Altering properties of transition metal dichalcogenides by bi-intercalation
Advanced synchrotron radiation spectroscopy helped to gain insight into the origins of phase transitions in Cu and Ni doped TiSe2 dichalcogenide.
This article is Open Access
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Transition metal dichalcogenides (TMDC) represent a family of materials typically in the form of MX2 where M is the transition metal and X is the chalcogen (S, Se, Te). Most TMDC bulk crystals are layered solids with strong bonding within the plane but weak interlayer bonding. The ability to tune the chemistry by choosing a unique combination of transition metals, chalcogen and dopant atoms along with controlling their properties by phase engineering allows realizing new functionalities with applications in semiconductor and data storage devices, spintronics, sensors, or in catalysis. Dopant atoms may intercalate between layers and appropriate dopant selection and concentration can improve the performance of TMDCs. On the other hand, an improper doping might have unwanted effects. It is therefore desirable to determine correlations between TMDC phase changes and the amount and nature of the intercalated atoms. Understanding the principles and mechanisms yielding the phases with desired or undesired properties is an important prerequisite to engineer materials with tailored functionalities.
While the effect of the intercalation of single metals into TMDCs is well understood nowadays, little is known about the compounds intercalated with several different metals. The team around Alexander Titov of the Institute of Metal Physics in Yekaterinburg (Russia) has been able to synthesize couple of new compounds of this peculiar class of TMDCs. “The transition from mono to poly-intercalation is like to watching colour TV after black-and-white one. Particularly interesting seems to be Cu and Ni co-intercalated TiSe2.” explains Alexander Titov. It is known that, in single metal intercalated TiSe2, Cu and Ni atoms occupy octahedral sites and hybridize with Ti atoms of the host lattice, while tetrahedral sites are energetically unstable. But which sites will be occupied if we introduce both Cu and Ni atoms simultaneously? Surprisingly, the researchers have discovered that introduction of Cu into Ni-intercalated (Ni)TiSe2 results in the displacement of the Ni atoms from octahedral to tetrahedral sites, even at room temperature. Why is this happening? What is at the origin of this phase transition? To answer these questions, the scientific team has turned to NFFA-EU infrastructures which offer advanced spectroscopic characterization of the electronic structure of solid state materials.
The electronic structure investigations were performed by photoemission and X-ray absorption spectroscopy at the BACH beamline of IOM-CNR developed in collaboration with synchrotron Elettra in Trieste (Italy).
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Fig. On-resonance valence band spectra before (left) and after (right) Cu intercalation show charge transfer to TiSe2 conduction band.
This project has received funding from the EU-H2020 research and innovation program under grant agreement No 654360 having benefitted from the Access provided by IOM-CNR in Trieste (Italy) within the framework of the NFFA-Europe Transnational Access Activity.
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This article is Open Access