Optical sensing of ammonia using ZnO nanostructure grown on a side-polished optical-fiber

Sensors and Actuators B 146 (2010) 331–336

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Optical sensing of ammonia using ZnO nanostructure grown on a side-polished optical-fiber A.Og. Dikovska a,∗ , G.B. Atanasova b , N.N. Nedyalkov a , P.K. Stefanov b , P.A. Atanasov a , E.I. Karakoleva c , A.Ts. Andreev c a b c

Institute of Electronics, Bulgarian Academy of Sciences, 72 Tsarigradsko Chaussee, Sofia 1784, Bulgaria Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, “Acad. G. Bonchev str.”, bld. 11, 1113 Sofia, Bulgaria Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tsarigradsko Chaussee, 1784 Sofia, Bulgaria

a r t i c l e

i n f o

Article history: Received 7 December 2009 Received in revised form 3 February 2010 Accepted 4 February 2010 Available online 11 February 2010 Keywords: Optical gas sensors ZnO nanostructures Distributed coupler PLD

a b s t r a c t In this work, thin ZnO films were produced by pulsed laser deposition on a side-polished single-mode fiber in view of optical gas sensor applications. The experimental conditions used for preparation of the samples were chosen so as to obtain smooth, porous and nanostructured films. The influence of the film structure on the sensitivity to ammonia was investigated. For all samples, a shift of the spectral position of the resonance minimum to the longer wavelengths was observed under gas exposure at room temperature. The nanostructured sensor element demonstrated a substantially higher sensitivity due to its structure compared to the only smooth and porous samples. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Gas sensors based on optical detection have focused significant attention due to the possibility to operate at room temperature, to measure low gas concentrations with fast response time and to be applicable even in explosive and/or electromagnetic environments. The optical gas sensing by metal oxide thin films utilize mainly the properties of planar optical waveguides formed by these films. The optical detection is based on the refractive index changes of the thin films caused by the gas components of the reacting medium. The measurements of the changes of the planar waveguide parameters can be carried out using standard integrated optic measurement techniques [1,2] or with an application of fiber-optic distributed couplers consisting of a side-polished fiber coupled with the planar waveguide investigated [3]. At present, the main efforts are focused on decreasing the minimum gas concentration necessary for optical detection. One possibility for improving the gas sensitivity is to make use of the advantages offered by nanotechnologies, in particular, the use of nanostructured planar waveguides. A high surface-to-bulk ratio in metal oxide nanostructures allows very sensitive transduction of the gas/surface interactions into a change in the electrical, optical or other properties [4,5]. In addition, the possibility to form a variety of morphologies and structures

∗ Corresponding author. Tel.: +359 2 979 59 11; fax: +359 2 975 32 01. E-mail address: [email protected] (A.Og. Dikovska). 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.02.018

offers many opportunities of tuning the gas sensing properties [6–8]. Among the various sintering methods, pulsed laser deposition (PLD) has been proven to be a simple and effective catalystfree method for preparation of nano-scale materials [9–12]. The main advantage of PLD is its ability to create high energy species which exhibit high reactivity and surface mobility when reaching the substrate. Several studies have been reported dealing with ZnO nanostructures growth by PLD and exploring a wide range of experimental parameters, such as choice of substrate, substrate temperature, background oxygen pressure, etc. [10,12,13]. For technological application, however, it is essential to develop a methodology for nanostructure fabrication which can be easily transferred and applied to real devices. In the present study, we attempted to synthesize nanostructured ZnO planar waveguide by pulsed laser deposition; to fabricate a simple and efficient sensor element consisting of a side-polished single-mode fiber and a nanostructured ZnO planar waveguide; and to test the gas sensing performance of the sensor element under ammonia gas exposure at room temperature. 2. Principle of sensor element operation The distributed coupling between a side-polished single-mode fiber and a planar waveguide is an efficient method for constructing sensor elements utilizing the sensor properties of planar optical waveguides. These sensors are an attractive addition to

