Large-scale synthesis of single-crystalline self-standing SnSe2 nanoplate arrays for wearable gas sensors

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.

Strong Covalent Coupling in Vertically Layered SnSe2/PTAA Heterojunctions Enabled High Performance Inorganic-Organic Hybrid Photodetectors

Controllable large-scale integration of two-dimensional (2D) materials with organic semiconductors and the realization of strong coupling between them still remain challenging. Herein, we demonstrate a wafer-scale, vertically layered SnSe2/PTAA heterojunction array with high light-trapping ability via a low-temperature molecular beam epitaxy method and a facile spin-coating process. Conductive probe atomic force microscopy (CP-AFM) measurements reveal strong rectification and photoresponse behavior in the individual SnSe2 nanosheet/PTAA heterojunction. Theoretical analysis demonstrates that vertically layered SnSe2/PTAA heterojunctions exhibit stronger C-Se covalent coupling than that of the conventional tiled type, which could facilitate more efficient charge transfer. Benefiting from these advantages, the SnSe2/PTAA heterojunction photodetectors with an optimized PTAA concentration show high performance, including a responsivity of 41.02 A/W, an external quantum efficiency of 1.31 × 104%, and high uniformity. The proposed approach for constructing large-scale 2D inorganic-organic heterostructures represents an effective route to fabricate high-performance broadband photodetectors for integrated optoelectronic systems.

Highly intrinsic carrier mobility in tin diselenide crystal accessed with ultrafast terahertz spectroscopy

Tin diselenide (SnSe2), a layered transition metal dichalcogenide (TMDC), stands out among other TMDCs for its extraordinary photoactive ability and low thermal conductivity. Consequently, it has stimulated many influential researches on photodetectors, ultrafast pulse shaping, thermoelectric devices, etc. However, the carrier mobility in SnSe2, as determined experimentally, remains limited to tens of cm2V-1s-1. This limitation poses a challenge for achieving high-performance SnSe2-based devices. Theoretical calculations, on the other hand, predict that the carrier mobility in SnSe2 can reach hundreds of cm2V-1s-1, approximately one order of magnitude higher than experimental value. Interestingly, the carrier mobility could be underestimated significantly in long-range transportation measurements due to the presence of defects and boundary scattering effects. To address this discrepancy, we employ optic pump terahertz probe spectroscopy to access the photoinduced dynamical THz photoconductivity of SnSe2. Our findings reveal that the intrinsic carrier mobility in conventional SnSe2 single crystal is remarkably high, reaching 353.2 ± 37.7 cm2V-1s-1, consistent with the theoretical prediction. Additionally, dynamical THz photoconductivity measurements reveal that the SnSe2 crystal containing rich defects efficiently capture photoinduced conduction-band electrons and valence-band holes with time constants of ∼20 and ∼200 ps, respectively. Meanwhile, we observe an impulsively stimulated Raman scattering at 0.60 THz. Our study not only demonstrates ultrafast THz spectroscopy as a reliable method for determining intrinsic carrier mobility and detection of low frequency coherent Raman mode in materials but also provides valuable reference for the future application of high-performance SnSe2-based devices.

Superior acetone sensor based on hetero-interface of SnSe2/SnO2 quasi core shell nanoparticles for previewing diabetes

To improve gas sensing performance of SnO₂ sensor, a heterostructure constructed by SnO₂ and SnSe₂ is designed and synthesized via hydrothermal method and post thermal oxidation treatment. The obtained SnSe₂/SnO₂ composite nanoparticles demonstrate a special core-shell structure with SnO₂ nanograins distributed in the shell and mixed SnSe₂ and SnO₂ nanograins in the core. Owning to the promoted charge transfer effect invited by SnSe₂, the sensor based on SnSe₂/SnO₂ composite nanoparticles exhibit expressively enhanced acetone sensing performance compared to the pristine SnO₂ sensor. At the working temperature of 300 °C, the SnSe₂/SnO₂ composite sensor with optimized composition exhibits superior sensing property towards acetone, including high response (10.77-100 ppm), low theoretical limit of detection (0.354 ppm), high selectivity and good reproducibility. Moreover, the sensor shows a satisfactory sensing performance in trace acetone gas detection under high humidity condition (relative humidity: 70-90%), making it a promising candidate to constructing exhaled breath sensors for acetone detection.

