The objective of this study is to quantitatively evaluate the impacts of LED components on the overdriving reliability of high power white LED chip scale packages (CSPs). The reliability tests under room temperature are conducted over 1000 h in this study on CSP LEDs with overdriving currents. A novel method is proposed to investigate the impact of various components, including blue die, phosphor layer, and substrate, on the lumen depreciation of CSP LEDs after aging test. The electro-optical measurement results show that the overdriving current can lead to both massive light output degradation and significant color shift of CSP LEDs. The quantitative analysis results show that the phosphor layer is the major contributor to the failure in early period aging test. For the long-term reliability, the degradations of phosphor and reflectivity of substrate contribute significantly on lumen depreciation. The proposed reliability assessment method with overdriving loadings can be usefully implemented for LED manufacturers to make a cost- and effective-decision before mass production.@en

The sensing behavior of monolayer tin sulfide (SnS) for four gas molecules (NH3, NO2, CO, and H2O) are studied by the first-principle calculation based on density-functional theory. We calculate adsorption energy, adsorption distance, and Hirshfeld charge to estimate the adsorption ability of monolayer SnS for these gas molecules. The results demonstrate that all the gas molecules show physisorption nature. We further calculate the current-voltage (I -V ) curves using the nonequilibrium Green's function formalism for evaluating the NO2 gas sensing properties. The monolayer SnS is found to be strongly sensitive to NO2 molecule dependent on moderate adsorption energy, excellent charge transfer, and significant change of I -V property before and after gas adsorption. Therefore, we suggest that monolayer SnS can be a prominent candidate for application as NO2 gas sensor.@en

Heat transfer across thermal interface material, such as graphene-polymer composite, is a critical issue for microelectronics thermal management. To improve its thermal performance, we use chemical functionalization on the graphene with hydrocarbon chains in this work. Molecular dynamics simulations are used to identify the thermal conductivity of monolayer graphene and graphene-polymer nanocomposites with and without grafted hydrocarbon chain. The influence of functionalization with hydrocarbon chains on the interfacial thermal conductance of graphene-polyethylene nanocomposites was investigated using a non-equilibrium molecular dynamics (NEMD) simulation. We also study the effects of the graft density (number of hydrocarbon chain) on the thermal conductivity of graphene and the nanocomposite.@en

This paper analyzes the mechanical properties of tungsten disulfide (WS2) by means of multiscale simulation, including density functional theory (DFT), molecular dynamic (MD) analysis, and finite element analysis (FEA). We first conducted MD analysis to calculate the mechanical properties (i.e., Young's modulus and critical stress) of WS2. The influence of different defect types (i.e., point defects and line defects) on the mechanical properties are discussed. The results reveal that WS2 has a high Young's modulus and high critical stress. Next, the effects of defect density and temperature on the mechanical properties of the material were analyzed. The results show that a lower defect density results in improved performance and a higher temperature results in better ductility, which indicate that WS2 can potentially be a strain sensor. Based on this result, FEA was employed to analyze the WS2 stress sensor and then fabricate and analyze the device for benchmarking. It is found that the FEA model proposed in this work can be used for further optimization of the device. According to the DFT results, a narrower band gap WS2 is found with the existence of defects and the applied strain. The proposed multiscale simulation method can effectively analyze the mechanical properties of WS2 and optimize the design. Moreover, this method can be extended to other 2D/nanomaterials, providing a reference for a rapid and effective systematic design from the nanoscale to macroscale. @en

