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Abstract
Infrared gas sensors hold great promise in the internet of things and artificial intelligence. Making infrared light sources with miniaturized size, reliable and tunable emission is essential but remains challenging. Herein, we present the tailorability of radiant power and the emergence of new emission wavelength of microelectromechanical system (MEMS)-based thermal emitters with nickel oxide (NiO) films. The coating of NiO on emitters increases top surface emissivity and induces the appearance of new wavelengths between 15 and 19 µm, all of which have been justified by spectroscopic methods. Furthermore, a sensor array is assembled for simultaneous monitoring of concentrations of carbon dioxide (CO2), methane (CH4), humidity, and temperature. The platform shows selective and sensitive detection at room temperature toward CO2 and CH4 with detection limits of around 50 and 1750 ppm, respectively, and also shows fast response/recovery and good recyclability. The demonstrated emission tailorability of MEMS emitters and their usage in sensor array provide novel insights for designing and fabricating optical sensors with good performance, which is promising for mass production and commercialization.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The ever-growing resource consumption caused by human activities poses severe energy and environmental related challenges for human beings. Forging smart cities with high sustainability and efficiency by utilizing intelligent electronic devices for monitoring and controlling all the infrastructures and associated services is perceived as an effective methodology for tackling the foregoing challenges [1]. To achieve so, sensors including gas sensors have to be widely deployed and interconnected in large quantities to collect information for informed decision-making [2]. Amongst the existing sensing technologies [3–18], nondispersive infrared (NDIR) gas sensors featuring distinctive characteristics such as good selectivity, fast response, and long lifetime, have gained substantial interests. Disadvantages like requiring periodic calibration and showing limited detection range also exist, these issues nevertheless can be alleviated by using effective light sources or photodetectors [19]. Fabrication of miniaturized sensors with reduced cost remains challenging. The working principle of NDIR sensors relies on the unique absorption fingerprints in mid-infrared (MIR) regime of gas molecules, whose absorbance is linearly proportional to their concentration according to the Beer-Lambert law [20]. As core components of NDIR gas sensors, MIR light sources with miniaturized size as well as reliable and tailorable radiation are highly desirable.
Thermal emitters based on microelectromechanical system (MEMS) technology offer an effective solution with compactness and reduced cost [3,21]. The thermal emitters can generate electromagnetic radiation under applied voltages. The radiant spectral profiles mainly depend on the chosen material and the operating temperature according to the Planck’s law and Wien's displacement law [22]. Recent studies have demonstrated the engineering of MEMS-based thermal emitters in terms of radiated power and/or spectral profile by judicious choices of architecture design and its constituent materials including tungsten (W), polysilicon (Si) or platinum (Pt) [21,23–25], as well as the additional coating of blackening layers composed of carbon nanotubes (CNTs), carbon nanoparticles, graphene, or graphene oxide (GO) [3,26–29]. However, therein still lies the issue of thermal stability of carbon-based materials under the ambient environment, restricting their long-term usage in optical gas detection. Nickel oxide (NiO) film has been reported showing good thermal stability and high emissivity contributed by its unique crystalline structure [30,31]. In the meanwhile, the potential for NiO film to be implemented on MEMS-based thermal emitter to achieve gas sensing with high performance remains to be further explored.
Herein, we experimentally demonstrate the tailorability of radiation intensity and emergence of new waveband from NiO-coated MEMS thermal emitters [Fig. 1(a)]. Integrating the thermal emitters with NiO films that exhibit enhanced chemical/thermal stability and emissivity not only increases the radiant power, but also induces the appearance of new wavelengths between 15 and 19 µm. The as-fabricated NiO-coated emitters serve as the light sources for NDIR carbon dioxide (CO2) sensors, which can be integrated with methane (CH4), humidity, and temperature sensors into a platform for monitoring multiple environmental parameters. The detection of CO2, which has been recognized as a greenhouse gas with significant influences on climate change and global ecosystems, holds great importance in various aspects including food quality determination and packaging, indoor air quality monitoring and auto-ventilation, as well as medical diagnostics in intensive care units (ICUs) [4–6]. Notably, the presented optical sensor displays selective and sensitive detection within seconds toward CO2 with a detection limit of around 50 ppm. Moreover, the platform can also be used for the simultaneous detection of CH4, temperature, and humidity, applicable to practical household and industrial environments. The detection limit of CH4 is around 1750 ppm. This study provides novel perspectives for emissivity enhancement on MEMS thermal emitters, which enable IR gas sensors with high sensitivity, fast response/recovery, and low power consumption promising for mass production and commercialization.
