Graphene-Like Two-Dimensional Materials

Electric-Field Control in Phosphorene-Based Heterostructures

Phosphorene is a graphene-like material with an intermediate band gap, in contrast to zero-gap graphene and large-gap dichalcogenides or hexagonal boron nitride (hBN), which makes it more suitable for nanoelectronic devices. However, inducing band-gap modulation in freestanding phosphorene nanoribbons (PNRs) is problematic, as high in-plane electric fields are necessary to close the gap. We perform here a detailed investigation concerning the substrate influence on the electric-field control exerted by an external gate, using the density functional theory-non-equilibrium Green's functions (DFT-NEGF) framework. It is established that the interaction with a hexagonal boron nitride supporting layer significantly enhances the gap modulation. Furthermore, we address the issue of contacting the PNRs, by using conducting graphene nanoribbons embedded in the support hBN layer. Within this setup, a measurable spin polarization is achieved owing to the anti-ferromagnetic coupling between the edges of the graphene nanoribbons.

Menezes L, Tittl A, Ren H, Maier SA. Optical Metasurfaces for Energy Conversion

Nanostructured surfaces with designed optical functionalities, such as metasurfaces, allow efficient harvesting of light at the nanoscale, enhancing light-matter interactions for a wide variety of material combinations. Exploiting light-driven matter excitations in these artificial materials opens up a new dimension in the conversion and management of energy at the nanoscale. In this review, we outline the impact, opportunities, applications, and challenges of optical metasurfaces in converting the energy of incoming photons into frequency-shifted photons, phonons, and energetic charge carriers. A myriad of opportunities await for the utilization of the converted energy. Here we cover the most pertinent aspects from a fundamental nanoscopic viewpoint all the way to applications.

2D electrene LaH2monolayer: an ideal ferrovalley direct semiconductor with room-temperature ferromagnetic stability

In developing nonvolatile valleytronic devices, ferromagnetic (FM) ferrovalley semiconductors are critically needed due to the existence of spontaneous valley polarization. At present, however, the known real materials have various drawbacks towards practical applications, including the in-plane FM ground state, low Curie temperature (TC), small valley polarization, narrow energy window with clean polarized valley, and indirect bandgap. From first-principles calculations, here we predict anideal ferrovalley semiconductor, honeycomb LaH₂monolayer (ML), whose intrinsic properties can overcome all these shortcomings. We demonstrate that LaH₂ML, having satisfied structural stability, is a FM half-semiconducting electrene (La3+2H-⋅e-) with its magnetic moments localized at the lattice interstitial sites rather than La atoms. At the same time, LaH₂ML holds the following desired attributes: a robust out-of-plane FM ground state with a highTC(334 K), a sizable valley polarization (166 meV), a wide energy window (137 meV) harboring clean single-valley carriers, and a direct bandgap. These results identify a much needed ideal ferrovalley semiconductor candidate, holding the promising application potential in valleytronics and spintronics devices.

Temperature dependent studies on centimeter-scale MoS2and vdW heterostructures

Transition metal dichalcogenides is an emerging 2D semiconducting material group which has excellent physical properties in the ultimately scaled thickness dimension. Specifically, van der Waals heterostructures hold the great promise in further advancing both the fundamental scientific knowledge and practical technological applications of 2D materials. Although 2D materials have been extensively studied for various sensing applications, temperature sensing still remains relatively unexplored. In this work, we experimentally study the temperature-dependent Raman spectroscopy and electrical conductivity of molybdenum disulfide (MoS₂) and its heterostructures with platinum dichalcogenides (PtSe₂and PtTe₂) to explore their potential to become the next-generation temperature sensor. It is found that the MoS₂-PtX₂heterostructure shows the great promise as the high-sensitivity temperature sensor.

Exfoliation of MoS2 Quantum Dots: Recent Progress and Challenges

Although, quantum dots (QDs) of two-dimensional (2D) molybdenum disulfide (MoS2) have shown great potential for various applications, such as sensing, catalysis, energy storage, and electronics. However, the lack of a simple, scalable, and inexpensive fabrication method for QDs is still a challenge. To overcome this challenge, a lot of attention has been given to the fabrication of QDs, and several fabrication strategies have been established. These exfoliation processes are mainly divided into two categories, the 'top-down' and 'bottom-up' methods. In this review, we have discussed different top-down exfoliation methods used for the fabrication of MoS2 QDs and the advantages and limitations of these methods. A detailed description of the various properties of QDs is also presented.

Surface Passivation of Layered MoSe2 via van der Waals Stacking of Amorphous Hydrocarbon

Development of efficient surface passivation methods for semiconductor devices is crucial to counter the degradation in their electrical performance owing to scattering or trapping of carriers in the channels induced by molecular adsorption from the ambient environment. However, conventional dielectric deposition involves the formation of additional interfacial defects associated with broken covalent bonds, resulting in accidental electrostatic doping or enhanced hysteretic behavior. In this study, centimeter-scaled van der Waals passivation of transition metal dichalcogenides (TMDCs) is demonstrated by stacking hydrocarbon (HC) dielectrics onto MoSe₂ field-effect transistors (FETs), thereby enhancing the electric performance and stability of the device, accompanied with the suppression of chemical disorder at the HC/TMDCs interface. The stacking of HC onto MoSe₂ FETs enhances the carrier mobility of MoSe₂ FET by over 50% at the n-branch, and a significant decrease in hysteresis, owing to the screening of molecular adsorption. The electron mobility and hysteresis of the HC/MoSe₂ FETs are verified to be nearly intact compared to those of the fabricated HC/MoSe₂ FETs after exposure to ambient environment for 3 months. Consequently, the proposed design can act as a model for developing advanced nanoelectronics applications based on layered materials for mass production.