Gas sensing devices based on two-dimensional materials: a review

Gas sensors have been widely utilized penetrating every aspect of our daily lives, such as medical industry, environmental safety testing, and the food industry. In recent years, two-dimensional (2D) materials have shown promising potential and prominent advantages in gas sensing technology, due to their unique physical and chemical properties. In addition, the ultra-high surface-to-volume ratio and surface activity of the 2D materials with atomic-level thickness enables enhanced absorption and sensitivity. Till now, different gas sensing techniques have been developed to further boost the performance of 2D materials-based gas sensors, such as various surface functionalization and Van der Waals heterojunction formation. In this article, a comprehensive review of advanced gas sensing devices is provided based on 2D materials, focusing on two sensing principles of charge-exchange and surface oxygen ion adsorption. Six types of typical gas sensor devices based on 2D materials are introduced with discussion of latest research progress and future perspectives.

Selenium Vacancies and Synergistic Effect of Near- and Far-Field-Enabled Ultrasensitive Surface-Enhanced Raman-Scattering-Active Substrates for Malaria Detection

Defect engineering with the active control of defect states brings remarkable enhancement on surface-enhanced Raman scattering (SERS) by magnifying semiconductor-molecule interaction. Such light-trapping architectures can increase the light path length, which promotes photon-analytes interactions and further improves the SERS sensitivity. However, by far the reported semiconductor SERS-active substrates based on these strategies are often nonuniform and commonly in the form of isolated laminates or random clusters, which limit their reliability and stability for practical applications. Herein, we develop self-grown single-crystalline "V-shape" SnSe_2-x (SnSe_1.5, SnSe_1.75, SnSe₂) nanoflake arrays (SnSe_2-x NFAs) with controlled selenium vacancies over large-area (10 cm × 10 cm) for ultrahigh-sensitivity SERS. First-principles density functional theory (DFT) is used to calculate the band gap and the electronic density of states (DOS). Based on the Herzberg-Teller theory regarding the vibronic coupling, the results of theoretical calculation reveal that the downshift of band edge and high DOS of SnSe_1.75 can effectively enhance the vibronic coupling within the SnSe_1.75-R6G system, which in turn enhances the photoinduced charge transfer resonance and contributes to the SERS activity with a remarkable enhancement factor of 1.68 × 10⁷. Furthermore, we propose and demonstrate ultrasensitive (10^-15 M for R6G), uniform, and reliable SERS substrates by forming SnSe_1.75 NFAs/Au heterostructures via a facile Au evaporation process. We attribute the superior performance of our SnSe_1.75 NFAs/Au heterostructures to the following reasons: (1) selenium vacancies and (2) synergistic effect of the near and far fields. In addition, we successfully build a detection platform to achieve rapid (∼15 min for the whole process), antibody-free, in situ, and reliable early malaria detection (100% detection rate for 10 samples with 160 points) in whole blood, and molecular hemozoin (<100/mL) can be detected. Our approach not only provides an efficient technique to obtain large-area, uniform, and reliable SERS-active substrates but also offers a substantial impact on addressing practical issues in many application scenarios such as the detection of insect-borne infectious diseases.

Atmospheric pressure chemical vapor deposition growth of vertically aligned SnS2 and SnSe2 nanosheets

Laminated metal dichalcogenides are candidates for different potential applications ranging from catalysis to nanoelectronics. However, efforts are still needed to optimize synthesis methods aiming to control the number of layers, morphology, and crystallinity, parameters that govern the properties of the synthesized materials. Another important parameter is the thickness and the length of the samples with the possibility of large-scale growth of target homogeneous materials. Here, we report a chemical vapor deposition method at atmospheric pressure to produce vertically aligned tin dichalcogenide based-materials. Tin disulfide (SnS2) and tin diselenide (SnSe2) vertically aligned nanosheets have been synthesized and characterized by different methods showing their crystallinity and purity. Homogenous crystalline 2H-phase SnS2 nanosheets with high purity were synthesized with vertical orientation on substrates; sulfur vacancies were observed at the edges of the sheets. Similarly, in the crystalline 2H phase SnSe2 nanosheets selenium vacancies were observed at the edges. Moreover, these nanosheets are larger than the SnS2 nanosheets, show lower nanosheet homogeneity on substrates and contamination with selenium atoms from the synthesis was observed. The synthesized nanomaterials are interesting in various applications where the edge accessibility and/or directionality of the nanosheets play a major role as for example in gas sensing or field emission.

