Search

Developing highly active catalysts for hydrogen evolution reaction based on earth-abundant materials is challenging. Nitrogen doping has recently been reported to improve catalytic properties by modifying the electrochemical properties of titanium carbide MXene. However, systematic doping engineering, such as optimization of doping concentration, doping site, and thermodynamic phase stabilization have not been systematically controlled, which retards the reliable production of high-activity MXene catalysts. In this study, the optimum doping concentration of nitrogen and doping process conditions on O-functionalized Ti2C MXene for hydrogen evolution reaction were investigated using density functional theory with thermodynamics. To confirm the optimum nitrogen concentration, the catalytic properties are examined considering the Gibbs free energy of hydrogen adsorption and conductivity for 2.2-11.0 at % nitrogen concentration. It was confirmed that 8.8 at % nitrogen-doped Ti2CO2 had optimum catalytic properties under standard conditions. Moreover, when the doping concentration was higher, the decrease in the adsorption energies of hydrogen and the transition in the energy dispersion of the conduction band led to deterioration of the catalytic properties. Through theoretical results, the feasible process conditions for optimum nitrogen concentration while maintaining the structure of MXene are presented using a thermodynamics model taking into account chemical reactions with various nitrogen sources. This study provides further understanding of the nitrogen-doping mechanism of Ti2CO2 for hydrogen evolution reactions.

A defect structure and osseointegration capacity of sodium and chloride co-substituted hydroxyapatite (NaClAp) were newly studied. The NaClAp was prepared by reacting H3PO4 and Ca(OH)2 with NaNO3 and NH4Cl followed by sintering; pure hydroxyapatite (HAp) was synthesized as a control. After sintering, the co-substitution of Ca and OH with Na and Cl, respectively, produced charged point defects at Ca and PO4 sites. Also, OH molecules partially adopted a head-on structure. The calculated total system energy of NaClAp was higher, whereas the binding energies between each constituent elements and system were lower than those of HAp. These results suggest that NaClAp was less stable than HAp, due to the formation of various defects by co-substitution of Na and Cl. Indeed, NaClAp exhibited higher dissolution behavior in simulated body fluid (SBF) compared with HAp. Accordingly, this increased the capability to produce low crystalline hydroxyl carbonate apatite, likely due to the increasing degree of apatite supersaturation in SBF. Besides, the NaClAp granules showed noticeable improvements in osseointegration capacity four weeks after in vivo test compared with HAp. Collectively, these results imply that the defects made by multiple ion substitutions are useful to increase osseointegration capacity of hydroxyapatite.

Sulfide solid electrolytes (SEs) with high Li-ion conductivities (σion) and soft mechanical properties have limited applications in wet casting processes for commercial all-solid-state batteries (ASSBs) because of their inherent atmospheric and chemical instabilities. In this study, we fabricated sulfide SEs with a novel core–shell structure via environmental mechanical alloying, while providing sufficient control of the partial pressure of oxygen. This powder possesses notable atmospheric stability and chemical resistance because it is covered with a stable oxysulfide nanolayer that prevents deterioration of the bulk region. The core–shell SEs showed a σion of more than 2.50 mS cm–1 after air exposure (for 30 min) and reaction with slurry chemicals (mixing and drying for 31 min), which was approximately 82.8% of the initial σion. The ASSB cell fabricated through wet casting provided an initial discharge capacity of 125.6 mAh g–1. The core–shell SEs thus exhibited improved powder stability and reliability in the presence of chemicals used in various wet casting processes for commercial ASSBs.

Metal–fluoride possesses a high potential as new high-performance water oxidation catalysts due to a highly polarized electronic configuration. However, low conductivity, related to high iconicity ...

