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X-ray photoelectron spectroscopy (XPS) is among the most utilized analytical methods for nitrogen-doped carbon materials. Clarifying the assignments of nitrogen-doped carbon materials with different degrees of carbonization, which relates to conjugated systems, is essential to correlate structures with the performance of various applications, but such precise assignments were challenging. In this work, precise analyses were conducted to overcome the difficulty to assign the peaks of carbon materials with different degrees of carbonization, different edges, and various nitrogen-containing functional groups in graphene nanoribbons (GNRs), such as nitrile, pyridinic, primary with/without charges, secondary with/without charge, tertiary without charges (graphitic nitrogen), and quaternary nitrogen. Electrons donated from hydrogen and Madelung potentials showed a higher correlation to the peak shift of C1s and N1s XPS spectra than Mulliken charges on nitrogen. Zigzag edges showed a greater influence on the peak shift of tertiary amine (graphitic nitrogen) than armchair edges on XPS spectra. Besides, as the degree of carbonization is increased from aromatic compound-like structures to GNRs, the peak positions of C–N in C1s and N1s XPS spectra shifted to higher binding energies. This research proved that XPS could be applied to the precise structural analyses of nitrogen-containing carbon materials.

Oxygen-containing carbon materials have been studied extensively because of their excellent dispersibility, absorptivity, separability, and supportability of catalysts. However, structural control by existing top-down methods is almost impossible. Our group has demonstrated that phloroglucinol (PG, 1,3,5-trihydroxybenzene) can be a promising raw material to synthesize structurally controlled oxygen-containing carbon materials. In this study, in addition to PG, hexahydroxybenzene (HHB), which has more oxygen and high symmetry, was used as the raw material, and a Lewis acid catalyst, tris (pentafluorophenyl) borane (TPB), was used to enhance the structural control rate and the removability of catalysts from the carbonized samples. The solubility of heat-treated HHB was lower than that of heat-treated PG, but the oxygen content of heat-treated HHB was higher than that of heat-treated PG even at 673 K. By adding TPB to PG, dibenzofuran-like structures formed, and the structural control rate increased up to 93.6%. Besides, the content of fluorine in the catalyst was reduced to 0%, indicating that TPB can be a promising recyclable catalyst to promote the structural control rate of carbonized PG.

X-ray photoelectron spectroscopy (XPS) is among the most powerful techniques to analyse structures of nitrogen-doped carbon materials. However, reported assignments of (1) graphitic nitrogen (N)/substitutional N, quaternary N (Q–N), or tertiary amine (T–N) and (2) pyrrolic N/secondary amine or T–N are questionable. Most reports assign peaks at ca. 401 eV as Q–N or graphitic N, whereas raw materials in most of those works contain neither counter anion nor halogen. Besides, the peak at ca. 400 eV has been assigned as pyrrolic N, but the presence of N–H is generally not confirmed. In this work, it was clarified that one of the reasons for the prevailing ambiguous assignments is the presence of N in heptagonal and pentagonal rings. The peaks at 400.1–401.2 eV were determined to be T–N, but not Q–N by analyzing graphitized polyimide (with the oxygen content of 0.01 at% or lower and the hydrogen content of 0 at%) using Raman spectroscopy, XPS, X-ray diffraction, total neutron scattering, elemental analysis, and molecular dynamics simulation. Besides, it was revealed that the peak at 400.1 eV originated from T–N on 5-membered rings or 7- and 5-membered rings, but not pyrrolic N because graphite including no hydrogen was used for analysis.

Carbonization process of pyromellitic dianhydride (PMDA)-4,4′-diaminodiphenyl ether (ODA)-type polyimide has been studied for decades. However, various reaction mechanisms have been proposed and the detailed mechanisms are still controversial. It is essential to understand the carbonized structures of PMDA-ODA-type polyimide before analyzing the defect structures in graphite. In this work, the carbonization mechanisms of polyimide heated at 1273 K or lower were unveiled and the methodology to analyze carbon materials containing nitrogen, oxygen, pentagons, and heptagons using computational spectral analysis combined with molecular dynamics simulation (ReaxFF) are exhibited. For example, the formations of isoimide and cyclic ether were estimated by ReaxFF, and the presence was supported by experimental and calculated X-ray photoelectron spectroscopy (XPS) and carbon-13 nuclear magnetic resonance (13C NMR) spectra of polyimide heated between 813 and 873 K. The formations of pentagons and heptagon, suggested by ReaxFF, were also supported by Raman and 13C NMR spectra of polyimide heated between 833 and 1273 K. This work also revealed that the previously reported structures were unstable and that amine in the basal plane was the most plausible structure in polyimide heated at 1073 K or higher, as clarified by XPS, ReaxFF, and the energy calculation.

