  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.
