Scalable lateral heterojunction by chemical doping of 2D TMD thin films

Lateral junctions of transition metal dichalcogenides as ballistic channels for straintronic applications

In the context of advanced nanoelectronics, two-dimensional semiconductors such as transition metal dichalcogenides (TMDs) are gaining considerable interest due to their ultimate thinness, clean surface and high carrier mobility. The engineering prospects offered by those materials are further enlarged by the recent realization of atomically sharp TMD-based lateral junctions, whose electronic properties are governed by strain effects arising from the constituents lattice mismatch. Although most theoretical studies considered only misfit strain, first-principles simulations are employed here to investigate the transport properties under external deformation of a three-terminal device constructed from a MoS2/WSe2/MoS2junction. Large modulation of the current is reported owing to the change in band offset, illustrating the importance of strain on the p-n junction characteristics. The device operation is demonstrated for both local and global deformations, even for ultra-short channels, suggesting potential applications for ultra-thin body straintronics.

2D nanostructures: Potential in diagnosis and treatment of Alzheimer's disease

Two-dimensional (2D) nanomaterials have garnered enormous attention seemingly due to their unusual architecture and properties. Graphene and graphene oxide based 2D nanomaterials remained the most sought after for several years but the quest to design superior 2D nanomaterials which can find wider application gave rise to development of non-graphene 2D materials as well. Consequently, in addition to graphene based 2D nanomaterials, 2D nanostructures designed using macromolecules (such as DNAs, proteins, peptides and peptoids), transition metal dichalcogenides, transition-metal carbides and/or nitrides (MXene), black phosphorous, chitosan, hexagonal boron nitrides, and graphitic carbon nitride, and covalent organic frameworks have been developed. Interestingly, these 2D nanomaterials have found applications in diagnosis and treatment of various diseases including Alzheimer's disease (AD). Although AD is one of the most debilitating neurodegenerative conditions across the globe; unfortunately, there remains a paucity of effective diagnostic and/or therapeutic intervention for it till date. In this scenario, nanomaterial-based biosensors, or therapeutics especially 2D nanostructures are emerging to be promising in this regard. This review summarizes the diagnostic and therapeutic platforms developed for AD using 2D nanostructures. Collectively, it is worth mentioning that these 2D nanomaterials would seemingly provide an alternative and intriguing platform for biomedical interventions.

Enhancing the Carrier Transport in Monolayer MoS2 through Interlayer Coupling with 2D Covalent Organic Frameworks

The coupling of different 2D materials (2DMs) to form van der Waals heterostructures (vdWHs) is a powerful strategy for adjusting the electronic properties of 2D semiconductors, for applications in opto-electronics and quantum computing. 2D molybdenum disulfide (MoS2 ) represents an archetypical semiconducting, monolayer thick versatile platform for the generation of hybrid vdWH with tunable charge transport characteristics through its interfacing with molecules and assemblies thereof. However, the physisorption of (macro)molecules on 2D MoS2 yields hybrids possessing a limited thermal stability, thereby jeopardizing their technological applications. Herein, the rational design and optimized synthesis of 2D covalent organic frameworks (2D-COFs) for the generation of MoS2 /2D-COF vdWHs exhibiting strong interlayer coupling effects are reported. The high crystallinity of the 2D-COF films makes it possible to engineer an ultrastable periodic doping effect on MoS2 , boosting devices' field-effect mobility at room temperature. Such a performance increase can be attributed to the synergistic effect of the efficient interfacial electron transfer process and the pronounced suppression of MoS2 's lattice vibration. This proof-of-concept work validates an unprecedented approach for the efficient modulation of the electronic properties of 2D transition metal dichalcogenides toward high-performance (opto)electronics for CMOS digital circuits.

Plasma Processing and Treatment of 2D Transition Metal Dichalcogenides: Tuning Properties and Defect Engineering

Two-dimensional transition metal dichalcogenides (TMDs) offer fascinating opportunities for fundamental nanoscale science and various technological applications. They are a promising platform for next generation optoelectronics and energy harvesting devices due to their exceptional characteristics at the nanoscale, such as tunable bandgap and strong light-matter interactions. The performance of TMD-based devices is mainly governed by the structure, composition, size, defects, and the state of their interfaces. Many properties of TMDs are influenced by the method of synthesis so numerous studies have focused on processing high-quality TMDs with controlled physicochemical properties. Plasma-based methods are cost-effective, well controllable, and scalable techniques that have recently attracted researchers' interest in the synthesis and modification of 2D TMDs. TMDs' reactivity toward plasma offers numerous opportunities to modify the surface of TMDs, including functionalization, defect engineering, doping, oxidation, phase engineering, etching, healing, morphological changes, and altering the surface energy. Here we comprehensively review all roles of plasma in the realm of TMDs. The fundamental science behind plasma processing and modification of TMDs and their applications in different fields are presented and discussed. Future perspectives and challenges are highlighted to demonstrate the prominence of TMDs and the importance of surface engineering in next-generation optoelectronic applications.

