Investigations on Work Functions of Gasochromic Color Dyes as Gate Materials for FET Based Gas Sensors

Investigations on Work Functions of Gasochromic Color Dyes as Gate Materials for FET Based Gas Sensors

Available online at www.sciencedirect.com ScienceDirect Procedia Engineering 87 (2014) 108 – 111 EUROSENSORS 2014, the XXVIII edition of the confere...

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Available online at www.sciencedirect.com

ScienceDirect Procedia Engineering 87 (2014) 108 – 111

EUROSENSORS 2014, the XXVIII edition of the conference series

Investigations on work functions of gasochromic color dyes as gate materials for FET based gas sensors Carolin Petera,*, Dominik Zimmermannb, Daniel Knopa, Sven Rademachera, Ina Schumachera, Ingo Freundb, Jürgen Wöllensteina a

Fraunhofer Institute for Physical Measurement Techniques, 79110 Freiburg, Germany b Micronas GmbH, 79110 Freiburg, Germany

Abstract We present investigations on gasochromic color dyes as gas sensitive gate materials for field-effect transistor (FET) based gas sensors. Therefore the work function of two color dyes N’,N’,N,N-tetramethyl-p-phenylenediamine (TMPD) and bromphenolblue (BPB) were characterized using a Kelvin probe. TMPD is selective to NO2 and shows changes in the output of the Kelvin probe of 50 mV during exposure to 0.5 ppm NO2. BPB is an indicator for ammonia. Even 3 ppm NH3 cause a change of 10 mV. In addition, the gasochromic materials are highly selective to only one gas. The results show, that these dyes are suitable as sensitive gate materials and offer the possibility to build a low-power FET, detecting NO2 and NH3 in air at room temperature. © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2014 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-reviewunder underresponsibility responsibility of the scientific committee of Eurosensors Peer-review of the scientific committee of Eurosensors 2014 2014. Keywords: gas sensor; low-power; gasochromic color dye, gate material; FET

1. Introduction Today the use of low-cost sensors with low-power consumption is becoming increasingly important in many applications. Especially for battery-operated systems low power gas sensors are required. Often metal oxide gas sensors (MOX sensor) are in use. They are cost effective, robust and sensitive, but with the lack of selectivity and high operation temperatures up to 450 °C.

* Carolin Peter. Tel.: +49-761-8857-731. E-mail address: [email protected]

1877-7058 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the scientific committee of Eurosensors 2014 doi:10.1016/j.proeng.2014.11.394

Carolin Peter et al. / Procedia Engineering 87 (2014) 108 – 111

GASFETs are a possible alternative to MOX-sensors. GASFETs can detect changes in conductivity, capacitance or work function of a gate material due to the gas exposure [1] at room temperature. Many gate materials can be used as gas sensitive layers, like metals, oxides, salts, polymers and organic color dyes. 2. Gasochromic color dye We investigated changes in the work function of two different color dyes during exposure to NO2 and NH3, respectively. For NO2 detection we used N’,N’,N,N-tetramethyl- p-phenylenediamine (TMPD). TMPD is a paraphenylenediamin and belongs to the group of chinonimin dyes, also known as Kovac’s reagent or Wurster’s blue. The color change is a double-stage reaction. Due to the oxidation caused by NO2, the para-phenylenediamin provides an electron and forms the blue cation. By providing a second electron, the cation forms colorless chinondiimin. The equilibrium is on the side of the first, blue cation. Fig. 1 shows the complete double-step reaction mechanism.

Fig. 1: Two-step oxidation of TMPD due to NO2. The reaction results in a reversible color change from brown to blue.

For NH3-detection the pH-indicator bromphenol blue (BPB) was used. Its reaction to gaseous ammonia is shown in fig. 2. The strong basicity of NH3 leads to a splitting of the hydroxyl group of acid BPB. This acid-base reaction results in a visible color change, as the protonated form is yellow and the deprotonated one blue.

Fig. 2: Acid-base reaction of gaseous ammonia to the pH-Indicator BPB. The color dye changes its color reversible from yellow (protonated form) to blue (deprotonated form).

