Group 6 transition metal dichalcogenide nanomaterials: synthesis, applications and future perspectives

Band alignment tuning of heptazine-g-C3N4/g-ZnO vdW heterostructure as a promising water-splitting photocatalyst

Van der Waals (vdW) heterostructures of two-dimensional monolayers are a relatively new class of materials with highly tunable band alignment, bandgap energy, and bandgap transition type. In this study, we performed density functional theory calculations to investigate how a vdW heterostructure of heptazine-based graphitic carbon nitride (hg-C₃N₄) and graphitic zinc oxide (g-ZnO) monolayers is formed (hg-C₃N₄/g-ZnO). This heterostructure is a potential solar-driven photocatalyst for the water-splitting reaction. Upon the formation of the heterostructure, a type-I indirect bandgap (E_g = 2.08 eV) is created with appropriate conduction band minimum and valence band maximum levels relative to the oxidation/reduction potentials for the water-splitting reaction. In addition, a very large electrostatic potential difference of 11.18 eV is generated across the heterostructure, leading to a large, naturally-formed, built-in electric field directing from hg-C₃N₄ to g-ZnO. The produced electric field forces photogenerated electrons in g-ZnO to transfer toward hg-C₃N₄, leading to a decrease in the electron-hole recombination rate. We also found that both g-ZnO and hg-C₃N₄ synergistically lead to higher light absorption of the heterostructure (λ_max = 387 nm). Furthermore, band alignment, bandgap energy, and transition type of the heterostructure can be tuned by applying external perpendicular electric fields and biaxial strains. It was found that a strain of +2% leads to a Z-scheme band alignment (E_g = 2.34 eV, direct) and an electric field of 1 V Å^-1 leads to a type-II heterostructure (E_g = 2.29 eV, indirect), which are both beneficial for efficient water-splitting photocatalysis.

Load-dependent energy dissipation induced by the tip-membrane friction on suspended 2D materials

The tip-membrane interface plays a critical role in characterizing the mechanical properties of ultrathin 2D materials by commonly employed nanoindentation based on atomic force microscopy (AFM). However, the reliability of the assumption that the tip-membrane interface remains pinned during nanoindentation remains unclear, which may introduce unignorable uncertainty in evaluating their true mechanical properties. In this work, it is reported that load-dependent frictional behavior would occur on the tip-membrane interface during nanoindentation tests on monolayer and multilayer suspended WS₂ and graphene, and the curve hysteresis could be well explained by the stick-slip behavior. Further analyses and finite element simulations demonstrated that the frictional energy dissipation should be mainly attributed to the frictional behavior along the direction parallel to the cantilever beam. Meanwhile, the in-plane membrane stiffness was mainly responsible for the different frictional behavior on monolayer and multilayer 2D materials. Based on these analyses, some suggestions were proposed to help reduce the uncertainty when extracting the mechanical properties of 2D materials. These findings not only facilitate the deep understanding of the origin of the curve hysteresis during nanoindentation, but also help to evaluate the mechanical properties of 2D materials in a more reliable way.

Growth Mechanisms and Morphology Engineering of Atomic Layer-Deposited WS2

Transition-metal dichalcogenides (TMDs) have attracted intense research interest for a broad range of device applications. Atomic layer deposition (ALD), a CMOS compatible technique, can enable the preparation of high-quality TMD films on 8 to 12 in. wafers for large-scale circuit integration. However, the ALD growth mechanisms are still not fully understood. In this work, we systematically investigated the growth mechanisms for WS₂ and found them to be strongly affected by nucleation density and film thickness. Transmission electron microscope imaging reveals the coexistence and competition of lateral and vertical growth mechanisms at different growth stages, and the critical thicknesses for each mechanism are obtained. The in-plane lateral growth mode dominates when the film thickness remains less than 5.6 nm (8 layers), while the vertical growth mode dominates when the thickness is greater than 20 nm. From the resulting understanding of these growth mechanisms, the conditions for film deposition were optimized and a maximum grain size of 108 nm was achieved. WS₂-based field-effect transistors were fabricated with electron mobility and on/off current ratio up to 3.21 cm² V^-1 s^-1 and 10⁵, respectively. Particularly, this work proves the capability of synthesis of TMD films in a wafer scale with excellent controllability of thickness and morphology, enabling many potential applications other than transistors, such as nanowire- or nanosheet-based supercapacitors, batteries, sensors, and catalysis.

Design and Numerical Investigation of a Lead-Free Inorganic Layered Double Perovskite Cs4CuSb2Cl12 Nanocrystal Solar Cell by SCAPS-1D

In the last decade, perovskite solar cells have made a quantum leap in performance with the efficiency increasing from 3.8% to 25%. However, commercial perovskite solar cells have faced a major impediment due to toxicity and stability issues. Therefore, lead-free inorganic perovskites have been investigated in order to find substitute perovskites which can provide a high efficiency similar to lead-based perovskites. In recent studies, as a kind of lead-free inorganic perovskite material, Cs₄CuSb₂Cl₁₂ has been demonstrated to possess impressive photoelectric properties and excellent environmental stability. Moreover, Cs₄CuSb₂Cl₁₂ nanocrystals have smaller effective photo-generated carrier masses than bulk Cs₄CuSb₂Cl₁₂, which provides excellent carrier mobility. To date, there have been no reports about Cs₄CuSb₂Cl₁₂ nanocrystals used for making solar cells. To explore the potential of Cs₄CuSb₂Cl₁₂ nanocrystal solar cells, we propose a lead-free perovskite solar cell with the configuration of FTO/ETL/Cs₄CuSb₂Cl₁₂ nanocrystals/HTL/Au using a solar cell capacitance simulator. Moreover, we numerically investigate the factors that affect the performance of the Cs₄CuSb₂Cl₁₂ nanocrystal solar cell with the aim of enhancing its performance. By selecting the appropriate hole transport material, electron transport material, thickness of the absorber layer, doping densities, defect density in the absorber, interface defect densities, and working temperature point, we predict that the Cs₄CuSb₂Cl₁₂ nanocrystal solar cell with the FTO/TiO₂/Cs₄CuSb₂Cl₁₂ nanocrystals/Cu₂O/Au structure can attain a power conversion efficiency of 23.07% at 300 K. Our analysis indicates that Cs₄CuSb₂Cl₁₂ nanocrystals have great potential as an absorbing layer towards highly efficient lead-free all-inorganic perovskite solar cells.