2D and nanostructured metal sulfide materials are promising in the advancement of several gas sensing applications due to the abundant choice of materials with easily tunable electronic, optical, physical, and chemical properties. These applications are particularly attractive for gas sensing in environmental monitoring and breath analysis. This review gives a systematic description of various gas sensors based on 2D and nanostructured metal sulfide materials. Firstly, the crystal structures of metal sulfides are introduced. Secondly, the gas sensing mechanisms of different metal sulfides based on density functional theory analysis are summarised. Various gas-sensing concepts of metal sulfide-based devices, including chemiresistors, functionalized metal sulfides, Schottky junctions, heterojunctions, field-effect transistors, and optical and surface acoustic wave sensors, are compared and presented. It then discusses the extensive applications of metal sulfide-based sensors for different gas molecules, including volatile organic compounds (i.e., acetone, benzene, methane, formaldehyde, ethanol, and liquefied petroleum gas) and inorganic gas (i.e., CO2, O2, NH3, H2S, SO2, NOx, CH4, H2, and humidity). Finally, a strengths-weaknesses-opportunities-threats (SWOT) analysis is proposed for future development and commercialization in this field. This journal is@en

In this work, the structural, electronic and optical properties of germanene and ZnSe substrate nanocomposites have been investigated using first-principles calculations. We found that the large direct-gap ZnSe semiconductors and zero-gap germanene form a typical orbital hybridization heterostructure with a strong binding energy, which shows a moderate direct band gap of 0.503 eV in the most stable pattern. Furthermore, the heterostructure undergoes semiconductor-to-metal band gap transition when subjected to external out-of-plane electric field. We also found that applying external strain and compressing the interlayer distance are two simple ways of tuning the electronic structure. An unexpected indirect-direct band gap transition is also observed in the AAII pattern via adjusting the interlayer distance. Quite interestingly, the calculated results exhibit that the germanene/ZnSe heterobilayer structure has perfect optical absorption in the solar spectrum as well as the infrared and UV light zones, which is superior to that of the individual ZnSe substrate and germanene. The staggered interfacial gap and tunability of the energy band structure via interlayer distance and external electric field and strain thus make the germanene/ZnSe heterostructure a promising candidate for field effect transistors (FETs) and nanoelectronic applications.@en

In this study, the structural, electronic and optical properties of a tungsten disulfide (WS2) hybrid with indium-gallium-zinc-oxide (IGZO) heterostructures were investigated based on density functional theory (DFT) calculations. According to the results of binding energy, charge density difference and electron localization function of heterostructures, we found that the WS2 and IGZO monolayers were bound to each other via non-covalent interactions with large binding energy. The calculated results illustrate that the AAii stacking pattern has an indirect band gap of 1.643 eV, while AAi and AB stacking patterns have maximum direct-gaps of 1.102 eV and 1.234 eV, respectively. Under an external E-field and mechanical strain, the response of the energy gap of the WS2/IGZO heterostructure monotonically decreased over a wide range, even with a semiconductor-metal transition. In addition, we investigated the optical properties of the heterostructure and found that it exhibits a much broad spectral responsivity (from visible light to deep UV light) and a more pronounced optical absorption than WS2 and IGZO monolayers. Moreover, the tensile strain could weaken the photoresponse of the heterostructure to the UV light and enhance the response for the visible light; under compressive strain, the heterostructure showed a strong absorption peak in the UV light. Meanwhile, a red-shift was observed under an external strain. All these unique and tunable properties indicate that the WS2/IGZO heterostructure is a good candidate for nanoelectronic and photoelectronic devices, such as field-effect transistors, flexible sensors, photodetectors and photonic devices.@en

In this study, the structural, electronic and optical properties of a tungsten disulfide (WS2) hybrid with indium-gallium-zinc-oxide (IGZO) heterostructures were investigated based on density functional theory (DFT) calculations. According to the results of binding energy, charge density difference and electron localization function of heterostructures, we found that the WS2 and IGZO monolayers were bound to each other via non-covalent interactions with large binding energy. The calculated results illustrate that the AAii stacking pattern has an indirect band gap of 1.643 eV, while AAi and AB stacking patterns have maximum direct-gaps of 1.102 eV and 1.234 eV, respectively. Under an external E-field and mechanical strain, the response of the energy gap of the WS2/IGZO heterostructure monotonically decreased over a wide range, even with a semiconductor-metal transition. In addition, we investigated the optical properties of the heterostructure and found that it exhibits a much broad spectral responsivity (from visible light to deep UV light) and a more pronounced optical absorption than WS2 and IGZO monolayers. Moreover, the tensile strain could weaken the photoresponse of the heterostructure to the UV light and enhance the response for the visible light; under compressive strain, the heterostructure showed a strong absorption peak in the UV light. Meanwhile, a red-shift was observed under an external strain. All these unique and tunable properties indicate that the WS2/IGZO heterostructure is a good candidate for nanoelectronic and photoelectronic devices, such as field-effect transistors, flexible sensors, photodetectors and photonic devices.@en

