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One-dimensional (1D)/two-dimensional (2D) heterostructures offer attractive opportunities for developing high-performance gas sensors.

Non-precious metals have been considered as suitable alternatives for high-performance hydrogen evolution reactions (HER). Although the incorporation of carbon substances is shown to improve the number of active sites, electron transfer pathways, and long-term stability, there have been rare reports on their single-step scalable production. Herein, we realize free-standing two-dimensional (2D) carbon sheets heterostructured with nickel (Ni) nanocatalysts by pyrolyzing ultrathin layers of acetate tetrahydrate (i.e. the single precursor for both Ni and C sources) over water-soluble salt crystals. Such a salt-templated methodology is environmentally friendly and readily scalable without the implementation of sophisticated equipment. The resulting 2D carbon sheets exhibit an average small thickness of ∼ 3 nm and lateral dimensions with tens of micrometers, where a large number of nano-sized Ni particles with an average diameter of 14 nm are uniformly dispersed. Such 2D Ni-C sheets demonstrate a small overpotential of 111 mV at 10 mA/cm

Metal-organic frameworks (MOFs) with exceptional features such as high structural diversity and surface area as well as controlled pore size has been considered a promising candidate for developing room temperature highly-sensitive gas sensors. In comparison, the hetero-metallic MOFs with redox-active open-metal sites and mixed metal nodes may create peculiar surface properties and synergetic effects for enhanced gas sensing performances. In this work, the Fe atoms in the Fe3 (Porous coordination network) PCN-250 MOFs are partially replaced by transition metal Co, Mn, and Zn through a facile hydrothermal approach, leading to the formation of hetero-metallic MOFs (Fe2IIIMII, M=Co, Mn, and Zn). While the PCN-250 framework is maintained, the morphological and electronic band structural properties are manipulated upon the partial metal replacement of Fe. More importantly, the room temperature NO2 sensing performances are significantly varied, in which Fe2Mn PCN-250 demonstrates the largest response magnitude for ppb-level NO2 gas compared to those of pure Fe3 PCN-250 and other hetero-metallic MOF structures mainly attributed to the highest binding energy of NO2 gas. This work demonstrates the strong potential of hetero-metallic MOFs with carefully engineered substituted metal clusters for power-saving and high-performance gas sensing applications.

2D Ag2SO4 nanosheets have been delaminated from bulk Ag2S using a two-step combined exfoliation method. Upon blue light irradiation, the 2D Ag2SO4-based sensor exhibits high-performance responses toward low-concentrated NO2 gas at room temperature.

Wide-bandgap oxide semiconductors are essential for the development of high-speed and energy-efficient transparent electronics. However, while many high-mobility n-type oxide semiconductors are known, wide-bandgap p-type oxides have carrier mobilities that are one to two orders of magnitude lower due to strong carrier localization near their valence band edge. Here, we report the growth of bilayer beta tellurium dioxide (β-TeO2), which has recently been proposed theoretically as a high-mobility p-type semiconductor, through the surface oxidation of a eutectic mixture of tellurium and selenium. The isolated β-TeO2 nanosheets are transparent and have a direct bandgap of 3.7 eV. Field-effect transistors based on the nanosheets exhibit p-type switching with an on/off ratio exceeding 106 and a field-effect hole mobility of up to 232 cm2 V−1 s−1 at room temperature. A low effective mass of 0.51 was observed for holes, and the carrier mobility reached 6,000 cm2 V−1 s−1 on cooling to −50 °C. Bilayer beta tellurium dioxide nanosheets with p-type characteristics can be formed through the surface oxidation of a mixture of tellurium and selenium, and used to create transistors with performance that matches their n-type oxide counterparts.

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.

The predicted strong piezoelectricity for monolayers of group IV monochalcogenides, together with their inherent flexibility, makes them likely candidates for developing flexible nanogenerators. Within this group, SnS is a potential choice for such nanogenerators due to its favourable semiconducting properties. To date, access to large-area and highly crystalline monolayer SnS has been challenging due to the presence of strong inter-layer interactions by the lone-pair electrons of S. Here we report single crystal across-the-plane and large-area monolayer SnS synthesis using a liquid metal-based technique. The characterisations confirm the formation of atomically thin SnS with a remarkable carrier mobility of ~35 cm2 V−1 s−1 and piezoelectric coefficient of ~26 pm V−1. Piezoelectric nanogenerators fabricated using the SnS monolayers demonstrate a peak output voltage of ~150 mV at 0.7% strain. The stable and flexible monolayer SnS can be implemented into a variety of systems for efficient energy harvesting., The presence of strong inter-layer interactions has hindered the synthesis efforts towards large-area and highly crystalline monolayer SnS. Here, the authors report synthesis of large-area monolayer SnS using a liquid metal-based technique, and fabricate piezoelectric nano-generators with average peak output voltage of 150 mV at 0.7% strain.