MoSe2 -VSe2 -NbSe2 Ternary Alloy Nanosheets to Boost Electrocatalytic Hydrogen Evolution Reaction

Alloying of transition metal dichalcogenides (TMDs) is a pioneering method for engineering electronic structures with expanded applications. In this study, MoSe₂ -VSe₂ -NbSe₂ ternary alloy nanosheets are synthesized via a colloidal reaction. The composition is successfully tuned over a wide range to adjust the 2H-1T phase transition. The alloy nanosheets consist of miscible atomic structures at all compositions, which is distinct from immiscible binary alloys. Compared to each binary alloy, the ternary alloys display higher electrocatalytic activity toward the hydrogen evolution reaction (HER) in an acidic electrolyte. The HER performance exhibits a volcano-type composition dependence, which is correlated with the experimental d-band center (ε_d ). Spin-polarized density functional theory (DFT) calculations consistently predict the homogenous atomic distributions. The Gibbs free energy of H adsorption (ΔG_H* ) and the activation barrier (E_a ) support that miscible ternary alloying greatly enhances the HER performance.

Interfacial Coupling and Modulation of van der Waals Heterostructures for Nanodevices

In recent years, van der Waals heterostructures (vdWHs) of two-dimensional (2D) materials have attracted extensive research interest. By stacking various 2D materials together to form vdWHs, it is interesting to see that new and fascinating properties are formed beyond single 2D materials; thus, 2D heterostructures-based nanodevices, especially for potential optoelectronic applications, were successfully constructed in the past few decades. With the dramatically increased demand for well-controlled heterostructures for nanodevices with desired performance in recent years, various interfacial modulation methods have been carried out to regulate the interfacial coupling of such heterostructures. Here, the research progress in the study of interfacial coupling of vdWHs (investigated by Photoluminescence, Raman, and Pump-probe spectroscopies as well as other techniques), the modulation of interfacial coupling by applying various external fields (including electrical, optical, mechanical fields), as well as the related applications for future electrics and optoelectronics, have been briefly reviewed. By summarizing the recent progress, discussing the recent advances, and looking forward to future trends and existing challenges, this review is aimed at providing an overall picture of the importance of interfacial modulation in vdWHs for possible strategies to optimize the device's performance.

MoS2 nanosheet induced destructive alterations in the Escherichia coli bacterial membrane

Two dimensional molybdenum disulfide (MoS₂) nanosheets have recently gained wide recognition for their efficient broad-spectrum antibacterial activity complemented with great biocompatibility and minimal bacterial resistance inducing capabilities. However, despite the numerous investigations, the molecular level interactions at the nano-bio interface responsible for their bactericidal activity remain obscure. Herein, through an atomistic molecular dynamics study, we attempt to seek an in-depth understanding of the atomic level details of the underlying mechanism of their antibacterial action against the Escherichia coli (E. coli) bacterial membrane. Our study reveals a two-step MoS₂ nanosheet interaction pathway with the bacterial membrane. The nanosheets spontaneously adhere to the membrane surface and prompt vigorous phospholipid extraction majorly via strong van der Waals interactions with lipid hydrophobic tails. The lipid extraction process originates a significant water intrusion in the bilayer hydrophobic region, signifying the onset of cytoplasmic leakage under realistic conditions. Further, a synergistic effect of lipid-lipid self-interactions and lipid-MoS₂ dispersion interactions drags the nanosheet to completely immerse in the bilayer hydrophobic core. The embedded nanosheets induce a layerwise structural rearrangement of the membrane lipids in their vicinity, thus altering the structural and dynamic features of the membrane in a localized manner by (i) increasing the lipid fatty acyl tail ordering and (ii) alleviating the lipid lateral dynamics. The detrimental efficacy of the nanosheets can be magnified by enlarging the nanosheet size or by increasing the nanosheet concentration. Our study concludes that the MoS₂ nanosheets can exhibit their antibacterial action through destructive phospholipid extraction as well as by altering the morphology of the membrane by embedding in the membrane core.

Facile hydrothermal synthesis of layered 1T′ MoTe2 nanotubes as robust hydrogen evolution electrocatalysts

Layered transition metal dichalcogenides (TMDs), such as molybdenum ditelluride (MoTe₂), have attracted much attention because of their novel structure-related physicochemical properties. In particular, semi-metallic-phase MoTe₂ (1T′) is considered as a competitive candidate for low-cost electrocatalysts for water splitting. However, there are few reports on the simple hydrothermal synthesis of MoTe₂ nanostructures compared with other layered TMDs. In this study, a facile one-step hydrothermal process was developed for the fabrication of layered MoTe₂, in which uniform nanotubes with a few layers of 1T′ MoTe₂ were fabricated at a lower temperature for the first time. The as-obtained MoTe₂ nanotubes were fully characterized using different techniques, which revealed their structure and indicated the presence of layered 1T′ nanocrystals. The efficient activity of MoTe₂ nanotubes for the electrocatalytic hydrogen evolution reaction (HER) in 0.5 M H₂SO₄ was demonstrated by the small Tafel slope of 54 mV/dec⁻¹ and endurable ability, which is attributed to the abundant active sites and remarkable conductivity of 1T′ MoTe₂ with a few-layer feature. This provides a facile method for the design and construction of efficient layered MoTe₂ based electrocatalysts.

