Ultrafast and Ultrasensitive Gas Sensors Derived from a Large Fermi-Level Shift in the Schottky Junction with Sieve-Layer Modulation

Gas sensors play an important role in numerous fields, covering a wide range of applications, including intelligent systems and detection of harmful and toxic gases. Even though they have attracted much attention, the response time on the order of seconds to minutes is still very slow. To circumvent the existing problems, here, we provide a seminal attempt with the integration of graphene, semiconductor, and an addition sieve layer forming a nanocomposite gas sensor with ultrahigh sensitivity and ultrafast response. The designed sieve layer has a suitable band structure that can serve as a blocking layer to prevent transfer of the charges induced by adsorbed gas molecules into the underlying semiconductor layer. We found that the sensitivity can be reduced to the parts per million level, and the ultrafast response of around 60 ms is unprecedented compared with published graphene-based gas sensors. The achieved high performance can be interpreted well by the large change of the Fermi level of graphene due to its inherent nature of the low density of states and blocking of the sieve layer to prevent charge transfer from graphene to the underlying semiconductor layer. Accordingly, our work is very useful and timely for the development of gas sensors with high performance for practical applications

Excess Random Laser Action in Memories for Hybrid Optical/Electric Logic

To surmount the scalability limitations of the nanoelectronics industry, the invention of resistance random access memory (RRAM) has drawn considerable attention in recent years for being a new-era memory. Nevertheless, the data transmission speed of RRAM is confined by virtue of its sequential reading nature. To improve upon this weakness, a hybrid optical/electric memory with ION/IOFF ratio up to 105 and laser-level optical signal is proposed. The device was engineered through an adroit design of integrating a random laser (RL) into the conducting bridge random access memory (CBRAM). According to the electrochemical metallization (ECM) effect of CBRAM, agglomerative silver nanoparticles form in the insulating layer during the ON/OFF switching process, which can serve as scattering centers. By adding CdSe/ZnS quantum dots (QDs) as the gain medium, a random laser system is obtained. Due to the quantum confinement effect, the device also features spectral tunable signal feedback by modulating the size of the QDs. In this study, devices with two different sizes of QDs are demonstrated such that a multiple-bit AND gate logic can be achieved. The innovation behind this RL-ECM memory might facilitate a key step toward the development of ultrahigh-speed information technology

Self-Powered, Self-Healed, and Shape-Adaptive Ultraviolet Photodetectors

The emergence of self-healing devices in recent years has drawn a great amount of attention in both academics and industry. Self-healed devices can autonomically restore a rupture as unexpected destruction occurs, which can efficiently prolong the life span of the devices; hence, they have an enhanced durability and decreased replacement cost. As a result, integration of wearable devices with self-healed electronics has become an indispensable issue in smart wearable devices. In this study, we present the first self-powered, self-healed, and wearable ultraviolet (UV) photodetector based on the integration of agarose/poly­(vinyl alcohol) (PVA) double network (DN) hydrogels, which have the advantages of good mechanical strength, self-healing ability, and tolerability of multiple types of damage. With the integration of a DN hydrogel substrate, the photodetector enables 90% of the initial efficiency to be restored after five healing cycles, and each rapid healing time is suppressed to only 10 s. The proposed device has several merits, including having an all spray coating, self-sustainability, biocompatibility, good sensitivity, mechanical flexibility, and an outstanding healing ability, which are all essential to build smart electronic systems. The unprecedented self-healed photodetector expands the future scope of electronic skin design, and it also offers a new platform for the development of next-generation wearable electronics

A Transferrable, Adaptable, Free-Standing, and Water-Resistant Hyperbolic Metamaterial

Hyperbolic metamaterials (HMMs) have attracted significant attention due to the profound manipulation of the photonic density of states, resulting in the efficient optoelectronic devices with the enhanced light–matter interaction. HMMs are conventionally built on rigid large-size substrates with poor conformability and the absence of flexibility. Here, we demonstrate a grating collageable HMM (GCHMM), which is composed of eight alternating layers of Au and poly­(methyl methacrylate) (PMMA) and PMMA grating nanostructure containing quantum dots (QDs). The QDs serve as a scattering gain medium performing a random laser action, and the grating nanostructure enhances the extraction of light from QDs. The GCHMM enhances laser action by 13 times, reduces lasing threshold by 46%, and increases differential quantum efficiency by 1.8 times as compared to a planar collageable HMM. In addition, the GCHMM can be retransferred multiple times to other substrates as well as provide sufficient protection in water and still retain an excellent performance. It also shows stable functionality even when transferred to a dental floss. The GCHMM, therefore, promises to become a versatile platform for foldable, adaptable, free-standing, and water-resistant optoelectronic device applications

