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Understanding charge transfer (CT) between two chemical entities andsubsequent change in their charge densities is essential not only for molecularspecies but also for various low-dimensional materials. Because of theirextremely high fraction of surface atoms, two-dimensional (2-D) materials aremost susceptible to charge exchange and exhibit drastically differentphysicochemical properties depending on their charge density. In this regard,spontaneous and uncontrollable ionization of graphene in the ambient air hascaused much confusion and technical difficulty in achieving experimentalreproducibility since its first report in 2004. Moreover, the same ambient holedoping was soon observed in 2-D semiconductors, which implied that a commonmechanism should be operative and apply to other low-dimensional materialsuniversally. In this Account, we review our breakthroughs in unraveling thechemical origin and mechanistic requirements of the hidden CT reactions using2-D crystals. We developed in-situ optical methods to quantify charge densityusing Raman and photoluminescence (PL) spectroscopy and imaging. Using gas andtemperature-controlled in-situ measurements, we revealed that the electricalholes are injected by the oxygen reduction reaction (ORR): $O_{2}$ + $4H^{+}$ +$4e^{-}$ $\rightleftharpoons$ $2H_{2}O$, which was independently verified by pHdependence in HCl solutions. In addition to oxygen and water vapor, the overallCT reaction requires hydrophilic dielectric substrates, which assist hydrationof the sample-substrate interface. The interface-localized reaction allowed usto visualized and control interfacial molecular diffusion and CT by varing the2-D gap spacing and introducing defects. The complete mechanism of thefundamental charge exchange summarized in this Account will be essential inexploring material and device properties of other low dimensional materials.

Redox and Molecular Diffusion in 2D van der Waals Space