Low-cost anion exchange membrane fuel cells have been investigated as a promising alternative to proton exchange membrane fuel cells for the last decade. The major barriers to the viability of anion exchange membrane fuel cells are their unsatisfactory key components—anion exchange ionomers and membranes. Here, we present a series of durable poly(fluorenyl aryl piperidinium) ionomers and membranes where the membranes possess high OH− conductivity of 208 mS cm−1 at 80 °C, low H2 permeability, excellent mechanical properties (84.5 MPa TS), and 2000 h ex-situ durability in 1 M NaOH at 80 °C, while the ionomers have high water vapor permeability and low phenyl adsorption. Based on our rational design of poly(fluorenyl aryl piperidinium) membranes and ionomers, we demonstrate alkaline fuel cell performances of 2.34 W cm−2 in H2-O2 and 1.25 W cm−2 in H2-air (CO2-free) at 80 °C. The present cells can be operated stably under a 0.2 A cm−2 current density for ~200 h., Developing high-performance anion exchange membranes and ionomers is crucial for low-cost alkaline fuel cells. Here, the authors explore rigid and high ion conductive poly(fluorenyl aryl piperidinium) copolymers, extending their applications to anion exchange membrane fuel cells.

The catalytic mechanism of the initial reaction of lithium oxides formation on buckled graphitic carbon nitrides (g-C3N4) as a catalyst for non-aqueous Li–O2 batteries was investigated using density function theory. The overpotential of heptazine were superior to triazine and precious metal. It was revealed that the difference in overpotential between heptazine and triazine was mainly a result of the gap of charge voltages. Using potential-dependent surface phase diagram, it was confirmed that the charge voltage was determined by the LiO2 formation. The hybridization of Li and bound N in the cavity revealed a clear distinction between substrates. Through the charge analysis, these results stemmed from the electron difference of bound N that was caused by the cavity structural property depending on the substrates. These findings may provide insight for designing of N-doped carbon materials as catalysts for Li–O2 batteries, which possess overpotential that are lower than existing catalyst materials.

In this study, the influence of the non-uniform surface of F- and O-functionalized Ti2C on the anchoring behavior of lithium polysulfide (LiPS) is investigated using density functional theory. In order to consider the non-uniform surface, the substitutional, vacancy, and S-trapped sites of F- and O-functionalized Ti2C are designed. The anchoring behavior is investigated considering the adsorption energy of LiPS, reactivity between Li atoms and the substrate, and the reduction state of the S atoms. On the F-substitutional site of the O-functionalized surface, it is confirmed that the suppressing mechanism changes from the neutralization of S atoms to the anchoring of LiPS. However, too strong of an interaction between Ti atoms exposed at the vacancy site and S atoms induces trapping of the S atom at the vacancies of both F- and O-functionalized surfaces. As a result of the trapping of the S atom, the use of active material decreases. In addition, the S-trapped site originated from the vacancy site does not affect the suppressing mechanism. In conclusion, to optimize the Ti2C-based MXene as an anchoring material for Li-S batteries, the preparation process should be focused on eliminating the vacancy of functional groups.

Co 3 O 4 is one of the most widely used materials in energy and environmental field due to its unintentional p-type nature, which depends on the preparation conditions. In this study, we investigated the origin of the unintentional p-type conductivity of Co 3 O 4 by calculating all possible intrinsic point defects. We found that the octahedral cobalt vacancy and tetrahedral cobalt vacancy are the sources of unintentional p-type doping. Using charge balance theory, we analyzed the effect of preparation condition on intrinsic defect-induced doping. In most of preparation condition, the formation of these cobalt vacancies plays a dominant role and the spontaneous formation of p-type doping is unavoidable. However, if there is ample oxygen and the temperature is low during the preparation, the unintentional p-type doping can be avoided. This theoretical work on defects provides a crucial clue to optimize Co 3 O 4 for various electrochemical applications.

MXenes have potential as a hydrogen evolution reaction (HER) catalyst based on their large surface area and excellent electrical properties. In this study, the change in HER catalytic properties according to the surface state of Ti2C-based MXene was analyzed theoretically based on first principles calculation. It was confirmed that the reactivity of the surrounding active sites is enhanced by the influence of isolated fluorine. The mechanism of this phenomenon was investigated, focusing on the electron migration and structural changes. In the MXene family, the Ti2C-based MXene best utilized the isolated fluorine effect. It was found that Ti2CO2 functionalized with a small amount of fluorine was superior as a HER catalyst to Ti2CO2 fully functionalized with oxygen, and the optimal proportion of fluorine was 1.5 to 3%.

