2p Hole Research Articles

Insight into the Role of Metal–Oxygen Bond and O 2p Hole in High-Voltage Cathode LiNixMn2–xO4

The role of transition-metal d and ligand p hybridization continues to be of immense interest in Li-ion battery cathode, and yet it is still poorly understood. Using combined experimental and theoretical soft X-ray absorption spectroscopic study and density functional theory calculation, we investigated the fundamental electronic structure of the high-voltage spinel LiNixMn2–xO4. An oxygen-participating charge rebalance between manganese and nickel ions was found; that is, the content of O 2p holes close to the Fermi level increases along with the increasing Ni content. Moreover, these unstable oxygen holes primarily congregate around the redox active dopants. The underlying mechanism accounting for this charge-compensated occurrence is the reverse of two bonding levels when manganese ions are oxidized from +3 to +4 states. On the basis of these new findings, we further exposed the role of oxygen in electrochemical performance. First, oxygen ions afford the charge variation together with the cations durin...

Multi-electron coincidence spectroscopy: Triple Auger decay of Ar 2p and 2s holes

The detailed study of single photon multiple ionization processes requires the detection in coincidence of all the electrons emitted from the same ionization event. The advent of magnetic bottle experiments made possible the development of this multi-electron coincidence spectroscopy. First we will briefly review the achievements of this technique. Then we will illustrate more specifically its high sensitivity taking as an example the decay by emission of three Auger electrons of 2p and 2s holes in argon. Our results show that the processes are completely different depending on the initial core hole: the three Auger electrons are emitted dominantly in a simultaneous path for the 2p case, but in cascade for the 2s one.

First-principles exploration of sp-electron digital magnetic heterostructures: The case for CaO δ-doped with 2p-block elements boron, carbon, and nitrogen

Most previous studies on digital magnetic heterostructures have been concerned with materials containing d-block and f-block magnetic ions. In this work, we predicted several alkaline-earth monoxide-based digital magnetic heterostructures via anionic substitution by main group elements, exhibiting sp-electron ferromagnetism. First-principles calculations were performed to investigate rock-salt CaO δ-doped with 2p-block elements B, C, and N atoms. Semiconducting and half-metallic ferromagnetic states were found and identified. Spin polarization was mainly derived from the 2p holes induced by substitution, i.e., replacing oxygen with boron, carbon, and nitrogen. The orbital-specific spin-polarized states were characterized and the relationships between the electronic structures and magnetic properties were described and explained. Moreover, the carriers were mostly concentrated within a narrowly confined area around the δ-doped layers, and were for the most part not acted on the host layers, showing a spin polarization of the two-dimensional hole gas.

Investigation of the metal-insulator transition in NdNiO3 films by site-selective X-ray absorption spectroscopy.

In this work, multifunctional oxide NdNiO3 (NNO) thin films grown on a SrTiO3 (STO) substrate using pulsed-laser deposition are studied. Temperature dependent resistivity measurements revealed that NNO/STO samples exhibit a sharp thickness dependent metal-insulator transition (MIT) over a range of 150-200 K. It is known that the electronic properties of correlated oxides are extremely complex and sensitive to changes in orbital occupancy. To evaluate the changes in the electronic and/or crystallographic structure responsible for the MIT, a site-selective (O, Ni and Nd) X-ray absorption near edge structure (XANES) analysis is performed above and below the transition temperature. Analysis of XANES spectra suggests that: (i) in NNO films nominally trivalent Ni ions exhibit multiple valency (bond disproportionation), (ii) intermetallic hybridization plays an important role, (iii) the presence of strong O 2p-O 2p hole correlation at low temperature results in the opening of the p-p gap and (iv) the valency of Nd ions matches well with that of Nd3+. For NNO films exhibiting a sharp MIT, Ni 3d electron localization and concurrent existence of Ni 3d8 and Ni 3d8L[combining low line]2 states are responsible for the observed transition. At temperatures below the MIT the O 2p-O 2p hole correlation is strong enough to split the O 2p band stabilizing insulating phase. Temperature and thickness dependent differences observed in the site-selective XANES data are discussed in terms of possible mechanisms for the MIT (negative charge-transfer type).

