Pseudocapacitance-Induced Synaptic Plasticity of Tribo-Phototronic Effect Between Ionic Liquid and 2D MoS2.

Contact-induced electrification, commonly referred to as triboelectrification, is the subject of extensive investigation at liquid-solid interfaces due to its wide range of applications in electrochemistry, energy harvesting, and sensors. This study examines the triboelectric between an ionic liquid and 2D MoS2 under light illumination. Notably, when a liquid droplet slides across the MoS2 surface, an increase in the generated current and voltage is observed in the forward direction, while a decrease is observed in the reverse direction. This suggests a memory-like tribo-phototronic effect between ionic liquid and 2D MoS2 . The underlying mechanism behind this tribo-phototronic synaptic plasticity is proposed to be ion adsorption/desorption processes resulting from pseudocapacitive deionization/ionization at the liquid-MoS2  interface. This explanation is supported by the equivalent electrical circuit modeling, contact angle measurements, and electronic band diagrams. Furthermore, the influence of various factors such as velocity, step size, light wavelength and intensity, ion concentration, and bias voltage is thoroughly investigated. The artificial synaptic plasticity arising from this phenomenon exhibits significant synaptic features, including potentiation/inhibition, paired-pulse facilitation/depression, and short-term memory (STM) to long-term memory (LTM) transition. This research opens up promising avenues for the development of synaptic memory systems and intelligent sensing applications based on liquid-solid interfaces.

Enhanced Sensitivity in Photovoltaic 2D MoS2/Te Heterojunction VOC Sensors.

Volatile organic compound (VOC) sensors have a broad range of applications including healthcare monitoring, product quality control, and air quality management. However, many such applications are demanding, requiring sensors with high sensitivity and selectivity. 2D materials are extensively used in many VOC sensing devices due to their large surface-to-volume ratio and fascinating electronic properties. These properties, along with their exceptional flexibility, low power consumption, room-temperature operation, chemical functionalization potential, and defect engineering capabilities, make 2D materials ideal for high-performance VOC sensing. Here, a 2D MoS2/Te heterojunction is reported that significantly improves the VOC detection compared to MoS2 and Te sensors on their own. Density functional theory (DFT) analysis shows that the MoS2/Te heterojunction significantly enhances the adsorption energy and therefore sensing sensitivity of the sensor. The sensor response, which denotes the percentage change in the sensor's conductance upon VOC exposure, is further enhanced under photo-illumination and zero-bias conditions to values up to ≈7000% when exposed to butanone. The MoS2/Te heterojunction is therefore a promising device architecture for portable and wearable sensing applications.

Multilayer WSe2/ZnO heterojunctions for self-powered, broadband, and high-speed photodetectors.

Self-powered broadband photodetectors have attracted great interest due to their applications in biomedical imaging, integrated circuits, wireless communication systems, and optical switches. Recently, significant research is being carried out to develop high-performance self-powered photodetectors based on thin 2D materials and their heterostructures due to their unique optoelectronic properties. Herein, a vertical heterostructure based on p-type 2D WSe2and n-type thin film ZnO is realized for photodetectors with a broadband response in the wavelength range of 300-850 nm. Due to the formation of a built-in electric field at the WSe2/ZnO interface and the photovoltaic effect, this structure exhibits a rectifying behavior with a maximum photoresponsivity and detectivity of ∼131 mA W-1and ∼3.92 × 1010Jones, respectively, under an incident light wavelength ofλ= 300 nm at zero voltage bias. It also shows a 3-dB cut-off frequency of ∼300 Hz along with a fast response time of ∼496µs, making it suitable for high-speed self-powered optoelectronic applications. Furthermore, the facilitation of charge collection under reverse voltage bias results in a photoresponsivity as high as ∼7160 mA W-1and a large detectivity of ∼1.18 × 1011Jones at a bias voltage of -5 V. Hence, the p-WSe2/n-ZnO heterojunction is proposed as an excellent candidate for high-performance, self-powered, and broadband photodetectors.

Trends in energy and charge transfer in 2D and integrated perovskite heterostructures.

Two-dimensional (2D) van der Waals (vdW) heterostructured transition metal dichalcogenides (TMDs) open up new possibilities for a wide range of optoelectronic applications. Interlayer couplings are responsible for several fascinating physics phenomena, which are in addition to the multifunctionalities that have been discovered in the field of optoelectronics. These couplings can influence the overall charge, or the energy transfer processes via stacking, separation, and dielectric angles. This focused review article summarizes the most recent and promising strategies for interlayer exciton emission in 2D or integrated perovskites and TMD heterostructures. These types of devices require a thorough comprehension and effective control of interlayer couplings in order to realize their functionalities and improve performance, which is demonstrated in this article with the energy or charge transfer mechanisms in the individual devices. An ideal platform for examining the interlayer coupling and the related physical processes is provided by a summary of the recent research findings in 2D perovskites and TMDs. Furthermore, it would encourage more investigation into the comprehension and regulation of excitonic effects and the related optoelectronic applications in vdW heterostructures over a broad spectral response range. Finally, the current challenges and prospects are summarized in this paper.

Bulk Photovoltaic Effect in Two-Dimensional Distorted MoTe2.