Possibility of regulating valley-contrasting physics and topological properties by ferroelectricity in functionalized arsenene

A two-dimensional (2D) multifunctional material, which couples multiple physical properties together, is both fundamentally intriguing and practically appealing. Here, based on first-principles calculations and tight-binding (TB) model analysis, the possibility of regulating the valley-contrasting physics and nontrivial topological properties via ferroelectricity is investigated in monolayer AsCH₂OH. Reversible electric polarization is accessible via the rotation operation on the ligand. The broken inversion symmetry and the spin-orbit coupling (SOC) would lead to valley spin splitting, spin-valley coupling and valley-contrasting Berry curvature. More importantly, the reversal of electric polarization can realize the nonvolatile control of valley-dependent properties. Besides, the nontrivial topological state is confirmed in the monolayer AsCH₂OH, which is robust against the rotation operation on the ligand. The magnitude of polarization, valley spin splitting and bulk band gap can be effectively modulated by the biaxial strain. The H-terminated SiC is demonstrated to be an appropriate candidate for encapsulating monolayer AsCH₂OH, without affecting its exotic properties. These findings provide insights into the fundamental physics for the coupling of the valley-contrasting phenomenon, topological properties and ferroelectricity, and open avenues for exploiting innovative device applications.

Full Composition Tuning of W1-xNbxSe2 Alloy Nanosheets to Promote the Electrocatalytic Hydrogen Evolution Reaction

Composition modulation of transition metal dichalcogenides is an effective way to engineer their crystal/electronic structures for expanded applications. Here, fully composition-tuned W_1-xNb_xSe₂ alloy nanosheets were produced via colloidal synthesis. These nanosheets ultimately exhibited a notable transition between WSe₂ and NbSe₂ hexagonal phases at x = 0.6. As x approaches 0.6, point doping is converted into cluster doping and eventually separated domains of WSe₂ and NbSe₂. Extensive density functional theory calculations predicted the composition-dependent crystal structures and phase transitions, consistently with the experiments. The electrocatalytic activity for the hydrogen evolution reaction (HER) in acidic electrolyte was significantly enhanced at x = 0.2, which was linked with the d-band center. The Gibbs free energy for the H adsorption at various basal and edge sites supported the enhanced HER performance of the metallic alloy nanosheets. We suggested that the dispersed doping structures of Nb atoms resulted in the best HER performance. Our findings highlight the significance of composition tuning in enhancing the catalytic activity of alloys.

Stable and flexible photodetector based on liquid-phase exfoliated titanium disulfide nanosheets

Herein, the TiS₂nanosheets (NSs) are prepared from the TiS₂bulk by the liquid-phase exfoliation to fabricate photoelectrochemical-type (PEC) photodetector. SEM images and Raman spectra show the successful acquisition of the TiS₂NSs. The as-prepared TiS₂photodetector shows self-powered ability with an applicable photoresponsivity that is about 0.37μA W^-1under zero bias potential and 80 mW cm^-2visible light, and the response time of rise is 0.67 s and the decay time is 2.81 s. In this case, the photodetector is made of ITO-coated polyethylene terephthalate (PET), so it can maintain stable performance under the bending conditions. These results display that the as-prepared photodetector has excellent photoelectric properties, which facilitates the development of TiS₂NSs in optoelectronic devices.

Engineering the Dynamics and Transport of Excitons, Trions, and Biexcitons in Monolayer WS2

The study of transport and diffusion dynamics of quasi-particles such as excitons, trions, and biexcitons in two-dimensional (2D) semiconductors has opened avenues for their application in high-speed excitonic and optoelectronic devices. However, long-range transport and fast diffusion of these quasi-particles have not been reported for 2D systems such as transition metal dichalcogenides (TMDCs). The reported diffusion coefficients from TMDCs are low, limiting their use in high-speed excitonic devices and other optoelectronic applications. Here, we report the highest exciton diffusion coefficient value in monolayer WS₂ achieved via engineering the radiative lifetime and diffusion lengths using static back-gate voltage and substrate engineering. Electrostatic doping is observed to modulate the radiative lifetime and in turn the diffusion coefficient of excitons by ∼three times at room temperature. By combining electrostatic doping and substrate engineering, we push the diffusion coefficient to an extremely high value of 86.5 cm²/s, which has not been reported before in TMDCs and is even higher than the values in some 1D systems. At low temperatures, we further report the control of dynamic and spatial diffusion of excitons, trions, and biexcitons from WS₂. The electrostatic control of dynamics and transport of these quasi-particles in monolayers establishes monolayer TMDCs as ideal candidates for high-speed excitonic circuits, optoelectronic, and photonic device applications.

