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Gallium (Ga) and Ga-based liquid metal (LM) alloys offer low toxicity, excellent electrical and thermal conductivities, and fluidity at or near room temperature. Ga-based LM particles (LMPs) synthesized from these LMs exhibit both fluidic and metallic properties and are suitable for versatile functionalization in therapeutics. Functionalized Ga-based LMPs can be actuated using physical or chemical stimuli for drug delivery, cancer treatment, bioimaging, and biosensing. However, many of the fundamentals of their unique characteristics for therapeutics remain underexplored. We present the most recent advances in Ga-based LMPs in therapeutics based on the underlying mechanisms of their design and implementation. We also highlight some future biotechnological opportunities for Ga-based LMPs based on their extraordinary advantages.

Synthesizing two dimensional (2D) nanomaterials with controlled sub-nanometer thicknesses from non-layered crystals presents both significant challenges and vast opportunities. However, mechanical exfoliation techniques and physical/wet chemical deposition processes are widely disadvantageous for applicability to non-layered structures. Here we have utilized a simple self-limiting approach to prepare large sheets of 2D zinc oxide (ZnO) at the metal-melt/air interface. These ultra-thin sheets demonstrated highly crystalline hexagonal structures. The specific ZnO hexagonal sheet thickness and its interaction with the substrate were found to have a critical impact on d33. This unusual structure resulted in an exceptionally high out of plane piezoelectricity, yielding a giant value of 80 ± 0.8 pm/V at 2.5 unit-cell thickness for d33, which is 5 Zn-O layers in the wurtzite crystal. This out of plane piezoelectricity value is approximately 8 times larger than that of the value for bulk ZnO. Theoretical studies were carried out to elucidate the impact of the thickness and the substrate’s role on the polarization of the layers. The existence of a large piezoelectricity offered by the synergy of the substrate and specific thickness of ultrathin films offers the opportunity for other groups of potentially piezoelectric materials to be explored.

Ultrathin transparent conductive oxides (TCOs) are emerging candidates for next-generation transparent electronics. Indium oxide (In2O3) incorporated with post-transition-metal ions (e.g., Sn) has been widely studied due to their excellent optical transparency and electrical conductivity. However, their electron transport properties are deteriorated at the ultrathin two-dimensional (2D) morphology compared to that of intrinsic In2O3. Here, we explore the domain of transition-metal dopants in ultrathin In2O3 with the thicknesses down to the single-unit-cell limit, which is realized in a large area using a low-temperature liquid metal printing technique. Zn dopant is selected as a representative to incorporate into the In2O3 rhombohedral crystal framework, which results in the gradual transition of the host to quasimetallic. While the optical transmittance is maintained above 98%, an electron field-effect mobility of up to 87 cm2 V-1 s-1 and a considerable sub-kΩ-1 cm-1 ranged electrical conductivity are achieved when the Zn doping level is optimized, which are in a combination significantly improved compared to those of reported ultrathin TCOs. This work presents various opportunities for developing high-performance flexible transparent electronics based on emerging ultrathin TCO candidates.

Two-dimensional (2D) crystals are promising materials for developing future nano-enabled technologies1-6. The cleavage of weak, interlayer van der Waals bonds in layered bulk crystals enables the production of high-quality 2D, atomically thin monolayers7-10. Nonetheless, as earth-abundant compounds, metal oxides are rarely accessible as pure and fully stoichiometric monolayers owing to their ion-stabilized 'lamellar' bulk structure11-14. Here, we report the discovery of a layered planar hexagonal phase of oxides from elements across the transition metals, post-transition metals, lanthanides and metalloids, derived from strictly controlled oxidation at the metal-gas interface. The highly crystalline monolayers, without the support of ionic dopants or vacancies, can easily be mechanically exfoliated by stamping them onto substrates. Monolayer and few-layered hexagonal TiO2 are characterized as examples, showing p-type semiconducting properties with hole mobilities of up to 950 cm2 V-1 s-1 at room temperature. The strategy can be readily extended to a variety of elements, possibly expanding the exploration of metal oxides in the 2D quantum regime.

On-chip optical switches have emerged as a new class of photonic components for high-performance optical communication networks and on-chip interconnects, in which the all-optical configuration without the incorporation of other control-means is highly desired. While two-dimensional (2D) ultrathin materials demonstrate their great potential in developing ultrafast all-optical switches owing to their unique light–matter interaction, such investigations have so far been limited to the fiber optic platform or free space. Here, we realize an all-optical on-chip switch from a silicon waveguide-based asymmetric Mach–Zehnder interferometer (MZI) structure enabled by 2D ultrathin Ga2S3. Upon the visible light excitation at 532 nm, excessive photocarriers in Ga2S3 cause a change of the refractive index and subsequently a phase variation between MZI arms at the 1550 nm operation wavelength, triggering on the optical switch. On the other hand, the switch is off without the visible light stimulation, as the phase variation is recovered due to the ultrafast photo-exciton relaxation behavior of Ga2S3. The Ga2S3-enabled all-optical switch is driven at an extremely small optical power density of 0.12 W cm−2 and exhibits a response and recovery time of 26.3 and 43.5 μs, respectively, which as a combination is superior to those of fiber optic-based all-optical switches enabled by 2D materials. This work may provide a viable approach to develop on-chip all-optical photonic components for practical integrated photonic chips.