N-Doped graphene (NG) has been widely used as a cathode material for lithium-sulfur (Li-S) batteries due to its strong interaction with lithium polysulfide (LiPS) species. However, strong interaction between the NG substrate and the LiPS molecules induces undesirable molecular structure decomposition of LiPS. Due to the strong interaction between Li and NG, Li-trapping occurs during battery operation. Therefore, in this study, Li-trapped NG (LiNG) is introduced as a possible structure of NG, and the structural stability of LiNG under applied electric potential is examined. The effect of Li-trapping on the properties of NG as an anchoring material for Li-S batteries is investigated using density functional theory calculations. Li-trapping relieves the strong interaction between NG and LiPS, thereby avoiding decomposition of the LiPS molecule. Although the interaction between the LiPS molecule and the substrate is weakened, additionally formed interaction after Li-trapping, which is between Li in the substrate and S in the molecule, enables LiNG to suppress the shuttle effect. LiNG shows advanced anchoring behavior that suppresses the shuttle effect without any molecular decomposition of LiPS. This finding provides a further understanding of the effect of Li-trapping on the anchoring properties of NG for Li-S batteries.

Controlling the properties of piezoelectric thin films is a key aspect for designing highly efficient flexible electromechanical devices. In this study, the crystallographic phenomena of PbZr1–x Ti x O3 (PZT) thin films caused by distinguished interfacial effects are deeply investigated by overlooking views, including not only an experimental demonstration but also ab initio modeling. The polymorphic phase balance and crystallinity, as well as the crystal orientation of PZT thin films at the morphotropic phase boundary (MPB), can be stably modulated using interfacial crystal structures. Here, interactions with MgO stabilize the PZT crystallographic system well and induce the texturing influences, while the PZT film remains quasi-stable on a conventional Al2O3 wafer. On the basis of this fundamental understanding, a high-output flexible energy harvester is developed using the controlled-PZT system, which shows significantly higher performance than the unmodified PZT generator. The voltage, current, and power densities are improved by 556%, 503%, and 822%, respectively, in comparison with the previous flexional single-crystalline piezoelectric device. Finally, the improved flexible generator is applied to harvest tiny vibrational energy from a real traffic system, and it is used to operate a commercial electronic unit. These results clearly indicate that atomic-scale designs can produce significant impacts on macroscopic applications.

The feasibility of designing two-dimensional (2D) materials was investigated using monolayer Cr 2 C, a member of the MXene family, with F and OH functional groups on each side of the surface. Interestingly, this study shows that the magnetic and electronic properties of Cr 2 C with F and OH functional groups (i.e., Cr 2 CF(OH), with one functional group on each side) can be predicted using previously known properties of Cr 2 C with only one kind of functional group (i.e., either F or OH on both sides). Unlike typical functionalized 2D materials with different functional groups, each functional group of Cr 2 CF(OH) has a negligible effect on the magnetic and electronic properties of the other side. This unique phenomenon results from the small differences in geometry between the functional groups, such as the coordination number, bond length, and thickness; the properties themselves come from those of Cr 2 CF 2 and Cr 2 C(OH) 2 , despite having different functional groups. Due to the preserved magnetic moments of the Cr atoms bound with each functional group, Cr 2 CF(OH) exhibits ferrimagnetism. Likewise, semiconducting characters are present because the atomic contributions of each energy level from Cr 2 CF 2 and Cr 2 C(OH) 2 are maintained. Thus, the present work demonstrates that a combination of functional groups that shows certain desired properties can be selected when designing Cr 2 C-based spintronic and electronic devices.

