Abstract
We report on the photostability of monolayer (1L) transition-metal dichalcogenides (TMDCs) in air and in aqueous solutions, as probed using photoluminescence spectroscopy. 1L-WSe2 was readily degraded under continuous irradiation of visible light in aqueous solutions, whereas 1L-MoS2 was relatively stable in both ambient air and aqueous solutions. The stability difference between these two materials was mainly ascribed to the oxidization reaction at the interface of 1L-TMDCs and the O2/H2O redox system induced by both band alignment and photogenerated holes. This interpretation was strongly supported by the observation of the lower degradation rate of 1L-WSe2 in the dark and in degassed water with a lower concentration of dissolved oxygen compared with the degradation rate of 1L-WSe2 in distilled water. Furthermore, the degradation rate was also nearly proportional to the number of photogenerated carriers. The degradation rate under acidic conditions was smaller than that under the basic conditions. The results are attributed to the oxidation/reduction potential of 1L-WSe2 and to the dissolution reaction of degraded species, both of which are strongly pH-dependent.
Highlights
Monolayers (1L) of transition-metal dichalcogenides (TMDCs) MX2, where M is a transition metal and X is a chalcogen, have emerged as new direct-gap semiconductors with truly twodimensional structures
The electronic properties have been studied using chemical doping and defect engineering.[27−29] the applications of 1LTMDCs as gas sensors or biosensors and photocatalysts for water splitting have been reported.[30−35] Electrochemical etching or oxidation reactions of atomically thin TMDCs under photoirradiation[36,37] or ozone exposure[38] conditions were studied. These previous studies suggest that deep understanding of the influence of surface reactions on the physical properties and stability of 1L-TMDCs is important for realizing their application in various environments such as ambient air or wet conditions
We report on the stability of the most typical 1L-TMDCs 1L-WSe2 and 1L-MoS2 in ambient air and acidic/basic aqueous solutions under visible-light irradiation
Summary
Monolayers (1L) of transition-metal dichalcogenides (TMDCs) MX2, where M is a transition metal (typically Mo or W) and X is a chalcogen (typically S, Se, or Te), have emerged as new direct-gap semiconductors with truly twodimensional structures. Because of the striking change in their electronic structures from indirect (>2L) to direct band gap (1L) depending on their number of layers,1−3 1L-TMDCs show interesting optical and electrical properties which have strong potential for use in future electronic and optoelectronic devices such as phototransistors,[4−9] photodetectors,[10−12] lightemitting devices,[13,14] light modulators,[15] and solar cells.[16−18] Tunable photoluminescence (PL) has far been demonstrated using chemisorption and physisorption,[19−22] covalent bonding,[23,24] and electrostatic methods.[25,26] The electronic properties have been studied using chemical doping and defect engineering.[27−29] the applications of 1LTMDCs as gas sensors or biosensors and photocatalysts for water splitting have been reported.[30−35] Electrochemical etching or oxidation reactions of atomically thin TMDCs under photoirradiation[36,37] or ozone exposure[38] conditions were studied These previous studies suggest that deep understanding of the influence of surface reactions on the physical properties and stability of 1L-TMDCs is important for realizing their application in various environments such as ambient air or wet conditions. This mechanism is effective for 1L-WSe2 and ineffective for MoS2 because of the difference in the relative alignment among the oxidation/reduction potentials of O2/ H2O and the levels of the valence and conduction bands of each 1L-TMDC
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