Tunable Infrared Metamaterial Emitter for Gas Sensing Application - MDPI
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nanomaterials Article Tunable Infrared Metamaterial Emitter for Gas Sensing Application Ruijia Xu and Yu-Sheng Lin * State Key Laboratory of Optoelectronic Materials and Technologies, School of Electronics and Information Technology, Sun Yat-Sen University, Guangzhou 510275, China; xurj6@mail2.sysu.edu.cn * Correspondence: linyoush@mail.sysu.edu.cn Received: 17 June 2020; Accepted: 22 July 2020; Published: 24 July 2020 Abstract: We present an on-chip tunable infrared (IR) metamaterial emitter for gas sensing applications. The proposed emitter exhibits high electrical-thermal-optical efficiency, which can be realized by the integration of microelectromechanical system (MEMS) microheaters and IR metamaterials. According to the blackbody radiation law, high-efficiency IR radiation can be generated by driving a Direct Current (DC) bias voltage on a microheater. The MEMS microheater has a Peano-shaped microstructure, which exhibits great heating uniformity and high energy conversion efficiency. The implantation of a top metamaterial layer can narrow the bandwidth of the radiation spectrum from the microheater to perform wavelength-selective and narrow-band IR emission. A linear relationship between emission wavelengths and deformation ratios provides an effective approach to meet the requirement at different IR wavelengths by tailoring the suitable metamaterial pattern. The maximum radiated power of the proposed IR emitter is 85.0 µW. Furthermore, a tunable emission is achieved at a wavelength around 2.44 µm with a full-width at half-maximum of 0.38 µm, which is suitable for high-sensitivity gas sensing applications. This work provides a strategy for electro-thermal-optical devices to be used as sensors, emitters, and switches in the IR wavelength range. Keywords: metamaterial; MEMS; IR emitter; IR sensor; mathematical fractal theory 1. Introduction Gas sensors play a vital role in industry and also have civil applications, including as semiconductor sensors [1–3], catalysis sensors [4], synergistic surface reaction sensors [5], and so on. Along with the development of the Internet of Things (IOT), gas sensors with high sensitivity and selectivity have been desired for monitoring air quality in real time. Recent advances in optical gas sensors provide a path for both miniaturized and high-response speed [6–8]. Many studies have reported optical gas sensors using infrared (IR) absorption [9], ultraviolet (UV) absorption [10], and light scattering [11]. For gas sensing applications, IR emitters exhibit electrical-thermal characteristics by using a microelectromechanical system (MEMS) microheater, owing to its low thermal mass [12]. It utilizes conventional blackbody thermal emission in the IR wavelength range, which provides a broad-band IR emission spectrum. The thermal radiation of the microheater can be driven in a short time based on Joule heating. However, a drawback is the low thermal emission efficiency of the microheater owing to the non-uniform temperature distribution. Furthermore, the optical performance of such emitters is strongly limited by the specific operating wavelength, and it cannot be used in practical gas sensing applications. To overcome the above-mentioned drawbacks, metamaterial is an alternative candidate for selection of a certain emission wavelength. Metamaterial has drawn a great deal of attention due to its unique optical properties, which enable it to control electromagnetic waves on subwavelength scales. The permittivity and permeability of metamaterials can be tailored by properly engineering the Nanomaterials 2020, 10, 1442; doi:10.3390/nano10081442 www.mdpi.com/journal/nanomaterials
Nanomaterials 2020, 10, 1442 2 of 11 geometrical dimensions and material compositions of their subwavelength periodic patterns. They are widely studied to realize thermal emitters and are perfect absorbers for energy harvesting, medical imaging, and high-sensitivity sensing applications [13–30]. By tailoring the geometrical dimensions, metamaterials can be designed to span broad operating wavelengths, including visible [31–33], IR [34–39], terahertz [40–44], and microwave light [45,46]. To provide metamaterials with more flexibility, there are many techniques proposed for tuning mechanisms using MEMS technology [47–54]: liquid crystal [55], photo-excited [56], phase-change materials [57,58], thermal annealing [59], and so on. One important tuning method for metamaterial emitters is the integration of microheaters and metamaterials owing to their high efficiency. Radiation intensity can be