2, Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California, United States
3, University of Pittsburgh, Pittsburgh, Pennsylvania, United States
4, George Mason University, Fairfax, Virginia, United States
5, The University of Texas at Dallas, Richardson, Texas, United States
Doping, as a fundamental technique to functionalize conventional semiconductors, holds unique promise to tune the properties of 2D materials. However, due to the ultra-low thickness and simple structure of 2D materials, the chemical environment (i.e. substrates) may drastically modify the properties of synthetic 2D materials, leading disagreements between the experiments and simulations. Indeed, realizing the in-situ substitutional doping of monolayer MoS2 by chemical vapor deposition necessitates the in-depth understanding of the substrates’ properties. Here, we present successful in-situ doping studies achieved by fine-tuning the substrate properties. First, the substrate surface inertness directly impacts on the doping efficacy of manganese (Mn). Substitutional Mn doping of monolayer of MoS2 can only be achieved on 2D substrates such as graphene due to the absence of polarized dangling bonds on the surface. On 3D substrates such as sapphire and SiO2, Mn is adsorbed by the surface dangling bond instead of being bonded in the MoS2 lattice (Nano Lett., 2015, 15 (10) 6586-6591). Second, the substrate surface termination guides the film/substrate charge transfer and limits carrier density tunability by foreign dopants. Aluminum terminated sapphire (c-sapphire) surface exhibits strong electron doping to the synthetic MoS2 monolayers, leading the 100x higher electron concentration in MoS2 than its counterpart grown on oxygen terminated sapphire (r-sapphire) surface, evident by field effect transistors (FETs) characterization. Meanwhile, the reduced electron doping and the interaction between Mo and surface oxygen on r-sapphire results in 100x enhancement of photoluminescence (PL) intensity and carrier lifetime (~1 ns) (Sci. Rep., 7 (1), 16938). Utilizing r- sapphire, nearly degenerately n-doped MoS2 is realized by 1 at% Re substitutional doping, evident by various electronic measurements including conductive atomic force microcopy, scanning tunneling microscopy and FETs, agreeing well with density function theory (DFT) calculations. In contrast, 1 at% Re doping is unable to effectively tune the electron concentration due to the strong electron doping from Al terminated sapphire. Surprisingly, X-ray photoelectron spectroscopy (XPS) and low temperature PL indicates that Re doping is also able to reduce the density of sulfur vacancies by 25% (Adv. Funct. Mater., 2018, 28, 1706950), demonstrating the possibility of multifunctional doping of 2D materials.