Hierarchical Nanoflowers of Colloidal WS2 and Their Potential Gas Sensing Properties for Room Temperature Detection of Ammonia

A one-step colloidal synthesis of hierarchical nanoflowers of WS2 is reported. The nanoflowers were used to fabricate a chemical sensor for the detection of ammonia vapors at room temperature. The gas sensing performance of the WS2 nanoflowers was measured using an in-house custom-made gas chamber. SEM analysis revealed that the nanoflowers were made up of petals and that the nanoflowers self-assembled to form hierarchical structures. Meanwhile, TEM showed the exposed edges of the petals that make up the nanoflower. A band gap of 1.98 eV confirmed a transition from indirect-to-direct band gap as well as a reduction in the number of layers of the WS2 nanoflowers. The formation of WS2 was confirmed by XPS and XRD with traces of the oxide phase, WO3. XPS analysis also confirmed the successful capping of the nanoflowers. The WS2 nanoflowers exhibited a good response and selectivity for ammonia.

Fast Polymeric Functionalization Approach for the Covalent Coating of MoS2 Layers

We present the covalent coating of chemically exfoliated molybdenum disulfide (MoS₂) based on the polymerization of functional acryl molecules. The method relies on the efficient diazonium anchoring reaction to provoke the in situ radical polymerization and covalent adhesion of functional coatings. In particular, we successfully implement hydrophobicity on the exfoliated MoS₂ in a direct, fast, and quantitative synthetic approach. The covalent functionalization is proved by multiple techniques including X-ray photoelectron spectroscopy and TGA-MS. This approach represents a simple and general protocol to reach dense and homogeneous functional coatings on 2D materials.

Colloidal Synthesis and Characterization of Molybdenum Chalcogenide Quantum Dots Using a Two-Source Precursor Pathway for Photovoltaic Applications

The drawbacks of utilizing nonrenewable energy have quickened innovative work on practical sustainable power sources (photovoltaics) because of their provision of a better-preserved decent environment which is free from natural contamination and commotion. Herein, the synthesis, characterization, and application of Mo chalcogenide nanoparticles (NP) as alternative sources in the absorber layer of QDSSCs is discussed. The successful synthesis of the NP was confirmed as the results from the diffractive peaks obtained from XRD which were positive and agreed in comparison with the standard. The diffractive peaks were shown in the planes (100), (002), (100), and (105) for the MoS₂ nanoparticles; (002), (100), (103), and (110) for the MoSe₂ nanoparticles; and (0002), (0004), (103), as well as (0006) for the MoTe₂ nanoparticles. MoSe₂ presented the smallest size of the nanoparticles, followed by MoTe₂ and, lastly, by MoS₂. These results agreed with the results obtained using SEM analysis. For the optical properties of the nanoparticles, UV-Vis and PL were used. The shift of the peaks from the red shift (600 nm) to the blue shift (270-275 nm and 287-289 nm (UV-Vis)) confirmed that the nanoparticles were quantum-confined. The application of the MoX₂ NPs in QDSSCs was performed, with MoSe₂ presenting the greatest PCE of 7.86%, followed by MoTe₂ (6.93%) and, lastly, by MoS₂, with the PCE of 6.05%.

Tuning Valleys and Wave Functions of van der Waals Heterostructures by Varying the Number of Layers: A First-Principles Study

In van der Waals heterostructures of 2D transition-metal dichalcogenides (2D TMDCs) electron and hole states are spatially localized in different layers forming long-lived interlayer excitons. Here, the influence of additional electron or hole layers on the electronic properties of a MoS₂ /WSe₂ heterobilayer (HBL), which is a direct bandgap material, is investigated from first principles. Additional layers modify the interlayer hybridization, mostly affecting the quasiparticle energy and real-space extend of hole states at the Γ and electron states at the Q valleys. For a sufficient number of additional layers, the band edges move from K to Q or Γ, respectively. Adding electron layers to the HBL leads to more delocalized K and Q states, while Γ states do not extend much beyond the HBL, even when more hole layers are added. These results suggest a simple and yet powerful way to tune band edges and the real-space extent of the electron and hole wave functions in TMDC heterostructures, potentially affecting strongly the lifetime and dynamics of interlayer excitons.

Assemble of Bi-doped TiO2 onto 2D MoS2: an efficient p-n heterojunction for photocatalytic H2 generation under visible light

Fabrication of noble-metal-free, efficient and stable hybrid photocatalyst is essential to address the rapidly growing energy crisis and environmental pollution. Here, MoS₂ has been used as the co-catalyst on Bi-doped TiO₂ to form a novel heterostructure to increase the utilization of the photogenerated charge carriers for improving photocatalytic H₂ evolution activity through water reduction. Significantly increased photocatalytic H₂ generation has been achieved on the optimized MoS₂/Bi-TiO₂ nanocomposite (∼512 μmol g^-1) after 4 h of visible light illumination, which is nine times higher than that of the pristine TiO₂ (∼57 μmol g^-1). The measurements of photocurrent, charge transfer resistance and photo-stability of MoS₂/Bi-TiO₂ photoanode imply that charge separation efficiency has been improved in comparison to the pure MoS₂ and TiO₂ photoanodes. Further, the Mott-Schottky study confirmed that a p-n heterojunction has been formed between n-type MoS₂ and p-type Bi-doped TiO₂, which provides a potential gradient to increase charge separation and transfer efficiency. On the basis of these experimental results, this enhanced photocatalytic activity of MoS₂/Bi-TiO₂ heterostructures could be ascribed to the significant visible light absorption and the efficient charge carrier separation. Thus, this work demonstrates the effect of p-n junction for achieving high H₂ evolution activity and photoelectrochemical water oxidation under visible light illumination.

