Integrated optic ammonia gas sensor using graphene covered silicon microring resonator is proposed. Ammonia gas adsorption increases the Fermi energy of graphene. The gas concentration is detected by the resonant-wavelength shift of microring resonator. The NH3 gas concentration from 0.5 to 1000 ppm is shown to be measured by the resonant wavelength from 1551.75 to 1551.27 nm. By applying voltage to the graphene, the Fermi energy is controlled to adjust the sensitivity and the range of detectable concentration

Goto, Nobuo

Kaito, Takamasa

Kishikawa, Hiroki

Liaw, Shien-Kuei

Sato, Masayasu

Yanagiya, Shin-ichiro

English

Tokushima University Institutional Repository

Japanese Journal of Applied Physics REGULAR PAPEROptical Ammonia Gas Sensor with Adjustable Sensitivity UsingSilicon Microring Resonator Covered with Monolayer GrapheneHiroki Kishikawa1∗, Masayasu Sato1†, Nobuo Goto1, Shin-ichiro Yanagiya1, TakamasaKaito2, and Shien-Kuei Liaw31Tokushima University, 2-1 Minamijosanjima-cho, Tokushima, 770-8506 Japan2KRI Inc., 5-11-151 Torishima, Konohana-ku, Osaka, 554-0051 Japan3National Taiwan University of Science and Technology, Taipei City, 10607 TaiwanIntegrated optic ammonia gas sensor using graphene covered silicon microring resonator is proposed.Ammonia gas adsorption increases the Fermi energy of graphene. The gas concentration is detected bythe resonant-wavelength shift of microring resonator. The NH3 gas concentration from 0.5 to 1000 ppm isshown to be measured by the resonant wavelength from 1551.75 to 1551.27 nm. By applying voltage to thegraphene, the Fermi energy is controlled to adjust the sensitivity and the range of detectable concentration.1. Intr oductionGraphene is a two-dimensional carbon atomic layered material and has attracted great atten-tion in electronic and opto-electronic devices due to its unique band structure.1–3) Specificgas molecules adsorbed on the graphene surface induce change of the electron distributionin the graphene,4,5) which induces change of the Fermi energy of graphene and then changeof the complex conductivity. Such graphene’s electron behavior is originated by its uniqueband structure. This phenomenon is used for gas sensing by detecting optical behavior6–9) orelectrical behavior.10–13) The optical detection using graphene has advantage in low-invasiveinteraction by optical evanescent fields over the electrical detection. The optical approachis also considered to be less affected by degradation of graphene quality over time than theelectrical approach.14)We focus on optical sensing with optical waveguide devices. In particular, the opticalphase change in the waveguide induced by the gas adsorption has been used as high-sensitivesensing. So far, various types of graphene based optical sensing devices have been proposedsuch as a Mach-Zehnder interferometer with a microfiber on a graphene covered substrate∗E-mail: kishikawa.hiroki@tokushima-u.ac.jp†H. Kishikawa and M. Sato equally contributed to this work.1/12The final authenticated version is available online at:https://doi.org/10.7567/1347-4065/ab24b7Jpn. J. Appl. Phys. REGULAR PAPERas its one arm,15) in-fiber Mach-Zehnder interferometer using interference between guidedmode and clad mode,16,17) side-by-side coupled microfibers,18) a graphene-covered opticalprism,8) a graphene-coated microfiber, a graphene-covered D-shape fiber.8,19) Micro-ring res-onator covered with graphene has been investigated from the viewpoint of resonant quality20)and optical modulation.21) Microring resonators22) have also been used for sensing withoutgraphene layer.23–25)In this paper, we propose a gas sensor consisting of graphene covered