tuned through the quantity of heating energy generated from the microheater. In this study, we propose an on-chip tunable metamaterial IR emitter for gas sensing applications. The IR emitter is composed of a MEMS microheater and IR metamaterial. For optimization of the microheater, two structural patterns of microheaters are discussed to realize consistent high performance and heating energy conversion. The microheater is then covered with a dielectric layer to ensure electrical insulation of the device. Since the electromagnetic waves generated by the microheater have multiple polarization directions, the top metamaterial layer is tailored to be polarization independent, which can ensure uniformity of the radiation power. By properly tailoring the metamaterial patterns, it can be tuned to different wavelengths in the IR spectrum. This provides a strategy to detect various kinds of gases due to the tunable IR emitter, which can be selected at the specific absorption spectrum from the analyte. Integration of the microheater and the metamaterial achieves a tunable narrow-band emitter, which is suitable for high-sensitivity gas sensing applications. 2. Designs and Methods Figure 1a shows the schematic drawing of the proposed on-chip tunable IR emitter. The proposed IR emitter was composed of a MEMS Peano-shaped microheater covered with a SiO2 dielectric layer on a Si substrate and a top metamaterial layer. Such MEMS Peano-shaped microheaters and periodic metamaterials were fabricated with gold (Au) materials at a thickness of 200-nm. The Peano-shaped microheater was covered with a SiO2 dielectric layer, and the metamaterial was fabricated on top to maintain electrical insulation and high energy conversion efficiency. The whole emission area of the device was 120 × 120 µm2 . According to the conventional Joule effect, heat energy will be generated from the microheater by driving a DC bias voltage on the device. The Peano-shaped microheater was designed with a concentrated heater pattern and highly conductive metal lines, which collected all flowing electric energy and then converted it into thermal energy. The microheater achieved high electrothermal conversion efficiency and had a rapidly increasing surface temperature. Such thermal radiation power produces a broadband electromagnetic spectrum. By integrating the top metamaterial, a radiated IR light was wavelength-selected and ultimately emitted, as illustrated in Figure 1a. For the metamaterials in the proposed design, the effective permittivity and the effective permeability are expressed by the following equations [60]: εe f f = ne f f /ze f f (1) µe f f = ne f f ze f f (2) where neff is the metamaterial effective refraction index, and zeff is the metamaterial effective impedance index. The effective refraction index and the effective impedance can be calculated as follows: 1 1 − r2 + t2 ne f f = cos−1 ( ) (3) kd 2t v t (1 + r)2 − t2 ze f f = (4) ( 1 − r ) 2 − t2
Nanomaterials 2020, 10, x FOR PEER REVIEW 3 of 11 Nanomaterials 2020, 10, 1442 (1 + ) − 3 of 11 = (4) (1 − ) − where r is where isthe thereflection reflectioncoefficient, coefficient,t ist the transmission is the transmission coefficient, d is the coefficient, d ismetamaterial thickness, the metamaterial and k thickness, is thek incident and wavevector. is the incident The corresponding wavevector. parameters The corresponding of the metamaterial parameters are shown in of the metamaterial areFigure shown1b–e. in While the Figure effective 1b–e. While refraction index the effective was indeed refraction indexpositive in the resonant was indeed positive range, in the the proposed resonant design range, the exhibited high proposed design transmission exhibited in thistransmission high range to realize in athis frequency-selective range to realizeapplication. The optical a frequency-selective properties ofThe application. the designed metamaterials optical properties of thewere studiedmetamaterials designed through finite difference time through were studied domain-based finite (FDTD) simulations. difference In the numerical time domain-based (FDTD)simulations, simulations.the Inincident electromagnetic the numerical wavethe simulations, propagated incident along the z-axis, which electromagnetic wave was perpendicular propagated alongtothe x–y plane. thez-axis, Periodic which boundary conditions was perpendicular to thewere x–y defined plane. in the x- and Periodic y-axis directions, boundary conditions and were perfectly definedmatched in the x-layer and (PML) y-axis boundary directions,conditions werematched and perfectly adopted layer z-axis boundary in the(PML) direction. conditions were adopted in the z-axis direction. Figure 1. Figure (a) Schematic 1. (a) Schematic drawing drawing of of tunable tunable IR IR (infrared) (infrared) emitter. emitter. The The calculated calculated (b) (b) effective effective permittivity, (c) permittivity, (c) effective effective permeability, permeability, (d) (d) effective effective refraction refraction index, index, and and (e) (e) effective effective impedance impedance index of index of the the metamaterial. metamaterial. 