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A gas sensor based on a ZnGa2O4(ZGO) thin film grown by metalorganic chemical vapor deposition operated under the different temperature from 25 °C to 300 °C is investigated in this study. This sensor shows great sensing properties at 300 °C. The sensitivity of this sensor is 22.21 as exposed to 6.25 ppm of NO and its response time is 57 s. Besides that, the sensitivities are 1.18, 1.27, 1.06, and 1.00 when exposed to NO2(500 ppb), SO2 (125 ppm), CO (125 ppm), and CO2 (1500 ppm), respectively. These results imply that the ZGO gas sensor not only has high sensitivity, but also has great selectivity for NO gas. Moreover, the obtained results suggest that ZGO sensors are suitable for the internet of things(IOT) applications.
Recently, gas sensors have been developed and applied in environmental monitoring, human security, medical applications, and automobiles applications1,2,3,4. Among those different target gases monitoring, the detection of nitric oxide (NO) has attracted considerable interest. NO is an extremely toxic oxidizing gas with a pungent odor. It is always released by the action of nitric acid on metals, such as in metal etching and pickling. Besides, it plays an important role in a human biological process such as cardiovascular, immune systems5,6,7. Moreover, NO also affects neuron operation, which causes neurodegenerative diseases8. Therefore, it is very important to develop NO gas sensors.
There are various types of gas sensor including electrochemical9,10,11, optical12,13 and semiconductor gas sensors14,15,16,17. Semiconductor gas sensors have great potential for commercial application in environmental monitoring and healthcare due to the properties low cost, low power consumption, long lifetime, and the ability to operate in harsh environments.
It is well known that metal oxide semiconductors, e.g. SnO2 and ZnO, have been extensively studied for gas sensors applications. The corresponding sensing mechanism is resulted from the oxygen vacancy, metal vacancy and the other defects existing in the thin film18,19,20,21. Most of the metal oxide semiconductors were deposited by sputtering and sol-gel methods. It results that the crystal structure of thin films was amorphous or polycrystal. These suggest the defects in the thin film were not easily controlled and repeated. Although the polycrystal metal oxide semiconductors showed good sensitivity to many gases, the main issues are the poor selectivity and long response time. Recently, the wide bandgap oxide materials are attracting more attention for their use in novel devices owing to their thermally and chemically inert properties. Owing to such material properties, ZnGa2O4 (ZGO) has been demonstrated and presents very promising applications from the viewpoint of device fabrication22,23,24. ZGO is a transparent and conductive oxide material with a wide bandgap of approximately 5.2 eV, and it can be grown by metalorganic chemical vapor deposition (MOCVD) and fabricated into a photodetector for deep-ultraviolet25,26,27 and power device applications28,29.
Although, some NOx gas sensors with nanorods, nanowires, nanosheet have been fabricated30,31,32,33,34. The nanostructure gas sensor has great sensitivity due to extremely high surface-to-volume ratios. However, most thin film NOx gas sensors have difficulties in sensing ppb-level of NO35,36,37 and in gas selectivity. In this study, the ZGO epilayers grown on the sapphire substrate was successfully fabricated as a channel material for NOx gas sensor. The sensitivity, selectivity, and responsivity to NO at different operating temperature will be studied in this work.
Figure 1 shows XRD patterns of the ZGO thin film that were grown by MOCVD. The diffraction peaks about 18.40°, 37.34° and 57.49° were identified as the (111), (222), and (333) crystal plane of ZGO thin film. In general, the (333) plane was not allowed diffraction plane in the spinel crystal and the peak is always attributed to (511). Here, it can be regarded as the (333) diffraction plane due to the lattice mismatch between ZGO and sapphire. It means the ZGO eiplayer is a single crystal structure.
Figure 2(a) shows an SEM image of the ZGO thin film. It can be observed that ZGO thin film had a spindle structure. This structure offers lots of areas to react with NO gas molecules. Figure 2(b) shows the enlarged image. The length and width of spindle dimensions are about 120 nm and 40 nm, respectively.
The sensor was operated at 25, 100, 150, 200, 250, 300 °C six different operating temperatures to evaluate the optimum operated temperature. Figure 3(a) illustrates the relationship between sensitivity and NO gas concentration with different operating temperature from 25 °C to 300 °C. It indicates that the sensor has the highest sensitivity at 300 °C. Figure 3(b) illustrates the sensitivity as a function of NO gas concentration as the gas sensor operated at 300 °C. The sensitivity (S) using the curve fitting has a linear relation to NO concentration (C) at 300 °C. The linear fitting denoted as
It is worthy to mention that the concentration of NO increasing to ppm level still presented a linear relationship between sensitivity and concentration.