Composition modulation by Ar bombardment on the Co0.5Cu0.5 alloy and the CoAl B2 phase was investigated on atomic scale by molecular dynamics simulation. The Ar bombardment on alloys at 300 K revealed that this bombardment induces a surface composition modulation in the layer-by-layer mode. In both the Co0.5Cu0.5 alloy and the CoAl B2 phase, the element of higher-sputtering yield is accumulated on the top surface layer, whereas it is depleted in lower layers, which is puzzling in the framework the conventional sputtering theory. A phenomenological kinetic model derived from the MD simulation results considering both the rearrangement and the sputtering of the substrate atoms successfully demonstrated that the rearrangement of the substrate atoms plays a significant role in the observed composition modulation.

Group-14 elemental two-dimensional materials are a key material for future supercapacitors because of their various advantages compared to activated carbon. However, the lack of density of states (DOS) near the Fermi level (EF) of these materials is one of the limiting factors for the performance of supercapacitors, especially for electric double-layer capacitors (EDLCs). In this study, Ti-doping was investigated as a strategy for providing a large amount of DOS near the EF. By using density functional theory calculations, it was confirmed that the d band of the Ti atom provides additional DOS to the materials. The calculated quantum capacitances and the surface charge densities of the doped systems were enhanced overall in both their gravimetric and specific aspects thanks to the increased DOS near the EF. In addition, it was revealed that Ti-doped silicene has superior characteristics at the low voltage range compared to other materials. These findings may provide practical guidelines for improving the performance of EDLCs.

The geometries and electronic characteristics of the graphene monoxide (GMO) bilayer are predicted via density functional theory (DFT) calculations. All the possible sequences of the GMO bilayer show the typical interlayer bonding characteristics of two-dimensional bilayer systems with a weak van der Waals interaction. The band gap energies of the GMO bilayers are predicted to be adequate for electronic device application, indicating slightly smaller energy gaps (0.418-0.448 eV) compared to the energy gap of the monolayer (0.536 eV). Above all, in light of the band gap engineering, the band gap of the GMO bilayer responds to the external electric field sensitively. As a result, a semiconductor-metal transition occurs at a small critical electric field (EC = 0.22-0.30 V/Å). It is therefore confirmed that the GMO bilayer is a strong candidate for nanoelectronics.

We use first-principles calculation to investigate the influence of lattice strain on the oxygen vacancy and the migration energies in La 1- x SrCoO 3- δ via a realistic approach using the Poisson's ratio. In the calculation of the vacancy formation energy, it was found that the oxygen sites in the major axis are favorable for achieving vacancy under tensile conditions, while those in the minor axis are in favor of vacancy formation under compressive conditions. The migration energy calculations for oxygen vacancy are also carried out and this reveals interesting strain dependence, higher activation energy when compressive strain is applied in the lateral direction.

In this study, the heterostructure of h-BN and Ni (111) is adopted as effective cathode catalysts for the Li–O2 battery using first-principles calculations. It was determined that h-BN/Ni (111) thermodynamically prefers a 2e− pathway despite the large adsorption energy of O2, even larger than Pt (111), and dissociation of O2 at the formation of the oxygen reduction reaction (ORR) intermediates of the Li–O2 battery on h-BN/Ni (111). In this respect, the result of h-BN/Ni (111) does not accord with previous studies that found that strong adsorption and dissociation of O2 indicate a reaction to proceed via the 4e− pathway. The reason for this behavior is identified as being adsorption of the ORR intermediates mainly conducted by strong ionic bonds between the B atoms of h-BN and the O atoms of the intermediates, while the Li atoms do not participate in the bonds. The electrochemical performance of h-BN/Ni (111) is remarkable with a maximum discharge potential of 1.93 V and a minimum charge potential of 3.83 V, comparable to noble metal based catalysts.

Transition metal layered double hydroxides (LDHs) have received much attention as high-performance oxygen evolution reaction (OER) catalysts due to their large number of active sites with favorable adsorption/desorption energies for intermittent reactants. However, the relatively sluggish charge transfer kinetics of transition metal LDHs due to their intrinsically low conductivity often hinders their use in practical applications as high-performance water oxidation catalysts. Here, we disclose a novel strategy of metalloid incorporation into transition metal LDHs, allowing us to simultaneously optimize surface electronic configuration and charge transfer between adsorbed reactants and catalyst surface. Importantly, incorporated metalloid can enhance the density of states (DOS) near the Fermi level and alter the nature of the chemical bonds in the catalytically active atoms, resulting in fast reaction kinetics. Thus, metalloid incorporation into transition metal LDHs can substantially improve the overall reaction kinetics and thermodynamics for water oxidation due to a large number of active sites and high conductivity, boosting OER performance of transition metal LDHs. The metalloid-incorporated transition metal LDHs far outperform their counterpart transition metal LDHs and even the noble metal catalyst RuO2.