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were deposited on the side-polished fiber in an on-axis PLD configuration. The substrate temperature was kept at 300 ◦ C during deposition. The second step consisted in deposition of ZnO on the as-created nuclei in an off-axis PLD configuration. In the off-axis deposition, the substrate repositioned away from the normal position in the PLD method at a distance of 1.5 cm. The laser fluence used was 3.5 J/cm2 . During the second step the substrate temperature and the oxygen pressure were kept constant at 300 ◦ C and 5 Pa, respectively. 3.2. Characterization of the samples The surface morphology of the as-deposited samples was characterized by atomic force microscopy (AFM). The morphology of the nanostructured sample was analyzed using high-resolution scanning electron microscopy (HR-SEM) Sirion-FEI. The surface composition and chemical state of the ZnO waveguiding films were investigated by X-ray photoelectron spectroscopy (XPS). The measurements were performed on a VG ESCALAB II electron spectrometer using MgK˛ radiation with energy of 1253.6 eV. The binding energies (BE) were determined with an accuracy of ±0.1 eV utilizing the C 1s line at 285.0 eV (from an adventitious carbon) as a reference. Fig. 1. (a) A schematic view and (b) cross-section of the side-polished fiber sensor element.

the standard integrated optic elements–prism and grating couplers, because of their inherent in-line fiber-optic construction. A schematic view of the sensor element is presented in Fig. 1a. A single-mode fiber is glued into a fused silica rectangular block inside a convex groove. The optical cladding of the fiber is mechanically ground and polished to about 1 ␮m of minimal residual cladding thickness to the fiber core. The planar waveguide is grown on the flat polished fiber surface so that a distributed evanescent wave coupling between the fiber and the planar waveguide is implemented. The mechanism of interaction between a side-polished single-mode fiber evanescently coupled to a planar waveguide has been already well described [14,15]. Light coupling takes place under the condition of waveguide resonance between the fiber mode and the corresponding planar waveguide mode, leading to the appearance of a channel-dropping filter in the spectral transmittance of the fiber [16,17]. The spectral position of the dropping filter is strongly sensitive to the parameters of the planar waveguide (thickness, refractive index of the film, refractive index of the superstrate) forming in this way the basis for sensor applications. Sensor elements consisting of a side-polished fiber coupled to a planar waveguide have been used for registration of different physical and chemical parameters [18–22]. 3. Experimental 3.1. ZnO planar waveguide deposition Thin ZnO films were grown by PLD on the side-polished fiber. A XeCl excimer laser ( = 308 nm,  = 30 ns full width at half maximum (FWHM), and rep rate of 2 Hz) at a fluence of 2 J/cm2 was used for ablation of the ZnO ceramic target. The target to substrate distance was fixed at 40 mm in a standard on-axis configuration. The films were grown at substrate temperatures of 300 ◦ C. All experiments were performed in oxygen atmosphere. Smooth films were prepared at oxygen pressure of 5 Pa and porous films – at 20 Pa. The nanostructured ZnO waveguides were fabricated via a two step process. As the first step, a ZnO film was prepared at oxygen pressure of 20 Pa in order to form growth nuclei. These nuclei