3. Results and Discussion 3. Results and Discussion Figure 2a,b shows the design of a traditional winding microheater and the proposed Figure 2a,b shows the design of a traditional winding microheater and the proposed Peano- Peano-shaped microheater to enable comparison with the electrothermal characteristics. The pattern shaped microheater to enable comparison with the electrothermal characteristics. The pattern of the of the Peano-shaped microheater is a famous space-filling curve in mathematical fractal theory. Peano-shaped microheater is a famous space-filling curve in mathematical fractal theory. For For microheater design, heating uniformity is a key factor to evaluate heating performance. High microheater design, heating uniformity is a key factor to evaluate heating performance. High heating heating uniformity provides high reliability of the device and prolongs its lifetime. It also results in less uniformity provides high reliability of the device and prolongs its lifetime. It also results in less thermal dissipation in real-world applications, which can greatly reduce energy consumption. Herein, thermal dissipation in real-world applications, which can greatly reduce energy consumption. Herein, both microheater designs exhibited great heating uniformity, as shown in the surface temperature both microheater designs exhibited great heating uniformity, as shown in the surface temperature distributions in Figure 2c,d. By driving a DC bias voltage, most of the heating power was concentrated distributions in Figure 2c,d. By driving a DC bias voltage, most of the heating power was in the central area of the device. The effective heating area was greater than 75% (120 × 120 µm2 ), concentrated in the central area of the device. The effective heating area was greater than 75% (120 × keeping a 90% maximum surface temperature, which makes such microheaters suitable for use as 120 µm2), keeping a 90% maximum surface temperature, which makes such microheaters suitable for on-chip IR light-emitting sources with high efficiency. use as on-chip IR light-emitting sources with high efficiency.
Nanomaterials 2020, 10, 1442 4 of 11 Nanomaterials 2020, 10, x FOR PEER REVIEW 4 of 11 Figure 2. Schematic Schematic drawings drawings of (a) winding microheater and (b) Peano-shaped microheater. (c,d) temperature Surface temperature distributions distributions of winding of winding microheater microheater and Peano-shaped and Peano-shaped microheater,microheater, respectively. respectively. dimensions Geometrical Geometricalare dimensions = dkept kept as d1are 2 = 6.0 as µm d1 =and 1 =µm d2 = l6.0 l2 =and 120 lµm. 1 = l2 = 120 µm. Figure 3a 3a shows shows the thesurface surfacetemperature temperaturedistributions distributionsofofthe themicroheater microheater with withmetal metal linewidths linewidths of dof1 and d1 and d2 . Itd2can . It becanclearly observed be clearly that such observed microheater that designs exhibited such microheater great heating designs exhibited uniformity great heating when the metal linewidth increased. It can also generate higher maximum uniformity when the metal linewidth increased. It can also generate higher maximum temperature temperature distribution on the surface. To distribution onfurther optimize the surface. the Peano-shaped To further optimize the microheater Peano-shaped withmicroheater high electrical-thermal efficiency, with high electrical- the Peano-shaped thermal efficiency,microheater was designed the Peano-shaped to facilitate microheater wascomparison designed to of six effective facilitate pattern densities comparison of six (D , effective eff the ratio of effective heating material to the whole area of microheater) according pattern densities (Deff, the ratio of effective heating material to the whole area of microheater) to mathematical fractal according theory. The material selected to mathematical was Au, fractal theory. Theand the Deff selected material values were was16%, Au, 20%, and the32%, Deff38%, 42%, values and16%, were 49%, respectively. 