silicon-waveguidemicroring resonator for NH3 gas.26) Since the optical wavelength used for sensing is in op-tical communication band, remote sensing through optical fiber transmission is performedwithout electro-magnetic affection. It is found that the gas concentration can be detected bythe resonant wavelength change. The Fermi energy is manipulated by applying voltage tothe graphene, which will result in adjusting the sensing sensitivity and the sensing range ofgas concentration. We discuss comprehensive description of the sensing principle and de-tailed derivation of equations based on our proposal.26) The optical sensing with grapheneand waveguide devices have advantages such as miniaturized system with small footprint,low power consumption, remote sensing, high-sensitivity, and good flexibility27) comparedto the frequently used techniques in commercial NH3 gas detectors based on metal- andsemiconductor-oxide sensors, catalytic metal sensors, conducting polymer sensors, and opti-cal absorbance analyzers.28)2. Device structureThe basic structure of the proposed device consists of a microring resonator with ring radiusRcovered by a single atomic layer of graphene as shown in Fig. 1. The graphene is assumed tocoat on top and side surfaces of the ring waveguide and on the substrate. Optical continuouswave at wavelengthλ is coupled from a straight waveguide. The silicon waveguides withwidth W and heightH are formed on a SiO2/Si substrate. The fabrication process describedin Ref.20) is a possible way for our proposed device. Selective patterning of graphene is alsodescribed in the literature, which is performed by reactive ion etching with an O2 gas flow.NH3 molecules adsorbed on graphene will act like a donor.5,8) As a result, the adsorptionof NH3 molecules changes the Fermi level of the graphene, which induces the conductivitychange. High sensitivity is caused by the fact that graphene has a low density of states nearthe Dirac point and small change in carrier density results in large change in Fermi energy.14)The resonant wavelength shift in the microring resonator is then induced due to the effectivindex change of the graphene covered waveguide.2/12Jpn. J. Appl. Phys. REGULAR PAPERREin EoutrWHV(a)dV(b)Fig. 1. (a) Graphene covered optical microring-resonator gas sensor and (b) cross-section of the ringresonator with applied voltage.The Fermi energy is also adjustable by applying voltage to the graphene as shown inFig. 1(b). The voltageVg is applied between the graphene and Si waveguide through a thininsulating layer of SiO2 with thicknessd. A thin Si layer is employed below the Si core toapply Vg.21) Thus, the applied voltage will expect to control the sensitivity and the sensingrange of the gas concentration.3. Sensitivity for NH 3 gas concentrationAssuming the resistivity and the conductivity of graphene without NH3 gas asρs andσs,respectively, we find∆σ/σs = (∆ρ/ρs)/[(∆ρ/ρs) + 1]. Graphene relative resistivity change∆ρ/ρs as a function of NH3 concentration is found from the experimental result of resistivityby Yavariet al.29) as shown in Fig.2. The relative conductivity change∆σ/σs calculated fromthe resistivity change is also plotted.The complex conductivityσ in the graphene is related to Fermi energyµc by30)σ(ω) =σ02(tanhℏω + 2µc4kBT+ tanhℏω − 2µc4kBT)− jσ02πln[(ℏω + 2µc)2(ℏω − 2µc)2 + (2kBT)2]+ j4σ0πµcℏω + jℏγ,(1)whereσ0 = q2/(4ℏ), q is the electron charge,kBT is the thermal energy,ℏω is the photon en-ergy,ℏγ is the electron relaxation energy, andγ is the intraband scattering rate. The complex3/12Jpn. J. Appl. Phys. REGULAR PAPERet alFig. 2. Resistivity and conductivity of graphene as a function of NH3 gas concentration from Yavariet al.29)ReImFig. 