In this study, we analyze the influence of the pore structure of an SBA-15 particle on the light emission from its inner adsorbed quantum dots (QDs) and outer light-emitting diode (LED) chips. It is found that the particle features of a high refractive index, comparable feature size of pore structure, and lower amount of QD adsorption help with QD light extraction, demonstrating a mechanism to suppress QD light propagating through pores and thus reducing the reabsorption loss. We consequently developed highly efficient QD white LEDs with wet-mixing QD/SBA-15 nanocomposite particles (NPs) by further optimizing the packaging methods and the introduced NP mass ratio. The LEDs demonstrated a record luminous efficacy (the ratio of luminous flux to electrical power) of 206.8 (entrusted test efficiency of 205.8 lm W-1 certificated by China National Accreditation Service) and 137.6 lm W-1 at 20 mA for white LEDs integrating only green QDs and green-red QD color convertors, respectively, with improved operating stability. These results are comparable to conventional phosphor-based white LEDs, which can be a starting point for white LEDs only using QDs as convertors toward commercialization in the near future.

The emergence of piezoelectric properties in two-dimensional (2D) layered transition metal dichalcogenides (TMDs) has triggered the intensive research on using low dimensional materials for conversion of mechanical stimuli into electrical signals or vice versa. While the bulk intrinsically presents no piezoelectric property, the origin of the piezoelectric responses in their 2D thin planes is ascribed to the loss of centrosymmetry. There are also other categories of 2D layered materials such as post-transition metal dichalcogenides (PTMDs) that might be of interests, which have been confirmed theoretically and are yet to be fully explored experimentally. In this work, we investigate the thickness-dependent piezoelectric responses of 2D tin disulfide (SnS2) nanosheets as a representative of layered PTMDs. The results indicate that the 2D SnS2 nanosheets with a thickness of ∼4 nm present an effective out-of-plane piezoelectric response of 2 ± 0.22 pm/V. Furthermore, the thickness dependence of the piezoelectric behavior at a resonant frequency shows that the piezoelectric coefficient decreases with increasing the thickness of 2D SnS2 nanosheets. Additionally, in reference to periodically poled lithium niobate piezoelectric crystal, the measured effective lateral piezoelectric coefficients at different voltages range from 0.61 to 1.55 pm/V with the average value at ∼1 pm/V. This study expands candidates for new piezoelectric materials in the 2D domain with comparable vertical and lateral coefficients, potentially opening a broader horizon for integration into sensors, actuators, and micro- and nanoelectromechanical systems.

The burgeoning research into two-dimensional (2D) materials opens a door to novel photonic and optoelectronic devices utilizing their fascinating electronic and photonic properties in thin-layered architectures. The hybrid integration of 2D materials onto integrated optics platforms thus becomes a potential solution to tackle the bottlenecks of traditional optoelectronic devices. In this paper, we present the recent advances of hybrid integration of a wide range of 2D materials on integrated optics platforms for developing high-performance photodetectors, modulators, lasers, and nonlinear optics. Such hybrid integration enables fully functional on-chip devices to be readily accessible researchers and technology developers, becoming a potential candidate for next-generation photonics and optoelectronics industries.

Nitrogen-doped carbon catalysts prepared from amino-functionalized metal–organic frameworks [amino-MIL-101(Al)] were investigated for the oxygen-reduction reaction (ORR) with special emphasis on el...

The library of true two-dimensional materials is limited since many transition metal compounds are not stratified and can thus not be easily isolated as nanosheets. Here, micron-sized ultrathin rutile TiO2 nanosheets featuring uniform thickness (2 ± 0.5 nm) with dielectric constant (e⊥ = 24) have been synthesized via a liquid metal synthesis strategy.

Metal chalcogenides are important members of the two-dimensional (2D) materials family and have been extensively investigated for high-performance electronic device applications. However, when they are produced on a large-scale, their carrier mobilities are strongly influenced by the surface conditions. Here, we print indium sulphide (In2S3) with the thickness down to the single unit cell limit on wafer-scale out of metallic indium liquid, in which structural indium vacancies are formed in an orderly fashion. First principles investigations reveal that the unique ordered-vacancy structure results in a highly dispersive conduction band with low effective electron mass, forming multiple band-like electronic transport channels sandwiched within the crystal structure which are less influenced by the surface conditions. Back-gated field effect transistors are fabricated, and the measured mobility is up to 58 cm2 V-1 s-1 with a high degree of reproducibility, which is amongst one of the highest reported for wafer-scale-grown ultra-thin metal chalcogenides. This establishes ordered-vacancy-enabled semiconductors in the 2D geometry as suitable alternatives for new generation high-performance electronic devices.

Facile and scalable synthesis of two-dimensional (2D) non-layered materials is highly desirable for novel applications. Iron compounds including hematite (α-Fe2O3) and iron phosphide (FeP) are an important group of semiconductors but their 2D ultrathin morphology has been rarely reported. Here, we develop a water dissolvable template-based synthesis route to produce free-standing ultrathin iron compounds. Such a method also enables the tunability of morphology from mesoporous nanosheet to meso-macroporous hierarchical nanonet utilizing the aging process, while its corresponding surface active area is reduced simultaneously. Ultrathin hematite is relatively inert to electrochemical hydrogen evolution reaction (HER). However, FeP exhibits excellent catalytic performances with a relatively low overpotential of 117 mV and a Tafel slope of 56 mV dec−1, which is as a whole improved over those of reported free-standing binary transition-metal phosphide nanostructures. This work extends the possibility to produce high-quality 2D non-layered materials, which are expected to exhibit unique properties compared to their bulk counterparts.