Figure 4 shows the relationship between operating temperature and the sensitivity of ZGO gas sensor when exposed to 6250 ppb of NO. It was found that the dynamic sensitivity curve shifted to the upper left corner as operating temperature increasing (black arrow). As the temperature ramps from 25 to 300 °C, the sensitivity increases from 1.11 to 22.21. Besides of that, the response time reduces from 10053 s to 57 s and recovery time reduces from 17646 s to 78 s. In other words, the sensing properties have been extraordinarily improved after ramping operating temperature to 300 °C.
Figure 5 illustrates the transient response of ZGO gas sensor with NO gas at 300 °C. The NO gas concentrations are 6250 ppb, 3125 ppb, 1000 ppb, 500 ppb, 250 ppb, and 125 ppb, respectively. As shown in the figure, the resistance increased on NO injection (gray region). NO gas molecules adsorbed onto ZnGa2O4 surface, and they captured electrons, leading to an increase in resistance. Figure 6 shows the sensitivity of ZGO gas sensor with 500 ppb of NO and NO2. It can be found that the ZGO gas sensor has a larger sensitivity of NO than that of NO2. The behavior can also support that the resistance increased abruptly and then decreased slowly as NO gas was injected to the chamber due to the NO gas transferring into NO2 in the air. The decrease in the resistance can possibly be ascribed to a decreasing in NO concentration, owing to the transformation of NO to NO2. By contrast, when NO gas was purged by fresh air (white region), the electrons returned to the conduction band of the ZGO thin film. Therefore, the resistance recovered to the original baseline. The sensitivity values are 22.21, 10.53, 5.03, 3.10, 2.01, and 1.88 with the NO concentrations of 6250 ppb, 3125 ppb, 1000 ppb, 500 ppb, 250 ppb, and 125 ppb, respectively.
To study the selectivity of the ZGO gas sensor, CO2, CO, and SO2 were injected with concentrations of 1500, 125 and 125 ppm at the same operating temperature (300 °C), respectively. Figure 7 shows the transient response of the sensor to those gases. The sensor hardly reacted with CO2 and CO. It did react with SO2, but it displayed a low sensitivity (1.27) against a high SO2 concentration (125 ppm). After comparing the gas concentration and the sensitivity, as shown in Fig. 8, the results imply that the ZGO gas sensor exhibits a high selectivity to NO at the operating temperature of 300 °C.
As concerning the mechanism of ZGO gas sensor being high responsivity for NO, it could be the fact that the dangling bonds on the surface of ZGO epilayer trapped oxygen molecules and turned them into adsorbed oxygen molecules. With different operating temperatures, the adsorbed oxygen molecules have different forms (O2−(ads) or O−(ads))38. The reactions of the adsorbed oxygen molecules are given by Eqs (2) and (3)39.
Figure 9(a) illustrates the interactions between the surface of the ZGO thin film and the adsorbed oxygen ions before NO gas injection. As mentioned above, the oxygen molecules dissociate and adsorb onto the ZGO surface of the thin film with the characteristic O2− or O− depending on the surface temperature. Both forms (O2−(ads) or O−(ads)) extract electrons from the conduction band of the semiconductor, leading to the creation of the depletion region in the ZGO thin film. Figure 9(b) shows the interactions between the thin-film surface and the NO gas molecules. When NO gases were introduced into the chamber, NO gas molecules trapped the electrons due to high electronegative property and became NO− which is shown in Eq. (4)40. On the other hand, NO gas molecules reacted with the adsorbed oxygen molecules as the Eq. (5) shows40. Both reactions further extracted the electrons, and that caused the conductivity to decrease. As shown in Fig. 3(a), there is an obvious enhancement on sensitivity to NO as operating temperature increases from 100 °C to 150 °C (red arrow). This is due to the fact that high temperature makes the particles originally adsorbed on the surface desorb which allows more states on ZGO surface to react with NO gas molecules. Furthermore, high temperature also changes the form of adsorbed oxygen molecules. As the temperature is low, O2− is the dominant adsorbed oxygen molecule. When the temperature ramps up, the dominant molecule becomes to O− which is more reactive. This makes NO gas molecules more easily react with adsorbed oxygen molecules and increase the sensitivity. On the other hand, high temperature provides more kinetic energy for gas molecules to move in the chamber, and also speeds up the process of reactions between NO gas molecules and adsorbed oxygen molecules. This resulted in a dramatically reducing on response time and recovery time as operating temperature increasing.