Synthesis and characterisation of thin-film platinum disulfide and platinum sulfide

Group-10 transition metal dichalcogenides (TMDs) are rising in prominence within the highly innovative field of 2D materials. While PtS₂ has been investigated for potential electronic applications, due to its high charge-carrier mobility and strongly layer-dependent bandgap, it has proven to be one of the more difficult TMDs to synthesise. In contrast to most TMDs, Pt has a significantly more stable monosulfide, the non-layered PtS. The existence of two stable platinum sulfides, sometimes within the same sample, has resulted in much confusion between the materials in the literature. Neither of these Pt sulfides have been thoroughly characterised as-of-yet. Here we utilise time-efficient, scalable methods to synthesise high-quality thin films of both Pt sulfides on a variety of substrates. The competing nature of the sulfides and limited thermal stability of these materials is demonstrated. We report peak-fitted X-ray photoelectron spectra, and Raman spectra using a variety of laser wavelengths, for both materials. This systematic characterisation provides a guide to differentiate between the sulfides using relatively simple methods which is essential to enable future work on these interesting materials.

Tailoring the structure of MoS2 using ball-milled MoO3 powders: hexagonal, triangular, and fullerene-like shapes

Single and few-layered MoS₂ materials have attracted attention due to their outstanding physicochemical properties with potential applications in optoelectronics, catalysis, and energy storage. In the past, these materials have been produced using the chemical vapor deposition (CVD) method using MoO₃ films and powders as Mo precursors. In this work, we demonstrate that the size and morphology of few-layered MoS₂ nanostructures can be controlled, modifying the Mo precursor mechanically. We synthesized few-layered MoS₂ materials using MoO₃ powders previously exposed to a high-energy ball milling treatment by the salt-assisted CVD method. The MoO₃ powders milled for 30, 120, and 300 min were used to synthesize sample MoS₂-30, MoS₂-120, and MoS₂-300, respectively. We found morphologies mainly of hexagons (MoS₂-30), triangles (MoS₂-120), and fullerenes (MoS₂-300). The MoS₂ nanostructures and MoO₃ powders were characterized by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, x-ray diffraction, and thermogravimetric analysis. It was found that MoO₃ milled powders exhibit oxygen loss and decrease in crystallite size as milling time increases. Oxygen deficiency in the Mo precursor prevents the growth of large MoS₂ crystals and a large number of milled MoO_3-x  + NaCl promote greater nucleation sites for the formation of MoS₂, achieving a high density of nanoflakes in the 2H and 3R phases, with diameter sizes in the range of ∼30-600 nm with 1-12 layers. Photoluminescence characterization at room temperature revealed a direct bandgap and exciting trends for the different MoS₂ samples. We envisage that our work provides a route for modifying the structure and optical properties for future device design via precursor engineering.

Stacking-Engineered Heterostructures in Transition Metal Dichalcogenides

The layer-by-layer assembly of 2D transition metal dichalcogenide monolayer blocks to form a 3D stack, with a precisely chosen sequence/angle, is the newest development for these materials. In this way, one can create "van der Waals heterostructures (HSs)," opening up a new realm of materials engineering and novel devices with designed functionalities. Herein, a detailed systematic review of transition metal dichalcogenide stacking-engineered heterostructures, from controllable fabrication to typical characterization, and stacking-correlated physical behaviors is presented. Furthermore, recent advances in stacking design, such as stacking sequence, twist angles, and moiré superlattice heterojunctions, are also comprehensively summarized. Finally, the remaining challenges and possible strategies for using stacking engineering to tune the properties of 2D materials are also outlined.

Dimensional engineering of a topological insulating phase in Half-Heusler LiMgAs

We propose a novel technique of dimensional engineering to realize low dimensional topological insulator from a trivial three dimensional parent. This is achieved by confining the bulk system to one dimension along a particular crystal direction, thus enhancing the quantum confinement effects in the system. We investigate this mechanism in the Half-Heusler compound LiMgAs with face-centered cubic (FCC) structure. At ambient conditions the bulk FCC structure exhibits a semi-conducting nature. But, under the influence of high volume expansive pressure (VEP) the system undergoes a topological phase transition (TPT) from semi-conducting to semi-metallic forming a Dirac cone. At a critical VEP we observe that, spin-orbit coupling (SOC) effects introduce a gap of [Formula: see text] 1.5 meV in the Dirac cone at high symmetry point [Formula: see text] in the Brillouin zone. This phase of bulk LiMgAs exhibits a trivial nature characterized by the [Formula: see text] invariants as (0,000). By further performing dimensional engineering, we cleave [111] plane from the bulk FCC structure and confine the system in one dimension. This low-dimensional phase of LiMgAs has structure similar to the two dimensional [Formula: see text] system. Under a relatively lower compressive strain, the low-dimensional system undergoes a TPT and exhibits a non-trivial topological nature characterized by the SOC gap of [Formula: see text] 55 meV and [Formula: see text] invariant [Formula: see text] = 1. Although both, the low-dimensional and bulk phase exhibit edge and surface states, the low-dimensional phase is far more superior and exceptional as compared to the bulk parent in terms of the velocity of Fermions ([Formula: see text]) across the surface states. Such a system has promising applications in nano-electronics.