3.3. Fabrication of the side-polished fiber An isotropic single-mode fiber (SM800-5.6-125; Fibercore) was used. The cross-section of the side-polished fiber is presented in Fig. 1b. The fiber was glued into a fused silica block with dimensions of 3 mm × 6 mm × 15 mm inside a convex groove. The radius of curvature at the groove bottom was 125 mm. Such a small radius was chosen in order to obtain a short interaction length between the optical-fiber and the planar waveguide (approximately 1 mm). A high temperature optical epoxy (Epo-Tek 354) was used for embedding the fiber into the silica block. The optical set-up for in situ control of the film thickness during the deposition process was already reported in [3]. The set-up for in situ control allowed us to convert the increase of the oxide film thickness during the deposition process into a time-dependence of the output fiber intensity. The deposition of the oxide film was stopped at the second minimum which corresponds to the interaction with the TM0 mode of the planar waveguide. The interaction with the TM0 mode was chosen for two reasons: (i) to reduce the time of film deposition and (ii) this interaction is more sensitive in comparison with the TE1 mode previously used in similar measurements [23]. The measurements of the spectral transmittance of the fiber samples were carried out using a halogen lamp as a light source and a monochromator. The light from the monochromator output was polarized by a Glan prism and then focused into the fiber by a microscope objective. The output signal from the fiber was measured by a lock-in-amplifier (SR 510, Stanford Research Systems). The spectral shapes of the receiving channel-dropping filters are presented in Fig. 2. The FWHM of the channel-dropping filters was evaluated to be approximately 25, 30 and 40 nm for smooth, porous and nanostructured films, respectively. The optical set-up for measuring the sensitivity to gaseous ammonia is presented in Fig. 3. The light source was a broadband superluminescent light emitting diode (SLD). The light from the SLD passed through a fiber polarizer and a fiber polarization controller for accurate polarization adjustment of the channeldropping filter. The output fiber was connected to the entrance of a 0.2 m monochromator (Photon Technology International, Model 101, grating 600 l/mm, 1000 nm blazed). The fiber core served as an entrance slit, the exit slit was adjusted to ensure 1 nm spectral resolution. The intensity of the passing light was measured by

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Fig. 2. Spectral dependence of the output signal from the sensor element with deposited: (a) smooth, (b) porous and (c) nanostructured ZnO film and interaction with the planar waveguide TM0 mode.

a large area silicon photodiode (PD) (Siemens BPW 34) mounted on the monochromator exit. The photodiode signal was registered by a lock-in amplifier. The monochromator grating was turned so that the intensity in the middle of the long wavelength slope of the channel-dropping filter could be measured. The measurements were carried out in a home-made vacuum chamber equipped with needle valves. The ammonia vapors were produced by evaporation of 0.2 ml water solution of 2.5, 5, 10 and 25% NH3 in a container with volume of 0.01 m3 . Ammonia concentrations of approximately 500, 1000, 2000 and 5 000 ppm were thus obtained. The vacuum chamber allows the reactive gas from the gas container to fill the test chamber, keeping the pressure at atmospheric value. All experiments were carried out at room temperature. The gas sensor tests were performed several times in order to check the reproducibility of the response. 4. Results and discussion 4.1. Surface morphology of the sensor element Our previous investigations showed that ZnO films prepared at 5 Pa oxygen pressure have smooth surface with root mean square (RMS) value of a few nanometers [24]. This is why in the present work these films are referred to as “smooth films”. As it is well known, raising the oxygen pressure enhances the porosity of the surface and increases the RMS value [25]. The films prepared at a higher oxygen pressure, 20 Pa, exhibit a rough surface with a high peak-to-valley ratio [24]. The films prepared

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Fig. 4. AFM image of the porous ZnO planar waveguide taken from an area 800 nm × 800 nm.

under these experimental conditions in our work are referred to as “porous films”. An AFM image of a porous ZnO film is presented in Fig. 4. The surface morphology is a very important parameter for formation of nanostructures. The rough surface plays the role of nuclei for nanostructured films growth [26]. Therefore, our first step of nanostructure preparation was formation of a thin ZnO film with a clearly expressed rough surface. A film with thickness lower than 30 nm was deposited at the same experimental conditions as the porous film in order to create the growth nuclei (see Fig. 4). In the second step, ZnO was deposited on the as-created nuclei. The ZnO growth follows the nuclei during the off-axis deposition and then a nanostructured film is formed. Fig. 5 shows the HR-SEM image of a nanostrucured ZnO film. The top view of the sample shows uniform and densely packed nanostructures with average diameter in the range of 40–70 nm. The chemical composition of the surface is of essential importance for gas sensor element’s performance. Thus, the surface composition of the samples was investigated by XPS. The XPS spectra of porous and nanostructured ZnO are presented in Fig. 6. For both samples, the Zn 2p3/2 peak is characterized by a binding energy of 1021.5 eV, which is typical for zinc in ZnO (see Fig. 6a). A mod´ Zn = BE Zn 2p + KE Zn LMM) was used to ified Auger parameter (␣ investigate the changes in the chemical environment of the Zn and O ions in the samples. The modified Auger parameter shows similar values (2010.6 and 2010.7 eV) for both samples which indicates that the chemical surrounding of Zn2+ ions is practically

Fig. 3. Optical set-up for measurement of the NH3 influence.

