Youichi Ohno

Interlayer interaction in misfit layer compounds MTS3 (M = Sn, Pb, La; T = Ti, Nb)

Abstract MTS 3 (M = Sn, Pb, La; T = Ti, Nb), rigorously denoted by (MS) 1+ x (TS 2 ), are misfit layer compounds with nonstoichiometric compositions. We may regard them as intercalation derivatives of layered TS 2 compounds, but also as materials having ultimately-thin one-dimensional superlattices with repeated distances of an atomic scale. The detailed investigation of interlayer interaction has been made, applying a difference technique to the sulfur K and niobium 2 X-ray absorption spectra. It is found that in all cases charge transfer occurs from MS to TS 2 layers, but interlayer interaction depends on the combination of two kinds of alternately-stacked layers. PbNbS 3 and SnNbS 3 have stronger interlayer interaction than PbTiS 3 and LaNbS 3 .

S K and P K absorption spectra and electronic structures of layered thiophosphates MPS3

Abstract The S K and P K absorption spectra of layered thiophosphates MPS3 (M = Mg, Mn, Fe, Ni, Zn, Cd, Sn) were measured. The general features of the S K absorption spectra resemble one another, but the intensity ratio of the first peak to a higher energy structure and the energy position of a shoulder vary, depending on the metal species. All the P K absorption spectra exhibit a prominent peak in the neighborhood of the threshold. It is found that (1) the spectra mainly reveal the p-like partial density of states of the unoccupied energy levels of a [P2S6]4- cluster and (2) the first peak arises predominantly from the electronic transitions to the antibonding levels of the PS bonds. The electronic structures and the optical spectra are discussed.

Raman and Infrared Spectra of Misfit Layer Compounds MNbS3 (M=Sn, Pb, La, Ce)

Raman scattering and infrared reflection spectra of misfit layer compounds MNbS3 (M=Sn, Pb, La, Ce) have been measured to study the interlayer interaction and the charge transfer from the MS layer to the NbS2 layer. Analysis of the Raman spectra indicates that the interlayer interaction is considerably stronger for the La and Ce compounds than for the Sn and Pb compounds, which may be related to the difference in the amount of the charge transfer.

Photoelectron spectra and surface valence fluctuation of Eu in the misfit-layer compound {(EUS)1.15}1.5NbS2

Abstract The 1.5Q/1 H type of the misfit layer compound {(EuS) 1.15 } 1.5 NbS 2 has been studied by X-ray photoelectron spectroscopy. Eu 3d, 4d, 4f and 5p spectra are measured with Al and MgKα radiation. Their intensities are analyzed, taking the escape depth of photoelectrons emitted from the core levels into account. The results indicate that valence transfer of nearly-half Eu 2+ ions to Eu 3+ happens in the outermost surface layer and a reduced Eu–S distance brings about surface rearrangement. It is confirmed that Eu 2+ and Eu 3+ coexist in a bulk, in consistency with bond valence calculations and Mossbauer spectra. The XPS spectra are well interpreted in terms of the multiplet structures of Eu 2+ and Eu 3+ .

Multiplet structures of metal L2,3 absorption spectra of ionic 3d transition-metal compounds

The L 2,3 absorption spectra of the metal ions in layered 3d transition-metal thiophosphates MPS 3 (M=Mn, Fe, Ni) have been measured. Results are discussed by comparing with the spectra of other transition-metal compounds and with the theoretical calculation by Gupta and Sen. The spectra are interpreted in terms of the multiplet structure which results from the interaction between a 2p hole and 3d electrons of a localized metal ion. Since the p-d interaction is reduced by the delocalization of metal 3d orbitals, the multiplet structure becomes inconspicuous as covalency is increased. It is concluded that the near-edge structure of the metal L 2.3 absorption spectrum of an ionic 3d transition-metal compound exhibits the multiplet structure, while that of a covalent compound represents the projected density of states of the conduction band.