Excitation wavelength-dependent photoluminescence (PL) in two-dimensional (2D) transition-metal chalcogenides enables a strong excitonic interaction for high-performance chemical and biological sensing applications. In this work, we explore the possible candidates in the domain of post-transition-metal chalcogenides. Few-layered 2D p-type tin monosulfide (SnS) nanoflakes with submicrometer lateral dimensions are synthesized from the liquid phase exfoliation of bulk crystals. Excitation wavelength-dependent PL is found, and the excitonic radiative lifetime is more than one order enhanced compared to that of the bulk counterpart because of the quantum confinement effect. Paramagnetic NO2 gas is selected as a representative to investigate the exciton-driven chemical-sensing properties of 2D SnS. Physisorption of NO2 results in the formation of dipoles on the surface of 2D SnS, causing the redistribution of photoexcited charges in the body and therefore modifying PL properties. For practical sensing applications, 2D SnS is integrated into a resistive transducing platform. Under light irradiation, the sensor exhibits excellent sensitivity and selectivity to NO2 at a relatively low operating temperature of 60 °C. The limit of detection is 17 parts per billion (ppb), which is significantly improved over other previously reported 2D p-type semiconductor-based NO2 sensors.

Microperoxidase-11 (MP-11) has been widely used in enzymatic reactions. Further improvement of its performance requires a better charge transfer and more exposure of its active site in the enzymatic conversions, which can be achieved by immobilisation of MP-11 into functional materials. However, conventional immobilisation techniques always suffer from non-specific and uncontrolled weak interactions and energy level of two entities in the hybrid is not perfectly matched, thus resulting in limited improvement of the system. In this work, a hybrid material of layered MoO3 and MP-11 was synthesised by a self-assembly technique through a covalent interaction. Physicochemical characterisation indicated that there is a charge transfer from MP-11 to MoO3 and a covalent bond is formed in the hybrid. A notable enhancement of biocatalysis and photocurrent conversion were observed in the studies, which are due to a synergistic effect and band alignment of the two entities in the hybrid. The superior combined properties provide a great opportunity for developing high performance enzymatic conversion systems.

Intriguing physical and chemical properties of atomically thin semiconductors provide avenues for the development of the next-generation electronics, optoelectronics, and sensing applications. Howe...

Molybdenum based compounds are an emerging class of non-metallic catalytic materials for the hydrogen evolution reaction (HER) in acidic media. However, most of them lose considerable catalytic performance and exhibit poor long-term stability in alkaline media. Here, planar molybdenum oxide, with high alkaline stability and ordered intracrystalline pores, is developed as the HER candidate. The pores with diameters in the order of ∼5–7 A are HER-active, and appear after an NH4+ doping-driven phase transition from the orthorhombic to hexagonal phase. Such a unique structure facilitates the diffusion of ionic entities and water molecules to the HER sites and helps in the removal of gaseous products, therefore improving the surface active area and reaction kinetics. These intracrystalline pores also reduce the long term stress on electrodes. The corresponding HER activity is extremely stable for >40 h in an alkaline medium at an overpotential of 138 mV with a Tafel slope of 50 mV dec−1. Such properties offer a superior combination compared to those of other reported molybdenum based nanostructures, hence providing a great opportunity for developing high-performance alkaline non-metal HER catalysts.

Extracellular vesicles and particles (EVPs), which contain the same surface proteins as their mother cells, are promising biomarkers for cancer liquid biopsy. However, most of the isolation methods of EVPs are time-consuming and complicated, and hence, sensitive detection and classification methods are required for EVPs. Here, we report a facile polyethylene glycol (PEG)-based method for isolating and classifying EVPs with label-free surface-enhanced Raman scattering (SERS) and pattern recognition algorithm. There are only three steps in the PEG-based isolation method, and it does not require ultracentrifugation, which makes it a low-cost and easy-to-use method. Three types of common male cancer cell lines, namely leukemia (THP-1), prostate cancer (DU-145), and colorectal cancer (COLO-205), and one healthy male blood sample, were utilized to isolate EVPs. To collect the SERS spectra of EVPs, a novel planar nanomaterial, namely amino molybdenum oxide (AMO) nanoflakes, was applied, with the enhancement factor being obtained as 3.2 × 102. Based on the principal component analysis and support vector machine (PCA-SVM) algorithm, cancer and normal EVPs were classified with 97.4% accuracy. However, among the cancer EVPs, the accuracy, precision, and sensitivity were found to be 90.0%, 90.9%, and 83.3% for THP-1; 86.7%, 80.0%, and 92.3% for DU-145; 96.7%, 83.3%, and 100% for COLO-205, respectively. Thus, this work will improve the isolation, detection, and classification of EVPs and promote the development of cancer liquid biopsies.

Highly confined and low-loss polaritons are known to propagate isotropically over graphene and hexagonal boron nitride in the plane, leaving limited degrees of freedom in manipulating light at the nanoscale. The emerging family of biaxial van der Waals materials, such as α-MoO3 and V2O5, support exotic polariton propagation, as their auxiliary optical axis is in the plane. Here, exploiting this strong in-plane anisotropy, we report edge-tailored hyperbolic polaritons in patterned α-MoO3 nanocavities via real-space nanoimaging. We find that the angle between the edge orientation and the crystallographic direction significantly affects the optical response, and can serve as a key tuning parameter in tailoring the polaritonic patterns. By shaping α-MoO3 nanocavities with different geometries, we observe edge-oriented and steerable hyperbolic polaritons as well as forbidden zones where the polaritons detour. The lifetime and figure of merit of the hyperbolic polaritons can be regulated by the edge aspect ratio of nanocavity., The possibility to manipulate the propagation of polaritons is limited by the isotropy of 2D materials like graphene and hexagonal boron nitride. Here, the authors exploit the anisotropy of α-MoO3 and study edge tailored hyperbolic polariton manipulation in α-MoO3 nanocavities via real space nanoimaging.

