Carrier Doping of Few-Layer MoS2 with Ionic Polymers and MoS2 Quantum Dots with Atmospheric Water

Thin layered MoS₂ has emerged as a remarkable 2D semiconductor material with great potential due to its direct band gap (thickness dependent), photoluminescence, and high on/off FET rectification.  Additionally, molecular gating has proven to be an effective technique to dope nanomaterials for specific electronic applications. Here, we show two avenues for molecular gating: (A) Carrier doping and electrical properties of few layer MoS₂ were controllably modulated by interfacing it with the polyanionic polymer, polystyrene sulfonate (PSS) and the polycationic polymer, polyallylamine hydrochloride (PAH); and (B) MoS₂ quantum dots interfaced with a microfiber of PAH were reversibly doped with water molecules by altering the local humidity. PSS interfacing with few layer MoS₂ resulted in a blue shift of the Raman E¹_2g and A_1g Raman peaks in MoS₂ corresponding to hole doping, as expected for negative potential gating. PAH adsorption on PSS doped MoS₂ led to a recovery of the A_1g peak, while the E¹_2g peak red shifted slightly attributed to over compensation by PAH and electron doping. Similar results were attained with PAH interfacing on MoS₂ few layer sheets resulting in electron doping. Electrical characterization confirmed the carrier doping observed by Raman. MoS₂ quantum dots interfaced with a microfiber of PAH were reversibly doped with water molecules by changing the local humidity. The p-doping of water absorbed by the polymer microfiber reduced the electron density in n-type MoS₂ quantum dots. The change in the carrier concentration (5.24 x 10¹¹ cm^-2) was confirmed by electrical measurements and Raman spectra were analyzed to understand the lattice bonding of MoS₂ during the self-assembly process onto PAH microfibers. Both of these avenues demonstrate chemical modification or interfacing providing a path for tuning the electronic properties of MoS₂ for use in future electronic devices.