Fig. 1. (a) Schematic representation of the NiO-coated MEMS emitter for tailorable emission and gas sensing. (b) Cross-sectional view of the MEMS emitter showing different layers. PECVD: plasma enhanced chemical vapor deposition. LPCVD: low pressure chemical vapor deposition. (c) SEM image of the bare MEMS emitter. The inset depicts the zoomed-in SEM image of the IDEs. (d, e) Top-view SEM images of the emitter after film growth. (f) Cross-sectional FIB-TEM image of the NiO-coated thermal emitter. Raman (g) and XPS (h) spectra of the NiO film grown on the MEMS emitter.
2. Experimental
2.1 Design and fabrication of MEMS emitters
MEMS emitters rely on the embedded micro-hotplates to generate electromagnetic radiation under applied voltages. Miniaturized MEMS emitters with a dimension size of 1.8 mm × 1.8 mm × 0.5 mm were fabricated using cleanroom technologies mentioned previously [3]. The MEMS emitters are fabricated on an 8-inch silicon wafer. The membrane is formed by multiple layers of thin films, from bottom to top, 1 µm-thick SiO2, 0.4 µm-thick Si3N4, 200 nm-thick Pt, 200 nm-thick Si3N4, and 200 nm-thick Pt. The upper Pt layer acts as interdigitated electrodes (IDEs) to guarantee a homogeneous distribution of heat and can potentially act as capacitive sensors [8].
2.2 Growth of NiO films on emitters
To prepare NiO films on emitters, nickel nitrate hexahydrate (2.7 × 10−3 M) and thiourea (2.14 × 10−2 M) were firstly dissolved in deionized water. Homogeneous solutions were obtained under stirring for 10 minutes. The MEMS emitters were rinsed twice by absolute ethanol with each cycle for 10 minutes, blown dry afterward under compressed dry air flow and eventually placed face-down in Teflon liners containing freshly prepared precursor solutions to avoid precipitation. In-situ growth of nickel hydroxide (Ni(OH)2) films on emitters was achieved at 100 °C for 10 hours in the above aqueous solutions. The as-synthesized films after thorough rinsing by deionized water were afterward heated at 140 °C overnight to induce the transformation of Ni(OH)2 to NiO.
2.3 Real-time radiation measurements
The schematic of the setup that monitors real-time radiations of emitters is provided in Supplement 1 , Fig. S3. All the measurements were conducted in air. The emitter was packaged on transistor outline (TO, model: TO-46) header with wire bonding, and placed within a compound parabolic concentrator (CPC) to supply collimated light. A signal generator (Tektronix AFG3102) was used to provide a driving voltage with a square wave of 10 Hz and 0.5 duty cycle. A unity signal gain amplifier was employed before the emitter to endow continuous delivery of voltage. In the meanwhile, a photodetector (Thorlabs PDA20H-EC) with limited detection wavelengths ranging from 1.5 to 4.8 µm was employed to record the radiation as CH4 and CO2 molecules show their absorption fingerprints within this range (peaks localized respectively at 3.5 µm and 4.2 µm) [20]. Three emitters have been fabricated and measured for real-time radiation. The best performer was selected for the following characterizations in sections 2.4 and 2.5.
2.4 Emission spectra acquisition and optical power measurements
The emitter mounted within the CPC acts as the IR source in air for the emission spectra acquisition using FTIR, with photo image of the setup shown in Supplement 1 , Fig. S7(a). In addition, the voltage with a continuous wave instead of a square wave was maintained during the entire measurement to guarantee a stable radiation. The radiant spectra profiles were recorded using an FTIR spectrometer (Bruker Vertex 70 series) with a calibrated HgCdTe detector, the resolution of which is 4 cm−1. The optical power was measured via a MIR power meter (Ophir, model: 3A-fs P/n 7z02628) in the wavelength range of 0.19-20 µm. The photo image of the optical power measurement is illustrated in Supplement 1, Fig. S7(b).