Nanoengineering Approaches Toward Artificial Nose

Significant scientific efforts have been made to mimic and potentially supersede the mammalian nose using artificial noses based on arrays of individual cross-sensitive gas sensors over the past couple decades. To this end, thousands of research articles have been published regarding the design of gas sensor arrays to function as artificial noses. Nanoengineered materials possessing high surface area for enhanced reaction kinetics and uniquely tunable optical, electronic, and optoelectronic properties have been extensively used as gas sensing materials in single gas sensors and sensor arrays. Therefore, nanoengineered materials address some of the shortcomings in sensitivity and selectivity inherent in microscale and macroscale materials for chemical sensors. In this article, the fundamental gas sensing mechanisms are briefly reviewed for each material class and sensing modality (electrical, optical, optoelectronic), followed by a survey and review of the various strategies for engineering or functionalizing these nanomaterials to improve their gas sensing selectivity, sensitivity and other measures of gas sensing performance. Specifically, one major focus of this review is on nanoscale materials and nanoengineering approaches for semiconducting metal oxides, transition metal dichalcogenides, carbonaceous nanomaterials, conducting polymers, and others as used in single gas sensors or sensor arrays for electrical sensing modality. Additionally, this review discusses the various nano-enabled techniques and materials of optical gas detection modality, including photonic crystals, surface plasmonic sensing, and nanoscale waveguides. Strategies for improving or tuning the sensitivity and selectivity of materials toward different gases are given priority due to the importance of having cross-sensitivity and selectivity toward various analytes in designing an effective artificial nose. Furthermore, optoelectrical sensing, which has to date not served as a common sensing modality, is also reviewed to highlight potential research directions. We close with some perspective on the future development of artificial noses which utilize optical and electrical sensing modalities, with additional focus on the less researched optoelectronic sensing modality.

Exploring the N2 Adsorption and Activation Mechanisms over the 2H/1T Mixed-Phase Ultrathin Mo1-xWxS2 Nanosheets for Boosting N2 Photosynthesis

Solar-driven conversion of nitrogen (N₂) to ammonia (NH₃) is highly appealing, yet in its infancy, the low photocatalytic efficiency and unclear adsorption and activation mechanisms of N₂ are still issues to be addressed. In this study, ultrathin alloyed Mo_1-xW_xS₂ nanosheets with tunable hexagonal (2H)/trigonal (1T) phase ratios were proposed to boost photoreduction N₂ efficiency, while the mechanisms of N₂ adsorption and activation were explored simultaneously. The alloyed Mo_1-xW_xS₂ nanosheets for the 1T phase concentration of 33.6% and Mo/W = 0.68:0.32 were proven to reach about 111 μmol g_cat^-1 h^-1 under visible light, which is 3.7 (or 3)-fold higher than that of pristine MoS₂ (or WS₂). With the aid of density functional theory calculations and in situ N₂ adsorption X-ray absorption near-edge fine structure techniques, the adsorption and activation behaviors of N₂ over the interface of Mo_1-xW_xS₂ nanosheets were investigated during the N₂ reduction process. The results show that the W doping causes a higher electron density state in W 5d orbitals, which can further polarize the adsorbed N₂ molecules for adsorption and activation. This work provides a new insight into the adsorption and activation mechanisms for the NH₃ synthesis.

Graphene/MoS2 Nanohybrid for Biosensors

Graphene has been studied a lot in different scientific fields because of its unique properties, including its superior conductivity, plasmonic property, and biocompatibility. More recently, transition metal dicharcogenide (TMD) nanomaterials, beyond graphene, have been widely researched due to their exceptional properties. Among the various TMD nanomaterials, molybdenum disulfide (MoS2) has attracted attention in biological fields due to its excellent biocompatibility and simple steps for synthesis. Accordingly, graphene and MoS2 have been widely studied to be applied in the development of biosensors. Moreover, nanohybrid materials developed by hybridization of graphene and MoS2 have a huge potential for developing various types of outstanding biosensors, like electrochemical-, optical-, or surface-enhanced Raman spectroscopy (SERS)-based biosensors. In this review, we will focus on materials such as graphene and MoS2. Next, their application will be discussed with regard to the development of highly sensitive biosensors based on graphene, MoS2, and nanohybrid materials composed of graphene and MoS2. In conclusion, this review will provide interdisciplinary knowledge about graphene/MoS2 nanohybrids to be applied to the biomedical field, particularly biosensors.

Transferability of Molecular Potentials for 2D Molybdenum Disulphide

An ability of different molecular potentials to reproduce the properties of 2D molybdenum disulphide polymorphs is examined. Structural and mechanical properties, as well as phonon dispersion of the 1H, 1T and 1T’ single-layer MoS2 (SL MoS2) phases, were obtained using density functional theory (DFT) and molecular statics calculations (MS) with Stillinger-Weber, REBO, SNAP and ReaxFF interatomic potentials. Quantitative systematic comparison and discussion of the results obtained are reported.

Atomic-scale evidence for highly selective electrocatalytic N-N coupling on metallic MoS2

Molybdenum sulfide (MoS₂) is the most studied two-dimensional (2D) material bar graphene. Current research on crystal-phase engineering focuses almost exclusively on the improvement of catalytic activity. However, the potential advantages of phase engineering toward regulation of selectivity control during multistep catalytic processes remain unexplored. Here, we report atomic-scale evidence on how metallic MoS₂ shows significantly higher selectivity compared to the semiconducting phase during multielectron reduction of nitrite to nitrous oxide. Namely, a reaction intermediate specific to metallic MoS₂ increases the selectivity by decoupling the proton and electron transfer steps. This has previously been shown to be a universal mechanism to enhance selectivity, and therefore, our work opens directions of the application of 2D materials toward selective electrocatalysis.

Molybdenum sulfide (MoS₂) is the most widely studied transition-metal dichalcogenide (TMDs) and phase engineering can markedly improve its electrocatalytic activity. However, the selectivity toward desired products remains poorly explored, limiting its application in complex chemical reactions. Here we report how phase engineering of MoS₂ significantly improves the selectivity for nitrite reduction to nitrous oxide, a critical process in biological denitrification, using continuous-wave and pulsed electron paramagnetic resonance spectroscopy. We reveal that metallic 1T-MoS₂ has a protonation site with a pK_a of ∼5.5, where the proton is located ∼3.26 Å from redox-active Mo site. This protonation site is unique to 1T-MoS₂ and induces sequential proton−electron transfer which inhibits ammonium formation while promoting nitrous oxide production, as confirmed by the pH-dependent selectivity and deuterium kinetic isotope effect. This is atomic-scale evidence of phase-dependent selectivity on MoS₂, expanding the application of TMDs to selective electrocatalysis.

