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.

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.

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.

Graphene Sandwich Stable Perovskite Quantum-Dot Light-Emissive Ultrasensitive and Ultrafast Broadband Vertical Phototransistors.

Dual-functional devices that can simultaneously detect light and emit light have a tremendous appeal for multiple applications, including displays, sensors, defense, and high-speed optical communication. Despite the tremendous efforts of scientists, the progress of integration of a phototransistor, where the built-in electric field separates the photogenerated excitons, and a light-emitting diode, where the radiative recombination can be enhanced by band offset, into a single device remains a challenge. Combining the superior properties of perovskite quantum dots (PQDs) and graphene, here we report a light-emissive, ultrasensitive, ultrafast, and broadband vertical phototransistor that can simultaneously act as an efficient photodetector and light emitter within a single device. The estimated value of the external quantum efficiency of the vertical phototransistor is ∼1.2 × 1010% with a photoresponsivity of >109 A W-1 and a response time of <50 µs, which exceed all the presently reported vertical phototransistor devices. We also demonstrate that the modulation of the Dirac point of graphene efficiently tunes both amplitude and polarity of the photocurrent. The device exhibits a green emission having a quantum efficiency of 5.6%. The moisture-insensitive and environmentally stable, light-emissive, ultrafast, and ultrasensitive broadband phototransistor creates a useful route for dual-functional optoelectronic devices.

Self-Healing Nanophotonics: Robust and Soft Random Lasers.

Self-healing technology promises a generation of innovation in cross-cutting subjects ranging from electronic skins, to wearable electronics, to point-of-care biomedical sensing modules. Recently, scientists have successfully pulled off significant advances in self-healing components including sensors, energy devices, transistors, and even integrated circuits. Lasers, one of the most important light sources, integrated with autonomous self-healability should be endowed with more functionalities and opportunities; however, the study of self-healing lasers is absent in all published reports. Here, the soft and self-healable random laser (SSRL) is presented. The SSRL can not only endure extreme external strain but also withstand several cutting/healing test cycles. Particularly, the damaged SSRL enables its functionality to be restored within just few minutes without the need of additional energy, chemical/electrical agents, or other healing stimuli, truly exhibiting a supple yet robust laser prototype. It is believed that SSRL can serve as a vital building block for next-generation laser technology as well as follow-on self-healing optoelectronics.

Single-Molecule-Based Electroluminescent Device as Future White Light Source.

During the last two decades, spectacular development of light-emitting diodes (LEDs) has been achieved owing to their widespread application possibilities. However, traditional LEDs suffer from unavoidable energy loss because of the down conversion of photons, toxicity due to the involvement of rare-earth materials in their production, higher manufacturing cost, and reduced thermal stability that prevent them from all-inclusive applications. To address the existing challenges associated with current commercially available white LEDs, herein, we report a broad-band emission originating from an intrinsic lanthanide-free single-molecule-based LED. Self-assembly of a butterfly-shaped strontium-based compound {[Sr(H2btc)2(MeOH)(H2O)2]·2H2O} (1) was achieved through the reaction of Sr(NO3)2 with 1,2,3-benzenetricarboxylic acid hydrate (1,2,3-H3btc) under hydrothermal conditions. A white LED based on this single molecule exhibited a remarkable broad-band luminescence spectrum with Commission Internationale de l'Eclairage (CIE) coordinates at (0.33, 0.32) under 30 mA current injection. Such a broad luminescence spectrum can be attributed to the simultaneous existence of several emission lines originating from the intramolecular interactions within the structure. To further examine the nature of the observed transitions, density functional theory (DFT) calculations were carried out to explore the geometric and electronic properties of the complex. Our study thus paves the way toward a key step for developing a basic understanding and the development of high performance broad-band light-emitting devices with environment-friendly characteristics based on organic-inorganic supramolecular materials.

A Highly-Efficient Single Segment White Random Laser.

Production of multicolor or multiple wavelength lasers over the full visible-color spectrum from a single chip device has widespread applications, such as superbright solid-state lighting, color laser displays, light-based version of Wi-Fi (Li-Fi), and bioimaging, etc. However, designing such lasing devices remains a challenging issue owing to the material requirements for producing multicolor emissions and sophisticated design for producing laser action. Here we demonstrate a simple design and highly efficient single segment white random laser based on solution-processed NaYF4:Yb/Er/Tm@NaYF4:Eu core-shell nanoparticles assisted by Au/MoO3 multilayer hyperbolic meta-materials. The multicolor lasing emitted from core-shell nanoparticles covering the red, green, and blue, simultaneously, can be greatly enhanced by the high photonic density of states with a suitable design of hyperbolic meta-materials, which enables decreasing the energy consumption of photon propagation. As a result, the energy upconversion emission is enhanced by ∼50 times with a drastic reduction of the lasing threshold. The multiple scatterings arising from the inherent nature of the disordered nanoparticle matrix provide a convenient way for the formation of closed feedback loops, which is beneficial for the coherent laser action. The experimental results were supported by the electromagnetic simulations derived from the finite-difference time-domain (FDTD) method. The approach shown here can greatly simplify the design of laser structures with color-tunable emissions, which can be extended to many other material systems. Together with the characteristics of angle free laser action, our device provides a promising solution toward the realization of many laser-based practical applications.

Ultra-high performance flexible piezopotential gated In1-xSnxSe phototransistor.

Flexible optoelectronic devices facilitated by the piezotronic effect have important applications in the near future in many different fields ranging from solid-state lighting to biomedicine. Two-dimensional materials possessing extraordinary mechanical strength and semiconducting properties are essential for realizing nanopiezotronics and piezo-phototronics. Here, we report the first demonstration of piezo-phototronic properties in In1-xSnxSe flexible devices by applying systematic mechanical strain under photoexcitation. Interestingly, we discover that the dark current and photocurrent are increased by five times under a bending strain of 2.7% with a maximum photoresponsivity of 1037 AW-1. In addition, the device can act as a strain sensor with a strain sensitivity up to 206. Based on these values, the device outperforms the same class of devices in two-dimensional materials. The underlying mechanism responsible for the discovered behavior can be interpreted in terms of piezoelectric potential gating, allowing the device to perform like a phototransistor. The strain-induced gate voltage assists in the efficient separation of photogenerated charge carriers and enhances the mobility of In1-xSnxSe, resulting in good performance on a freeform surface. Thus, our multifunctional device is useful for the development of a variety of advanced applications and will help meet the demand of emerging technologies.