Charge-Transfer Interaction between Few-Layer MoS2and Tetrathiafulvalene

van der Waals 2D transition metal dichalcogenide/organic hybridized heterostructures: recent breakthroughs and emerging prospects of the device

The near-atomic thickness and organic molecular systems, including organic semiconductors and polymer-enabled hybrid heterostructures, of two-dimensional transition metal dichalcogenides (2D-TMDs) can modulate their optoelectronic and transport properties outstandingly. In this review, the current understanding and mechanism of the most recent and significant breakthrough of novel interlayer exciton emission and its modulation by harnessing the band energy alignment between TMDs and organic semiconductors in a TMD/organic (TMDO) hybrid heterostructure are demonstrated. The review encompasses up-to-date device demonstrations, including field-effect transistors, detectors, phototransistors, and photo-switchable superlattices. An exploration of distinct traits in 2D-TMDs and organic semiconductors delves into the applications of TMDO hybrid heterostructures. This review provides insights into the synthesis of 2D-TMDs and organic layers, covering fabrication techniques and challenges. Band bending and charge transfer via band energy alignment are explored from both structural and molecular orbital perspectives. The progress in emission modulation, including charge transfer, energy transfer, doping, defect healing, and phase engineering, is presented. The recent advancements in 2D-TMDO-based optoelectronic synaptic devices, including various 2D-TMDs and organic materials for neuromorphic applications are discussed. The section assesses their compatibility for synaptic devices, revisits the operating principles, and highlights the recent device demonstrations. Existing challenges and potential solutions are discussed. Finally, the review concludes by outlining the current challenges that span from synthesis intricacies to device applications, and by offering an outlook on the evolving field of emerging TMDO heterostructures.

Room-temperature highly sensitive and selective NH3gas sensor using vertically aligned WS2nanosheets

Here, we report the room temperature (35 °C) NH₃gas sensor device made from WS₂nanosheets obtained via a facile and low-cost probe sonication method. The gas-sensing properties of devices made from these nanosheets were examined for various analytes such as ammonia, ethanol, methanol, formaldehyde, acetone, chloroform, and benzene. The fabricated gas sensor is selective towards NH₃and exhibits excellent sensitivity, faster response, and recovery time in comparison to previously reported values. The device can detect NH₃down to 5 ppm, much below the maximum allowed workspace NH₃level (20 ppm), and have a sensing response of the order of 112% with a response and recovery time of 54 s and 66 s, respectively. On the other hand, a sensor made from nanostructures has a bit longer recovery time than a device made from nanosheets. This was attributed to the fact that NH₃molecules adsorbed on the surface site and those trapped in between WS₂layers may have different adsorption energies . In the latter case, desorption becomes difficult and may give rise to slower recovery as noticed. Further, stiffened Raman modes upon exposure to NH₃reveal strong electron-phonon interaction between NH₃and the WS₂channel. The present work highlights the potential use of scaled two-dimensional nanosheets in sensing devices and particularly when used with inter-digitized electrodes, may offer enhanced performance.

Transition Metal Dichalcogenides (TMDC)-Based Nanozymes for Biosensing and Therapeutic Applications

Nanozymes, a type of nanomaterial with enzyme-like properties, are a promising alternative to natural enzymes. In particular, transition metal dichalcogenides (TMDCs, with the general formula MX2, where M represents a transition metal and X is a chalcogen element)-based nanozymes have demonstrated exceptional potential in the healthcare and diagnostic sectors. TMDCs have different enzymatic properties due to their unique nano-architecture, high surface area, and semiconducting properties with tunable band gaps. Furthermore, the compatibility of TMDCs with various chemical or physical modification strategies provide a simple and scalable way to engineer and control their enzymatic activity. Here, we discuss recent advances made with TMDC-based nanozymes for biosensing and therapeutic applications. We also discuss their synthesis strategies, various enzymatic properties, current challenges, and the outlook for future developments in this field.