20%, 32%, 38%, Figure 42%, 3bandshows the experimental 49%, respectively. Figure results for Au-based 3b shows Peano-shaped the experimental results microheaters for Au-based under Peano-shapeddifferentmicroheaters driving voltages. The inserted under different images driving showThe voltages. topinserted views of optical images microscopes show top viewsfor of six Peano-shaped optical microscopes microheaters. It can be microheaters. for six Peano-shaped observed thatItacan higher Deff can realize be observed that a higher D surface eff can temperature. realize higherElectrical-thermal surface temperature. efficiency increased along Electrical-thermal with the efficiency increments increased along Deff . the ofwith Theincrements maximum heating of Deff. The temperature maximumcan reach temperature heating 660 K under can a driving reach voltage of 5 Vafor 660 K under Deff = voltage driving 49%. The of fundamental 5 V for Deff = electrothermal 49%. The fundamental behavior is the processbehavior electrothermal of energyisconversion the processto ofthermal power. In the energy conversion ideal situation, to thermal power. without considering thermal dissipation, thermal emission can be explained In the ideal situation, without considering thermal dissipation, thermal emission can be explained by Planck’s law (also by known Planck’saslaw the (also blackbody known radiation law). According as the blackbody to the radiation blackbody law). According radiation to the law, emission blackbody wavelength radiation law, is only related emission to temperature. wavelength Emission is only related wavelength has to temperature. a broadwavelength Emission bandwidth,has andaemissivity is varied broad bandwidth, by andthe corresponding emissivity is variedwavelength. Blackbody radiation by the corresponding wavelength. can Blackbody be expressed by radiation can be expressed by π hc 88πhc 11 ρρλ dλ dλ λ == dλdλ (5) (5) λλ55 hc λ kT) − exp((hc/λkT exp ) −11 where ρρλλ isisthe where theemission emissionpower powerperper unitunit volume, λ is the volume, λ isemission wavelength, the emission h is the Planck wavelength, h is theconstant, Planck cconstant, is the velocity of light in vacuum, k is the Boltzmann constant, and T is temperature. c is the velocity of light in vacuum, k is the Boltzmann constant, and T is temperature. The heat The flux heat flowed mainly flux flowed outward mainly on the outward onupper and lower the upper surfaces. and lower Thus,Thus, surfaces. half of halftheofradiated power the radiated was power ideally generated was ideally though generated the the though upper surface upper to to surface bebe the theradiation radiationsource sourceofofthe the proposed proposed IR IR emitter. emitter. Relationship between maximum Relationship between maximum surface surface temperatures temperatures and and applying applying DC DC bias bias voltages voltages for for both both the the winding microheater and the Peano-shaped microheater are plotted in Figure 3c. Since high- efficiency thermal conduction among metals can improve radiated power, it is also worth discussing
Nanomaterials 2020, 10, 1442 5 of 11 Nanomaterials 2020, 10, x FOR PEER REVIEW 5 of 11 winding microheater and the Peano-shaped microheater are plotted in Figure 3c. Since high-efficiency the effectconduction thermal of microheaters amongmade metals from candifferent improve metals. radiatedThe thermal power, conductivities it is also of Al and worth discussing Au are the effect of 238 W/(m·K) and 317 W/(m·K), respectively. By applying a bias voltage, the microheaters made from different metals. The thermal conductivities of Al and Au are 238 W/(m·K) surface temperature increased gradually and 317 W/(m·K), from a room respectively. Bytemperature of 293 applying a bias K. The voltage, theareas surfaceof both microheater temperature patterns increased were gradually kept fromthe same temperature a room at 120 × 120 µm 2 of 293. TheK.maximum The areassurface of bothheating temperature microheater patternsof were the Al-based kept thewinding same at microheater was 2 571 K, and it was 598 K for the Al-based Peano-shaped microheater. 120 × 120 µm . The maximum surface heating temperature of the Al-based winding microheater was The maximum surface temperature 571 K, and it was 598 of a Au-based K for the Al-based Peano-shaped Peano-shaped microheater microheater.canThereach 660 K, while maximum surfacethat of a Au- temperature based windingPeano-shaped of a Au-based microheater can only reach microheater can625 K under reach 660 K, the same while thatdriving voltagewinding of a Au-based of 5 V. This shows microheater that the radiation efficiencies of both microheaters were better when Au materials were can only reach 625 K under the same driving voltage of 5 V. This shows that the radiation efficiencies used, as the Peano-shaped microheater