New Generation Cathode Materials Realizing Both Safety and High Energy Density

Introduction A lithium ion battery is the most promising battery for the next generation electric vehicle (xEV). The cathode material is a critical element of the lithium ion battery, since it determines the performance. Generally, it has been known that cathode materials have trade-off relation between the energy density and the safety. We proposed in our previous study that the formation of oxygen 2p hole in a layer-type cathode material, LiNi0.5Mn0.5O2, plays an important role in the charge compensation during battery operation[1]. Furthermore, we have cultivated the understanding about the durability and the safety and arrived at a hypothesis that these performances strongly depend on the fixing level of oxygen 2p electron. The excess oxygen 2p hole would cause the crystal degradation or O2 gas generation typically at the surface where mass transfer occurs easily. On the basis of these, we came up with the “trade-on relation” materials possessing both high energy density and safety. We introduced a layer-type LiNi0.33Mn0.16Co0.33Ge0.17O2 as a typical example of the “trade-on relation” materials. Experimental Active materials, LiNi0.33Mn0.16Co0.33Ge0.17O2 (LNMCGO) and LiNi0.33Mn0.34Co0.33O2 (NMC), were obtained by using a liquid-phase synthesis technique. Electrochemical measurements were conducted using a 2032-type coin cell. Cathode-slurry was prepared by mixing the active material, carbon black, and polyvinylidenefluoride with a weight ratio of 80:10:10 in N-methlpyrrolidone solution. The slurry was spread onto an aluminum foil current collector and dried in a 120 °C vacuum oven. The cathode and Li metal anode were separated by a PE membrane. 1M LiPF6 in ethylene carbonate/diethyl carbonate (3:7 v/v) was used as the electrolyte. Cells were assembled in an argon-filled grove box. The charge-discharge operations were repeated twice at a rate of 0.02 C within the range of 2.6-4.5 V using constant current mode. The rest time was set to 10 minutes. Results and Discussion Figure 1 shows the 1st and 2nd cycle performances for LNMCGO and NMC during 2.6 - 4.5 V at room temperature. It is clearly indicated that the coulomb efficiency of LNMCGO is far superior to that of NMC and the difference between the discharge capacities of these materials at the 2nd cycle is almost none. It is considered that the stable high energy capacity bases on the charge compensation due to the oxygen 2p hole formation and the Ge-O strong bonding which limits the excessive over-formation of oxygen 2p hole typically at the surface. We also comfirmed the suprior safety as a battery for LNMCGO by using the TPD-MS technique which could measure the starting temeperature of oxygen elimination reaction and the amount of oxygen on the charged active materials. We plan to talk about the details of the characterization and the battery performances of the “trade-on relation” materials, e.g. the rate performance and the durability, at PRiME 2016. Conclusion On the basis of theoretical understanding about the function of cathode materials, we came up with the “trade-on relation materials” possessing both high energy density and safety. We introduced a layer-type LiNi0.33Mn0.16Co0.33Ge0.17O2 as a typical example of the “trade-on relation” materials. Reference [1] Y. Satou, S. Komine and S. Itou, LIBD2015 (France), abstract Figure 1

Molecular cascade Auger decays following Si KL23L23 Auger transitions in SiCl4

Cascade Si LVV Auger electron spectra at the photoexcitation of the Si 1s electron in a SiCl4 molecule have been measured using an electron spectrometer combined with monochromatized undulator radiation. In the instance of the resonant excitation of the Si 1s electron into the vacant molecular orbital a peak with high yield is observed at about 106 eV, an energy considerably higher than the energies of the normal LVV Auger electron. This peak is presumed to originate from the participator decay from the state with two 2p holes and one excited electron into the state with one 2p hole and one valence hole. Following the normal KL23L23 Auger transition, the cascade spectrum shows several peak structures, e.g. 63 eV, 76 eV and 91 eV. The peak at 91 eV is probably assigned to the second step Auger decay into states having a 2p hole together with two valence holes. These findings are similar to experimental results of SiF4. The former two peaks (63 eV and 76 eV) are ascribed to Auger transitions of Si atomic ions produced through molecular ion dissociation after the first step cascade decays, although the peak heights of atomic ions are lower than those of SiF4.

Electric‐field‐induced magnetism of first‐row d0 semiconductor nanowires and nanotubes (Phys. Status Solidi B 3/2015)

Recent observations of high-temperature ferromagnetism in d0 semiconductors (such as ZnO) have excited immediate interest since these d0 semiconductors provide a new platform for the study of semiconductor spintronics. Gangxu Gu et al. (pp. 484–489) demonstrate that, without dopants, lattice defects or polar surfaces, d0 semiconductor nanowires and nanotubes can be induced to be magnetic by simply applying an external electric field F. The authors' first-principles calculations reveal that under F the band gap is reduced, and the mixing of the valence band edge states is different from that of the conduction band edge states due to the different delocalization tendencies of the bands. After the band gap is nominally closed at a critical F value, separation of charges and localization of holes occur owing to the exotic electronic structure. Quantitative studies indicate that the presence of spontaneous magnetization is caused by localization of sufficient 2p holes around O at one side of the nanostructures, and the critical F value decreases as the diameter increases. These results provide an all-electric way for inducing and tuning magnetic properties of semiconductor nanostructures.