Phosphides of transition metals (TMPs) are a developing class of materials for hydrogen evolution reaction (HER) as an alternative to expensive noble metals to produce clean energy. Herein, the nitrogen-doped molybdenum oxide (MoOx) is developed via a facile and simple hydrothermal method, followed by annealing in the N2 atmosphere and phosphorization to form a nitrogen-doped oxygenated molybdenum phosphide (N-MoP) sphere-shaped structure. The developed N-doped phosphide structure depicts enhanced HER activity by reaching a current density of 10 mA cm-2 at a very low overpotential of only 87 mV, which is much better than annealed nitrogen-doped molybdenum oxide (A-MoOx) 138 mV in alkaline medium. N-MoP is a highly efficient electrocatalyst for HER attributed to a more exposed surface, large electrode/electrolyte interface and appropriate binding energies for reactants. This study extends the opportunity of developing nitrogen-doped TMPs, which can display exceptional properties as compared to their oxides.

Achieving high-performance in low temperature is challenging for conventional two-dimensional gas-sensing materials. With an optimized liquid-phase-exfoliation approach, we present the first atomically-thin Janus metal oxychalcogenide heterojunctions, exhibiting extraordinary gas sensitivity at room-temperature with visible-light excitation.

Noble metal palladium (Pd) has been widely used in hydrogen-related catalytic reactions. However, its performances toward hydrogen evolution reactions (HER) are intrinsically restricted due to a strong bonding of Pd–H thus make the hydrogen desorption difficult. In this work, being as an electron–cocatalyst, Pd nanoparticles are anchored on our well-established amorphous MoSx/TiO2 nanotube arrays (TNAs) electrocatalyst system through an electrochemical deposition technique. The unique electronic structure in the S-vacancies and/or unsaturated S atoms of MoSx significantly weaken the Pd–H bonding in the electrocatalytic process, facilitating the hydrogen desorption process. In the meantime, conductivity of MoSx/TNAs is largely improved due to incorporation of Pd nanoparticles into the system, which enables the charge transfer from electrode to active site of MoSx more efficient. The synergetic coupling of Pd and MoSx/TNAs result in a superior electrocatalytic activity, achieving an onset overpotential of −29 mV, overpotentials of −64 and −88 mV at −10 and −20 mA cm−2, which are equivalent to that from Pt catalysts.

Seeking for an efficient and stable electrocatalyst in a wide potential range is vital for the electrocatalytic reduction of CO2 into high-value added liquid fuels. Herein, the nanoporous Sn/SnO2 (np-Sn/SnO2) composites with high mesoporosity are fabricated through a two-step dealloying strategy. At all the applied potentials, the as-prepared np-Sn/SnO2 composites show obviously higher Faradaic efficiency of formate relative to porous Sn structures. More importantly, the np-Sn/SnO2 composites exhibit high FE HCOO − of >70% at a wide potential range from −0.8 to −1.4 V vs. RHE. In addition, np-Sn/SnO2 composites possess an excellent long-term stability over 58 h at −0.8 V vs. RHE. As compared to the porous Sn structures, the superiority of np-Sn/SnO2 composites toward electroreduction of CO2 to formate could be mainly attributed to their unique mesoporous structures with high-density grain boundaries and large surface area.

We report an enhancement in the efficiency of agitative liquid-phase exfoliation of stratified WS2 crystals under the influence of an electric field. We have been successfully able to increase the exfoliation efficiency of WS2 nanoflakes using a co-applied alternating electric field. Loss of centrosymmetry at the facial layers of WS2, together with the matching between the dispersive component of this material and solvent that weakens the van der Waals forces, augment the exfoliation process. However, this is not seen for MoS2 and graphene. The outcomes provide the base for future investigations on the influence of electric field for the exfoliation of layered structures.

Metal molybdates, such as lead molybdate (PbMoO4), are wide bandgap crystals with favourable photophysical and photocatalytic properties and also high stability. If these crystals are synthesized in two dimensional (2D) morphologies, they can offer the needed large surface-area for photo-reactions. However, this category of materials does not constitute natural stratified crystals, and hence cannot be readily formed into 2D sheets using conventional methods. Here we present a two-step synthesis approach. First, we exfoliate stratified alpha-MoO3 into alpha-MoO3-x. Subsequently, these defect rich 2D alpha-MoO3-x nanosheets are transformed into stable 2D PbMoO4 nanosheets by dipping-pulling. We show that the transformed 2D PbMoO4 display trap states within its bandgap, allowing its efficient performance as a photocatalyst under the visible light condition. The presented method in this work can be extended to establish a variety of highly stable defect rich 2D metal molybdates, which are otherwise challenging to achieve, for visible light region photo-reactions.

