Self-Formed Channel Devices Based on Vertically Grown 2D Materials with Large-Surface-Area and Their Potential for Chemical Sensor Applications

Recent Advances in Engineering of 2D Layered Metal Chalcogenides for Resistive-Type Gas Sensor

2D nanomaterials have triggered widespread attention in sensing applications. Especially for 2D layered metal chalcogenides (LMCs), the unique semiconducting properties and high surface area endow them with great potential for gas sensors. The assembly of 2D LMCs with guest species is an effective functionalization method to produce the synergistic effects of hybridization for greatly enhancing the gas-sensing properties. This review starts with the synthetic techniques, sensing properties, and principles, and then comprehensively compiles the advanced achievements of the pristine 2D LMCs gas sensors. Key advances in the development of the functionalization of 2D LMCs for enhancing gas-sensing properties are categorized according to the spatial architectures. It is systematically discussed in three aspects: surface, lattice, and interlayer, to comprehend the benefits of the functionalized 2D LMCs from surface chemical effect, electronic properties, and structure features. The challenges and outlooks for developing high-performance 2D LMCs-based gas sensors are also proposed.

A Terbium Ion-Based Dual-Ligand Fluorescent Probe for Highly Selective and Sensitive Detection of Cu2+ Ions

In this study, a fluorescent probe (GMP-Tb-SSA) utilizing lanthanide coordination polymer nanoparticles, GMP-Tb, as a sensing platform, and 5-sulfosalicylic acid (SSA) as a cofactor ligand was proposed for the detection of copper ions (Cu2+). GMP-Tb was synthesized by the self-assembly of guanine monophosphate (GMP) and terbium ion (Tb3+), and SSA was introduced as a sensitizer into the GMP-Tb network. Cu2+ could efficiently inhibit the electron transfer from the ligand GMP to the central ion, Tb3+, leading to a significant quench of fluorescence of Tb3+. The method is highly selective with a linear range of 0 to 21 µM and a detection limit of 300 nM. It is not interfered by metal ions, amino acids, and other species, and can be successfully applied to the detection of Cu2+ in real water samples.

Enhanced NO2 Sensitivity of Vertically Stacked van der Waals Heterostructure Gas Sensor and Its Remarkable Electric and Mechanical Tunability

Nanodevices based on van der Waals heterostructures have been predicted, and shown, to have unprecedented operational principles and functionalities that hold promise for highly sensitive and selective gas sensors with rapid response times and minimal power consumption. In this study, we fabricated gas sensors based on vertical MoS2/WS2 van der Waals heterostructures and investigated their gas sensing capabilities. Compared with individual MoS2 or WS2 gas sensors, the MoS2/WS2 van der Waals heterostructure gas sensors are shown to have enhanced sensitivity, faster response times, rapid recovery, and a notable selectivity, especially toward NO2. In combination with a theoretical model, we show that it is important to take into account created trapped states (flat bands) induced by the adsorption of gas molecules, which capture charges and alter the inherent built-in potential of van der Waals heterostructure gas sensors. Additionally, we note that the performance of these MoS2/WS2 heterostructure gas sensors could be further enhanced using electrical gating and mechanical strain. Our findings highlight the importance of understanding the effects of altered built-in potentials arising from gas molecule adsorption induced flat bands, thus offering a way to enhance the gas sensing performance of van der Waals heterostructure gas sensors.

2D Materials in Flexible Electronics: Recent Advances and Future Prospectives

Flexible electronics have recently gained considerable attention due to their potential to provide new and innovative solutions to a wide range of challenges in various electronic fields. These electronics require specific material properties and performance because they need to be integrated into a variety of surfaces or folded and rolled for newly formatted electronics. Two-dimensional (2D) materials have emerged as promising candidates for flexible electronics due to their unique mechanical, electrical, and optical properties, as well as their compatibility with other materials, enabling the creation of various flexible electronic devices. This article provides a comprehensive review of the progress made in developing flexible electronic devices using 2D materials. In addition, it highlights the key aspects of materials, scalable material production, and device fabrication processes for flexible applications, along with important examples of demonstrations that achieved breakthroughs in various flexible and wearable electronic applications. Finally, we discuss the opportunities, current challenges, potential solutions, and future investigative directions about this field.