Electric‐field‐induced magnetism of first‐row d0 semiconductor nanowires and nanotubes

Our first‐principles calculations reveal that a transverse electric field (F) can induce and modulate spontaneous magnetization in defect‐free first‐row d0 semiconductor (ZnO, GaN and InN) nanowires (NWs) and nanotubes (NTs). Under F, the band gap is reduced, and the mixing of the valence band edge (VBE) states is different from that of the conduction band edge (CBE) states due to the different delocalization tendencies of the bands. After the band gap is closed at a critical F value, separation of charges and localization of holes occur owing to the exotic electronic structure. Quantitative studies indicate that the presence of spontaneous magnetization is caused by localization of sufficient 2p holes around O/N at one side of the nanostructures, and the critical F value decreases as the diameter increases. Since it is easy to apply and unapply an external electric field in practice, our results provide a viable way for inducing and tuning magnetic properties of d0 semiconductor nanostructures.

Electronic State of P2-Type NaxMO2 (M = Mn and Co) as Investigated by In situ X-ray Absorption Spectroscopy

P2-type NaxMO2 (M = Mn and Co) with a layered structure is a promising cathode material for low-cost sodium ion secondary batteries (SIBs). Here, we performed an in situ X-ray absorption spectroscopy (XAS) of NaxMnO2 and NaxCoO2 near the Mn and Co K-edges against Na concentration (x). In both compounds, the red shift of the main peak with x indicates the electron doping on the Mn 3d (Co 3d) orbital. We further analyzed the pre-edge XAS profiles with several configuration models. In NaxMnO2, the spectral weights of the doublet components suggest the high-spin (HS) state of Mn. In NaxCoO2, the single peak feature suggests that the ground-state configurations of Co4+ and Co3+ have significant weights of d6L and d7L, where L denotes an oxygen 2p hole.

Jahn-Teller effects in transition-metal compounds with small charge-transfer energy

We have studied Jahn-Teller effects in Cs2Au2Br6, ACu3Co4O12(A=Ca or Y), and IrTe2 in which the ligand p-to-transition-metal d charge-transfer energy is small or negative. The Au+/Au3+ charge disproportionation of Cs2Au2Br6 manifests in Au 4f photoemission spectra. In Cs2Au2Br6 with negative Δ and intermediate U, the charge disproportionation can be described using effective d orbitals constructed from the Au 5d and Br 4p orbitals and is stabilized by the Jahn-Teller distortion of the Au3+ site with low-spin d8 configuration. In ACu3Co4O12, Δs for Cu3+ and Co4+ are negative and Us are very large. The Zhang-Rice picture is valid to describe the electronic state, and the valence change from Cu2+/Co4+ to Cu3+/Co3+ can be viewed as the O 2p hole transfer from Co to Cu or d9 + d6L → d9L + d6. In IrTe2, both Δ and U are small and the Ir 5d and Te 5p electrons are itinerant to form the multi-band Fermi surfaces. The ideas of band Jahn-Teller transition and Peierls transition are useful to describe the structural instabilities.

Cascade Auger decays following Si KL23L23 Auger transitions in SiF4

Cascade Si LVV Auger decays following KL(23)L(23) Auger transitions have been measured in SiF(4) molecule using an electron spectrometer combined with monochromatized undulator radiation. Molecular cascade processes from the two 2p holes states largely generate wide band structures in the spectra due to sequential electron emission leading to multiple valence holes. However, a peak with high yield is observed for the first time at about 103 eV, an energy being considerably higher than the energies of the normal LVV Auger electron, in the instance of the resonant excitation of Si 1s electron into the vacant molecular orbital. This peak is presumed to originate from the participator decay from the state with two 2p holes and one excited electron into the state with one 2p hole and one valence hole. A similar peak with less intensity is detected in the photoexcitation of the 1s electron into a Rydberg orbital. After the normal KL(23)L(23) Auger transition, the resultant cascade spectrum shows several peaks, e.g., 61 eV, 76 eV, and 82 eV. The former two peaks are assigned to the Auger transitions of Si atoms produced through molecular ion dissociation after cascade decays, and the latter is probably ascribed to the second step Auger decay into states having a 2p hole together with two valence holes.

