Effects of Steric Factors on Molecular Doping to MoS$_2$

Surface functionalization of two-dimensional (2D) materials with organic electron donors (OEDs) is a powerful method to modulate the electronic properties of the material. However, our fundamental understanding of the doping mechanism is largely limited to the categorization of molecular dopants as n- or p-type based on the relative position of the molecule's redox potential in relation to the Fermi level of the 2D host. Our limited knowledge about the impact of factors other than the redox properties of the molecule on doping makes it challenging to controllably use molecules to dope 2D materials and design new OEDs. Here, we functionalize monolayer MoS$_2$ using two molecular dopants, Me- and $^t$Bu-OED, which have the same redox potential but different steric properties to probe the effects of molecular size on the doping level of MoS$_2$. We show that, for the same functionalization conditions, the doping powers of Me- and $^t$Bu-OED are 0.22 - 0.44 and 0.11 electrons per molecule, respectively, demonstrating that the steric properties of the molecule critically affect doping levels. Using the stronger dopant, Me-OED, a carrier density of 1.10 +/- 0.37 x 10$^{14}$ cm$^{-2}$ is achieved in MoS$_2$, the highest doping level to date for MoS$_2$ using surface functionalization. Overall, we establish that tuning of the steric properties of the dopant is essential in the rational design of molecular dopants.