior of GaN surfaces.
"Possible Polymerization of PS4 at a Li3PS4/FePO4 Interface with Reduction of the FePO4 Phase ",
The Journal of Physical Chemistry C, 121, 9698-9704 (2017).
An important issue about developing all solid-state Li-ion batteries is to lower the high ionic interfacial resistance between a cathode and an electrolyte. An origin of the interfacial resistance is hypothesized due to a Li-depleted layer at the interface. Our computation has shown that the Li-depleted layer was the result of redox reaction at the interface in the charging process. In this subsequent theoretical study, we validate this redox reaction between the FePO4 phase and the Li3PS4 phase from the viewpoint of their band alignment through the density functional theory with the hybrid functional (HSE06). In addition, we demonstrate that the Li-depleted layer grows up to a defective layer at a Li3PS4/FePO4 interface by exothermic radical polymerization of PS4 anions in the oxidized Li3PS4 phase with the volume reduction. This decrease in Li-ion sites due to the PS4 polymerization makes the Li-depleted region long-lived and has the potential as an origin of the resistance against the Li-ion diffusion near the interface.
"First-principles analysis on role of spinel (111) phase boundaries in Li4+3xTi5O12 Li-ion battery anodes ",
Physcial Chemistry Chemical Physics , 8, 23383-23388 (2016).
The practical anode material Li4+3xTi5O12 is known to undergo a two-phase separation into Li7Ti5O12 and Li4Ti5O12 during charging/discharging. This phase-separated Li4+3xTi5O12 exhibits electron conduction, although individual phases are expected to be insulators. To elucidate the role played by spinel (111) phase boundaries on these physical properties, first principles calculations were carried out using the GGA+U method. Two-phase Li7Ti5O12/Li4Ti5O12 models are found to exhibit metallic characteristics near their phase boundaries. These boundaries provide conduction paths not only for electrons, but also for Li ions. Judging from the formation energy of Li vacancies/interstitials, the phase boundaries preferentially uptake or release Li via in-plane conduction and then continuously shift in a direction perpendicular to the phase boundary planes. The continuous phase boundary shift leads to a constant electrode potential. A three-dimensional network of cubic {111} planes may contribute to smooth electrochemical reactions.
"Charged and Discharged States of Cathode/Sulfide Electrolyte Interfaces in All-Solid-State Lithium Ion Batteries",
The Journal of Physical Chemistry C, 120, 13332-13339 (2016).
Interfaces between cathodes and sulfide electrolytes exhibit high resistance in all-solid-state lithium ion batteries. In this paper, to elucidate the origin of the high interface resistance we have theoretically investigated the properties of the cathode interfaces with the sulfide electrolyte and oxide electrolyte for comparison. From the density functional molecular dynamics simulations of the LiFePO4/Li3PS4 interface in both discharged and charged states, we have demonstrated the instability of the sulfide interface in the charged state, that is, the lithium depletion and oxidation on the sulfide side near the interface, in contrast to the oxide interfaces. The obtained results imply the formation of a Li-depleted layer around the sulfide interfaces during charging and support the validity of the insertion of oxide buffer layers at the interface to reduce the interface resistance.
"Multi-Spin-State at a Li3PO4 (100)/LiCoO2 (104) Interface",
Physcial Chemistry Chemical Physics, 18, 4316-4319 (2016).
(Open access)
We have found the disproportion between the intermediate spin (IS) and low spin (LS) configurations of Co atoms at a Li3PO4/LiCoO2 (104) interface through density functional molecular dynamics (DF-MD). The manifold of the spin state at the interface, however, does not affect the band alignment between the Li3PO4 and LiCoO2 regions.
"Theoretical Insight into Charging Process in a Li3PO4 (100)/LiFePO4 (010) Coherent Interface System",
Solid State Ionics, 285, 59-65 (2016).
We have investigated the introduction of Li vacancy (VLi× in the Kröger-Vink notation) into a Li3PO4 (100)/LiFePO4 (010) coherent interface with the density functional theory in order to gain an insight into the initial stage of the charging process. The VLi× introduction results in the formation of a Li ion vacancy (VLi') and hole (h). When one VLi× is introduced in each bulk LiFePO4 and Li3PO4 materials (both VLi' and h are donated to the position in the proximity of the initial VLi× position), the formation energies of the Li vacancy (EV) are quite different between them. On the other hand, when introduced into the Li3PO4/LiFePO4 interface system, the values of EV in the LiFePO4 and Li3PO4 bulk regions of the interface become almost equal. This is because irrespective of the introduced VLi× position, the associated h is always donated to the LiFePO4 region while the Li3PO4 region keeps its insulating properties. Although the Li atoms near the interface have smaller EV than in the bulk regions, it is suggested that only a fraction of them may be extracted at the initial stage of charging which is not enough to lead to the Li depletion at the interface and as a result the effect of the space charge layer may be negligible in the Li3PO4/LiFePO4 interface.
"Theoretically Designed Li3PO4 (100)/LiFePO4 (010) Coherent Electrolyte/Cathode Interface for All Solid-State Li Ion Secondary Batteries" ,
The Journal of Physical Chemistry C, 119, 14-22 (2015).
