WS2 monolayer-based light-emitting devices in a vertical p-n architecture

Electroluminescence from Megasonically Solution-Processed MoS2 Nanosheet Films

Due to their superior optoelectronic properties, monolayer two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted significant attention for electroluminescent devices. However, challenges in isolating optoelectronically active TMD monolayers using scalable liquid phase exfoliation have precluded electroluminescence in large-area, solution-processed TMD films. Here, we overcome these limitations and demonstrate electroluminescence from molybdenum disulfide (MoS2) nanosheet films by employing a monolayer-rich MoS2 ink produced by electrochemical intercalation and megasonic exfoliation. Characteristic monolayer MoS2 photoluminescence and electroluminescence spectral peaks at 1.88-1.90 eV are observed in megasonicated MoS2 films, with the emission intensity increasing with film thickness over the range 10-70 nm. Furthermore, employing a vertical light-emitting capacitor architecture enables uniform electroluminescence in large-area devices. These results indicate that megasonically exfoliated MoS2 monolayers retain their direct bandgap character in electrically percolating thin films even following multistep solution processing. Overall, this work establishes megasonicated MoS2 inks as an additive manufacturing platform for flexible, patterned, and miniaturized light sources that can likely be expanded to other TMD semiconductors.

Temperature Dependence of Excitonic Auger Recombination in Excitonic-Complex-Free Monolayer WS2 by Considering Auger Broadening and Generation Efficiency

Monolayer transition metal dichalcogenides (TMDs) have been extensively studied for their optoelectronic properties and applications. However, even at moderate exciton densities, their light-emitting capability is severely limited by Auger-type exciton-exciton annihilation (EEA). Previous work on EEA used oversimplified models in the presence of excitonic complexes, resulting in seriously underestimated values for the Auger coefficient. In this work, we transferred monolayer WS₂ on a gold substrate with hBN encapsulation, where excitons persist as the main species at 3-300 K via metal proximity. We numerically solved the rate equation for excitons to accurately determine the Auger coefficient as a function of temperature by considering laser pulse width and spatially inhomogeneous exciton distribution. We found that the Auger coefficient consists of temperature-dependent and independent terms, consistent with a theoretical model for direct and exchange processes, respectively. We believe that our results provide a guide for enhancing the luminescence quantum yield of TMDs.

Silver nanowire electrodes for transparent light emitting devices based on WS2monolayers

Transition metal dichalcogenide (TMDC) monolayers with their direct band gap in the visible to near-infrared spectral range have emerged over the past years as highly promising semiconducting materials for optoelectronic applications. Progress in scalable fabrication methods for TMDCs like metal-organic chemical vapor deposition (MOCVD) and the ambition to exploit specific material properties, such as mechanical flexibility or high transparency, highlight the importance of suitable device concepts and processing techniques. In this work, we make use of the high transparency of TMDC monolayers to fabricate transparent light-emitting devices (LEDs). MOCVD-grown WS₂is embedded as the active material in a scalable vertical device architecture and combined with a silver nanowire (AgNW) network as a transparent top electrode. The AgNW network was deposited onto the device by a spin-coating process, providing contacts with a sheet resistance below 10 Ω sq^-1and a transmittance of nearly 80%. As an electron transport layer we employed a continuous 40 nm thick zinc oxide (ZnO) layer, which was grown by atmospheric pressure spatial atomic layer deposition (AP-SALD), a precise tool for scalable deposition of oxides with defined thickness. With this, LEDs with an average transmittance over 60% in the visible spectral range, emissive areas of several mm²and a turn-on voltage of around 3 V are obtained.

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

Light-emitting diodes (LEDs) are used widely, but when operated at a low-voltage direct current (DC), they consume unnecessary power because a converter must be used to convert it to an alternating current (AC). DC flow across devices also causes charge accumulation at a high current density, leading to lowered LED reliability. In contrast, gallium-nitride-based LEDs can be operated without an AC-DC converter being required, potentially leading to greater energy efficiency and reliability. In this study, we developed a multicolor AC-driven light-emitting device by integrating a WSe₂ monolayer and AlGaInP-GaInP multiple quantum well (MQW) structures. The CVD-grown WSe₂ monolayer was placed on the top of an AlGaInP-based light-emitting diode (LED) wafer to create a two-dimensional/three-dimensional heterostructure. The interfaces of these hybrid devices are characterized and verified through transmission electron microscopy and energy-dispersive X-ray spectroscopy techniques. More than 20% energy conversion from the AlGaInP MQWs to the WSe₂ monolayer was observed to boost the WSe₂ monolayer emissions. The voltage dependence of the electroluminescence intensity was characterized. Electroluminescence intensity-voltage characteristic curves indicated that thermionic emission was the mechanism underlying carrier injection across the potential barrier at the Ag-WSe₂ monolayer interface at low voltage, whereas Fowler-Nordheim emission was the mechanism at voltages higher than approximately 8.0 V. These multi-color hybrid light-emitting devices both expand the wavelength range of 2-D TMDC-based light emitters and support their implementation in applications such as chip-scale optoelectronic integrated systems, broad-band LEDs, and quantum display systems.