Functionalized MoS2 Nanoflowers with Excellent Near-Infrared Photothermal Activities for Scavenging of Antibiotic Resistant Bacteria

Presently, antibiotic resistant bacteria (ARB) have been commonly found in environment, such as air, soil and lakes. Therefore, it is urgent and necessary to prepare antimicrobial agents with excellent anti-antibiotic resistant bacteria. In our research, poly-ethylene glycol functionalized molybdenum disulfide nanoflowers (PEG-MoS2 NFs) were synthesized via a one-step hydrothermal method. As-prepared PEG-MoS2 NFs displayed excellent photothermal conversion efficiency (30.6%) and photothermal stability. Under 808 nm NIR laser irradiation for 10 min, the inhibition rate of tetracycline-resistant Bacillus tropicalis and Stenotrophomonas malphilia reached more than 95% at the concentration of 50 μg/mL. More interestingly, the photothermal effect of PEG-MoS2 NFs could accelerate the oxidation of glutathione, resulting in the rapid death of bacteria. A functionalized PEG-MoS2 NFs photothermal anti-antibiotic resistant system was constructed successfully.

Recent Advances in Interface Engineering of Transition-Metal Dichalcogenides with Organic Molecules and Polymers

The interface engineering of two-dimensional (2D) transition-metal dichalcogenides (TMDs) has been regarded as a promising strategy to modulate their outstanding electrical and optoelectronic properties because of their inherent 2D nature and large surface-to-volume ratio. In particular, introducing organic molecules and polymers directly onto the surface of TMDs has been explored to passivate the surface defects or achieve better interfacial properties with neighboring surfaces efficiently, thus leading to great opportunities for the realization of high-performance TMD-based applications. This review provides recent progress in the interface engineering of TMDs with organic molecules and polymers corresponding to the modulation of their electrical and optoelectronic characteristics. Depending on the interfaces between the surface of TMDs and dielectric, conductive contacts or the ambient environment, we present various strategies to introduce an organic interlayer from materials to processing. In addition, the role of native defects on the surface of TMDs, such as adatoms or vacancies, in determining their electrical characteristics is also discussed in detail. Finally, the future challenges and opportunities associated with the interface engineering are highlighted.

Charge carrier injection and transport engineering in two-dimensional transition metal dichalcogenides

Ever since two dimensional-transition (2D) metal dichalcogenides (TMDs) were discovered, their fascinating electronic properties have attracted a great deal of attention for harnessing them as critical components in novel electronic devices. 2D-TMDs endowed with an atomically thin structure, dangling bond-free nature, electrostatic integrity, and tunable wide band gaps enable low power consumption, low leakage, ambipolar transport, high mobility, superconductivity, robustness against short channel effects and tunneling in highly scaled devices. However, the progress of 2D-TMDs has been hampered by severe charge transport issues arising from undesired phenomena occurring at the surfaces and interfaces. Therefore, this review provides three distinct engineering strategies embodied with distinct innovative approaches to optimize both carrier injection and transport. First, contact engineering involves 2D-metal contacts and tunneling interlayers to overcome metal-induced interface states and the Fermi level pinning effect caused by low vacancy energy formation. Second, dielectric engineering covers high-k dielectrics, ionic liquids or 2D-insulators to screen scattering centers caused by carrier traps, imperfections and rough substrates, to finely tune the Fermi level across the band gap, and to provide dangling bond-free media. Third, material engineering focuses on charge transfer via substitutional, chemical and plasma doping to precisely modulate the carrier concentration and to passivate defects while preserving material integrity. Finally, we provide an outlook of the conceptual and technical achievements in 2D-TMDs to give a prospective view of the future development of highly scaled nanoelectronic devices.

Photoactivated Mixed In-Plane and Edge-Enriched p-Type MoS2 Flake-Based NO2 Sensor Working at Room Temperature