Tunable Schottky and ohmic contacts in the Ti2NF2/α-Te van der Waals heterostructure

Two dimensional α-Te holds great promise in optoelectronic devices because of its high mobility and excellent environmental stability. In this study, the electronic structures and interfacial contact characteristics of the Ti₂NF₂/α-Te van der Waals heterostructure are investigated by means of first-principles calculations. It is found that p-type Schottky contacts with a Schottky barrier (SB) of 0.21 eV are formed at the Ti₂NF₂/α-Te interface. By applying external electric fields or controlling the interlayer coupling between the Ti₂NF₂ and α-Te monolayers, the SB height can be effectively tuned, and all the n-type Schottky, p-type Schottky, n-type ohmic and p-type ohmic contacts can be achieved. Such an extremely high tunability is further found to be closely associated with the charge transfer at the interface, as well as the interface dipole and the potential step. Our results provide an avenue for the design of future α-Te-based electronic devices with high performance.

Tuning the structural stability and electrochemical properties in graphene anode materials by B doping: a first-principles study

The first-principles method of density functional theory (DFT) is used to study the structural stability and electrochemical properties of B doped graphene with concentrations of 3.125%, 6.25% and 18.75% respectively, and their lithium storage mechanism and characteristics are further studied. The results show that the doped systems all have negative adsorption energy, indicating that the structures can exist stably, and the adsorption energy of lithium ions on graphene decreases with the increase of B doping concentration. Among them, the B₆C₂₆ structure has the lowest adsorption energy and can adsorb more lithium ions. The density of states indicates that doping with B can increase the conductivity of graphene greatly. Subsequently, the CI-NEB method to search for the transition state of the doped structure is used, showing that the B₆C₂₆ structure has the lowest diffusion barrier and good rate performance. Therefore, these findings provide a certain research foundation for the development and application of lithium-ion battery anode materials.

Energy Interplay in Materials: Unlocking Next-Generation Synchronous Multisource Energy Conversion with Layered 2D Crystals

Layered 2D crystals have unique properties and rich chemical and electronic diversity, with over 6000 2D crystals known and, in principle, millions of different stacked hybrid 2D crystals accessible. This diversity provides unique combinations of properties that can profoundly affect the future of energy conversion and harvesting devices. Notably, this includes catalysts, photovoltaics, superconductors, solar-fuel generators, and piezoelectric devices that will receive broad commercial uptake in the near future. However, the unique properties of layered 2D crystals are not limited to individual applications and they can achieve exceptional performance in multiple energy conversion applications synchronously. This synchronous multisource energy conversion (SMEC) has yet to be fully realized but offers a real game-changer in how devices will be produced and utilized in the future. This perspective highlights the energy interplay in materials and its impact on energy conversion, how SMEC devices can be realized, particularly through layered 2D crystals, and provides a vision of the future of effective environmental energy harvesting devices with layered 2D crystals.

Liquid-Phase Exfoliation of Nonlayered Non-Van-Der-Waals Crystals into Nanoplatelets

For nearly 15 years, researchers have been using liquid-phase exfoliation (LPE) to produce 2D nanosheets from layered crystals. This has yielded multiple 2D materials in a solution-processable form whose utility has been demonstrated in multiple applications. It was believed that the exfoliation of such materials is enabled by the very large bonding anisotropy of layered materials where the strength of intralayer chemical bonds is very much larger than that of interlayer van der Waals bonds. However, over the last five years, a number of papers have raised questions about our understanding of exfoliation by describing the LPE of nonlayered materials. These results are extremely surprising because, as no van der Waals gap is present to provide an easily cleaved direction, the exfoliation of such compounds requires the breaking of only chemical bonds. Here the progress in this unexpected new research area is examined. The structure and properties of nanoplatelets produced by LPE of nonlayered materials are reviewed. A number of unexplained trends are found, not least the preponderance of isotropic materials that have been exfoliated to give high-aspect-ratio nanoplatelets. Finally, the applications potential of this new class of 2D materials are considered.

Transfer-free, scalable vertical heterostructure FET on MoS2/WS2continuous films

Taking into account the novel layered structure and unusual electronic properties of MoS₂and WS₂on the side the lack of dangling bonds between these two components and donor-acceptor linkage effects, growth of the MoS₂/WS₂vertical heterojunction film on the amorphous SiO₂/Si substrate have created high demand. In this study, we reported the continuous, scalable, and vertical MoS₂/WS₂heterostructure film by using a sputtering without a transfer step. The WS₂film was continuously grown on MoS₂and eventually led to the formation of the MoS₂/WS₂vertical heterojunction film. Dozens of FETs fabricated on MoS₂/WS₂continuous heterojunction film were created on the same substrate in a single lithographic fabrication step, allowing them to be commercialized and not only used in research applications. RAMAN spectra proved the formation of the MoS₂/WS₂heterostructure film. In XPS measurements, it was shown that a separate MoS₂and WS₂layer was grown instead of the alloy structure. The polarity behavior of the MoS₂/WS₂heterostructure FET was found to be modulated with different drain voltages as p-type to ambipolar and finally n-type conductivity because of the transition of band structure and Schottky barrier heights at different drain voltages. Electron mobility (7.2 cm²V.s^-1) and on/off ratio (10⁴-10⁵) exhibited by the MoS₂/WS₂heterostructure FETs displayed a more improved electrical performance than that of individual WS₂, MoS₂devices. It was observed that the mobility value of MoS₂/WS₂FET was approximately 514 times greater than WS₂FET and 800 times greater than MoS₂FET. Additionally, the MoS₂/WS₂FET on/off ratio was larger than 2 order MoS₂FET and 1 order WS₂FET. The film of continuous vertical heterojunctions as in the MoS₂/WS₂currents in the study would be a promising candidate for nanoelectronics fields. This work demonstrated the progress towards realizing carrier-type controlled high-performance MoS₂/WS₂heterojunction-based FETs for future logic devices.