The bromination reactivity of various types of polycyclic aromatic hydrocarbons (PAHs) with oxygen atoms and graphene with oxygen atoms was estimated by density functional theory calculation and experimentally clarified by analyzing bromination of PAHs using gas chromatography–mass spectrometry. In the experimental and theoretical bromination reactivity of PAHs, the presence of hydroxyl group increased the reactivity of PAHs because of electron-donating nature of the hydroxyl group but the other oxygen-containing functional groups such as lactone, ether, and ketone decreased the reactivity due to the electron-withdrawing nature of those groups. These effects of functional groups on the reactivity were also confirmed in graphene. The tendency of theoretical bromination reactivity of graphene was graphene with hydroxyl group > graphene with no group > graphene with lactone group > graphene with ether group > graphene with ketone group. Our study on the estimation of bromination reactivity of graphene edges provides the groundwork for the bromination of graphene edges.

The performance of graphene-based electronic devices depends critically on the existence of topological defects such as heptagons. Identifying heptagons at the atomic scale is important to completely understand the electronic properties of these materials. In this study, we report an atomic-scale analysis of graphene nanoflakes with two to eight isolated or connected heptagons, using simulated C 1s X-ray photoelectron spectroscopy (XPS) to estimate the XPS profiles depending on the density and the position of the heptagons. The introduction of up to 24% of isolated heptagons shifted the peak position toward high binding energies (284.0 to 284.3 eV), whereas the introduction of up to 39% of connected heptagons shifted the calculated peak position toward low binding energies (284.0 to 283.5 eV). The presence of heptagons also influenced the full width at half-maximum (FWHM). The introduction of 24% of isolated heptagons increased the FWHMs from 1.25 to 1.50 eV. However, the introduction of connected heptagons did not increase the FWHMs above 1.40 eV. The FWHMs increased to 1.40 eV for 19% of connected heptagons, but did not increase further as the percentage of connected heptagons increased to 39%. Based on the calculated results, the XPS profiles of graphene nanoflakes containing heptagons with different densities and positions can be obtained. Our precise identification of heptagons in graphene nanoflakes by XPS lays the groundwork for the analysis of graphene.

Oxygen-containing carbon materials such as graphene oxide have been extensively studied because of their high dispersibility. However, the oxygen-containing functional groups in most carbon materials are not controlled. Uncontrollability of the synthesis is also one of factors that prevent industrialization. Carbon materials derived from phloroglucinol (PG), which show high solubility/dispersibility and controllability of functional groups, have been developed recently by our group. The high performance of carbonized PG originates from the thermally stable backbone structure of the benzene ring with hydroxy groups of PG. However, the degree of carbonization was low. In this study, five heteropoly acids (HPAs), which are thermally stable homogeneous strong acid catalysts, were used to promote carbonization of PG without losing the controllability of functional groups and the dispersibility. HPAs promoted etherification of hydroxy groups followed by C=C coupling reactions (furan cyclization) at 523 K. Furthermore, it was confirmed that particularly furan structures, which contribute to solubility/dispersibility in solvents, and thermal stability in air, could be maintained at 673 K as suggested by spectroscopies and thermogravimetric-differential thermal analysis. Among five HPAs, phosphotungstic acid worked as the excellent catalyst to promote carbonization of PG containing furan structures, exhibiting high solubility/dispersibility and high thermal stability in air.

Pentagons in carbon materials have attracted attentions because of the potential high chemical reactivity, band gap control, and electrochemical activity. However, it is challenging to prepare a carbon film with high pentagon density because of the curvature and the high reactivity caused by the presence of pentagons, and it is also challenging to estimate the percentage of pentagons in carbon materials because of the limitation of current analytical techniques. In this work, the percentage of pentagons in carbon materials was experimentally estimated for the first time using experimental and calculated C1s X-ray photoelectron spectroscopy and elemental analysis. Carbon films with 7% of pentagons (40% of pentagons compared to the raw material) with electrical resistivity of 1.1 × 104 Ω meter were prepared by heat treatment of corannulene at 873 K. On the other hand, fluoranthene and fullerene remained as non-film solid and powder without forming films at 873 K. Experimental and calculated Raman and IR spectra revealed the peaks of different types of pentagons. Decrement of pentagons in corannulene and fluoranthene heated at high temperatures can be explained mainly by the scission of C=C bond in pentagons, as suggested by the results of reactive molecular dynamics simulation (ReaxFF).

Oxygen-containing carbon materials such as graphene oxide have been studied intensively for a decade because of the high oxygen content, which is beneficial to disperse carbon materials in solutions and to support either metals or metal oxides on carbon materials. However, various oxygen-containing functional groups exist on carbon materials and controlling the structures is almost impossible. In this work, phloroglucinol (PG), which has a symmetrical structure with three hydroxyl groups relative to six aromatic carbon atoms, was found to be the best precursor among PG, cyanuric acid, trimesic acid, and melamine because of the high yield (63 wt%) at 573 K even in an open system which is essential for mass production. The materials synthesized from PG also showed the high dispersibility and/or solubility in solvents (N-methyl-2-pyrrolidone and N,N-dimethylformamide) and the low temperature to form carbon materials (573 K), which can be explained from (002) and (10) in X-ray diffraction pattern. X-ray photoelectron spectroscopy, Fourier transform infrared spectrometer, carbon-13 nuclear magnetic resonance with the aid of calculation of both spectra and carbonization mechanisms revealed that the high solubility of carbonized PG originates from the presence of ether and cyclic ether, which were formed from dehydration of hydroxy groups, and also some remained hydroxyl groups in carbonized PG. Oxygen-containing groups in carbonized PG were effective as an antioxidant. In addition, the coating of carbonized PG on silica nanoparticles imparted conductivity and lubricity to silica nanoparticles.