Indium tin oxide (ITO) is a transparent conductor used in applications such as touch screens, smart windows and displays. A key limitation of ITO is its brittle nature, which prohibits its use in flexible electronics. The commercial deposition of high-quality ITO also currently relies on a costly vacuum manufacturing approach. Here we report the centimetre-scale synthesis of flexible two-dimensional ITO using a low-temperature liquid metal printing technique. The approach can directly deposit monolayer or bilayer ITO onto desired substrates, with the resulting bilayer samples offering a transparency above 99.3% and a sheet resistance as low as 5.4 kΩ □−1. We also show that the bilayer ITO features a stratified structure with a pronounced van der Waals spacing. To illustrate the capabilities of the technique, we develop a capacitive touch screen using centimetre-sized monolayer ITO sheets. A liquid metal printing technique can be used to create monolayer and bilayer indium tin oxide, with the bilayer samples offering a transparency above 99.3% and a sheet resistance as low as 5.4 kΩ □−1.

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.

Cancer is a complex disease associated with multiple gene mutations and malignant phenotypes, and multi-target drugs provide a promising therapy idea for the treatment of cancer. Natural products with abundant chemical structure types and rich pharmacological characteristics could be ideal sources for screening multi-target antineoplastic drugs. In this paper, 50 tumor-related targets were collected by searching the Therapeutic Target Database and Thomson Reuters Integrity database, and a multi-target anti-cancer prediction system based on mt-QSAR models was constructed by using naïve Bayesian and recursive partitioning algorithm for the first time. Through the multi-target anti-cancer prediction system, some dominant fragments that act on multiple tumor-related targets were analyzed, which could be helpful in designing multi-target anti-cancer drugs. Anti-cancer traditional Chinese medicine (TCM) and its natural products were collected to form a TCM formula-based natural products library, and the potential targets of the natural products in the library were predicted by multi-target anti-cancer prediction system. As a result, alkaloids, flavonoids and terpenoids were predicted to act on multiple tumor-related targets. The predicted targets of some representative compounds were verified according to literature review and most of the selected natural compounds were found to exert certain anti-cancer activity in vitro biological experiments. In conclusion, the multi-target anti-cancer prediction system is very effective and reliable, and it could be further used for elucidating the functional mechanism of anti-cancer TCM formula and screening for multi-target anti-cancer drugs. The anti-cancer natural compounds found in this paper will lay important information for further study.

Metal-organic frameworks (MOFs) with exceptional features such as high structural diversity and surface area as well as controlled pore size has been considered a promising candidate for developing room temperature highly-sensitive gas sensors. In comparison, the hetero-metallic MOFs with redox-active open-metal sites and mixed metal nodes may create peculiar surface properties and synergetic effects for enhanced gas sensing performances. In this work, the Fe atoms in the Fe

Fiber-optic gas sensors have been considered a low-cost, effective, and robust approach for monitoring nitrogen dioxide (NO

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.

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.

The family of crystals constituting covalently bound strings, held together by van der Waals forces, can be exfoliated into smaller entities, similar to crystals made of van der Waals sheets. Depending on the anisotropy of such crystals, as well as the spacing between their strings in each direction, van der Waals sheets or ribbons can be obtained after the exfoliation process. In this work, we demonstrate that ultrathin nanoribbons of bismuth sulfide (Bi2S3) can be synthesized via a high-power sonication process. The thickness and width of these ribbons are governed by the van der Waals spacings around the strings within the parent crystal. The lengths of the nanoribbons are initially limited by the dimensions of the starting bulk particles. Interestingly, these nanoribbons change stoichiometry and composition and are elongated when the duration of agitation increases because of Ostwald ripening. An application of the exfoliated van der Waals strings is presented for optical biosensing using photoluminescence of Bi2S3 nanoribbons, reaching detection limits of less than 10 nM L-1 in response to bovine serum albumin. The concept of exfoliating van der Waals strings could be extended to a large class of crystals for creating bodies ranging from sheets to strings, with optoelectronic properties different from that of their bulk counterparts.