Strategies for Improving the Sensing Performance of Semiconductor Gas Sensors for High-Performance Formaldehyde Detection: A Review

Formaldehyde is a poisonous and harmful gas, which is ubiquitous in our daily life. Long-term exposure to formaldehyde harms human body functions; therefore, it is urgent to fabricate sensors for the real-time monitoring of formaldehyde concentrations. Metal oxide semiconductor (MOS) gas sensors is favored by researchers as a result of their low cost, simple operation and portability. In this paper, the mechanism of formaldehyde detection by gas sensors is introduced, and then the ways of ameliorating the response of gas sensors for formaldehyde detection in recent years are summarized. These methods include the control of the microstructure and morphology of sensing materials, the doping modification of matrix materials, the development of new semiconductor sensing materials, the outfield control strategy and the construction of the filter membrane. These five methods will provide a good prerequisite for the preparation of better performing formaldehyde gas sensors.

Chemical reactions on surfaces for applications in catalysis, gas sensing, adsorption-assisted desalination and Li-ion batteries: opportunities and challenges for surface science

The study of chemical processes on solid surfaces is a powerful tool to discover novel physicochemical concepts with direct implications for processes based on chemical reactions at surfaces, largely exploited by industry. Recent upgrades of experimental tools and computational capabilities, as well as the advent of two-dimensional materials, have opened new opportunities and challenges for surface science. In this Perspective, we highlight recent advances in application fields strictly connected to novel concepts emerging in surface science. Specifically, we show for selected case-study examples that surface oxidation can be unexpectedly beneficial for improving the efficiency in electrocatalysis (the hydrogen evolution reaction and oxygen evolution reaction) and photocatalysis, as well as in gas sensing. Moreover, we discuss the adsorption-assisted mechanism in membrane distillation for seawater desalination, as well as the use of surface-science tools in the study of Li-ion batteries. In all these applications, surface-science methodologies (both experimental and theoretical) have unveiled new physicochemical processes, whose efficiency can be further tuned by controlling surface phenomena, thus paving the way for a new era for the investigation of surfaces and interfaces of nanomaterials. In addition, we discuss the role of surface scientists in contemporary condensed matter physics, taking as case-study examples specific controversial debates concerning unexpected phenomena emerging in nanosheets of layered materials, solved by adopting a surface-science approach.

Tin-selenide as a futuristic material: properties and applications

SnSe/SnSe2 is a promising versatile material with applications in various fields like solar cells, photodetectors, memory devices, lithium and sodium-ion batteries, gas sensing, photocatalysis, supercapacitors, topological insulators, resistive switching devices due to its optimal band gap. In this review, all possible applications of SnSe/SnSe2 have been summarized. Some of the basic properties, as well as synthesis techniques have also been outlined. This review will help the researcher to understand the properties and possible applications of tin selenide-based materials. Thus, this will help in advancing the field of tin selenide-based materials for next generation technology.

Two-dimensional inorganic nanosheets: production and utility in the development of novel electrochemical (bio)sensors and gas-sensing applications

This review (with 178 references) focuses on inorganic layered materials (ILMs) and the use of their two-dimensional nanosheets in the development of novel electrochemical (bio)sensors, analytical devices, and gas-phase sensing applications. The text is organized in three main sections including the presentation of the most important families of ILMs, a comprehensive outline of various "bottom-up", "top-down," and hydro(solvo)thermal methods that have been used for the production of ILM nanosheets, and finally an evaluative survey on their utility for the determination of analytes with interest in different sectors of contemporary analysis. Critical discussion on the effect of the production method on their electronic properties, the suitability of each nanomaterial in different sensing technologies along with an assessment of the performance of the (bio)sensors and devices that have been proposed within the last 5 years, is enclosed. The perspectives of further improving the utility of 2D inorganic nanosheets in sensing applications, in real-world samples, are also discussed.