In future solar cell technologies, the thermodynamic Shockley-Queisser limit for solar-to-current conversion in traditional p-n junctions could potentially be overcome with a bulk photovoltaic effect by creating an inversion broken symmetry in piezoelectric or ferroelectric materials. Here, we unveiled mechanical distortion-induced bulk photovoltaic behavior in a two-dimensional (2D) material, MoTe2, caused by the phase transition and broken inversion symmetry in MoTe2. The phase transition from single-crystalline semiconducting 2H-MoTe2 to semimetallic 1T'-MoTe2 was confirmed using X-ray photoelectron spectroscopy (XPS). We used a micrometer-scale system to measure the absorption of energy, which reduced from 800 to 63 meV during phase transformation from hexagonal to distorted octahedral and revealed a smaller bandgap semimetallic behavior. Experimentally, a large bulk photovoltaic response is anticipated with the maximum photovoltage VOC = 16 mV and a positive signal of the ISC = 60 µA (400 nm, 90.4 Wcm-2) in the absence of an external electric field. The maximum values of both R and EQE were found to be 98 mAW-1 and 30%, respectively. Our findings are distinctive features of the photocurrent responses caused by in-plane polarity and its potential from a wide pool of established TMD-based nanomaterials and a cutting-edge approach to optimize the efficiency in converting photons-to-electricity for power harvesting optoelectronics devices.

MXene-modified electrodes and electrolytes in dye-sensitized solar cells.

Dye-sensitized solar cells (DSSCs) have attracted much attention as promising tools in renewable energy conversion technology. This is mainly because of their beneficial qualities, such as their impressive efficiency levels and low-cost fabrication techniques. An overview of MXene-modified electrodes in DSSCs is given in this review article. MXenes are two-dimensional (2D) transition metal carbides or nitrides with remarkable properties such as high conductivity and large surface area. MXenes' properties make them an appealing material for various applications, including energy storage, catalysis, and electronic devices. MXene integration enhances ion transport, dye adsorption, and charge transport in DSSC electrodes. In-depth analysis of the use of 2D Mxene and integration with carbon nanotubes (CNTs), reduced graphene oxide (rGO), 2D MoS2, and hybrids like 2D-2D heterostructures for electrode modification in photovoltaics (PVs), including anodes, photoanodes, composite decorated electrodes, counter electrodes (CEs), and electrolytes, is provided in this review article. The effects on the performance metrics of various deposition techniques are discussed and assessed. The use of MXene-modified electrodes in DSSCs suggests potential for enhancing the performance and efficiency of these solar cells in general. The article examines this strategy's potential advantages and implications, illuminating the fascinating advancements in the area and emphasizing MXenes' potential as a valuable substance for renewable energy applications. We also discuss the difficulties and potential benefits of using MXene-modified electrodes in DSSCs and emphasize the need for additional study to enhance stability, optimize MXene integration techniques, and enhance long-term device performance. The scalability and potential of MXene-based electrode modifications for commercial applications are also covered, addressing issues and prospects for the future, focusing on the necessity of more study. Electrodes modified with MXenes can improve DSSC performance and advance sustainable energy conversion.

Unique Photoactivated Time-Resolved Response in 2D GeS for Selective Detection of Volatile Organic Compounds.

Volatile organic compounds (VOCs) sensors have a broad range of applications including healthcare, process control, and air quality analysis. There are a variety of techniques for detecting VOCs such as optical, acoustic, electrochemical, and chemiresistive sensors. However, existing commercial VOC detectors have drawbacks such as high cost, large size, or lack of selectivity. Herein, a new sensing mechanism is demonstrated based on surface interactions between VOC and UV-excited 2D germanium sulfide (GeS), which provides an effective solution to distinguish VOCs. The GeS sensor shows a unique time-resolved electrical response to different VOC species, facilitating identification and qualitative measurement of VOCs. Moreover, machine learning is utilized to distinguish VOC species from their dynamic response via visualization with high accuracy. The proposed approach demonstrates the potential of 2D GeS as a promising candidate for selective miniature VOCs sensors in critical applications such as non-invasive diagnosis of diseases and health monitoring.

Self-Powered, Broadband Photodetector Based on Two-Dimensional Tellurium-Silicon Heterojunction.

As a new class of two-dimensional (2D) materials and a group-VI chalcogen, tellurium (Te) has emerged as a p-type semiconductor with high carrier mobility. Potential applications include high-speed opto-electronic devices for communication. One method to enhance the performance of 2D material-based photodetectors is by integration with a IV group of semiconductors such as silicon (Si). In this work, we demonstrate a self-powered, high-speed, broadband photodetector based on the 2D Te/n-type Si heterojunction. The fabricated Te/n-type Si heterojunction exhibits high performance in the UV-vis-NIR light with a high responsivity of up to ∼250 mA/W and a photocurrent-to-dark current ratio (I on/I off) of ∼106, fast response time of 8.6 µs, and superior repeatability and stability. The results show that the fabricated Te/n-type Si heterojunction photodetector has a strong potential to be utilized in ultrafast, broadband, and efficient photodetection applications.

Ultrasensitive rapid cytokine sensors based on asymmetric geometry two-dimensional MoS2 diodes.

The elevation of cytokine levels in body fluids has been associated with numerous health conditions. The detection of these cytokine biomarkers at low concentrations may help clinicians diagnose diseases at an early stage. Here, we report an asymmetric geometry MoS2 diode-based biosensor for rapid, label-free, highly sensitive, and specific detection of tumor necrosis factor-α (TNF-α), a proinflammatory cytokine. This sensor is functionalized with TNF-α binding aptamers to detect TNF-α at concentrations as low as 10 fM, well below the typical concentrations found in healthy blood. Interactions between aptamers and TNF-α at the sensor surface induce a change in surface energy that alters the current-voltage rectification behavior of the MoS2 diode, which can be read out using a two-electrode configuration. The key advantages of this diode sensor are the simple fabrication process and electrical readout, and therefore, the potential to be applied in a rapid and easy-to-use, point-of-care, diagnostic tool.