Table 1 presents the response time and recovery time of the ZGO gas sensor for different concentrations of NO gas at the operating temperature of 300 °C. The results imply that the sensor rapidly detected NO gas. The response times to all gas concentrations were shorter than 60 s, and no relation was observed between the NO gas concentration and response time. The response time of 125, 250, and 500 ppb are almost the same. This implies that NO gas molecules could adsorb onto ZGO thin film surface very easily in this concentration range. However, as the gas concentration increases (>1000 ppb), there are more and more molecules try to adsorb onto the ZGO surface. The molecules have to spend more time to find the unoccupied states on the surface. Therefore, the response time increases. By contrast, it was found that the recovery time increased with decreasing gas concentration. This could possibly be ascribed to the fact that the surface completely absorbed the NO under high concentration. Owing to purging by fresh air, the gas sensor surface desorbed NO immediately. The recovery time was related to the concentration difference (reference is the background concentration). The recovery time was shorter when the concentration difference was high.
The work functions of the clean ZGO (111) surface and the adsorption bonding of NO on the ZGO (111) surface are summarized in Table 2. The work function of the clean ZGO(111) surface is 4.04 eV, and it was used as a reference for the work function change calculations herein. Figure 10 shows the energy diagram of Model N-Ga, which in turn shows the work function, 4.15 eV, between the vacuum level EVAC and the Fermi level EF. The work function changes in the cases of Models N-Ga and N-Zn were 0.11 and 0.04 eV, respectively, indicating a more sensitive adsorption site of atomic Ga on the ZGO(111) surface. For two NO molecules, we verified that the work function changes of the models in terms of magnitude follows the order: Model 2N-Ga (0.26 eV) > Model 2N-Ga-Zn (0.23 eV) > Model 2N-Zn (0.17 eV). This ordering remarkably demonstrates that high concentrations of NO gas exhibit high selective gas adsorption for NO onto ZGO thin film.
A NO gas sensor based on a ZGO epilayer grown by MOCVD was investigated in this work. The results indicated that ZGO gas sensor exhibited high sensitivity, reversibility, and selectivity in detecting NO at the operating temperature of 300 °C. When exposed to 125 ppb NO, a sensitivity of 1.88 was observed. The response time and recovery time were 36 and 208 s, respectively. The sensor has high sensitivity to NO, but it hardly reacts with CO2, CO, and SO2. Besides, ZGO also shows a larger response to NO than to NO2. Moreover, the results of a first-principles simulation proved that the ZGO gas sensor exhibits a great response to NO gas because of the large change in work function when NO gas molecules are adsorbed onto the ZGO thin-film surface. The above results prove that the proposed ZGO thin film gas sensor has the potential for use in IOT applications.
ZGO thin films with a thickness of 100 nm were grown on a c-plane (0001) sapphire substrate at 600 °C by MOCVD. The precursors of Zn and Ga are diethylzinc (DEZn) and triethylgallium (TEGa), respectively. Purified Ar and oxygen were employed as the carrier gas and oxide source, respectively. The thickness of epilayer was approximately 100 nm under a growth rate of 0.8 nm/min. After epilayer growth, the process commenced with mesa isolation in an induced coupled plasma etching system by BCl3/Cl2/Ar. The mesa isolation process etched the epilayer onto the sapphire substrate and left the plateau. The electrodes were composed of Ti/Al/Ni (50/75/25 nm) multilayer metals deposited using an E-gun evaporator, and they were patterned by a lift-off process. The channel length L and width W were 30 and 250 µm, respectively. After the completion of the above mentioned processes, the device was annealed under 700 °C for 1 h. A schematic of the gas sensor measurement system and the device structure are shown in Fig. 11.
Regarding the sensing characteristics of the gas sensor, sensitivity and response time are important parameters. Sensitivity can be defined as Ra/Rg for reducing gases and Rg/Ra for oxidizing gases, where Ra and Rg denoted as the resistance of the gas sensor with dry air and that to the target gases, respectively41.
Response time and recovery time are defined as the time required for the sensor to reach 90% of its steady resistance and back to 10% of the value42.