Double-sided packages for heat dissipation are an efficient thermal management mechanism for power semiconductor devices. A fan-out panel-level packaging (FOPLP), as one of the double-sided forms, exhibits excellent electro–thermal characteristics and provides low stray inductance and thermal resistance. Besides, the temperature at each point within the structure is closely related to its thermo–mechanical properties and device reliability. However, thermal resistance is limited in describing the temperature distribution. Finite element analysis (FEA) requires time-consuming construction of 3D models. Therefore, to depict the temperature distribution of FOPLP rapidly and accurately, a numerical heat transfer model was proposed for the double-sided package structure. The solution was obtained from the steady-state thermal balance Laplace equation using the separation of variables method. Several boundaries were analyzed to determine the specific parameters in the model. Finally, the temperature field predicted by the derived numerical model was compared with finite element simulation results. The proposed model was consistent with both Silicon (Si) and Silicon Carbide (SiC) FOPLP structures within the error of 15 % at the center of the device, which verified the validity and accuracy of the numerical model for double-sided heat dissipation. The proposed models and results could contribute to the development of effective thermal design tools for double-sided thermal power modules.@en

In this paper, tin oxidation (SnO x )/tin-sulfide (SnS) heterostructures are synthesized by the post-oxidation of liquid-phase exfoliated SnS nanosheets in air. We comparatively analyzed the NO2 gas response of samples with different oxidation levels to study the gas sensing mechanisms. The results show that the samples oxidized at 325 °C are the most sensitive to NO2 gas molecules, followed by the samples oxidated at 350 °C, 400 °C and 450 °C. The repeatabilities of 350 °C samples are better than that of 325 °C, and there is almost no shift in the baseline. Thus this work systematically analyzed the gas sensing performance of SnO x/SnS-based sensor oxidized at 350 °C. It exhibits a high response of 171% towards 1 ppb NO2, a wide detecting range (from 1 ppb to 1 ppm), and an ultra-low theoretical detection limit of 5 ppt, and excellent repeatability at room temperature. The sensor also shows superior gas selectivity to NO2 in comparison to several other gas molecules, such as NO, H2, SO2, CO, NH3, and H2O. After X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscope, and electron paramagnetic resonance characterizations combining first principle analysis, it is found that the outstanding NO2 sensing behavior may be attributed to three factors: The Schottky contact between electrodes and SnO x/SnS; active charge transfer in the surface and the interface layer of SnO x/SnS heterostructures; and numerous oxygen vacancies generated during the post-oxidation process, which provides more adsorption sites and superior bandgap modulation. Such a heterostructure-based room-temperature sensor can be fabricated in miniaturized size with low cost, making it possible for large-scale applications.@en

In this paper, four composite coatings of nano-SnS/polyvinylbutyral (PVB), nano-MoS2/PVB, nano-SnS-Zn/PVB, and nano-MoS2-Zn/PVB were prepared, and their anti-corrosion mechanism was analyzed by experimental and theoretical calculations. The results of the electrochemical experiments show that the effect of nano-MoS2 on the corrosion protection performance of PVB coating is better than that of nano-SnS in 3% NaCl solution, and that the addition of Zn further enhances this effect, which is consistent with the results of weight loss measurements. Furthermore, the observation of the corrosion matrix by the field emission scanning electron microscope (FESEM) further confirmed the above conclusion. At last, the molecular dynamics (MD) simulation were carried out to investigate the anti-corrosion mechanism of the nanofillers/PVB composites for the copper surface. The results show that both nano-SnS and nano-MoS2 are adsorbed strongly on the copper surface, and the binding energy of nano-MoS2 is larger than that of nano-SnS.@en

