Electrical Rectifying and Photosensing Property of Schottky Diode Based on MoS2

Moisture Power Generation: From Material Selection to Device Structure Optimization

Moisture power generation (MPG) technology, producing clean and sustainable energy from a humid environment, has drawn significant attention and research efforts in recent years as a means of easing the energy crisis. Despite the rapid progress, MPG technology still faces numerous challenges with the most significant one being the low power-generating performance of individual MPG devices. In this review, we introduce the background and underlying principles of MPG technology while thoroughly explaining how the selection of suitable materials (carbons, polymers, inorganic salts, etc.) and the optimization of the device structure (pore structure, moisture gradient structure, functional group gradient structure, and electrode structure) can address the existing and anticipated challenges. Furthermore, this review highlights the major scientific and engineering hurdles on the way to advancing MPG technology and offers potential insights for the development of high-performance MPG systems.

GaN Nano Air Channel Diodes: Enabling High Rectification Ratio and Neutron Robust Radiation Operation

Nano air channel transistors (NACTs) provide numerous advantages over traditional silicon devices, including faster switching speeds, higher operating frequencies, and enhanced radiation hardness attributable to the ballistic transport of electrons. In the development of field-emission-based integrated circuits, low-power consumption rectifying nano air channel diodes (NACDs) play a crucial role. However, achieving rectification characteristics in NACDs is challenging due to their structural and material symmetry. This paper proposes a vertical GaN NACD with a consistent nano air channel fabricated using IC-compatible processes. The GaN NACD exhibits an exceptionally low turn-on voltage of 0.3 V while delivering a high output current of 5.02 mA at 3 V. Notably, it demonstrates a high rectification ratio of up to 2.2 × 105, attributing to significant work function disparities within the GaN-Au structure, coupled with the reduction of Au surface roughness to minimize reverse current. Furthermore, the junction-free structure and superior material properties of GaN enable the NACD to be suitable for use in radiation-rich environments. With its potential as a fundamental component of ultrafast and ultrahigh-frequency integrated circuits, this intriguing and cost-effective rectifying diode is anticipated to garner widespread interest within the electronics community.

Van der Waals Schottky Junction Photodetector with Ultrahigh Rectifying Ratio and Switchable Photocurrent Generation

Metal-semiconductor junctions play an important role in the development of electronic and optoelectronic devices. A Schottky junction photodetector based on two-dimensional (2D) materials is promising for self-powered photodetection with fast response speed and large signal-to-noise ratio. However, it usually suffers from an uncontrolled Schottky barrier due to the Fermi level pinning effect arising from the interface states. In this work, all-2D Schottky junctions with near-ideal Fermi level depinning are realized, attributed to the high-quality interface between 2D semimetals and semiconductors. We further demonstrate asymmetric diodes based on multilayer graphene/MoS2/PtSe2 with a current rectification ratio exceeding 105 and an ideality factor of 1.2. Scanning photocurrent mapping shows that the photocurrent generation mechanism in the heterostructure switches from photovoltaic effect to photogating effect at varying drain biases, indicating both energy conversion and optical sensing are realized in a single device. In the photovoltaic mode, the photodetector is self-powered with a response time smaller than 100 μs under the illumination of a 405 nm laser. In the photogating mode, the photodetector exhibits a high responsivity up to 460 A/W originating from a high photogain. Finally, the photodetector is employed for single-pixel imaging, demonstrating its high-contrast photodetection ability. This work provides insight into the development of high-performance self-powered photodetectors based on 2D Schottky junctions.

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.

Fully rubbery Schottky diode and integrated devices

A fully rubbery stretchable diode, particularly entirely based on stretchy materials, is a crucial device for stretchable integrated electronics in a wide range of applications, ranging from energy to biomedical, to integrated circuits, and to robotics. However, its development has been very nascent. Here, we report a fully rubbery Schottky diode constructed all based on stretchable electronic materials, including a liquid metal cathode, a rubbery semiconductor, and a stretchable anode. The rubbery Schottky diode exhibited a forward current density of 6.99 × 10-3 A/cm2 at 5 V and a rectification ratio of 8.37 × 104 at ±5 V. Stretchy rectifiers and logic gates based on the rubbery Schottky diodes were developed and could retain their electrical performance even under 30% tensile stretching. With the rubbery diodes, fully rubbery integrated electronics, including an active matrix multiplexed tactile sensor and a triboelectric nanogenerator-based power management system, are further demonstrated.