Synthesis of MoS2 Thin Film by Ionized Jet Deposition: Role of Substrate and Working Parameters

The lack of scalable synthesis of transition metal dichalcogenides, such as molybdenum disulfide (MoS2), has proved to be a significant bottleneck in realization of fundamental devices and has hindered the commercialization of these materials in technologically relevant applications. In this study, a cost-efficient and versatile thin-film fabrication technique based on ionized jet deposition (IJD), i.e., a technique potentially providing high processing efficiency and scalability, is used to grow MoS2 thin films on silicon substrates. The operating conditions of IJD were found to influence mainly the ablation efficiency of the target and only slightly the quality of the deposited MoS2 thin film. All as-deposited films show chemical properties typical of MoS2 with an excess of free, elemental sulfur that can be removed by post-deposition annealing at 300–400 °C, which also promotes MoS2 crystallization. The formation of an interface comprised of several silicon oxide species was observed between MoS2 and the silicon substrate, which is suggested to originate from etching and oxidizing processes of dissociated water molecules in the vacuum chamber during growth. The present study paves the way to further design and improve the IJD approach for TMDC-based devices and other relevant technological applications.

Effect of temperature and exfoliation time on the properties of chemically exfoliated MoS2 nanosheets

A systematic investigation of the experimental conditions for the chemical exfoliation of MoS₂ using n-butyllithium as intercalating agent has been carried out to unravel the effect of reaction time and temperature for maximizing the percentage of monolayer thick-flakes and achieve a control over the content of metallic 1T vs. semiconductive 2H phases, thereby tuning the electrical properties of ultrathin MoS₂ few-layer thick films.

Prediction of topological nontrivial semimetals and pressure-induced Lifshitz transition in 1T'-MoS2 layered bulk polytypes

Recently, bulk MoS2 crystals stacked by 1T'-MoS2 monolayers have been synthesized successfully, but little is known about their stacking sequences and topological properties. Based on first-principles calculations and symmetry-based indicator theory, we discovered that three predicted bulk structures of MoS2 (named 2M-, 1T'- and β-MoS2) stacked by 1T' monolayers are topological insulators and nodal line semimetals with and without spin-orbit coupling. Their stacking stability, electronic structure and the topology origin were systematically investigated. Further research proves that in the absence of SOC the open- and closed-type nodal lines can coexist in the momentum space of 2M-MoS2, which also possesses drumhead-like surface state. Moreover, we predicted a pressure-induced Lifshitz transition at about 1.3 GPa in 2M-MoS2. Our findings greatly enrich the topological phases of MoS2 and probably bring MoS2 to the rapidly growing family of layered topological semimetals.

Application of a 2D Molybdenum Telluride in SERS Detection of Biorelevant Molecules

Two-dimensional (2D) transition-metal dichalcogenides have become promising candidates for surface-enhanced Raman spectroscopy (SERS), but currently very few examples of detection of relevant molecules are available. Herein, we show the detection of the lipophilic disease marker β-sitosterol on few-layered MoTe₂ films. The chemical vapor deposition (CVD)-grown films are capable of nanomolar detection, exceeding the performance of alternative noble-metal surfaces. We confirm that the enhancement occurs through the chemical enhancement (CE) mechanism via formation of a surface-analyte complex, which leads to an enhancement factor of ≈10⁴, as confirmed by Fourier transform infrared (FTIR), UV-vis, and cyclic voltammetry (CV) analyses and density functional theory (DFT) calculations. Low values of signal deviation over a seven-layered MoTe₂ film confirms the homogeneity and reproducibility of the results in comparison to noble-metal substrate analogues. Furthermore, β-sitosterol detection within cell culture media, a minimal loss of signal over 50 days, and the opportunity for sensor regeneration suggest that MoTe₂ can become a promising new SERS platform for biosensing.

Graphene to Advanced MoS2: A Review of Structure, Synthesis, and Optoelectronic Device Application

In contrast to zero-dimensional (0D), one-dimensional (1D), and even their bulk equivalents, in two-dimensional (2D) layered materials, charge carriers are confined across thickness and are empowered to move across the planes. The features of 2D structures, such as quantum confinement, high absorption coefficient, high surface-to-volume ratio, and tunable bandgap, make them an encouraging contestant in various fields such as electronics, energy storage, catalysis, etc. In this review, we provide a gentle introduction to the 2D family, then a brief description of transition metal dichalcogenides (TMDCs), mainly focusing on MoS2, followed by the crystal structure and synthesis of MoS2, and finally wet chemistry methods. Later on, applications of MoS2 in dye-sensitized, organic, and perovskite solar cells are discussed. MoS2 has impressive optoelectronic properties; due to the fact of its tunable work function, it can be used as a transport layer, buffer layer, and as an absorber layer in heterojunction solar cells. A power conversion efficiency (PCE) of 8.40% as an absorber and 13.3% as carrier transfer layer have been reported for MoS2-based organic and perovskite solar cells, respectively. Moreover, MoS2 is a potential replacement for the platinum counter electrode in dye-sensitized solar cells with a PCE of 7.50%. This review also highlights the incorporation of MoS2 in silicon-based heterostructures where graphene/MoS2/n-Si-based heterojunction solar cell devices exhibit a PCE of 11.1%.

The Properties and Preparation Methods of Different Boron Nitride Nanostructures and Applications of Related Nanocomposites

Due to special non-metallic polar bond between the III group (with certain metallic properties) element boron (B) and the V group element nitrogen (N), boron nitride (BN) has unique physical and chemical properties such as strong high-temperature resistance, oxidation resistance, heat conduction, electrical insulation and neutron absorption. Its unique lamellar, reticular and tubular morphologies and physicochemical properties make it attractive in the fields of adsorption, catalysis, hydrogen storage, thermal conduction, insulation, dielectric substrate of electronic devices, radiation protection, polymer composites, medicine, etc. Therefore, the synthesis and properties of BN derived materials become the main research hotspots of low-dimensional nanomaterials. This paper reviews the synthetic methods, overall properties, and applications of BN nanostructures and nanocomposites. In addition, challenges and prospect of this kind of materials are discussed.

