Defects usually play an important role in tailoring various properties of two-dimensional materials. Defects in two-dimensional monolayer molybdenum disulphide may be responsible for large variation of electric and optical properties. Here we present a comprehensive joint experiment–theory investigation of point defects in monolayer molybdenum disulphide prepared by mechanical exfoliation, physical and chemical vapour deposition. Defect species are systematically identified and their concentrations determined by aberration-corrected scanning transmission electron microscopy, and also studied by ab-initio calculation. Defect density up to 3.5 × 1013 cm−2 is found and the dominant category of defects changes from sulphur vacancy in mechanical exfoliation and chemical vapour deposition samples to molybdenum antisite in physical vapour deposition samples. Influence of defects on electronic structure and charge-carrier mobility are predicted by calculation and observed by electric transport measurement. In light of these results, the growth of ultra-high-quality monolayer molybdenum disulphide appears a primary task for the community pursuing high-performance electronic devices.
The development of non precious metal
based electrocatalysts for
the hydrogen evolution reaction (HER) holds a decisive key to a spectrum
of energy conversion technologies. Previous studies have established
layered molybdenum chalcogenides as promising candidates. In this
work, we prepared ultrathin MoS2(1–x)Se2x
alloy nanoflakes with monolayer
or few-layer thickness and fully tunable chemical composition for
maximum HER activity. Spectroscopic characterizations corroborate
the progressive evolution of their structures and properties as x increases from 0 to 1 without any noticeable phase separation.
In particular, it is evidenced that the introduction of selenium continuously
modulates the d band electronic structure of molybdenum, probably
leading to tuned hydrogen adsorption free energy and consequently
electrocatalytic activity. Electrochemical measurements show that
all MoS2(1–x)Se2x
nanoflakes are highly active and durable for HER with small
overpotentials in the range of 80–100 mV and negligible activity
loss up to 10000 cycles. Most importantly, alloyed nanoflakes, especially
with the chemical composition of MoSSe, exhibit improved performance
in comparison to either MoS2 or MoSe2. Given
their overall similar nanoflake morphologies, we believe such improvements
reflect the higher intrinsic activity of alloyed catalysts with the
hydrogen adsorption free energy closer to thermoneutral.
Semiconducting MoS₂(₁-x) Se₂x mono-layers where x = 0-0.40 are successfully grown over large areas. A random arrangement of the S and Se atoms and a tunable bandgap photoluminescence are observed. Atomically thin, 2D semiconductor alloys with tunable bandgaps have potential applications in nano- and opto-electronics. Field-effect transistors fabricated with the monolayers exhibit high on/off ratios of >10(5).
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