Understanding charge transfer (CT) between two chemical entities and subsequent change in their charge densities is essential not only for molecular species but also for various low-dimensional materials. Because of their extremely high fraction of surface atoms, two-dimensional (2-D) materials are most susceptible to charge exchange and exhibit drastically different physicochemical properties depending on their charge density. In this regard, spontaneous and uncontrollable ionization of graphene in the ambient air has caused much confusion and technical difficulty in achieving experimental reproducibility since its first report in 2004. Moreover, the same ambient hole doping was soon observed in 2-D semiconductors, which implied that a common mechanism should be operative and apply to other low-dimensional materials universally. In this Account, we review our breakthroughs in unraveling the chemical origin and mechanistic requirements of the hidden CT reactions using 2-D crystals. We developed in-situ optical methods to quantify charge density using Raman and photoluminescence (PL) spectroscopy and imaging. Using gas and temperature-controlled in-situ measurements, we revealed that the electrical holes are injected by the oxygen reduction reaction (ORR): $O_{2}$ + $4H^{+}$ + $4e^{-}$ $rightleftharpoons$ $2H_{2}O$, which was independently verified by pH dependence in HCl solutions. In addition to oxygen and water vapor, the overall CT reaction requires hydrophilic dielectric substrates, which assist hydration of the sample-substrate interface. The interface-localized reaction allowed us to visualized and control interfacial molecular diffusion and CT by varing the 2-D gap spacing and introducing defects. The complete mechanism of the fundamental charge exchange summarized in this Account will be essential in exploring material and device properties of other low dimensional materials.
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