Nanostructured materials have attracted more and more attention in the applications of gas sensing due to their high specific surface area, numerous surface-active sites, as well as the effect of crystal facets with high surface reactivity. These kinds of gas sensors are mainly used for detecting air quality, environment situation, and breath analysis. Among different gas sensors, metal sulfide-based sensors have generated considerable interest in recent years because of their excellent sensitivity, fast response, and good selectivity. Alternatively, driven by the increasing demand for environmental and health monitoring, the sensors are required to have low limit of detection (LOD) in ppb-level, higher response and selectivity, and real-time recording. There are several ways to improve the sensing performance, such as functionalizing metal sulfide (defects, dopants), constructing heterojunction (Schottky junction, p-n, n-n, and p-p semiconductor junction), and using field-effect transistor (FET) gas sensor. Herein, my research aims to explore high-performance gas sensors through these techniques and to research the fundamental mechanism of the gas sensing process for metal sulfides devices. A comprehensive literature review of the state-of-the-art of metal sulfide-based gas sensor is presented in chapter 2. It includes the basic crystal structures, synthesis methods, device fabrication methods, and the gas sensing performances of various metal sulfide-based gas sensors. Since metal sulfides have a shallow valence band and different shapes, sizes, crystalline forms, chemical compositions, they have excellent sensing performance. It is found that the devices based on Schottky diode, metal oxide/metal sulfide heterojunction, and transistor have enhanced gas-sensing performance. Thus in this work, I analyzed the sensing behaviour of an SnS-Ti Schottky contact humidity sensor, an SnOx/SnS heterostructures-based NO2 gas sensor with rich oxygen vacancies, and a WS2/IGZO-based thin film transistor for NO2 gas sensing. To improve the humidity sensing performance, an SnS-Ti Schottky-contacted sensor is designed and analyzed in chapter 3. The SnS nanoflakes were mechanically exfoliated and then transferred on a rigid or flexible substrate. The as-fabricated sensor exhibited high response of 67600% towards 10% RH and 2491000% towards 99% RH, wide RH range from 3% RH to 99% RH, and fast response/recovery time of 6 s /4 s. The flexible humidity sensor shows a similar performance. Through the density functional theory (DFT) analysis and band alignment analysis, it is found that excellent sensing performance is attributed to the Schottky nature of SnS-Ti contact. H2O absorption moves the Fermi level of SnS toward the conduction band, decreasing the Schottky barrier (φB) byΔφB, resulting in thinning of the φB and an increase of the device current. Different relative humidity levels induce different ΔφB and sensitivity. The recovery mechanism is also attributed to the φB. When air flows out of the chamber, the water molecule shifts from the adsorption sites, and the conductivity decreases due to the increased φB. To extend the device’s application, a smart home system based on the sensors is proposed to process the signal from breath and finger touch experiments for noncontact controlling and respiration monitoring. To further improve the LOD and sensitivity for humidity and NO2 gas, four types of SnS-based gas sensors, including liquid phase exfoliated (LPE) SnS nanosheets, SnO2 nanosheets, SnO2/SnS nanocomposites, and SnOx/SnS heterostructure, are explored and comparatively analyzed in chapter 4. The results show that the sensor based on SnOx/SnS heterostructure that formed by the post-oxidation of LPE-SnS nanosheets in air, has excellent humidity sensing response among these four types of sensors. Accordingly, the SnOx/SnS is also used for detecting NO2 gas, which exhibits