Black Phosphorus-IGZO van der Waals Diode with Low-Resistivity Metal Contacts

Homogeneous in-plane WSe2 P-N junctions for advanced optoelectronic devices

Conventional doping schemes of silicon (Si) microelectronics are incompatible with atomically thick two-dimensional (2D) transition metal dichalcogenides (TMDCs), which makes it challenging to construct high-quality 2D homogeneous p-n junctions. Herein, we adopt a simple yet effective plasma-treated doping method to seamlessly construct a lateral 2D WSe₂ p-n homojunction. WSe₂ with ambipolar transport properties was exposed to O₂ plasma to form WO_x on the surface in a self-limiting process that induces hole doping in the underlying WSe₂via electron transfer. Different electrical behaviors were observed between the as-exfoliated (ambipolar) region and the O₂ plasma-treated (p-doped) region under electrostatic modulation of the back-gate bias (V_BG), which produces a p-n in-plane homojunction. More importantly, a small contact resistance of 710 Ω μm with a p-doped region transistor mobility of ∼157 cm² V^-1 s^-1 was achieved due to the transformation of Schottky contact into Ohmic contact after plasma treatment. This effectively avoids Fermi-level pinning and significantly improves the performance of photodetectors. The resultant WSe₂ p-n junction device thus exhibits a high photoresponsivity of ∼7.1 × 10⁴ mA W^-1 and a superior external quantum efficiency of ∼228%. Also, the physical mechanism of charge transfer in the WSe₂ p-n homojunction was analyzed. Our proposed strategy offers a powerful route to realize low contact resistance and high photoresponsivity in 2D TMDC-based optoelectronic devices, paving the way for next-generation atomic-thickness optoelectronics.

Low-Power Negative-Differential-Resistance Device for Sensing the Selective Protein via Supporter Molecule Engineering

Van der Waals (vdW) heterostructures composed of atomically thin two-dimensional (2D) materials have more potential than conventional metal-oxide semiconductors because of their tunable bandgaps, and sensitivities. The remarkable features of these amazing vdW heterostructures are leading to multi-functional logic devices, atomically thin photodetectors, and negative differential resistance (NDR) Esaki diodes. Here, an atomically thin vdW stacking composed of p-type black arsenic (b-As) and n-type tin disulfide (n-SnS2 ) to build a type-III (broken gap) heterojunction is introduced, leading to a negative differential resistance device. Charge transport through the NDR device is investigated under electrostatic gating to achieve a high peak-to-valley current ratio (PVCR), which improved from 2.8 to 4.6 when the temperature is lowered from 300 to 100 K. At various applied-biasing voltages, all conceivable tunneling mechanisms that regulate charge transport are elucidated. Furthermore, the real-time response of the NDR device is investigated at various streptavidin concentrations down to 1 pm, operating at a low biasing voltage. Such applications of NDR devices may lead to the development of cutting-edge electrical devices operating at low power that may be employed as biosensors to detect a variety of target DNA (e.g., ct-DNA) and protein (e.g., the spike protein associated with COVID-19).

Self-biased wavelength selective photodetection in an n-IGZO/p-GeSe heterostructure by polarity flipping

Transparent semiconductor oxides with two-dimensional (2D) heterostructures have been extensively studied as new materials for thin-film transistors and photosensors due to their remarkable photovoltaic characteristics, making them useful for newly developed optoelectronics. Here we demonstrate the fabrication and characterization of an ITO/n-IGZO/p-GeSe transparent selective wavelength photodetector. The wavelength-dependent photovoltaic behavior of the n-IGZO/p-GeSe heterostructure under UV-Visible laser light shifts the I-V curves down with positive V_oc and negative I_sc values of about 0.12 V and -49 nA and 0.09 V and -17 nA, respectively. Interestingly, when an NIR laser irradiated the device, the I-V curves shifted up with negative V_oc and positive I_sc values of about -0.11 V and 45 nA, respectively. This behavior is attributed to the free carrier concentration induced by photogenerated carriers across the device at different points that varied with the wavelength-dependent photon absorption. Consequently, the direction of the electric field polarity across the junction can be flipped. This study demonstrates a zero-bias near-infrared (NIR) photodetector with a high photoresponsivity of 538.9 mA W^-1, a fast rise time of 25.2 ms, and a decay time of 25.08 ms. Furthermore, we observed a detectivity (D) of 8.4 × 10⁹ Jones, a normalized photocurrent to dark current ratio (NPDR) of 2.8 × 10¹⁰ W^-1, and a noise equivalent power (NEP) of 2.2 × 10^-14 W Hz^-1/2. Our strategy opens alternative possibilities for scalable, low-cost, multifunctional transparent near-infrared photosensors with selective wavelength photodetection.