Herein, as a binder (or catalyst ionomer) for AEMFCs, we investigated the effect of two different cationic copolymers based on poly(phenylene oxide) (PPO) with N-cyclic quaternary ammonium (QA) groups, including six-membered dimethyl piperidinium (DMP) and bis-six-membered azonia-spiro undecane (ASU). An earlier report on the same polymers for membranes in AEMFCs indicated the better electrochemical performance of PPO-ASU compared with PPO-DMP. Therefore, we would like to investigate these two polymers for catalyst ionomers. The outcome in this study using these two copolymers as catalyst ionomers indicates the opposite result; the electrochemical performance of the PPO-DMP ionomer is much better than the PPO-ASU ionomer. The commercial Fumion ionomer was used for the qualitative comparison. The density functional theory (DFT) calculation of the adsorption energy according to different orientations of the cationic groups on the catalyst surface shows that there is no difference between the adsorption energy of DMP and ASU cations, in compliance with the orientations of the cations. Although the PPO-ASU ionomer membrane has the highest hydroxide conductivity at 60 °C in liquid water, the hydrogen oxidation/reduction (HOR) activity of PPO-DMP and PPO-ASU showed similar values with the Fumion ionomer. While the PPO-DMP ionomer membrane shows relatively large fuel gas (hydrogen) permeability in dry and wet conditions, due to the chain flexibility and the presence of two methyl groups compared to the single methyl groups and lower flexibility of the PPO-ASU and Fumion ionomers. The electrochemical performance of a membrane electrode assembly (MEA) using the PPO-DMP ionomer exhibited an exceptional peak power density of 335 mW cm−2 compared to lower peak power densities the of PPO-ASU and Fumion ionomers under 60 °C and a fully humidified condition (H2/O2). The SEM images of MEAs after testing supports the conclusion that the PPO-DMP ionomer forms a uniform catalyst interface that is very well bound between the electrode and membrane, unlike the PPO-ASU and Fumion ionomers. The PPO-DMP ionomer membrane also showed better tensile strength and elongation at break than the PPO-ASU ionomer membrane. Therefore, we conclude that the well-prepared three-phase boundary structure played a critical role for the catalyst ionomer in each electrode, overcoming one of the critical performance-limiting factors.

As a strategy to improve the catalytic performances of two-dimensional transition metal dichalcogenide materials, doping and defect engineering are widely used. However, it is extremely challenging to find proper new materials and doping/defect engineering conditions relying only on experimental trial-and-error. In this study, use of vanadium disulfide (VS2) was suggested as an effective cathode catalyst for Li-air batteries (LABs), under the condition of proper doping engineering to optimize electrochemical performances. To investigate the dopant screening and doping processing optimization of VS2, a theoretical concept, which combines first-principles calculations and thermodynamic modeling, was first derived and suggested. Through our approach, it is worth noting that F- and N-doping on H-VS2 shows superior overpotential properties (0.68 V and 0.76 V) than the carbon-based cathode and NH3 and O2 as reactant for T-VS2, and N2 and H2 for H-VS2 are necessary to control the N doping on VS2. Our theoretical work provides the guideline for application of VS2 to cathode of LABs and leads to further insights for designing a new cathode materials based on two-dimensional materials.

