Plasmon resonances of highly doped two-dimensional MoS₂

Tuning d-Orbital Electronic Structure via Au-Intercalated Two-Dimensional Fe3GeTe2 to Increase Surface Plasmon Activity

While extensive research has been dedicated to plasmon tuning within non-noble metals, prior investigations primarily concentrated on markedly augmenting the inherently low concentration of free carriers in materials with minimal consideration given to the influence of electron orbitals on surface plasmons. Here, we achieve successful intercalation of Au atoms into the layered structure of Fe3GeTe2 (FGT), thereby exerting control over the orbital electronic states or structure of FGT. This intervention not only amplifies the charge density and electron mobility but also mitigates the loss associated with interband transitions, resulting in increased two-dimensional FGT surface plasmon activity. As a consequence, Au-intercalated FGT detects crystal violet molecules as a surface-enhanced Raman scattering substrate, and the detection lines are 3 orders of magnitude higher than before Au intercalation. Our work provides insight for further studies on plasmon effects and the relation between surface plasmon resonance behavior and electronic structures.

Optical properties and plasmons in moiré structures

The discoveries of numerous exciting phenomena in twisted bilayer graphene (TBG) are stimulating significant investigations on moiré structures that possess a tunable moiré potential. Optical response can provide insights into the electronic structures and transport phenomena of non-twisted and twisted moiré structures. In this article, we review both experimental and theoretical studies of optical properties such as optical conductivity, dielectric function, non-linear optical response, and plasmons in moiré structures composed of graphene, hexagonal boron nitride (hBN), and/or transition metal dichalcogenides. Firstly, a comprehensive introduction to the widely employed methodology on optical properties is presented. After, moiré potential induced optical conductivity and plasmons in non-twisted structures are reviewed, such as single layer graphene-hBN, bilayer graphene-hBN and graphene-metal moiré heterostructures. Next, recent investigations of twist-angle dependent optical response and plasmons are addressed in twisted moiré structures. Additionally, we discuss how optical properties and plasmons could contribute to the understanding of the many-body effects and superconductivity observed in moiré structures.

Componential and molecular-weight-dependent effects of natural organic matter on the colloidal behavior, transformation, and toxicity of MoS2 nanoflakes

The potential widespread applications in water processing have rendered the necessity for investigations of the fate and hazard of molybdenum disulfide (MoS2) nanosheets. Herein, it was found that humic acid (HA) had better performances toward stabilizing pure 2H phase MoS2 and chemical-exfoliated MoS2 (ce-MoS2) in electrolyte solutions than fulvic acid (FA), and molecular weight (MW)-dependent manners were disclosed due to steric repulsions. Compared with darkness, the extent to which the aggregation and sedimentation of ce-MoS2 facilitated by visible light irradiation was greater in the presence of HA and FA fractions, likely due to the introduction of stronger plasmonic dipole-dipole interaction and Van der Waals attraction forces. HA-triggered structural disintegration of nanosheets was performed after irradiation and it was observed to be more significant with the increase in MWs, whereas the MW-dependent dissolution of MoS2 caused by FA was much quicker than that by HA owing to the higher generation of singlet oxygen. Moreover, FA lowered the bioavailability of MoS2 and relieved its toxicity to zebrafish more effectively than HA. Our findings boost the insights into the effects of organic molecules on the fates and hazards of MoS2, providing guidance for the MoS2-based nanotechnological development on environment.

Boosting Plant Photosynthesis with Carbon Dots: A Critical Review of Performance and Prospects

Artificially augmented photosynthesis in nano-bionic plants requires tunable nano-antenna structures with physiochemical and optoelectronic properties, as well as unique light conversion capabilities. The use of nanomaterials to promote light capture across photosystems, primarily by carbon dots, has shown promising results in enhancing photosynthesis through tunable uptake, translocation, and biocompatibility. Carbon dots possess the ability to perform both down and up-light conversions, making them effective light promoters for harnessing solar energy beyond visible light wavelengths.This review presents and discusses the recent progress in fabrication, chemistry, and morphology, as well as other properties such as photoluminescence and energy conversion efficiency of nano-antennas based on carbon dots. The performance of artificially boosted photosynthesis is discussed and then correlated with the conversion properties of carbon dots and how they are applied to plant models. The challenges related to the nanomaterial delivery and the performance evaluation practices in modified photosystems, consideration of the reliability of this approach, and the potential avenues for performance improvements through other types of nano-antennas based on alternative nanomaterials are also critically evaluated. It is anticipated that this review will stimulate more high-quality research in plant nano-bionics and provide avenues to enhance photosynthesis for future agricultural applications.

Microenvironment Restruction of Emerging 2D Materials and their Roles in Therapeutic and Diagnostic Nano-Bio-Platforms

Engineering advanced therapeutic and diagnostic nano-bio-platforms (NBPFs) have emerged as rapidly-developed pathways against a wide range of challenges in antitumor, antipathogen, tissue regeneration, bioimaging, and biosensing applications. Emerged 2D materials have attracted extensive scientific interest as fundamental building blocks or nanostructures among material scientists, chemists, biologists, and doctors due to their advantageous physicochemical and biological properties. This timely review provides a comprehensive summary of creating advanced NBPFs via emerging 2D materials (2D-NBPFs) with unique insights into the corresponding molecularly restructured microenvironments and biofunctionalities. First, it is focused on an up-to-date overview of the synthetic strategies for designing 2D-NBPFs with a cross-comparison of their advantages and disadvantages. After that, the recent key achievements are summarized in tuning the biofunctionalities of 2D-NBPFs via molecularly programmed microenvironments, including physiological stability, biocompatibility, bio-adhesiveness, specific binding to pathogens, broad-spectrum pathogen inhibitors, stimuli-responsive systems, and enzyme-mimetics. Moreover, the representative therapeutic and diagnostic applications of 2D-NBPFs are also discussed with detailed disclosure of their critical design principles and parameters. Finally, current challenges and future research directions are also discussed. Overall, this review will provide cutting-edge and multidisciplinary guidance for accelerating future developments and therapeutic/diagnostic applications of 2D-NBPFs.