of both microheaters generated were better whena Au more complex materials weremagnetic used, as field the to induce electromagnetic Peano-shaped microheater heating. The maximum applied DC bias was set as 5 V to avoid breakdown of generated a more complex magnetic field to induce electromagnetic heating. The maximum applied the device. Therefore, DC abias higher surface heating temperature can be achieved by using a Au-based Peano-shaped was set as 5 V to avoid breakdown of the device. Therefore, a higher surface heating temperature microheater, which can becan generate achieved highera radiated by using Au-basedpower. Au-based Peano-shaped Peano-shaped microheater, which microheaters can generate arehigher very suitable radiated for use in tunable IR emitter design. power. Au-based Peano-shaped microheaters are very suitable for use in tunable IR emitter design. Figure Figure3.3.(a) (a)Surface Surfacetemperature temperature distributions of of distributions microheaters with microheaters metal with linewidths metal of dof linewidths 1 and d2. (b) d1 and d2 . Temperature (b) Temperature as a function of driving voltage for six Peano-shaped microheater designs withdifferent as a function of driving voltage for six Peano-shaped microheater designs with different pattern density.(c)(c) pattern density. Relationships Relationships between between applied applied DC DC bias bias and voltage voltage and surface maximum maximum surface temperature temperature for winding for winding microheaters microheaters and Peano-shaped and Peano-shaped microheaters.microheaters. CorrespondingCorresponding materials are materials selected asare Al selected and Au.as Al and Au. The The relationships relationships between between thethe applied applied DC DC bias bias voltages voltages and and effective effective radiated radiated powers powers were were calculated calculatedand andplotted plottedin in Figure Figure 4.4. The Thetotal totalradiated radiatedpower powerwas wasbelow below20 20µW µW when when applying applying DC DC bias biasvoltage voltagefrom from00toto33V. V. By By increasing increasing the the DC DC bias bias voltage voltagetoto44V, V, the the maximum maximum radiated radiated power power was 48.6 µW. When the driving DC bias voltage was 5 V, the corresponding radiated power increased to 111.1 µW, and the emitted wavelength blue-shifted to the wavelength of 5.25 µm. The inset in
Nanomaterials 2020, 10, 1442 6 of 11 Nanomaterials was 48.6 µW. 2020, 10, x FOR When PEER REVIEW the driving DC bias 6 of 11 voltage was 5 V, the corresponding radiated power increased to 111.1 µW, and the emitted wavelength blue-shifted to the wavelength of 5.25 µm. The inset in Figure 4 shows the thermal radiation image measured by a thermal radiometer (Sentris thermal Figure 4 shows the thermal radiation image measured by a thermal radiometer (Sentris thermal imaging microscope, OPTOTHERM Co. Ltd., Sewickley, PA, USA). It can be clearly observed that imaging microscope, OPTOTHERM Co. Ltd., Sewickley, PA, USA). It can be clearly observed the temperature distribution of the Peano microheater was uniform. Moreover, the full-width at half- that the temperature distribution of the Peano microheater was uniform. Moreover, the full-width maximum (FWHM) of the radiated power spectrum was 5.4 µm. It is a broadband and at half-maximum (FWHM) of the radiated power spectrum was 5.4 µm. It is a broadband and omnidirectional IR radiation spectrum. omnidirectional IR radiation spectrum. Figure 4. Figure Blackbodyradiated 4. Blackbody radiatedpower powerof ofaa Peano-shaped Peano-shapedmicroheater microheaterunder underdifferent different applied applied DC DC bias bias voltage. Inset voltage. Inset is is IR IR thermal thermal radiation radiation measured measured with with aa thermal thermal radiometer. radiometer. To narrow To narrow down down the the FWHM FWHM value value ofof the the IR IR radiation radiation spectrum, spectrum, we we proposed proposed two two metamaterial metamaterial designs of designs of Peano-shaped Peano-shapedmicroheater microheatersurfaces surfacesencapsulated encapsulated bybyinsulated insulated layers on the layers on top. Figure the top. 5a,b Figure shows the proposed circle-shaped and ring-shaped metamaterials. To 5a,b shows the proposed circle-shaped and ring-shaped metamaterials. To investigate the influence investigate the influence of the geometrical of dimensions the geometrical of the metamaterial dimensions on the emitted of the metamaterial on thewavelength, the deformedthe emitted wavelength, metamaterial deformed period and outer circle diameter divided by the initial metamaterial metamaterial period and outer circle