Tunable Infrared Metamaterial Emitter for Gas Sensing Application

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; [email protected] * Correspondence: [email protected]

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 Figure1a 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 Figure1a. 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:

2 2 1 1 1 r + t n = cos− ( − ) (3) e f f kd 2t tv (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 + 𝑟) −𝑡 3 of 11 𝑧 = (4) (1 − 𝑟) −𝑡 wherewhere r isis thethe reflection reflection coe coefficient,fficient, t is t theis the transmission transmission coe coefficient,fficient, d is thed is metamaterialthe metamaterial thickness, thickness, and k andis the k incidentis the incident wavevector. wavevector. The corresponding The corresponding parameters parameters of the metamaterial of the metamaterial are shown inare Figure shown1b–e. in FigureWhile the1b–e. eff ectiveWhile refractionthe effective index refraction was indeed index positive was indeed in the resonantpositive range,in the theresonant proposed range, design the proposedexhibited highdesign transmission exhibited inhigh this transmission range to realize in athis frequency-selective range to realize application. a frequency-selective The optical application.properties of The the designedoptical properties metamaterials of the were des studiedigned metamaterials through finite diwerefference studied time through domain-based finite difference(FDTD) simulations. time domain-based In the numerical (FDTD) simulations, simulations. the In incidentthe numerical electromagnetic simulations, wave the propagated incident electromagneticalong the z-axis, whichwave waspropagated perpendicular along tothe the zx-axis,–y plane. which Periodic was perpendicular boundary conditions to the were x–y definedplane. Periodicin the x- andboundaryy-axis directions,conditions andwere perfectly defined matched in the x- layer and (PML)y-axis boundarydirections, conditions and perfectly were matched adopted layerin the (PML)z-axis boundary direction. conditions were adopted in the z-axis direction.

FigureFigure 1. (a(a) )Schematic Schematic drawing drawing of of tunable tunable IR IR (i (infrared)nfrared) emitter. The The calculated calculated ( (bb)) effective effective permittivity,permittivity, ( (cc)) effective effective permeability, permeability, ( (d)) effective effective refraction index, and and ( (ee)) effective effective impedance impedance indexindex of of the the metamaterial. metamaterial. 3. Results and Discussion 3. Results and Discussion Figure2a,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 Figure2c,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 drawings of ( a) winding microheater and (b) Peano-shapedPeano-shaped microheater. ( (c,,d)) Surface temperaturetemperature distributions distributions of windingof winding microheater microheater and Peano-shaped and Peano-shaped microheater, microheater, respectively.

Geometricalrespectively. dimensionsGeometrical are dimensions kept as d1 are= d kept2 = 6.0 as µdm1 = andd2 = l6.01 = µml2 = and120 lµ1 m.= l2 = 120 µm.