Toxic gases are produced during the burning of fossil fuels. Room temperature (RT) fast detection of toxic gases is still challenging. Recently, MoS₂ transition metal dichalcogenides have sparked great attention in the research community due to their performance in gas sensing applications. However, MoS₂ based gas sensors still suffer from long response and recovery times, especially at RT. Considering this challenge, here, we report photoactivated highly reversible and fast detection of NO₂ sensors at room temperature (RT) by using mixed in-plane and edge-enriched p-MoS₂ flakes (mixed MoS₂). The sensor showed fast response with good sensitivity of ∼10.36% for 10 ppm of NO₂ at RT without complete recovery. However, complete recovery was obtained with better sensor performance under UV light illumination at RT. The UV assisted NO₂ sensing showed improved performance in terms of fast response and recovery kinetics with enhanced sensitivity to 10 ppm NO₂ concentration. The sensor performance is also investigated under thermal energy, and a better sensor performance with reduced sensitivity and high selectivity toward NO₂ was observed. A detailed gas sensing mechanism based on the density functional theory (DFT) calculations for favorable NO₂ adsorption sites on in-plane and edge-enriched MoS₂ flakes is proposed. This study revealed the role of favorable adsorption sites in MoS₂ flakes for the enhanced interaction of target gases and developed a highly sensitive, reversible, and fast gas sensor for next-generation toxic gases at room temperature.

Two-dimensional transition metal dichalcogenides: interface and defect engineering

This review summarizes the recent advances in understanding the effects of interface and defect engineering on the electronic and optical properties of TMDCs, as well as their applications in advanced (opto)electronic devices.

Fabrication of Z-scheme magnetic MoS2/CoFe2O4 nanocomposites with highly efficient photocatalytic activity

MoS₂ thin nanosheets decorated with CoFe₂O₄ nanoparticles have been successfully synthesized via a simple hydrothermal method. The nanocomposites are characterized by XRD, TEM, HRTEM, BET, XPS, UV-Vis DRS, PL and magnetic property analysis. The Z-scheme mechanism at the interface of MoS₂ and CoFe₂O₄ is formed. When the mass ratio of MoS₂ and CoFe₂O₄ is 1:3, the MoS₂/CoFe₂O₄ nanocomposites present excellent photocatalytic performance. The degradation rate of rhodamine B (RhB) and congo red (CR) is 93.80% and 94.43% in 90 and 50 min, respectively, under visible light irradiation. The highly photocatalytic activity could be mainly ascribed to the formed Z-scheme mechanism which facilitates the separation of photoinduced electron-hole pairs. Besides, the MoS₂ thin nanosheets not only provide the most active sites for photocatalytic reactions, but also act as the backing material for CoFe₂O₄ nanoparticles to effectively disperse and avoid the magnetic aggregation. Moreover, the MoS₂/CoFe₂O₄ nanocomposites present a good recyclability and the degradation rate of RhB and CR is still beyond 82% after seven runs. In addition, the nanocomposites can be easily separated by an external magnet.

Electrostatic Origin of Stabilization in MoS2-Organic Nanocrystals

Negatively charged molybdenum disulfide layers form stable organic-inorganic layered nanocrystals when reacted with organic cations in solution. The reasons why this self-assembly process leads to a single-phase compound with a well-defined interlayer distance in given conditions are, however, poorly understood to date. Here, for the first time, we quantify the interactions determining the cation packing and stability of the MoS₂-organic nanocrystals and find that the main contribution arises from Coulomb forces. The study was performed on the series of new layered compounds of MoS₂ with naphthalene derivatives, forming several distinct phases depending on reaction conditions. Starting with structural models derived from powder X-ray diffraction data and TEM, we evaluate their cohesion energy by modeling layer separation with periodic PW-DFT-D calculations. The results provide a reliable approach for estimation of the stability of MoS₂-based heterolayered compounds.

Recent Advances in Doping of Molybdenum Disulfide: Industrial Applications and Future Prospects

Owing to their excellent physical properties, atomically thin layers of molybdenum disulfide (MoS₂ ) have recently attracted much attention due to their nonzero-gap property, exceptionally high electrical conductivity, good thermal stability, and excellent mechanical strength, etc. MoS₂ -based devices exhibit great potential for applications in optoelectronics and energy harvesting. Here, a comprehensive review of various doping strategies is presented, including wet doping and dry doping of atomically crystalline MoS₂ thin layers, and the progress made so far for their doping-based prospective applications is also discussed. Finally, several significant research issues for the prospects of doped-MoS₂ in industry, as a guide for 2D material community, are also provided.