diameter divided by the initial metamaterial parameter parameter is defined as n, i.e., is the metamaterial deformation ratio. The subscripts 1 and 2 indicate circle-shaped defined as n, i.e., the metamaterial deformation ratio. The subscripts 1 and 2 indicate circle-shaped and ring-shaped metamaterials, and ring-shapedrespectively. metamaterials,Figure 5c,d shows respectively. Figurethe 5c,dtransmission spectra of circle-shaped shows the transmission spectra of circle-and ring-shaped metamaterials with shaped and ring-shaped metamaterials with different n and n 1 different 2 values, respectively. In Figure 5c, the transmission n1 and n2 values, respectively. In Figure 5c, the transmission spectrum gradually red-shifted from 2.16 µmwhen spectrum gradually red-shifted from 2.16 µm to 4.29 µm n1µm to 4.29 increased when nfrom 1 to 2. The tuning 1 increased from 1 to 2. range The was 2.13 tuning µm. range wasThe corresponding 2.13 transmissiontransmission µm. The corresponding intensity was enhanced intensity was10% (from 75% enhanced 10%to(from 85%) by increasing n from 1 to 2. These tuning transmission spectra are not perfect 75% to 85%) by1 increasing n1 from 1 to 2. These tuning transmission spectra are not perfect single- single-band resonances in theresonances band IR wavelength in therange. It can be clearly IR wavelength range. observed that there It can be clearly were several observed that therelower-order were severalresonances lower- at short IR wavelengths, which will result in interference with the IR order resonances at short IR wavelengths, which will result in interference with the IR emitter emitter and will also reduceand the radiated power. In order to solve this problem, a ring-shaped metamaterial will also reduce the radiated power. In order to solve this problem, a ring-shaped metamaterial was was designed as shown in Figure 5d.asThe designed ring-shaped shown in Figure metamaterial 5d. The exhibited ring-shaped resonance at the wavelength metamaterial of 2.61 µm at exhibited resonance withthea transmission intensity of 76% wavelength of 2.61 µm with a transmission for n2 = 1. By increasing the n value to 2, the resonance intensity of 76% for2 n2 = 1. By increasing the n2 value to 2, red-shifted the resonance red-shifted to the wavelength of 5.83intensity to the wavelength of 5.83 µm with a transmission µm with ofa74%. The tuning transmission range was intensity 3.22 The of 74%. µm. It was enhanced tuning range was 1.093.22 µm µm. compared It waswith that of the enhanced 1.09 circle-shaped µm compared metamaterial. with that Itofis theworth mentioning circle-shaped that the transmission intensity was kept stable by changing the n metamaterial. It is worth mentioning that the transmission intensity2was kept stable by changing value. The variation was onlythe 2%. The lower-order resonances were greatly reduced. The relationships n2 value. The variation was only 2%. The lower-order resonances were greatly reduced. The between resonances and deformation ratios relationships between areresonances summarized andin deformation Figure 6a,b for circle-shaped ratios and ring-shaped are summarized metamaterials, in Figure 6a,b for circle- respectively. To understand the physical mechanism of these shaped and ring-shaped metamaterials, respectively. To understand the physical mechanismtunable metamaterials, the electric (E) of these and magnetic tunable (H) field distributions metamaterials, the electric (E)ofand circle-shaped magnetic (H) andfield ring-shaped metamaterials distributions are illustrated of circle-shaped and ring- in the insets of Figure 6a,b, respectively. E- and H-field distributions were shaped metamaterials are illustrated in the insets of Figure 6a,b, respectively. E- and H-field highly concentrated around the Au patterns distributions wereandhighly interacted with incident concentrated electromagnetic around the Au patterns wavesand to provide a strong interacted withresonance. incident The relationshipswaves electromagnetic between to resonances provide a strongand deformation resonance.ratios were linear. This The relationships meansresonances between that IR emitters and deformation ratios were linear. This means that IR emitters can be designed at different IR wavelengths by modifying the metamaterial period. The correlation coefficients were 0.9999 for both
