Variations of paramagnetic defects and dopants in geo-MoS₂ from diverse localities probed by ESR

Exfoliated flakes from molybdenite crystals often still serve as benchmark substrates for two-dimensional MoS₂ fundamental and device-oriented research. In this article, results are reported of a multi-frequency electron paramagnetic resonance (EPR) study on a series of natural 2H MoS₂ crystals taken from various (seven) geological sites with the intent to explore the variations in quality and properties in terms of occurring paramagnetic point defects, with particular focus on the assessment of the predominant type of impurity dopant. The sample set covers three types of overall doping regimes, i.e., p-type, n-type, and mixed (n-type and p-type parts in one sample). The doping type appears primarily governed by substitutional impurities as evidenced by the observed As and N acceptor (both substituting for S) and Re donor (substituting for Mo) signals. For all p-type specimens, doping is found to be ruled by As where, however, a strong variation is revealed in doping uniformity, which appears not directly correlated with the As dopant density. Without specific precautions taken, surface contamination related EPR signals are observed in virtually all As-excavated geo-MoS₂ specimens. While several of these signals are of unassigned origin, two prominent ones are identified, one as concerning oxo-Mo⁵⁺ compounds and the other Mn²⁺ centers. The geo-MoS₂ sample with the foremost n-type doping shows, besides the prime Re donor EPR signal, an intense powder-pattern signal, tentatively typified by g_∥ = 2.076, g_⊥ = 2.253, which is suggested to originate from intercalation-related defects. The results bear out the necessity of rigorous surface cleaning, even including invasive removal of surface layers, to obtain pristine MoS₂ parent crystals suitable for enabling exfoliation of high quality flakes.