Figure3 3aa shows shows the the surface surface temperature temperature distributions distributions of of the the microheater microheater with with metal metal linewidths linewidths of dof1 andd1 andd2. Itd2 can. It becan clearly be clearly observed observed that such that microheater such microheater designs exhibiteddesigns exhibited great heating great uniformity heating whenuniformity the metal when linewidth the metal increased. linewidth It canincreased. also generate It can higheralso generate maximum higher temperature maximum distribution temperature on thedistribution surface. Toon furtherthe surface. optimize To further the Peano-shaped optimize the microheater Peano-shaped with microheater high electrical-thermal with high electrical- efficiency, thethermal Peano-shaped efficiency, microheater the Peano-shaped was designed microheater to facilitate was comparison designed to of sixfacilitate effective comparison pattern densities of six (effectiveDeff, the ratiopattern of edensitiesffective heating (Deff, the material ratio of toeffective the whole heating area ofmaterial microheater) to the whole according area toof mathematicalmicroheater) fractalaccording theory. to mathematical The material selectedfractal theory. was Au, The and material the Deff selectedvalues were was 16%, Au, 20%,and the 32%, Deff 38%, values 42%, were and 16%, 49%, respectively.20%, 32%, 38%, Figure 42%,3 andb shows 49%, therespectively. experimental Figure results 3b shows for Au-based the experimental Peano-shaped results microheatersfor Au-based underPeano-shaped different microheaters driving voltages. under different The inserted driving images voltages. show The top inserted views of images optical show microscopes top views for of sixoptical Peano-shaped microscopes microheaters. for six Peano-shaped It can be microheaters. observed that It acan higher be observedDeff can realizethat a higher D surfaceeff can temperature.realize higher Electrical-thermalsurface temperature. efficiency Electrical-therm increasedal along efficiency with the increased increments along of withDeff. the The increments maximum heatingof Deff. The temperature maximum can heating reach temperature 660 K under can a driving reach voltage660 K under of 5 V a for drivingDeff = voltage49%. The of fundamental5 V for Deff = electrothermal49%. The fundamental behavior electrothermal is the process behavior of energy is conversionthe process toof thermalenergy conversion power. In theto thermal ideal situation, power. withoutIn the ideal considering situation, thermalwithout dissipation,considering thermal dissipation, emission can thermal be explained emission by can Planck’s be explained law (also by knownPlanck’s as law the (also blackbody known radiation as the blackbody law). According radiatio ton the law). blackbody According radiation to the law,blackbody emission radiation wavelength law, isemission only related wavelength to temperature. is only related Emission to temperat wavelengthure. has Emission a broad wavelength bandwidth, has and a emissivity broad bandwidth, is varied byand the emissivity corresponding is varied wavelength. by the corresponding Blackbody radiationwavelength. can Blackbody be expressed radiation by can be expressed by π ρλ= 818πhchc 1 λ ρλ ddλdλ = dλ (5)(5) λλλ55 expexp((hchc/λkT kT) )−1 1 − where ρρλλ isis the the emission emission power power per per unit unit volume, volume,λ is theλ is emission the emission wavelength, wavelength,h is the Planckh is the constant, Planck cconstant,is the velocity c is the ofvelocity light inof vacuum,light in vacuum,k is the k Boltzmann is the Boltzmann constant, constant, and T andis temperature. T is temperature. The heatThe fluxheat flowedflux flowed mainly mainly outward outward on the on upper the upper and lowerand lower surfaces. surfaces. Thus, Thus, half ofhalf the of radiated the radiated power power was ideallywas ideally generated generated though though the the upper upper surface surface to to be be the the radiation radiation source source of of thethe proposedproposed IR emitter. emitter. Relationship between maximum surface temperatures and applying DC bias voltages for both 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 Figure3c. Since high-e fficiency thethermal effect conduction of microheaters among made metals from can different improve meta radiatedls. The power, thermal it is alsoconductivities worth discussing of Al and the eAuffect are of 238microheaters W/(m·K) and made 317 from W/(m·K), different respectively. metals. The By thermal applying conductivities a bias voltage, of Al the and surface Au are temperature 238 W/(m K) · increasedand 317 W gradually/(m K), respectively. from a room By temperature applying a bias of 293 voltage, K. The the areas surface of both temperature microheater increased patterns gradually were · 2 keptfrom the a roomsame temperatureat 120 × 120 µm of 293. The K. maximum The areas surface of both heating microheater temperature patterns of werethe Al-based kept the winding same at microheater120 120 µm was2. The 571 maximumK, and it was surface 598 K heating for the temperature Al-based Peano-shaped of the Al-based microheater. winding microheater The maximum was × surface571 K,and temperature it was 598 of K fora Au-based the Al-based Peano-shaped Peano-shaped microheater microheater. can The reach maximum 660 K, while surface that temperature of a Au- basedof a Au-based winding Peano-shaped microheater can microheater only reach can 625 reach K under 660 K, the while same that driving of a Au-based voltage winding of 5 V. This microheater shows thatcan the only radiation reach 625 efficiencies K under the of sameboth microheaters driving voltage were of 5better V. This when shows Au that materials the radiation were used, efficiencies as the Peano-shapedof both microheaters microheater were bettergenerated when a Aumore materials complex were magnetic used, asfield the to Peano-shaped induce electromagnetic microheater heating.generated The a moremaximum complex applied magnetic DC fieldbias was to induce set as electromagnetic 5 V to avoid breakdown heating. The of the maximum device. appliedTherefore, DC abias higher was surface set as 5 heating V to avoid temperature breakdown can of be the achiev device.ed by Therefore, using a Au-based a higher surfacePeano-shaped heating microheater, temperature whichcan be can achieved generate by usinghigher a radiated Au-based power. Peano-shaped Au-based microheater, Peano-shaped which microheaters can generate are higher very suitable radiated forpower. use in Au-based tunable IR Peano-shaped emitter design. microheaters are very suitable for use in tunable IR emitter design.