Nanomaterials 2020, 10, 1442 7 of 11 can be designed at different IR wavelengths by modifying the metamaterial period. The correlation Nanomaterials 2020, 10, x FOR PEER REVIEW 7 of 11 coefficients were 0.9999 for both circle-shaped and ring-shaped metamaterials. These results provide an effective approach circle-shaped to high-precision and ring-shaped and wavelength-selective metamaterials. These results provideemission applications an effective approachintothe IR high- wavelength range. precision and wavelength-selective emission applications in the IR wavelength range. Nanomaterials 2020, 10, x FOR PEER REVIEW 7 of 11 Figure 5. Schematic drawings of (a) circle-shaped and (b) ring-shaped metamaterials. Transmission Figure 5. Schematic drawings of (a) circle-shaped and (b) ring-shaped ring-shaped metamaterials. Transmission spectra spectra withwith different different n different n1 1 andnn 12 and n values for (c) circle-shaped and (d) ring-shaped metamaterials. 2 values for (c) circle-shaped and (d) ring-shaped metamaterials. Initial values 2for (c) circle-shaped Initial geometrical dimensions geometrical dimensions R11 ==RR2R2=1=1=1µm, were were 2 =P1 Rµm, P 11 = P2P=2P2=1µm, =µm, P2r ==r2 =2 µm, =µm, 0.2 µm. 0.2 r = 0.2 µm. µm. Figure Relationships 6. 6. Figure between Relationships resonances between and deformation resonances ratios forratios and deformation (a) circle-shaped and (b) for (a) circle-shaped and (b) ring- ring-shaped metamaterials. The inserted images show the corresponding E- and H-field distributions. shaped Figure metamaterials. 6. Relationships Theresonances between inserted images show theratios and deformation corresponding E- and H-field for (a) circle-shaped and (b) distributions. ring- According to the above results, the maximum radiated power of the Peano-shaped microheater occurred According shaped to the above metamaterials. Theresults, insertedthe maximum images show radiated the power corresponding of E- the and Peano-shaped H-field microheater distributions. occurredat at thethe wavelength wavelength of of 5.25 5.25 µmµmbyby driving driving a DCa DC biasbias voltage voltage of 5of V.5We V. We further further integrated integrated the the Peano-shaped Accordingmicroheater Peano-shaped to the abovewith results, the maximum a ring-shaped radiated power metamaterial of theatPeano-shaped operating a wavelengthmicroheater of 5.25 µm. occurredmicroheater with a ring-shaped at the wavelength metamaterial operating at a of wavelength of 5.25 µm. It was thepromising to It was promising to provide of 5.25 µm by maximum driving radiated a DC power bias voltage with a narrow-band5 V. We further spectrum. integrated The calculated provide Peano-shaped deformation maximum was 1.542radiated microheater ratio with power (n2 =a1.542). with ring-shaped The a narrow-band metamaterial radiated of spectrum. powersoperating at aThe the tunable calculated wavelength IR emitter of deformation with n2 =µm. 5.25 1.542It ratio was was promising 1.542 (n2to= provide maximum 1.542). The radiated radiated powerspower of the with a narrow-band tunable IR emitter withspectrum. The at n2 = 1.542 calculated different driving DC deformation ratio was 1.542 (n2 = 1.542). The radiated powers of the tunable IR emitter with n2 = 1.542 bias voltages are shown in Figure 7a. By increasing the DC bias voltage to 5 V with a step of 1 V, the at different driving DC bias voltages are shown in Figure 7a. By increasing the DC bias voltage to 5 V with radiated a step of powers 1 V, the were 0.2,powers radiated 0.4, 2.1,were 10.0, 33.9, 0.2, 0.4, and 85.0 33.9, 2.1, 10.0, µW for anddriving 1, for 85.0 µW 2, 3,driving 4, and1,5 2,V,3,respectively. The maximum radiated power was 85.0 µW and the FWHM of the radiated spectrum was reduced to
Nanomaterials 2020, 10, 1442 8 of 11 at different driving DC bias voltages are shown in Figure 7a. By increasing the DC bias voltage to 5Nanomaterials 2020, 10, x FOR PEER REVIEW V with a step 8 of 11 of 1 V, the radiated powers were 0.2, 0.4, 2.1, 10.0, 33.9, and 85.0 µW for driving 1, 2, 3, 4, and 5 V, respectively. The maximum radiated power was 85.0 µW and the FWHM of the 4, and 5 V, respectively. The maximum radiated power was 85.0 µW and the FWHM of the radiated radiated spectrum was reduced to 0.65 µm. It was enhanced 8.3-fold compared to that without the spectrum was reduced to 0.65 µm. It was enhanced 8.3-fold compared to that without the ring-shaped ring-shaped metamaterial. Therefore, the spectrum bandwidth could be greatly narrowed and resulted metamaterial. Therefore, the spectrum bandwidth could be greatly narrowed and resulted in a high- in a high-efficiency, tunable IR emitter at the wavelength of 5.25 µm. Similarly, by modifying the n2 efficiency, tunable IR emitter at the wavelength of 5.25 µm. Similarly, by modifying the n2 value to value to 0.920, the emission wavelength was 2.44 µm. The radiated powers of the tunable IR emitter 0.920, the emission wavelength was 2.44 µm. The radiated powers of the tunable