a high response of 161% towards 1 ppb NO2, wide detecting range (from 1 ppb to 1 ppm), an ultra-low theoretical LOD of 5 ppt, and excellent repeatability. To the best of my knowledge, such a LOD is the lowest among metal sulfide-based and metal oxide-based gas sensors. The sensor also shows excellent gas selectivity to NO2 with comparison to several other gas molecules, such as NO, H2, CO, NH3}, and H2O. The gas sensing mechanism analysis based on experiments and DFT calculations reveals that oxygen vacancies provide more adsorption sites, superior band gap modulation, and more active charge transfer in the sensing interface layer. Metal oxide/metal sulfide heterojunction is a great potential candidate for gas sensing applications. Thus we vertically stacked a p-type narrow bandgap semiconductor (WS2) and an N-type wide bandgap semiconductor (IGZO) to form a type I heterojunction WS2/ IGZO in chapter 5. The straddling gap results in both electrons and holes accumulating on the same side, and sensitive to the external stimulations. First of all, the structural, electronic, and optical properties of WS2/IGZO heterostructure are analyzed by DFT calculation under different E-field, mechanical strain, and gas molecules. The results demonstrate that the band gap of WS2/IGZO heterostructure shows a near-linear decrease with the increase of the E-field both in the negative and positive direction, resulting in a semiconductor-metal transition, revealing an application for the FET. The heterostructure exhibits broad spectral responsivity (from visible light to deep UV wavelengths) and enhanced optical properties under mechanical strain. The tensile strain can weaken the photoresponse of the heterostructure to the UV light and improve the response for the visible light; while for compressive strain, the heterostructure shows a sharp absorption peak in UV light. Moreover, the gas adsorption energy of NH3 and NO2 on the WS2/IGZO heterostructure are calculated, which shows high gas adsorption energy with NO2, indicating the potential application in NO2 gas sensor. The unique and tunable properties based on DFT calculation endow that the WS2/IGZO heterostructure is a good candidate for transistor and gas sensors. Thus, CVD-WS2/IGZO heterojunction-based devices are designed and investigated in two modes, chemiresistor, and transistor mode. The device has a maximum response of 18170% in the chemiresistor mode, and 499400% in the transistor mode under 300 ppm NO2 after applying -20 V gate bias. The heterojunction device is much better than that of only WS2 and IGZO. Moreover, the sensor shows excellent gas selectivity toward NO2 with comparison to several gas vapors such as CO, NH3, and humidity. The superior gas sensing performance could benefit from the heterojunction of WS2 and IGZO and the external electric field under the back gate voltage. In addition, the transistor notably presents a typical ambipolar-behaviour under dry air, while the transistor becomes p-type as the amount of NO2 increases. The mobility, on/off ratio, and subthreshold slope of the device is modulated by the NO2 gas concentration. The unique tunable behaviour can be associated with the doping effects of NO2 on the heterojunction and the modulated Schottky barrier value at the WS2 and IGZO with a metal contact interface. This thesis is concluded with summarizing the main obtained results and providing suggestions for future research opportunities in the field of 2D/nano- metal sulfides materials-based devices. The research for 2D/nanomaterials based device is still at an early stage. It is full of challenges to exploring high-quality materials suitable for gas sensors to guarantee the reliability and long-term stability of the device, to evaluate/test the sample accurately, and to integrate the sensor with the existing system. These fundamental research challenges need to be resolved in the future. @en