Oxygen Reduction Reaction in the Field of Water Environment for Application of Nanomaterials

Water pollution has caused the ecosystem to be in a state of imbalance for a long time. It has become a major global ecological and environmental problem today. Solving the potential hidden dangers of pollutants and avoiding unauthorized access to resources has become the necessary condition and important task to ensure the sustainable development of human society. To solve such problems, this review summarizes the research progress of nanomaterials in the field of water aimed at the treatment of water pollution and the development and utilization of new energy. The paper also tries to seek scientific solutions to environmental degradation and to create better living environmental conditions from previously published cutting edge research. The main content in this review article includes four parts: advanced oxidation, catalytic adsorption, hydrogen, and oxygen production. Among a host of other things, this paper also summarizes the various ways by which composite nanomaterials have been combined for enhancing catalytic efficiency, reducing energy consumption, recycling, and ability to expand their scope of application. Hence, this paper provides a clear roadmap on the status, success, problems, and the way forward for future studies.

Monomolecular covalent honeycomb nanosheets produced by surface-mediated polycondensation between 1,3,5-triamino benzene and benzene-1,3,5-tricarbox aldehyde on Au(111)

Fabrication of a two-dimensional covalent network of honeycomb nanosheets comprising small 1,3,5-triamino benzene and benzene-1,3,5-tricarboxaldehyde aromatic building blocks was conducted on Au(111) in a pH-controlled aqueous solution. In situ scanning tunneling microscopy revealed a large defect-free and homogeneous honeycomb π-conjugated nanosheet at the Au(111)/liquid interface. An electrochemical potential dependence indicated that the nanosheets were the result of thermodynamic self-assembly based not only on the reaction equilibrium but also on the adsorption (partition) equilibrium, which was controlled by the building block surface coverage as a function of electrode potential.

Crossing interfacial conduction in nanometer-sized graphitic carbon layers

Graphitic carbon layers (GCLs), exemplified by graphene, have been proposed for potential application in high-performance functional devices due to superior electrical properties, e.g., high electron mobility. In state-of-the-art electronics, it is required that GCLs are miniaturized to nanometer scales and incorporated into the integrated circuits to exhibit novel functions at nanometer scales. However, the implementation of nanometer-scale GCLs is suspended; the function in devices is deteriorated by increasing contact resistance in miniaturized GCL/electrode interfaces. In this study, nanometer-sized GCL/gold (Au) interfaces were fabricated via atomistic visualization of nanomanipulation, and simultaneously their contact resistance was measured. We showed that the contact resistivity of the interfaces was decreased to the order of 10^-10Ω cm², which was 10⁴ times smaller than that of micrometer-sized or larger GCL/metal interfaces. In addition, it was revealed that peculiar electrical conduction at the nanometer-sized GCL/Au interfaces emerged; current flows throughout the entire area of the interfaces unlike micrometer-sized or larger GCL/metal interfaces. These results directly contribute to actual application of GCLs in advanced nanodevices.

Assessing the role of plasma-engineered acceptor-like intra- and inter-grain boundaries of heterogeneous WS2-WO3 nanosheets for photocurrent characteristics

High-temperature annealing in tungsten disulfide resulted in heterogeneous WS₂-WO₃ in which intra- (within WS₂ and WO₃) and inter- (between WS₂ and WO₃) grain boundaries were observed, which were highly critical for charge transport and recombination. The heterogeneous WS₂-WO₃ phase was evidenced by observing the coexistence of d-spacing values of 0.26 nm (WS₂) and 0.37 nm (WO₃) in transmission electron microscopic (TEM) studies. Further systematic high-resolution TEM studies elucidated that intra-grain boundaries separated crystallites within WS₂ and WO₃, while inter-grain boundaries separated WS₂ from WO₃. As WS₂ and WO₃ are both n-type, these defects are acceptor-like in the grain boundaries and they actively participate in the capture (trapping) process, which impedes charge transport characteristics in the heterogeneous WS₂-WO₃ films. Plasma treatment in the heterogeneous WS₂-WO₃ film, for 60 minutes using argon, energetically modulated the defects in the intra/inter-grain boundaries, as evidenced from detailed comparative photocurrent characteristics obtained individually in (i) pristine WS₂, (ii) heterogeneous WS₂-WO₃ and (iii) Ar plasma-treated heterogeneous WS₂-WO₃ films under blue and green lasers, along with AM1.5 (1 sun) illumination. Detrimental roles (trapping/de-trapping and scattering) of grain boundary states on photoelectrons were seen to be significantly suppressed under the influence of plasma.

Cu Nano-Roses Self-Assembly from Allium cepa, L., Pyrolysis by Green Synthesis of C Nanostructures

Carbon nanostructures are achieved by bio-waste Allium cepa, L., (onion vulgaris) peels through pyrolysis at 900 °C. They contain dispersed elements derived by their bio-precursors, like Mg, Ca, S, Na, K, and Cu. Here, we report the self-assembly of new Cu flower-shaped nanostructures organized as nano-roses. Remarkably, the nano-roses show rolled-up petals of Cu0 with a high chemical stability in air, exhibiting an intrinsic pure Cu crystalline phase. This suggests the exceptional potentiality to synthesize Cu0 nanostructures with novel physical/chemical properties. The size, morphology, and chemical composition were obtained by a combination of high-resolution scanning electron microscopy, energy dispersive X-ray spectroscopy, energy dispersive X-ray diffraction, and Raman spectroscopy.