To study the mechanism of the reaction between NO gas and ZGO, the reactions of the gas with different surface structures were simulated by first-principle calculations. In general, the sensor response is typically characterized by work function changes in gas-sensitive materials. If we assume that the gap between the conduction band and the Fermi level in the bulk is not affected by gas adsorption at the surface, the work function changes because of the adsorption process of oxidizing gases as opposed to those caused by the clean surface can be written as follows43.
where Δχ denotes changes in electron affinity, and the second term corresponds to changes in band bending. Here, k and T are Boltzmann constant and temperature, respectively. Equation (6) shows that the work function changes can be described in terms of sensitivity (Rg/Ra for oxidizing gases). Moreover, we present ab initio simulations of NO adsorption behavior onto ZGO (111) thin film to elucidate the sensitivity of our gas sensor. Our simulations were based on the density functional theory (DFT), as implemented in the Vienna ab initio simulation package code44,45,46. The projector-augmented wave method and the generalized gradient approximation (GGA) with the Perdew-Wang (PW91) exchange-correlation functional were employed to efficiently treat ion-electron interactions47,48. The electronic configurations of the valence electrons were N: 2 s2/2p3, O: 2s2/2p4, Zn: 4s2/3d10, and Ga: 4s2/4p1. The ZGO (space group: 227 Fd-3 m) alloy and NO gas (space group: 99 P4mm) were constructed using the bulk crystalline and the gas configurations, respectively. In the ZGO(111) surface slab models, we adopted a (sqrt{2}times sqrt{2}) basal setting (11.85 Å × 11.85 Å) for all adsorption calculations. The repeated slab geometry layers fixed at Zn16Ga32O64 were separated by vacuum regions equivalent to a thickness of 20 Å. Ga-Zn-O-terminated ZGO (111) surfaces are preferred with a low surface energy of 0.096 eV/Å2, and therefore, such surfaces were adopted in the present work49. Reactions of NO molecules on Ga-Zn-O-terminated ZnGa2O4(111) surfaces were modeled to calculate the work function changes or the NO sensitivity. The Brillouin zones were created using a 3 × 3 × 1 Gamma-Center grid and a 400-eV energy cutoff in the surface reaction models to obtain the optimized adsorption bonding of NO molecules on Ga-Zn-O-terminated ZnGa2O4(111) surfaces (Fig. 12). As one NO molecule approached the Ga-Zn-O-terminated ZnGa2O4(111) surface, the nitrogen of NO bonded with the gallium atom on the ZGO (111) surface shown in Model N-Ga. In Model N-Zn, the nitrogen of NO bonded with the zinc atom on the ZGO (111) surface. To compare the concentrations of NO, we constructed Model 2N-Ga, which showed that each of nitrogen of two NO molecules was bonded to the gallium atoms on the ZGO (111) surface. In Model 2N-Ga-Zn, each nitrogen of two NO molecules were bonded to one zinc atom and one gallium atom on the ZGO (111) surface. In Model 2N-Zn, each nitrogen of two NO molecules were bonded to the zinc atoms on the ZGO (111) surface.
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This work was supported by the Ministry of Science and Technology (MOST), Taiwan, R.O.C., under Grants MOST 105–2221-E-009-183-MY3, 106-2745-M009-001-ASP, 107-2221-E-009-117-MY3, 107-2218-E-009-056, 107-3017-F009-003, Ministry of Education, Taiwan (SPROUT Project-Center for Emergent Functional Matter Science of National Chiao Tung University) and TYNYEK Corp. We are grateful for the use of facilities in the National Nano Device Laboratory of Taiwan. A number of people have improved this work by offering their expertise and skills.
M.R.W., W.Z.L. and C.Y.T. conceived and designed the experiments. C.Y.H. contributed to growing the ZnGa2O4 films by MOCVD technique. Y.H.C. and P.L.L. do the simulation. R.H.H. designed experiments, analyzed, verified the data and wrote paper. All authors read and approved the final version of the manuscript to be submitted.
The authors declare no competing interests.
Correspondence to Ray-Hua Horng.
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Two-dimensional (2D) molybdenum disulphide (MoS2) atomic layers have a strong potential to be used as 2D electronic sensor components. However, intrinsic synthesis challenges have made this task difficult. In addition, the detection mechanisms for gas molecules are not fully understood. Here, we report a high-performance gas sensor constructed using atomic-layered MoS2 synthesised by chemical vapour deposition (CVD). A highly sensitive and selective gas sensor based on the CVD-synthesised MoS2 was developed. In situ photoluminescence characterisation revealed the charge transfer mechanism between the gas molecules and MoS2, which was validated by theoretical calculations. First-principles density functional theory calculations indicated that NO2 and NH3 molecules have negative adsorption energies (i.e., the adsorption processes are exothermic). Thus, NO2 and NH3 molecules are likely to adsorb onto the surface of the MoS2. The in situ PL characterisation of the changes in the peaks corresponding to charged trions and neutral excitons via gas adsorption processes was used to elucidate the mechanisms of charge transfer between the MoS2 and the gas molecules.