Field Effect Transistor Gas Sensors Based on Mechanically Exfoliated Van der Waals Materials

The high surface-to-volume ratio and flatness of mechanically exfoliated van der Waals (vdW) layered materials make them an ideal platform to investigate the Langmuir absorption model. In this work, we fabricated field effect transistor gas sensors, based on a variety of mechanically exfoliated vdW materials, and investigated their electrical field-dependent gas sensing properties. The good agreement between the experimentally extracted intrinsic parameters, such as equilibrium constant and adsorption energy, and theoretically predicted values suggests validity of the Langmuir absorption model for vdW materials. Moreover, we show that the device sensing behavior depends crucially on the availability of carriers, and giant sensitivities and strong selectivity can be achieved at the sensitivity singularity. Finally, we demonstrate that such features provide a fingerprint for different gases to quickly detect and differentiate between low concentrations of mixed hazardous gases using sensor arrays.

The Road for 2D Semiconductors in the Silicon Age

Continued reduction in transistor size can improve the performance of silicon integrated circuits (ICs). However, as Moore's law approaches physical limits, high-performance growth in silicon ICs becomes unsustainable, due to challenges of scaling, energy efficiency, and memory limitations. The ultrathin layers, diverse band structures, unique electronic properties, and silicon-compatible processes of 2D materials create the potential to consistently drive advanced performance in ICs. Here, the potential of fusing 2D materials with silicon ICs to minimize the challenges in silicon ICs, and to create technologies beyond the von Neumann architecture, is presented, and the killer applications for 2D materials in logic and memory devices to ease scaling, energy efficiency bottlenecks, and memory dilemmas encountered in silicon ICs are discussed. The fusion of 2D materials allows the creation of all-in-one perception, memory, and computation technologies beyond the von Neumann architecture to enhance system efficiency and remove computing power bottlenecks. Progress on the 2D ICs demonstration is summarized, as well as the technical hurdles it faces in terms of wafer-scale heterostructure growth, transfer, and compatible integration with silicon ICs. Finally, the promising pathways and obstacles to the technological advances in ICs due to the integration of 2D materials with silicon are presented.

Novel Hydrazone Chromophore Sensor for Metallochromic Determination of Cadmium Ions

For the detection of Cd(II) in aquatic media, a novel dicyanomethylene dihydrofuran hydrazone(DCDHFH)-based colorimetric chemosensor was developed. DCDHFH was prepared by an azo-coupling process involving the diazonium chloride of 2, 4-dichloroaniline and a dicyanomethylene dihydrofuran heterocyclic moiety bearing an active methyl group. The DCDHFH chromophore showed strong solvatochromism depending on solvent polarity due to electronic delocalization. The pH sensory effects of the DCDHFH chromophore were also explored. DCDHFH could be used to identify Cd(II) in the presence of other competitive metals, as indicated by variations in color and absorbance spectra. In the presence of cadmium ions, the synthesized DCDHFH probe with hydrazone recognition moiety exhibited a significant sensitivity and selectivity to cadmium ions at the ppm concentration level (10–250 ppm). A DCDHFH-immobilized paper test strip was also prepared and effectively used for the detection of cadmium in aqueous media at various concentrations. According to CIE Lab’s criteria, colorimetric strength (K/S), and the UV–Vis absorbance spectra, the cadmium detection abilities of the DCDHFH-immobilized paper strips were evaluated. The optimal pH range for the determination of Cd(II) was monitored in the area of 5.5–6.3, with a fast chromogenic change from yellow to red relying on the Cd(II) concentration. The deposition of dicyanomethylene dihydrofuran hydrazone onto the paper strip’s surface was studied by scanning electron microscopy (SEM).