3. The complex conductivityσ = σ′ + jσ′′(= Re[σ] + jIm[σ]) as a function of the Fermi energyµcusing eq.(1).Fig. 4. The evaluatedµc for the graphene in NH3 atmosphere.conductivityσ as a function of the Fermi energyµc using eq.(1) is plotted in Fig. 3. Theµcfor the graphene in NH3 atmosphere is evaluated using the conductivity from Fig. 2 as shownin Fig. 4. From Figs. 3 and 4, the complex conductivityσ = σ′ + jσ′′ is found as in Fig. 5.The complex refractive indexn = n′+ jn′′ of the graphene is calculated fromσ = σ′+ jσ′′4/12Jpn. J. Appl. Phys. REGULAR PAPERReImFig. 5. The complex conductivityσ = σ′ + jσ′′(= Re[σ] + jIm[σ]) for the graphene in NH3 atmosphere.Re nIm nFig. 6. The theoretical complex refractive index of graphene as a function of the Fermi energy.by15)n′ =√12ω∆ε0√σ′′ +√σ′′2 + σ′2, n′′ = − σ′2ε0ω∆1n′, (2)whereε0 is the dielectric constant under vacuum,∆ is the graphene thickness. We assume∆to be 0.4 nm.31) Fig. 6 plots the complex refractive index as a function of the Fermi energy.The evaluated complex refractive index of graphene as a function of NH3 concentrationin air is finally obtained as shown in Fig. 7.Optical guided modes were analyzed by finite element method (FEM) with COMSOLsoftware. The guided modes in silicon ridge waveguide ofW = 500 nm andH = 220 nmwith 0.4-nm thickness graphene cover were calculated at wavelengthλ = 1551.2 nm. The thinSi layer below the Si core for voltage application was not considered in the calculation. Meshgrids in the cross-section of the waveguide for the FEM calculation are shown in Fig. 8(a),where the dense grids are used to model thin mono-layer graphene. The detailed grids at theright-lower corner of the silicon core are shown in Fig. 8(b).5/12Jpn. J. Appl. Phys. REGULAR PAPERRe nIm nFig. 7. The evaluated complex refractive indexn = n′ + jn′′(= Re[n] + jIm[n]) of graphene as a function ofNH3 concentration.(a) (b)Fig. 8. Mesh grids for FEM calculation, (a) modeled waveguide area and (b) at the right-lower corner ofsilicon core.The x, y, andz components of the calculated electric fields in TE-like mode in the caseof NH3 gas concentration of 0.5 ppm are plotted in Figs. 9(a), (b), and (c), respectively. Thetotal electric field|E| is also plotted in Fig. 9(d).The calculated complex eff ctive indexN = Nr + jNi of the TE-like mode is shown inFig. 10. BothNr and |Ni | decrease with the NH3 concentration. The decrease of|Ni | meansdecrease of propagation loss of the guided mode.Now, we consider the ring resonator. The transmittance of the resonator is obtained by32)∣∣∣∣∣EoutEin∣∣∣∣∣2 = r2 + a2 − 2ar cosϕ1+ a2r2 − 2ar cosϕ, (3)wherer is the uncoupled (bar-path) coefficient between the ring and the straight waveguide,a = exp(2πk0NiR) corresponds to the ring propagation loss,k0 = 2π/λ is the wave number,andϕ = −2πk0NrR is the round-trip phase shift experienced in the ring. The radiusR of thering is assumed as 50µm. We also assume the uncoupled coefficientr as 0.95. The transmit-tance drops at nearly periodical wavelengths. Figure 11 shows the transmitted output intensity6/12Jpn. J. Appl. Phys. REGULAR PAPER(a) (b)(c) (d)Fig. 9. Calculated electric fields in TE like mode in the case of NH3 concentration of 0.5 ppm, (a)|Ex|, (b)|Ey|, (c) |Ez|, and (d)|E|.Fig. 10. Effective indexN = Nr + Ni of the guided TE-like mode as a function of NH3 concentration.