Figure 3. (a) Surface temperature distributions of microheaters with metal linewidths of d1 and d2. (b) Figure 3. (a) Surface temperature distributions of microheaters with metal linewidths of d1 and d2. Temperature(b) Temperature as a as function a function of driving of driving voltage voltage for for six six Peano-shaped Peano-shaped microheater microheater designs designs with with different diﬀerent patternpattern density.density. (c)(c Relationships) Relationships between between applied applied DC bias DC voltage bias andvoltage maximum and surfacemaximum temperature surface temperaturefor winding for microheaters winding microheaters and Peano-shaped and Peano-sh microheaters.aped microheaters. Corresponding Corresponding materials are materials selected as are Al selectedand Au. as Al and Au.

TheThe relationships relationships between between the the applied applied DC DC bias bias voltages voltages and and effective eﬀective radiated radiated powers powers were were calculatedcalculated and and plotted plotted in in Figure Figure 44.. The total radi radiatedated power was below 20 µWµW when when applying applying DC DC biasbias voltage voltage from from 0 0 to to 3 3 V. V. By By increasing increasing the the DC DC bias bias voltage voltage to to 4 4 V, 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

Nanomaterialswas 48.6 µW. 2020 When, 10, x FOR the drivingPEER REVIEW DC bias voltage was 5 V, the corresponding radiated power increased6 of 11 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 Figure4 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.

FigureFigure 4. 4. BlackbodyBlackbody radiated radiated power power of of a a Peano-shaped Peano-shaped microheater microheater under under different diﬀerent applied applied DC DC bias bias voltage.voltage. Inset Inset is is IR IR thermal thermal radiatio radiationn measured measured with with a a thermal thermal radiometer.

ToTo narrow down the FWHM value of of the the IR IR radi radiationation spectrum, spectrum, we we proposed proposed two two metamaterial metamaterial designsdesigns of Peano-shapedPeano-shaped microheatermicroheater surfaces surfaces encapsulated encapsulated by by insulated insulated layers laye onrs theon top.the top. Figure Figure5a,b 5a,bshows shows the proposedthe proposed circle-shaped circle-shaped and ring-shapedand ring-sha metamaterials.ped metamaterials. To investigate To investigate the influence the influence of the ofgeometrical the geometrical dimensions dimensions of the metamaterial of the metamate on therial emitted on the wavelength, emitted wavelength, the deformed the metamaterial deformed metamaterialperiod and outer period circle and diameter outer circle divided diameter by the divided initial metamaterial by the initial parameter metamaterial is defined parameter as n, i.e., is definedthe metamaterial as n, i.e., the deformation metamaterial ratio. deformation The subscripts ratio. 1 The and subscripts 2 indicate circle-shaped1 and 2 indicate and circle-shaped ring-shaped andmetamaterials, ring-shaped respectively.metamaterials, Figure respectively.5c,d shows Figure the 5c,d transmission shows the spectratransmission of circle-shaped spectra of circle- and ring-shaped metamaterials with different n and n values, respectively. In Figure5c, the transmission shaped and ring-shaped metamaterials with1 different2 n1 and n2 values, respectively. In Figure 5c, the spectrum gradually red-shifted from 2.16 µm to 4.29 µm when n increased from 1 to 2. The tuning transmission spectrum gradually red-shifted from 2.16 µm to 4.291 µm when n1 increased from 1 to 2. µ Therange tuning was 2.13rangem. was The 2.13 corresponding µm. The corresponding transmission transmission intensity was intensity enhanced was 10% enhanced (from 75% 10% to (from 85%) by increasing n from 1 to 2. These tuning transmission spectra are not perfect single-band resonances 75% to 85%) by1 increasing n1 from 1 to 2. These tuning transmission spectra are not perfect single- bandin the resonances IR wavelength in the range. IR wavelength It can be clearly range. observedIt can be clearly that there observed were several that there lower-order were several resonances lower- orderat short resonances IR wavelengths, at short which IR wavelengths, will result inwhich interference will result with in the interference IR emitter with and willthe IR also emitter reduce and the willradiated also reduce power. the In order radiated to solve power. this In problem, order to a solve ring-shaped this problem, metamaterial a ring-shaped was designed metamaterial as shown was in µ designedFigure5d. as The shown ring-shaped in Figure metamaterial 5d. The exhibitedring-shaped resonance metamaterial at the wavelength exhibited ofresonance 2.61 m at with the a transmission intensity of 76% for n = 1. By increasing the n value to 2, the resonance red-shifted wavelength of 2.61 µm with a transmission2 intensity of 76% for2 n2 = 1. By increasing the n2 value to 2, µ µ theto the resonance wavelength red-shifted of 5.83 tom the with wavelength a transmission of 5.83 intensity µm with of a 74%. transmission The tuning intensity range wasof 74%. 3.22 Them. µ tuningIt was enhancedrange was 1.09 3.22m µm. compared It was with enhanced that of the1.09 circle-shaped µm compared metamaterial. with that Itof is the worth circle-shaped mentioning metamaterial.that the transmission It is worth intensity mentioning was keptthat the stable transm by changingission intensity the n2 wasvalue. kept The stable variation by changing was only the 2%. The lower-order resonances were greatly reduced. The relationships between resonances and n2 value. The variation was only 2%. The lower-order resonances were greatly reduced. The relationshipsdeformation ratiosbetween are resonances summarized and in deformation Figure6a,b for ra circle-shapedtios are summarized and ring-shaped in Figure metamaterials,6a,b for circle- shapedrespectively. and ring-shaped To understand metamaterials, the physical respectively mechanism. To of understand these tunable the metamaterials, physical mechanism the electric of these (E) tunableand magnetic metamaterials, (H) field distributionsthe electric (E) of and circle-shaped magnetic (H) and field ring-shaped distributions metamaterials of circle-shaped are illustrated and ring- in shapedthe insets metamaterials of Figure6a,b, are respectively. illustrated E- in and the H-field insets distributionsof Figure 6a,b, were respectively. highly concentrated E- and aroundH-field distributionsthe Au patterns were and highly interacted concentrated with incident around electromagnetic the Au patterns waves and to provide interacted a strong with resonance. incident electromagneticThe relationships waves between to resonancesprovide a strong and deformation resonance. ratios The relationships were linear. This between means resonances 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 circle-shapedan effective approach and ring-shaped to high-precision metamaterials. and wavelength-selective These results provide emission an effective applications approach in to thehigh- IR wavelengthprecision and range. 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-shapedring-shaped metamaterials.metamaterials. Transmission spectra with different n and n values for (c) circle-shaped and (d) ring-shaped metamaterials. Initial spectra with different diﬀerent n11 and n12 valuesvalues 2for for ( (cc)) circle-shaped circle-shaped and and ( (d) ring-shaped metamaterials. Initial Initial geometricalgeometrical dimensions dimensions were wereR1 = RR 2R2 =1= 1= 1 µm,Rµ2m, = P 1P1 1=µm, =P2P = 2 P2=1 µm, =2 µPm, 2r ==r 20.2= µm, 0.2µm.µ r m. = 0.2 µm.