Background : Colorectal cancer (CRC) is the third most commonly diagnosed cancer in males and the second in females, whose survival ratio and indicating biomarkers are limited. The rapid development of multiple immunofluorescence gives rise to widespread applications of this newly advanced technology called multiplex immunohistochemistry (mIHC), which makes it possible to detect several fluorescent proteins on the same tumor tissue microarray (TMA) within the same time and spatial organization. Methods : By taking advantage of this mIHC technology, we detected three tumor-associated antigens (TAA) including the human epidermal growth factor receptor 2 (HER2), the cluster of differentiation 133 (CD133), the programmed death ligand-1 (PD-L1) and one immune-associated macrophage marker, the cluster of differentiation 68 (CD68) in cancer tissues versus para-carcinomatous normal tissues derived from a cohort of 84 CRC patients. Results: All the four markers were upregulated in cancer tissue compared with normal tissues. And the expressions of CD133, HER2, PD-L1 and CD68 were correlated with pathological grade, T stage, tumor size, metastasis, respectively. Accordingly, CD133 and PD-L1 could be applied as potential diagnostic biomarkers for CRC at early stage, while the enrichment of HER2 might act as an advanced indicator in aggressive cancer status of CRC; whereas, CD68 could be potentially considered as an advanced diagnostic indicator in CRC patients, as well as a metastatic promoter in CRC-related TME. Conclusions : The differential expression of these four proteins, as well as their clinicopathological correlation indicates that these four proteins could be utilized as diagnostic and prognostic biomarkers in CRC patients.

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.

The detection of cancer cells at the single-cell level enables many novel functionalities such as next-generation cancer prognosis and accurate cellular analysis. While surface-enhanced Raman spectroscopy (SERS) has been widely considered as an effective tool in a low-cost and label-free manner, however, it is challenging to discriminate single cancer cells with an accuracy above 90% mainly due to the poor biocompatibility of the noble-metal-based SERS agents. Here, we report a dual-functional nanoprobe based on dopant-driven plasmonic oxides, demonstrating a maximum accuracy above 90% in distinguishing single THP-1 cell from peripheral blood mononuclear cell (PBMC) and human embryonic kidney (HEK) 293 from human macrophage cell line U937 based on their SERS patterns. Furthermore, this nanoprobe can be triggered by the bio-redox response from individual cells towards stimuli, empowering another complementary colorimetric cell detection, approximately achieving the unity discrimination accuracy at a single-cell level. Our strategy could potentially enable the future accurate and low-cost detection of cancer cells from mixed cell samples.

Two-dimensional (2D) crystals are promising materials for developing future nano-enabled technologies

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 (ε⊥ = 24) have been synthesized via a liquid metal synthesis strategy.

Silicon photonics has been demonstrated as a suitable platform for developing next generation on-chip biochemical micro- /nano-sensors due to its compactness, ultra-sensitivity, CMOS technology compatibility and extremely low power consumption, which other optical sensing platforms cannot provide as a combination. These sensors rely heavily on the change of ambient-refractive-index upon the immobilization of specific biomolecules via surface functionalization. However, if the immobilized biochemical species (e.g., ionic species) do not produce sufficient influence on the ambient-refractive-index, these sensors become "blind", hence hindering their wide applicability.In this paper, we attempt to divert the research interest by introducing the concept of functionalization-free silicon photonic biochemical sensors coupled with two-dimensional (2D) tunable plasmonic materials for ultra-sensitive ionic sensing. Here, 2D molybdenum oxide is chosen as its plasmon resonance wavelength is close to the operation wavelength of the silicon chip. At the same time, the induced scattering and absorption losses are kept low enough at the operation wavelength, maintaining relatively high optical performance of the silicon chip and hence sustaining excellent sensitivity. These important features make it the first successful report on the integration of 2D material onto the silicon photonic platform for sensing applications.Through the demonstration of a pH sensing model, the exposure of ionic analytes results in the modification of ion-driven plasmonic properties, hence changing the refractive index and plasmonic absorption properties of 2D molybdenum oxide. Consequently, the phase and amplitude of the light transmitted through the silicon photonic device are affected due to the evanescent interaction between the guided optical mode and the plasmonic material. Such a dual sensing output has never been successfully achieved by other optical sensing platform and results in the great enhancement of the sensor robustness. More importantly, this unique sensor demonstrates a up to 7 orders improvement on sensitivity per unit area over all other optical pH sensors, therefore generates new modalities for on-chip biochemical sensors in the micro/nano-scale. In addition, the possible combination with conventional biorecognition elements will potentially widen the applicability of silicon photonic based sensors and have a notorious impact in the development of more advanced and sophisticated biosensors.