Determination of the number of bromo groups and brominated positions of edges on carbon materials is essential for selective functionalization because the brominated positions influence their electronic structures. However, the determination is challenging because the analysis of introduced bromo groups on carbon materials is conventionally limited to quantitative analysis by X-ray photoelectron spectroscopy. In this work, infrared spectra (IR) of brominated aromatic compounds were analyzed experimentally and theoretically using reference aromatic compounds to verify that the calculated peak positions can be used to estimate the experimental peak positions of brominated aromatic compounds. IR spectra of brominated graphene nanoribbons (GNRs) with zigzag, armchair, and other edges were simulated as one of the carbon materials with clear edge structures. Their characteristic peak positions and tendencies of shifts were explained from the point of view of Mulliken charge, C–H and C=C length, and orbital hybridization. As the number of Br on GNRs increased, the peak positions basically shifted further, indicating that the density of Br and introduced positions of Br can be estimated from IR spectra. The detailed assignments obtained in this work will lead to selective functionalization of carbon materials.

Identifying pentagons and heptagons in graphene nanoflake (GNF) structures at the atomic scale is important to completely understand the chemical and physical properties of these materials. Herein, we used X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy to analyze the spectral features of GNFs according to the position of pentagons and heptagons introduced onto their zigzag and armchair edges. The XPS peak maxima were shifted to higher binding energies by introducing the pentagons or heptagons on armchair rather than zigzag edges, and the structures could be distinguished depending on the positions of the introduced pentagons or heptagons. Raman spectroscopic analyses also revealed that the position of edges with introduced pentagons or heptagons could also be identified using Raman spectroscopy, with characteristic bands appearing at 800-1200 cm-1, following the introduction of either pentagons or heptagons on armchair edges. This precise spectroscopic identification of pentagons and heptagons in GNFs provides the groundwork for the analysis of graphene-related materials.

The optical properties of monolayer materials can be indirectly tuned by the dielectric properties of surrounding materials. Although proof-of-concepts for this so-called Coulomb engineering have a...

Monolayer materials are strongly affected by their potential fluctuations, which can be induced by an intrinsic corrugation or the surface roughness of the substrate. We compare the effective exciton-exciton annihilation (EEA) rate constants of monolayer ${\mathrm{WS}}_{2}$ on substrates with different surface topographies. We show that the ${\mathrm{WS}}_{2}$ monolayers on the substrates with atomically flat terraces have small effective EEA rate constants that deviate from the overall tendency and exhibit multiple exciton decay components, which cannot be accounted for by the conventional EEA model. To obtain a correct description, it is important to use a quantized EEA model. The intrinsic EEA rate constants for the flat-terrace substrates determined by this model are comparable to that of hBN-encapsulated monolayer ${\mathrm{WS}}_{2}$.

We characterized the temperature dependence of the complex dielectric susceptibility spectrum of a perovskite $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Pb}{\mathrm{Br}}_{3}$ single crystal by using heterodyne interference spectroscopy. Owing to a time-domain measurement that is not influenced by incoherent lattice relaxations, we were able to detect a low-energy indirect-gap band. We observed two peaks in the energy region below the exciton transition at the $R$ point in the imaginary part of the spectrum. The temperature dependences of the energies of these peaks were different from the dependence of the energy of the direct-transition exciton. The trends of both the splitting width and peak intensities can be explained by assuming a Rashba spin-split band.

Isotropic pitch-based carbon fiber has been utilized for various applications such as thermal insulation materials for high-temperature furnace and additives for slide member, but the structure of the carbon fiber is still under debate. This is because the precursor pitch contains various aromatic compounds in addition to the complicated oxidation and carbonization reaction during production process of carbon fiber. In this work, oxidation processes of model compounds of pitch such as pyrene (sp2C H with zigzag-like edges), triphenylene (sp2C H with armchair edges), fluorene (pentagon with sp3CH2), and 9-methylanthracene (hexagon with sp3CH3) were analyzed using mass spectrometry, elemental analysis, infrared (IR), Raman, and X-ray photoelectron spectroscopy (XPS) in addition to calculation such as molecular dynamic simulation with a reactive force field (ReaxFF) and simulation of IR, Raman, and XPS spectra (Gaussian09). Combination of calculations and experiments revealed that oxidation proceeded in the order of zigzag-like edges > sp3CH3 > sp2CH2 > armchair edges, and the quinone formed by oxidation worked as a precursor for a crosslinking reaction. It is expected that the results of this work, which revealed the origin of the crosslinking, could lead to improve properties as well as to estimate the detailed structures of isotropic pitch-based carbon fiber.