Large-Area, High-Specific-Power Schottky-Junction Photovoltaics from CVD-Grown Monolayer MoS2

The deployment of two-dimensional (2D) materials for solar energy conversion requires scalable large-area devices. Here, we present the design, modeling, fabrication, and characterization of monolayer MoS₂-based lateral Schottky-junction photovoltaic (PV) devices grown by using chemical vapor deposition (CVD). The device design consists of asymmetric Ti and Pt metal contacts with a work function offset to enable charge separation. These early stage devices show repeatable performance under 1 sun illumination, with V_OC of 160 mV, J_SC of 0.01 mA/cm², power conversion efficiency of 0.0005%, and specific power of 1.58 kW/kg. An optoelectronic model for this device is developed and validated with experimental results. This model is used to understand loss mechanisms and project optimized device designs. The model predicts that a 2D PV device with ∼70 kW/kg of specific power can be achieved with minimum optimization to the current devices. By increasing the thickness of the absorber layer, we can achieve even higher performance devices. Finally, a 25 mm² area solar cell made with a 0.65 nm thick MoS₂ monolayer is demonstrated, showing V_OC of 210 mV under 1 sun illumination. This is the first demonstration of a large-area PV device made with CVD-grown scalable 2D materials.

Asymmetric Nanofractures Determined the Nonreciprocal Peeling for Self-Aligned Heterostructure Nanogaps and Devices

Planar heterostructures composed of two or more adjacent structures with different materials are a kind of building blocks for various applications in surface plasmon resonance sensors, rectifiers, photovoltaic devices, and ambipolar devices, but their reliable fabrication with controllable shape, size, and positioning accuracy remains challenging. In this work, we propose a concept for fabricating planar heterostructures via directional stripping and controlled nanofractures of metallic films, with which self-aligned, multimaterial, multiscale heterostructures with arbitrary geometries and sub-20 nm gaps can be obtained. By using a split ring as the template, the asymmetric nanofracture of the deposited film at the split position results in nonreciprocal peeling of the film in the split ring. Compared to the conventional processes, the final heterostructures are defined only by their outlines, thus providing the ability to fabricate complex heterostructures with higher resolutions. We demonstrate that this method can be used to fabricate heterodimers, multimaterial oligomers, and multiscale asymmetrical electrodes. An Ag-MoS₂-Au photodiode with a strong rectification effect is fabricated based on the nanogap heterostructures prepared by this method. This technology provides a unique and reliable approach to define nanogap heterostructures, which are supposed to have potential applications in nanoelectronics, nanoplasmonics, nano-optoelectronics, and electrochemistry.

High performance sub-bandgap photodetection via internal photoemission based on ideal metal/2D-material van der Waals Schottky interface

Two-dimensional (2D) materials have been demonstrated to be promising candidates to design high performance photodetectors owing to their strong light-matter interaction. However, the performance of 2D material photodetectors is still unsatisfactory, such as slow response speed due to defects and vulnerable contact interface, which impede their rapid development in the field of optoelectronics. In this paper, we obtained the ideal and large photosensitive van der Waals Schottky interface by the laminating-flipping method. Hence, a fast response speed (<1 ms) and high detectivity (>10¹² Jones) are observed on the van der Waals Schottky junction photodiode. More importantly, benefiting from the flat Schottky interface (the roughness ∼0.6 nm), a sub-bandgap light response modulated by the Schottky barrier height (cut-off edge at 1050 nm) has been detected based on the large Au/MoSe₂ sensitive Schottky interface internal photoemission. As a result, a universal strategy for the sub-bandgap near-infrared van der Waals Schottky junction detector of 2D materials was obtained.

ReSe2/metal interface for hydrogen gas sensing

The Fermi level alignment between electrodes and two-dimensional (2D) materials is significant in characterizing sensors based on their reversibility, response time, sensitivity, and long-term stability. Here, we have demonstrated that the modulation of the Schottky barrier height between the interface of metal (Pd/Au) and multilayered ReSe₂ nanoflakes caused the change in the transfer curve (I_ds-V_bg) of FETs based devices and rectifying characteristics (I_ds-V_ds) of the Schottky diodes at various hydrogen concentrations at T = 22 °C, fluctuating from 50 to 350 ppm with a response (R%) from 669 to 1198%, respectively. Sensors based on a mono- or bilayer system did not exhibit sensitivity to hydrogen gas owing to metal electrodes diffused into materials. The value of the ideality factor of the Schottky diode-based sensor changed from 4 to 1.6 as the hydrogen concentration was changed from 50 to 900 ppm, while the relative response increased from 0 to 3.5 as the hydrogen concentration was increased from 0 to 900 ppm. This research can offer a real solution for developing cost-effective, faster, and room temperature sensors based on 2D materials.