Excitation dependent bidirectional electron transfer in phthalocyanine-functionalised MoS2 nanosheets

Two-dimensional (2D) transition metal chalcogenides such as 2D MoS₂ are considered prime candidate materials for the design of next generation optoelectronics. Functionalisation of these materials is considered to be a key step in tailoring their properties towards specific applications and unlocking their full potential. Here we present a van der Waals functionalisation strategy for creating MoS₂ nanosheets decorated with free base phthalocyanine chromophores. The semiconducting sheets are found to intimately interact with these optoelectronically active chromophores, resulting in an electronic heterostructure that exhibits enhanced optoelectronic properties and exploitable charge transfer. We show that by utilising laterally confined MoS₂ nanosheets, the conduction band of the semiconductor could be positioned between the chromophore's S1 and S2 states. Consequently, bidirectional photoinduced electron transfer processes are observed, with excitation of the functionalised nanosheet's semiconductor transition resulting in electron transfer to the phthalocyanine's LUMO, and excitation of the chromophore's S2 state leading to electron injection into the MoS₂ conduction band. However, charge transfer from the dye's S1 transition to the MoS₂ nanosheet is found to be thermodynamically unfavourable, resulting in intense radiative recombination. These findings may enable controlling and tuning the charge carrier density of semiconducting nanosheets via optical means through the exploitation of photoinduced electron transfer. Furthermore this work provides access to 2D semiconductor-hybrids with tailored absorption profiles and photoluminescence.

Two-dimensional inorganic analogues of graphene: transition metal dichalcogenides

The discovery of graphene marks a major event in the physics and chemistry of materials. The amazing properties of this two-dimensional (2D) material have prompted research on other 2D layered materials, of which layered transition metal dichalcogenides (TMDCs) are important members. Single-layer and few-layer TMDCs have been synthesized and characterized. They possess a wide range of properties many of which have not been known hitherto. A typical example of such materials is MoS2 In this article, we briefly present various aspects of layered analogues of graphene as exemplified by TMDCs. The discussion includes not only synthesis and characterization, but also various properties and phenomena exhibited by the TMDCs.This article is part of the themed issue 'Fullerenes: past, present and future, celebrating the 30th anniversary of Buckminster Fullerene'.

Tetrathiafulvalene-containing polymers for simultaneous non-covalent modification and electronic modulation of MoS2 nanomaterials

Polymers with pendent tetrathiafulvalene groups for solubilization and electronic modification of MoS₂ nanosheets.

Transition metal dichalcogenides (TMDCs) such as MoS₂ comprise an important class of 2D semiconductors with numerous interesting electronic and mechanical features. Full utilization of TMDCs in materials and devices, however, necessitates robust functionalization methods. We report well-defined tetrathiafulvalene (TTF)-based polymers, exploiting synthetic routes that overcome challenges previously associated with these systems. These platforms enable basal plane coordinative interactions with MoS₂, conceptually in parallel with pyrene-containing platforms for graphene and carbon nanotube modification. Not yet reported for TMDCs, these non-covalent interactions are universal and effective for MoS₂ irrespective of the lattice structure, affording significantly enhanced solution stabilization of the nanosheets. Additionally, the TTF-functionalized polymers offer electronic structure modulation of MoS₂ by ground state charge transfer and work function reduction, demonstrated using Kelvin probe force microscopy (KPFM). Notably, coordination and electronic effects are amplified for the TTF–polymers over TTF itself. Experiments are supported by first-principles density functional theory (DFT) calculations that probe polymer–TTF surface interactions with MoS₂ and the resultant impact on electronic properties.

Electronic transport properties of transition metal dichalcogenide field-effect devices: surface and interface effects

Recent explosion of interest in two-dimensional (2D) materials research has led to extensive exploration of physical and chemical phenomena unique to this new class of materials and their technological potential. Atomically thin layers of group 6 transition metal dichalcogenides (TMDs) such as MoS2 and WSe2 are remarkably stable semiconductors that allow highly efficient electrostatic control due to their 2D nature. Field effect transistors (FETs) based on 2D TMDs are basic building blocks for novel electronic and chemical sensing applications. Here, we review the state-of-the-art of TMD-based FETs and summarize the current understanding of interface and surface effects that play a major role in these systems. We discuss how controlled doping is key to tailoring the electrical response of these materials and realizing high performance devices. The first part of this review focuses on some fundamental features of gate-modulated charge transport in 2D TMDs. We critically evaluate the role of surfaces and interfaces based on the data reported in the literature and explain the observed discrepancies between the experimental and theoretical values of carrier mobility. The second part introduces various non-covalent strategies for achieving desired doping in these systems. Gas sensors based on charge transfer doping and electrostatic stabilization are introduced to highlight progress in this direction. We conclude the review with an outlook on the realization of tailored TMD-based field-effect devices through surface and interface chemistry.