One-Step Synthesis of Transition Metal Dichalcogenide Quantum Dots Using Only Alcohol Solvents for Indoor-Light Photocatalytic Antibacterial Activity

In this study, we report a one-step direct synthesis of molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂) quantum dots (QDs) through a solvothermal reaction using only alcohol solvents and efficient Escherichia coli (E. coli) decompositions as photocatalytic antibacterial agents under visible light irradiation. The solvothermal reaction gives the scission of molybdenum-sulfur (Mo-S) and tungsten-sulfur (W-S) bonding during the synthesis of MoS₂ and WS₂ QDs. Using only alcohol solvent does not require a residue purification process necessary for metal intercalation. As the number of the CH₃ groups of alcohol solvents among ethyl, isopropyl, and tert(t)-butyl alcohols increases, the dispersibility of MoS₂/WS₂ increases. The CH₃ groups of alcohols minimize the surface energy, leading to the effective exfoliation and disintegration of the bulk under heat and pressure. The bulky t-butyl alcohol with the highest number of methyl groups shows the highest exfoliation and yield. MoS₂ QDs with a lateral size of about 2.5 nm and WS₂ QDs of about 10 nm are prepared, exhibiting a strong blue luminescence under 365 nm ultraviolet (UV) light irradiation. Their heights are 0.68-3 and 0.72-5 nm, corresponding to a few layers of MoS₂ and WS₂, respectively. They offer a highly efficient performance in sterilizing E. coli as the visible-light-driven photocatalyst.

All-Optical Reconfigurable Excitonic Charge States in Monolayer MoS2

Excitons are quasi-particles composed of electron-hole pairs through Coulomb interaction. Due to the atomic-thin thickness, they are tightly bound in monolayer transition metal dichalcogenides (TMDs) and dominate their optical properties. The capability to manipulate the excitonic behavior can significantly influence the photon emission or carrier transport performance of TMD-based devices. However, on-demand and region-selective manipulation of the excitonic states in a reversible manner remains challenging so far. Herein, harnessing the coordinated effect of femtosecond-laser-driven atomic defect generation, interfacial electron transfer, and surface molecular desorption/adsorption, we develop an all-optical approach to manipulate the charge states of excitons in monolayer molybdenum disulfide (MoS₂). Through steering the laser beam, we demonstrate reconfigurable optical encoding of the excitonic charge states (between neutral and negative states) on a single MoS₂ flake. Our technique can be extended to other TMDs materials, which will guide the design of all-optical and reconfigurable TMD-based optoelectronic and nanophotonic devices.

Nanobionics-Driven Synthesis of Molybdenum Oxide Nanosheets with Tunable Plasmonic Resonances in Visible Light Regions

Nanobionics-driven synthesis offers a process of designing and synthesizing functional materials on a nanoscale based on the structures and functions of biological systems. An approach such as this is environmentally friendly and sustainable, providing a viable option for synthesizing functional nanomaterials for catalysis and nanoelectronic components. In this work, we present a facile and green nanobionics approach to synthesize plasmonic H_xMoO₃ by interacting chloroplasts extracted from spinach with two-dimensional (2D) MoO₃ nanoflakes. The generated plasmon resonances can be modulated in the visible wavelength ranges, and the efficiency to form the plasmonic materials is enhanced by 90% within 45 min of light excitation compared to reactions without chloroplast involvement. Such a characteristic is ascribed to the interfacial carrier dynamics between the two entities in the reactions, in which highly doped metal oxides with quasi-metallic properties can be formed to generate optical absorptions in the visible light region. The green synthesized plasmonic materials show high photocatalytic activities without the coupling of semiconductors, providing a promising nanoelectronics unit, based on the nanobionics-driven synthesized plasmonic materials.

Plasmonic metal oxides and their biological applications

Metal oxides modified with dopants and defects are an emerging class of novel materials supporting the localized surface plasmon resonance across a wide range of optical wavelengths, which have attracted tremendous research interest particularly in biological applications in the past decade. Compared to conventional noble metal-based plasmonic materials, plasmonic metal oxides are particularly favored for their cost efficiency, flexible plasmonic properties, and improved biocompatibility, which can be important to accelerate their practical implementation. In this review, we first explicate the origin of plasmonics in dopant/defect-enabled metal oxides and their associated tunable localized surface plasmon resonance through the conventional Mie-Gans model. The research progress of dopant incorporation and defect generation in metal oxide hosts, including both in situ and ex situ approaches, is critically discussed. The implementation of plasmonic metal oxides in biological applications in terms of therapy, imaging, and sensing is summarized, in which the uniqueness of dopant/defect-driven plasmonics for inducing novel functionalities is particularly emphasized. This review may provide insightful guidance for developing next-generation plasmonic devices for human health monitoring, diagnosis and therapy.

Impact of sulfhydryl ligands on the transformation of silver ions by molybdenum disulfide and their combined toxicity to freshwater algae

The transformation of silver ions (Ag⁺) mediated by engineered nanomaterials (ENMs) influences the biosafety of Ag-containing products in natural environments. Actually, modification of biomolecules to ENMs in aquatic ecosystems alters their interactions with Ag⁺. This study discovered that surface functionalization of glutathione (GSH, a sulfhydryl compound ubiquitous in natural waters) on molybdenum disulfide (MoS₂) nanoflakes suppressed the redox reaction between 1 T components and Ag⁺, inhibiting the MoS₂-mediated reduction of Ag⁺ to Ag nanoparticles (AgNPs) in aqueous phase in the dark. However, AgNPs formation (from 2.32 ± 0.35-3.25 ± 0.29 mg/L per day, pH 7.0) and oxidation of MoS₂ were remarkably accelerated after GSH binding under light conditions. The dominant electron donator of MoS₂ to Ag⁺ was transformed from the electron-hole pairs to surface ligands driven by the introduction of chromophoric groups was authenticated as the cause for the elevated Ag⁺ reduction. These processes also occurred between Ag⁺ and MoS₂ at low levels (50 μg/L). Additionally, the joint algal toxicity of GSH-modified MoS₂ with Ag⁺ was weaker than that of pristine MoS₂ due to increased retention of free Ag⁺ and AgNPs formation. Our findings improve the understanding of the interaction between ENMs and Ag⁺ in aquatic ecosystems.

