Solution-Processed Doping of Trilayer WSe2with Redox-Active Molecules

The development of processes to controllably dope two-dimensional semiconductors is critical to achieving next-generation electronic and optoelectronic devices. In this study, n- and p-doping of highly uniform large-area trilayer WSe2is achieved by treatment with solutions of molecular reductants and oxidants. The sign and extent of doping can be conveniently controlled by the redox potential of the (metal−)organic molecules, the concentration of dopant solutions, and the treatment time. Threshold voltage shifts, the direction of which depends on whether a p- or n-dopant is used, and tunable channel current are observed in doped WSe2field-effect transistors. Detailed physical characterization including photoemission (ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy) and Raman spectroscopy provides fundamental understanding of the underlying mechanism. The origin of the doping is the electron-transfer reactions between molecular dopants and 2D semiconductors and results in a shift of the Fermi level relative to the valence band due both to state filling/emptying and to large surface dipoles between the dopant ions and the oppositely charged WSe2. These two effects both contribute to large work function changes of up to ±1 eV.