Reconfigurable carrier type and photodetection of MoTe2 of various thicknesses by deep ultraviolet light illumination

Tuning of the Fermi level in transition metal dichalcogenides (TMDCs) leads to devices with excellent electrical and optical properties. In this study, we controlled the Fermi level of MoTe₂ by deep ultraviolet (DUV) light illumination in different gaseous environments. Specifically, we investigated the reconfigurable carrier type of an intrinsic p-MoTe₂ flake that gradually transformed into n-MoTe₂ after illumination with DUV light for 30, 60, 90, 120, 160, 250, 500, 900, and 1200 s in a nitrogen (N₂) gas environment. Subsequently, we illuminated this n-MoTe₂ sample with DUV light in oxygen (O₂) gas and reversed its carrier polarity toward p-MoTe₂. However, using this doping scheme to reveal the effect of DUV light on various layers (3-30 nm) of MoTe₂ is challenging. The DUV + N₂ treatment significantly altered the polarity of MoTe₂ of different thicknesses from p-type to n-type under the DUV + N₂ treatment, but the DUV + O₂ treatment did not completely alter the polarity of thicker n-MoTe₂ flakes to p-type. In addition, we investigated the photoresponse of MoTe₂ after DUV light treatment in N₂ and O₂ gas environments. From the time-resolved photoresponsivity at different polarity states of MoTe₂, we have shown that the response time of the DUV + O₂ treated p-MoTe₂ is faster than that of the pristine and doped n-MoTe₂ films. These carrier polarity modulations and photoresponse paves the way for wider applications of MoTe₂ in optoelectronic devices.

Precise and Prompt Analyte Detection via Ordered Orientation of Receptor in WSe2-Based Field Effect Transistor

Field-effect transistors (FET) composed of transition metal dichalcogenide (TMDC) materials have gained huge importance as biosensors due to their added advantage of high sensitivity and moderate bandgap. However, the true potential of these biosensors highly depends upon the quality of TMDC material, as well as the orientation of receptors on their surfaces. The uncontrolled orientation of receptors and screening issues due to crossing the Debye screening length while functionalizing TMDC materials is a big challenge in this field. To address these issues, we introduce a combination of high-quality monolayer WSe₂ with our designed Pyrene-based receptor moiety for its ordered orientation onto the WSe₂ FET biosensor. A monolayer WSe₂ sheet is utilized to fabricate an ideal FET for biosensing applications, which is characterized via Raman spectroscopy, atomic force microscopy, and electrical prob station. Our construct can sensitively detect our target protein (streptavidin) with 1 pM limit of detection within a short span of 2 min, through a one-step functionalizing process. In addition to having this ultra-fast response and high sensitivity, our biosensor can be a reliable platform for point-of-care-based diagnosis.

Flexible Memory Device Composed of Metal-Oxide and Two-Dimensional Material (SnO2/WTe2) Exhibiting Stable Resistive Switching

Two-terminal, non-volatile memory devices are the fundamental building blocks of memory-storage devices to store the required information, but their lack of flexibility limits their potential for biological applications. After the discovery of two-dimensional (2D) materials, flexible memory devices are easy to build, because of their flexible nature. Here, we report on our flexible resistive-switching devices, composed of a bilayer tin-oxide/tungsten-ditelluride (SnO2/WTe2) heterostructure sandwiched between Ag (top) and Au (bottom) metal electrodes over a flexible PET substrate. The Ag/SnO2/WTe2/Au flexible devices exhibited highly stable resistive switching along with an excellent retention time. Triggering the device from a high-resistance state (HRS) to a low-resistance state (LRS) is attributed to Ag filament formation because of its diffusion. The conductive filament begins its development from the anode to the cathode, contrary to the formal electrochemical metallization theory. The bilayer structure of SnO2/WTe2 improved the endurance of the devices and reduced the switching voltage by up to 0.2 V compared to the single SnO2 stacked devices. These flexible and low-power-consumption features may lead to the construction of a wearable memory device for data-storage purposes.

A review on two-dimensional (2D) perovskite material-based solar cells to enhance the power conversion efficiency

With perovskite materials, rapid progress in power conversion efficiency (PCE) to reach 25% has gained a significant amount of attention from the solar cell industry. Since the development of solid-state perovskite solar cells, rapid research development and investigation on structure design, device fabrication and fundamental studies have contributed to solid-state perovskite solar cells to be a strong candidate for next-generation solar energy. The promising efficiency with low-cost materials is the key point over the other material-based solar cells. The power conversion efficiency (PCE) of two-dimensional (2D) perovskite materials is yet to be enhanced in order to contest with the 3D perovskite-based solar cells. Their enormous variety compromises better prospects and possibilities for research. Two-dimensional (2D) perovskites play a multi-functional role within a solar cell, such as a capping layer, passivating layer, prime cell absorber, and in a hybrid 3D/2D perovskite-based solar cell absorber. This review summarizes the evolution of solar cells that are based on 2D perovskites and their prominent character in solar cells, along with the significant trends. The fundamental configuration and the optoelectronic characteristics, including the band orientation and the transportation of the charges, are discussed in detail. The 2D perovskites are analyzed to study the confined charges within the inorganic structure due to the dielectric and quantum confinement influence. Furthermore, the importance of cesium cation (Cs⁺) doped with 2D substance (BA)₂(MA₃) PbI₃ approach has been discussed to attain high power conversion efficiency (PCE). These attributes offer an efficient step towards air-stable and small-sized perovskites as a new group of renewable energy sources.