Reduced dopant-induced scattering in remote charge-transfer-doped MoS2 field-effect transistors

Efficient doping for modulating electrical properties of two-dimensional (2D) transition metal dichalcogenide (TMDC) semiconductors is essential for meeting the versatile requirements for future electronic and optoelectronic devices. Because doping of semiconductors, including TMDCs, typically involves generation of charged dopants that hinder charge transport, tackling Coulomb scattering induced by the externally introduced dopants remains a key challenge in achieving ultrahigh mobility 2D semiconductor systems. In this study, we demonstrated remote charge transfer doping by simply inserting a hexagonal boron nitride layer between MoS₂ and solution-deposited n-type dopants, benzyl viologen. A quantitative analysis of temperature-dependent charge transport in remotely doped devices supports an effective suppression of the dopant-induced scattering relative to the conventional direct doping method. Our mechanistic investigation of the remote doping method promotes the charge transfer strategy as a promising method for material-level tailoring of electrical and optoelectronic devices based on TMDCs.

Recent progress in 2D hybrid heterostructures from transition metal dichalcogenides and organic layers: properties and applications in energy and optoelectronics fields

Atomically thin transition metal dichalcogenides (TMDs) present extraordinary optoelectronic, electrochemical, and mechanical properties that have not been accessible in bulk semiconducting materials. Recently, a new research field, 2D hybrid heteromaterials, has emerged upon integrating TMDs with molecular systems, including organic molecules, polymers, metal-organic frameworks, and carbonaceous materials, that can tailor the TMD properties and exploit synergetic effects. TMD-based hybrid heterostructures can meet the demands of future optoelectronics, including supporting flexible, transparent, and ultrathin devices, and energy-based applications, offering high energy and power densities with long cycle lives. To realize such applications, it is necessary to understand the interactions between the hybrid components and to develop strategies for exploiting the distinct benefits of each component. Here, we provide an overview of the current understanding of the new phenomena and mechanisms involved in TMD/organic hybrids and potential applications harnessing such valuable materials in an insightful way. We highlight recent discoveries relating to multicomponent hybrid materials. Finally, we conclude this review by discussing challenges related to hybrid heteromaterials and presenting future directions and opportunities in this research field.

Lateral layered semiconductor multijunctions for novel electronic devices

Layered semiconductors, represented by transition metal dichalcogenides, have attached extensive attention due to their unique and tunable electrical and optical properties. In particular, lateral layered semiconductor multijunctions, including homojunctions, heterojunctions, hybrid junctions and superlattices, present a totally new degree of freedom in research on electronic devices beyond traditional materials and their structures, providing unique opportunities for the development of new structures and operation principle-based high performance devices. However, the advances in this field are limited by the precise synthesis of high-quality junctions and greatly hampered by ambiguous device performance limits. Herein, we review the recent key breakthroughs in the design, synthesis, electronic structure and property modulation of lateral semiconductor multijunctions and focus on their application-specific devices. Specifically, the synthesis methods based on different principles, such as chemical and external source-induced methods, are introduced stepwise for the controllable fabrication of semiconductor multijunctions as the basics of device application. Subsequently, their structure and property modulation are discussed, including control of their electronic structure, exciton dynamics and optical properties before the fabrication of lateral layered semiconductor multijunction devices. Precise property control will potentially result in outstanding device performances, including high-quality diodes and FETs, scalable logic and analog circuits, highly efficient optoelectronic devices, and unique electrochemical devices. Lastly, we focus on several of the most essential but unresolved debates in this field, such as the true advantages of few-layer vs. monolayer multijunctions, how sharp the interface should be for specific functional devices, and the superiority of lateral multijunctions over vertical multijunctions, highlighting the next-phase strategy to enhance the performance potential of lateral multijunction devices.