Lithium stabilizes square-two-dimensional metal sheets: a computational exploration

Based on the M₄-square-containing M₄Li₂ (M = Al, Ga, In, Tl, Ge, Sn, Pb, Sb, Bi, Cu, Ag, Au, and Hg) clusters, we computationally designed two-dimensional (2D) M₂Li sheets consisting of M₄-square motifs. The four M₂Li-I (M = Sb, Bi, Ag, and Au) monolayers with Li square sublayer sandwiched between two M square sublayers (P4/mmm space group) were confirmed to be stable (high cohesive energies, positive vibrational frequencies, moderate Young's moduli, and structural integrity during first-principles molecular dynamics simulations at 500 K), and the particle swarm optimization (PSO) method identified these constructed monolayers as the global minima in the 2D space. The three M₂Li-I (M = Sb, Bi, and Ag) monolayers demonstrated a half-auxetic behavior. Ag₂Li-I could well activate CO₂ and convert it into HCOOH by following the path * → *CO₂ → *OCHO → *HCOOH → *+HCOOH. Particularly, Ag₂Li-I shows great promise as an electrocatalyst for CO₂ reduction as its limiting potential is as low as 0.40 (0.27) V without (with) considering the solvent effect. Our theoretical explorations reveal that lithium can stabilize the square metal monolayers, and the stable square binary metal sheets exhibit diverse mechanical and electrochemical properties, which can be used in the fields of mechanics and electrochemical catalysis.

Advances in Two-Dimensional Materials for Optoelectronics Applications

The past one and a half decades have witnessed the tremendous progress of two-dimensional (2D) crystals, including graphene, transition-metal dichalcogenides, black phosphorus, MXenes, hexagonal boron nitride, etc., in a variety of fields. The key to their success is their unique structural, electrical, mechanical and optical properties. Herein, this paper gives a comprehensive summary on the recent advances in 2D materials for optoelectronic approaches with the emphasis on the morphology and structure, optical properties, synthesis methods, as well as detailed optoelectronic applications. Additionally, the challenges and perspectives in the current development of 2D materials are also summarized and indicated. Therefore, this review can provide a reference for further explorations and innovations of 2D material-based optoelectronics devices.

Understanding the linear and nonlinear optical responses of few-layer exfoliated MoS2and WS2nanoflakes: experimental and simulation studies

Understanding the linear and nonlinear optical (NLO) responses of two-dimensional nanomaterials is essential to effectively utilize them in various optoelectronic applications. Here, few-layer MoS₂and WS₂nanoflakes with lateral size less than 200 nm were prepared by liquid-phase exfoliation, and their linear and NLO responses were studied simultaneously using experimental measurements and theoretical simulations. Finite-difference time-domain (FDTD) simulations confirmed the redshift in the excitonic transitions when the thickness was increased above 10 nm indicating the layer-number dependent bandgap of nanoflakes. WS₂nanoflakes exhibited around 5 times higher absorption to scattering cross-section ratio than MoS₂nanoflakes at various wavelengths. Open aperture Z scan analysis of both the MoS₂and WS₂nanoflakes using 532 nm nanosecond laser pulses reveals strong nonlinear absorption activity with effective nonlinear absorption coefficient (β_eff) of 120 cm GW^-1and 180 cm GW^-1, respectively, which was attributed to the combined contributions of ground, singlet excited and triplet excited state absorption. FDTD simulation results also showed the signature of strong absorption density of few layer nanoflakes which may be account for their excellent NLO characteristics. Optical limiting threshold values of MoS₂and WS₂nanoflakes were obtained as ∼1.96 J cm^-2and 0.88 J cm^-2, respectively, which are better than many of the reported values. Intensity dependent switching from saturable absorption (SA) to reverse SA was also observed for MoS₂nanoflakes when the laser intensity increased from 0.14 to 0.27 GW cm^-2. The present study provides valuable information to improve the selection of two-dimensional nanomaterials for the design of highly efficient linear and nonlinear optoelectronic devices.

Recent Progress in Phase Regulation, Functionalization, and Biosensing Applications of Polyphase MoS2

The disulfide compounds of molybdenum (MoS₂ ) are layered van der Waals materials that exhibit a rich array of polymorphic structures. MoS₂ can be roughly divided into semiconductive phase and metallic phase according to the difference in electron filling state of the 4d orbital of Mo atom. The two phases show completely different properties, leading to their diverse applications in biosensors. But to some extent, they compensate for each other. This review first introduces the relationship between phase state and the chemical/physical structures and properties of MoS₂ . Furthermore, the synthetic methods are summarized and the preparation strategies for metastable phases are highlighted. In addition, examples of electronic and chemical property designs of MoS₂ by means of doping and surface modification are outlined. Finally, studies on biosensors based on MoS₂ in recent years are presented and classified, and the roles of MoS₂ with different phases are highlighted. This review offers references for the selection of materials to construct different types of biosensors based on MoS₂ , and provides inspiration for sensing performance enhancement.