Designing doping strategy in arsenene monolayer for spintronic and optoelectronic applications: a case study of germanium and nitrogen as dopants

In this work, the structural, electronic, and magnetic properties of arsenene monolayer doped with germanium (Ge) and nitrogen (N) atoms are investigated using density functional theory calculations. Pristine monolayer is dynamically stable and it possesses a wide indirect band gap. Ge doping induces magnetic semiconductor (MS) nature generated by the semiconductor behavior in both spin channels with significant spin asymmetry around the Fermi level. The dopant produces mainly magnetic properties. Upon increasing the doping concentration, different doping configurations along armchair, zigzag edges, and hexagonal ring have been proposed. The MS nature is retained with an odd number of Ge atoms, meanwhile an eVen number leads to the disappearance of magnetism. In contrast, N doping induces a gap reduction of 11.80%, preserving the non-magnetic nature. At higher doping level, different electronic features including semiconductor, nearly semimetallic, and metallic natures are obtained depending on the doping concentration and configurations. In addition, the formation energy and cohesive energy are calculated to analyze the systems' stability. Our results show that different doping arrangements induce novel features in arsenene monolayer for applications in spintronic and optoelectronic devices.

Temperature-Dependent n-p-n Switching and Highly Selective Room-Temperature n-SnSe2/p-SnO/n-SnSe Heterojunction-Based NO2 Gas Sensor

Many toxic gases are mixed into the atmosphere because of increased air pollution. An efficient gas sensor is required to detect these poisonous gases with its ultrasensitive ability. We employed the thermal evaporation method to deposit an n-SnSe₂/p-SnO/n-SnSe heterojunction and observed a temperature-dependent n-p-n switching NO₂ gas sensor with high selectivity working at room temperature (RT). The structural and morphological properties of the material were studied using the characterization techniques such as XRD, SEM, Raman spectroscopy, XPS, and HRTEM, respectively. At RT, the device response was 256% for 5 ppm NO₂. The response/recovery times were 34 s/272 s, respectively. The calculated limit of detection (LOD) was ∼115 ppb with a 38% response. The device response was better with NO₂ gas than with SO₂, NO, H₂S, CO, H₂, and NH₃. The mechanism of temperature-dependent n-p-n switching, fast response, recovery, and selective detection of NO₂ at RT has been discussed on the basis of physisorption and charge transfer. Thus, this work will add a new dimension to 2D materials as selective gas detectors at room temperature.

High-Performance Boron Nitride Based Membranes for Water Purification

In recent years, nanotechnology-based approaches have resulted in the development of new alternative sustainable technologies for water purification. Two-dimensional (2D) nanomaterials are an emerging class of materials for nanofiltration membranes. In this work, we report the production, characterisation and testing of a promising nanofiltration membrane made from water-exfoliated boron nitride (BN) 2D nanosheets. The membranes have been tested for water purification and removal of typical water-soluble dyes such as methyl orange, methylene blue and Evans blue, with the water-exfoliated BN membranes achieving retention values close to 100%. In addition, we compared the performance of membranes made from water-exfoliated BN with those produced from BN using sonication-assisted liquid exfoliation in selected organic solvents such as 2-propanol and N-methyl-2-pyrrolidone. It was found that membranes from the water-exfoliated BN showed superior performance. We believe this research opens up a unique opportunity for the development of new high-performance environmentally friendly membranes for nanofiltration and new sustainable separation technologies.

Au Nanoparticle Modification Induces Charge-Transfer Channels to Enhance the Electrocatalytic Hydrogen Evolution Reaction of InSe Nanosheets

Electrocatalytic water splitting for hydrogen production is an efficient, clean, and sustainable strategy to solve energy and environmental problems. As the important alternative materials for noble metals (Pt, Ir, etc.), two-dimensional (2D) materials have been widely applied for electrocatalysis, although the practical performance is restricted by low carrier mobility and slow reaction kinetics. Here, we adopt the strategy of Au nanoparticle modification to achieve the enhanced hydrogen evolution reaction (HER) performance of InSe nanosheets. Experimental results prove that the HER performance of InSe nanosheets is significantly enhanced under the modification of Au nanoparticles, and the overpotential (392 mV) and Tafel slope (59 mV/dec) are significantly reduced compared to sole InSe nanosheets (580 mV and 148.2 mV/dec). First-principles calculations have found that the InSe/Au system exhibits metallicity because the free electrons provided by the Au particles are injected into the InSe, thereby improving its conductivity. The difference charge density and localized charge density of InSe/Au show that Au nanoparticle loading can induce the formation of Au-Se electron-transfer channels with electrovalent bond characteristics, which effectively promotes the charge transfer. Meanwhile, the standard free-energy calculation of the HER process shows that the InSe/Au heterojunction has a H* adsorption/desorption Gibbs free energy [(|ΔG_H*|) = 0.59 eV] closer to the optimal value. This study reveals the theoretical mechanism of metal modification to improve the performance of electrocatalytic HER and is expected to motivate the development of a new strategy for enhancing the catalytic activity of 2D semiconductor materials.

