Emerging transition metal dichalcogenides (TMDCs) offer an attractive platform for investigating functional light-emitting devices, such as flexible devices, quantum and chiral devices, high-performance optical modulators, and ultralow threshold lasers. In these devices, the key operation is to control the light-emitting position, that is, the spatial position of the recombination zone to generate electroluminescence, which permits precise light guides/passes/confinement to ensure favorable device performance. Although various structures of TMDC light-emitting devices have been demonstrated, including the transistor configuration and heterostructured diodes, it is still difficult to tune the light-emitting position precisely owing to the structural device complexity. In this study, we fabricated two-terminal light-emitting devices with chemically synthesized WSe 2 , MoSe 2 , and WS 2 monolayers, and performed direct observations of their electroluminescence, from which we discovered a divergence in their light-emitting positions. Subsequently, we propose a method to associate spatial electroluminescence imaging with transport properties among different samples; consequently, a common rule for determining the locations of recombination zones is revealed. Owing to dynamic carrier accumulations and p−i−n junction formations, the light-emitting positions in electrolyte-based devices can be tuned continuously. The proposed method will expand the device applicability for designing functional optoelectronic applications based on TMDCs.
The diverse series of transition metal dichalcogenide (TMDC) materials has been employed in various optoelectronic applications, such as photodetectors, light‐emitting diodes, and lasers. Typically, the detection or emission range of optoelectronic devices is unique to the bandgap of the active material. Therefore, to improve the capability of these devices, extensive efforts have been devoted to tune the bandgap, such as gating, strain, and dielectric engineering. However, the controllability of these methods is severely limited (typically ≈0.1 eV). In contrast, alloying TMDCs is an effective approach that yields a composition‐dependent bandgap and enables light emissions over a wide range. In this study, a color‐tunable light‐emitting device using compositionally graded TMDC alloys is fabricated. The monolayer WS2/WSe2 alloy grown by chemical vapor deposition shows a spatial gradient in the light‐emission energy, which varies from 2.1 to 1.7 eV. This alloy is incorporated in an electrolyte‐based light‐emitting device structure that can tune the recombination zone laterally. Thus, a continuous and reversible color‐tunable light‐emitting device is successfully fabricated by controlling the light‐emitting positions. The results provide a new approach for exploring monolayer semiconductor‐based broadband optical applications.
Light‐Emitting Devices
A color‐tunable light‐emitting diode is realized by Jiang Pu, Yasumitsu Miyata, Taishi Takenobu, and co‐workers in article number 2203250 using compositionally graded monolayer transition metal dichalcogenide alloys. By controlling the light‐emitting positions in the alloys, the composition gradient of the bandgap enables continuous and reversible light emission with energies ranging from 2.1 to 1.7 eV. The results provide a new approach for exploring monolayer semiconductor alloy based broadband optoelectronic device applications.
Fabrication of high-performance optoelectronic devices is an important aspect of the application research of transition metal dichalcogenides (TMDCs). In this study, heterostructures of TMDCs and hexagonal boron nitrides (hBN) were successfully fabricated into light-emitting devices. Monolayer and artificially stacked homobilayer WS2 were prepared on hBN, respectively. They were then deposited with electrodes and covered by the ion gels to function as light-emitting devices. Both devices showed clear electroluminescence (EL) with voltages of ~3 V. In monolayer device, a symmetric EL peak was observed with suppressed inhomogeneity. The bilayer device showed spectra that agreed with the natural bilayer samples. These results indicate the enhancement of the optical performance of TMDCs and the heterostructure could expand the potential of TMDC-based light-emitting devices.
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