Effective film surface treatment for improving external quantum efficiency of photomultiplication type organic photodetector

A simple yet effective method based on methanol treatment is proposed to enhance the external quantum efficiency (EQE) of the photomultiplication type organic photodetector with a structure of Glass/ITO/PEDOT:PSS/P3H:PC71BM (100:1, wt./wt.)/Al. By modifying the PEDOT:PSS film surface with methanol, the EQE of photodetector significantly improved within a broad wavelength range of 300–700 nm. The maximum EQE of 25300% occurs at the wavelength of 350 nm in the methanol-treated device under −9 V bias, which more than doubles that (11500%) of the device without treatment. In addition, as a result of the methanol treatment, the detectivity of the device improved from 3.72 × 1012 to 7.24 × 1012 Jones at −9 V under 350 nm light illumination. The large improvement is attributed to the fact that the methanol treatment can improve the electrical performance of the PEDOT:PSS by removing the insulator PSS within the film and also result in PC71BM aggregations in the active layer. The latter can enhance the tunneling hole injection by the intensified energy-level bending, which is induced by both the trapped electrons in these aggregations and accumulated ones near Al electrode. As a result, the modification of both the PEDOT:PSS layer and the active layer increases the response current, resulting in the EQE improvement.

Asymmetric Schottky Contacts in van der Waals Metal-Semiconductor-Metal Structures Based on Two-Dimensional Janus Materials

Physical and electronic asymmetry plays a crucial role in rectifiers and other devices with a directionally variant current-voltage (I-V) ratio. Several strategies for practically creating asymmetry in nanoscale components have been demonstrated, but complex fabrication procedures, high cost, and incomplete mechanistic understanding have significantly limited large-scale applications of these components. In this work, we present density functional theory calculations which demonstrate asymmetric electronic properties in a metal-semiconductor-metal (MSM) interface composed of stacked van der Waals (vdW) heterostructures. Janus MoSSe has an intrinsic dipole due to its asymmetric structure and, consequently, can act as either an n-type or p-type diode depending on the face at the interior of the stacked structure (SeMoS-SMoS vs. SMoSe-SMoS). In each configuration, vdW forces dominate the interfacial interactions, and thus, Fermi level pinning is largely suppressed. Our transport calculations show that not only does the intrinsic dipole cause asymmetric I-V characteristics in the MSM structure but also that different transmission mechanisms are involved across the S-S (direct tunneling) and S-Se interface (thermionic excitation). This work illustrates a simple and practical method to introduce asymmetric Schottky barriers into an MSM structure and provides a conceptual framework which can be extended to other 2D Janus semiconductors.

Au-InSe van der Waals Schottky junctions with ultralow reverse current and high photosensitivity

The Schottky junction, composed of a rectifying metal-semiconductor interface, is an essential component for microelectronic and optoelectronic devices. However, due to the considerable reverse tunneling current, typical Schottky junctions cannot be widely applied in devices requiring high signal-to-noise ratios, such as photodetectors with high detectivity. Here, a van der Waals (vdW) Schottky junction is constructed by mechanically stacking a gold (Au) electrode onto a multilayer indium selenide (InSe) nanosheet, which shows an ultralow reverse current in sub-picoamperes and an excellent rectification ratio exceeding 10⁶ at room temperature. The reverse current, which corresponds to the thermionic emission transport model, is independent of the applied reverse bias. As a result, the Au-InSe vdW Schottky junction device can function as an ultrasensitive photodetector with a photodetectivity over 2.4 × 10¹⁵ Jones, corresponding to a photoresponsivity of 853 A W^-1 and a light on/off ratio exceeding 1 × 10⁷. The work offers an idea for investigating electronic and optoelectronic devices with high signal-to-noise ratios based on vdW Schottky junctions.

Enhanced electrical and broad spectral (UV-Vis-NIR) photodetection in a Gr/ReSe2/Gr heterojunction

Excellent electrical and photoelectrical study of vertical integration by layered two-dimensional materials having gate tunable broad spectral (UV-Vis-NIR) light detection response.