Graphene nanoribbons (GNRs) have recently emerged as alternative 2D semiconductors owing to their fascinating electronic properties that include tunable band gaps and high charge-carrier mobilities. Identifying the atomic-scale edge structures of GNRs through structural investigations is very important to fully understand the electronic properties of these materials. Herein, we report an atomic-scale analysis of GNRs using simulated X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Tetracene with zigzag edges and chrysene with armchair edges were selected as initial model structures, and their XPS and Raman spectra were analyzed. Structurally expanded nanoribbons based on tetracene and chrysene, in which zigzag and armchair edges were combined in various ratios, were then simulated. The edge structures of chain-shaped nanoribbons composed only of either zigzag edges or armchair edges were distinguishable by XPS and Raman spectroscopy, depending on the edge type. It was also possible to distinguish planar nanoribbons consisting of both zigzag and armchair edges with zigzag/armchair ratios of 4:1 or 1:4, indicating that it is possible to analyze normally synthesized GNRs because their zigzag to armchair edge ratios are usually greater than 4 or less than 0.25. Our study on the precise identification of GNR edge structures by XPS and Raman spectroscopy provides the groundwork for the analysis of GNRs.

Carbon materials such as graphene and graphene nanoribbon with zigzag and armchair edges have attracted much attention because of various applications such as electronics, batteries, adsorbents, and catalyst supports. Preparation of carbon materials with different edge structures at a large scale is essential for the future of carbon materials, but it is generally difficult and expensive because of the necessity of organic synthesis on metal substrates. This work demonstrated a simple preparation method of carbon materials with zigzag and armchair edges with/without nonmetallic silica supports from aromatic compounds such as tetracene with zigzag edges and chrysene with armchair edges and also determined the edge structures in detail by three types of analyses such as (1) reactive molecular dynamic simulation with a reactive force field, (2) Raman and infrared (IR) spectra combined with calculation of spectra, and (3) reactivity analyzed by oxidative gasification using thermogravimetric analysis. Two different types of carbon materials with characteristic Raman and IR spectra could be prepared. These carbon materials with different edge structures also clearly showed different tendency in oxidative gasification. This work did not only show the simple preparation method of carbon materials with different edge structures, but also contributes to the development of detailed analyses for carbon materials.

Cu-Ni/SiO2 catalyst was developed by impregnation assisted with citric acid for the vapor-phase hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL). The prepared catalysts were characterized by using X-ray powder diffraction (XRD), transmission electron microscope (TEM), temperature-programed reduction, thermogravimetry analysis, and X-ray fluorescence analysis. The XRD analysis and the TEM observation of the catalyst samples indicate that the usage of citric acid in the preparation process of impregnation improved the dispersion of Cu and Ni metal on SiO2. A Cu-Ni/SiO2 catalyst with Cu loading of 4 wt.% and Ni of 16 wt.% showed the best performance in the hydrogenation of LA to GVL under ambient H2 pressure at a high weight hourly space velocity (WHSV) of 13.2 h−1 and 250 °C: the conversion was 98.2% and the GVL selectivity was 97.6%. The catalyst prepared by citric acid-assisted impregnation showed the resistance to catalyst deactivation.

We studied the optical responses of organic–inorganic halide perovskite CH3NH3PbBr3 single crystals doped with heterovalent Bi3+ ions (electron densities up to 2.3 × 1012 cm–3). The Bi doping causes no significant changes in the band gap energy but leads to an enhanced Urbach tail and photoluminescence blue shift. On the basis of the time-resolved photoluminescence measurements, we attribute the PL response to a shorter carrier lifetime induced by Bi doping, which results in a reduced photon recycling effect (i.e., the repeated emission and reabsorption of photons inside the crystal). We discuss the physical relation between Bi concentration and the optical and electric properties of Bi-doped CH3NH3PbBr3 and reveal the unique nature of the Bi3+ ion as a dopant in halide perovskites.

Despite recent advances in the carbonization of organic crystalline solids like metal-organic frameworks or supramolecular frameworks, it has been challenging to convert crystalline organic solids into ordered carbonaceous frameworks. Herein, we report a route to attaining such ordered frameworks via the carbonization of an organic crystal of a Ni-containing cyclic porphyrin dimer (Ni2-CPDPy). This dimer comprises two Ni–porphyrins linked by two butadiyne (diacetylene) moieties through phenyl groups. The Ni2-CPDPy crystal is thermally converted into a crystalline covalent-organic framework at 581 K and is further converted into ordered carbonaceous frameworks equipped with electrical conductivity by subsequent carbonization at 873–1073 K. In addition, the porphyrin’s Ni–N4 unit is also well retained and embedded in the final framework. The resulting ordered carbonaceous frameworks exhibit an intermediate structure, between organic-based frameworks and carbon materials, with advantageous electrocatalysis. This principle enables the chemical molecular-level structural design of three-dimensional carbonaceous frameworks., Carbon-based materials are promising alternatives to noble metal catalysts, but their structures are typically disordered and difficult to control. Here, the authors obtain ordered carbonaceous frameworks with advantageous electrocatalytic properties via the carbonization of nickel-containing porphyrin dimer networks.

