Controlled
substitutional doping of two-dimensional transition-metal
dichalcogenides (TMDs) is of fundamental importance for their applications
in electronics and optoelectronics. However, achieving p-type conductivity in MoS2 and WS2 is challenging
because of their natural tendency to form n-type
vacancy defects. Here, we report versatile growth of p-type monolayer WS2 by liquid-phase mixing of a host tungsten
source and niobium dopant. We show that crystallites of WS2 with different concentrations of substitutionally doped Nb up to
1014 cm–2 can be grown by reacting solution-deposited
precursor film with sulfur vapor at 850 °C, reflecting the good
miscibility of the precursors in the liquid phase. Atomic-resolution
characterization with aberration-corrected scanning transmission electron
microscopy reveals that the Nb concentration along the outer edge
region of the flakes increases consistently with the molar concentration
of Nb in the precursor solution. We further demonstrate that ambipolar
field-effect transistors can be fabricated based on Nb-doped monolayer
WS2.
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