Two-dimensional amorphous MoSx (a-MoSx) has been confirmed to be a highly active and economic electrocatalyst for hydrogen evolution reaction (HER). The development of its hybrid cocatalyst is envisioned to bestow more active sites with appropriate crystal engineering and modified electronic properties for enhancing catalytic performance. In this work, a composite cocatalyst comprising a-MoSx (x = 1.78) and well-ordered anodized TiO2 nanotube arrays (TNAs) is successfully developed through a facile electrodeposition route. The synergistic coupling of the unique vector charge transfer effect of TNAs and proliferation of active sites in a-MoSx derived from the space confinement effect and curved interface growth of TNAs lead to a significant enhancement of HER activity, compared to those of other forms of MoS2-based electrodes that have been previously reported. The MoSx/TNAs electrode exhibits the relatively small onset overpotential of 88 mV and presents an overpotential of 157 mV at 10 mA cm–2 HER curren...

Ingestible sensors are potentially a powerful tool for monitoring human health. Sensors have been developed that can, for example, provide pH and pressure readings or monitor medication, but capsules that can provide key information about the chemical composition of the gut are still not available. Here we report a human pilot trial of an ingestible electronic capsule that can sense oxygen, hydrogen, and carbon dioxide. The capsule uses a combination of thermal conductivity and semiconducting sensors, and their selectivity and sensitivity to different gases is controlled by adjusting the heating elements of the sensors. Gas profiles of the subjects were obtained while modulating gut microbial fermentative activities by altering their intake of dietary fibre. Ultrasound imaging confirmed that the oxygen-equivalent concentration profile could be used as an accurate marker for the location of the capsule. In a crossover study, variations of fibre intake were found to be associated with differing small intestinal and colonic transit times, and gut fermentation. Regional fermentation patterns could be defined via hydrogen gas profiles. Our gas capsule offers an accurate and safe tool for monitoring the effects of diet of individuals, and has the potential to be used as a diagnostic tool for the gut. In a human pilot trial, ingestible electronic capsules, which can sense oxygen, hydrogen, and carbon dioxide gas in the gut, have been shown to be capable of monitoring changes in a person’s diet.

The electrochemical conversion of the greenhouse gas, carbon dioxide (CO2), to energy fuels and value-added chemicals presents one of the most valuable approaches to harvest pollutants and produce renewable energy. However, the stable molecular structure of CO2 and the sluggish reaction kinetics make CO2 reduction reaction (CO2RR) formidably challenging to achieve reaction rate and selectivity practical in industry. Graphene and its derivatives have been considered a group of intriguing materials to develop advanced CO2RR electrocatalysts due to their large specific surface area, remarkable electron transfer ability, superior stability, and easy tunability of the structure and surface properties. Herein, we comprehensively discuss the state-of-the-art electrocatalysts constructed with graphene and derivatives for active and selective CO2RR within the recent five years, mainly including the electrocatalysts with both metal-based (e.g., noble, non-noble, or combined thereof) and non-metal (e.g., doped, modified, defected, or composited) catalytic sites. To present the versatile, high-performance metal-based CO2RR electrocatalysts constructed with graphene, we further subdivide them according to the sizes, oxidation states, metal species synergies, dimensionalities, and versatility. Finally, we provide the challenges and perspectives in this emerging area of utilising CO2 to produce various carbon-based fuels and chemicals via graphene chemistry.

Graphene oxide-based membrane (GOM) has been massively studied in the dye/water separation field, predominately relying on their 1 nm-sized interlayer distance spacing (D-spacing). However, the inefficient removal of sub-nanosized dye molecules and swelling-induced weak long-term stability are currently two key challenges faced by GOM. In this work, we intercalate the soft polymeric polypyrrole (PPy) nanoparticles into GOM to address such gaps. The strong dye adsorption ability of PPy results in the increase of the sub-nanosized MB molecule rejection rate from 60% for GOM to 97% for GO-PPy membrane (GPM) after the initial filtration treatment. Simultaneously, the nanochannels of GPM are partially expanded, leading to an approximately a ~14 times water permeability (21.14 L m-2h−1 bar−1) improvement over GOM (1.56 L m-2h−1 bar−1). More importantly, the strong hydrogen-bond, electrostatic, and π-π interactions between GO and PPy improve the membrane mechanical stability and further reduce the D-spacing, while the enhanced separation ability of nano-sized dye molecules is also exhibited evidenced by the > 99% rejection rates of CV, EBT, CR, and TB. We consider that our strategy could promote the design and development of the high-performance GOM for nanofiltration applications.

This work presents the development of a highly selective wireless NO 2 gas sensor based on a resonant antenna circuit using a low temperature co-fired ceramic (LTCC) technique. The wireless LTCC based gas sensor consists of a planar inductor-interdigital capacitor (LC) resonant antenna, which incorporates two dimensional (2D) SnS 2 nanoflakes as a model NO 2 gas selective material. The fabrication of the LTCC platform is fully described, a brief description on the characterization of the gas sensitive material is presented and eventually the operation of the system for the sensing of NO 2 gas is investigated. The response of the LTCC template was associated to the changes of its resistance and capacitance due to the alterations in the properties of 2D SnS 2 as a result of physical adsorption of NO 2 gas molecules onto its surface. The wireless gas sensing performance under different operating temperatures and NO 2 gas concentrations are presented. The obtained LC gas sensor is successfully used for the wireless sensing of NO 2 in the background of atmospheric gas at concentrations lower than 0.6 ppm. The developed technology has a great promise for establishing wireless gas sensors for harsh environments