Direct Observation of Two Electron Holes in a Hematite Photoanode during Photoelectrochemical Water Splitting

Visible light active photoelectrodes for hydrogen generation by solar photoelectrochemical water splitting have been under scrutiny for many decades. In particular, the role of electron holes and charge transfer remains controversial. We have investigated the oxygen evolution of hematite in alkaline aqueous electrolyte under a bias potential during visible light illumination in a photoelectrochemical cell operando with soft X-ray (O 1s) spectroscopy. Only under these conditions, two new spectral signatures evolve in the valence band, which we identify as an O 2p hole transition into the charge transfer band and an Fe 3d type hole into the upper Hubbard band. Quantitative analysis of their spectral weight and comparison with the photocurrent reveals that both types of holes, contrary to earlier speculations and common perception, contribute to the photocurrent.

Post collision interaction probed by multi-electron coincidences: Application to the Ar 2s inner-shell photoionization

Post-collisional interaction following inner-shell ionization of atomic systems has been investigated by multi-coincidence electron spectroscopy, with a special emphasis on the decay by emission of two Auger electrons. The case of the Ar 2p hole is first reviewed. After Ar 2s inner-shell photoionization, the dominant decay channel is cascade Coster–Kronig/Auger decay. The distortion of the photoelectron peak shows a sensitivity to both the energy of the Coster–Kronig electron and to the lifetime of the intermediate Ar2+ (2p−13l−1) state that is well reproduced by a theoretical model based on the eikonal approach. The dynamics of cascade double Auger decay can be clearly understood thanks to this study.

First principles study of the magnetism driven by cation defects in CeO2: the important role of O2p states

The magnetism driven by cation defects in undoped CeO2 bulk and thin films is studied by the density functional theory corrected for on-site Coulomb interactions (DFT+U) with U = 5 eV for the Ce4f states and U = 7 eV for the O2p states. It is found that the Ce vacancies can induce a magnetic moment of the ∼ 4 μB/supercell, which arises mainly from the 2p hole state of the nearest neighbouring O atom (∼ 1 μB on per oxygen) to the Ce vacancy. The effect of the methodology is investigated, indicating that U = 7 eV for the O2p state is necessary to obtain the localized O2p hole state in defective ceria with cation vacancies.

Is N-doped SrO magnetic? A first-principles view

N-doped SrO seems to be one of the model systems for d0 magnetism, in which magnetism (or ideally, ferromagnetism) was ascribed to the localized N 2p spins mediated by delocalized O 2p holes. Here, we offer a different view, using density functional calculations. We find that N-doped SrO with solely substitutional N impurities as widely assumed in the literature is unstable and instead that a pairing state of substitutional and interstitial N impurities is significantly more stable and has a much lower formation energy than the former by 6.7 eV. The stable (Nsub−Nint)2− dimers behave like a charged (N2)2− molecule and have each a molecular spin = 1. However, their spin-polarized molecular levels lie well inside the wide band gap of SrO, and thus, the exchange interaction is negligibly weak. As a consequence, N-doped SrO could not be ferromagnetic but paramagnetic.

A many-body theory of the Ti M 2,3-VV Auger-photoelectron coincidence spectroscopy (APECS) spectrum of TiO 2(1 1 0)

We calculated the Ti M 2,3-VV Auger-photoelectron coincidence spectroscopy spectrum of TiO 2(1 1 0) by a many-body theory. The spectral main line is governed by the DOS of the two O 2p holes living longer than the Ti ∣ cd 2 L 2〉 → ∣ L 2〉 super Coster-Kronig (sCK) decay. The two O 2p holes are created by the charge transfer core–hole screening at the Ti atomic site. Here c and L are the Ti 3p hole and the ligand O 2p hole, respectively. Analysis of the spectrum shows that the two (or three) CT O 2p holes in the π bonding states are localized, whereas those in the σ bonding states are delocalized. The three localized CT O 2p ( π) holes in ∣ cd 3 L 3〉 in Ti M 2,3 main line (or satellite) of TiO 2(1 1 0) live longer than the Ti ∣ cd 3 L 3〉 → ∣d 1 L 3〉 sCK decay so that the Coulomb repulsion from the surrounding Ti ions gives the O + ion desorption from the surface.