Ultrafast and stable planar photodetector based on SnS2 nanosheets/perovskite structure

Two-dimensional (2D) transition metal dichalcogenides are promising candidates of photodetectors where they are commonly grown parallel to the substrate due to their 2D characteristics in micrometer scales from exfoliation of bulk crystals or through high temperature chemical vapor deposition (CVD) methods. In this study, semi-hexagonal vertical nanosheets of SnS2 layered have been fabricated on FTO substrate without using Sn source through CVD method at relatively low temperature (500 °C). Due to exceptional band alignment of triple cation lead perovskite (TCLP) with semi-hexagonal SnS2 nanosheets, an improved photodetector has been fabricated. This type of photodetectors fabricated through lithography-free and electrodes metallization free approach with remarkable fast response (20.7 µs/31.4 µs as rising /falling times), showed high photoresponsivity, external quantum efficiency and detectivity of 1.84 AW-1, 513% and 1.69 × 1011, respectively under illumination of incident light with wavelength of 445 nm. The stability of the photodetectors has been studied utilizing a protective PMMA layer on the perovskite layer in 100% humidity. The introduced growth and fabrication process of the planar photodetector, including one/two dimensional interface through the edges/basal planes of layered materials with perovskite film, paves a way for the large scale, cost-effective and high-performance optoelectronic devices.

SF6 decomposed gas sensing performance of van der Waals layered cobalt oxyhydroxide: insights from a computational study

The detection of SF₆ decomposition products plays a significant part in identifying and assessing the electric discharge faults in SF₆ insulation equipment. We performed dispersion corrected density functional theory calculations to study the adsorption performance of CoOOH upon SO₂, SF₄, SOF₂, CF₄, and SO₂F₂ toxic gases, to investigate their potential application as a gas sensor. The results clearly show a weak force between the CoOOH sheet, and the molecular gas with moderate adsorption strength enhances the desorption processes. According to Löwdin charge population analysis, electrons transfer from the molecular gas to the CoOOH surface, where the molecular gas behaves like an electron donor. The lower bandgap energy of the adsorption systems compared with pristine CoOOH significantly increases its electrical conductivity and gas sensing performance. The higher charge transfer and adsorption energy of the SOF₂ adsorption system compared with the other four molecular gas is due to orbital hybridization around the Fermi energy. The theoretical computed adsorption energy with ultrahigh sensitivity and fast recovery time suggests that SF₆ decomposed gases reusability is achieved with CoOOH as a resistance-type gas sensor.

Preparation of defected SWCNTs decorated with en-APTAS for application in high-performance nitric oxide gas detection

We demonstrate highly sensitive and selective chemiresistive-type NO gas detection using defected single-walled carbon nanotubes (SWCNTs) decorated with N-[3-(trimethoxysilyl)propyl]ethylene diamine (en-APTAS) molecules. The defected SWCNTs were prepared via furnace annealing at 700 °C and confirmed by transmission electron microscopy. A single en-APTAS molecule has two amine groups acting as adsorption sites for NO gas, which can improve the NO response. The NO response was further enhanced when the defected SWCNTs were utilized because NO sensing reactions could occur on both the inner and outer walls of the defected SWCNTs. The en-APTAS decoration improved the NO response of the SWCNT-based gas sensing devices by 2.5 times; when the defected SWCNTs were used, the NO response was further improved by 3 times. Meanwhile, the recovery performance in a time-resolved response curve was significantly improved (45 times) via a simple rinsing process with ethanol. Specifically, the fabricated device did not respond to carbon monoxide (CO) or BTEX gas (i.e., a mixture of benzene, toluene, ethyl benzene, and xylene), indicating its high selectivity to NO gas. The results show the possibility of a high-performance SWCNT-based NO gas sensor applicable to healthcare fields requiring ppb-level detection, such as in vitro diagnostics (IVDs) of respiratory diseases.

