Encapsulation strategies on 2D materials for field effect transistors and photodetectors

Inert Liquid Exfoliation and Langmuir-Type Thin Film Deposition of Semimetallic Metal Diborides

Graphite is one of only a few layered materials that can be exfoliated into nanosheets with semimetallic properties, which limits the applications of nanosheet-based electrodes to material combinations compatible with the work function of graphene. It is therefore important to identify additional metallic or semimetallic two-dimensional (2D) nanomaterials that can be processed in solution for scalable fabrication of printed electronic devices. Metal diborides represent a family of layered non-van der Waals crystals with semimetallic properties for all nanosheet thicknesses. While previous reports show that the exfoliated nanomaterial is prone to oxidation, we demonstrate a readily accessible inert exfoliation process to produce quasi-2D nanoplatelets with intrinsic material properties. For this purpose, we demonstrate the exfoliation of three representative metal diborides (MgB2, CrB2, and ZrB2) under inert conditions. Nanomaterial is characterized using a combination of transmission electron microscopy, scanning electron microscopy, atomic force microscopy, IR, and UV-vis measurements, with only minimal oxidation indicated postprocessing. By depositing the pristine metal diboride nanoplatelets as thin films using a Langmuir-type deposition technique, the ohmic behavior of the networks is validated. Furthermore, the material decomposition is studied by using a combination of electrical and optical measurements after controlled exposure to ambient conditions. Finally, we report an efficient, low-cost approach for sample encapsulation to protect the nanomaterials from oxidation. This is used to demonstrate low-gauge factor strain sensors, confirming metal diboride nanosheets as a suitable alternative to graphene for electrode materials in printed electronics.

Performance Limits and Advancements in Single 2D Transition Metal Dichalcogenide Transistor

Two-dimensional (2D) transition metal dichalcogenides (TMDs) allow for atomic-scale manipulation, challenging the conventional limitations of semiconductor materials. This capability may overcome the short-channel effect, sparking significant advancements in electronic devices that utilize 2D TMDs. Exploring the dimension and performance limits of transistors based on 2D TMDs has gained substantial importance. This review provides a comprehensive investigation into these limits of the single 2D-TMD transistor. It delves into the impacts of miniaturization, including the reduction of channel length, gate length, source/drain contact length, and dielectric thickness on transistor operation and performance. In addition, this review provides a detailed analysis of performance parameters such as source/drain contact resistance, subthreshold swing, hysteresis loop, carrier mobility, on/off ratio, and the development of p-type and single logic transistors. This review details the two logical expressions of the single 2D-TMD logic transistor, including current and voltage. It also emphasizes the role of 2D TMD-based transistors as memory devices, focusing on enhancing memory operation speed, endurance, data retention, and extinction ratio, as well as reducing energy consumption in memory devices functioning as artificial synapses. This review demonstrates the two calculating methods for dynamic energy consumption of 2D synaptic devices. This review not only summarizes the current state of the art in this field but also highlights potential future research directions and applications. It underscores the anticipated challenges, opportunities, and potential solutions in navigating the dimension and performance boundaries of 2D transistors.

Two-Step Conversion of Metal and Metal Oxide Precursor Films to 2D Transition Metal Dichalcogenides and Heterostructures

The widely studied class of two-dimensional (2D) materials known as transition metal dichalcogenides (TMDs) are now well-poised to be employed in real-world applications ranging from electronic logic and memory devices to gas and biological sensors. Several scalable thin film synthesis techniques have demonstrated nanoscale control of TMD material thickness, morphology, structure, and chemistry and correlated these properties with high-performing, application-specific device metrics. In this review, the particularly versatile two-step conversion (2SC) method of TMD film synthesis is highlighted. The 2SC technique relies on deposition of a solid metal or metal oxide precursor material, followed by a reaction with a chalcogen vapor at an elevated temperature, converting the precursor film to a crystalline TMD. Herein, the variables at each step of the 2SC process including the impact of the precursor film material and deposition technique, the influence of gas composition and temperature during conversion, as well as other factors controlling high-quality 2D TMD synthesis are considered. The specific advantages of the 2SC approach including deposition on diverse substrates, low-temperature processing, orientation control, and heterostructure synthesis, among others, are featured. Finally, emergent opportunities that take advantage of the 2SC approach are discussed to include next-generation electronics, sensing, and optoelectronic devices, as well as catalysis for energy-related applications.

MXene-based electrochemical devices applied for healthcare applications

The initial part of the review provides an extensive overview about MXenes as novel and exciting 2D nanomaterials describing their basic physico-chemical features, methods of their synthesis, and possible interfacial modifications and techniques, which could be applied to the characterization of MXenes. Unique physico-chemical parameters of MXenes make them attractive for many practical applications, which are shortly discussed. Use of MXenes for healthcare applications is a hot scientific discipline which is discussed in detail. The article focuses on determination of low molecular weight analytes (metabolites), high molecular weight analytes (DNA/RNA and proteins), or even cells, exosomes, and viruses detected using electrochemical sensors and biosensors. Separate chapters are provided to show the potential of MXene-based devices for determination of cancer biomarkers and as wearable sensors and biosensors for monitoring of a wide range of human activities.

