Harnessing Quantum Capacitance in 2D Material/Molecular Layer Junctions for Novel Electronic Device Functionality.

Two-dimensional (2D) materials promise advances in electronic devices beyond Moore's scaling law through extended functionality, such as non-monotonic dependence of device parameters on input parameters. However, the robustness and performance of effects like negative differential resistance (NDR) and anti-ambipolar behavior have been limited in scale and robustness by relying on atomic defects and complex heterojunctions. In this paper, we introduce a novel device concept that utilizes the quantum capacitance of junctions between 2D materials and molecular layers. We realized a variable capacitance 2D molecular junction (vc2Dmj) diode through the scalable integration of graphene and single layers of stearic acid. The vc2Dmj exhibits NDR with a substantial peak-to-valley ratio even at room temperature and an active negative resistance region. The origin of this unique behavior was identified through thermoelectric measurements and ab initio calculations to be a hybridization effect between graphene and the molecular layer. The enhancement of device parameters through morphology optimization highlights the potential of our approach toward new functionalities that advance the landscape of future electronics.

Fabry-Perot Oscillation and Resonance Energy Transfer: Mechanism for Ultralow-Threshold Optically and Electrically Driven Random Laser in Quasi-2D Ruddlesden-Popper Perovskites.

The recently emerged metal-halide hybrid perovskite (MHP) possesses superb optoelectronic features, which have obtained great attention in solid-state lighting, photodetection, and photovoltaic applications. Because of its excellent external quantum efficiency, MHP has promising potential for the manifestation of ultralow threshold optically pumped laser. However, the demonstration of an electrically driven laser remains a challenge because of the vulnerable degradation of perovskite, limited exciton binding energy (Eb), intensity quenching, and efficiency drop by nonradiative recombinations. In this work, based on the paradigm of integration of Fabry-Perot (F-P) oscillation and resonance energy transfer, we observed an ultralow-threshold (∼250 µWcm-2) optically pumped random laser from moisture-insensitive mixed dimensional quasi-2D Ruddlesden-Popper phase perovskite microplates. Particularly, we demonstrated an electrically driven multimode laser with a threshold of ∼60 mAcm-2 from quasi-2D RPP by judicious combination of a perovskite/hole transport layer (HTL) and an electron transport layer (ETL) having suitable band alignment and thickness. Additionally, we showed the tunability of lasing modes and color by driving an external electric potential. Performing finite difference time domain (FDTD) simulations, we confirmed the presence of F-P feedback resonance, the light trapping effect at perovskite/ETL, and resonance energy transfer contributing to laser action. Our discovery of an electrically driven laser from MHP opens a useful avenue for developing future optoelectronics.

Exciton Manipulation for Enhancing Photoelectrochemical Hydrogen Evolution Reaction in Wrinkled 2D Heterostructures.

2D materials' junctions have demonstrated capabilities as metal-free alternatives for the hydrogen evolution reaction (HER). To date, the HER has been limited to heterojunctions of different compositions or band structures. Here, the potential of local strain modulation based on wrinkled 2D heterostructures is demonstrated, which helps to realize photoelectrocatalytically active junctions. By forming regions of high and low tensile strain in wrinkled WS2 monolayers, local modification of their band structure and internal electric field due to piezoelectricity is realized in the lateral direction. This structure produces efficient electron-hole pair generation due to light trapping and exciton funneling toward the crest of the WS2 wrinkles and enhances exciton separation. Additionally, the formation of wrinkles induces an air gap in-between the 2D layer and substrate, which reduces the interfacial scattering effect and consequently improves the charge-carrier mobility. A detailed study of the strain-dependence of the photocatalytic HER process demonstrates a 2-fold decrease in the Tafel slope and a 30-fold enhancement in exchange current density. Finally, optimization of the light absorption through functionalization with quantum dots produces unprecedented photoelectrocatalytic performance and provides a route toward the scalable formation of strain-modulated WS2 nanojunctions for future green energy generation.

All-carbon stretchable and cavity-free white lasers.