Thermoplasmonics in Solar Energy Conversion: Materials, Nanostructured Designs, and Applications

The indispensable requirement for sustainable development of human society has forced almost all countries to seek highly efficient and cost-effective ways to harvest and convert solar energy. Though continuous progress has advanced, it remains a daunting challenge to achieve full-spectrum solar absorption and maximize the conversion efficiency of sunlight. Recently, thermoplasmonics has emerged as a promising solution, which involves several beneficial effects including enhanced light absorption and scattering, generation and relaxation of hot carriers, as well as localized/collective heating, offering tremendous opportunities for optimized energy conversion. Besides, all these functionalities can be tailored via elaborated designs of materials and nanostructures. Here, first the fundamental physics governing thermoplasmonics is presented and then the strategies for both material selection and nanostructured designs toward more efficient energy conversion are summarized. Based on this, recent progress in thermoplasmonic applications including solar evaporation, photothermal chemistry, and thermophotovoltaic is reviewed. Finally, the corresponding challenges and prospects are discussed.

Super strong wide TM Mie bandgaps tolerating disorders

This study demonstrates the appearance of super intense and wide Mie bandgaps in metamaterials composed of tellurium, germanium, and silicon rods in air that tolerate some disordering of rod position and rod radius under transverse magnetic (TM) polarized light waves. 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This strong tolerance of disordering of TM modes in tellurium, germanium, and silicon metamaterials opens a new way to design small, high-efficient, and feasible fabrication optical devices for optical integrated circuits.

2D Material-Based Optical Biosensor: Status and Prospect

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

Synergistic Effect of Metal Cations and Visible Light on 2D MoS2 Nanosheet Aggregation

Aggregation significantly influences the transport, transformation, and bioavailability of engineered nanomaterials. Two-dimensional MoS₂ nanosheets are one of the most well-studied transition-metal dichalcogenide nanomaterials. Nonetheless, the aggregation behavior of this material under environmental conditions is not well understood. Here, we investigated the aggregation of single-layer MoS₂ (SL-MoS₂) nanosheets under a variety of conditions. Trends in the aggregation of SL-MoS₂ are consistent with classical Derjaguin-Landau-Verwey-Overbeek (DLVO) colloidal theory, and the critical coagulation concentrations of cations follow the order of trivalent (Cr³⁺) < divalent (Ca²⁺, Mg²⁺, Cd²⁺) < monovalent cations (Na⁺, K⁺). Notably, Pb²⁺ and Ag⁺ destabilize MoS₂ nanosheet suspensions much more strongly than do their divalent and monovalent counterparts. This effect is attributable to Lewis soft acid-base interactions of cations with MoS₂. Visible light irradiation synergistically promotes the aggregation of SL-MoS₂ nanosheets in the presence of cations, which was evident even in the presence of natural organic matter. The light-accelerated aggregation was ascribed to dipole-dipole interactions due to transient surface plasmon oscillation of electrons in the metallic 1T phase, which decrease the aggregation energy barrier. These results reveal the phase-dependent aggregation behaviors of engineered MoS₂ nanosheets with important implications for environmental fate and risk.

Ultra-narrow, highly efficient power splitters and waveguides that exploit the TE01 Mie-resonant bandgap

In this paper, ultra-narrow and highly-efficient straight and Ω-shaped waveguides, and Y-shaped and T-shaped optical power splitters composed of two rows of two-dimensional germanium rods in air are designed and simulated. The position-disordering effect on the waveguides is considered. Finite-difference time-domain numerical simulation results for two rows of straight and Ω-shaped waveguides with no position disordering at the normalized frequency of a λ=0.327 show optical transmission of 90%, and two rows of Y-shaped and T-shaped power splitters with no position disordering have transmissions >46% for each output branch at the normalized frequency of a λ=0.327. Also, the straight and Ω-shaped waveguides with four rows of germanium rods tolerated position disordering of η = 10%. The proposed ultra-narrow waveguides and power splitters are vital components in high-density and all-dielectric optical integrated circuits.

Two-dimensional MoS2 2H, 1T, and 1T' crystalline phases with incorporated adatoms: theoretical investigation of electronic and optical properties

Although there has been progress in studying the electronic and optical properties of monolayer and near-monolayer (two-dimensional, 2D) MoS₂ upon adatom adsorption and intercalation, understanding the underlying atomic-level behavior is lacking, particularly as related to the optical response. Alkali atom intercalation in 2D transition metal dichalcogenides (TMDs) is relevant to chemical exfoliation methods that are expected to enable large scale production. In this work, focusing on prototypical 2D MoS₂, the adsorption and intercalation of Li, Na, K, and Ca adatoms were investigated for the 2H, 1T, and 1T^' phases of the TMD by the first principles density functional theory in comparison to experimental characterization of 2H and 1T 2D MoS₂ films. Our electronic structure calculations demonstrate significant charge transfer, influencing work function reductions of 1-1.5 eV. Furthermore, electrical conductivity calculations confirm the semiconducting versus metallic behavior. Calculations of the optical spectra, including excitonic effects using a many-body theoretical approach, indicate enhancement of the optical transmission upon phase change. Encouragingly, this is corroborated, in part, by the experimental measurements for the 2H and 1T phases having semiconducting and metallic behavior, respectively, thus motivating further experimental exploration. Overall, our calculations emphasize the potential impact of synthesis-relevant adatom incorporation in 2D MoS₂ on the electronic and optical responses that comprise important considerations toward the development of devices such as photodetectors or the miniaturization of electroabsorption modulator components.

Photoinduced transformation of silver ion by molybdenum disulfide nanoflakes at environmentally relevant concentrations attenuates its toxicity to freshwater algae

The transformation of Ag⁺ is strongly correlated with its risks in aquatic environment. Considering the wide application of molybdenum disulfide (MoS₂) and the inevitable release into the environment, the effects of MoS₂ on Ag⁺ transformation and toxicity are of great concerns. This study revealed the pH-dependent reduction of Ag⁺ (0.5 mM) to Ag nanoparticles (AgNPs) by MoS₂ (50 mg/L) and solar irradiation obviously accelerates the AgNPs formation (2.638 mg/L per day, pH=7.0) compared with dark condition (0.637 mg/L per day), ascribing to the electrons capture from electron-hole pairs of MoS₂ by Ag⁺. Ionic strengths and natural organic matter decreased the AgNPs yield. Metallic 1 T phase of MoS₂ primarily participated in AgNPs formation and was oxidized to soluble ions (MoO₄^2-) due to the oxygen generation in valance band. The above processes also occurred between Ag⁺ and MoS₂ at environmentally relevant concentrations. Further, photoinduced transformation of Ag⁺ by MoS₂ (10-100 μg/L) significantly lowered its toxicity to freshwater algae. The AgNPs formation on MoS₂ reduced the bioavailability of Ag⁺ to algae, which was the mechanism for attenuated Ag⁺ toxicity. The provided data are helpful for better understanding the roles of MoS₂ on the environmental fates and risks of metal ions under natural conditions.