Carbon-doping-induced energy-band modification and vacancies in SnS2 nanosheets for room-temperature ppb-level NO2 detection

Carbon-doping mediated synergistic effect of energy-band modification and vacancy provides a new solution for developing high-performance LMDs-based gas sensors.

Strategy and Future Prospects to Develop Room-Temperature-Recoverable NO2 Gas Sensor Based on Two-Dimensional Molybdenum Disulfide

Nitrogen dioxide (NO2), a hazardous gas with acidic nature, is continuously being liberated in the atmosphere due to human activity. The NO2 sensors based on traditional materials have limitations of high-temperature requirements, slow recovery, and performance degradation under harsh environmental conditions. These limitations of traditional materials are forcing the scientific community to discover future alternative NO2 sensitive materials. Molybdenum disulfide (MoS2) has emerged as a potential candidate for developing next-generation NO2 gas sensors. MoS2 has a large surface area for NO2 molecules adsorption with controllable morphologies, facile integration with other materials and compatibility with internet of things (IoT) devices. The aim of this review is to provide a detailed overview of the fabrication of MoS2 chemiresistance sensors in terms of devices (resistor and transistor), layer thickness, morphology control, defect tailoring, heterostructure, metal nanoparticle doping, and through light illumination. Moreover, the experimental and theoretical aspects used in designing MoS2-based NO2 sensors are also discussed extensively. Finally, the review concludes the challenges and future perspectives to further enhance the gas-sensing performance of MoS2. Understanding and addressing these issues are expected to yield the development of highly reliable and industry standard chemiresistance NO2 gas sensors for environmental monitoring.

Enhanced NO2 Sensitivity in Schottky-Contacted n-Type SnS2 Gas Sensors

Layered materials are highly attractive in gas sensor research due to their extraordinary electronic and physicochemical properties. The development of cheaper and faster room-temperature detectors with high sensitivities especially in the parts per billion level is the main challenge in this rapidly developing field. Here, we show that sensitivity to NO₂ (S) can be greatly improved by at least two orders of magnitude using an n-type electrode metal. Unconventionally for such devices, the ln(S) follows the classic Langmuir isotherm model rather than S as is for a p-type electrode metal. Excellent device sensitivities, as high as 13,000% for 9 ppm and 97% for 1 ppb NO₂, are achieved with Mn electrodes at room temperature, which can be further tuned and enhanced with the application of a bias. Long-term stability, fast recovery, and strong selectivity toward NO₂ are also demonstrated. Such impressive features provide a real solution for designing a practical high-performance layered material-based gas sensor.

Wafer-Scale, Conformal, and Low-Temperature Synthesis of Layered Tin Disulfides for Emerging Nonplanar and Flexible Electronics

Two-dimensional (2D) metal dichalcogenides have drawn considerable interest because they offer possibilities for the implementation of emerging electronics. The emerging electronics are moving toward two major directions: vertical expansion of device space and flexibility. However, the development of a synthesis method for 2D metal dichalcogenides that meets all the requirements remains a significant challenge. Here, we propose a promising method for wafer-scale, conformal, and low-temperature (≤240 °C) synthesis of single-phase SnS₂ via the atomic layer deposition technique. There is a trade-off relationship between the crystallinity and orientation preference of SnS₂, which is efficiently eliminated by the two-step growth occurring at different temperatures. Consequently, the van der Waals layers of the highly crystalline SnS₂ are parallel to the substrate. Thin-film transistors (TFTs) comprising the SnS₂ layer show reasonable electrical performances (field-effect mobility: ∼0.8 cm² V^-1 s^-1 and on/off ratio: ∼10⁶), which are comparable to that of a single-crystal SnS₂ flake. Moreover, we demonstrate nonplanar and flexible TFTs to identify the feasibility of the implementation of future electronics. Both the diagonal-structured TFT and flexible TFT fabricated without a transfer process show electrical performances comparable to those of rigid and planar TFTs. Particularly, the flexible TFT does not exhibit substantial degradation even after 2000 bending cycles. Our work would provide decisive opportunities for the implementation of future electronic devices utilizing 2D metal chalcogenides.

