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Strategies for chemical modification of transition metal halides MX3 (M = Cr, Mo, Rh, Ru; X = Cl, Br, I) as 2D layered materials: rational synthesis planning, crystal growth and chemical/physical properties

Subject Area Solid State and Surface Chemistry, Material Synthesis
Term from 2020 to 2023
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 437046793
 
Final Report Year 2024

Final Report Abstract

It was the aim of the project to verify the existence of selected solid solutions of transition metal trihalides (TMTH or MX3) with a 2-dimensional crystal structure, their synthesis and the deposition of nanocrystals. In detail, the following solid solutions were realized: CrCl3 - CrBr3, CrBr3 - CrI3, RhCl3 - RhBr3, RhBr3 - RhI3, CrCl3 - RuCl3 and CrCl3 - MoCl3. Simulations of the phase equilibria proved to be suitable for determining the requirements for the thermodynamic stability of the mixed phases and for estimating suitable conditions for the phase-pure synthesis and crystal growth of representatives of the mixed crystals with a defined composition. The systems of transition metal trihalides (TMTH and MX3) each show a very individual behavior in the formation of heterogeneous equilibria and the subsequent mechanisms of gas phase deposition. Reactions could be identified as sublimation, autotransport or regular chemical vapor transport. Their processes, especially with regard to the composition of the mixed phases, can be regulated by the parameters composition, temperature and temperature gradient. By using powder XRD the phase purity of the samples was confirmed and the course of the lattice parameters within the solid solution series was examined. With the exception of CrCl3 - MoCl3, all mixtures showed Vegardian behavior. Powder samples with defined compositions were further used for the deposition of nanocrystals on a substrate by chemical vapor transport (CVT). The nanocrystals generally have heights in the low 2-double digit nm range. In the RhCl3 - RhBr3 system, the direct deposition of monolayers was demonstrated - an absolute highlight of the project. The compositions of the deposited crystals were analyzed using EDX and possible enrichment effects were investigated. Enrichment occurred particularly in the cationic solid solutions, while congruent transport predominated in the anionic ones. The high quality of the deposited crystals was verified using TEM, SAED and XPS. In addition, selected material properties such as the photoluminescence behavior of the CrCl3 - CrBr3 and CrBr3 - CrI3 series and the optical band gap of the RhCl3 - RhBr3 and RhBr3 - RhI3 systems were investigated. In the CrCl3 - MoCl3 system, deviations from the ideal course of the lattice parameters were observed with a differentiation of several phase regions in the solid solution series. Possible structural models for the formation of differently ordered Mo-Mo dimers were investigated using IR spectroscopy or SQUID measurements and confirmed within the scope of the significance of the measurement methods. As a result of the project, a comprehensive methodological approach for the simulative and experimental investigation of TMTH can be presented, which also can be transferred to other solid-state systems.

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