FigureFigure 6. 6.Relationships Relationships between between resonances resonances and deformation and deformation ratios for ratios (a) circle-shaped for (a) circle-shaped and (b) and (b) ring- ring-shaped metamaterials. The inserted images show the corresponding E- and H-ﬁeld distributions. Figureshaped 6. Relationships metamaterials. between The resonances inserted images and deformation show the ratios correspondin for (a) circle-shapedg E- and H-field and (b) distributions.ring- shapedAccordingAccording metamaterials. to to the the above above The results, inserted results, the images the maximum maximum show the radiated radiatedcorresponding power power E- of ofand the the H-field Peano-shaped Peano-shaped distributions. microheater microheater occurred occurredat atthe the wavelength wavelength of of5.25 5.25 µmµ mby by driving driving a DC a DCbias bias voltage voltage of 5 ofV. 5We V. Wefurther further integrated integrated the thePeano-shaped Peano-shapedAccording microheater to the above with results, a ring-shaped the maximum metamaterial radiated power operating of the at Peano-shaped a wavelength microheater of 5.25 µm. microheater with a ring-shaped metamaterial operating at a wavelength of 5.25 µm. It was promising to occurredIt was promising at the wavelength to provide of maximum 5.25 µm by radiated driving power a DC withbias voltage a narrow-band of 5 V. We spectrum. further Theintegrated calculated the Peano-shapeddeformationprovide ratio microheater maximum was 1.542 radiated with (n2 = a1.542 ring-shaped power). The with radiated metamaterial a narrow-band powers operating of spectrum. the tunable at a The wavelength IR emittercalculated with of deformation5.25n2 = µm.1.542 It ratio was was promising1.542 (n 2to = provide1.542). Themaximu radiatedm radiated powers power of the withtunable a narrow-band IR emitter with spectrum. n2 = 1.542 The atcalculated 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 radiateda step of powers1 V, the wereradiated 0.2, powers 0.4, 2.1, were 10.0, 0.2, 33.9, 0.4, and2.1, 10.0,85.0 33.9,µW forand driving 85.0 µW 1, for2, 3, driving 4, and 1, 5 2,V, 3, respectively. The maximum radiated power was 85.0 µW and the FWHM of the radiated spectrum was reduced to