Over the past few decades, metal oxide semiconductors have been applied as conventional chemical sensing materials because of their high sensitivity and relatively low cost,2,. However, they still have some critical drawbacks. First, metal oxide semiconductors exhibit poor sensitivity and selectivity at room temperature. This obstacle has led to the development of alternative materials such as carbon nanotubes, graphene, and transition metal dichalcogenides (TMDs),,,,10,. Recently, 2D TMDs have attracted much attention for use in next-generation nanoelectronic devices,,, with a single-layer MoS2 transistor having been reported to exhibit outstanding performance. The intrinsic merits of TMDs, including their high surface-to-volume ratio and semiconducting properties, have accelerated the development of a diverse range of applications of these materials as chemical sensors. A recent flurry of research involving MoS2-based gas detection has mitigated the wide chasm between metal oxide materials and alternatives,,,,10,. However, the fundamental mechanism of chemical sensing using MoS2 remains unclear, limiting its practical applications. Here, we demonstrate highly sensitive and selective gas detection of NO2 and NH3 using uniform wafer-scale MoS2 nanofilms synthesised by thermal chemical vapour deposition (CVD). We elucidate the charge transfer mechanism of MoS2 gas adsorption using in situ photoluminescence (PL) and computational calculations involving first-principles density functional theory. The peak intensities from the positively charged trions (A+) and neutral excitons (A0) in the PL spectrum show trade-off phenomena by adsorption of each different gas molecule (NO2 or NH3) onto the MoS2. The electron depletion of MoS2 by NO2 adsorption leads to an increase in the intensity of the A+ peak and a suppression of the intensity of the A0 peak, whereas electron accumulation by NH3 adsorption suppresses the intensity of the A+ peak and increases the intensity of the A0 peak. These in situ PL characterisation results clarify the mechanisms of charge transfer between the MoS2 and the gas molecules. These findings will help to implement future gas sensing technologies using diverse two dimensional TMDs nanomaterials.
Most approaches use direct/indirect sulphurisation of Mo-containing thin films to synthesise atomic-layered MoS2 thin films. The precursor is a key factor in the synthesis of MoS2. In previous studies, most authors adopted one of three precursors: molybdenum thin films; molybdenum trioxide; or ammonium thiomolybdate. However, previous methods have involved complex precursor preparations, yielding films with inconsistent quality. In our search for strategies for synthesising uniform wafer-scale MoS2 (see schematic in Fig. 1a), we have focused on the development of a thermal CVD system and process. Atomic-layered MoS2 was grown using molybdenum trioxide (MoO3) deposited onto a sapphire substrate and a sulphur powder source. The sublimated sulphur served as a precursor to sulphurise the MoO3 film. To achieve our overall goal of preparing MoS2 films of consistent quality on the desired substrates, we turned our attention to pressure control during the CVD reaction. A recent report indicated that an increase in the amount of either Mo or S atoms results in increased formation of energetically favourable defects on the MoS2 surface during film growth. Thus, we systematically controlled the reaction pressure to provide sufficient sublimated sulphur using a custom-made automatic pressure control system (Supplementary Fig. S1).
Large-scale synthesis of MoS2.(a) Schematic of the atomic-layered MoS2. The quasi-2D MoS2 was occupied by one Mo (a trigonal prismatic structure) and two S atoms (hexagonal planes). (b) Image of the as-synthesised MoS2 film on the 2-inch sapphire substrate. The as-synthesised MoS2 film was semi-transparent. (c) Cross-sectional TEM images of the as-grown MoS2 films. The image clearly demonstrates that the synthesised MoS2 films consisted of three layers of MoS2. (d) Raman spectrum of the triple-layered MoS2. The spectrum reveals a strong in-plane vibrational mode for the Mo and S atoms (E2g) and an out-of-plane vibrational mode for the S atoms (A1g). The peak position difference (Δ) between the E2g and A1g bands is approximately 22.9, indicating triple-layered MoS2. (e, f) Raman maps of E2g (blue) and A1g (red), respectively. The Raman mapping area was 50 × 50 μm2 with 0.3 μm steps. The Raman images show the spatial distribution on the surface of the substrates.