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Fig. 5. 45◦ HR-SEM view of the nanostructured ZnO film.

identical. Similar results were obtained for the smooth waveguide film. The O 1s peaks of both samples are wide and asymmetric, as it is clearly seen in Fig. 6b. The O 1s peak was deconvoluted by a Lorentzian–Gaussian curve fitting into two components at 530.1 and 531.8 eV, respectively [27]. They are attributed to O2− ions in the ZnO lattice and to oxygen bound in OH− groups, respectively. A significant difference was found in the values of the OOH− /O2− intensity ratio for the nanostructured sample (0.89), and for the porous one (0.41). This can be attributed to the higher specific surface area of nanostructured ZnO as compared to the porous sample resulting in the adsorption of a larger amount of OH− on its surface. 4.2. Ammonia gas sensing results For all the three samples, a shift of the spectral position of the resonance minima to the longer wavelengths was observed after

Fig. 7. Typical response of the nanostructured ZnO sensor element to various ammonia gas concentrations: (a) 500 ppm, (b) 1000 ppm, (c) 2000 ppm and (d) 5000 ppm.

exposure to ammonia vapors. Fig. 7 presents the changes in the intensity of the fiber output power as a result of ammonia exposure for the sample with a nanostructured waveguide film. A relatively fast response was observed – in the range of 20–30 s for the higher gas concentration. The time for gas reaction slightly increased as the concentration was decreased (500 ppm). The time of the full signal restoration after opening the test chamber to the atmospheric air was measured to be about 20 min for the samples with smooth and porous films and about 30–35 min for the sample with a nanostructured film. For the latter, sample fluctuations in the intensity during the restoration process were typical. We relate this observation with the fact that no air flow was applied around the samples so that back absorption of ammonia molecules was possible. Because of the different slopes of the samples’ curves, a transformation

Fig. 6. XPS of the: (a) Zn 2p and (b) O 1s core level on the surface of the: (1) porous and (2) nanostructured ZnO planar waveguides.

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were studied experimentally under exposure to ammonia. The nanostructured waveguide demonstrated a substantially higher sensitivity, which allows us to predict a low concentration limit of 50 ppm for NH3 in air. Acknowledgment This work was supported in part by the Bulgarian Ministry of Education and Science under Contract DO 02-293/08.

References

Fig. 8. Wavelength shifts of the smooth, porous and nanostructured ZnO sensor elements in dependence of the ammonia concentration.

of the intensity changes to the corresponding wavelength shifts was performed to enable a comparison of the samples sensitivities. The results are presented in Fig. 8. The nanostructured waveguide demonstrated substantially higher sensitivity, confirming the assumptions that the physical absorption is the main gas sensing mechanism. Using the data in Fig. 8, a calculation of the refractive index changes of the waveguiding films was performed. For example, for the highest concentration (5000 ppm) these changes are 0.81 × 10−3 , 2.14 × 10−3 and 6.35 × 10−3 for the smooth, porous and nanostructured films, respectively. The new sentences are added in the revised manuscript. No nitrogen was observed in the XPS spectra after NH3 exposure. This result also indicates that physical absorption of gas molecules on the surface is the basic sensing mechanism. The nanostructured thin film provides a larger gas interaction surface which makes this film a promising material for designing optical ammonia sensors. The optical set-up used permits us to register intensity changes corresponding to spectral shifts as small as 0.02 nm. This set-up can be used successfully in a laboratory but is very sensitive to variations of the light intensity and polarization outside the sensing area. Direct spectral measurements with depolarized light are better suited to real sensor systems. The spectral measurements eliminate the influence of the intensity changes so that no limit is imposed on the distance between the sensor element and the signal processing electronics. In accordance with the width of the resonance dropping filters (∼30–40 nm), a spectral resolution of 0.2 nm is achievable with commercially available compact fiber-optics spectrometers. With such resolution, the sensitivity reported here would yield a low concentration limit of approximately 50 ppm for NH3 in air. 5. Conclusions A simple and effective sensor element was fabricated consisting of a side-polished single-mode fiber and a PLD ZnO planar waveguide. The sensor element operation principle is based on a distributed coupling between the fiber mode and the corresponding mode of the metal oxide planar waveguide. The change of the spectral behavior of the as-obtained channel-dropping filter under gas exposure allows optical detection of the gas molecules. The planar waveguide structure is essential in achieving high sensitivity of the sensor element. The difference between the performance of porous and nanostuctred ZnO waveguides is mainly determined by the capability of absorbing large amounts of gas on their surfaces. The sensing properties of the element at room temperature