Flexible, stretchable, and bendable electronics and optoelectronics have a great potential for wide applications in smart life. An environmentally friendly, cost effective and wide-angle emission laser is indispensable for the emerging technology. In this work, circumvent the challenge issue, cavity-free and stretchable white light lasers based on all carbon materials have been demonstrated by integration of fluorescent carbon quantum dots (CQDs) and crumpled graphene. The typical emission spectrum of the cavity-free laser based on all-carbon materials has a CIE chromaticity coordinate of (0.30, 0.38) exhibiting an intriguing broadband white-light emission. The unprecedented and non-toxic stretchable and white light cavity-free lasers based on all-carbon materials can serve as next-generation optoelectronic devices for a wide range application covering solid-state lighting and future wearable technologies.

All-carbon stretchable and cavity-free white lasers: publisher's note.

This publisher's note contains corrections to [Opt. Express30, 20213 (2022)10.1364/OE.457921].

Chemical vapor deposition merges MoS2 grains into high-quality and centimeter-scale films on Si/SiO2.

Two-dimensional molybdenum disulfide (MoS2) has attracted increasing attention due to its promise for next-generation electronics. To realize MoS2-based electronics, however, a synthesis method is required that produces a uniform single-layer material and that is compatible with existing semiconductor fabrication techniques. Here, we demonstrate that uniform films of single-layer MoS2 can be directly produced on Si/SiO2 at wafer-scale without the use of catalysts or promoters. Control of the precursor transport through oxygen dosing yielded complete coverage and increased connectivity between crystalline MoS2 domains. Spectroscopic characterization and carrier transport measurements furthermore revealed a reduced density of defects compared to conventional chemical vapor deposition growth that increased the quantum yield over ten-fold. To demonstrate the impact of enhanced scale and optoelectronic performance, centimeter-scale arrays of MoS2 photosensors were produced that demonstrate unprecedentedly high and uniform responsivity. Our approach improves the prospect of MoS2 for future applications.

Enhancing the Photoelectrochemical Hydrogen Evolution Reaction through Nanoscrolling of Two-Dimensional Material Heterojunctions.

The clean production of hydrogen from water using sunlight has emerged as a sustainable alternative toward large-scale energy generation and storage. However, designing photoactive semiconductors that are suitable for both light harvesting and water splitting is a pivotal challenge. Atomically thin transition metal dichalcogenides (TMD) are considered as promising photocatalysts because of their wide range of available electronic properties and compositional variability. However, trade-offs between carrier transport efficiency, light absorption, and electrochemical reactivity have limited their prospects. We here combine two approaches that synergistically enhance the efficiency of photocarrier generation and electrocatalytic efficiency of two-dimensional (2D) TMDs. The arrangement of monolayer WS2 and MoS2 into a heterojunction and subsequent nanostructuring into a nanoscroll (NS) yields significant modifications of fundamental properties from its constituents. Spectroscopic characterization and ab initio simulation demonstrate the beneficial effects of straining and wall interactions on the band structure of such a heterojunction-NS that enhance the electrochemical reaction rate by an order of magnitude compared to planar heterojunctions. Phototrapping in this NS further increases the light-matter interaction and yields superior photocatalytic performance compared to previously reported 2D material catalysts and is comparable to noble-metal catalyst systems in the photoelectrochemical hydrogen evolution reaction (PEC-HER) process. Our approach highlights the potential of morphologically varied TMD-based catalysts for PEC-HER.

QD/2D Hybrid Nanoscrolls: A New Class of Materials for High-Performance Polarized Photodetection and Ultralow Threshold Laser Action.