Effect of Temperature and Capping Agents on Structural and Optical Properties of Tin Sulphide Nanocrystals

SnS nanocrystals were synthesized using bis(phenylpiperazine dithiocarbamate)tin(II) in oleic acid (OA) and octadecylamine (ODA) at three different temperatures (150, 190, and 230°C). XRD diffraction pattern confirms that OASnS and ODASnS nanoparticles are in the orthorhombic phase and the type of capping agent used affects the crystallinity. Transmission electron microscopy (TEM) images shows spherically shaped nanocrystals for oleic acid capped SnS (OASnS) while octadecylamine (ODASnS) are cubic. Monodispersed SnS of size range 10.67–17.74 nm was obtained at 150°C for OASnS while the biggest-sized nanocrystals were obtained at 230°C for ODASnS. Temperature and capping agents tuned the crystallite sizes and shapes of the as-prepared nanocrystals. Electron dispersive X-ray spectroscopy indicates the formation of tin sulphide with the presence of Sn and S peaks in the nanocrystals. Flowery and agglomerated spherical-like morphology were observed for ODASnS and OASnS nanocrystals, respectively, using a SEM (scanning electron microscope). Direct electronic band gaps of the synthesized SnS nanocrystals are 1.71–1.95 eV and 1.93–2.81 eV for OASnS and ODASnS nanocrystals, respectively.

Photo-enhanced gas sensing of SnS2 with nanoscale defects

Recently a SnS₂ based NO₂ gas sensor with a 30 ppb detection limit was demonstrated but this required high operation temperatures. Concurrently, SnS₂ grown by chemical vapor deposition is known to naturally contain nanoscale defects, which could be exploited. Here, we significantly enhance the performance of a NO₂ gas sensor based on SnS₂ with nanoscale defects by photon illumination, and a detection limit of 2.5 ppb is achieved at room temperature. Using a classical Langmuir model and density functional theory simulations, we show S vacancies work as additional adsorption sites with fast adsorption times, higher adsorption energies, and an order of magnitude higher resistance change compared with pristine SnS₂. More interestingly, when electron–hole pairs are excited by photon illumination, the average adsorption time first increases and then decreases with NO₂ concentration, while the average desorption time always decreases with NO₂ concentration. Our results give a deep understanding of photo-enhanced gas sensing of SnS₂ with nanoscale defects, and thus open an interesting window for the design of high performance gas sensing devices based on 2D materials.

A photon assisted SnS₂-based gas sensor with an ultra-high sensitivity of 3 ppb NO₂ has been achieved at room temperature.

Two-dimensional SnS2 nanosheets exfoliated from an inorganic–organic hybrid with enhanced photocatalytic activity towards Cr(vi) reduction

Thin SnS₂ nanosheets with {001} facets dominating were obtained with the liquid-exfoliation method and exhibit largely improved photocatalytic activity for Cr(vi) reduction.

Synthesis of Numerous Edge Sites in MoS2 via SiO2 Nanorods Platform for Highly Sensitive Gas Sensor

The utilization of edge sites in two-dimensional materials including transition-metal dichalcogenides (TMDs) is an effective strategy to realize high-performance gas sensors because of their high catalytic activity. Herein, we demonstrate a facile strategy to synthesize the numerous edge sites of vertically aligned MoS₂ and larger surface area via SiO₂ nanorod (NRs) platforms for highly sensitive NO₂ gas sensor. The SiO₂ NRs encapsulated by MoS₂ film with numerous edge sites and partially vertical-aligned regions synthesized using simple thermolysis process of [(NH₄)₂MoS₄]. Especially, the vertically aligned MoS₂ prepared on 500 nm thick SiO₂ NRs (500MoS₂) shows approximately 90 times higher gas-sensing response to 50 ppm NO₂ at room temperature than the MoS₂ film prepared on flat SiO₂, and the theoretical detection limit is as low as ∼2.3 ppb. Additionally, it shows reliable operation with reversible response to NO₂ gas without degradation at an operating temperature of 100 °C. The use of the proposed facile approach to synthesize vertically aligned TMDs using nanostructured platform can be extended for various TMD-based devices including sensors, water splitting catalysts, and batteries.