Solution-Processed p-SnSe/n-SnSe2 Hetero-Structure Layers for Ultrasensitive NO2 Detection

Photostimulated Pyrothermoelectric Coupling in Two-Dimensional Tin Monoselenide Enabling Zero-Biased Multimodal Transducers

Despite the advancement of the Internet of Things (IoT) and portable devices, the development of zero-biased sensing systems for the dual detection of light and gases remains a challenge. As an emerging technology, direct energy conversion driven by intriguing physical properties of two-dimensional (2D) materials can be realized in nanodevices or a zero-biased integrated system. In this study, we unprecedentedly attempted to exploit the photostimulated pyrothermoelectric coupling of two-dimensional SnSe for use in zero-biased multimodal transducers for the dual detection of light and gases. We synthesized homogeneous, large-area 6 in SnSe multilayers via a rational synthetic route based on the thermal decomposition of a solution-processed single-source precursor. Zero-biased SnSe transducers for the dual monitoring of light and gases were realized by exploiting the synergistic coupling of the photostimulated pyroelectric and thermoelectric effects of SnSe. The extracted photoresponsivity at 532 nm and NO2 gas responsivity of the SnSe-based transducers corresponded to 1.07 × 10-6 A/W and 13263.6% at 0 V, respectively. To bring universal applicability of the zero-biased SnSe transducers, the wide operation bandwidth photoelectrical properties (visible to NIR) and dynamic current responses toward two NO2/NH3 gases were systematically evaluated.

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.

A high-sensitivity SnSe/Si heterojunction position-sensitive detector for ultra-low power detection

Position-sensitive detectors (PSDs) based on the lateral photovoltaic effect are crucial components in non-contact distance measurement, process control, guidance systems, and other related applications. However, PSDs are challenging due to the narrow spectral range and low sensitivity, limiting further practical application. Here, we present an ultra-sensitive SnSe/Si PSD device. A large-area uniform SnSe nanorod (NR) array film was grown on Si using a glancing-angle magnetron sputtering deposition technique and a SnSe/Si heterojunction PSD device was fabricated. PSDs exhibit an excellent photoresponse in a wide spectral range of 405-980 nm, showing an ultrahigh position sensitivity of 1517.4 mV mm-1 and an excellent spectral sensitivity of 4 × 104 V W-1. More importantly, the detection limit power of the device is as low as 10 nW, indicating the outstanding potential for weak light detection. Based on the unique structural features and interface effect, the mechanisms for the remarkable performance of the fabricated SnSe/Si PSD device are clarified. This work indicates the large potential of SnSe/Si heterojunctions as a promising material for ultrasensitive optical position-sensitive devices.

Wearable Nano-Based Gas Sensors for Environmental Monitoring and Encountered Challenges in Optimization

With a rising emphasis on public safety and quality of life, there is an urgent need to ensure optimal air quality, both indoors and outdoors. Detecting toxic gaseous compounds plays a pivotal role in shaping our sustainable future. This review aims to elucidate the advancements in smart wearable (nano)sensors for monitoring harmful gaseous pollutants, such as ammonia (NH3), nitric oxide (NO), nitrous oxide (N2O), nitrogen dioxide (NO2), carbon monoxide (CO), carbon dioxide (CO2), hydrogen sulfide (H2S), sulfur dioxide (SO2), ozone (O3), hydrocarbons (CxHy), and hydrogen fluoride (HF). Differentiating this review from its predecessors, we shed light on the challenges faced in enhancing sensor performance and offer a deep dive into the evolution of sensing materials, wearable substrates, electrodes, and types of sensors. Noteworthy materials for robust detection systems encompass 2D nanostructures, carbon nanomaterials, conducting polymers, nanohybrids, and metal oxide semiconductors. A dedicated section dissects the significance of circuit integration, miniaturization, real-time sensing, repeatability, reusability, power efficiency, gas-sensitive material deposition, selectivity, sensitivity, stability, and response/recovery time, pinpointing gaps in the current knowledge and offering avenues for further research. To conclude, we provide insights and suggestions for the prospective trajectory of smart wearable nanosensors in addressing the extant challenges.

Macroporous SnO2/MoS2 inverse opal hierarchitecture for highly efficient trace NO2 gas sensing

The innovation of NO₂ gas sensors is highly desirable in environmental monitoring and human safety. Herein, a macroporous SnO₂/MoS₂ inverse opal hierarchitecture has been constructed with substantial interface charge transfer, which realizes the efficient and stable detection of NO₂ with an enhanced response, fast kinetics, and high selectivity at low temperatures.