Perpendicular magnetic anisotropy (PMA) in thin film is the key factor to obtain good properties for high density storage devices. Although the importance of the properties of PMA is well-known for Co/Pd thin film, it is still unclear which surface structure and composition indicate that PMA is present. In this work, the surface structure and magnetic properties of L10-ordered Co/Pd (001) were analyzed using density functional theory calculations. It was confirmed that only Pd-rich A among the facile surface structures indicates PMA properties. However, according to the calculated surface energy, not only Pd-rich A but also Co-rich B is among the most energetically stable structures. The density of states showed a clear distinct electronic structure between Pd-rich A and Co-rich B derived from the structural difference. This result indicates that PMA is not always present in a sandwich structure such as Pd-rich A. The results provide a useful guide to magnetic devices created using L10-ordered Co/Pd (001) on substrate Pd.

Research on the cathode catalysts of lithium–oxygen (Li–O 2 ) batteries is one of the most important branches to commercialize these batteries to overcome the sluggish kinetics during both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). In this study, a high performance catalyst based on a bimetallic Pt–Cu alloy is investigated for Li–O 2 batteries using first-principles calculation. The theoretical prediction shows that the Pt–Cu alloy is much more effective than the pure Pt according to the electrochemical performance. In particular, the effectiveness of the catalytic property is maximized in the case of the PtCu (111) surface which greatly reduces the large overpotentials of the original Li–O 2 batteries during the OER/ORR. It is identified for the first time that the charge overpotentials are affected mainly by the inherent surface charge character of the alloy catalyst. It is observed that the more negatively charged PtCu (111) surface can act as a weakly positively charged surface for the adsorption of Li–O intermediates and thus result in weak ionic bonding of the intermediates on the surface. As a result, the dominant factor improving the catalytic performance is clearly demonstrated, providing insight into the design of an efficient catalyst for Li–O 2 battery technologies.

The cell performance of lithium–oxygen batteries using nitrogen doped graphene as a catalytic cathode has been validated in recent research, but the cathode reaction mechanism of lithium and oxygen still remains unclear. Since the oxygen reduction reaction (ORR) mechanism by ionic lithium and catalytic surface is predicted to be distinct for different defective sites such as graphitic, pyridinic, and pyrrolic, it is necessary to observe the behavior of ionic lithium and oxygen gas at each defective site in nitrogen doped graphene. In this study, density functional theory (DFT) calculations are adopted to analyze at an atomic scale how effectively each defective site acts as a catalytic cathode. Interestingly, unlike pyridinic or pyrrolic N is known to be the most effective catalytic site for ORR in fuel cells. Among the other defective sites, it is found that the graphitic N site is the most effective catalytic media activating ORR by ionic lithium in lithium–oxygen batteries due to the electron accepting the reaction of Li–O formation by the graphitic N site.

The lithium–oxygen (Li–O2) battery is one of the most promising technologies for energy storage due to its extremely high-energy density. However, the design still faces many challenges for practical use including the decomposition of cathodes, which are typically composed of carbon-based materials. In this study, a carbon-free and two-dimensional cathode structure based on silicene is first proposed for Li–O2 batteries using density functional theory calculations. In contrast to graphene, oxygen reduction reactions (ORR) and oxygen evolution reactions (OER) can occur on the pristine form of silicene without any defect sites. In addition, it was found that reactions on silicene strongly correlate with strong adsorptions of the ORR intermediates, which are caused not only by ionic bonding between the oxygen atoms in the ORR intermediates and silicene but also by the structural stabilization of silicene. Theoretical observations demonstrate the great potential of silicene as a carbon-free cathode structure for Li–O2 batteries and provide further insights for designing a new cathode material architecture based on two-dimensional structured materials.

In the present paper, the band gap characteristics of oxygen functionalized-monolayer scandium carbide (monolayer Sc2CO2) under a perpendicular external electric field (E-field) were studied using DFT calculations for the potential application of MXene in optoelectronic and optical nanodevices. In contrast to general pristine single-layer materials under an external E-field, monolayer Sc2CO2 undergoes an indirect to direct band gap transition under a positive E-field, and the band gap value changes sharply after the band gap transition. Remarkable variations of the band gap properties are induced by the distinct sensitivity between the Γ and K points in the lowest conduction band to the perpendicular E-field, and different types of orbital lead to the dissimilar response of each point. The present work clearly suggests an effective direction to obtain attractive band gap properties in monolayer MXene using an external E-field for next generation optoelectronic and optical devices.