Gas-Phase Formation of Highly Luminescent 2D GaSe Nanoparticle Ensembles in a Nonequilibrium Laser Ablation Process

Interest in layered two-dimensional (2D) materials has been escalating rapidly over the past few decades due to their promising optoelectronic and photonic properties emerging from their atomically thin 2D structural confinements. When these 2D materials are further confined in lateral dimensions toward zero-dimensional (0D) structures, 2D nanoparticles and quantum dots with new properties can be formed. Here, we report a nonequilibrium gas-phase synthesis method for the stoichiometric formation of gallium selenide (GaSe) nanoparticles ensembles that can potentially serve as quantum dots. We show that the laser ablation of a target in an argon background gas condenses the laser-generated plume, resulting in the formation of metastable nanoparticles in the gas phase. The deposition of these nanoparticles onto the substrate results in the formation of nanoparticle ensembles, which are then post-processed to crystallize or sinter the nanoparticles. The effects of background gas pressures, in addition to crystallization/sintering temperatures, are systematically studied. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), photoluminescence (PL) spectroscopy, and time-correlated single-photon counting (TCSPC) measurements are used to study the correlations between growth parameters, morphology, and optical properties of the fabricated 2D nanoparticle ensembles.

Defect Engineering of Two-Dimensional Molybdenum Disulfide

Two‐dimensional (2D) molybdenum disulfide (MoS₂) holds great promise in electronic and optoelectronic applications owing to its unique structure and intriguing properties. The intrinsic defects such as sulfur vacancies (SVs) of MoS₂ nanosheets are found to be detrimental to the device efficiency. To mitigate this problem, functionalization of 2D MoS₂ using thiols has emerged as one of the key strategies for engineering defects. Herein, we demonstrate an approach to controllably engineer the SVs of chemically exfoliated MoS₂ nanosheets using a series of substituted thiophenols in solution. The degree of functionalization can be tuned by varying the electron‐withdrawing strength of substituents in thiophenols. We find that the intensity of 2LA(M) peak normalized to A_1g peak strongly correlates to the degree of functionalization. Our results provide a spectroscopic indicator to monitor and quantify the defect engineering process. This method of MoS₂ defect functionalization in solution also benefits the further exploration of defect‐free MoS₂ for a wide range of applications.

Environmental Analysis with 2D Transition-Metal Dichalcogenide-Based Field-Effect Transistors

Field-effect transistors (FETs) present highly sensitive, rapid, and in situ detection capability in chemical and biological analysis. Recently, two-dimensional (2D) transition-metal dichalcogenides (TMDCs) attract significant attention as FET channel due to their unique structures and outstanding properties. With the booming of studies on TMDC FETs, we aim to give a timely review on TMDC-based FET sensors for environmental analysis in different media. First, theoretical basics on TMDC and FET sensor are introduced. Then, recent advances of TMDC FET sensor for pollutant detection in gaseous and aqueous media are, respectively, discussed. At last, future perspectives and challenges in practical application and commercialization are given for TMDC FET sensors. This article provides an overview on TMDC sensors for a wide variety of analytes with an emphasize on the increasing demand of advanced sensing technologies in environmental analysis.

Study on the Growth Parameters and the Electrical and Optical Behaviors of 2D Tungsten Disulfide

Transition-metal dichalcogenides (TMDCs) with atomic thickness are promising materials for next-generation electronic and optoelectronic devices. Herein, we report uniform growth of triangular-shaped (∼40 μm) monolayer WS₂ using the atmospheric-pressure chemical vapor deposition (APCVD) technique in a hydrogen-free environment. We have studied the optical and electrical behaviors of as-grown WS₂ samples. The absorption spectrum of monolayer WS₂ shows two intense excitonic absorption peaks, namely, A (∼630 nm) and B (∼530 nm), due to the direct gap transitions at the K point. Photoluminescence (PL) and fluorescence studies reveal that under the exposure of green light, monolayer WS₂ gives very strong red emission at ∼663 nm. This corresponds to the direct band gap and strong excitonic effect in monolayer WS₂. Furthermore, the efficacy of the synthesized WS₂ crystals for electronic devices is also checked by fabricating field-effect transistors (FETs). FET devices exhibit an electron mobility of μ ∼ 6 cm² V^-1 s^-1, current ON/OFF ratio of ∼10⁶, and subthreshold swing (SS) of ∼641 mV decade^-1, which are comparable to those of the exfoliated monolayer WS₂ FETs. These findings suggest that our APCVD-grown WS₂ has the potential to be used for next-generation nanoelectronic and optoelectronic applications.

Influence of the Substrate on the Optical and Photo-electrochemical Properties of Monolayer MoS2

Substrates influence the electrical and optical properties of monolayer (ML) MoS₂ in field-effect transistors and photodetectors. Photoluminescence (PL) and Raman spectroscopy measurements have shown that conducting substrates can vary the doping concentration and influence exciton decay channels in ML-MoS₂. Doping and exciton decay dynamics are expected to play a major role in the efficiency of light-driven chemical reactions, but it is unclear to what extent these factors contribute to the photo(electro)catalytic properties of ML-MoS₂. Here, we report spatially resolved PL, Raman, and photo-electrochemical current measurements of 5-10 μm-wide ML-MoS₂ triangles deposited on pairs of indium-doped tin oxide (ITO) electrodes that are separated by a narrow insulating quartz channel [i.e., an ITO interdigitated array (IDA) electrode]. Optical microscopy images and atomic force microscopy measurements revealed that the ML-MoS₂ triangles lie conformally on the quartz and ITO substrates. The PL spectrum of MoS₂ shifts and decreases in intensity in the ITO region, which can be attributed to differences in nonradiative and radiative exciton decay channels. Raman spectra showed no significant peak shifts on the two substrates that would have indicated a substrate-induced doping effect. We spatially resolved the photo-electrochemical current because of hole-induced iodide oxidation and observed that ML-MoS₂ produces lower photocurrents in the quartz region than in the ITO region. The correlated PL, Raman, and photocurrent mapping data show that the MoS₂/quartz interface diminishes fast nonradiative exciton decay pathways but the photocurrent response in the quartz region is likely limited by inefficient in-plane carrier transport to the ITO electrode because of carrier recombination with S vacancies in MoS₂ or charged impurities in the quartz substrate. Nonetheless, the experimental methodology presented herein provides a framework to evaluate substrate engineering strategies to tune the (photo)electrocatalytic properties of two-dimensional materials.