Attributing to their distinct thickness and surface dependent physicochemical properties, two dimensional (2D) nanostructures have become an area of increasing interest for interfacial interactions. Effectively, properties such as high surface-to-volume ratio, modulated surface activities and increased control of oxygen vacancies make these types of materials particularly suitable for gas-sensing applications. This work reports a facile wet-chemical synthesis of 2D tungsten oxide nanosheets by sonication of tungsten particles in an acidic environment and thermal annealing thereafter. The resultant product of large nanosheets with intrinsic substoichiometric properties is shown to be highly sensitive and selective to nitrogen dioxide (NO2) gas, which is a major pollutant. The strong synergy between polar NO2 molecules and tungsten oxide surface and also abundance of active surface sites on the nanosheets for molecule interactions contribute to the exceptionally sensitive and selective response. An extraordinary response factor of ∼30 is demonstrated to ultralow 40 parts per billion (ppb) NO2 at a relatively low operating temperature of 150 °C, within the physisorption temperature band for tungsten oxide. Selectivity to NO2 is demonstrated and the theory behind it is discussed. The structural, morphological and compositional characteristics of the synthesised and annealed materials are extensively characterised and electronic band structures are proposed. The demonstrated 2D tungsten oxide based sensing device holds the greatest promise for producing future commercial low-cost, sensitive and selective NO2 gas sensors.

The development of earth-abundant electrocatalysts for hydrogen evolution, with high activity and stability, is of great interest in the field of clean energy. The highly tunable chemical and physical properties of earth-abundant molybdenum oxides make them versatile for their incorporation into electrochemical and catalytic systems. Due to the layered crystal arrangement of orthorhombic α-MoO3, this material can be exfoliated into two dimensional (2D) nanosheets, featuring a large surface area. Variations in the oxidation states of molybdenum facilitate the crystal structure, morphology and oxygen vacancy tuning, making these oxide compounds suitable for electrochemical activities. Here, oxygen deficient 2D α-MoO3−x nanosheets (x = 0.045) are successfully synthesised, using a liquid phase exfoliation method, which display superior activity for the electrocatalytic hydrogen evolution reaction (HER) with a low overpotential and fast electron transfer. In alkaline media, the 2D compound exhibits an overpotential value of 142 mV at the standard current density of 10 mA cm−2 with excellent stability. Here, the 2D morphology, structural defects and oxygen vacancies in the planar construction of molybdenum oxide nanosheets significantly increase the active sites of the catalyst, which act as key factors to promote the HER performance. This work presents 2D α-MoO3−x nanosheets as strong candidates for the HER.

Layered transition metal dichalcogenides (TMDs) are important members in the family of two-dimensional (2D) materials. The large surface-to-volume ratio, combined with the fascinating tunable electronic and optical properties, low toxicity, unique van der Waals layered structure, and engineerable surface structure, renders 2D TMDs highly valuable for next-generation biosensing applications. Herein, the recent progress in the development of 2D TMDs-based biosensors is comprehensively reviewed, with special focus on the implementation of the structural, electronic and optical properties of 2D TMDs in the realization of high-performance biosensors with different configurations for a wide spectrum of bioanalytes and bio-species. In addition, the comparison on biosensing performances with graphene as the currently most studied 2D candidate is critically discussed. Finally, future perspectives are provided along the development progress of 2D TMDs-based biosensors which are currently undergoing an intense study. This work will lead researchers to explore more novel sensing candidates within the category of TMDs with exotic chemical composition, structure, morphologies, dimensionalities, and properties.

Atomically-thin Janus heterojunctions exhibit extraordinary electronic and optoelectronic properties, mainly due to their intrinsic built-in electric field. However, current investigations are limited within layered metal chalcogenides. Here, a free-standing ultra-thin Janus metal oxychalcogenide is realized from non-layered In2S3. In the presence of strong mechanical agitation in the liquid medium, the original tetragonal crystal is cleaved. Ambient oxygen atoms are subsequently diffused and incorporated into limited part of the crystal structure, forming the oxysulfide phase with the gradual transition into a hexagonal structure. While the integrity of the covalent bonding system is maintained, a tensile strain is generated as a result of the crystal coordination mismatching between the pure sulfide and oxysulfide phases, leading to a more than two orders enhancement on visible-light-driven exciton lifetime compared to that of pure In2S3. Such an impressive excitonic interaction provides the fundamentals to establish an ultra-sensitive and power-saving optoelectronic chemical sensing platform. As an example, dipoles generated by the surface adsorbed NO2 molecules cause significant re-distribution of photoinduced charges in the anisotropic non-layered Janus structure under visible light low-power excitation, resulting in a superior sub-ppb detection limit of NO2 gas at room temperature.

Graphene oxide based nanofiltration membranes (GONMs) have attracted tremendous attention for water purification applications given their excellent mechanical properties and tunable inter-layer distance spacing (D-spacing). However, the desalination performance is significantly degraded in the pressure-driven filtration. In this work, we find that the pressure-driven water desalination properties are nanochannel spacing and length dependent, in which the cation-π interaction possibly plays a critical factor in the desalination procedure. Through the demonstration of surface weak-reduction and membrane thickness tuning, the D-spacing and molecule transport route length of GONMs are identified to affect the quality and quantity of cation-π interaction, respectively, which are innovatively regulated by the intercalation of nano-sized lattices into large GO sheets. As a result, the regulated membrane improves the rejection rates of MgCl2, NaCl, MgSO4 and Na2SO4 up to 41.10%, 64.14%, 84.62% and 93.19%, respectively, compared to unregulated counterparts of 13.72%, 15.93%, 34.58% and 40.99%, respectively. In addition, the regulated membrane also exhibits excellent removal efficiency of >93% against tested dyes with different molecular weight and surface charge properties. We consider that our approach could lead to the development of a powerful GONMs based platform for future water treatment.