Adsorption properties of small gas molecules on SnSe2 monolayer supported with transition metal: first-principles calculations

The adsorption properties of CH₄, H₂S, SO₂, CO, H₂O and NO molecules on transition metal-supported SnSe₂ surface are investigated by the first-principles method. The calculation results show that the transition metal (TM = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu) has the lowest adsorption energy when supporting at the Sn site of SnSe₂, indicating the system is relatively stable. Also, we find that CH₄, SO₂ and H₂O molecules tend to adsorb on Sc-supported SnSe₂ surface, H₂S and NO molecules prefer to adsorb on V-supported SnSe₂ surface, while CO molecule and Fe-supported SnSe₂ surfaces have strong interaction. And, CH₄, H₂S and H₂O molecules act as donors to provide electrons to the substrate, while SO₂, CO and NO molecules act as acceptors to gain electrons from the substrate. An analysis of charge difference density and density of states reveals that the adsorption energies of gas molecules are related to charge transfer and orbital hybridization. We hope that this work not only provides a promising sensor material, but also provides a new idea for the rational design of two-dimensional materials.

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.

2D SnSe2 nanoflakes decorated with 1D ZnO nanowires for ppb-level NO2 detection at room temperature

The detection of air pollutant nitrogen dioxide (NO₂) is of great importance arising from its great harm to the ecological environment and human health. However, the detection range of most NO₂ sensors is ppm-level, and it is still challenging to achieve lower concentration (ppb-level) NO₂ detection. Herein, 2D tin diselenide nanoflakes decorated with 1D zinc oxide nanowires (SnSe₂/ZnO) heterojunctions were first reported by facile hydrothermal and ultra-sonication methods. The response of the fabricated SnSe₂/ZnO sensor enhances 3.41 times on average compared with that of pure SnSe₂ sensor to 50-150 ppb NO₂ with a high detection sensitivity (22.57 ppm^-1) at room temperature. In addition, the SnSe₂/ZnO sensor has complete recovery, negligible cross-sensitivity, and small relative standard deviation (6.98%) during the 1 month sensing test, which can meet the requirements for NO₂ detection in environmental monitoring. The enhanced NO₂ sensing performance can be attributed to the n-n heterojunction constructed between SnSe₂ and ZnO. The as-prepared sensor based on SnSe₂/ZnO hybrid significantly promotes the development of the low detection limit of the NO₂ sensor at room temperature.

Ligand-assisted deposition of ultra-small Au nanodots on Fe2O3/reduced graphene oxide for flexible gas sensors

The development of flexible room-temperature gas sensors is important in environmental monitoring and protection. In this contribution, by using 1-octadecanethiol (ODT) as a surface ligand, Au nanodots (NDs) with ultra-small size of ∼1.7 nm were deposited on the surface of α-Fe₂O₃/reduced graphene oxide (rGO). The Au ND-ODT/α-Fe₂O₃/rGO composite was fabricated into flexible gas sensors, which could detect NO₂ gas down to 200 ppb at room temperature. Compared with α-Fe₂O₃/rGO, Au ND-ODT/α-Fe₂O₃/rGO showed enhanced sensing performance because of the beneficial effects of Au NDs, including facilitating the adsorption of NO₂ molecules and forming ohmic-like contact with rGO and α-Fe₂O₃. In addition, the sensing performance of the composite was also influenced by the surface ligands of the Au NDs. Ligands with less polar terminal groups were found to be beneficial to charge transfer in the sensing film. Moreover, Au ND-ODT/α-Fe₂O₃/rGO-based flexible sensors showed negligible performance deterioration under moderately bent conditions, suggesting their potential to be used in portable and wearable devices.

Tin Diselenide (SnSe2) Van der Waals Semiconductor: Surface Chemical Reactivity, Ambient Stability, Chemical and Optical Sensors