A mixture of acetylacetone and water was condensed in low-temperature Ar matrices at 5 K, and infrared spectra were observed. It was found that the H2O molecules formed hydrogen bonds with the acetylacetone. The structure of the H2O-C5H8O2 complex was estimated with the aid of a density functional calculation. A similar sample was prepared using deuterium-substituted water to confirm the structure of the complex. Upon annealing the matrix-isolated sample, aggregation of H2O molecules was observed and the amount of the complex remained unchanged.

To investigate the interactions of 57Fe atoms with sulfur hexafluoride SF6 molecules, in-beam Mossbauer spectra were measured for 57Mn implanted SF6 at 9 K and 65 K. Isolated 57Fe atoms or ions produced by β-decay were not trapped within the SF6 matrix. At 9 K, monomeric FeF2 and FeF3 molecules were produced by reaction of 57Fe with F atoms released by decomposition of the SF6 molecules. When the temperature of the SF6 solid was increased to 65 K, FeF4 was formed in addition to FeF2 and FeF3. Density functional calculations were performed to confirm the assignments of the candidate species identified in the Mossbauer spectra.

Iron nitrides with a high nitrogen content are metastable and can be prepared in the form of fine particles or as a film. An arc discharge of iron metal in a reactive atmosphere provides a convenient method to produce iron-based films. In this study, iron nitride films were formed on an aluminum substrate by arc deposition of iron in a nitrogen atmosphere under a controlled nitrogen flow rate (10–30 sccm). The samples were analyzed by transmission Mossbauer spectroscopy and X-ray diffraction. When the temperature of the aluminum substrate during deposition was maintained at 298 K, e-Fe3-2N was obtained for nitrogen flow rates in the range 20 to 30 sccm. When the aluminum substrate was heated to 573 K during deposition with nitrogen flow rates of 15 to 30 sccm, a highly crystalline e-Fe2N film was formed.

Metastable iron carbide thin films (χ-Fe5C2 and o-Fe7C3) were produced by pulsed laser deposition of Fe in a CH4 atmosphere, and their Mossbauer spectra and X-ray diffraction patterns were measured. Films consisting of amorphous Fe-C carbide were obtained when the substrate temperature was kept at 300 K during deposition, while crystalline films were produced for a substrate temperature of 573 K. The effect of CH4 pressure was investigated. Films produced below 4.0 Pa consisted of a combination of χ-Fe5C2 and α-Fe, and single-phase carbide films were produced at higher pressure: pure χ-Fe5C2 and o-Fe7C3 films were produced at 4.0 and 6.0 Pa, respectively. At 13 Pa, a film was produced consisting of o-Fe7C3 and paramagnetic amorphous Fe-C containing a large amount of C atoms. The film produced at the highest pressure of 40 Pa consisted solely of paramagnetic amorphous Fe-C.

Halide perovskites are attracting attentions as a novel class of semiconductor materials for devices such as solar cells, light-emitting diodes, lasers, and photodetectors. High-quality perovskite crystals can be fabricated with different forms such as bulk crystals, thin films and nanocrystals. These different geometries influence photoluminescence (PL) properties, and the characteristic differences of PL spectra between bulk single crystals and polycrystalline thin films are originated from the carrier diffusion and PL reabsorption. Nanocrystals exhibit optical properties different from their bulk counterparts due to size effects. Furthermore, the bandgap energies of perovskites can be tuned over a wide range by controlling the constituents. Unique anti-Stokes PL and high-order harmonic generation have also been demonstrated in nanocrystals in addition to bulk crystals and thin films. In this review article, we provide a brief summary of the PL and nonliear light emission properties of perovskite semiconductors. We first discuss the influence of the carrier diffusion and PL reabsorption on the PL spectra and dynamics of thick samples, and then the impact of the electron–phonon interactions on PL spectra in bulk crystals and nanocrystals. The last part of this review is devoted to the light emission phenomena induced by nonlinear optical processes in perovskites.

In-beam Mossbauer spectra of 57Mn implanted into LiAlH4 were measured at different temperatures between 17 and 300 K. The Mossbauer spectrum measured at 17 K showed two sets of doublets, which were assigned to 57Fe atoms at substitutional sites at Al3+ and Li+ sites. The Debye temperatures θM for the 57Fe atoms at Al3+-substituted and Li+-substituted sites were estimated to be 194 K and 117 K, respectively. The assignments were confirmed by density functional theory calculations.

Carrier mobility is one of the most fundamental material parameters of semiconductors and requisite for device applications and interpretation of physical phenomena. We determined the electron and hole mobilities of a CH3NH3PbBr3 single crystal in the high-carrier density regime by combining AC Hall measurements under photoexcitation and two-carrier analysis. Both electron and hole mobilities were significantly enhanced by photo-doping and exceeded 300 cm2 V−1 s−1, which are comparable to the electron Hall mobilities of conventional inorganic semiconductors. Our experimental results indicate that charged dislocation scattering dominates the carrier transport at room temperature in the low-carrier density regime.