Immunoassay-Amplified Responses Using a Functionalized MoS2-Based SPR Biosensor to Detect PAPP-A2 in Maternal Serum Samples to Screen for Fetal Down's Syndrome

Background

Due to educational, social and economic reasons, more and more women are delaying childbirth. However, advanced maternal age is associated with several adverse pregnancy outcomes, and in particular a high risk of Down’s syndrome (DS). Hence, it is increasingly important to be able to detect fetal Down’s syndrome (FDS).

Methods

We developed an effective, highly sensitive, surface plasmon resonance (SPR) biosensor with biochemically amplified responses using carboxyl-molybdenum disulfide (MoS₂) film. The use of carboxylic acid as a surface modifier of MoS₂ promoted dispersion and formed specific three-dimensional coordination sites. The carboxylic acid immobilized unmodified antibodies in a way that enhanced the bioaffinity of MoS₂ and preserved biorecognition properties of the SPR sensor surface. Complete antigen pregnancy-associated plasma protein-A2 (PAPP-A2) conjugated with the carboxyl-MoS₂-modified gold chip to amplify the signal and improve detection sensitivity. This heterostructure interface had a high work function, and thus improved the efficiency of the electric field energy of the surface plasmon. These results provide evidence that the interface electric field improved performance of the SPR biosensor.

Results

The carboxyl-MoS₂-based SPR biosensor was used successfully to evaluate PAPP-A2 level for fetal Down’s syndrome screening in maternal serum samples. The detection limit was 0.05 pg/mL, and the linear working range was 0.1 to 1100 pg/mL. The women with an SPR angle >46.57 m° were more closely associated with fetal Down’s syndrome. Once optimized for serum Down’s syndrome screening, an average recovery of 95.2% and relative standard deviation of 8.5% were obtained. Our findings suggest that carboxyl-MoS₂-based SPR technology may have advantages over conventional ELISA in certain situations.

Conclusion

Carboxyl-MoS₂-based SPR biosensors can be used as a new diagnostic technology to respond to the increasing need for fetal Down’s syndrome screening in maternal serum samples. Our results demonstrated that the carboxyl-MoS₂-based SPR biosensor was capable of determining PAPP-A2 levels with acceptable accuracy and recovery. We hope that this technology will be investigated in diverse clinical trials and in real case applications for screening and early diagnosis in the future.

Anomalous plasmons in a two-dimensional Dirac nodal-line Lieb lattice

Plasmons in two-dimensional (2D) Dirac materials feature an interesting regime with a tunable frequency, and long propagating length and lifetime, but are rarely achieved in the visible light regime. Using a tight-binding (TB) model in combination with first-principles calculations, we investigated plasmon modes in a 2D Lieb lattice with a Dirac nodal-line electronic structure. In contrast to conventional 2D plasmons, anomalous plasmons in the Lieb lattice exhibit the unique features of a carrier-density-independent frequency, being Landau-damping free in a wide-range of wave vectors, a high frequency, and high subwavelength confinement. Remarkably, by using first-principles calculations, we proposed a candidate material, 2D Be₂C monolayer, to achieve these interesting plasmon properties. The plasmons in the Be₂C monolayer can survive up to the visible frequency region and propagate to large momentum transfer that has rarely been reported. The anomalous plasmons revealed in the Lieb lattice offer a promising platform for the study of 2D plasmons as well as the design of 2D plasmonic materials.

Two-Dimensional Material-Based Biosensors for Virus Detection

Viral infections are one of the major causes of mortality and economic losses worldwide. Consequently, efficient virus detection methods are crucial to determine the infection prevalence. However, most detection methods face challenges related to false-negative or false-positive results, long response times, high costs, and/or the need for specialized equipment and staff. Such issues can be overcome by access to low-cost and fast response point-of-care detection systems, and two-dimensional materials (2DMs) can play a critical role in this regard. Indeed, the unique and tunable physicochemical properties of 2DMs provide many advantages for developing biosensors for viral infections with high sensitivity and selectivity. Fast, accurate, and reliable detection, even at early infection stages by the virus, can be potentially enabled by highly accessible surface interactions between the 2DMs and the analytes. High selectivity can be obtained by functionalization of the 2DMs with antibodies, nucleic acids, proteins, peptides, or aptamers, allowing for specific binding to a particular virus, viral fingerprints, or proteins released by the host organism. Multiplexed detection and discrimination between different virus strains are also feasible. In this Review, we present a comprehensive overview of the major advances of 2DM-based biosensors for the detection of viruses. We describe the main factors governing the efficient interactions between viruses and 2DMs, making them ideal candidates for the detection of viral infections. We also critically detail their advantages and drawbacks, providing insights for the development of future biosensors for virus detection. Lastly, we provide suggestions to stimulate research in the fast expanding field of 2DMs that could help in designing advanced systems for preventing virus-related pandemics.

Influence of Size and Phase on the Biodegradation, Excretion, and Phytotoxicity Persistence of Single-Layer Molybdenum Disulfide

The increasing applications of single-layer molybdenum disulfide (SLMoS₂) pose great potential risks associated with environmental exposure. This study found that metallic-phase SLMoS₂ with nanoscale (N-1T-SLMoS₂, ∼400 nm) and microscale (M-1T-SLMoS₂, ∼3.6 μm) diameters at 10-25 mg/L induced significant algal growth inhibition (maximum 72.7 and 74.6%, respectively), plasmolysis, and oxidative damage, but these alterations were recoverable. Nevertheless, membrane permeability, chloroplast damage, and chlorophyll biosynthesis reduction were persistent. By contrast, the growth inhibition (maximum 55.3%) and adverse effects of nano-sized semiconductive-phase SLMoS₂ (N-2H-SLMoS₂, ∼400 nm) were weak and easily alleviated after 96 h of recovery. N-1T-SLMoS₂ (0.011 μg/h) and N-2H-SLMoS₂ (0.008 μg/h) were quickly biodegraded to soluble Mo compared with M-1T-SLMoS₂ (0.004 μg/h) and excreted by algae. Incomplete biodegradation of SLMoS₂ (26.8-43.9%) did not significantly mitigate its toxicity. Proteomics and metabolomics indicated that the downregulation of proteins (50.7-99.2%) related to antioxidants and photosynthesis and inhibition of carbon fixation and carbohydrate metabolism contributed to the persistent phytotoxicity. These findings highlight the roles and mechanisms of the size and phase in the persistent phytotoxicity of SLMoS₂, which has potential implications for risk assessment and environmental applications of nanomaterials.