|Eout/Ein|2 for the cases of NH3 concentration of 0.5, 2, 10, 40, 200, and 1000 ppm. The effec-tive index at each of the wavelength is calculated by FEM in order to consider the wavelengthdependence ofNr andNi due to the large dispersion of Si waveguide. The minimum outputintensity (dip) shifts to shorter wavelength direction by increasing the gas concentration. Theresonant dip is relatively broad due to the propagation loss of the guided mode. The free spec-tral range (FSR) is around 1.8 nm. The Q factor is calculated to be around 1700. The Q factorwithout graphene overlay is calculated to be 52500 by using Eq. (20) in Ref.33) The grapheneoverlay reduces the Q factor. In Ref.,20) the Q factor for the ring resonator ofR = 10 µmreduces from 7900 to 1200 by overlaying graphene in the length from 0 to 20µm.One of the wavelength drop varies as a function of NH3 concentration as shown in Fig. 12.Thus, the gas concentration is measured by the wavelength shift of the dip. The dip wave-7/12Jpn. J. Appl. Phys. REGULAR PAPERFig. 11. Wavelength dependence of the output intensity from the resonator for NH3 concentration of200 ppm and 1000 ppm.Fig. 12. The dip wavelength of the resonant as a function of the NH3 concentration.length decreases monotonically with the NH3 concentration. The sensing sensitivity varieswith the gas concentration. The sensitivity from 0.5 to 2 ppm is evaluated to be 67 pm/ppm,whereas the value decreases to 0.69 pm/p from 40 to 200 ppm.Finally, we discuss controllability of the measurement sensitivity. The Fermi energyµc ischanged by applying voltage to the graphene. The carrier density in the graphene is changedby applying electric voltage. This is caused by Fermi energy shift induced by carriers. Thechange of Fermi energyVD is given by14)VD =ℏνFq√πεVgqd, (4)wherevF is the Fermi velocity (=106 m/s), ε/ε0 for SiO2 is 3.8. Fig. 13(a) shows the cal-8/12Jpn. J. Appl. Phys. REGULAR PAPER(a)(b)Fig. 13. Adjustment of sensitivity by applying voltageVg; (a) Fermi energy change by applying voltage tographene, (b) Fermi energy change as a function of NH3 concentration.culatedVD with SiO2 thicknessd as a parameter. The Fermi energy as a function of NH3concentration shown in Fig. 4 is expected to be adjusted by applying the voltageVg as il-lustrated in Fig. 13(b). The sensitivity for higher NH3 concentration might be adjusted byapplying a negative voltageVg according to the gas concentration range to be measured. Notethat the sign ofq is changed with the sign ofVg since the dominant carrier for the negativevoltage case is hole.34)4. ConclusionAn optical NH3 gas sensor consisting of graphene covered microring resonator was proposed.The sensitivity was analyzed by FEM. It was clarified from the simulation using experimentaldata on graphene resistivity that the gas concentration from 0.5 to 1000 ppm was successfullymeasured by the resonant wavelength shift of the microring resonator. The sensitivity from0.5 to 2 ppm was evaluated to be 67 pm/ pm. The sensitivity of the gas detection mightbe adjusted by applying electric voltage to the graphene to shift its Fermi energy. There is9/12Jpn. J. Appl. Phys. REGULAR PAPERa trade-off between the sensitivity and the extinction ratio calculated from the spectral dipdepth at the resonant wavelength due to the relation of graphene length and optical loss.Although the required sensitivity and extinction ratio depend on the application, we willconsider optimized designs and performances for the proposed optical NH3 gas sensor. Wewill also numerically evaluate the resonator characteristics of the proposed sensor in futureby using finite difference time domain method.10/12Jpn. J. Appl. Phys. REGULAR PAPERReferences1) A. K. Geim and K. S. Novoselov, Nature Materials,6, 183 (2007).2) K. S. Novoselov, V. I. Fal’ko, L. Colombo, P. R. Gellert, M. G. Schwab, and K. Kim,Nature,490, 192 (2012).3) B. Sensale-Rodriguez, J. Lightwave Technol., 33, 1100 (2015).4) F. Schedin, A. K.Geim, S. V. Morozov, E. W. Hill, P. Blake, M. I. Katsnelson, and K. S.Novoselov, Nature Materials,6 652 (2007).5) O. Leenaerts, B. Partoens, and F. M. Peeters, Phys. Rev. B,77, 125416 (2008).6) M. Harnaez, C. R. Zamarreno, S. Melendi-Espina, L. R. Bird, A. G. Mayes, and F. J.Arregui, Sensors,17, 155 (2017).7) Y. Zhao, X.