Metal sulfides for gas sensing applications: devices and mechanisms

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.