Boosted interfacial charge transfer in SnO2/SnSe2 heterostructures: toward ultrasensitive room-temperature H2S detection

Novel Sn atom cosharing SnO₂/SnSe₂ heterostructures with a high-quality interface were synthesized via in situ thermal oxidation of SnSe. The boosted interfacial charge transfer endows the material with excellent H₂S sensing performance.

Design of hierarchical SnSe2 for efficient detection of trace NO2 at room temperature

Nanosheet-assembled hierarchical SnSe2 could serve as a new suitable candidate for high-performance room-temperature NO2 gas sensing.

SnSe2-Zn-Porphyrin Nanocomposite Thin Films for Threshold Methane Concentration Detection at Room Temperature

Nanocomposite thin films, sensitive to methane at the room temperature (25–30 °C), have been prepared, starting from SnSe2 powder and Zn(II)-5,10,15,20-tetrakis-(4-aminophenyl)- -porphyrin (ZnTAPP) powder, that were fully characterized by XRD, UV-VIS, FT-IR, Nuclear Magnetic Resonance (1H-NMR and 13C-NMR), Atomic Force Microscopy (AFM), SEM and Electron Paramagnetic Resonance (EPR) techniques. Film deposition was made by drop casting from a suitable solvent for the two starting materials, after mixing them in an ultrasonic bath. The thickness of these films were estimated from SEM images, and found to be around 1.3 μm. These thin films proved to be sensitive to a threshold methane (CH4) concentration as low as 1000 ppm, at a room temperature of about 25 °C, without the need for heating the sensing element. The nanocomposite material has a prompt and reproducible response to methane in the case of air, with 50% relative humidity (RH) as well. A comparison of the methane sensing performances of our new nanocomposite film with that of other recently reported methane sensitive materials is provided. It is suitable for signaling gas presence before reaching the critical lower explosion limit concentration of methane at 50,000 ppm.

Towards a Large-Area Freestanding Single-Crystal Ferroelectric BaTiO3 Membrane

The fabrication and transfer of freestanding single-crystal ferroelectric membranes deserve intensive investigations as to their potential applications in flexible wearable devices, such as flexible data storage devices and varied sensors in E-skin configurations. In this report, we have shown a comprehensive study approach to the acquisition of a large-area freestanding single-crystal ferroelectric BaTiO3 by the Sr3Al2O6 scarification layer method. By controlling the thickness of the BaTiO3 and Sr3Al2O6, the exposed area of the Sr3Al2O6 interlayer, and the utilization of an additional electrode La2/3Sr1/3MnO3 layer, the crack density on the freestanding BaTiO3 can be dramatically decreased from 24.53% to almost none; then, a more than 700 × 530 μm2 area high-quality freestanding BaTiO3 membrane can be achieved. Our results offer a clear and repeatable technology routine for the acquisition of a flexible large-area ferroelectric membrane, which should be instructive to other transition metal oxides as well. Our study can confidently boost flexible device fabrication based on single-crystal transition metal oxides.

Self-Assembled SnO2/SnSe2 Heterostructures: A Suitable Platform for Ultrasensitive NO2 and H2 Sensing

By means of experiments and theory, the gas-sensing properties of tin diselenide (SnSe₂) were elucidated. We discover that, while the stoichiometric single crystal is chemically inert even in air, the nonstoichiometric sample assumes a subnanometric SnO₂ surface oxide layer once exposed to ambient atmosphere. The presence of Se vacancies induces the formation of a metastable SeO₂-like layer, which is finally transformed into a SnO₂ skin. Remarkably, the self-assembled SnO₂/SnSe_2-x heterostructure is particularly efficient in gas sensing, whereas the stoichiometric SnSe₂ sample does not show sensing properties. Congruently with the theoretical model, direct sensing tests carried out on SnO₂/SnSe_2-x at an operational temperature of 150 °C provided sensitivities of (1.06 ± 0.03) and (0.43 ± 0.02) [ppm]^-1 for NO₂ and H₂, respectively, in dry air. The corresponding calculated limits of detection are (0.36 ± 0.01) and (3.6 ± 0.1) ppm for NO₂ and H₂, respectively. No detectable changes in gas-sensing performances are observed in a time period extended above six months. Our results pave the way for a novel generation of ambient-stable gas sensor based on self-assembled heterostructures formed taking advantage on the natural interaction of substoichiometric van der Waals semiconductors with air.