Tailoring the Electrical Characteristics of MoS2 FETs through Controllable Surface Charge Transfer Doping Using Selective Inkjet Printing

Surface charge transfer doping (SCTD) has been regarded as an effective approach to tailor the electrical characteristics of atomically thin transition metal dichalcogenides (TMDs) in a nondestructive manner due to their two-dimensional nature. However, the difficulty of achieving rationally controlled SCTD on TMDs via conventional doping methods, such as solution immersion and dopant vaporization, has impeded the realization of practical optoelectronic and electronic devices. Here, we demonstrate controllable SCTD of molybdenum disulfide (MoS₂) field-effect transistors using inkjet-printed benzyl viologen (BV) as an n-type dopant. By adjusting the BV concentration and the areal coverage of inkjet-printed BV dopants, controllable SCTD results in BV-doped MoS₂ FETs with elaborately tailored electrical performance. Specifically, the suggested solvent system creates well-defined droplets of BV ink having a volume of ∼2 pL, which allows the high spatial selectivity of SCTD onto the MoS₂ channels by depositing the BV dopant on demand. Our inkjet-printed SCTD method provides a feasible solution for achieving controllable doping to modulate the electrical and optical performances of TMD-based devices.

2D Material-Based Optical Biosensor: Status and Prospect

The combination of 2D materials and optical biosensors has become a hot research topic in recent years. Graphene, transition metal dichalcogenides, black phosphorus, MXenes, and other 2D materials (metal oxides and degenerate semiconductors) have unique optical properties and play a unique role in the detection of different biomolecules. Through the modification of 2D materials, optical biosensor has the advantages that traditional sensors (such as electrical sensing) do not have, and the sensitivity and detection limit are greatly improved. Here, optical biosensors based on different 2D materials are reviewed. First, various detection methods of biomolecules, including surface plasmon resonance (SPR), fluorescence resonance energy transfer (FRET), and evanescent wave and properties, preparation and integration strategies of 2D material, are introduced in detail. Second, various biosensors based on 2D materials are summarized. Furthermore, the applications of these optical biosensors in biological imaging, food safety, pollution prevention/control, and biological medicine are discussed. Finally, the future development of optical biosensors is prospected. It is believed that with their in-depth research in the laboratory, optical biosensors will gradually become commercialized and improve people's quality of life in many aspects.

Rapid Degradation of the Electrical Properties of 2D MoS2 Thin Films under Long-Term Ambient Exposure

The MoS₂ thin film has attracted a lot of attention due to its potential applications in flexible electronics, sensors, catalysis, and heterostructures. Understanding the effect of long-term ambient exposure on the electrical properties of the thin film is important for achieving many overreaching goals of this material. Here, we report for the first time a systematic study of electrical property variation and stability of MoS₂ thin films under ambient exposure of up to a year. The MoS₂ thin films were grown via the sulfurization of 6 nm thick molybdenum films. We found that the resistance of the samples increases by 114% just in 4 weeks and 430% in 4 months and they become fully insulated in a year of ambient exposure. The dual-sweep current-voltage (I-V) characteristic shows hysteretic behavior for a 4-month-old sample which further exhibits pronounced nonlinear I-V curves and hysteretic behavior after 8 months. The X-ray photoelectron spectroscopy measurements show that the MoS₂ thin film gradually oxidizes and 13.1% of MoO₃ and 11.8% oxide of sulfur were formed in 4 months, which further increased to 23.1 and 12.7% in a year, respectively. The oxide of the sulfur peak was not reported in any previous stability studies of exfoliated and chemical vapor deposition-grown MoS₂, suggesting that the origin of this peak is related to the distinct crystallinity of the MoS₂ thin film due to its smaller grain sizes, abundant grain boundaries, and exposed edges. Raman studies show the broadening of E_2g ¹ and A_1g peaks with increasing exposure time, suggesting an increase in the disorder in MoS₂. It is also found that coating the MoS₂ thin film with polymethylmethacrylate can effectively prevent the electrical property degradation, showing only a 6% increase in resistance in 4 months and 40% over a year of ambient exposure.

Photodetectors of 2D Materials from Ultraviolet to Terahertz Waves

2D materials are considered to be the most promising materials for photodetectors due to their unique optical and electrical properties. Since the discovery of graphene, many photodetectors based on 2D materials have been reported. However, the low quantum efficiency, large noise, and slow response caused by the thinness of 2D materials limit their application in photodetectors. Here, recent progress on 2D material photodetectors is reviewed, covering the spectrum from ultraviolet to terahertz waves. First the interaction of 2D materials with light is analyzed in terms of optical physics. Then the present methods to improve the performance of 2D material photodetectors are summarized, such as defect engineering, p-n junctions and hybrid detectors, and the issue of serious overestimation of the performance in reported photodetectors based on 2D materials is discussed. Next, a comparison of 2D material photodetectors with traditional commercially available detectors shows that it is difficult to balance the current 2D material photodetectors with regard to having simultaneously both high sensitivity and fast response. Finally, a possible novel EIW mechanism is suggested to advance the performance of 2D material photodetectors in the future.