2.5 Optical gas sensing
The schematic of optical gas sensing setup is provided in Supplement 1, Fig. S8. The MEMS thermal emitters developed by us, commercial photodetectors (HAMAMATSU InAsSb photovoltaic detectors P13243 series), commercial temperature and humidity sensors (Bosch BME280) were assembled within a metallic gas chamber to form a sensor array. They were controlled and actuated by an electronic circuit board mounted on top of the metallic gas chamber, as shown in Supplement 1, Fig. S8. Optical filters were mounted on photodetectors to avoid interference of other molecules and enable the selectivity of the NDIR gas sensor. The metallic gas chamber has a length of 8 cm and a diameter of 1 cm. It has one gas input port and one gas output port for gas flow. The input port was connected to gas cylinder through mass flow controllers to modulate the flow speed as well as gas concentration. The output port was connected to a gas scrubber for exhausting the outflow gas. The output of the photodetectors was processed by a microcontroller that communicates with the computer for data recording.
For gas sensing measurements, we first purged the gas chamber with solvent gas (e.g. N2) for about 20 mins until reaching the low humidity level (<5%). Then we use mass flow controllers to mix gases with targeted concentration. For each concentration of gas, we kept the mixed gas flowing for 2 mins, purged the chamber using solvent gas for another 2 mins, and then entered into next cycle with different concentration.
3. Results and discussion
The schematic in Fig. 1(b) illustrates the cross-sectional view of the fabricated emitter. Figure 1(c) presents the top view scanning electron microscopy (SEM) image of the bare emitter. Zoomed-in SEM image in Fig. 1(c) depicts the IDEs with an electrode width of 20 µm and a spacing of 10 µm. The fabrication process of the MEMS thermal emitters is provided in Supplement 1, Note 2. More fabrication details can be found in [3]. Figures 1(d) and 1(e) present the top view SEM images of the MEMS emitter with NiO film. A layer composed of interpenetrated nanosheets and protruding ditch-like islands aggregated by nanoflakes totally covers the surface. The presence of protruding ditch-like islands are due to the existence of height difference between the area with and without IDE layer underneath. Focus ion beam coupled transmission electron microscopy (FIB-TEM) image [Fig. 1(f)] indicates that the film with a thickness of around 3.4 µm is well bonded to the underneath substrate. This would undoubtedly avoid delamination of the NiO films under heating, and endow a long lifetime of such emitters. Raman spectrum of the film as presented in Fig. 1(g) clearly shows a peak localized at 530 cm−1, which represents the typical stretching vibration of NiO lattice [30]. X-ray photoelectron spectroscopy (XPS) spectra of the layer [Fig. 1(h) and Supplement 1, Figs. S1-S2] confirm the photosplitting of Ni2+, i.e., Ni 2p3/2, Ni 2p1/2, as well as the existence of O2- anions related with the NiO lattice [30]. Moreover, the good symmetric shape of Ni 2p peaks rules out the presence of metal Ni, consistent with our foregoing inference.
Real-time radiation of emitters without and with NiO film under variable voltages in air was monitored by employing a square-wave modulated stimulator (U = 1.0 V, f = 10 Hz, duty cycle = 0.5). The details of characterization methods are provided in Supplement 1, Note 3. Figure 2(a) compares the dynamic radiation intensity ranging from 1.5 to 4.8 µm of the bare emitter with that of the NiO-coated emitter under 1 V. Comparisons under U = 0.8, 1.2 and 1.4 V are available in Supplement 1, Fig. S4. The rise times (from 10% to 90%) of the emitters without and with NiO film were measured to be 9.1 ms and >23.8 ms, respectively; whereas their corresponding fall times were 2.7 ms and 8.4 ms. Please note the rise time of emitter with NiO coating should be larger than 23.8 ms, considering that the emission has not reached the saturation yet under 10 Hz modulation speed. The discrepancy in response time between the emitter with and without NiO coating is probably caused by their disparate thermal losses and thermal mass. The radiant power of the NiO-coated emitter triples that of the bare emitter under 1 V within the measured wavelength range. In the meanwhile, their resistance values are quite comparable (Supplement 1, Fig. S5), ruling out the possibility that the observed radiation enhancement is induced by the variation of their resistances. In addition, two additional NiO-coated emitters have been made and tested. Their real-time radiation intensity plots under 1 V are provided in Supplement 1, Fig. S6.
