Spatial Control of Dynamic p–i–n Junctions in Transition Metal Dichalcogenide Light-Emitting Devices

Abstract

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.

Continuous Color‐Tunable Light‐Emitting Devices Based on Compositionally Graded Monolayer Transition Metal Dichalcogenide Alloys

“…In this device, the light‐emitting positions can be continuously tuned by the bias conditions, based on the principle that we established in our previous report. [ 39 ] Thus, when we laterally control the light‐emitting position inside the channels, their EL energy would vary according to the gradient in the composition‐dependent bandgap (Figure 3b). We removed the residual flakes from outside the channel regions by plasma etching to define precisely the electrical current path parallel to the channel length, as shown in the optical microscopy image in Figure 3b (sample #3).…”

“…To observe the EL, electrical measurements were performed using a semiconductor parameter analyzer (Agilent Technologies, Inc. E5270) with a custom‐made spectroscopy setup inside an N 2 ‐filled glove box. [ 39 ] The device was set on a microscope equipped with a CCD camera (SHODENSHA) and a spectrometer (Hamamatsu Photonics K.K. ); a half mirror was placed above the device to guide the emitted light in two separate directions.…”

“…Understanding the additional effects such as electro-optic and manybody effects to generate a high energy peak of EL could also expand the possibility to achieve white-color EL (Figure S7, Supporting Information). Furthermore, although our devices are still inferior for precise control of EL, we believe that managing EL emissions with arbitrary shapes, such as by introducing an optical waveguide [42] or patterning channels/electrodes, [39,43] would make it possible to further exploit the broadband optical functionality in alloy LEDs. For example, if the alloy device/ channel is constructed on 1D photonic crystals with periodic trenches/structures, one can expect bright light emission on those suspended areas and can expect fine control of the lightemitting position with the composition gradient.…”

“…We fabricated an ion‐gel‐based LED using this monolayer alloy, which could laterally tune the recombination zone. [ 39 ] Consequently, we demonstrated the operation of a continuous and reversible color‐tunable light‐emitting device by controlling the light‐emitting positions in the compositionally graded alloys. Our results offer a new approach for exploring TMDC alloy‐based broadband optoelectronic applications.…”

AC-driven multicolor electroluminescence from a hybrid WSe2 monolayer/AlGaInP quantum well light-emitting device

“…In such structures, electrons and holes are injected simultaneously into a TMDC from two electrode terminals when a bias voltage is applied. 23–29…”