As an increasing attention towards sustainable development of energy and environment, the power electronics (PEs) are gaining more and more attraction on various energy systems. The insulated gate bipolar transistor (IGBT), as one of the PEs with numerous advantages and potentials for development of higher voltage and current ratings, has been used in a board range of applications. However, the continuing miniaturization and rapid increasing power ratings of IGBTs have remarkable high heat flux, which requires complex thermal management. In this paper, studies of the thermal management on IGBTs are generally reviewed including analyzing, comparing, and classifying the results originating from these researches. The thermal models to accurately calculate the dynamic heat dissipation are divided into analytical models, numerical models, and thermal network models, respectively. The thermal resistances of current IGBT modules are also studied. According to the current products on a number of IGBTs, we observe that the junction-to-case thermal resistance generally decreases inversely in terms of the total thermal power. In addition, the cooling solutions of IGBTs are reviewed and the performance of the various solutions are studied and compared. At last, we have proposed a quick and efficient evaluation judgment for the thermal management of the IGBTs depended on the requirements on the junction-to-case thermal resistance and equivalent heat transfer coefficient of the test samples.@en

In this paper, the heat transfer performance of the multi-chip (MC) LED module is investigated numerically by using a general analytical solution. The configuration of the module is optimized with genetic algorithm (GA) combined with a response surface methodology. The space between chips, the thickness of the metal core printed circuit board (MCPCB), and the thickness of the base plate are considered as three optimal parameters, while the total thermal resistance (Rtot) is considered as a single objective function. After optimizing objectives with GA, the optimal design parameters of three types of MC LED modules are determined. The results show that the thickness of MCPCB has a stronger influence on the total thermal resistance than other parameters. In addition, the sensitivity analysis is performed based on the optimum data. It reveals thatRtot increases with the increased thickness of MCPCB, and reduces as the space between chips increases. The effect of the thickness of base plate is far less than that of the thickness of MCPCB. After optimization, three types of MC LED modules obtain lower Tj andRtot. Moreover, the optimized modules can emit large luminous energy under high-power input conditions. Therefore, the optimization results are of great significance in the selection of configuration parameters to improve the performance of the MC LED module.@en

Humidity sensors based on flexible sensitive nanomaterials are very attractive in noncontact healthcare monitoring. However, the existing humidity sensors have some shortcomings such as limited sensitivity, narrow relative humidity (RH) range, and a complex process. Herein, we show that a tin sulphide (SnS) nanoflakes-based sensor presents high humidity sensing behaviour both in rigid and flexible substrate. The sensing mechanism based on the Schottky nature of a SnS-metal contact endows the as-fabricated sensor with a high response of 2491000% towards a wide RH range from 3% RH to 99% RH. The response and recovery time of the sensor are 6 s and 4 s, respectively. Besides, the flexible SnS nanoflakes-based humidity sensor with a polyimide substrate can be well attached to the skin and exhibits stable humidity sensing performance in the natural flat state and under bending loading. Moreover, the first-principles analysis is performed to prove the high specificity of SnS to the moisture (H2O) in the air. Benefiting from its promising advantages, we explore some application of the SnS nanoflakes-based sensors in detection of breathing patterns and non-contact finger tips sensing behaviour. The sensor can monitor the respiration pattern of a human being accurately, and recognize the movement of the fingertip speedily. This novel humidity sensor shows great promising application in physiological and physical monitoring, portable diagnosis system, and noncontact interface localization.@en

Humidity sensors based on flexible sensitive nanomaterials are very attractive in noncontact healthcare monitoring. However, the existing humidity sensors have some shortcomings such as limited sensitivity, narrow relative humidity (RH) range, and a complex process. Herein, we show that a tin sulphide (SnS) nanoflakes-based sensor presents high humidity sensing behaviour both in rigid and flexible substrate. The sensing mechanism based on the Schottky nature of a SnS-metal contact endows the as-fabricated sensor with a high response of 2491000% towards a wide RH range from 3% RH to 99% RH. The response and recovery time of the sensor are 6 s and 4 s, respectively. Besides, the flexible SnS nanoflakes-based humidity sensor with a polyimide substrate can be well attached to the skin and exhibits stable humidity sensing performance in the natural flat state and under bending loading. Moreover, the first-principles analysis is performed to prove the high specificity of SnS to the moisture (H2O) in the air. Benefiting from its promising advantages, we explore some application of the SnS nanoflakes-based sensors in detection of breathing patterns and non-contact finger tips sensing behaviour. The sensor can monitor the respiration pattern of a human being accurately, and recognize the movement of the fingertip speedily. This novel humidity sensor shows great promising application in physiological and physical monitoring, portable diagnosis system, and noncontact interface localization.@en