The electronic properties of 3d transition metal (TM)-decorated silicene were investigated by using density functional calculations in an attempt to replace graphene in electronic applications, owing to its better compatibility with Si-based technology. Among the ten types of TM-doped silicene (TM-silicene) studied, Ti-, Ni-, and Zn-doped silicene became semiconductors, whereas Co and Cu doping changed the substrate to a half-metallic material. Interestingly, in cases of Ti- and Cu-doped silicene, the measured band gaps turned out to be significantly larger than the previously reported band gap in silicene. The observed band-gap openings at the Fermi level were induced by breaking the sublattice symmetry caused by two structural changes, that is, the Jahn-Teller distortion and protrusion of the TM atom. The present calculation of the band gap in TM-silicene suggests useful guidance for future experiments to fabricate various silicene-based applications such as a field-effect transistor, single-spin electron source, and nonvolatile magnetic random-access memory.

Using First-principle calculations, the substrate effect of O-terminated ( 3 × 3 ) MgO (1 1 1) on graphene was investigated for spintronics application. It was found that the graphene could be turned into a spin-polarized semiconductor due to alternative sp3 chemisorption with the substrate. The majority spin channel has an insulating band gap of 2 eV, while the minority spin channel has a semiconducting band gap of 0.3 eV. These results imply that the graphene with a tailored pattern of chemisorption could be highly efficient for introducing a totally spin-polarized current and controlling its on/off switching.

MXene has not yet been investigated in optical applications because it is a newly suggested two-dimensional material. In the present work, the first investigation of the prospects of MXene as a novel optical nanodevice was done by applying strain to monolayer Sc2CO2 using first-principles density-functional theory. This single-layer material experiences an indirect to direct band gap transition with variation of the band gap size at a relatively small critical strain of about 2%. The present work emphasizes that monolayer MXene can become a promising material for an optical nanodevice by modulating the band gap properties using strain engineering.

The magnetic property of a graphene-adsorbed Fe adatom was observed under an external electric field effect (−0.3–0.3 eV/A) using density functional theory (DFT) calculations. In this study, it was demonstrated that the magnetic moment of Fe on graphene was changed linearly according to the electric field. The density of states and differential planar-averaged charge-density indicated that the changing electronic structure was due to a redistribution of valence electrons under external electric field that induces a continuous change in the localized magnetic moment of the Fe adatom. This research suggests that the magnetic property of the adatom on graphene is tunable by an electric field. Furthermore, these results may be applicable to the spintronic memory device industry.

An external strain is suggested as an effective means to finely control the magnetic properties of a candidate medium for future spintronics devices. To demonstrate the potential of this concept, the biaxial strain effects on the magnetic moment of Co adatom on pyridinic N-doped graphene (PNG) sheet were investigated using density functional theory calculations. Under the strain from -5% to 5%, the magnetic moment of the Co adatom on PNG was increased continuously from 1.72 to 1.95 μ B . Also, Co adatoms are expected to be dispersed on the PNG surface without metal clustering in this range of applied strain due to its strong binding with the pyridinic nitrogen defects. From these results, it is anticipated that reliable control of magnetism in Co decorated PNG system is available by use of an external strain.

In this paper, a comparative study of the adsorption behavior of single metal atoms (Li, Sc, Ti, Co, Ni, and Cu) on two systems, a free-standing hexagonal boron nitride (h-BN) sheet and an h-BN/Ni (1 1 1) surface, was performed using density functional theory calculations. It was found that the Ni (1 1 1) supporting layer under the h-BN sheet could significantly improves the adsorption energies for single metal adatoms with h-BN. In particular, in the case of Li and Sc, the improved adsorption energies were higher than the cohesive energies of their atoms. The mechanism for these strong adsorptions was primarily due to the charge transfer increases from the adsorbed metal atoms to the h-BN, except for the case of Ni. On the other hand, the adsorption behavior was greatly affected by the interface interaction between the h-BN and Ni (1 1 1) for the adsorption of a single Ni atom. These results may provide fundamental information on the interaction between the adsorbed metal atoms between the h-BN based systems, and suggest that the use of a metal-adsorbed h-BN/Ni (1 1 1) system has good potential for nanosensors and nanocatalysts.