Flowerlike Tin Diselenide Hexagonal Nanosheets for High-Performance Lithium-Ion Batteries

SnSe2 nanosheet is a common anode for lithium-ion batteries (LIBs), but its severe agglomeration hinders its practical application. Herein, a three-dimensional (3D) SnSe2 nanoflower (F-SnSe2) composed of non-stacking vertical upward hexagonal nanosheets was prepared through a colloidal method as an anode material for LIBs. Benefiting from the advantages of fast reaction-diffusion kinetics and buffering unavoidable volume variation during cycling, the F-SnSe2 electrode displays remarkable specific capacity of 795 mAh g-1 after 100 cycles at 100 mA g-1 and superior rate performance (282 mAh g-1 at 2,000 mA g-1). This work provides an effective way to get non-stacking nanosheets in energy storage field.

Electrical Properties of Two-Dimensional Materials Used in Gas Sensors

In the search for gas sensing materials, two-dimensional materials offer the possibility of designing sensors capable of tuning the electronic band structure by controlling their thickness, quantity of dopants, alloying between different materials, vertical stacking, and the presence of gases. Through materials engineering it is feasible to study the electrical properties of two-dimensional materials which are directly related to their crystalline structure, first Brillouin zone, and dispersion energy, the latter estimated through the tight-binding model. A review of the electrical properties directly related to the crystalline structure of these materials is made in this article for the two-dimensional materials used in the design of gas sensors. It was found that most 2D sensing materials have a hexagonal crystalline structure, although some materials have monoclinic, orthorhombic and triclinic structures. Through the simulation of the mathematical models of the dispersion energy, two-dimensional and three-dimensional electronic band structures were predicted for graphene, hexagonal boron nitride (h-BN) and silicene, which must be known before designing a gas sensor.

Enhanced visible light absorption performance of SnS2 and SnSe2 via surface charge transfer doping

The layered two-dimensional (2D) SnS₂ and SnSe₂ have received intensive attention due to their sizable band gaps and potential properties. However, it has been shown that the visible light absorption of SnS₂ and SnSe₂ are restricted as photocatalysts and light-harvesting material absorbers for water splitting and high-performance optoelectronic devices. Herein, to enhance the visible light absorption performance of SnS₂ and SnSe₂, we performed a systematic investigation on tuning the electronic and optical properties of monolayers SnS₂ and SnSe₂ via surface charge transfer doping (SCTD) with the adsorption of molybdenum trioxide (MoO₃) and potassium (K) as surface dopants based on density functional theory. Our calculations reveal that MoO₃ molecules and K atoms can draw/donate electrons from/to SnS₂ and SnSe₂ as acceptors and donors, respectively. The adsorption of MoO₃ molecules introduces a new flat impurity state in the gap of the monolayers SnS₂/SnSe₂, and the Fermi level moves correspondingly to the top of valence band, resulting in a p-type doping of the monolayer SnS₂/SnSe₂. With the adsorption of K atoms, the electrons can transfer from K atoms to the monolayer of SnS₂ and SnSe₂, making K an effective electron-donating dopant. Meanwhile, the bandgaps of monolayers SnS₂ and SnSe₂ decrease after the MoO₃ and K doping, which leads to the appearance of appreciable new absorption peaks at around ∼650/480 and ∼600/680 nm, respectively, and yielding an enhanced visible light absorption of SnS₂ and SnSe₂. Our results unveil that SCTD is an effective way to improve the photocatalytic and light-harvesting performance of SnS₂ and SnSe₂, broadening their applications in splitting water and degrading environmental pollutants under sunlight irradiation.