-G. Li, X. Zhou, Y.-N. Zhang, Sensor and Actuators B,231, 324 (2016).8) B. N. Shivananju, W. Yu, Y. Liu, Y. Zhang, B. Lin,S.Li, and Q. Bao, AdvancedFunctional Materials,27, 19, 1 (2017).9) Y. Wu, B. Yao, C. Yu, and Y. Rao, Sensors,18, 941 (2018).10) S. S. Varghese, S. Lonkar, K. K. Singh, S. Swaminathan, and A. Abdala, Sensors andActuators B,218, 160 (2015).11) Y. Dan, Y. Lu, N. J Kybert, Z. Luo, and A. T. C. Johnson, Nano Letters,9, 1472 (2009).12) G. Ko, H.-Y. Kim, J. Ahn, Y.-M. Park, K.-Y. Lee, and J. Kim, Current Applied Physics,10, 1002 (2010).13) R. Kumar, R. Singh, D. Hui, L. Feo, F. Fraternali, Composites, Part B,134, 193 (2018).14) A. Maliakal, L. Reith, and S. Cabot, Appl. Phys. Lett.,108, 153109 (2016).15) B. Yao, Y. Wu, Y. Cheng, A. Zhang, Y. Gong, Y.-J. Rao, Z. Wang, and Y. Chen, Sensorsand Actuators B,194, 142 (2014).16) T. Hao and K. S. Chiang, IEEE Photonic. Tech. Lett.29, 2035 (2017).17) H. Fu,Y.Jiang, J. Ding, J. Zhang, M. Zhang, Y. Zhu, H. Li, Sensor and Actuators B,254,239 (2018).18) Y. Wu, B.-C. Yao, Y. Cheng, Y.-J. Rao, Y. Gong, W. Zhang, Z. Wang, and Y Chen, IEEEJ. Selected Topics in Quantum Electronics,20, 4400206 (2014).19) Q. Bao, H. Zhang, B. Wang, Z. Ni, C. Haley, Y. X. Lim, Y. Wang, D. Y. Tang, and K. P.Loh, Nature Photonics,5, 411 (2011).20) R. Kou, S. Tanabe, T. Tsuchizawa, T. Yamamoto, H. Hibino, H. Nakajima, and K.Yamada, Appl. Phys. Lett.,104, 091122 (2014).21) C. Qiu, W. Gao, R. Vajtai, P. M. Ajayan, J. Kono, and Q. Xu, Nano Lett.,2014, 681111/12Jpn. J. Appl. Phys. REGULAR PAPER(2014).22) W. Bogaerts, P. De Heyn, T. Van Vaerenbergh, K. De Vos, S. K. Selvaraja, T. Claes, P.Dumon, P. Bienstman, D. Van Thourhout, and R. Baets, Laser Photonics Rev.,6, 47(2012).23) A. F. Gavela, D. G. Garcia, J. C. Ramirez, and L. M. Lechuga, Sensors,16, 285 (2016).24) K. De Vos, I. Bartolozzi, E. Schacht, P. Bienstman, and R. Baets, Opt. Exp.,15, 7610(2007).25) M. K. Park, J. S. Kee, J. Y. Quah, V. Netto, J. Song, Q. Fang, E. M. La Fosse, G.-Q. Lo,Sensors and Actuators B,176, 552 (2013).26) M. Sato, H. Kishikawa, N. Goto, S. Yanagiya, and S.-K. Liaw, 23rd MicroOpticsConference (MOC2018), Taipei, P-68 (2018).27) Y. Wu, B. Yao, C. Yu, and Y. Rao, Sensors,18, 941, (2018).28) B. Timmer, W. Olthuis, and A. V. D. Berg, Sensors and Actuators B,107 666, (2005).29) F. Yavari, E. Castillo, H. Gullapalli, P. M. Ajayan, and N. Korathar, Appl. Phys. Lett.,100, 203120 (2012).30) Y.-C. Chang, C.-H. Liu, C.-H. Liu, Z. Zhong, and T. B. Norris, Appl. Phys. Lett.,104,261909 (2014).31) Y. Yao, L. Ren, S. Gao, and S. Li, J. Materials Science and Technology,33, 815 (2017).32) J. E. Heebner, V. Wong, A. Schweinsberg, R. W. Boyd, and D. J. Jackson, IEEE J. ofQuantum Electron.,40, 726 (2004).33) W. Bogaerts, P. D. Heyn, T. V. Vaerenbergh, K. D. Vos, S. K. Selvaraja, T. Claes, P.Dumon, P. Bienstman, D. V. Thourhout, and R. Baets, Laser and Photonics Reviews,6,47, (2012).34) Z. Q. Li, E. A. Henriksen, Z. Jiang, Z. Hao, M. C. Martin, P. Kim, H. L. Stormer, and D.N. Basov, Nature Physics,4 532, (2008).12/12

Optical Ammonia Gas Sensor with Adjustable Sensitivity Using Silicon Microring Resonator Covered with Monolayer Graphene

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Optical Ammonia Gas Sensor with Adjustable Sensitivity Using Silicon Microring Resonator Covered with Monolayer Graphene

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