Nanoscrolls are a class of nanostructures where atomic layers of 2D materials are stacked consecutively in a coaxial manner to form a 1D spiral topography. Self-assembly of chemical vapor deposition grown 2D WS2 monolayer into quasi-1D van der Waals scroll structure instigates a plethora of unique physiochemical properties significantly different from its 2D counterparts. The physical properties of such nanoscrolls can be greatly manipulated upon hybridizing them with high-quantum-yield colloidal quantum dots, forming 0D/2D structures. The efficient dissociation of excitons at the heterojunctions of QD/2D hybridized nanoscrolls exhibits a 3000-fold increased photosensitivity compared to the pristine 2D-material-based nanoscroll. The synergistic effects of confined geometry and efficient QD scatterers produce a nanocavity with multiple feedback loops, resulting in coherent lasing action with an unprecedentedly low lasing threshold. Predominant localization of the excitons along the circumference of this helical scroll results in a 12-fold brighter emission for the parallel-polarized transition compared to the perpendicular one, as confirmed by finite-difference time-domain simulation. The versatility of hybridized nanoscrolls and their unique properties opens up a powerful route for not-yet-realized devices toward practical applications.

Intrinsic Ultralow-Threshold Laser Action from Rationally Molecular Design of Metal-Organic Framework Materials.

Metal-organic frameworks (MOFs) are superior for multiple applications including drug delivery, sensing, and gas storage because of their tunable physiochemical properties and fascinating architectures. Optoelectronic application of MOFs is difficult because of their porous geometry and conductivity issues. Recently, a few optoelectronic devices have been fabricated by a suitable design of integrating MOFs with other materials. However, demonstration of laser action arising from MOFs as intrinsic gain media still remains challenging, even though some studies endeavor on encapsulating luminescence organic laser dyes into the porous skeleton of MOFs to achieve laser action. Unfortunately, the aggregation of such unstable laser dyes causes photoluminescence quenching and energy loss, which limits their practical application. In this research, unprecedently, we demonstrated ultralow-threshold (∼13 nJ/cm2) MOF laser action by a judicious choice of metal nodes and organic linkers during synthesis of MOFs. Importantly, we also demonstrated that the white random lasing from the beautiful microflowers of organic linkers possesses a porous network, which is utilized to synthesize the MOFs. The highly luminescent broad-band organic linker 1,4-NDC, which itself exhibits a strong white random laser, is used not only to achieve the stimulated emission in MOFs but also to reduce the lasing threshold. Such white lasing has multiple applications from bioimaging to the recently developed versatile Li-Fi technology. In addition, we showed that the smooth facets of MOF microcrystals can show Fabry-Perot resonant cavities having a high quality factor of ∼103 with excellent photostability. Our unique discovery of stable, nontoxic, high-performance MOF laser action will open up a new route for the development of new optoelectronic devices.

Heavy Mediator at Quantum Dot/Graphene Heterojunction for Efficient Charge Carrier Transfer: Alternative Approach for High-Performance Optoelectronic Devices.

Two-dimensional (2D) material nanocomposites have emerged as a material system for discovering new physical phenomena and developing novel devices. However, because of the low density of states of most two-dimensional materials such as graphene, the heterostructure of nanocomposites suffers from an enhanced depletion region, which can greatly reduce the efficiency of the charge carrier transfer and deteriorate the device performance. To circumvent this difficulty, here we propose an alternative approach by inserting a second 2D mediator with a heavy effective mass having a large density of states in-between the heterojunction of 2D nanocomposites. The mediator can effectively reduce the depletion region and form a type-II band alignment, which can speed up the dissociation of electron-hole pairs and enhance charge carrier transfer. To illustrate the principle, we demonstrate a novel stretchable photodetector based on the combination of graphene/ReS2/perovskite quantum dots. Two-dimensional ReS2 acts as a mediator in-between highly absorbing perovskite quantum dots and a high-mobility graphene channel and a thiol-based linker between the ReS2 and the perovskite. It is found that the optical sensitivity can be enhanced by 22 times. This enhancement was ascribed to the improvement of the charge transfer efficiency as evidenced by optical spectroscopy measurements. The produced photosensors are capable of reaching the highest reported value of photoresponsivity (>107 A W-1) and detectivity compared to previously studied stretchable devices. Mechanical robustness with tolerable strain up to 100% and excellent stability make our device ideal for future wearable electronics.