Redox-Governed Charge Doping in WS₂ and Graphene

Low dimensional materials often undergo spontaneous hole doping in the ambient conditions, the detailed mechanism of which has yet to be revealed. In this work, we propose a mechanism based on a redox couple of O₂/H₂O and verified it for two model systems: photoluminescence (PL) modulation in single-layer WS₂ and thermally-activated phonon hardening in graphene with both supported on silica substrates. The PL modulation was directly correlated with the concentration of oxygen both in gaseous and aqueous states. Wide-field PL imaging, however, showed distinctively different spatial propagations of the modulation for the two states, revealing the microscopic picture of the charge doping in WS₂. The mechanistic details and thermodynamic driving force for the charge doping will also be discussed in conjunction with the activated hole doping in graphene probed by Raman spectroscopy.