There is ongoing interest in exploring new two-dimensional materials and exploiting their functionalities. Here, a top-down approach is used for developing a new morphology of ultrathin nanosheets from highly ordered bismuth sulfide crystals. The efficient chemical delamination method exfoliates the bulk powder into a suspension of corrugated ultrathin sheets, despite the fact that the Bi2S3 fundamental layers are made of atomically thin ribbons that are held together by van der Waals forces in two dimensions. Morphological analyses show that the produced corrugated sheets are as thin as 2.5 nm and can be as large as 20 μm across. Determined atomic ratios indicate that the exfoliation process introduces sulfur vacancies into the sheets, with a resulting stoichiometry of Bi2S2.6. It is hypothesized that the nanoribbons were cross-linked during the reduction process leading to corrugated sheet formation. The material is used for preparing field effect devices and was found to be highly p-doped, which is att...

Single- or multi-layer transition metal oxides (TMOs) have a relatively longer history than other atomically thin materials. TMOs comprise many earth-abundant minerals and have been used for millenia as construction materials, pigments, lubricants, for heat management and many other applications. In TMOs, the transition metal s electrons are strongly pulled by oxygen, and consequently the structural, physical and chemical properties are mostly determined by the strongly correlated d electrons. TMOs are also highly tunable owing to the diversity of their chemical composition, crystal structure and relative ease in inducing oxygen defects. Two dimensional (2D) TMOs often show different physical and chemical properties in comparison to their bulk counterparts. These differences give rise to a variety of remarkable electronic properties such as high temperature superconductivity and multiferroicity and also unique optical, mechanical and thermal phenomena. Additionally, reducing the thickness of TMOs can alter their catalytic and chemical characteristics. Despite their unique properties, single- and few-layer TMOs have received relatively little attention compared to other recent families of atomically thin materials such as layered transition metal dichalcogenides. The overarching aim of this review is to bring the unique aspects of TMOs to the attention of the research community to establish a strong future for research on 2D and layered TMOs. In this review, a comprehensive overview regarding 2D and layered TMOs will be presented. The fundamentals and applications of planar TMOs are discussed. The manuscript will also present future prospects and pathways to new developments that are offered by such TMOs.

Liquid marbles are entities made of a small amount of liquid encapsulated by micro/nano scale particles. The functionality of liquid marbles depends on both the core liquid and particle coating. In this work, functional silver nanoparticle (Ag NPs)/oleic acid liquid marbles are developed using Ag NPs to encapsulate micro-sized oleic acid liquid spheres, which is achieved via sonication. We demonstrate that both the size of the encapsulated spheres and the order of the Ag NPs coating are a function of the sonication time. These micro-sized liquid marbles are investigated for sensing an organic thiolate as a model material. Their capacity for increasing solar light absorption and energy conversion into heat is also studied. The liquid marbles generated after sonication for 60 min show the strongest Raman peak signals which originate from the thiolate. They also demonstrate the highest solar energy absorption. We ascribe these enhancements to the increased surface area of the Ag NPs/oleic acid spheres and the optimum order of Ag NPs to produce the strongest surface enhanced Raman scattering. The liquid marbles can be used for the development of future biosensors and solar-thermal absorbers.

Molybdenum disulfide (MoS2) in its two-dimensional (2D) morphology has favorable catalytic properties for the hydrogen evolution reaction (HER). To fully take advantage of this capability and in order to enhance this material’s HER performance, viable methods for synthesizing and tuning electronic properties of 2D MoS2 should be developed. Here, we demonstrate a facile and nonhazardous approach to partially intercalate Li+ ions into the 2D MoS2 host structure. We show that such an intercalation is possible in a nonhazardous saline Li+ containing solution using a grinding/sonication-assisted exfoliation method in both dark and simulated sun irradiation conditions. A partial phase transformation from semiconducting (2H) to metallic (1T) phase is observed for the majority of flakes, after the process. We observe a notable enhancement of the HER activity for samples prepared under light illumination in the presence of a Li+ containing solution, in comparison to both pristine 2D MoS2 samples and samples proces...

Developing scalable methods of growing two dimensional molybdenum disulphide (2D MoS2) with strong optical properties, on any desired substrates, is a necessary step towards industrial uptake of this material for optical applications. In this study, Si/SiO2 substrates were functionalised using self-assembled monolayers of three different aminosilanes with various numbers of amine groups and molecular lengths as underlayers for enhancing the adherence of the molybdenum precursor. The tetrahedral [MoS4](2-) anion groups from the molybdenum precursor were bonded on these silanised Si/SiO2 substrates afterwards. The substrates were then treated with a combined thermolysis and sulphurisation step. The results showed that silanisation of the substrates using the longest chains and the largest number of amine groups provided a good foundation to grow quasi 2D MoS2 made from adjacent flakes in a mosaic formation. Microscopy and spectroscopy investigations revealed that these quasi 2D MoS2 formed using this long chain aminosilane resulted in flakes with lateral dimensions in micron and submicron ranges composed of adjoining MoS2 pieces of 20 to 60 nm in lateral dimensions, dominantly made of 3 to 5 MoS2 fundamental layers. The obtained quasi 2D MoS2 shows a high internal quantum efficiency of 2.6% associated with the quantum confinement effect and high stoichiometry of the adjoining nanoflakes that form the structure of the sheets. The synthesis technique in this study is reliable and facile and offers a procedure to form large, scalable and patternable quasi 2D MoS2 sheets on various substrates with enhanced optical properties for practical applications.