Covalent State and the Electronic and Transport Properties of CaCu3Ni4O12: A First-Principles Study

The structure stability and physical properties of CaCu 3 Ni 4 O 12 have been studied by first-principles calculations. It shows that CaCu 3 Ni 4 O 12 is stable both thermodynamically and mechanically. The electronic structure analysis reveals that the Cu covalence is formally +3 and that the Cu ions have the d 9 L configuration (L: an O 2p hole) that corresponds to the Zhang―Rice singlet state. It is predicted to be a ferromagnetic half-metal, comparable to the metallic character of its isostructural compound of CaCu 3 Co 4 O 12 . Electron correlation plays a vital role in stabilizing the half-metallic property of CaCu 3 -Ni 4 O 12 . The Fermi level shifts toward the middle of the minority-spin gap with the increase of U, demonstrating its stability of the half metallicity against structural distortion. Moreover, it is found that O atoms carry unusual large magnetic moments due to the enhanced exchange splitting with the increase of U. All the predicted properties for CaCu 3 Ni 4 O 12 are comparable with the corresponding existing compound CaCu 3 Co 4 O 12 . Therefore, interesting physical properties are also expected from CaCu 3 Ni 4 O 12 .

Single and multiple photoionisation of H2S by 40–250 eV photons

Multi-electron coincidence measurements on photoionisation of H(2)S have been carried out at photon energies from 40 to 250 eV. They quantify molecular field effects on the Auger process in detail and are in good agreement with the existing theory. Spectra of core-valence double ionisation of H(2)S are presented and partially analysed. Auger decays from the core-valence states produce triply charged product spectra with unexplained and surprising intensity distributions. Triple ionisation by the double Auger process from 2p hole states shows little effect of the molecular field splitting, but includes a substantial contribution from cascade processes, some involving dissociation in intermediate states. The onset of triple ionisation at the molecular geometry is determined as 61 ± 0.5 eV.

X-Ray spectra and electronic correlations of FeSe1–xTex

Critical issues concerning emerging Fe-based superconductors include the degree of electron correlation and the origin of the superconductivity. X-Ray absorption spectra (XAS) and resonant inelastic X-ray scattering spectra (RIXS) of FeSe(1-x)Te(x) (x = 0-1) single crystals were obtained to study their electronic properties that relate to electron correlation and superconductivity. The linewidth of Fe L(2,3)-edges XAS of FeSe(1-x)Te(x) is narrower than that of Fe-pnictides, revealing the difference between their hybridization effects and localization character and those of other Fe-pnictides. While no significant differences exist between the Fe L-edge XAS and RIXS of FeSe(1-x)Te(x) and those of Fe-pnictides, Se K-edge and Te K-edge XAS exhibit substantial edge shift, suggesting that the superconductivity in an Fe-Se superconductor is strongly associated with the ligand states. A comparison of the Se K-edge and Te K-edge spectra reveals that the charge transfer may occur between Se and Te. Given the Coulomb interaction and the bandwidth, the spectral results indicate that FeSe(1-x)Te(x) is unlikely to be a weakly correlated system unlike the Fe-pnictides of the "1111" and "122" families. The spectral results further demonstrate that superconductivity in this class of Fe-based compounds is strongly associated with the ligand 4p hole state.

Dependence of Si + and Si 2+ sputtering yields on residual oxygen impurity

The Si + and Si 2+ sputtering yields have been measured by time-of-flight (TOF) method under the bombardment of a pulsed 11 keV Ar 0 beam on a nominally clean Si(1 1 1) surface. Although the residual oxygen impurity is below the detection limit of Auger electron analysis (∼0.01 monolayer), the SiO + ions are clearly observed and its yield can be regarded as a measure of the residual O impurity on the Si surface region. The Si + TOF spectrum changes its shape and the Si + yield increases linearly with the SiO + yield, while the shape of the Si 2+ TOF spectrum and the Si 2+ yield remain almost unchanged. The change in the Si + TOF spectrum is attributed to the increasing SiH + yield, which may be caused by the H 2O adhesion to the Si surface during the TOF measurement. The increase in the adsorbing H 2O may also lead to the enhancement of SiO + and Si + yields; Si + ions may be created through the charge exchange process between O and Si due to difference in their electron affinity. The insensitivity of the Si 2+ yield to the residual O impurity is consistent with the Si 2+ formation by the Auger ionization process of the excited Si + having the 2p hole, which is produced in the collision cascades.