Fig. 2. (a) Dynamic radiation intensity in the wavelength range of 1.5-4.8 µm of the MEMS emitters without and with NiO coating under a modulated driving voltage (U = 1.0 V, f = 10 Hz, duty cycle = 0.5). It is worth mentioning that the measured intensity in such a wavelength range is limited by the instrument. (b) Total optical power of the emitters without and with NiO coating (from 0.19 to 20 µm) as a function of driving voltage. (c) Surface reflectance and emissivity spectra of the emitters without and with NiO coating. (d) Radiation spectra of the above emitters under 1.0 V and 1.4 V. Red-dotted lines indicate the emerging emission wavelength range due to NiO coating. Difference in wavelength range between (c) and (d) is caused by the different photodetectors in the infrared microscope and FTIR.
The radiant power of the above emitters under different bias voltages exhibiting a square-wave modulated signal with a frequency of 10 Hz and a duty cycle of 50% was measured in air via a thermal detector coupled with a power meter. Their radiant power follows a parabolic increase as the applied voltage increases, and the radiant power of the NiO-coated emitter surpasses the bare emitter under identical voltages [Fig. 2(b)]. The NiO-coated emitter exhibits a radiant power of 0.92 mW under 1.4 V, compared with a value of 0.24 mW of the bare emitter.
To justify the observed radiation enhancement of the NiO-coated emitter, we conducted the relative surface reflectance measurements of the relevant emitters in air by an infrared microscope (Shimadzu AIM-9000) using aluminum (Al) mirror as reference. Considering the Al mirror has reflectance close to 1 in MIR wavelength range, the reflectance of the bare emitter varies from 71% to 98%, whereas the reflectance of the NiO-coated emitter fluctuates from 1% to 38% [Fig. 2(c)]. Given the fact that both emitters are non-transparent in MIR wavelength range and the light illuminated on them is either absorbed or reflected, the absorption spectra can thus be obtained by subtracting the reflectance from 1. In addition, the surface emissivity equals to the absorption at the thermal equilibrium state based on the Kirchoff’s law [32,33]. Therefore, the emissivity spectra can be estimated as shown in Fig. 2(c). A higher emissivity of the NiO-coated emitter contributes to its enhanced radiation in comparison with that of the bare emitter. Such an increase in emissivity probably stems from the distinctive structure of NiO and its rough film morphology as observed in Figs. 1(d) and 1(e). The increased emissivity and good thermal stability of NiO enable them to be suitable coating materials for MEMS thermal emitters.
The emission spectra of the emitters without and with NiO film under different voltages were also obtained using a Fourier-transform infrared spectroscopic (FTIR) system (Bruker VERTEX 70 series) with the IR light source replaced by the presented emitters placed in air. As depicted in Fig. 2(d), their spectra with peaks centered at around 10 µm are all localized within the MIR regime, and the radiation intensities all increase as the driving voltage increases. Additionally, the radiation intensity of the NiO-coated emitter surpasses that of the bare one under the same voltage. More intriguingly, new wavelengths residing between 15 and 19 µm emerge for the NiO-coated emitter. Such a wavelength range, which corresponds to 660-530 cm−1, matches well with the relatively low reflectivity (or high emissivity) band of NiO thin film as reported previously [31]. It is correlated with the new radiative modes introduced by the vibration in NiO lattice [31]. The emerging wavelength of 15-19 µm contains fingerprint of organic chemicals such as alkene, alkynes and alkyl halides [34], and hence can be used for sensing of these chemicals. The radiation enhancement of the NiO-coated emitter can be elucidated from the following aspect. The additional coating of NiO on the emitter increases the surface emissivity by altering the chemical composition and surface roughness of the underneath substrate, which can be corroborated by the spectrum in Fig. 2(c).