The Hidden Flower in WS2 Flakes: A Combined Nanomechanical and Tip-Enhanced Raman Exploration

We report on tungsten disulfide (WS₂) flakes grown by chemical vapor deposition (CVD), which exhibit a flower-like surface structure above the primary few-layer flake with a triangular shape. The fine structure is only revealed in the mechanical, chemical, and electronic properties of the flake but not in the topography. The origin of this structure is the peculiar one-step growth during the CVD process that permits to control the sulfur concentration at any time. A high concentration of S at the onset of the deposition process leads to a rapid growth of the flake, resulting in tungsten vacancies. Reducing the sulfur concentration toward the end of the growth slows down the reaction and leads to sulfur vacancies. These microscale domains were studied by confocal- and tip-enhanced Raman spectroscopy revealing their chemical composition with high spatial resolution. A strong quenching of the photoluminescence in the tungsten-vacancy domains is observed. Atomic force microscope measurements, performed in intermittent contact mode, force modulation mode (including lateral force mode), and PeakForce quantitative nanomechanics mode, show that the mechanical properties of these domains differ. Within the tungsten-vacancy domains, the adhesion force is reduced, while the friction force increased. Kelvin probe force microscopy measurements show that the electronic properties of the flakes are modulated by these domains. The combined nanomechanical and nanospectroscopy measurements provide detailed insights on the inhomogeneous surface properties of the single WS₂ flake, further highlighting how its multidomain properties can be finely tuned using CVD.

Giant Photoluminescence Enhancement and Resonant Charge Transfer in Atomically Thin Two-Dimensional Cr2Ge2Te6/WS2 Heterostructures

Hybridization of two-dimensional (2D) magnetic semiconductors with transition-metal dichalcogenides (TMDC) monolayers can significantly engineer the light-matter interactions and provide a promising platform for enhanced excitonic systems with artificially tailored band alignments. Here, we report the fabrication of heterostructures with monolayer WS₂ on 2D Cr₂Ge₂Te₆ (CGT), which displayed giant photoluminescence enhancement at specific CGT layer numbers. The highly enhanced quantum yield obtained can be explained by novel photoexcited carrier dynamics, facilitated by alternate relaxation channels, resulting in resonance charge transfer at the heterointerface. 2D CGT revealed a strongly layer-dependent work function (up to ∼750 meV), which greatly modulates the band positioning in the heterostructure. These heterostructures conceived both type I and type II band alignments, which are verified by Kelvin probe force microscopy and PL measurements. In addition to layer modulation, we uncover temperature and power dependence of the resonance charge transfer in the multilayer heterostructure. Our findings provide further insights into the ultrafast charge dynamics occurring at the atomic interfaces. The results may pave the way for novel optoelectronics based on van der Waals heterostructures.

Operational Limits and Failure Mechanisms in All-2D van der Waals Vertical Heterostructure Devices with Long-Lived Persistent Electroluminescence

Various 2D materials can be assembled into vertical heterostructure stacks that emit strong electroluminescence. However, to date, most work is done using mechanical exfoliated materials, with little insights gained into the operation limits and failure mechanisms due to the limited number of devices produced and the device-to-device variances. However, when using chemical vapor deposition (CVD) grown 2D crystals, it is possible to construct dozens of devices to generate statistics and ensemble insights, providing a viable way toward scalable industrialization of 2D optoelectronics. In particular, the operation lifetime/duration of electroluminescence and subsequent failure mechanisms are poorly understood. Here, we demonstrate that all-2D vertical layered heterostructure (VLH) devices made using CVD-grown materials (Gr:h-BN:WS₂:h-BN:Gr) can generate strong red electroluminescence (EL) with continuous operation for more than 2 h in ambient atmospheric conditions under constant bias. Layer-by-layer controlled assembly is used to achieve graphene top and bottom electrodes in a crossbar geometry, with few layered h-BN continuous films as tunnel barriers for direct carrier injection into semiconducting monolayer WS₂ single crystals with direct band gap recombination. Tens of the devices were fabricated in a single chip, with strong EL routinely measured under both positive and negative graphene electrode bias. The success rate for EL emission in devices is over 90%. EL starts to be detected at bias values of ∼5 V, with bright red emission located at the crossbar intersection site, with intensity increasing with applied bias. Long-lived persistent EL is demonstrated for more than 2 h without significant degradation of WS₂ under high bias conditions of 20 V. In cycling tests, the EL signal peak position and intensity stay almost the same after several ON/OFF cycles with high bias, which proves that our device has good stability and durability when pulsed. Breakdown of the device is shown to occur at a bias value of ∼35 V, whereby current reduces to zero and EL abruptly stops, due to breakdown of the top graphene electrode, associated with local heating accumulation. This study provides a viable way for wafer-scale fabrication of high-performance 2D EL arrays for ultrathin optoelectronic devices and sheds light on the mechanisms of failure and operation limits of EL devices in ambient conditions.

Luminescence enhancement and dual-color emission of stacked mono-layer 2D materials

With additional precursor soaking, a thin Al₂O₃ dielectric layer can be grown on mono-layer MoS₂ by using atomic layer deposition (ALD). Similar optical characteristics are observed before and after ALD growth for the mono-layer MoS₂, which indicates that minor damage to the thin 2D material film is introduced during the growth procedure. With the thin separation layer, luminescence enhancement and dual-color emission are observed by transferring MoS₂ and WS₂ mono-layer 2D materials to 5 nm Al₂O₃/mono-layer MoS₂ samples, respectively. The results demonstrate that with careful treatment of the interfaces of 2D crystals with other materials, different stacked structures can be established.