Sub-9 nm high-performance and low-power transistors based on an in-plane NbSe2/MoSe2/NbSe2 heterojunction

Due to the ability to reduce the gate length of field-effect transistors (FETs) down to sub-10 nm without obviously affecting the performance of the device, the utilization of two-dimensional (2D) semiconductor materials as channel materials for FETs is of great interest. However, in-plane 2D/2D heterojunction FETs have received less attention in previous studies than vertical van der Waals heterojunction devices. Based on the above reasons, this study has investigated the transport properties of an in-plane NbSe2/MoSe2/NbSe2 heterojunction FET with different gate lengths by using ab initio quantum transport simulation. The results reveal that a gate length of sub-9 nm gives the device a low subthreshold swing down to 62 mV dec-1 and a high on-state current up to 1040 μA μm-1. Most importantly, the on-state current, delay time, and power dissipation of the FET with the optimized channel length can nearly meet or even exceed the high-performance and low-power requirements of the International Technology Roadmap for Semiconductors. The findings for this FET can provide the design and development guidance for other in-plane heterojunction electrical devices in the post-Moore era.

Antioxidative 2D Bismuth Selenide via Halide Passivation for Enhanced Device Stability

The topological insulator 2D Bi₂Se₃ is promising for electronic devices due to its unique electronic properties; however, it is challenging to prepare antioxidative nanosheets since Bi₂Se₃ is prone to oxidation. Surface passivation using ligand agents after Bi₂Se₃ exfoliation works well to protect the surface, but the process is time-consuming and technically challenging; a passivation agent that is stable under a highly biased potential is significant for in situ passivation of the Bi₂Se₃ surface. In this work, the roles of halide anions (Cl^-, Br^-, and I^-) in respect of the chemical properties of synthetic Bi₂Se₃ nanosheets during electrochemical intercalated exfoliation were investigated to determine the antioxidation capacity. It was found that Bi₂Se₃ nanosheets prepared in a solution of tetrabutylammonium chloride (TBA⁺ and Cl^-) have the best oxidation resistance via the surface bonding of Bi with Cl, which promotes obtaining better device stability. This work paves an avenue for adjusting the components of the electrolyte to further promote the stability of 2D Bi₂Se₃-nanosheet-based electronic devices.

High Performance GaN-Based Ultraviolet Photodetector via Te/Metal Electrodes

Photodetectors (PDs) based on two-dimensional (2D) materials have promising applications in modern electronics and optoelectronics. However, due to the intralayer recombination of the photogenerated carriers and the inevitable surface trapping stages of the constituent layers, the PDs based on 2D materials usually suffer from low responsivity and poor response speed. In this work, a distinguished GaN-based photodetector is constructed on a sapphire substrate with Te/metal electrodes. Due to the metal-like properties of tellurium, the band bending at the interface between Te and GaN generates an inherent electric field, which greatly reduces the carrier transport barrier and promotes the photoresponse of GaN. This Te-enhanced GaN-based PD show a promising responsivity of 4951 mA/W, detectivity of 1.79 × 10¹⁴ Jones, and an external quantum efficiency of 169%. In addition, owing to the collection efficiency of carriers by this Te-GaN interface, the response time is greatly decreased compared with pure GaN PDs. This high performance can be attributed to the fact that Te reduces the contact resistance of the metal electrode Au/Ti to GaN, forming an ohmic-like contact and promoting the photoresponse of GaN. This work greatly extends the application potential of GaN in the field of high-performance photodetectors and puts forward a new way of developing high performance photodetectors.

Combination of Polymer Gate Dielectric and Two-Dimensional Semiconductor for Emerging Field-Effect Transistors

Two-dimensional (2D) materials are considered attractive semiconducting layers for emerging field-effect transistors owing to their unique electronic and optoelectronic properties. Polymers have been utilized in combination with 2D semiconductors as gate dielectric layers in field-effect transistors (FETs). Despite their distinctive advantages, the applicability of polymer gate dielectric materials for 2D semiconductor FETs has rarely been discussed in a comprehensive manner. Therefore, this paper reviews recent progress relating to 2D semiconductor FETs based on a wide range of polymeric gate dielectric materials, including (1) solution-based polymer dielectrics, (2) vacuum-deposited polymer dielectrics, (3) ferroelectric polymers, and (4) ion gels. Exploiting appropriate materials and corresponding processes, polymer gate dielectrics have enhanced the performance of 2D semiconductor FETs and enabled the development of versatile device structures in energy-efficient ways. Furthermore, FET-based functional electronic devices, such as flash memory devices, photodetectors, ferroelectric memory devices, and flexible electronics, are highlighted in this review. This paper also outlines challenges and opportunities in order to help develop high-performance FETs based on 2D semiconductors and polymer gate dielectrics and realize their practical applications.

Multi-Dimensional Molecular Self-Assembly Strategy for the Construction of Two-Dimensional Mesoporous Polydiaminopyridine and Carbon Materials

Two-dimensional (2D) mesoporous polymers, combining the advantages of organic polymers, porous materials, and 2D materials, have received great attention in adsorption, catalysis, and energy storage. However, the synthesis of 2D mesoporous polymers is not only challenged by the complex 2D structure construction, but also by the low yield and difficulty in controlling the dynamics of the assembly during the generation of mesopores. Herein, a facile multi-dimensional molecular self-assembly strategy is reported for the preparation of 2D mesoporous polydiaminopyridines (MPDAPs), which features tunable pore sizes (17-35 nm) and abundant N content up to 18.0 at%. Benefitting from the abundant N sites, 2D nanostructure, and uniform-large mesopores, the 2D MPDAPs exhibit excellent catalytic performance for the Knoevenagel condensation reaction. After calcination under N₂ atmosphere, the obtained 2D N-doped mesoporous carbon (NMCs) with large and uniform pore sizes, high surface areas, abundant N content (up to 23.1%), and a high ratio of basic N species (57.0% pyridinic N and 35.9% pyrrolic N) can show an excellent CO₂ uptake density (11.7 µmol m^-2 at 273 K), higher than previously reported porous materials.