0.65 µm. It was enhanced 8.3-fold compared to that without the ring-shaped metamaterial. Therefore, the spectrum bandwidth could be greatly narrowed and resulted in a high-efficiency, tunable IR emitter Commented [ZB17]: Please check meaning has been retained at the wavelength of 5.25 µm. Similarly, by modifying the n2 value to 0.920, the emission wavelength was

2.44 µm. The radiated powers of the tunable IR emitter with n2 = 0.920 at different driving DC bias Commented [Y. S. Lin18R17]: Yes, it is correct. voltages are shown in Figure 7b. Emission wavelength is matched to the absorption wavelength of CO 2 Commented [ZB19]: Please check accuracy of edit gas at 2.369 µm. Such design is suitable for CO2 gas sensing applications. The FWHM of the radiated Commented [Y. S. Lin20R19]: Yes, it is correct.

Nanomaterials 2020, 10, 1442 8 of 11 at different driving DC bias voltages are shown in Figure7a. By increasing the DC bias voltage to 5Nanomaterials V with a step2020, 10 of, x 1 FOR V, the PEER radiated REVIEW powers were 0.2, 0.4, 2.1, 10.0, 33.9, and 85.0 µW for driving8 of 11 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 n = 0.920 at different driving DC bias voltages are shown in Figure7b. Emission wavelength 0.920 at2 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.

Figure 7.7. Radiated powers of the tunabletunable emitteremitter withwith ((aa)) nn22 == 1.542 and ((b)) nn22 = 0.920 at didifferentﬀerent driving DC bias voltages for IRIR emittingemitting andand gasgas sensingsensing applications.applications.

4. Conclusions In conclusion,conclusion, an on-chip tunable IR emitter is proposedproposed byby usingusing aa Peano-shapedPeano-shaped microheatermicroheater and IR metamaterial for gas sensing applications. By applying a DC bias voltage, high heat energy is generated viavia a Au-baseda Au-based Peano-microheater Peano-microheater with highwith temperature high temperature uniformity. uniformity. A surface temperatureA surface oftemperature 655 K is realizedof 655 K owing is realized to the owing high electrothermalto the high electrothermal conversion econversionﬃciency, which efficiency, generates which a broadbandgenerates a spectrum.broadband Byspectrum. properly By tailoring properly the ta topiloring metamaterial, the top metamaterial, a narrow-band a narrow-band IR emission IR is achieved.emission is The achieved. maximum The radiatedmaximum power radiated of the power proposed of the IRproposed emitter IR reached emitter 85.0 reachedµW. The 85.0 thermally µW. The radiatedthermally emission radiated was emission achieved wasat achieved a wavelength at a wave of 2.44lengthµm, of which 2.44 µm, greatly which reduced greatly the reduced FWHM the of µ µ 0.38FWHMm, of and 0.38 this µm, matched and this the matched absorption the wavelength absorption ofwavelength CO2 gas at of 2.369 CO2 gasm and at 2.369 can potentially µm and can be usedpotentially as a high-sensitivity be used as a gashigh-sensitivity sensor. Such emittersgas sensor. are promisingSuch emitters for widespreadare promising applications for widespread such as inapplications high-eﬃciency such IRas emissionin high-efficiency and high-sensitivity IR emission gasand sensing. 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.; writing—originalvalidation, R.X. and draft Y.-S.L.; preparation, investigation, R.X. and R.X. Y.-S.L.; and writing—reviewY.-S.L.; resources, and Y.-S.L.; editing, data R.X. curation, and Y.-S.L.; R.X. supervision,and Y.-S.L.; Y.-S.L.;writing—original project administration, draft preparation, Y.-S.L.; R.X. funding and Y.-S.L.; acquisition, writing—review Y.-S.L. All and authors editing, have R.X. and read Y.-S.L.; and agreed supervision, to the publishedY.-S.L.; project version administration, of the manuscript. Y.-S.L.; funding acquisition, Y.-S.L. All authors have read and agreed to the Funding:publishedThis version research of the was 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 ConflictsProgram of China Interest: (2019YFA0705000).The authors declare no conflict of interest.

Conflicts of Interest: The authors declare no conflict of interest.

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