The new CVD system design was very effective for the uniform synthesis of MoS2 films on 2-inch sapphire substrates, as illustrated in Fig. 1b. Cross-sectional transmission electron microscopy (TEM) was used to examine the number of layers formed by CVD (Fig. 1c). The MoS2 films contained double, triple, and, in some cases, more than three layers (additional TEM images, TEM energy-dispersive X-ray spectroscopy (EDS) maps, TEM EDS point spectra, atomic force microscopy images, X-ray photoelectron spectra, and absorption spectra are provided in Supplementary Figs. S2–7). The Raman spectrum in Fig. 1d shows the in-plane vibrational mode of the Mo and S atoms (E2g) and the out-of-plane vibrational mode of S atoms (A1g) in the as-synthesised MoS2 films. The difference in peak position (Δ) between the E2g and A1g bands, which is a strong indicator of the number of layers, was approximately 22.9. This result indicates that the as-synthesised MoS2 was mainly composed of three layers,,. To confirm the wafer-scale synthesis of MoS2, we conducted a large-scale structural analysis using Raman mapping and an imaging technique. The Raman mapping area was 50 × 50 μm2 with 0.3 μm steps (the original Raman mapping spectra are shown in Supplementary Fig. S8). The corresponding Raman images revealed the spatial distribution of MoS2 over a 250-μm2 area of the substrate (Figs. 1e and f). The blue and red models show the spatial distributions of the E2g and A1g bands, respectively. The as-synthesised MoS2 was highly uniform over a large area of the surface (Figs. 1e and f). Thus, systematic pressure control during the CVD process resulted in highly uniform MoS2 films on the wafer scale.
The uniform atomic-layered MoS2 films were used for gas molecule detection (Fig. 2a). Transient resistance responses were investigated using two analyte gases (NO2 or NH3 at concentrations from 1.2 to 50 ppm). The gas sensitivity was calculated using ΔR/Ra = (Rg - Ra)/Ra, where Ra and Rg represent the resistances of the device to air and the analyte gas, respectively. In the NO2 gas mode, the resistance increased (positive sensitivity) (Fig. 2b). The NO2 sensitivity values were comparable to those in a previous report. NO2 acts as an electron acceptor, resulting in p-doping (Supplementary Fig. S12). The NO2 molecules on the surface of MoS2 bring the Fermi level closer to the valence-band edge. During the desorption process, thermal energy (heating to 100°C) enhances the rate of desorption of the NO2 molecules from the MoS2 film (Fig. 2b, red line). We next compared the gas sensing characteristics for NH3 to those for NO2 (Fig. 2c). In contrast to the resistance recorded for NO2 molecules, the resistance of the MoS2 sensing device decreased with the adsorption of NH3 gas molecules, i.e., negative sensitivity was observed. NH3 acts as an electron donor (i.e., n-doping) such that it shifts the Fermi level of the MoS2 to the conduction-band edge. However, theoretical calculations indicated that the Fermi-level shift induced by the NH3 molecules is negligible (Supplementary Fig. S12). The measured overall NH3 sensitivities were lower than those of NO2 (Figs. 2b and c) because of the smaller charge transfer of NH3 compared to that of NO2. The dependence of the gas response on the gas concentration at different operating temperatures is plotted in Fig. 2d. The surface chemical reaction between the MoS2 channel and the NO2 molecules saturated at approximately 20 ppm, irrespective of the operating temperature (Fig. 2d, red and blue lines). By contrast, in the case of NH3, the sensitivity at RT gradually increased from 5 to 50 ppm and the sensitivity under 5 ppm was undetectable. However, the sensing signal at 100°C was imperceptible at all concentrations (Supplementary Fig. S9). Thus, the recovery rate of NO2 at 100°C is clearly superior to that of NH3, which is closely related to the faster desorption process of NO2 molecules as a result of the thermal energy,. At 20 ppm and 100°C, we obtained the best selectivity for NO2 relative to NH3 (~400% increase compared to that at RT).
Gas-sensing using the MoS2 device.(a) 3D schematic of the MoS2 gas-sensing device for NO2 and NH3. (b) Transient NO2 gas response at 1.5 to 50 ppm gas at operating temperatures of RT and 100°C. In the NO2 gas mode, the resistance increases (positive sensitivity). The recovery rate of NO2 is higher at 100°C than at RT. (c) The transient NH3 gas response at 1.5 to 50 ppm gas at operating temperatures of RT and 100°C. The resistance decreases with the adsorption of NH3 gas molecules (negative sensitivity). The NH3 sensing signal is negligible at 100°C. (d) Comparison of the NO2 and NH3 sensitivities at different gas concentrations and operating temperatures. The highest selectivity of NO2 to NH3 was obtained when the concentration reached 20 ppm at 100°C.