Physical Processes during the Formation of Silicon-Lithium p-i-n Structures Using Double-Sided Diffusion and Drift Methods

In this paper, we described a method of double-sided diffusion and drift of lithium-ions into monocrystalline silicon for the formation of the large-sized, p-i-n structured Si(Li) radiation detectors. The p-i-n structure is a p-n junction with a doped region, where the “i-region” is between the n and the p layers. A well-defined i-region is usually associated with p or n layers with high resistivities. The p-i-n structure is mostly used in diodes and in some types of semiconductor radiation detectors. The uniqueness of this method is that, in this method, the processes of diffusion and drift of lithium-ions, which are the main processes in the formation of Si(Li) p-i-n structures, are produced from both flat sides of cylindrical-shaped monocrystalline silicon, at optimal temperature (T = 420 °C) conditions of diffusion, and subsequently, with synchronous supply of temperature (from 55 to 100 °C) and reverse bias voltage (from 70 to 300 V) during drift of lithium-ions into silicon. Thus, shortening the manufacturing time of the detector and providing a more uniform distribution of lithium-ions in the crystal volume. Since, at present, the development of manufacturing of large-sized Si(Li) detectors is hindered due to difficulties in obtaining a uniformly compensated large area and time-consuming manufacturing process, the proposed method may open up new possibilities in detector manufacturing.

Thermal Conductivity of Nano-Crystallized Indium-Gallium-Zinc Oxide Thin Films Determined by Differential Three-Omega Method

The temperature dependence thermal conductivity of the indium-gallium-zinc oxide (IGZO) thin films was investigated with the differential three-omega method for the clear demonstration of nanocrystallinity. The thin films were deposited on an alumina (α-Al₂O₃) substrate by direct current (DC) magnetron sputtering at different oxygen partial pressures ([PO₂] = 0%, 10%, and 65%). Their thermal conductivities at room temperature were measured to be 1.65, 1.76, and 2.58 Wm⁻¹K⁻¹, respectively. The thermal conductivities decreased with an increase in the ambient measurement temperature. This thermal property is similar to that of crystalline materials. Electron microscopy observations revealed the presence of nanocrystals embedded in the amorphous matrix of the IGZO films. The typical size of the nanocrystals was approximately 2–5 nm with the lattice distance of about 0.24–0.26 nm. These experimental results indicate that the nanocrystalline microstructure controls the heat conduction in the IGZO films.

High performance IGZO-based phototransistors by BN/BP interface engineering

Some advances have been achieved in developing heterojunctions consisting of indium-gallium-zinc oxide (a-IGZO) films and two dimensional (2D) van der Waals materials for optoelectronic applications in recent years, however, the improvement of IGZO channel itself via constructing such heterojunctions is rarely reported. Here, we report the huge improvement in photoresponse performances for the IGZO phototransistor devices by introducing boron nitride (BN)/black phosphorus (BP) interface engineering. By creating an appropriate band bending and an efficient photo-generated carrier transfer path between IGZO and BP, the recombination of the photo-generated carriers in the IGZO channel is significantly suppressed. As a result, the corresponding photoresponsivity at a wavelength of 447 nm can be promoted from 0.05 A W^-1 to 0.3 A W^-1. A corresponding maximum external quantum efficiency of 83.4% was obtained for the BN/BP decorated IGZO phototransistor. The results imply that such interface engineering via 2D materials can be used as a general route to high performance oxide-semiconductor based optoelectronic devices.

Novel NiCl2 Nanosheets Synthesized via Chemical Vapor Deposition with High Specific Energy for Thermal Battery

Two-dimensional (2D) nanomaterials possessing a unique sheet structure, compared to correlative bulk materials, exhibit excellent properties, especially in the energy storage and energy conversion field. In this case, NiCl₂ nanosheets with thicknesses of 2-8 nm are first prepared by a simple chemical vapor deposition method. For the Li-B/LiF-LiCl-LiBr/NiCl₂ thermal battery, the specific energy of NiCl₂ nanosheets increases from 510 W h kg^-1 (NiCl₂ rods) to 616 W h kg^-1 at an operation temperature of 500 °C and a current density of 0.2 A cm^-2. The 2D morphology and large numbers of defects not only improve the redox reaction rates and the lithium storage capacity, but also enhance the adsorption capacity with the flake-like binder MgO, which prolong the discharge time by suppressing the discharge product diffusion to the electrolyte. These results indicate that NiCl₂ nanosheets have a great possibility to become a desirable candidate of cathode materials for assisting in the development of high energy output and provide a new way to restrain the immersion between the electrode and electrolyte.