Tin diselenide (SnSe2) is a layered semiconductor with broad application capabilities in the fields of energy storage, photocatalysis, and photodetection. Here, we correlate the physicochemical properties of this van der Waals semiconductor to sensing applications for detecting chemical species (chemosensors) and millimeter waves (terahertz photodetectors) by combining experiments of high-resolution electron energy loss spectroscopy and X-ray photoelectron spectroscopy with density functional theory. The response of the pristine, defective, and oxidized SnSe2 surface towards H2, H2O, H2S, NH3, and NO2 analytes was investigated. Furthermore, the effects of the thickness were assessed for monolayer, bilayer, and bulk samples of SnSe2. The formation of a sub-nanometric SnO2 skin over the SnSe2 surface (self-assembled SnO2/SnSe2 heterostructure) corresponds to a strong adsorption of all analytes. The formation of non-covalent bonds between SnO2 and analytes corresponds to an increase of the magnitude of the transferred charge. The theoretical model nicely fits experimental data on gas response to analytes, validating the SnO2/SnSe2 heterostructure as a suitable playground for sensing of noxious gases, with sensitivities of 0.43, 2.13, 0.11, 1.06 [ppm]-1 for H2, H2S, NH3, and NO2, respectively. The corresponding limit of detection is 5 ppm, 10 ppb, 250 ppb, and 400 ppb for H2, H2S, NH3, and NO2, respectively. Furthermore, SnSe2-based sensors are also suitable for fast large-area imaging applications at room temperature for millimeter waves in the THz range.

Heterostructures between a tin-based intermetallic compound and a layered semiconductor for gas sensing

SnS2 nanoplates are used as sacrificial templates to facilitate the in situ growth of intermetallic compound Pt3Sn nanoparticles. The Pt3Sn/SnS2 heterostructures show promise for selective NO2 sensing due to the favored gas adsorption and gas-solid charge transfer on Pt3Sn, combined with the optimized film conductance and formation of ohmic-type Pt3Sn/SnS2 heterointerfaces.

AuPt Bimetal-Functionalized SnSe2 Microflower-Based Sensors for Detecting Sub-ppm NO2 at Low Temperatures

A novel chemiresistive-type sensor for detecting sub-ppm NO₂ has been fabricated using AuPt bimetal-decorated SnSe₂ microflowers, which was synthesized by the hydrothermal treatment followed by in situ chemical reduction of the bimetal precursors on the surface of the petals of the microflowers. The as-prepared sensor registers a superior performance in detection of sub-ppm concentration of NO₂. Functionalized by the AuPt bimetal, the SnSe₂ microflower-based sensor shows a response of approximately 4.62 to 8 ppm NO₂ at 130 °C. It is significantly higher than those of the sensors using the pristine SnSe₂ (∼2.29) and the modified SnSe₂ samples by a single metal, either Au (∼3.03) or Pt (∼3.97). The sensor demonstrates excellent long-term stability, signal repeatability, and selectivity to some typical interfering gaseous species including ammonia, acetone, formaldehyde, ethanol, methanol, benzene, CO₂, SO₂, and CO. The remarkable improvement of the sensitive characteristics could be induced by the electronic and chemical sensitization and the synergistic effect of the AuPt bimetal. Density functional theory (DFT) is implemented to calculate the adsorption states of NO₂ on the sensing materials and thus to possibly reveal the sensing mechanism. The significantly enhanced response of the SnSe₂-based sensor decorated with AuPt bimetallic nanoparticles has been found to be possibly caused by the orbital hybridization of O, Au, and Pt atoms leading to the redistribution of electrons, which is beneficial for NO₂ molecules to obtain more electrons from the composite material.

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.

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.

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.

Dynamic Epitaxial Crystallization of SnSe2 on the Oxidized SnSe Surface and Its Atomistic Mechanisms

Surface oxidation of SnSe sharply reduces its thermoelectric properties though the bulk single-crystalline materials of SnSe claim the record high zT values. Investigation on the oxidation behaviors of SnSe together with the subsequent phase transition and element migration is fundamentally important to maintaining the ultrahigh zT values, with a potential for further improvement. In this work, we disclose the dynamic epitaxial crystallization of SnSe2 on the amorphous surface of partially oxidized SnSe crystals and the corresponding atomistic mechanisms via transmission electron microscopy (TEM). It is revealed that the thermally annealed amorphous surface crystallized to SnO2 and SnSe2 in the outermost and secondary layers, respectively, forming distinctive SnSe/SnSe2/SnO2 multilayer heterostructures with specific orientation relationships between the two selenides. By means of in situ scanning TEM (STEM), the dynamic epitaxial crystallization process of SnSe2 was revealed when the oxidized SnSe surface was subjected to electron beam irradiation. Through the atomic-scale characterization and modeling analysis, we find that the exposed dangling Se diatoms on the SnSe surface serve as nucleation sites for lateral epitaxial crystallization of SnSe2. The same valence and similar coordination configuration of Se atoms in these two phases are supposed to facilitate the sharing of Se atoms, with lattice distortions in the SnSe2/SnSe interface. These findings are valuable for understanding the surface oxidation behavior of SnSe and revealing the interface structures of SnSe2/SnSe heterojunctions and also offering new routes for SnSe-related multilayer or heterostructure system design.