2D Material Optoelectronics for Information Functional Device Applications: Status and Challenges

Graphene and the following derivative 2D materials have been demonstrated to exhibit rich distinct optoelectronic properties, such as broadband optical response, strong and tunable light-mater interactions, and fast relaxations in the flexible nanoscale. Combining with optical platforms like fibers, waveguides, grating, and resonators, these materials has spurred a variety of active and passive applications recently. Herein, the optical and electrical properties of graphene, transition metal dichalcogenides, black phosphorus, MXene, and their derivative van der Waals heterostructures are comprehensively reviewed, followed by the design and fabrication of these 2D material-based optical structures in implementation. Next, distinct devices, ranging from lasers to light emitters, frequency convertors, modulators, detectors, plasmonic generators, and sensors, are introduced. Finally, the state-of-art investigation progress of 2D material-based optoelectronics offers a promising way to realize new conceptual and high-performance applications for information science and nanotechnology. The outlook on the development trends and important research directions are also put forward.

Surface Plasmon Resonance-Enhanced Near-Infrared Absorption in Single-Layer MoS2 with Vertically Aligned Nanoflakes

The development of MoS₂ with two- or three-dimensional heterostructures can provide a significant breakthrough for the enhancement of photodetection abilities such as increase in light absorption and expanding the detection ranges. Till date, although the synthesis of a MoS₂ layer with three-dimensional nanostructures using a chemical vapor deposition (CVD) process has been successfully demonstrated, most studies have concentrated on electrochemical applications that utilize structural strengths, for example, a large specific surface area and electrochemically active sites. Here, for the first time, we report spectral light absorption induced by plasmon resonances in single-layer MoS₂ (SL-MoS₂) with vertically aligned nanoflakes grown by a CVD process. Treatment with oxygen plasma results in the formation of a substoichiometric phase of MoO_x in the vertical nanoflakes, which exhibit a high electron density of 4.5 × 10¹³ cm^-2. The substoichiometric MoO_x with a high electron-doping level that is locally present on the SL-MoS₂ surface induces an absorption band in the near-infrared (NIR) wavelength range of 1000-1750 nm because of the plasmon resonances. Finally, we demonstrate the enhancement of photodetection ability by broadening the detection range from the visible region to the NIR region in oxygen-treated SL-MoS₂ with vertically aligned nanoflakes.

Coupled plasmon-phonon modes in monolayer MoS2

We present a theoretical study of the plasmon-phonon coupling in a suspended monolayer MoS₂ and a MoS₂ substrate system using a diagrammatic self-consistent field theory. The four coupled plasmon-phonon modes and the four plasmon-surface phonon modes are observed due to the spin-orbit and electron-optic phonon interactions. The two of coupled plasmon-phonon and plasmon-surface phonon modes are optic-like and the other two are acoustic-like. The plasmon are strongly coupled with the optic-phonon in MoS₂ and the surface optic-phonon in the substrates as the electron density or wave-vector increases. The strong plasmon-phonon coupling shows that the optoelectronic properties of monolayer MoS₂ are evidently modulated by electron-phonon interactions. The hybrid plasmon-phonon polaritons can be achieved by strong light-mater interactions. This study is relevant to the application of MoS₂ as novel plasmonic and nanophotonic devices.

A self-driven approach for local ion intercalation in vdW crystals

Intercalation has proven to be a powerful strategy for physical and chemical property modulation in two dimensional (2D) van der Waals (vdW) materials. Traditional gaseous and chemical intercalation methods offer the ability for mass production, and the electrochemical method provides reversible fine tuning for in situ material investigation. Spatial control, or even direct patterning, of ions is widely required for practical device fabrication and integration; yet it is not realized. Here we demonstrate a self-driven ion (Co2+, Sn4+, and Cu2+) intercalation approach with patterning ability on vdW α-MoO3. It is proved that the self-driven intercalation was enabled by the formation of a local galvanic cell and could be controlled by the metal electrode potential and the solution concentration. The universality of self-intercalation was confirmed in various types of 2D materials (MoS2, WS2, MoSe2, WSe2 and graphene). Furthermore, the feasibility of building heterostructures by multiple species (Sn & Co) intercalation in a single nanosheet was demonstrated for broadband photodetection. The enhancement of conductivity and photoresponse was found to be due to the synergistic effect of lattice distortion from Sn intercalation and the d orbital from the Co atom. This approach offers a feasible way for direct nano-fabrication in 2D vdW material and functional device integration.

Simultaneous exfoliation and colloidal formation of few-layer semiconducting MoS2 sheets in water

A simple method allowing for simultaneous liquid-phase exfoliation of MoS₂ crystals into few-layer sheets and colloidal formation of the exfoliated sheets is reported.

Challenges and recent advancements of functionalization of two-dimensional nanostructured molybdenum trioxide and dichalcogenides

Atomically thin two-dimensional (2D) semiconductors are the thinnest functional semiconducting materials available today. Among them, both molybdenum trioxide and chalcogenides (MT&Ds) represent key components within the family of different 2D semiconductors for various electronic, optoelectronic and electrochemical applications due to their unique electronic, optical, mechanical and electrochemical properties. However, despite great progress in research dedicated to the development and fabrication of 2D MT&Ds observed within the last decade, there are significant challenges that affected their charge transport behavior and fabrication on a large scale as well as there is high dependence of the carrier mobility on the thickness. In this article, we review the recent progress in the carrier mobility engineering of 2D MT&Ds and elaborate devised strategies dedicated to the optimization of MT&D properties. Specifically, the latest physical and chemical methods towards the surface functionalization and optimization of the major factors influencing the extrinsic transport at the electrode-2D semiconductor interface are discussed.