δ-FeOOH nanoparticles were synthesized via the oxidation of precipitates obtained from the reaction of FeCl2 and N2H4 in the presence of sodium tartrate and gelatin in an alkaline condition. These δ-FeOOH particles were subsequently examined using transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), powder X-ray diffraction (XRD), Mossbauer spectroscopy, and superconducting quantum interference device (SQUID) assessment. The average size of the δ-FeOOH nanoparticles was below 10 nm, and these particles exhibited superparamagnetic behavior as a result of this small size. The precursors of the δ-FeOOH nanoparticles were also characterized as a means of elucidating the reaction mechanism. Precipitates prior to oxidation upon rinsing with water and ethanol were analyzed by obtaining XRD patterns and Mossbauer spectra of wet and frozen samples, respectively. The precipitates obtained by the reaction of FeCl2 and N2H4 were found to consist of a mixture of Fe3O4 and Fe(OH)2, and it is believed that these species then rapidly oxidized into δ-FeOOH nanoparticles.

Iron phosphides, FeP2 and FeP4, have been synthesized and characterized for the application to non-aqueous Na-ion battery. FeP2 shows no significant electrochemical reactivity in Na-cell. However, FeP4 composite electrode with sodium polyacrylate binder delivers a reversible capacity of 1137 mAh·g−1 and a Coulombic efficiency of 84.0% during the first cycle under a current density of 89 mA·g−1. The high capacity is maintained for 30 cycles. Moreover, FeP4 composite electrode presents a good rate capability. Although the sodiation mechanism of FeP4 has not been fully understood, FeP4 is a new promising negative electrode material for Na-ion batteries with both high-power and high-energy densities. Keywords: Na-ion battery, Negative electrode material, Conversion material, Transition metal phosphide

Selective doping of nitrogen into carbon materials in the absence of catalysts is a key for various applications of carbon materials. It is well known that most nitrogen-containing raw materials generate unnecessary functional groups during carbonization. Many researchers have noticed the significance of the selective nitrogen doping, whereas only a few works have reported the selective doping. In addition, those few works used catalysts to synthesize nitrogen-doped carbon materials, but the presence of catalysts limits the applications of nitrogen-doped carbon materials. This study found an unusual aromatic compound, imidazo[1,2-a]pyridine (IP), which maintained 88 % of the functional groups of as-received IP even after carbonization at 673 K in the absence of catalysts, and the functional groups were further maintained up to 773 K. The percentage of remaining functional groups was revealed using our state-of-the-art techniques of simulated X-ray photoelectron spectroscopy and Raman spectroscopy combined with transition state calculation using density functional theory. The low carbonization temperature as well as selective doping of nitrogen was achieved because of the low activation energy of dehydrogenation reaction among IP molecules compared to the high activation energy of radical formation for scission of C–N bonding.

We have investigated the dynamic optical properties of CH3NH3PbI3 (MAPbI3) perovskite thin films at low temperatures using time-resolved photoluminescence, optical transient absorption (TA), and THz TA spectroscopy. Optical spectroscopic results indicate that the high-temperature tetragonal phase still remains in the MAPbI3 thin films at low temperatures in addition to the major orthorhombic phase. The fast charge transfer from the orthorhombic phase to the tetragonal phase is likely to suppress the formation of excitons in the orthorhombic phase. Consequently, the near-band-edge optical responses of the photocarriers in both the tetragonal and orthorhombic phases of the MAPbI3 thin films are more accurately described by a free-carrier model, rather than an excitonic model even at low temperatures.

Although O3-NaFe1/2 Mn1/2 O2 delivers a large capacity of over 150 mAh g-1 in an aprotic Na cell, its moist-air stability and cycle stability are unsatisfactory for practical use. Slightly Na-deficient O3-Na5/6 Fe1/2 Mn1/2 O2 (O3-Na5/6 FeMn) and O3-Na5/6 Fe1/3 Mn1/2 Me1/6 O2 (Me = Mg or Cu, O3-FeMnMe) are newly synthesized. The Cu and Mg doping provides higher moist-air stability. O3-Na5/6 FeMn, O3-FeMnCu, and O3-FeMnMg deliver first discharge capacities of 193, 176, and 196 mAh g-1 , respectively. Despite partial replacement of Fe with redox inactive Mg, oxide ions in O3-FeMnMg participate in the redox reaction more apparently than O3-Na5/6 FeMn. X-ray diffraction studies unveil the formation of a P-O intergrowth phase during charging up to >4.0 V.

Lead halide perovskite semiconductors are promising materials for optical devices due to their excellent optoelectronic properties. In addition to large light absorption coefficients, which originate from a direct band gap structure, they exhibit almost no Stokes shift between absorption edge and photoluminescence (PL) peak. Therefore, the effect of PL reabsorption plays an important role for their emission spectra in bulk crystals. In this review, we analyze the PL dynamics in CH3NH3PbX3 (X = I, Br, Cl) single crystals to provide a clear picture of the dynamical effects of PL reabsorption that occur in perovskites. This knowledge is essential for accurate characterization via PL and photon management for optical devices. We also show that the grain size dependence of PL spectra obtained from polycrystalline thin films can be interpreted by PL reabsorption.