Graphene-Based Ion-Selective Field-Effect Transistor for Sodium Sensing

Field-effect transistors have attracted significant attention in chemical sensing and clinical diagnosis, due to their high sensitivity and label-free operation. Through a scalable photolithographic process in this study, we fabricated graphene-based ion-sensitive field-effect transistor (ISFET) arrays that can continuously monitor sodium ions in real-time. As the sodium ion concentration increased, the current–gate voltage characteristic curves shifted towards the negative direction, showing that sodium ions were captured and could be detected over a wide concentration range, from 10⁻⁸ to 10⁻¹ M, with a sensitivity of 152.4 mV/dec. Time-dependent measurements and interfering experiments were conducted to validate the real-time measurements and the highly specific detection capability of our sensor. Our graphene ISFETs (G-ISFET) not only showed a fast response, but also exhibited remarkable selectivity against interference ions, including Ca²⁺, K⁺, Mg²⁺ and NH₄⁺. The scalability, high sensitivity and selectivity synergistically make our G-ISFET a promising platform for sodium sensing in health monitoring.

Localized π Surface States on 2D Molybdenum Disulfide from Carbene-Functionalization as a Qubit Design Strategy

Surface chemistry is increasingly important for a number of applications, from catalysis to molecular qubits. For the qubit application, it is imperative that the energy levels of surface species involved in the process of interest are energetically distinct-that is addressable and not buried below or coincident with the substrate energy levels. One way to afford this is through chemical functionalization with derivatives that impart the property of choice to the interface. In this Letter, we report on the nature of the bond between a carbene moiety and an MoS2 surface. With density functional theory (DFT) and spin-polarized calculations, we first observe states in the band structure that pertain to the carbene group and then prove their origin. Importantly, we find localized π-states in the band gap that are due to π-electrons that are part of a diene attached to the carbene carbon and are not available in bonding configurations without the π conjugation. These lead to open-shell monocationic structures involving midgap HOMOs with densities on the carbene moiety. Both neutral and cationic forms of the carbenes are energetically separate from the MoS2 substrate, thus useful for optical manipulation. We explore several different choices of the carbene moieties, and show that those based on fused thiophene and bithiophene structures are the most favorable for localization, while purely carbon-based aromatic structures lead to states that are delocalized onto the MoS2 and thus less useful. These findings are potentially of interest to the design and synthesis of future molecular qubit candidates for device fabrication.

First-Principles Study of Irn (n = 3-5) Clusters Adsorbed on Graphene and Hexagonal Boron Nitride: Structural and Magnetic Properties

Magnetic clusters have attracted great attention and interest due to their novel electronic properties, and they have potential applications in nanoscale information storage devices and spintronics. The interaction between magnetic clusters and substrates is still one of the challenging research focuses. Here, by using the density functional theory (DFT), we study the structural stability and magnetic properties of iridium clusters (Ir_n, n = 3–5) adsorbed on two-dimensional (2D) substrates, such as graphene and hexagonal boron nitride (hBN). We find that the most favorable configurations of free Ir_n clusters change when adsorbed on 2D substrates. In the meantime, the magnetic moments of the most stable Ir_n reduce to 53% (graphene) and 23.6% (hBN) compared with those of the free−standing ones. Interestingly, about 12-times enlargement on the magnetic anisotropy energy can be found on hBN substrates. These theoretical results indicate that the cluster–substrate interaction has vital effects on the properties of Ir_n clusters.

Host-Guest Intercalation Chemistry for the Synthesis and Modification of Two-Dimensional Transition Metal Dichalcogenides

Intercalation chemistry is of great importance in solid-state physics and chemistry for the ability to modulate electronic structures for constructing new materials with exotic properties. This ancient and versatile discipline has recently become prevailing in the synthesis and regulation of 2D transition metal dichalcogenides (TMDs) with atomic thickness due to diverse host-guest configurations and their impact on layered frameworks, which bring in extensive applications in electronics, optoelectronics, and other energy-based devices. In order to prepare 2D TMD materials with desired structure and properties, it is essential to gain in-depth understanding of the key role the intercalation chemistry plays in the preparation process. A focused review on recent advances regarding 2D TMD materials through intercalation exfoliation from the view of host, guest, and solvent interactions is provided. The effect of intercalation chemistry on TMD nanosheets synthesis and modification is comprehensively reviewed. The interactions between host and guest from the aspects of lattice strain, interlayer distance, and carrier density are considered. Finally, a prospectus of the future research opportunities for the intercalation chemistry of 2D materials is provided.