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Biographies Anna Og. Dikovska graduated in 1996 from the Faculty of Physics of St. Kliment Ohridski University of Sofia with M.S. degree in Engineering Physics, and received in 1997 a Ph.D. degree in Physics of Wave Processes at the Institute of Electronics, Bulgarian Academy of Sciences. Her field of research is pulsed laser deposition of oxide materials for optical applications. She is currently involved in the development of nanostructured thin films. She has authored more than 25 papers and communications.

Plamen K. Stefanov is a Senior Research Associate and Head of the Laboratory of Electron Spectroscopy at the Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences. His research interests include gas sensors and thin film catalysts for environmental applications. Petar A. Atanasov is a Corresponding Member of the Bulgarian Academy of Sciences, Full Professor and Head of the Gas Lasers and Laser Technologies Laboratory at the Institute of Electronics, Bulgarian Academy of Sciences, Sofia, Bulgaria. He graduated in Physics from the Faculty of Physics of the University of Sofia in 1967, and received Ph.D. degree in 1977 and Dr. Sc degree in 1990 at the Institute of Electronics. He was Visiting Professor several times at Keio University, Chiba University, Japan, and Instituto de Optica, CSIC, Madrid. His scientific and application activities are in the field of micro- and nano-photonics, optoelectronics. He is author or co-author of more than 280 contributions.

Genoveva B. Atanasova graduated in 1996 from the Faculty of Physics, St. Kliment Ohridski University of Sofia with M.S. degree in Engineer Physics, got qualification “Quantum Electronics and Lasers Techniques” and “Measurement Electronics”. She has been Research Scientist in the Laboratory of Electron Spectroscopy, Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, since 2000.

Elka I. Karakoleva received in 1978 the B.S. degree from the Faculty of Physics of the University of Sofia. In 1979 she joined the Department of Nuclear Reactor Physics at the Institute of Nuclear Physics and Nuclear Energy of the Bulgarian Academy of Sciences, Sofia. Since 1990 she has been working as a Research Associate at the Department of Optics and Spectroscopy, Institute of Solid State Physics of the Bulgarian Academy of Sciences, Sofia. Her current interests are in the field of the development of fiber-optics elements and the optical waveguide modeling.

Nikolay N. Nedyalkov received in 1998 M.S. degree in Quantum Electronics at the Faculty of Physics of St. Kliment Ohridski University of Sofia. He obtained Ph.D. degree in Physics of Wave Processes in 2005 in the Institute of Electronics, Bulgarian Academy of Sciences. His research topics include laser–matter interaction, plasmonics, optical properties of metal nanoparticles, ultrashort laser nanostructuring. N. Nedyalkov holds a permanent position as a Senior Research Associate at the Institute of Electronics, Bulgarian Academy of Sciences.

Andrey Ts. Andreev graduated in 1975 from the Faculty of Physics of St. Kliment Ohridski University of Sofia. He received his Ph.D. degree in the Lebedev Institute of Physics, Russian Academy of Sciences, Moscow, in 1981, in the field of optical-fiber loss measurements. Since 1981 he has been working as a Research Associate and Senior Research Associate (1994) in the Department of Optics and Spectroscopy at the Institute of Solid State Physics, Bulgarian Academy of Sciences. His current interests are in the field of fiber-optics components and sensors.