We performed terahertz time-domain spectroscopy for methylammonium (MA) lead halide perovskite single crystals and characterized the longitudinal optical (LO) phonons directly. We found that the effective LO phonon wave number does not change in the wide temperature range between 10 and 300 K. However, the coupling between MA cation modes and the LO phonon mode derived from lead halide cages induces a mode splitting at low temperatures and a damping of the LO phonon mode at high temperatures. These results influence the interpretation of electron-LO phonon interactions in perovskite semiconductors, as well as the interpretations of mobility, carrier diffusion, and polaron formation.

The local structures of 122-type paradium arsenides, namely BaPd2As2 and SrPd2As2, are examined by As K-edge extended x-ray absorption fine structure measurements to find a possible correlation between the variation of their superconducting transition temperature and the local structure. The local atomic distances are found to be consistent with average distances measured by diffraction techniques. The temperature dependence of mean square relative displacements reveal that, while BaPd2As2 is characterized by a local As-Pd soft mode, albeit with larger atomic disorder, SrPd2As2 shows anomalous As-Pd correlations with a kink at similar to 160 K due to hardening by raising temperature. We have discussed implications of these results and possible mechanisms of differing superconducting transition temperature in relation with the structural instability.

Mn-doped feroxyhyte (δ-FeOOH) nanoparticles were synthesized using a wet chemical method, starting from a mixture of iron and manganese salts. The particles obtained were needle-like, around 100 nm in length, and formed had a nano-urchin structure. The compositions of the four samples obtained were calculated to be δ-Fe0.92Mn0.08OOH, δ-Fe0.75Mn0.25OOH, δ-Fe0.56Mn0.44OOH, and δ-Fe0.32Mn0.68OOH. The superparamagnetic behavior of the nanoparticles was determined from their room-temperature Mossbauer spectra. The hyperfine magnetic field of the Mn-doped δ-FeOOH nanoparticles decreased when iron atoms were substituted by Mn atoms. Furthermore, Mn atoms were locally doped into δ-FeOOH.

Cu-doped maghemite (γ-Fe2O3) and Cu-doped feroxyhyte (δ-FeOOH) nanoparticles were synthesized using a wet chemical method. The synthesis was performed starting from a mixture of iron and copper salts. Cu-doped γ-Fe2O3 and Cu metal nanoparticles were obtained from a solution with a high Cu/Fe ratio, whereas Cu-doped δ-FeOOH nanoparticles were obtained from a solution with a low Cu/Fe ratio. The size of the nanoparticles was below 10 nm. The superparamagnetic behavior of the nanoparticles was determined from their Mossbauer spectra. The hyperfine magnetic field of Cu-doped δ-FeOOH nanoparticles was larger than that of undoped δ-FeOOH nanoparticles. The Cu content of the intermediate species plays an important role in the oxidation process. The intermediate species in the precipitates before rinsing was speculated to be Cu1-xFeIIxFeIII2O4 (x = 0.4–0.7), which was subsequently oxidized in air to form Cu-doped γ-Fe2O3 or Cu-doped δ-FeOOH, depending on the amount of Cu.

Highly efficient three-phase to three-phase matrix converters using Gallium nitride (GaN) bidirectional switches with both high current and high breakdown voltage are demonstrated. The GaN switch with dual gates works as a bidirectional switch by a single device, while a conventional bidirectional switch consists of four devices by two Insulated Gate Bipolar Transistors (IGBTs) and two diodes. In addition, the GaN bidirectional switch is also free from the voltage offsets for the current conduction so that the GaN-based matrix converter enables small size and highly efficient AC/AC conversion. Improvement of the device performance including the introduction of the recessed gate enables the low on-state resistance with stable operation free from current collapse. The maximum drain current reaches 100 A together with the breakdown voltage of 1340 V. The fabricated three-phase to three-phase matrix converter exhibits the maximum conversion efficiency of 98% at 1 kW output power with the expectation that the maximum output power can reach 10 kW or more by the high current device.

C1s X-ray photoelectron spectroscopy (XPS) spectra of graphene with two to eight pentagons and fullerene pentagons were simulated using density functional theory calculation. Peak shifts and full width at half maximum (FWHM) of calculated C1s spectra were compared with those of actual C1s spectra. Introduction of up to four isolated pentagons had no influence on shifts of the calculated peak maxima of graphene (284.0 eV), whereas the introduction of six or more pentagons shifted the calculated peak maximum toward low binding energies because the number of connected pentagons increased. The presence of pentagons also influenced FWHMs. Introduction of six pentagons increased the calculated FWHMs from 1.25 to 1.45 eV, whereas introduction of eight or more pentagons decreased the FWHMs. The FWHM reached at 1.15 eV by introducing twelve pentagons (fullerene). These calculated shifts and FWHMs were close to the actual shifts of graphite (284.0 eV) and fullerene (282.9 eV) and FWHMs of graphite (1.25 eV) and fullerene (1.15 eV). Based on the calculated and the actual results, we proposed peak shifts and FWHMs of graphene with the different number of pentagons, which can be utilized for analyzing actual XPS spectra. Proposed FWHMs can be adjusted by measuring actual FWHMs using each device.