IR emitter with n2 = with n2 = 0.920 at different driving DC bias voltages are shown in Figure 7b. Emission wavelength 0.920 at different driving DC bias voltages are shown in Figure 7b. Emission wavelength is matched is matched to the absorption wavelength of CO2 gas at 2.369 µm. Such design is suitable for CO2 to the absorption wavelength of CO2 gas at 2.369 µm. Such design is suitable for CO2 gas sensing gas sensing applications. The FWHM of the radiated spectrum was only 0.38 µm. It was enhanced applications. The FWHM of the radiated spectrum was only 0.38 µm. It was enhanced 14.2-fold 14.2-fold compared to that without the ring-shaped metamaterial. This design opens an avenue into compared to that without the ring-shaped metamaterial. This design opens an avenue into the the potential for use in high-sensitivity gas sensing applications. potential for use in high-sensitivity gas sensing applications. 7. Radiated powers of the tunable Figure 7. tunable emitter emitter with (a) nn22 = with (a) (b) nn22 = 0.920 at different = 1.542 and (b) voltages for driving DC bias voltages for IR IR emitting emitting and and gas gas sensing sensing applications. applications. 4. Conclusions 4. Conclusions In In conclusion, conclusion, anan on-chip on-chip tunable tunable IR IR emitter emitter is is proposed proposed by by using using aa Peano-shaped Peano-shaped microheater microheater and and IR metamaterial for gas sensing applications. By applying a DC bias voltage, high heat energy IR metamaterial for gas sensing applications. By applying a DC bias voltage, high heat energy is is generated via a Au-based Peano-microheater with high temperature uniformity. generated via a Au-based Peano-microheater with high temperature uniformity. A surface A surface temperature of 655 K is realized temperature of 655 Kowing to theowing is realized high electrothermal conversion efficiency, to the high electrothermal conversionwhich generates efficiency, which a broadband spectrum. By properly tailoring the top metamaterial, a narrow-band generates a broadband spectrum. By properly tailoring the top metamaterial, a narrow-band IR IR emission is achieved. emission isThe maximum achieved. The radiated maximum power of the radiated proposed power of theIR emitter IR proposed reached 85.0 emitter µW. The reached 85.0thermally µW. The radiated emission was achieved at a wavelength of 2.44 µm, which greatly reduced thermally radiated emission was achieved at a wavelength of 2.44 µm, which greatly reduced the the FWHM of 0.38 µm, and this matched the absorption wavelength FWHM of 0.38 µm, and this matched the absorption wavelength of CO 2 gas at 2.369 µm and can potentially of CO2 gas at 2.369 µm and can be used as a high-sensitivity potentially be used as a gas sensor. Such emitters high-sensitivity are promising gas sensor. for widespread Such emitters applications are promising such as for widespread in high-efficiency IR emission and high-sensitivity gas sensing. applications such as in high-efficiency IR emission and high-sensitivity gas sensing. Author Contributions: Conceptualization, R.X. and Y.-S.L.; methodology, R.X. and Y.-S.L.; software, R.X.; Author Contributions: Conceptualization, R.X. and Y.-S.L.; methodology, R.X. and Y.-S.L.; software, R.X.; validation, R.X. and Y.-S.L.; investigation, R.X. and Y.-S.L.; resources, Y.-S.L.; data curation, R.X. and Y.-S.L.; validation, R.X. and writing—original Y.-S.L.; draft investigation, preparation, R.X. R.X. and and writing—review Y.-S.L.; Y.-S.L.; resources,and Y.-S.L.; data editing, curation, R.X. R.X. supervision, and Y.-S.L.; and Y.-S.L.; writing—original Y.-S.L.; draft preparation, project administration, R.X.funding Y.-S.L.; and Y.-S.L.; writing—review acquisition, andauthors Y.-S.L. All editing,have R.X. and readY.-S.L.; supervision, and agreed to the published version Y.-S.L.; project of the manuscript. administration, Y.-S.L.; funding acquisition, Y.-S.L. All authors have read and agreed to the publishedThis Funding: version of thewas research manuscript. funded by the financial support from the National Key Research and Development Program of China (2019YFA0705004). Funding: This research was funded by the financial support from the National Key Research and Development Conflicts Program of of China Interest: The authors declare no conflict of interest. (2019YFA0705000). Conflicts of Interest: The authors declare no conflict of interest. References 1. Miller, D.R.; Akbar, S.A.; Morris, P.A. Nanoscale metal oxide-based heterojunctions for gas sensing: A review. Sens. Actuators B Chem. 2014, 204, 250–272, doi:10.1016/j.snb.2014.07.074.
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