Low-Threshold Lasing from 2D Homologous Organic–Inorganic Hybrid Ruddlesden–Popper Perovskite Single Crystals

Organic–inorganic hybrid two-dimensional (2D) perovskites have recently attracted great attention in optical and optoelectronic applications due to their inherent natural quantum-well structure. We report the growth of high-quality millimeter-sized single crystals belonging to homologous two-dimensional (2D) hybrid organic–inorganic Ruddelsden–Popper perovskites (RPPs) of (BA)₂(MA)_{<i><i>n</i></i>−1}Pb_{<i><i>n</i></i>}I_{3<i><i>n</i></i>+1} (<i>n</i> = 1, 2, and 3) by a slow evaporation at a constant-temperature (SECT) solution-growth strategy. The as-grown 2D hybrid perovskite single crystals exhibit excellent crystallinity, phase purity, and spectral uniformity. Low-threshold lasing behaviors with different emission wavelengths at room temperature have been observed from the homologous 2D hybrid RPP single crystals. Our result demonstrates that solution-growth homologous organic–inorganic hybrid 2D perovskite single crystals open up a new window as a promising candidate for optical gain media

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

Generation of Silver Metal Nanocluster Random Lasing

Atomically precise molecular-like metal nanoclusters (MNCs) exhibit unique properties, such as strong photoluminescence and absorption with inherent biocompatibility, which enable us to extend their applications to chemical sensing, biomedical imaging, optoelectronics, and many other areas. However, stimulated laser emission is greatly desirable to upgrade their more advanced functionalities. Here we provide a plausible approach to achieve this outstanding characteristic from MNCs. Quite interestingly, by integrating hyperbolic metamaterials (HMMs) with highly luminescent silver metal nanoclusters (Ag-TSA MNCs), a strong stimulated emission (random lasing action) with a low threshold of ∼0.5 kW cm–2 is discovered. The light emission is enhanced by ∼35 times when the solid-state assembly of Ag-TSA MNCs is integrated with HMM in comparison with that with a silicon substrate. The high-k modes excited by the HMM offer the possibility of forming the coherent closed feedback loops necessary for random lasing actions, thereby decreasing the energy loss associated with the photons’ propagation in the matrix. The simulations derived from the finite-difference time-domain method support the experimental results. Our study shown here makes an initial step to demonstrate stimulated laser action from metal nanoclusters. It is believed that there exist many other alternatives for exploring this emerging research topic for the future development of cost-effective and biocompatible optoelectronic devices

Highly Efficient Photodetection in Metal Nanocluster/Graphene Heterojunctions

The molecule-like metal nanoclusters gained wide attention from biomedical to energy applications in recent years owing to their discrete spectra. These atomically precise metal nanoclusters exhibit a significant band opening and consequently the possibility for strong light emission. Based upon previous reports on conventional semiconductors, the semiconducting nature of these nanoclusters combined with two-dimensional semimetals can have a huge impact on optoelectronic devices. The present work demonstrates that a hybrid structure of glutathione stabilized gold nanoclusters (GSH-Au NCs) with monolayer graphene can serve as a highly sensitive photodetector. The underlying mechanism can be well understood by the fact that the photoexcited carriers in GSH-Au NCs enable them to effectively transfer into the highly conductive graphene transporting layer. Under 325 nm laser illumination, a photoresponsivity of 7 A W−1 estimated in GSH-Au NCs photodetector has been enhanced to 2 × 105 A W−1 through the hybrid GSH-Au NCs/graphene photodetector, which is the highest value among metal nanoclusters based devices. Additionally, the compatibility of metal nanocluster film on flexible substrates has been demonstrated. The GSH-Au NCs exhibit a stable photoresponse under the application of systematic mechanical strain, manifesting their excellent mechanical stability. Thus, our work establishes the outstanding photodetecting property of gold nanocluster thin films, which holds a promising potential for future development of cost-effective and solution-processed optoelectronic devices, including emerging wearable technology