The bromination reactivity of azulene, naphthalene, and graphene with pentagonal, hexagonal zigzag and armchair, and heptagonal edges was theoretically estimated by density functional theory calculation and experimentally clarified by analyzing bromination of azulene and naphthalene using gas chromatography–mass spectrometry and ultraviolet–visible spectroscopy. The experimental and theoretical bromination reactivity of azulene with one pentagon and one heptagon was higher than that of naphthalene with two hexagons because of electron-rich carbon atoms on the pentagon. On the other hand, the tendency of theoretical bromination reactivity of pentagonal, hexagonal, and heptagonal edges on graphene was totally opposite to that on azulene and naphthalene. The order of the bromination reactivity of graphene edges was hexagonal zigzag > pentagonal > heptagonal and hexagonal armchair edges. The highest reactivity of hexagonal zigzag edges can be explained by the largest amount of electrons of carbon atoms among all of edges of graphene.

In-beam Mossbauer spectroscopy of 57Fe using a short-lived 57Mn (T1/2 = 89 s) implantation was applied to study the chemical products of localized Fe atoms in ethylene and acetylene matrices at low temperatures. The spectra obtained in both matrices were analyzed as three doublets. In ethylene at 14 K, Fe(C2H4)2 (Fe0, S = 1), [Fe(C2H4)3]+ (Fe+, S = 3/2), and [Fe(C2H4)2]+ (Fe+, S = 3/2) were assigned as derived from the Mossbauer parameters and density functional theory calculations. On the other hand, the products of [Fe(C2H2)2]+ (Fe+, S = 3/2) and [(C2H2)FeCCH2]+ (Fe+, S = 3/2) were determined in the acetylene matrix at 17 K.

Photoelectronic responses of organic-inorganic hybrid perovskite CH 3 NH 3 PbI 3 on mesoporous TiO 2 electrodes are investigated. On the basis of near-band-edge optical absorption and photoluminescence spectra, the bandgap energy and exciton binding energy as a function of temperature are obtained. The exciton binding energy is much smaller than thermal energy at room temperature, which means that most excitons are thermally dissociated, and optical processes are determined by the photoexcited electrons and holes. We determined the temperature dependence of exciton binding energy, which changes from ~30 meV at 13 K to 6 meV at 300 K. In addition, the bandgap energy and the exciton binding energy show abrupt changes at 150 K due to structural phase transition. Our fundamental optical studies provide essential information for improving the device performance of solar cells based on halide perovskite semiconductors.

This paper describes an ultracompact GaN 3 × 3 matrix power converter with drive-by-microwave (DBM) technology, which comprises a radio frequency (RF)-triggered GaN-gate injection transistor (GIT) bidirectional power switches integration chip with co-integrated RF rectifiers, novel isolated dividing couplers in a printed circuit board to reduce complicated gate lines, and low-consumption GaN/Si DBM gate driver on a chip that controls nine bi-directional power switches. The proposed 4.0-kW GaN 3 × 3 matrix power converter is extremely compact, measuring only 18 × 25 mm due to the use of GaN-GIT power device integration technology and DBM technology that provides isolated gate signals by microwave wireless power transmission and eliminates the need for photo-couplers and isolated power supplies. The GaN/Si DBM driver realizes low power consumption of only 2.0 W as a result of gate power sharing with three RF oscillators. The sequential switching operation by the fabricated GaN 3 × 3 matrix converter was successfully achieved.

Synchrotron-Radiation-based $^{\mathrm {149}}$ Sm Mossbauer spectroscopy was applied to Sm intermetallics, SmBe13 and SmTi2Al20. Temperature dependence of the Mossbauer parameters in SmBe13 indicate the Sm valence state is purely trivalent. SmBe13 also showed second-order Doppler shift in synchrotron-radiation-based $^{\mathrm {149}}$ Sm Mossbauer spectroscopy. The Mossbauer parameters obtained in SmTi2Al20 suggest that the Sm valence is fluctuating and the magnitude of the magnetic moment is reduced by hybridization between 4f and conduction electrons and/or effect of crystal electric field.

Laser ablation in liquid is a useful mean of producing nanoparticles, based on both laser ablation (LA) and laser irradiation (LI) effects. In order to investigate the mechanism by which iron carbide nanoparticles are generated in ethanol, iron carbide nanoparticles were produced by LA of an iron block in a flowing ethanol solvent, which enabled separation and collection of the nanoparticles immediately following the process. These same particles were subsequently subjected to LI while suspended in stagnant ethanol. Both the LA and LA/LI nanoparticles were assessed using Mossbauer spectroscopy, X-ray diffraction and transmission electron microscopy. LA in flowing ethanol was found to produce nanoparticles composed of cementite (Fe3C) and other metastable iron carbides with an average size of 16 nm, dispersed in amorphous carbon. LI of the LA nanoparticles suspended in ethanol increased the particle size to 38 nm and changed the composition to pure Fe3C.