To explore the gas adsorption characteristics of the MoS2, we adopted theoretical and experimental approaches. First-principles density functional theory (DFT) calculations were conducted using the screened hybrid functional of Heyd-Scuseria-Ernzerhof with the D2 correction for van der Waals interactions26, (see the detailed methods in the Supplementary Information). To simulate NO2 and NH3 adsorption onto the MoS2 monolayer, supercells containing 16 Mo and 32 S atoms with NO2 and NH3 were employed using a 2 × 2 × 1 k-point grid. The most stable configurations of NO2 and NH3 reported in a recent study that compared the total energy between different adsorption configurations were considered. The NO2 and NH3 molecules were preferentially adsorbed onto the top of the hexagon of the MoS2 (Figs. 3a and b). The adsorption energies of the NO2 and NH3 gas molecules were evaluated using , where is the total energy of a supercell containing both an MoS2 monolayer and a gas molecule (NO2 or NH3), is the total energy of the host MoS2 supercell, and E(molecule) is the total energy of a supercell containing a gas molecule. The calculated adsorption energies of NO2 and NH3 were −0.14 eV and −0.16 eV, respectively. These values were ~0.1 eV smaller than the values obtained using the local density approximation (LDA) because the LDA functional overestimates the adsorption energy. The negative adsorption energies indicate that the adsorption process is exothermic. Thus, NO2 and NH3 molecules are likely to be adsorbed onto the surface of MoS2.
Adsorption configurations and in situ PL.(a, b) Top views of the most favourable configurations for NO2 (a) and NH3 (b) on the MoS2. The calculated adsorption energies were −0.14 eV for NO2 and −0.16 eV for NH3. The negative adsorption energies indicate that the adsorption process is exothermic, indicating that NO2 and NH3 molecules are likely to be adsorbed onto the surface of the MoS2. (c, d) In situ PL spectra recorded from the MoS2 with NO2 (c) and NH3 (d) molecules. The overall intensity of the PL spectra changes in the presence of NO2 and NH3 molecules. The PL intensities of the A+ trions and A0 excitons are either suppressed or increased by changes in the concentrations of the charge carriers. (e, f) Schematics of the charge density differences for MoS2 in the presence of NO2 (e) and NH3 (f) gas molecules. NO2 molecules on the surface of MoS2 act as electron acceptors, whereas NH3 molecules act as electron donors.
Next, we turned our attention to the in situ characterisation of PL to study the sensing mechanism in depth. The high temperature during film growth can induce unintentional defects on the substrate/MoS2 interface. To prevent defects, we transferred the as-grown MoS2 from the sapphire substrate to a SiO2/p + Si substrate. Interestingly, the atomic-layered MoS2 transferred onto the SiO2/p + Si substrate exhibited opposite gas sensitivity (i.e., p-type behaviour) compared to the n-type behaviour of MoS2 on the sapphire substrate (see Supplementary Fig. S13 for details). Dangling bonds at the semiconductor/substrate interface can redefine the effective Fermi levels within gap states, modulating the conductive properties of the MoS229. The dangling oxygen bonds on the SiO2 surface can result in a p-type MoS2 semiconductor29. In the in situ PL characterisation with the two analyte gases (see Supplementary Fig. S10 for details), we observed various intensity changes in the PL spectra for NO2 and NH3 molecules (Figs. 3c and d). The atomic-layered MoS2 had two main PL peaks associated with the A and B excitons (Supplementary Figs. S11). Spin-orbit coupling-induced valence-band splitting can give rise to A and B excitons. After the gas molecules are adsorbed, the PL intensities of the A and B excitons can either be suppressed or increased by changing the concentrations of the charge carriers,. We here focused on the signal peak of the A exciton. The relatively low-energy A exciton signal expands to two features: a trion of A−/+ (two electrons to a hole, resulting in a negatively charged exciton, or an electron to two holes, resulting in a positively charged exciton) and a neutral exciton of A0,. With the emergence of the trion, we assumed that the exciton is coupled to either another electron or to a hole at the Fermi level. In the case of MoS2, the A+ and A0 peaks correspond to the trions (1.8413 eV) and to the neutral excitons (1.8424 eV), respectively. The bare MoS2 notwithstanding, the positive trion (in this case, an electron to two holes, resulting in positively charged excitons, A+) emission dominates the PL spectra because the bare MoS2 on the SiO2 substrate exhibits p-type characteristics, as previously mentioned. As schematically shown in Fig. 3e, the NO2 molecules on the surface of the MoS2 act as electron acceptors (p-type dopants), whereas the NH3 molecules act as electron donors (n-type dopants). By adsorption of NO2 gas molecules, a neutral exciton (A0) can be converted into a quasi-particle (A+) because of excessive holes generated by electron extraction from the MoS2. As a result, the A+ peak in the PL spectrum increases in intensity and the A0 peak is suppressed (Fig. 3c). By contrast, when additional electrons are introduced from the NH3 molecules, the intensity of the A+ peak in the PL spectrum is suppressed because of dissociation of the positive trions from the neutral excitons, resulting in increasing neutral excitons (A0), as shown in Fig. 3d. The in situ PL characterisation clarifies the mechanisms of charge transfer between the MoS2 and the gas molecules.