Comparative Study of Potential Applications of Graphene, MoS2, and Other Two-Dimensional Materials in Energy Devices, Sensors, and Related Areas

Novel properties of graphene have been well documented, whereas the importance of nanosheets of MoS2 and other chalcogenides is increasingly being recognized over the last two to three years. Borocarbonitrides, BxCyNz, with insulating BN and conducting graphene on either side are new materials whose properties have been attracting attention. These two-dimensional (2D) materials contain certain common features. Thus, graphene, MoS2, and borocarbonitrides have all been used in supercapacitor applications, oxygen reduction reactions (ORRs), and lithium-ion batteries. It is instructive, therefore, to make a comparative study of some of the important properties of these layered materials. In this article, we discuss properties related to energy devices at length. We examine the hydrogen evolution reaction facilitated by graphene, MoS2, and related materials. We also discuss gas and radiation sensors based on graphene and MoS2 as well as gas storage properties of graphene and borocarbonitrides. The article should be useful in making a judicious choice of which 2D material to use for a particular application.

Small molecules make big differences: molecular doping effects on electronic and optical properties of phosphorene

Systematical computations on the density functional theory were performed to investigate the adsorption of three typical organic molecules, tetracyanoquinodimethane (TCNQ), tetracyanoethylene (TCNE) and tetrathiafulvalene (TTF), on the surface of phosphorene monolayers and thicker layers. There exist considerable charge transfer and strong non-covalent interaction between these molecules and phosphorene. In particular, the band gap of phosphorene decreases dramatically due to the molecular modification and can be further tuned by applying an external electric field. Meanwhile, surface molecular modification has proven to be an effective way to enhance the light harvesting of phosphorene in different directions. Our results predict a flexible method toward modulating the electronic and optical properties of phosphorene and shed light on its experimental applications.

A novel biosensor based on single-layer MoS2 nanosheets for detection of Ag(+)

In this work, we use for the first time single layer MoS2 as the fluorescence quencher to design a detection method for Ag(+) with excellent robustness, selectivity and sensitivity. To maintain the ultrathin MoS2, bulk MoS2 materials have been exfoliated by intercalation with lithium followed by reaction with water. As-prepared two-dimensional MoS2 not only has good water-solubility but also obtains high fluorescence quenching efficiency within 5 min. Importantly, the detection limit of this assay for Ag(+) (1 nM) was lower than the maximum limitation guided by the United States Environmental Protection Agency (EPA) and the World Health Organization (WHO). Further, this new Ag(+) probe was demonstrated in monitoring Ag(+) in lake water samples with satisfactory results.

Seeing diabetes: visual detection of glucose based on the intrinsic peroxidase-like activity of MoS2 nanosheets

Molybdenum disulfide (MoS2) has attracted increasing research interest recently due to its unique physical, optical and electrical properties, correlated with its 2D ultrathin atomic-layered structure. Until now, however, great efforts have focused on its applications such as lithium ion batteries, transistors, and hydrogen evolution reactions. Herein, for the first time, MoS2 nanosheets are discovered to possess an intrinsic peroxidase-like activity and can catalytically oxidize 3,3',5,5'-tetramethylbenzidine (TMB) by H2O2 to produce a color reaction. The catalytic activity follows the typical Michaelis-Menten kinetics and is dependent on temperature, pH, H2O2 concentration, and reaction time. Based on this finding, a highly sensitive and selective colorimetric method for H2O2 and glucose detection is developed and applied to detect glucose in serum samples. Moreover, a simple, inexpensive, instrument-free and portable test kit for the visual detection of glucose in normal and diabetic serum samples is constructed by utilizing agarose hydrogel as a visual detection platform.