Humidity sensors based on flexible sensitive nanomaterials are very attractive in noncontact healthcare monitoring. However, the existing humidity sensors have some shortcomings such as limited sensitivity, narrow relative humidity (RH) range, and a complex process. Herein, we show that a tin sulphide (SnS) nanoflakes-based sensor presents high humidity sensing behaviour both in rigid and flexible substrate. The sensing mechanism based on the Schottky nature of a SnS-metal contact endows the as-fabricated sensor with a high response of 2491000% towards a wide RH range from 3% RH to 99% RH. The response and recovery time of the sensor are 6 s and 4 s, respectively. Besides, the flexible SnS nanoflakes-based humidity sensor with a polyimide substrate can be well attached to the skin and exhibits stable humidity sensing performance in the natural flat state and under bending loading. Moreover, the first-principles analysis is performed to prove the high specificity of SnS to the moisture (H2O) in the air. Benefiting from its promising advantages, we explore some application of the SnS nanoflakes-based sensors in detection of breathing patterns and non-contact finger tips sensing behaviour. The sensor can monitor the respiration pattern of a human being accurately, and recognize the movement of the fingertip speedily. This novel humidity sensor shows great promising application in physiological and physical monitoring, portable diagnosis system, and noncontact interface localization.@en

In this work, one layout model of insulated gate bipolar transistor (IGBT) module are built by using a general analytical solution, which is used to analyze the effect of thermal spreading resistance on the whole temperature distribution of a rectangular board with multiple eccentric heat sources. IGBT power electronic module is optimized by changing the arrangement of IGBT and FRD chips on the cooling system with consideration of junction temperature (T j ), temperature uniformity. According to the thermal analysis and optimization on the layout models, quantitative designs of IGBT power electronic module for chip arrangement are achieved.@en

SnS monolayer has sparked intensive attention due to its unique electronic and optical properties. We systemically investigate the electronic properties of SnS by first-principles calculation. Our results show that the monolayer possesses indirect bandgap. We further perform mechanical strain to adjust the electronic structure of SnS, corresponding results display an indirect-direct transition of band gap when subjected to proper external strain. Interestingly, the bandgap can be linearly increase under tensile strain from 0% to 7%, while the bandgap reduced under compressive strain. For biaxial strain, the band gap changes more remarkable compared with that under uniaxial strain (zigzag x or armchair y direction). Furthermore, we demonstrate that the gas molecules (CO2, H2S, C2H4 and NO2) adsorption property on SnS monolayer can be modulated through biaxial strain. Especially, the NO2 adsorption is further enhanced on the SnS monolayer under biaxial tensile strain. These results may provide guidance for fabricating SnS-based strained gas sensor.@en

In this work, a thin-film transistor gas sensor based on the p-N heterojunction is fabricated by stacking chemical vapor deposition-grown tungsten disulfide (WS2) with a sputtered indium-gallium-zinc-oxide (IGZO) film. To the best of our knowledge, the present device has the best NO2 gas sensor response compared to all the gas sensors based on transition-metal dichalcogenide materials. The gas-sensing response is investigated under different NO2 concentrations, adopting heterojunction device mode and transistor mode. High sensing response is obtained of p-N diode in the range of 1-300 ppm with values of 230% for 5 ppm and 18 170% for 300 ppm. On the transistor mode, the gas-sensing response can be modulated by the gate bias, and the transistor shows an ultrahigh response after exposure to NO2, with sensitivity values of 6820% for 5 ppm and 499 400% for 300 ppm. Interestingly, the transistor has a typical ambipolar behavior under dry air, while the transistor becomes p-type as the amount of NO2 increases. The assembly of these results demonstrates that the WS2/IGZO device is a promising platform for the NO2-gas detection, and its gas-modulated transistor properties show a potential application in tunable engineering for two-dimensional material heterojunction-based transistor device.@en