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Optoelectronic properties of atomically-thin MoS₂flakes are modulated using photochromic diarylethene molecules. The layer-number-dependent charge transfer behaviors at the hybrid heterointerface are elucidated. 

Abstract Transition metal dichalcogenides (TMDCs) have received much attention for optoelectronic applications because of their band gap transition from indirect to direct as they decrease from multilayer to monolayer. Recent studies have experimented with the use of photochromic molecules to optically control the charge transport of two-dimensional (2D) TMDCs. In this work, a numerical study using density functional theory has been performed to test the possibility to control the optical property of 2D TMDC monolayers with various photochromic molecules. When the photochromic molecule’s highest occupied molecular orbital (HOMO) or lowest unoccupied molecular orbital (LUMO) energy levels are within the band gap of 2D TMDC monolayers, holes or electrons will transport to the photochromic molecules, resulting in the reduction of excitons in the 2D TMDC monolayers. The reduced optical response can be recovered by going through reverse isomerization of the photochromic molecules. Molybdenum disulfide (MoS2) and tungsten diselenide (WSe2) monolayers were tested with various photochromic molecules including azobenzene, spiropyran, and diarylethenes (DAE 2 ethyl, DAE 5 ethyl, DAE 5 methyl). The systematic study presented in this work displays that MoS2-Spiropyran and every diarylethene derivative used in this study except MoS2-DAE 5 methyl exhibited photo-switchable behavior. 

Abstract The ability to modulate optical and electrical properties of two-dimensional (2D) semiconductors has sparked considerable interest in transition metal dichalcogenides (TMDs). Herein, we introduce a facile strategy for modulating optoelectronic properties of monolayer MoSe₂with external light. Photochromic diarylethene (DAE) molecules formed a 2-nm-thick uniform layer on MoSe₂, switching between its closed- and open-form isomers under UV and visible irradiation, respectively. We have discovered that the closed DAE conformation under UV has its lowest unoccupied molecular orbital energy level lower than the conduction band minimum of MoSe₂, which facilitates photoinduced charge separation at the hybrid interface and quenches photoluminescence (PL) from monolayer flakes. In contrast, open isomers under visible light prevent photoexcited electron transfer from MoSe₂to DAE, thus retaining PL emission properties. Alternating UV and visible light repeatedly show a dynamic modulation of optoelectronic signatures of MoSe₂. Conductive atomic force microscopy and Kelvin probe force microscopy also reveal an increase in conductivity and work function of MoSe₂/DAE with photoswitched closed-form DAE. These results may open new opportunities for designing new phototransistors and other 2D optoelectronic devices.