Comparing the emissivity in Fig. 2(c), the NiO-coated emitter exhibits higher emissivity than the bare one by a factor of 6 to 7, depending on different wavelength range. In the meanwhile, comparing the optical power in Fig. 2(b), the NiO-coated emitter has higher emission power than the bare one by a factor of 4. Considering that the emission power is determined by the surface emissivity and temperature, the results in Figs. 2(b) and 2(c) indicate that the surface temperature of NiO-coated emitter should be lower than the bare emitter. Such lower surface temperature of NiO-coated emitter can be explained by the increased loss, in terms of electromagnetic radiation loss and heat transfer loss to surrounding air. The increased electromagnetic radiation loss is contributed by the increased surface emissivity. Besides, the increased heat transfer loss to air is contributed by the increased surface area of NiO film compared with the bare emitter. Hence, it is nature to induce that the NiO-coated emitter has lower surface temperature compared with bare emitter, which is in consistent with the measurement results obtained in Fig. 2. Furthermore, the surface temperatures of both NiO-coated emitter and bare emitter have been estimated by using an IR camera. Under different emissivity settings based on Fig. 2(c), the surface temperatures range from 300 °C to 420 °C with 1.4 V bias voltage.
Fig. 3. (a) Schematic of the sensor array composed of CO2, CH4, humidity, and temperature sensors. (b) Real-time response of the sensor equipped with the NiO-coated emitter toward different concentrations of CO2. (c) Response of the optical sensor versus CO2 concentration. The continual solid curve stands for the exponential fitting. C1 to C5 denote the concentrations of the diluent. The C1-C5 concentrations for acetone are 240, 432, 624, 816 and 1008 ppm, respectively. The C1-C5 concentrations for CO are 160, 288, 416, 544 and 672 ppm, respectively. The C1-C5 concentrations for CH4, are 200, 360, 520, 680 and 840 ppm, respectively. (d) Stability of the optical sensor equipped with the NiO-coated emitter toward 2500 ppm CO2. (e) Zoomed-in real-time response plot of the optical sensor equipped with the NiO-coated emitter toward CO2. (f) Real-time monitoring of temperature and humidity within the gas sensing chamber. (g) Real-time response of the optical sensor toward CH4 concentration. (h) Sensing property comparison of the presented CO2 sensor with the recently investigated sensors [3–15] (reference order is matched with the order on x-axis). The active materials are listed in x-axis, and the sensor type is indicated by different colors. OPT: optical; CAP: capacitive; PZT: piezoelectric; MOS: metal-oxide semiconductor; PAVB: poly(1-allyl-3-vinylimidazolium bromide); NA: (active material) not applicable; F-MOF: fluorinated metal-organic framework; PEI-GO: polyethyleneimine-graphene oxide. The data points on the red curve stand for the operating temperature of listed sensors (from right y-axis, red connecting line is used to guide the eye), and the blue vertical bars represent the concentration range tested in those studies (from left y-axis).