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

Two-dimensional (2D) tungsten disulfide (WS2) nanoflakes were synthesised and hybridised with carbon dots (CDs) using a facile two-step method of exfoliation of bulk tungsten disulphide followed by microwave irradiation of nanoflakes in a solution of citric acid. Physicochemical characterisation indicated that the hybrid consists of graphitic carbon dots with diameters of approximately 2–5 nm, attached to monolayer tungsten disulphide via electrostatic attraction forces. This synthesised hybrid material was investigated for photocatalytic applications. We found that within one hour approximately 30% more of the model organic dye was photodegraded by the hybrid material compared with the pristine 2D WS2. This enhancement was associated to the affinity of the CDs to the organic dye rather than heterojunctioning. Comparisons of the photocatalytic efficacy of this hybrid material with those of recently reported 2D transition metal dichalcogenides and their hybrids showed a significantly higher turnover frequency. Additionally, the presented microwave based synthesis method for developing hybrids of 2D WS2 and CDs, without making significant changes to the base 2D crystal structure and its surface chemistry, has not been demonstrated before. Altogether, the hybrid 2D material provides great potential for photocatalysis applications.

Substoichiometric molybdenum disulphide (MoSx) nanosheets are successfully synthesised following a novel reductive route using hydrazine salts. The resulting two dimensional crystals are found to be highly monodispersed in thickness, forming exclusively 1.9 ± 0.2 nm thick bilayers. The lateral dimensions of the nanosheets are governed by the precursor bulk particle's size. Exploring a range of hydrazine derivatives with various degrees of steric hindrance leads to the conclusion that intercalation does not occur during the process and that exfoliation is instead facilitated by the reduction of Mo centres leading to the exfoliation of substoichiometric bilayers with distorted lattices. The lattice distortion is found to be persistent across all samples with XPS analysis pointing towards a S to Mo ratio of 1.2. The resulting material features an electronic bandgap of 2.1 eV, which is wider than that of pristine monolayer MoS2 with relatively longer radiative decay time.

The design and preparation of nanoporous metal (NPM)/metallic oxides (or hydroxides) electrodes with good flexibility as well as high energy storage performance has become a great challenge for applications in wearable electronic products. Herein, we report a novel strategy to synthesize flexible nickel oxide/hydroxide coated nanoporous nickel (np-NiOxHy@Ni) electrode containing metallic glass (MG) interlayer (np-NiOxHy@Ni/MG/np-NiOxHy@Ni sandwich) by one-step dealloying of Ni40Zr20Ti40 MG ribbons. Benefiting from the ductile MG interlayer, the sandwich-like electrode presents excellent flexibility, which has not been reported for other dealloyed porous ribbons. Electrochemical measurements show that the electrode is essentially a battery-type material while it also exhibits pseudocapacitive behavior. Due to an integrated sandwich structure and np-NiOxHy@Ni shell@core network, the electrode displays an enhanced capacitance of 778 F cm−3 at 1 A cm−3 in KOH solutions (achieves 3536.01 F cm−3 when just considering the volume of the dealloyed layer) and a remarkable rate performance (80.3 % retention when the current density increase by 128-fold), as well as an outstanding cycle stability (100 % capacitance retention after 8000 cycles). Furthermore, the cable-like all-solid-state flexible supercapacitor (CAFS) device assembled by this flexible electrode can bear a bending of 0–180° without significant performance change. The highest volumetric energy density of 28.52 mW h cm−3 with a power density of 0.10 W cm−3 is obtained, which far exceeds commercially available supercapacitors (

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.

Two-dimensional piezotronics will benefit from the emergence of new crystals featuring high piezoelectric coefficients. Gallium phosphate (GaPO4) is an archetypal piezoelectric material, which does not naturally crystallise in a stratified structure and hence cannot be exfoliated using conventional methods. Here, we report a low-temperature liquid metal-based two-dimensional printing and synthesis strategy to achieve this goal. We exfoliate and surface print the interfacial oxide layer of liquid gallium, followed by a vapour phase reaction. The method offers access to large-area, wide bandgap two-dimensional (2D) GaPO4 nanosheets of unit cell thickness, while featuring lateral dimensions reaching centimetres. The unit cell thick nanosheets present a large effective out-of-plane piezoelectric coefficient of 7.5 ± 0.8 pm V−1. The developed printing process is also suitable for the synthesis of free standing GaPO4 nanosheets. The low temperature synthesis method is compatible with a variety of electronic device fabrication procedures, providing a route for the development of future 2D piezoelectric materials., Two-dimensional piezoelectric materials hold promise for nano-electromechanical technologies, yet it is challenging to prepare them in large areas with high sample homogeneity. Syed et al. surface print GaPO4 sheets with unit cell thickness over centimetres using a liquid metal-based synthesis process.

The unique properties of two-dimensional molybdenum disulfide (2D MoS2) have so far led to immense research regarding this material’s fundamentals, applications, and, more recently, its potential for biosensing. 2D MoS2 has properties that make it of great interest for developing biosensors. These properties include large surface area, tunable energy band diagrams, a comparatively high electron mobility, photoluminescence, liquid media stability, relatively low toxicity, and intercalatable morphologies. In this Review, the current progress on 2D MoS2 based biosensors is presented and the prospects for future possibilities of expanding its applications for a variety of biosensing applications are discussed.