H.E. Brauer

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.

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.

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.

Low temperature adsorption of Cs on layered TiS2 studied by photoelectron spectroscopy

Abstract The adsorption of Cs on a TiS 2 cleavage plane at 110 K was studied by core level and valence band photoelectron spectroscopy, using synchrotron radiation. The Cs 4d core level spectra, together with LEED observations, reveal that the Cs at low coverage forms a disordered phase on the surface, but condenses to form ordered islands as the coverage is increased. A small amount of Cs was seen to intercalate even at 110 K. As the sample was allowed to warm up, the ordered Cs phase melted to form a disordered phase again, although with the binding energy being different from the disordered low-temperature phase. At this stage the intercalation process accelerated, and before reaching room temperature most of the Cs had intercalated. The S 2p and Ti 3p core level spectra exhibits shifts and broadenings, and the valence band spectra provide clear evidence for charge transfer from both adsorbed and intercalated Cs to the host layers.

Na and Cs intercalation of 2H-TaSe2 studied by photoemission

The electronic structure of the layered compound 2H-TaSe2 has been studied using angle-resolved photoemission before and after in situ intercalation with Na and Cs. Core level spectra verified that Na and Cs both intercalate easily at room temperature, with only small amounts remaining on the surface. Valence band spectra revealed changes in the electronic band structure which were much more extensive than predicted by the rigid band model, but which were in reasonable agreement with theoretical bands calculated by the LAPW method. Some discrepancies between the experimental and calculated results are probably due to intercalation induced changes in the stacking of host layers. A general similarity with results from transition metal dichalcogenides with 1T structure indicates that the intercalation properties are not critically dependent on the internal structure of the host layers.

Band mapping of in situ Cs intercalated transition metal dichalcogenides

Abstract The valence band structure of in situ Cs intercalated layered transition metal dichalcogenides has been studied with photoemission spectroscopy. The intercalation was achieved by room temperature deposition of Cs onto clean sample surfaces in UHV. The changes in the valence band structure upon intercalation were more extensive than predicted by the rigid band model, with the most profound change being a 3D-to-2D transition of the valence band. The band gaps of structurally similar and iso-electronic semiconducting samples were differently affected by the intercalation; in TiS 2 the gap was significantly widened, while in ZrSe 2 a slight decrease was observed. Very good overall agreement was found when comparing the experimental results with self-consistent LAPW band calculations.

Valence Band Photoemission Study of Cs Intercalated VSe2

Abstract We have used photoemission spectroscopy to study the valence band structure of clean and in situ Cs intercalated VSe2. The results show that the valence band structure of VSe2 is transformed, from initially being of 3D character, to become essential 2D, as Cs is intercalated. The changes in the electronic structure are to a large extent understandable as caused by intercalation-induced de-coupling of the VSe2 layers, and electronic charge transfer from Cs to the host material. Comparison with band structure calculations supports this picture. Our findings indicate that the Cs/VSe2 system may be of great value in studies of “reduced dimensionality” phenomena.