Fabrication of Troponin I Biosensor Composed of Multi-Functional DNA Structure/Au Nanocrystal Using Electrochemical and Localized Surface Plasmon Resonance Dual-Detection Method

In the present study, we fabricated a dual-mode cardiac troponin I (cTnI) biosensor comprised of multi-functional DNA (MF-DNA) on Au nanocrystal (AuNC) using an electrochemical method (EC) and a localized surface plasmon resonance (LSPR) method. To construct a cTnI bioprobe, a DNA 3 way-junction (3WJ) was prepared to introduce multi-functionality. Each DNA 3WJ arm was modified to possess a recognition region (Troponin I detection aptamer), an EC-LSPR signal generation region (methylene blue: MB), and an anchoring region (Thiol group), respectively. After an annealing step, the multi-functional DNA 3WJ was assembled, and its configuration was confirmed by Native-TBM PAGE for subsequent use in biosensor construction. cTnI was also expressed and purified for use in biosensor experiments. To construct an EC-LSPR dual-mode biosensor, AuNCs were prepared on an indium-tin-oxide (ITO) substrate using an electrodeposition method. The prepared multi-functional (MF)-DNA was then immobilized onto AuNCs by covalent bonding. Field emission scanning electron microscope (FE-SEM) and atomic force microscopy (AFM) were used to analyze the surface morphology. LSPR and electrochemical impedance spectroscopy (EIS) experiments were performed to confirm the binding between the target and the bioprobe. The results indicated that cTnI could be effectively detected in the buffer solution and in diluted-human serum. Based on the results of these experiments, the loss on drying (LOD) was determined to be 1.0 pM in HEPES solution and 1.0 pM in 10% diluted human serum. Additionally, the selectivity assay was successfully tested using a number of different proteins. Taken together, the results of our study indicate that the proposed dual-mode biosensor is applicable for use in field-ready cTnI diagnosis systems for emergency situations.

Dissolved Oxygen and Visible Light Irradiation Drive the Structural Alterations and Phytotoxicity Mitigation of Single-Layer Molybdenum Disulfide

Understanding environmental fate is a prerequisite for the safe application of nanoparticles. However, the fundamental persistence and environmental transformation of single-layer molybdenum disulfide (SLMoS₂, a 2D nanosheet attracting substantial attention in various fields) remain largely unknown. The present work found that the dissolution of SLMoS₂ was pH and dissolved oxygen dependent and that alterations in phase composition significantly occur under visible light irradiation. The 1T phase was preferentially oxidized to yield soluble species (MoO₄^2- and SO₄^2-), and the 2H phase remained as a residual. The transformed SLMoS₂ exhibited a ribbon-like and multilayered structure and low colloidal stability due to the loss of surface charge. Dissolved oxygen competitively captured the electrons of SLMoS₂ to generate superoxide radicals and accelerated the dissolution of nanosheets. Compared to pristine 1T-phase SLMoS₂, the transformed 2H-phase SLMoS₂ could not easily enter algal cells and induced a low developmental inhibition, oxidative stress, plasmolysis, photosynthetic toxicity and metabolic perturbation. The downregulation of amino acids and upregulation of unsaturated fatty acids contributed to the higher toxicity of 1T-phase SLMoS₂. The dissolved ions did not induce apparent phytotoxicity. The connections between environmental transformation (phase change and ion release) and phytotoxicity provide insights into the safe design and evaluation of 2D nanomaterials.

Active control of terahertz plasmon-induced transparency in the hybrid metamaterial/monolayer MoS2/Si structure

Active control of terahertz waves is critical to the development of terahertz devices. Two-dimensional materials with excellent optical properties provide more choices for opto-electrical devices due to the advancement in their preparation technology. We proposed a hybrid structure of a metamaterial/monolayer MoS2/Si and investigated its optical properties in the terahertz range. The plasmon-induced transparency (PIT) effect was observed in the transmission spectra, resulting from the near-field coupling of two bright modes. According to the simulated results, this phenomenon confirmed its dependency on the length of the cutwire and the distance between DSSRs. Furthermore, an external optical field supported by a 1064 nm laser could exert a switch effect on the sample. The resonances of the PIT metamaterial disappeared when the optical power was further increased, as the excited carriers in the MoS2/Si substrate blocked the coupling effect. In addition, the experimental results indicated that the PIT metamaterial enhanced the interaction of infrared light with the monolayer MoS2/Si substrate.

Two-Dimensional Materials on the Rocks: Positive and Negative Role of Dopants and Impurities in Electrochemistry

Two-dimensional (2D) materials, such as graphene and transition-metal chalcogenides, were shown in many works as very potent catalysts for industrially important electrochemical reactions, such as oxygen reduction, hydrogen and oxygen evolution, and carbon dioxide reduction. We critically discuss here the development in the field, showing that not only dopants but also impurities can have dramatic effects on catalysis. Note here that the difference between dopant and impurity is merely semantic-dopant is an impurity deliberately added to the material. We contest the general belief that all doping has a positive effect on electrocatalysis. We show that in many cases, dopants actually inhibit the electrochemistry of 2D materials. This review provides a balanced view of the field of 2D materials electrocatalysis.

Sb2Te3 topological insulator: surface plasmon resonance and application in refractive index monitoring

Topological insulators as new emerging building blocks in electronics and photonics present promising prospects for exciting surface plasmons and enhancing light-matter interaction. Thus, exploring the visible-range plasmonic response of topological insulators is significant to reveal their optical characteristics and broaden their applications at high frequencies. Herein, we report the experimental demonstration of a visible-range surface plasmon resonance (SPR) effect on an antimony telluride (Sb2Te3) topological insulator film. The results show that the SPR can be excited with a relatively small incident angle in the Kretschmann configuration based on the Sb2Te3 film. Especially, we develop an impactful digital holographic imaging system based on the topological insulator SPR and realize the dynamic monitoring of refractive index variation. Compared with the traditional SPR, the Sb2Te3-based SPR possesses a broader measurement range. Our findings open a new avenue for exploring the optical physics and practical applications of topological insulators, such as environmental and biochemical sensing.

