H. I. Starnberg

Mapping the excited-state bands above the vacuum level with VLEED: principles, results for Cu, and the connection to photoemission

Experimental photoelectron spectra are usually interpreted using rather crude approximations for the upper states into which the electrons are excited. Better knowledge about these excited states could substantially improve the accuracy of valence band mapping by photoelectron spectroscopy. We here demonstrate that VLEED measurements are ideally suited for accurate determination of the desired upper states. This is illustrated by model calculations including absorption and self-energy corrections. The close correspondence between so-called irregularity points of the excited-state bands and the total electron reflectivity is established, which opens up the possibility for direct mapping of irregularity points by comparison with experimental VLEED spectra, and for fitting of the whole excited-state bands between these points. The proposed scheme is finally used to determine the excited-state bands of Cu along from measurements on Cu(111).

In situ intercalation of the layered compounds TiS2, ZrSe2 and VSe2

We report photoelectron spectroscopy studies of the valence band structure of the layered compounds TiS 2 , ZrSe 2 , and VSe 2 , and of changes induced by in situ intercalation with Cs. The pure compounds crystallize with the same structure, the 1T-CdI 2 structure, but their electronic properties are different; TiS 2 is a narrow-gap semiconductor (E g ∼ 0.2 eV), ZrSe 2 is a semiconductor (E g ∼ eV) and VSe 2 is a semimetal. Despite their different electronic properties, the results show that the character of their valence bands changes from 3D to 2D upon intercalation with Cs. The observed changes, supported by LAPW band calculations, go far beyond the rigid band model.

Core level and valence band studies of layered Ti3SiC2 by high resolution photoelectron spectroscopy

The ternary compund Ti3SiC2 is a prominent representative of a new class of layered ceramics whose extraordinary physical properties has attracted much attention in recent years. Ti3SiC2 is electrically and thermally highly conductive, elastically rigid, lightweight, and maintains its strength to high temperatures. It is furthermore damage tolerant and oxidation resistant. We have studied fractured surfaces of coarse-grained Ti3SiC2 by means of photoelectron spectroscopy at the MAX-lab synchrotron radiation facility in Lund, Sweden. High-resolution C 1s, Si 2p, Ti 2p, Ti 3s and Ti 3p core-level spectra are reported and interpreted in terms of crystallographic and electronic structure. Valence band spectra confirm the validity of recent band calculations.

Modifying the electronic structure of TiS2 by alkali metal intercalation

Angle-resolved photoelectron spectroscopy has been used to study in situ intercalation of the layered compound TiS2 with Na and Cs. The intercalation was verified by core-level spectroscopy. The valence bands of pure TiS2 and the intercalated compounds NaxTiS2 and CsxTiS2 were characterized and compared to self-consistent LAPW band-structure calculations. Remarkable agreement between experimental and calculated bands was found for the dispersion along the layers. The calculations predicted perpendicular dispersion also for the intercalation compounds, although significantly reduced. No significant perpendicular dispersion was seen in normal-emission spectra, which might be an effect of intercalation-induced stacking disorder. Charge transfer to the TiS2 host layers was evident from the much increased conduction band emission, and from the asymmetric S 2p core-level lineshape after intercalation. The intercalation produced electronic structure changes which are not well described by the rigid-band model, but as these changes occur at an early stage, the model can still be used, with modified bands, to describe the continued intercalation.

The electronic structure of ZrSe2 and CsxZrSe2 studied by angle-resolved photoelectron spectroscopy

We report an angle-resolved photoelectron spectroscopy study of the layered semiconductor ZrSe2, and of changes in its electronic structure induced by in situ intercalation with Cs. The results show that the valence band structure of ZrSe2 is initially of 3D character, but is transformed to become essentially 2D upon Cs intercalation. The observed changes are supported by self-consistent LAPW band calculations, and are not compatible with the rigid-band model. Changes in the Se 3d core level lineshape are attributed to an intercalation-induced increase in the carrier density in the lowest conduction band, which produces a different screening of the core hole.

Conduction band structure of VSe2 studied by inverse photoemission, secondary electron emission and total current spectroscopies

The authors have studied the unoccupied electronic states of VSe2 using three different techniques: inverse photoemission spectroscopy (IPES), secondary electron emission spectroscopy (SEES) and target current spectroscopy (TCS). The experimentally determined electron bands are in good agreement with band structure calculations, but on comparison the different techniques reveal significantly different aspects of the conduction band structure. The reasons for these differences are discussed.

Absolute determination of the layer-perpendicular band structure of and by combined very-low-energy electron diffraction and photoemission

The layer-perpendicular dispersions of the typical layered TMDCs and were studied by combining determination of the upper unoccupied bands by very-low-energy electron diffraction (VLEED) with mapping of the lower occupied bands by photoemission (PE). We found that the upper bands of these materials are very complicated, and are compatible neither with the free-electron, nor with the ground-state approximation. Knowledge of the upper bands allowed us to carry out a PE experiment optimized for the -resolved mapping of the lower bands. The PE data were consistently rationalized, using a map of the PE intensity as a function of the binding energy and the photon energy . We found that the PE intensity is well described by direct, -conserving, transitions, with minor shifts of PE peaks being basically a consequence of their broadening due to finite electron and hole lifetime. Finally the lower bands were mapped explicitly, using the PE peaks with minimal shifts and the experimental upper bands. The obtained is very consistent, and shows overall agreement with full-potential LAPW calculations.

Exchange reaction between intercalated Na and K studied by synchrotron radiation

An exchange reaction between intercalated Na and K has been observed with core-level spectroscopy using synchrotron radiation. Both alkali metals were found to intercalate the layered compound VSe2 when deposited onto the sample (0001) surface in UHV at room temperature, although the rate of intercalation was markedly different. Sodium intercalated easily, leaving only a small amount trapped at the surface, while for potassium a much larger part remained at the surface and the rate of intercalation was significantly lower. When Na was deposited onto VSe2 intercalated with K, an exchange reaction was observed, in that Na replaced most of the intercalated K, which was forced to deintercalate back to the surface. For a similar deposition of K onto VSe2 intercalated with Na, some of the K did intercalate, forcing some of the Na to larger intercalation depths but not back to the surface, where the largest part of the deposited K was found.

Catalytic Nitridation of a III-V Semiconductor Using Alkali Metal

The room temperature adsorption of molecular nitrogen on a InP(110) surface modified by a potassium overlayer is investigated by means of valence band and core level photoemission spectroscopy using synchrotron radiation. The results indicate no reaction between N2 and the clean InP(110), while the potassium-covered surface exhibits nitrogen uptake. Chemical shift at the P 2p core level suggests bonding with the anion and formation of InPNx nitride complex. In strong contrast with alkali-promoted silicon surfaces, reaction between potassium and the surface is found to be essential for the catalytic nitridation which is an indication of the role of defects. This study brings the first example of catalytic nitridation of a III-V semiconductor.

Stacking transformation and defect creation in Cs intercalated TiS2 single crystals

Abstract The intercalation of TiS 2 single crystals with Cs was studied by transmission electron microscopy. Evidence was found for an intercalation induced 1T→3R structure transformation. The Cs + ions formed an a 3 ×a 3 structure. In the thinnest parts of the crystals, the intercalation induced cracks and moire fringes. In thicker parts the 1T→3R transformation was frustrated, with formation of intercalation ribbons and dislocation loops. Air exposure resulted in de-intercalation and oxidation of Cs. The results emphasise the connections between defects and intercalation.