Date Published: February 21, 2018
Publisher: John Wiley and Sons Inc.
Author(s): Dihan Hasan, Chengkuo Lee.
Application of two major classes of CO2 gas sensors, i.e., electrochemical and nondispersive infrared is predominantly impeded by the poor selectivity and large optical interaction length, respectively. Here, a novel “hybrid metamaterial” absorber platform is presented by integrating the state‐of‐the‐art complementary metal–oxide–semiconductor compatible metamaterial with a smart, gas‐selective‐trapping polymer for highly selective and miniaturized optical sensing of CO2 gas in the 5–8 µm mid‐IR spectral window. The sensor offers a minimum of 40 ppm detection limit at ambient temperature on a small footprint (20 µm by 20 µm), fast response time (≈2 min), and low hysteresis. As a proof‐of‐concept, net absorption enhancement of 0.0282%/ppm and wavelength shift of 0.5319 nm ppm−1 are reported. Furthermore, the gas‐ selective smart polymer is found to enable dual‐mode multiplexed sensing for crosschecking and validation of gas concentration on a single platform. Additionally, unique sensing characteristics as determined by the operating wavelength and bandwidth are demonstrated. Also, large differential response of the metamaterial absorber platform for all‐optical monitoring is explored. The results will pave the way for a physical understanding of metamaterial‐based sensing when integrated with the mid‐IR detector for readout and extending the mid‐IR functionalities of selective polymers for the detection of technologically relevant gases.
Increasing rate of environmental pollution is causing monumental and irreversible damage to the earth as the rapid urbanization and industrialization take place across the globe. Air pollution is currently regarded as the most harmful form of environmental pollution which is caused by a myriad of natural and manmade agents. Among them, emission of greenhouse gases (e.g., CO2) causing global warming has been a critical environmental concern over the last decade.1, 2 Apart from emission control, precision sensing of CO2 gas is indispensable for indoor air quality (IAQ) monitoring and heating, ventilation and air conditioning systems, industrial storage and refrigeration, and medical applications such as capnography. Many efforts have been invested for the development of application‐specific CO2 sensors. Two major approaches include (i) electrochemical sensing using gas‐selective ceramics3 and (ii) nondispersive infrared (NDIR) sensing using optical elements, i.e., microheaters, filters, and infrared sensors.4 Although, electrochemical sensors are widely used in market and could be ideally further downsizing for portable electronic applications, such sensors strongly suffer from poor selectivity and large power consumption and hysteresis. Current NDIR platform promises lower hysteresis, but selectivity is still limited in a mixed sensing environment where various pollutant gases including water vapor coexist interfering at the same operating wavelength. More importantly, NDIR sensors require centimeter long optical interaction length with low roughness sidewall in order to achieve ppm level detection limit with high signal‐to‐noise ratio (S/N ratio). Therefore, present NDIR devices are bulky and limited for personalized applications. Recently, 10 000 ppm detection limit is achieved in an NDIR setup with a ≈7.5 mm long optical interaction length, where it also reports characterization of metamaterial emitter.5 While metamaterial emitters for producing radiation of wavelength with high‐quality factor have been investigated by a few groups,6, 7 enhanced absorption characteristics in mid‐IR spectrum by using metamaterials absorber for gas detection with high selectivity has not been reported. In contrast with metamaterial emitter, metamaterial absorber enables the conversion of light into heat for read out by the integrated system. With the aid of metamaterials absorber, the gas sensing system can achieve high wavelength selectivity, polarization dependence, and controlled light‐matter interaction.8, 9, 10, 11, 12, 13, 14 However, due to the small size of CO2 molecule (232 pm), the near field coupling between the metamaterial absorber and gas is still limited. Besides, the all kind of surrounding gas molecules including water vapor will again contribute signature peaks and wavelength shift in this sensing platform when we apply it in practical. It means the selective detection of particular gas is not achievable in the metamaterial absorber‐based NDIR sensing platform. By incorporating a gas‐selective‐trapping polymer into the existing NDIR sensing platform, we can overcome this grand challenge of gas sensing.
Figure1 represents the mid‐IR metamaterial platform for CO2 sensing application. The emerging issue of greenhouse CO2 emission is illustrated in Figure 1a for which massively deployable, CMOS compatible sensors are necessary. Conceptual integration of the hybrid metamaterial platform for CO2 sensing with the state‐of‐the‐art NDIR system (Figure 1b) is presented in Figure 1c.20 Figure 1c shows the post‐CMOS integration of gas‐selective, enrichment layer (polyethylenimine, PEI). The representative metamaterial pattern in this work is shown in Figure 1d. AFM (atomic force microscopy) image in Figure 1e and roughness plot in Figure 1f strongly suggest the uniformity of the spin‐coated PEI film on metamaterial patterns. The metamaterial absorber is consist of metal‐dielectric spacer‐metal layers. The dimensional parameters (length: l and width: w) of the metal patterns at the top layer are shown in Figure 1g. In this work, the width of the pattern is kept fixed at ≈287 nm being limited by the critical dimension of the lithography process. The thicknesses of the top metal layer, dielectric spacer, and bottom metal layer are fixed at 100, 200, and 200 nm, respectively. The resonant E‐field distribution at the device plane (XY plane) and H‐field distribution at the device cross‐section are given in Figure 1h,i, respectively, in which a plane wave light source is incident on the 3D structure from the top. The polarization of the incoming radiation is fixed along x‐axis while the probe wavelength is set to be 6.5 µm.
In summary, we demonstrate a novel solution for miniaturized gas sensor with ppm range of detection limit and high selectivity by coupling a smart, gas‐selective material with the metamaterial absorber platform. The hybrid metamaterial absorber‐based sensor further offers fast response time, large differential response for all‐optical monitoring, and low hysteresis and holds promise for direct integration with the existing NDIR system. As an added advantage, a dual mode sensing mechanism is demonstrated harnessing the full potential of the gas‐selective material. In particular, the infrared active functional groups of the gas‐selective polymer allow multiple unique sensing characteristics at low and high gas concentration as determined by the judicial selection of operating wavelength and spectral bandwidth. Saturation behavior of the thin film‐based sensor in continuous mode is expected to be furhter improved by (i) increasing the effective sensing area, (ii) engineering the gas‐selective layer for higher molecular weight, and (iii) strengthening light‐matter interaction by metamaterial absorber structure with nanogap supported field enhancement. The developed principle is expected to be extended further for the detection and sensing of various other greenhouse gases utilizing appropriate gas‐selective polymers. Finally, CMOS compatibility of the core absorber layer will be a great advantage for low‐cost implementation of the hybrid scheme in large‐scale gas sensing under the framework of the internet of things.
The authors declare no conflict of interest.