Advances and emerging challenges in MXenes and their nanocomposites for biosensing applications

Two-dimensional materials have unique properties and their better functionality has created new paradigms in the field of sensing. Over the past decade, a new family of 2D materials known as MXenes has emerged as a promising material for numerous applications, including biosensing. Their metallic conductivity, rich surface chemistry, hydrophilicity, good biocompatibility, and high anchoring capacity for biomaterials make them an attractive candidate to detect a variety of analytes. Despite such notable properties, there are certain limitations associated with them. This review aims to present a detailed survey of MXene's synthesis; in particular, their superiority in the field of biosensing as compared to other 2D materials is addressed. Their low oxidative stability is still an open challenge, and recent investigations on MXene's oxidation are summarized. The hexagonal stacking network of MXenes acts as a distinctive matrix to load nanoparticles, and the embedded nanoparticles can bind an excess number of biomolecules (e.g., antibodies) thereby improving biosensor performance. We will also discuss the synthesis and corresponding performance of MXenes nanocomposites with noble metal nanoparticles and magnetic nanoparticles. Furthermore, Nb and Ti2C-based MXenes, and Ti3C2-MXene sandwich immunoassays are also reviewed in view of their importance. Different aspects and challenges associated with MXenes (from their synthesis to final applications) and the future perspectives described give new directions to fabricate novel biosensors.

Elemental Two-Dimensional Materials for Li/Na-Ion Battery Anode Applications

Two-dimensional (2D) nanostructure is currently the subject in the fields of new energy storage and devices. During the past years, a broad range of 2D materials represented by graphene have been developed and endow with excellent electrochemical properties. Among them, elemental 2D materials (Xenes) are an emerged material family for Li/Na-ion battery (LIB/SIB) anodes. Compared with other 2D materials and bulk materials, Xenes may exhibit some great superiorities for Li/Na storage, including excellent conductivity, fast ion diffusion and large active sites exposure. In this review, we provide a systematic summary of the recent progress and achievements of Xenes as well as their applications in LIBs/SIBs. The broad categorization of Xenes from group IIIA to VIA has been concisely outlined, and the related details in syntheses, structures and Li/Na-ion storage properties are reviewed. Further, the latest research progress of Xenes in Li/Na ion batteries are summarized, together with mechanism discussions. Finally, the challenges and prospects of Xenes applied to Li/Na ion battery are proposed based on its current developments.

Two-Dimensional Ultrathin Silica Films

Two-dimensional (2D) ultrathin silica films have the potential to reach technological importance in electronics and catalysis. Several well-defined 2D-silica structures have been synthesized so far. The silica bilayer represents a 2D material with SiO₂ stoichiometry. It consists of precisely two layers of tetrahedral [SiO₄] building blocks, corner connected via oxygen bridges, thus forming a self-saturated silicon dioxide sheet with a thickness of ∼0.5 nm. Inspired by recent successful preparations and characterizations of these 2D-silica model systems, scientists now can forge novel concepts for realistic systems, particularly by atomic-scale studies with the most powerful and advanced surface science techniques and density functional theory calculations. This Review provides a solid introduction to these recent developments, breakthroughs, and implications on ultrathin 2D-silica films, including their atomic/electronic structures, chemical modifications, atom/molecule adsorptions, and catalytic reactivity properties, which can help to stimulate further investigations and understandings of these fundamentally important 2D materials.

Oxygen-Induced Dissociation of a Single Water Molecule in Confined 2-D Layers: A Semiempirical study

Semiempirical quantum mechanical methods provides a middle ground between a computationally demanding full ab initio quantum chemistry calculations and force-field calculations in application to molecule-surface interactions. In this study, PM7 semiempirical method is used to evaluate adsorption energy values of X@h-BN monolayer [X= O, OH, and H 2 O] followed by a mechanistic study of oxygen-induced water dissociation on a free-standing h-BN monolayer. Based on oxygen adsorption configurations, two reaction pathways for water dissociation are studied that yield two distinct configurations of double OH-functionalized h-BN monolayer. We then investigated the effect of a cover layer graphene on these proposed mechanistic pathways by placing the graphene cover layer on the top of the h-BN monolayer and continuously tuning the separation ( d Gr/h-BN ) between these two layers.

Mode-locked ytterbium-doped fiber laser with zinc phthalocyanine thin film saturable absorber

A stable mode-locked laser was demonstrated using a newly developed zinc phthalocyanine (ZnPc) thin film as passive saturable absorber (SA) in ytterbium-doped fiber laser (YDFL). The ZnPc thin film was obtained using a casting method and then inserted between the two fiber ferrules of a YDFL ring cavity to generate mode-locked pulses. The resulting pulsed laser operated at a wavelength of 1034.5 nm having a repetition rate of 3.3 MHz. At pump power of 277 mW, the maximum output power and pulse energy are achieved at 4.92 mW and 1.36 nJ, respectively. ZnPc has a high chemical and photochemical stability, and its significance for use as a potential SA in a mode-locked laser is reported in this work.

