Factors Controlling Intercalation of Metal Atoms into WS2: Implications for Electronically Tunable Semiconductors

The ability to tune the carrier transport properties of semiconducting two-dimensional materials is critical toward their eventual integration into nanoelectronic devices. Intercalation of transition-metal atoms and complexes into the interlayer galleries of two-dimensional transition-metal dichalcogenides (TMDs) can have a significant impact on the electronic structure and transport properties of the host material. However, because of the redox lability of transition metals, the intercalation process is frequently accompanied by undesired side reactions, including secondary nucleation, reduction, and degradation. In this work, we perform a systematic study on the intercalation of transition-metal cations into lithiated WS2 (LixWS2) in order to elucidate synthetic and structural design rules for achieving selective intercalation over secondary nucleation. We show that the intercalation process, driven by charge transfer and electrostatic interactions, can be controlled by tuning the charge density and reducibility of the transition-metal precursor complex. For less reducible precursors, a higher charge density enables a stronger interaction between the cation and the anionic WS2 sheet, resulting in intercalation of solvated ions with no secondary nucleation of metal oxide nanoparticles. For more reducible cations, reduction and desolvation of the precursor complex occur first followed by zerovalent intercalation of the desolvated atom. Kelvin probe force microscopy measurements show that intercalated metal ions raise the Fermi level of WS2 by up to 90 meV compared to the LixWS2 precursor, resulting in improvements in both the conductivity and carrier activation energy.