An Ultrasensitive Silicon Photonic Ion Sensor Enabled by 2D Plasmonic Molybdenum Oxide

Silicon photonics has demonstrated great potential in ultrasensitive biochemical sensing. However, it is challenging for such sensors to detect small ions which are also of great importance in many biochemical processes. A silicon photonic ion sensor enabled by an ionic dopant-driven plasmonic material is introduced here. The sensor consists of a microring resonator (MRR) coupled with a 2D restacked layer of near-infrared plasmonic molybdenum oxide. When the 2D plasmonic layer interacts with ions from the environment, a strong change in the refractive index results in a shift in the MRR resonance wavelength and simultaneously the alteration of plasmonic absorption leads to the modulation of MRR transmission power, hence generating dual sensing outputs which is unique to other optical ion sensors. Proof-of-concept via a pH sensing model is demonstrated, showing up to 7 orders improvement in sensitivity per unit area across the range from 1 to 13 compared to those of other optical pH sensors. This platform offers the unique potential for ultrasensitive and robust measurement of changes in ionic environment, generating new modalities for on-chip chemical sensors in the micro/nanoscale.

Wearable, stable, highly sensitive hydrogel-graphene strain sensors

A stable and highly sensitive graphene/hydrogel strain sensor is designed by introducing glycerol as a co-solvent in the formation of a hydrogel substrate and then casting a graphene solution onto the hydrogel in a simple, two-step method. This hydrogel-based strain sensor can effectively retain water in the polymer network due to the formation of strong hydrogen bonding between glycerol and water. The addition of glycerol not only enhances the stability of the hydrogel over a wider temperature range, but also increases the stretchability of the hydrogel from 800% to 2000%. The enhanced sensitivity can be attributed to the graphene film, whereby the graphene flakes redistribute to optimize the contact area under different strains. The careful design enables this sensor to be used in both stretching and bending modes. As a demonstration, the as-prepared strain sensor was applied to sense the movement of finger knuckles. Given the outstanding performance of this wearable sensor, together with the proposed scalable fabrication method, this stable and sensitive hydrogel strain sensor is considered to have great potential in the field of wearable sensors.

CO2 -Assisted Conversion of Crystal Two-Dimensional Molybdenum Oxide to Amorphism with Plasmon Resonances

Localized surface plasmon resonances (LSPRs) of ultra-thin two-dimensional (2D) nanomaterials have opened up a new regime in plasmonics in the last several years. 2D plasmonic materials are currently concentrated on the crystal structure, with amorphous materials hardly being reported because of their limited preparation methods rather than undesired plasmonic properties. Taking molybdenum oxides as an example, herein, we elaborate the 2D amorphous plasmons prepared with the assistance of supercritical CO₂ . In brief, we examine the reported characteristic plasmonic properties of molybdenum oxides, and applications of supercritical CO₂ in formations of 2D layer materials as well as introduced phase and disorder engineering based on our research. Furthermore, we propose our perspective on the development of 2D plasmons, especially for amorphous layer materials in the future.

3D SERS substrate based on Au-Ag bi-metal nanoparticles/MoS2 hybrid with pyramid structure

It is very vital to construct the dense hot spots for the strong surface-enhanced Raman scattering (SERS) signals. We take full advantage of the MoS₂ edge-active sites induced from annealing the Ag film on the surface of the MoS₂. Furthermore, the composite structure of Au-Ag bi-metal nanoparticles (NPs)/MoS₂ hybrid with pyramid structure is obtained by the in situ grown AuNPs around AgNPs, which serves the optimal SERS performance (enhancement factor is ~9.67 × 10⁹) in experiment. Due to the introduction of AuNPs with the simple method, the denser hot spots contribute greatly to the stronger local electric field, which is also confirmed by the finite-different time-domain (FDTD) simulation. Therefore, the ultralow limit of detection (the LOD of 10^-13 and 10^-12 M respectively for the resonant R6G and non-resonant CV), quantitative detection and excellent reproducibility are achieved by the proposed SERS substrate. For practical application, the melamine molecule is detected with the LOD of 10^-10 M using the proposed SERS substrate that has the potential to be a food security sensor.

Synthesis, properties, and optoelectronic applications of two-dimensional MoS2 and MoS2-based heterostructures

As a two-dimensional (2D) material, molybdenum disulfide (MoS2) exhibits unique electronic and optical properties useful for a variety of optoelectronic applications including light harvesting. In this article, we review recent progress in the synthesis, properties and applications of MoS2 and related heterostructures. Heterostructured materials are developed to add more functionality or flexibility compared to single component materials. Our focus is on their novel properties and functionalities as well as emerging applications, especially in the areas of light energy harvesting or conversion. We highlight the correlation between structural properties and other properties including electronic, optical, and dynamic. Whenever appropriate, we also try to provide fundamental insight gained from experimental as well as theoretical studies. Finally, we discuss some current challenges and opportunities in technological applications of MoS2.

Plasmonics with two-dimensional semiconductors: from basic research to technological applications

Herein, we explore the main features and the prospect of plasmonics with two-dimensional semiconductors. Plasmonic modes in each class of van der Waals semiconductors have their own peculiarities, along with potential technological capabilities. Plasmons of transition-metal dichalcogenides share features typical of graphene, due to their honeycomb structure, but with damping processes dominated by intraband rather than interband transitions, unlike graphene. Spin-orbit coupling strongly affects the plasmonic spectrum of buckled honeycomb lattices (silicene and germanene), while the anisotropic lattice of phosphorene determines different propagation of plasmons along the armchair and zigzag directions. Black phosphorus is also a suitable material for ultrafast plasmonics, for which the active plasmonic response can be initiated by photoexcitation with femtosecond pulses. We also review existing applications of plasmonics with two-dimensional materials in the fields of thermoplasmonics, biosensing, and plasma-wave Terahertz detection. Finally, we consider the capabilities of van der Waals heterostructures for innovative low-loss plasmonic devices.