Recent Development of Morphology-Controlled Hybrid Nanomaterials for Triboelectric Nanogenerator: A Review

Being cognizant of modern electronic devices, the scientists are continuing to investigate renewable green-energy resources for a decade. Amid different energy harvesting systems, the triboelectric nanogenerators (TENGs) have been found to be the most promising mechanical harvesting technology and have drawn attention to generate electrical energy. Thanks to its instant output power, choice to opt for wide-ranging materials, low maintenance cost, easy fabrication process and environmentally friendly nature. Due to numerous working modes of TENGs, it is dedicated to desired application at ambient conditions. In this review, an advance correlation of TENGs have been explained based on the variety of nanostructures, including 0D, 1D, 2D, 3D, metal organic frameworks (MOFs), coordination polymers (CPs), covalent organic frameworks (COFs), and perovskite materials. Moreover, an overview of previous and current perspectives of various nanomaterials, synthesis, fabrication and their applications in potential fields have been discussed in detail.

Novel Van Der Waals Heterostructures Based on Borophene, Graphene-like GaN and ZnO for Nanoelectronics: A First Principles Study

At present, the combination of 2D materials of different types of conductivity in the form of van der Waals heterostructures is an effective approach to designing electronic devices with desired characteristics. In this paper, we design novel van der Waals heterostructures by combing buckled triangular borophene (tr-B) and graphene-like gallium nitride (GaN) monolayers, and tr-B and zinc oxide (ZnO) monolayers together. Using ab initio methods, we theoretically predict the structural, electronic, and electrically conductive properties of tr-B/GaN and tr-B/ZnO van der Waals heterostructures. It is shown that the proposed atomic configurations of tr-B/GaN and tr-B/ZnO heterostructures are energetically stable and are characterized by a gapless band structure in contrast to the semiconductor character of GaN and ZnO monolayers. We find the phenomenon of charge transfer from tr-B to GaN and ZnO monolayers, which predetermines the key role of borophene in the formation of the features of the electronic structure of tr-B/GaN and tr-B/ZnO van der Waals heterostructures. The results of the calculation of the current-voltage (I-V) curves reveal that tr-B/GaN and tr-B/ZnO van der Waals heterostructures are characterized by the phenomenon of current anisotropy: the current along the zigzag edge of the ZnO/GaN monolayers is five times greater than along the armchair edge of these monolayers. Moreover, the heterostructures show good stability of current to temperature change at small voltage. These findings demonstrate that r-B/GaN and tr-B/ZnO vdW heterostructures are promising candidates for creating the element base of nanoelectronic devices, in particular, a conducting channel in field-effect transistors.

Functionalization of 2D MoS2 Nanosheets with Various Metal and Metal Oxide Nanostructures: Their Properties and Application in Electrochemical Sensors

Two-dimensional transition metal dichalcogenides (2D TMDs) have gained considerable attention due to their distinctive properties and broad range of possible applications. One of the most widely studied transition metal dichalcogenides is molybdenum disulfide (MoS₂). The 2D MoS₂ nanosheets have unique and complementary properties to those of graphene, rendering them ideal electrode materials that could potentially lead to significant benefits in many electrochemical applications. These properties include tunable bandgaps, large surface areas, relatively high electron mobilities, and good optical and catalytic characteristics. Although the use of 2D MoS₂ nanosheets offers several advantages and excellent properties, surface functionalization of 2D MoS₂ is a potential route for further enhancing their properties and adding extra functionalities to the surface of the fabricated sensor. The functionalization of the material with various metal and metal oxide nanostructures has a significant impact on its overall electrochemical performance, improving various sensing parameters, such as selectivity, sensitivity, and stability. In this review, different methods of preparing 2D-layered MoS₂ nanomaterials, followed by different surface functionalization methods of these nanomaterials, are explored and discussed. Finally, the structure-properties relationship and electrochemical sensor applications over the last ten years are discussed. Emphasis is placed on the performance of 2D MoS₂ with respect to the performance of electrochemical sensors, thereby giving new insights into this unique material and providing a foundation for researchers of different disciplines who are interested in advancing the development of MoS₂-based sensors.

A General Synthetic Method for High-Entropy Alloy Subnanometer Ribbons

High-entropy alloys (HEAs) are attracting intensive attention due to their broad compositional tunability and interesting catalytic properties. However, precisely shaping the HEAs into suprathin low-dimensional nanostructures for achieving diverse applications remains an enormous challenge owing to their intrinsic thermodynamic instability. Herein we propose a new and general low-temperature method for incorporating up to eight metallic elements into one single-phase subnanometer ribbon to achieve the thinnest HEA metal materials in the world. We experimentally demonstrate that synthetic processes for suprathin HEA subnanometer ribbons (SNRs) include (1) different metal atom nucleation via galvanic exchange reaction between different metal precursors and Ag nanowire template, (2) co-reduction of different metal precursors on nanowire template, and (3) the removal of the inner Ag core. Density functional theory (DFT) calculations reveal that the crystallization and stabilization of HEA SNRs strongly depend on the "highly dynamic" Ag from the template, and the crystallization levels of HEA subnanometer ribbons are closely correlated with the concentration of Pt and Pd. We demonstrate that the present synthetic method enables the flexible control of components and concentrations in HEAs SNRs for achieving a library of HEA SNRs and also superior electrocatalytic properties. The well-designed HEA SNRs show great improvement in catalyzing the oxygen reduction reaction of fuel cells and also high discharge capacity, low charge overpotential, and excellent durability for Li-O₂ batteries. DFT calculations reveal the superior electrochemical performances are attributed to the strong reduction capability from high-concentration reductive elements in HEAs, while the other elements guarantee the site-to-site efficient electron transfer.