The gas sensing characteristics of wafer-scale layered MoS2 fabricated by CVD were determined. The gas sensor based on the CVD-fabricated MoS2 exhibited excellent sensitivity and high selectivity. The in situ PL characterisation and theoretical studies elucidated the charge-transfer mechanism between the gas molecules and the MoS2. In-depth PL studies verified that the electron depletion of the MoS2 by NO2 adsorption increased the intensity of the A+ peak and suppressed that of the A0 peak, whereas electron accumulation by NH3 adsorption suppressed the intensity of the A+ peak and increased that of the A0 peak. Intensive PL characterisation clarified the charge transfer phenomena between the MoS2 and the gas molecules. The results of this study will enable more extensive applications of gas sensing using two dimensional transition metal dichalcogenides nanomaterials.
MoS2 nanofilms were synthesised using chemical vapour deposition (CVD) (Teraleader Co., Ltd., South Korea) (Supplementary Fig. S1). First, C-plane sapphire substrates were prepared using a typical cleaning process (sonication in acetone, isopropyl alcohol, and deionised water for 10 min each). MoO3 films (5 nm) were deposited onto the clean substrates using a thermal evaporator. The pre-deposited MoO3 samples were placed at the centre of the furnace, and ~1 g of sulphur powder, which was used as a sulphur precursor, was subsequently loaded into a quartz boat in an independently temperature-controllable flange heater located near the inlet of the furnace. The furnace and the flange heater were heated to ~850°C and ~180°C, respectively, for 1 h. The process was maintained for an additional 1 h under flowing Ar/H2 gas (volume ratio: Ar:H2 = 85:15%) at a chamber pressure of 760 torr. The MoO3 film was converted into a MoS2 nanofilm via a two-step reaction (the reduction of MoO3 by hydrogen gas, followed by sulphurisation of the reduced MoO3 with sublimated sulphur gases). Finally, the furnace was rapidly cooled to room temperature by opening the chamber box after the furnace was turned off.
The SiO2 (300 nm)/C-plane sapphire was cleaned using a typical cleaning process (sequential sonication in acetone, isopropyl alcohol, and deionised water for 10 min each). A MoO3 film (~5 nm thick) was patterned with an active shadow mask using a thermal evaporator. The patterned MoO3 film was converted into a MoS2 nanofilm by CVD. Using a thermal evaporator and a shadow mask with an interdigitated electrode array structure consisting of two opposing comb-shaped electrodes with a width of 400 μm and a gap of 100 μm, we deposited an Ag film (100 nm; used for the electrodes) onto the MoS2 nanofilm. Detailed fabrication schemes are provided in Supplementary Fig. S14.
See the detailed methods in the Supplementary Information.
B.C., M.G.H. and D.-H.K. designed and supervised the experiments. B.C., M.G.H. and A.R.K. synthesised the MoS2. B.C., M.G.H., J.Y., A.R.K. and Y.-J.L. characterised the MoS2. B.C. and A.R.K. fabricated the MoS2-based devices. B.C., M.G.H., S.L., T.J.Y. and C.G.K. measured the MoS2 sensing devices. M.G.H. and M.C. performed the first-principles DFT calculations for the MoS2 with gas molecules. B.C., M.G.H., M.C., S.-G.P., J.-D.K., C.S.K., M.S., Y.J., K.-S.N., B.H.L., H.C.K., P.M.A. and D.-H.K. analysed the data. B.C., M.G.H. and D.-H.K. co-wrote the paper. All authors discussed the results and commented on the manuscript.
Charge-transfer-based Gas Sensing Using Atomic-layer MoS2
This study was supported financially by the Fundamental Research Program (PNK3770 and PNK4060) of the Korean Institute of Materials Science (KIMS) and by the “Gyeongsangnam, Changwon Science Research Park Project” of the Grant of the Korean Ministry of Science, ICT and Future Planning. M. G. H. and B. C. are grateful for support from the Basic Science Research Program of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2014R1A1A1006214 and NRF-2014R1A1A1036139). MC was supported by Global Frontier Program through the Global Frontier Hybrid Interface Materials (GFHIM) of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2013M3A6B1078872).