Resonant 2 p →3 d Photoemission Measurement of MPS 3 (M=Mn, Fe, Ni)

Resonant 2 p →3 d photoemission (RPE) spectra in the valence band region of the transition-metal thiophosphates MPS 3 ( M=Mn, Fe, Ni) have been measured by using synchrotron radiation. These spectra show largely enhanced resonated profiles and spread over in the wide range (∼15 eV) at the incident photon energy of the transition-metal 2 p 3/2 absorption peak. Comparing the obtained spectra with the result of the cluster calculation of RPE spectra for transition-metal monoxides carried out by Tanaka and Jo, we argued the differences of the two parameters, charge transfer energy Δ and d - d Coulomb interaction energy U d d of these materials. It is suggested that MnPS 3 and FePS 3 are Mott-Hubbard type insulator with Δ > U d d , while NiPS 3 is a charge transfer type insulator with Δ < U d d .

Mechanisms of Field Ionization and Field Evaporation on Semiconductor Surfaces

This paper describes the mechanisms of field ionization (FI) and field evaporation (FE) on semiconductor surfaces. By including experimental results obtained up to the present, it is demonstrated that field penetration (FP) into the surface of these semiconductors has significant influence on these mechanisms. The best image field and the ion current generation for these materials are discussed on the basis of the FI theory in which FP is taken into account. The evaporation field, the ionization states of evaporated ions and preferential FE for these materials are also discussed on the basis of the FE theory in which FP is taken into account. Moreover, by obtaining the ion current-voltage characteristics of an FIM by means of image photometry, it has been made clear that the ion current at the best image field for these materials depends not on the gas supply, but principally on the ionization probability of the imaging gas.

Scanning Tunneling Microscopy, X-Ray Photoelectron Spectroscopy and Electron Energy-Loss Spectroscopy Studies of the Misfit Layer Compounds {(Pb1-xSbxS)1+y}mTS2 (T=Ti and Nb; m=1 and 2)

The misfit-layer compounds {(Pb 1- x Sb x S) 1+ y } m TS 2 (T=Ti and Nb; m =1 and 2) are constructed of the regular stacking of single or double two-atom-thick Pb 1- x Sb x S (Q) layers and a three-atom-thick TS 2 (H) layer. They are studied by means of the scanning tunneling microscope (STM) and the x-ray photoelectron spectroscopy (XPS) and the electron-energy-loss spectroscopy (EELS) methods. For the 2Q/1H ( m =2) compounds, atomic-scale STM images are obtained from both the Q and H layers, whereas for the 1Q/1H compounds no STM images are obtained from a Q layer even if a Q layer consists of a (Pb,Sb)S layer. The electronic structures of the 2Q/1H compounds as well as the 1Q/1H compounds are discussed. It is found that as m increases, the x-ray photoemission threshold or the Fermi energy position shifts to lower binding energy by charge transfer from a Q layer.

X-ray absorption spectroscopy of the first-row transition-metal intercalates of layered transition-metal disulfides

Abstract The X-ray absorption spectra of the first-row transition-metal intercalates denoted by M 1/3 TS 2 (M = Ti, V, Cr, Mn, Fe, Co, Ni; T = Ti, Zr, V, Nb) were measured. The appearance as a whole is not greatly affected by intercalation. In this sense the rigid-band model can be said to be valid. However, variations in the fine structure are found namely, the larger band-width and reduced crystal field splitting. These effects do not depend so much on the host materials, but on the intercalated species. The charge transfer appears to occur from the intercalated species to the conduction band of the host layers.

X-ray absorption spectroscopy of layered 4D transition-metal dichalcogenides

The authors have measured the metal LIII absorption spectra of layered 4d transition-metal dichalcogenides with the use of a Yohan-type curved-crystal spectrometer. The spectra showed that the metal d orbitals are strongly admixed with the chalcogen p orbitals to form the covalent bonding. The rigid-band model which has been suggested by Wilson and Yoffe (1969) appears to be valid in the first approximation for the unoccupied bands to about 5 eV above the Fermi level, but is not valid for the higher-energy bands. As the atomic number of the chalcogen increases, the bands broaden and overlap each other, probably due to the spin-orbit interaction. The absorption-edge shifts observed can be explained in terms of the energy differences of the bottom of the conduction band or the lowest empty state.