The effect of gas adsorption on the change in magnetic properties of platinum doped graphene (Pt-graphene) system was investigated using first-principles density-functional theory (DFT). Four chemisorbed gas molecules (N2, O2, NO2, SO2) on Pt-graphene each induced a different type of magnetic properties. For N2 adsorption, there was no spin polarization. However, for the other cases, magnetic properties were altered by occurring spin polarization. O2 adsorption led to local polarization on the gas molecule, and two types of complete polarization were introduced on Pt-graphene by NO2 and SO2 adsorption. Also, in the latter two cases, an interesting difference was found in the spin direction of gas and Pt-graphene. NO2 adsorption induced the same spin direction on the adsorbate and substrate, while SO2 adsorption introduced the opposite spin directions. Thus, these differences in magnetic properties of the Pt-graphene according to the type of adsorbed gas molecules are expected to play a vital role in application as gas sensor or spintronic devices.

Adsorptions of gas molecules were found to alter the directions and magnitudes of magnetic moments of transition metal (Co, Fe) atoms adsorbed on graphene. Using first-principles calculations, we demonstrated that magnetism of surface atoms can be used to identify the kind of existing gas molecules via spin-reorientation and/or demagnetizations caused by the reconfigurations of 3d electron energy levels of Co and Fe. We suggest for the first time that magnetic properties of transition metal-embedded nanostructures can be used in highly selective gas-sensing applications.

The effect of substituting sodium for calcium on enhanced osteoconductivity of hydroxyapatite was newly investigated. Sodium-substituted hydroxyapatite was synthesized by reacting calcium hydroxide and phosphoric acid with sodium nitrate followed by sintering. As a control, pure hydroxyapatite was prepared under identical conditions, but without the addition of sodium nitrate. Substitution of calcium with sodium in hydroxyapatite produced the structural vacancies for carbonate ion from phosphate site and hydrogen ion from hydroxide site of hydroxyapatite after sintering. The total system energy of sodium-substituted hydroxyapatite with structural defects calculated by ab initio methods based on quantum mechanics was much higher than that of hydroxyapatite, suggesting that the sodium-substituted hydroxyapatite was energetically less stable compared with hydroxyapatite. Indeed, sodium-substituted hydroxyapatite exhibited higher dissolution behavior of constituent elements of hydroxyapatite in simulated body fluid (SBF) and Tris-buffered deionized water compared with hydroxyapatite, which directly affected low-crystalline hydroxyl-carbonate apatite forming capacity by increasing the degree of apatite supersaturation in SBF. Actually, sodium-substituted hydroxyapatite exhibited markedly improved low-crystalline hydroxyl-carbonate apatite forming capacity in SBF and noticeably higher osteoconductivity 4 weeks after implantation in calvarial defects of New Zealand white rabbits compared with hydroxyapatite. In addition, there were no statistically significant differences between hydroxyapatite and sodium-substituted hydroxyapatite on cytotoxicity as determined by BCA assay. Taken together, these results indicate that sodium-substituted hydroxyapatite with structural defects has promising potential for use as a bone grafting material due to its enhanced osteoconductivity compared with hydroxyapatite.

Density functional theory was used to investigate the electrostatic effect of various oxygen impurities at the interface of MgO/Ag, including interstitial oxygen defects, substitutional oxygen defects, and reconstructed substitutional oxygen defects. When interstitial and reconstructed substitutional oxygen impurities were generated at the interface, an additional bond with the film was formed and the work function of the interface increased. On the other hand, in the case of substitutional oxygen generated at the interface, the oxygen impurities migrated into the Ag subinterface layer and the work function of the interface was slightly decreased. It can be inferred that the origin of the work function change is the dipole moment induced by oxygen impurities. The results of this study indicate that the work function of MgO/Ag (001) can be finely tuned with interfacial oxygen impurities.