Affinity capture surface carboxyl-functionalized MoS2 sheets to enhance the sensitivity of surface plasmon resonance immunosensors

The development of functionalized molybdenum disulfide (MoS₂) has led to a new trend in the biosensing field, owing to its high sensitivity and bio-affinity characteristics with regards to the simple synthesis of carboxyl-functionalized MoS₂ nanocomposites. In this study, we used monochloroacetic acid (MCA) to successfully modify carboxyl-MoS₂. The efficiency of this MCA modification method showed a higher -COOH group content of 30.1%, mainly due to chlorine atoms occupying the MoS2 sulfur vacancy to allow for the formation of a strong bonding effect. This then enhanced the surface area of -COOH and improved the formation of covalent bonds between proteins. We demonstrated that MoS₂-COOH-based surface plasmon resonance (SPR) chips can provide excellent sensitivity and high affinity for immunoassay biomolecules detected in a low sample volume of 20 μl. With respect to the shifts of the SPR angles of the chips, the high binding affinity at a BSA concentration of 14.5 nM for a MoS₂-COOH chip, a MoS₂ chip and a traditional SPR chip are 4.69 m°, 2.49 m° and 1.53 m°, respectively. In addition, the MoS₂-COOH chip could amplify the SPR angle response by 3.1 folds and enhance the high association rate of ka by 212 folds compared to MoS₂ and traditional SPR chips. The results thus obtained revealed that the overall affinity binding value, K_A, of the MoS₂-COOH chip can be significantly enhanced by up to ∼ 6.5 folds that of the MoS₂ chip. In summary, the excellent binding affinity, biocompatible and high sensitivity suggest the potential of the clinical application of this MoS₂-COOH-based SPR chip detection method for in vitro diagnostic and point-of-care testing devices.

Environmental Transformations and Algal Toxicity of Single-Layer Molybdenum Disulfide Regulated by Humic Acid

The environmental transformations of nanomaterials are correlated with their behaviors and ecological risks. The applications of single-layer molybdenum disulfide (SLMoS₂) have rapidly developed in environmental fields, but the potential transformations and biological effects of SLMoS₂ remain largely unknown. This study revealed that humic acid (HA, over 10 mg/L) induced the scrolling of SLMoS₂ with light irradiation over a 56-day incubation. The colloidal stability of SLMoS₂ increased, and the aggregation ratio decreased from 0.59 ± 0.07 to 0.08 ± 0.01 nm/min after HA hybridization. Besides, compared with pristine SLMoS₂, the chemical dissolution rate of SLMoS₂ was up to 4.6-fold faster with HA exposure. These results demonstrate that HA affects the environmental fate and transformations of SLMoS₂. SLMoS₂-HA possessed a significantly widened direct band gap (2.06 eV) compared with that of SLMoS₂ (1.8 eV). SLMoS₂ acted as an electronic acceptor from HA, resulting in the separation of electron-hole pairs. Consequently, SLMoS₂-HA exhibited stronger peroxidase-like catalytic activity, which was approximately 2-fold higher than that of SLMoS₂. Moreover, the morphology and layered structure of SLMoS₂ changed, and the damage SLMoS₂ inflicted on microalgae was significantly reduced. This work provides insights into the behaviors and related biological risks of SLMoS₂ in aqueous environments.

Two-Dimensional Transition Metal Oxide and Chalcogenide-Based Photocatalysts

Two-dimensional (2D) transition metal oxide and chalcogenide (TMO&C)-based photocatalysts have recently attracted significant attention for addressing the current worldwide challenges of energy shortage and environmental pollution. The ultrahigh surface area and unconventional physiochemical, electronic and optical properties of 2D TMO&Cs have been demonstrated to facilitate photocatalytic applications. This review provides a concise overview of properties, synthesis methods and applications of 2D TMO&C-based photocatalysts. Particular attention is paid on the emerging strategies to improve the abilities of light harvesting and photoinduced charge separation for enhancing photocatalytic performances, which include elemental doping, surface functionalization as well as heterojunctions with semiconducting and conductive materials. The future opportunities regarding the research pathways of 2D TMO&C-based photocatalysts are also presented.

First-principles study of a MXene terahertz detector

2D transition metal carbides, nitrides, and carbonitrides called MXenes have attracted increasing attention due to their outstanding properties in many fields. By performing systematic density functional theory calculations, here we show that MXenes can serve as excellent terahertz detecting materials. Giant optical absorption and extinction coefficients are observed in the terahertz range in the most popular MXene, namely, Ti₃C₂, which is regardless of the stacking degree. Various other optical properties have been investigated as well in the terahertz range for in-depth understanding of its optical response. We find that the thermoelectric figure of merit (ZT) of stacked Ti₃C₂ flakes is comparable to that of carbon nanotube films. Based on excellent terahertz absorption and decent thermoelectric efficiency in MXenes, we finally suggest the promise of MXenes in terahertz detection applications, which includes terahertz bolometers and photothermoelectric detectors. Possible ZT improvements are discussed in large-scale MXene flake films and/or MXene-polymer composite films.

Giant Valley Splitting and Valley Polarized Plasmonics in Group V Transition-Metal Dichalcogenide Monolayers

Two-dimensional group VI transition-metal dichalcogenides (TMDs) provide a promising platform to encode and manipulate quantum information in the valleytronics. However, the two valleys are energetically degenerate, protected by time-reversal symmetry (TRS). To lift this degeneracy, one needs to break the TRS by either applying an external magnetic field or using a magnetic rare-earth oxide substrate. Here, we predict a different strategy to achieve this goal. We propose that the ferromagnetic group V TMD monolayer, in which the TRS is intrinsically broken, can produce a larger valley and spin splitting. A polarized ZnS(0001) surface is also used as a substrate, which shifts the valleys to the low-energy regime (near the Fermi level). Moreover, by calculating its collective electronic excitation behaviors, we show that such a system hosts a giant valley polarized terahertz plasmonics. Our results demonstrate a new way to design and use valleytronic devices, which are both fundamentally and technologically significant.

Plasmon modes of bilayer molybdenum disulfide: a density functional study

We explore the collective electronic excitations of bilayer molybdenum disulfide (MoS₂) using density functional theory together with random phase approximation. The many-body dielectric function and electron energy-loss spectra are calculated using an ab initio based model involving material-realistic physical properties. The electron energy-loss function of the bilayer MoS₂ system is found to be sensitive to either electron or hole doping and this is due to the fact that the Kohn-Sham band dispersions are not symmetric for energies above and below the zero Fermi level. Three plasmon modes are predicted, a damped high-energy mode, one optical mode (in-phase mode) for which the plasmon dispersion exhibits [Formula: see text] in the long wavelength limit originating from low-energy electron scattering and finally a highly damped acoustic mode (out-of-phase mode).