  We investigate the excitonic species in WS$_{2}$ monolayers transferred onto
III-V semiconductor substrates with different surface treatments. When the
III-V substrates were covered with amorphous native oxides, negatively charged
excitons dominate the spectral weight in low-temperature near-resonance
photoluminescence (PL) measurements. However, when the native oxides of the
III-V substrates were reduced, neutral excitons begin to dominate the spectral
weight, indicating a reduction in the electron density in the WS$_{2}$
monolayers. The removal of the native oxides enhanced the electron transfer
from the WS$_{2}$ monolayer to the III-V substrate. In addition, an additional
shoulder-like PL feature appeared $\sim$50 meV below the emission of neutral
excitons, which can be attributed to the emission of localized excitons. When
the III-V substrate surface was passivated by sulfur after the reduction of the
native oxides, neutral excitons still dominated the spectral weight. However,
the low energy PL shoulder disappeared again, suggesting the effective
delocalization of excitons through the substrate surface passivation. Surface
engineering of the semiconductor substrates for two-dimensional (2D) materials
can provide a novel approach to control the carrier density of the 2D
materials, to implement deterministic carrier localization or delocalization
for the 2D materials, and to facilitate the interlayer transfer of charge,
spin, and valley currents. These findings open the avenue for novel device
concepts and phenomena in mixed-dimensional semiconductor heterostructures.
