Exchange reaction between Li and Na intercalated into TiS2

doi:10.1016/S0039-6028(99)00665-2

Surface Science

Volume 436, Issues 1–3, 10 August 1999, Pages 213-219

https://doi.org/10.1016/S0039-6028(99)00665-2 Get rights and content

Abstract

Single crystals of TiS₂(0001) were intercalated sequentially by Li and Na. The intercalation occurred across the Van der Waals planes, by deposition onto the basal plane of TiS₂. The investigation took place in UHV by soft X-ray photoelectron spectroscopy. Deposition of Na onto Li-intercalated TiS₂(0001) caused the Li to move further into the bulk of the substrate. However, deposition of Li onto Na-intercalated TiS₂(0001) forced Na to move out towards the surface by an exchange mechanism. These interactions between intercalated Li and Na into TiS₂(0001) and their subsequent behaviour have been explained by a correlation of thermodynamic (Gibbs free energy), kinetic (diffusion coefficient) and electrostatic effects.

Introduction

The transition metal dichalcogenide (TMDC) layer compounds have been extensively studied in recent years. They offer ideal surfaces by cleavage along the Van der Waals planes. Nevertheless, TMDCs are not only interesting in fundamental research but are also promising materials in technological applications, such as solar energy conversion, battery systems and superconductivity [1], [2], [3], [4], [5]. Alkali metals deposited on TMDC(0001) surfaces are intercalated into their bulk [6], [7], [8]. The intercalation reaction depends on different characteristics related to the system, such as thermodynamics and kinetics [5], [6], [7], [8]. The intercalation reaction is also affected by external conditions. The presence of electronegative elements on the basal plane of an intercalated TMDC layer compound causes a deintercalation of the intercalated electropositive elements from the bulk to the surface of the TMDC [9], [10]. The evolution of structural and electronic features of both host and guest material can be investigated by surface science techniques such as, soft X-ray photoelectron spectroscopy (SXPS) and low energy electron diffraction (LEED) in ultra high vacuum (UHV). In a recent work, Starnberg et al. studied the interaction of Na and K into the VSe₂ layer compound [11]. The information from this study was rather limited and, according to the authors, further studies of similar systems are needed to understand these interactions. In the present work we investigate the alternative adsorption of Li and Na on TiS₂(0001) TMDC in UHV by SXPS using synchrotron radiation as the excitation source.

Section snippets

Experimental

The experiments were performed in a commercial ultra high vacuum (UHV) (P<5×10⁻¹¹ mbar) angle-resolved system (VG ADES 500) using synchrotron radiation as the excitation source (BESSY-TGM 7, hν=10–120 eV) for photoelectron spectroscopy. A −6 V bias was applied to the sample in order to separate the sample's secondary electrons from those of the analyser and to allow the work function measurements to be taken. All spectra were measured in normal emission and were calibrated to the photon flux. The

Deposition of Na on Li-intercalated TiS₂(0001)

Fig. 1 shows the Li 1s core level after (a) Li deposition at room temperature, (b) 10 h later and (c) after subsequent Na deposition at room temperature. The Li 1s peak appears initially at a binding energy (BE) of 55.2 eV and FWHM of 0.7 eV. These values of BE and FWHM are in agreement with those reported for Li intercalated into TMDCs [6], [7], [8]. As the coverage of Li increases, the Li 1s core level shifts gradually to higher BE by 0.2 eV. Such a shift has been also observed previously during

Conclusion

In this work we studied the interaction of Li and Na intercalated into TiS₂. The investigation took place experimentally in two stages: (a) Li was deposited on the basal plane of Na-intercalated TiS₂ and (b) Na was deposited on the same plane of Li-intercalated TiS₂. The experiment was performed in UHV and monitored by soft X-ray photoelectron spectroscopy, using synchrotron radiation as the excitation source. The results led us to the conclusion that the interaction between the different

Acknowledgements

The authors would like to thank the BMFT of the funding of their research program, the technician J. Lehmann for his support, and Dr Y. Tomm for offering the single layer compounds crystals. C.A.P., M.K. and D.C.P. wish to thank EC for grant NoCHGE-CT-930027, enabling them to make measurements at BESSY. The authors are grateful to Prof. Gudat and Dr Brown for their support.

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Exchange reaction between Li and Na intercalated into TiS2

Abstract

Introduction

Section snippets

Experimental

Deposition of Na on Li-intercalated TiS2(0001)

Conclusion

Acknowledgements

Progr. Solid State Chem.

Chem. Phys. Lett.

Surf. Sci.

Surf. Sci.

J. Cryst. Growth

Surf. Sci.

Surf. Sci.

Adv. Phys.

Exchange reaction between Li and Na intercalated into TiS₂

Deposition of Na on Li-intercalated TiS₂(0001)