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.

Atomically thin, semiconducting transition and post transition metal oxides are emerging as a promising category of materials for high-performance oxide optoelectronic applications. However, the wafer-scale synthesis of crystalline atomically thin samples has been a challenge, particularly for oxides that do not present layered crystal structures. Herein we use a facile, scalable method to synthesise ultrathin bismuth oxide nanosheets using a liquid metal facilitated synthesis approach. Monolayers of α-Bi2O3 featuring sub-nanometre thickness, high crystallinity and large lateral dimensions could be isolated from the liquid bismuth surface. The nanosheets were found to be n-type semiconductors with a direct band gap of ∼3.5 eV and were suited for developing ultra violet (UV) photodetectors. The developed devices featured a high responsivity of ∼400 AW-1 when illuminated with 365 nm UV light and fast response times of ∼70 μs. The developed methods and obtained nanosheets can likely be developed further towards the synthesis of other bismuth based atomically thin chalcogenides that hold promise for electronic, optical and catalytic applications.

Nanoporous plasmonic nanostructures based on noble metals have received extensive attention on the high-performance detection of chemical analytes. However, the size of such pores is rarely at the angstrom scale given the limitation of current fabrication methods. This leads to a relatively poor performance in the detection of ions. Here, we demonstrate the formation of angstrom-scale pores in ultra-thin plasmonic ammonium doped molybdenum oxide through crystal nucleation. The molybdenum oxide octahedra assemble into hexagonal rings, in which dopants fill in parts of the rings to stabilize the crystal structure and simultaneously generate a broad plasmon resonance across the visible to near-infrared regions. As a result, the unfilled centers of the rings effectively become angstrom-scale pores. Na+ ion sensing capability is investigated by integrating plasmonic ammonium doped molybdenum oxide onto D -shaped optical fibers. The ions are facilely accommodated within the pores and induce a charge re-distribution in the host. This alters the plasmon resonance behavior and modulates the optical output of the fiber transducing platform, through a strong light-matter interaction. The structure is sensitive to a wide concentration range of Na+ ions from subnanomolar (sub-nM) to submolar (sub-M) with the limit of detection (LOD) of ~5 fM, and high selectivity in both the aqueous solution and simulated serum conditions, which is a superior sensitivity over other reported optical ion sensors. This work demonstrates the strong potential of angstrom-scale porous plasmonic materials for chemical detection, and the possibility of being integrated with popular optical transducing platforms for practical high-performance sensing.

The emerging two-dimensional (2D) materials have led to the revolution across many fields in optics, electronics, optoelectronics, and sensors. Physical sensors such as photodetector and chemical sensors like gas and biological sensors play important roles in optical communications, imaging, environmental monitoring, remediation, as well as healthcare and medical industries. The implementation of 2D materials can significantly enhance the performances of such sensors due to their ultra-thin planar surface, large surface-to-volume ratio, and unique physiochemical properties. Peculiar features such as tunable band structures and relatively large charge carrier mobilities in certain 2D materials further provide additional dimensions to realize high-performance sensors from optical, electronic, and optoelectronic transducing platforms. Enabled by the weak van der Waals (vdW) force, individual 2D materials can be artificially stacked to realize the atomically thin 2D heterostructures, producing unprecedented features which are not accessible in the individual 2D counterparts or other low-dimensional heterostructures. Here, the atomically thin 2D heterostructures are comprehensively reviewed by firstly summarizing their controllable and scalable synthesis. Meanwhile, the band structure alignment and interfacial charge transfer behavior of 2D heterostructures are specifically introduced and their influences on physical and chemical sensing are revealed. In addition, the state-of-the-art progress is reported and critically discussed. Finally, the current challenges and prospects of 2D heterostructure-based sensors are provided.

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.

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.