In order to demonstrate the reliability and practicability of the aforementioned MEMS emitters, we utilized the as-fabricated emitters as the light sources for photonic gas detection. A sensor array was also assembled for simultaneous monitoring of multiple parameters including CO2 and CH4 concentrations, temperature, and humidity [Fig. 3(a)]. The commercial photodetector, temperature sensor and humidity sensor were integrated onto the same sensor board. Their brand and model numbers are provided in earlier section 2.5 optical gas sensing part. Figure 3(b) depicts the dynamic response of the sensor coupled with the NiO-coated emitter upon CO2 ranging from 50 to 2500 ppm. Clearly, the sensor with NiO-coated emitter exhibits a sensitive response [defined as (Vo-Vanalyte)/Vo, whereas Vo and Vanalyte denote the voltages of photodetector under pure N2 and the analyte, respectively] toward variable concentrations of CO2, with respective responses of 0.68% and 13% upon 50 and 2500 ppm. The zoomed-in illustration [Fig. 3(e)] presents its dynamic response toward CO2 ranging from 2000 to 2400 ppm. Apparently, the response and recovery times of the gas sensor remain within few seconds, promising for practical sensing scenarios. The response with respect to CO2 concentration is plotted in Fig. 3(c). The fitting result with an R-square value of 0.998 indicates that the response with respect to CO2 follows an exponential relationship, consistent with the Beer-Lambert law [20]. The limit of detection (LOD) can be estimated to be around 50 ppm by multiplying the derivation between the experimental and the fitted data by a factor of 3 and then mapping the concentration on the fitted curve [35]. Such LOD can be further reduced with the optimization of the sensing system (e.g. gas chamber length). Here we just demonstrate the application of fabricated emitter. Since the selectivity is a key performance factor for commercial gas sensors in practical applications, the selectivity of the presented CO2 sensor, which is contributed by the spectrum filter on its photodetector, is evaluated by inspecting the interference of other analytes toward CO2 sensing. The co-existence of variable amounts of gases including acetone or CH4 does not have any influence toward the detection of CO2 under variable concentrations [Fig. 3(c)]. The good selectivity makes the sensors promising for practical applications. Moreover, the sensitivity of the IR CO2 sensors toward 2500 ppm CO2 exhibits a minor variation within 11 weeks [Fig. 3(d)], suggesting their good stability and repeatability. The minor variation is correlated with the environmental change, e.g., variations of ambient temperature and/or humidity.
Simultaneously, humidity and temperature within the gas chamber can be monitored and recorded [Fig. 3(f)]. The slight variation of humidity is induced by the purging of the CO2 gas. The presence of moisture has minor impact on the CO2 sensing since the water molecules show nearly no absorption at the wavelength of 4.2 µm, which is the center wavelength of the optical bandpass filter for CO2 detection. In addition, the real-time response toward variable concentrations of CH4 is plotted in Fig. 3(g). The platform shows an acceptable response toward CH4 in the range of 5000 to 25000 ppm with a LOD of 1750 ppm. Furthermore, the CH4 sensor exhibits the advantages of fast response/recovery and room temperature operation.
The performance of our CO2 sensor equipped with the NiO-coated emitter is benchmarked with the recently demonstrated CO2 sensors in terms of the detected concentration range and operating temperature [Fig. 3(h)] [3–15]. The presented CO2 sensor exhibits high sensitivity especially compared with the widely investigated sensors based on metal oxide semiconductors (MOSs).
4. Conclusion
In conclusion, the tailorability of radiant power and the emergence of new emission wavelength of MEMS-based thermal emitters coated with NiO films are demonstrated. The NiO coating on the emitter enables high surface emissivity, accompanied by the emerging of new wavelengths between 15 and 19 µm. The appearance of new waveband stems from the new radiative modes introduced by the vibration in NiO lattice. To demonstrate its reliability and practicability, the NiO-coated thermal emitter was integrated into a sensor array for simultaneous monitoring the multiple parameters including CO2 and CH4 concentrations, temperature, and humidity. The platform exhibits selective and sensitive detection at room temperature toward CO2 and CH4 with detection limits of around 50 ppm and 1750 ppm, respectively, and also shows fast response/recovery (within seconds) with good recyclability. The CO2 sensor outperforms most of the CO2 sensors. In addition, the presented CH4 sensor shows the advantages of fast response/recovery and room temperature operation. The demonstrated emission tailorability of NiO-coated MEMS emitters and their usage in simultaneous detection of multiple environmental factors provide novel insights for making optical sensors with miniaturized size and high performance promising for mass production and commercialization.
Funding
Agency for Science, Technology and Research (IAF-PP A1789a0024); National Research Foundation Singapore (NRF-CRP18-2017-02).
Acknowledgments
The authors thank Cheam Daw Don, Zhonghua Gu and Michelle Bi-Rong Chew for their kindly assistance on MEMS emitter fabrication and SEM characterization. We acknowledge the assistance from Yu Xi Cui, Dr. Hui Liu, Dr. Jie Fang and Dr. Fuu Ming Kai for the gas sensing tests. We are also indebted to Dr. Yuandong Gu and Dr. Stephanie Yang for the initiation of this project.
Disclosures
The authors declare no conflicts of interest.
Supplemental document
See Supplement 1 for supporting content.
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