Electron-doping-enhanced trion formation in monolayer molybdenum disulfide functionalized with cesium carbonate

We report effective and stable electron doping of monolayer molybdenum disulfide (MoS2) by cesium carbonate (Cs2CO3) surface functionalization. The electron charge carrier concentration in exfoliated monolayer MoS2 can be increased by about 9 times after Cs2CO3 functionalization. The n-type doping effect was evaluated by in situ transport measurements of MoS2 field-effect transistors (FETs) and further corroborated by in situ ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy, and Raman scattering measurements. The electron doping enhances the formation of negative trions (i.e., a quasiparticle comprising two electrons and one hole) in monolayer MoS2 under light irradiation and significantly reduces the charge recombination of photoexcited electron-hole pairs. This results in large photoluminescence suppression and an obvious photocurrent enhancement in monolayer MoS2 FETs.

Graphene analogues of inorganic layered materials

The discovery of graphene has created a great sensation in chemistry, physics, materials science, and related areas. The unusual properties of graphene have aroused interest in other layered materials, such as molybdenum sulfide and boron nitride. In the last few years, single- as well as few-layer as well as chalcogenides and other inorganic materials have been prepared and characterized by a variety of methods. These materials possess interesting properties, and some have potential applications. This Review provides an up-to-date account of these emerging two-dimensional nanomaterials. Not only are the synthesis and characterization covered, but also important aspects such as spectroscopic and optical properties, magnetic and electrical properties, as well as applications. Salient features of the composites formed from the layered inorganic structures with graphene and polymers are presented along with a brief description of borocarbonitrides.

MoS2 nanocrystals confined in a DNA matrix exhibiting energy transfer

We report the wet chemical synthesis of MoS2 nanocrystals (NCs), a transition-metal dichalcogenide, using DNA as a host matrix. As evidenced from transmission electron microscopy (TEM), the NCs are highly crystalline, with an average diameter of ~5 nm. Ultraviolet-visible (UV-vis) absorption studies along with band gap calculations confirm that NCs are in quantum confinement. A prominent red shift of the optical absorption bands has been observed upon formation of the thin film using hexadecyltrimethylammonium chloride (CTAC), i.e., in the case of MoS2@DNA-CTAC. In the thin film, strong electron-phonon coupling arises because of the resonance effect, which is reflected from the emergence of intense first-, second-, and third-order Raman peaks, whenever excited with the 488 nm line. We have established that our as-synthesized MoS2 NCs quench the fluorescence of a well-known DNA minor groove binding probe, Hoechst 33258. Unprecedented fluorescence quenching (94%) of donor (Hoechst 33258) emission and efficient energy transfer (89%) between Hoechst 33258 and MoS2 NCs (acceptor) are obtained. The donor-acceptor distance of these conjugates has been described by a Förster resonance energy transfer (FRET)-based model. Furthermore, employing a statistical method, we have estimated the probability of the distance distribution between the donor and acceptor. We believe that the study described herein may enable substantial advances in fields of optoelectronics, photovoltaics, catalysis, and many others.

Sensing behavior of atomically thin-layered MoS2 transistors

Most of recent research on layered chalcogenides is understandably focused on single atomic layers. However, it is unclear if single-layer units are the most ideal structures for enhanced gas-solid interactions. To probe this issue further, we have prepared large-area MoS2 sheets ranging from single to multiple layers on 300 nm SiO2/Si substrates using the micromechanical exfoliation method. The thickness and layering of the sheets were identified by optical microscope, invoking recently reported specific optical color contrast, and further confirmed by AFM and Raman spectroscopy. The MoS2 transistors with different thicknesses were assessed for gas-sensing performances with exposure to NO2, NH3, and humidity in different conditions such as gate bias and light irradiation. The results show that, compared to the single-layer counterpart, transistors of few MoS2 layers exhibit excellent sensitivity, recovery, and ability to be manipulated by gate bias and green light. Further, our ab initio DFT calculations on single-layer and bilayer MoS2 show that the charge transfer is the reason for the decrease in resistance in the presence of applied field.