Controllable Synthesis of Two-Dimensional Molybdenum Disulfide (MoS2 ) for Energy-Storage Applications

Lamellar molybdenum disulfide (MoS₂ ) has attracted a wide range of research interests in recent years because of its two-dimensional layered structure, ultrathin thickness, large interlayer distance, adjustable band gap, and capability to form different crystal structures. These special characteristics and high anisotropy have made MoS₂ widely applicable in energy storage and harvesting. In this Minireview, a systematic and comprehensive introduction to MoS₂ , as well as its composites, is presented. It is aimed to summarize the various synthetic methods of MoS₂ -based composites and their application in energy-storage devices (lithium-ion batteries, sodium-ion batteries, lithium-sulfur batteries, and supercapacitors) in detail. Based on recent studies, this Minireview provides important and comprehensive guidelines for further study and development efforts in the MoS₂ in energy-storage field.

Radical enhancement of molecular thermoelectric efficiency

There is a worldwide race to find materials with high thermoelectric efficiency to convert waste heat to useful energy in consumer electronics and server farms. Here, we propose a radically new method to enhance simultaneously the electrical conductance and thermopower and suppress heat transport through ultra-thin materials formed by single radical molecules. This leads to a significant enhancement of room temperature thermoelectric efficiency. The proposed strategy utilises the formation of transport resonances due to singly occupied spin orbitals in radical molecules. This enhances the electrical conductance by a couple of orders of magnitude in molecular junctions formed by nitroxide radicals compared to the non-radical counterpart. It also increases the Seebeck coefficient to high values of 200 μV K^-1. Consequently, the power factor increases by more than two orders of magnitude. In addition, the asymmetry and destructive phonon interference that was induced by the stable organic radical side group significantly decreases the phonon thermal conductance. The enhanced power factor and suppressed thermal conductance in the nitroxide radical lead to the significant enhancement of room temperature ZT to values ca. 0.8. Our result confirms the great potential of stable organic radicals to form ultra-thin film thermoelectric materials with unprecedented thermoelectric efficiency.

Applications of Two-Dimensional Nanomaterials in Breast Cancer Theranostics

Breast cancer is the leading cause of cancer-related mortality among women. Early stage diagnosis and treatment of this cancer are crucial to patients' survival. In addition, it is important to avoid severe side effects during the process of conventional treatments (surgery, chemotherapy, hormonal therapy, and targeted therapy) and increase the patients' quality of life. Over the past decade, nanomaterials of all kinds have shown excellent prospects in different aspects of oncology. Among them, two-dimensional (2D) nanomaterials are unique due to their physical and chemical properties. The functional variability of 2D nanomaterials stems from their large specific surface area as well as the diversity of composition, electronic configurations, interlayer forces, surface functionalities, and charges. In this review, the current status of 2D nanomaterials in breast cancer diagnosis and therapy is reviewed. In this respect, sensing of the tumor biomarkers, imaging, therapy, and theranostics are discussed. The ever-growing 2D nanomaterials are building blocks for the development of a myriad of nanotheranostics. Accordingly, there is the possibility to explore yet novel properties, biological effects, and oncological applications.

Direct Growth of Continuous and Uniform MoS2 Film on SiO2/Si Substrate Catalyzed by Sodium Sulfate

Because of its unique electronic band structure, molybdenum disulfide (MoS₂) has been regarded as a star semiconducting material. However, direct growth of continuous and high-quality MoS₂ films on SiO₂/Si substrates is still very challenging. Here, we report a facile chemical vapor deposition (CVD) method based on synergistic modulation of precursor and Na₂SO₄ catalysis, realizing the centimeter scale growth of a continuous MoS₂ film on SiO₂/Si substrates. The as-grown MoS₂ film had an excellent spatial homogeneity and crystal quality, with an edge length of the composite domain as large as 632 μm. Both experimental and theoretical results proved that Na tended to bond with SiO₂ substrates rather than to interfere with as-grown MoS₂. Thus, they showed decent and uniform electrical performance, with electron mobilities as high as 5.9 cm² V^-1 s^-1_. We believe our method will pave a new way for MoS₂ toward real application in modern electronics.

Solution-Based Synthesis of Few-Layer WS2 Large Area Continuous Films for Electronic Applications

Unlike MoS2 ultra-thin films, where solution-based single source precursor synthesis for electronic applications has been widely studied, growing uniform and large area few-layer WS2 films using this approach has been more challenging. Here, we report a method for growth of few-layer WS2 that results in continuous and uniform films over centimetre scale. The method is based on the thermolysis of spin coated ammonium tetrathiotungstate ((NH4)2WS4) films by two-step high temperature annealing without additional sulphurization. This facile and scalable growth method solves previously encountered film uniformity issues. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) were used to confirm the few-layer nature of WS2 films. Raman and X-Ray photoelectron spectroscopy (XPS) revealed that the synthesized few-layer WS2 films are highly crystalline and stoichiometric. Finally, WS2 films as-deposited on SiO2/Si substrates were used to fabricate a backgated Field Effect Transistor (FET) device for the first time using this precursor to demonstrate the electronic functionality of the material and further validate the method.

2D material broadband photodetectors

Our review provides a comprehensive overview of the latest evolution of broadband photodetectors (BBPDs) based on 2D materials (2DMs). We begin with BBPDs built on various 2DM channels, including narrow-bandgap 2DMs, 2D topological semimetals, 2D charge density wave compounds, and 2D heterojunctions. Then, we introduce defect-engineered 2DM BBPDs, including vacancy engineering, heteroatom incorporation, and interfacial engineering. Subsequently, we summarize 2DM based mixed-dimensional (0D-2D, 1D-2D, 2D-3D, and 0D-2D-3D) BBPDs. Finally, we provide several viewpoints for the future development of this burgeoning field.