(Open access)
Controlling the electrolyte/electrode interface is of great importance to promote new-generation solid-state Li ion secondary batteries. In this paper, we report a theoretically designed electrolyte/cathode coherent interface at the density functional theory level, where γ-Li3PO4 and LiFePO4 are used as an electrolyte and a cathode, respectively. At the stoichiometric Li3PO4 (100)/LiFePO4 (010) coherent interface, there are vacant Li-sites that give the chance for Li ions to migrate. From the density functional molecular dynamics at 1500 K, it is found that this interface is stable and no impurity phase is produced, and also that Li ions in the Li3PO4 phase around the interface can diffuse with large diffusion coefficients. The dynamic behavior of these Li ions is also reflected in the layered phonon spectra of Li ions; the diffusible Li ions around the interface have the same spectrum.
"Electronic structure of acetonitrile adsorbed on the anatase TiO2 (101) surface",
Chemical Physics Letters, 556, 225-229 (2013).
We have investigated detailed electronic states of acetonitrile (MeCN) molecule adsorption on the anatase (101) TiO2 surface by using the density functional theory with a hybrid exchange correlation functional for deeper understanding of dye-sensitized solar cells interfaces. Our analysis indicates that the 7a1 orbital of MeCN hybridizes with the non-bonding orbitals of threefold coordinated oxygen (O3C) on the surface, not directly to the fivefold coordinated Ti (Ti5C). This mechanism accounts for the stable adsorption geometry of MeCN and the coverage of MeCN molecules on the surface.
"Protonated Carboxyl Anchor for Stable Adsorption of Ru N749 Dye (Articles Dye) on a TiO2 Anatase (101) Surface",
The Jornal of Physical Chemistry Letters, 3, 472-477 (2012).
We have investigated the adsorption stability of ruthenium N749 dye [black dye (BD)], a highly efficient dye for dye-sensitized solar cells (DSCs), through protonated and deprotonated carboxyl group anchors on a TiO2 anatase (101) surface by using first-principles calculations. Geometry optimizations of the surface system with a supercell and the UV-visible spectrum calculation of the optimized dye structure were carried out. Among the configurations with one and two anchors, the BD adsorption anchored with one protonated carboxyl group was found to be the most stable, in contrast to most previous reports. Hydrogen bonding between the proton retained in BD and the surface oxygen is responsible for the stability of the protonated anchor. We confirmed that the calculated UV-visible spectrum of the most stable dye structure shows the best consistency with the experimental data. It is also demonstrated that the electronic density of states largely depends on the proton position. This novel aspect of adsorption via a protonated carboxyl anchor gives a new perspective for interfacial electronic processes of DSCs.
"Water Contamination Effect on Liquid Acetonitrile / TiO2 Anatase (101) Interface for Durable Dye-sensitized Solar Cell",
J. Phys. Chem. C, 115, 19849-19855 (2011).
We have investigated structural and electronic properties of liquid acetonitrile (MeCN) / TiO2 anatase (101) interfaces involving a water molecule in order to analyze effect of ubiquitous water contamination in the typical electrolyte solution on the durability of dye-sensitized solar cell (DSSC), by using density-functional molecular dynamics simulations at room temperature. Our results show that H2O does not dive into the unbound Ti5C sites on the (101) surface kinetically, once the coverage of MeCN is saturated (Θ ∼ 0.6). On the other hand, H2O adsorption through hydrogen bond with surface O2C sites, not a Ti5C site, is found the most stable if MeCN solvent is introduced to H2O-preadsorbed TiO2 anatase (101) surface. This no-adsorption character of Ti5C site in the aprotic solvent MeCN is in stark contrast to the case where the (101) surface is immersed by H2O layer or liquid H2O. The adsorbed H2O molecule via the hydrogen bond between O2C and HW has its 1b1 orbital at an energy just below the valence band maximum of TiO2. Therefore, this H2O has a sufficient possibility to become a cation radical by capturing hole generated by irradiation, which may attack the dye molecules towards the desorption. We thus demonstrate that removing H2O from the anatase (101) surface prior to introduction of the MeCN electrolyte solution is crucial to make the DSSCs more durable and efficient.
"Interface Water on TiO2 Anatase (101) and (001) Surfaces: First-Principles Study with TiO2 Slabs Dipped in Bulk Water",
J. Phys. Chem. C, 114, 18529-18537 (2010).
We investigated the TiO2 anatase (101) and (001) interfaces dipped in bulk water on the atomic scale by first-principles density-functional molecular dynamics simulations. We verified that the water adsorption models proposed in the previous studies with less than a couple of water layers on the vacuum surfaces still hold. On the contrary, novel adsorption structures of interfacial water molecules are also found. Our results indicate that water molecules around the interface between the TiO2 and bulk water can be described by the following two-layer model: The first water layer can be defined as the water molecules adsorbed at Ti5C sites moleculary on the anatase (101) surfaces and dissociatively on the (001). The second layer on the anatase (101) surface can be defined as the water molecules adsorbed to O2C or adsorbed water to Ti5C via strong HB. Second layer on the (001) consists of water molecules bound to the first-layer water molecules via the strong and weak HBs. Contour maps of the atomic densities show that water molecules in the second layers are relatively mobile. This two-layer model well accounts for the experimental results of solid-state 1H NMR.