3. Experimental This section describes the development of the two different color dye matrices and the sample preparation for Kelvin probe measurements. 3.1. Sample preparation of kelvin probes To obtain stable gas sensitive films the color dyes have been embedded into gas permeable and optically transparent polymer matrices. TMPD has been embedded into a poly vinyl chloride (PVC) matrix. Therefore, 10 mg of PVC powder were diluted bit by bit in 100 ml THF under ambient condition. After complete dilution 2 g TMPD were added. To avoid rough and cracked films, 10 ml plasticizer (Hexamoll Dinch, provided by BASF) were diluted too.

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The BPB solution was made under similar conditions. Instead of PVC we used an ethyl cellulose matrix. Therefore, 125 mg ethyl cellulose were stirred into 5 ml ethanol under ambient conditions for 1 hour. After complete dilution we added 425 µl tributyl-phosphate as plasticizer and 20 mg BPB. For sample preparation 100 µl of each solution were pipetted directly on the substrates. After drying for 24 hours to provide evaporation of solvent residues, the substrates are prepared for the measurement. 3.2. Screen-printing of NH3 matrix Investigations have been performed on screen-printing of the films as an alternative deposition method. Screenprinting offers the possibility of simultaneous deposition of many samples. For this technique, the matrix has to by highly viscous. The color dye was also diluted into a matrix of ethyl cellulose. But, compared to matrix above, 4 g ethyl cellulose were stirred into 40 ml ethanol. After complete dilution 1.5 ml plasticizer (tributyl-phosphate) and 20 mg pH indicator were added. For first tests we added the pH indicator BCG (bromcresol green) instead of BPB. They have the same working principle, but different points of changes. The meshes of the used screen have the dimensions of 0.6x1.35 mm2. There are always two meshes directly next to each other with a space of 0.2 mm. Conventional Si-wafers were used as substrates. The squeegee was pressed with 2.5 bar over the substrate. Fig. 3 (left) shows a picture of the screen-printed layer directly after the printing process, fig. 3 (right) the according profilometer scan in transverse direction. The resulting layers are homogeneous and have an average thickness of 1.8 µm. The edge regions are, by the drying of the paste, wavy, with a lateral extent of a few microns.

Fig. 3: Left: picture of the screen-printed layer directly after printing. Right: profilometer scan of one rectangle in transverse direction. The layers have an average thickness of 1,8 µm.

4. Measurements and results The following measurements were carried out using a Kelvin probe in ambient filtered pressurized air with 40% r.H. and ambient room conditions. Fig. 4 (left) shows the measured work function changes of TMPD under NO2 atmosphere, in steps of 0.3 ppm, 0.5 ppm, 1 ppm, 3 ppm and 10 ppm NO2 each one for 30 minutes. After gas reaction, the layers were exposed to synthetic air again. The gas exposure leads to changes of 50 mV for 0.5 ppm and 350 mV for 10 ppm NO2. Lower gas concentrations than 0.5 ppm generate only a slow and time shifted output signal. Fig. 4 (right) shows the according changes in the work function of BPB exposed to different NH3 concentrations from 3 ppm, 5 ppm, 10 ppm, 30 ppm to 50 ppm, each one also for 30 minutes. The exposure of 3 ppm leads to changes in the work function of 10 mV, higher gas concentrations of 50 ppm to higher voltages of 40 mV.

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Fig. 4: Left: changes in the work function of TMPD according to the NO2 concentration between 0.3 ppm and 10 ppm. Right: according changes in the work function of BPB under NH3 deposition at different concentrations between 3 ppm and 50 ppm. All measurement were taken out in filtered pressurized air at 40 % r.H..

5. Conclusions The results show that the work function change of gasochromic color dyes are suitable for measuring gas concentrations. In addition, the gas reaction is highly selective to the target gas. A changing humidity influences the sensor signal due to the water adsorption of the polymer matrix. These investigations show the high capability of gasochromic materials as sensitive layers for FET based gas sensors. Acknowledgements We would like to thank the German BMBF and the Leading Edge Cluster Microtec Südwest for funding within the Project Minergy. References [1] I. Eisele, T. Doll and M. Burgmair, Low power gas detection with FET sensors, Sensors and Actuators B 78 (2001) 19-25 [2] C. Peter, S. Schulz, M. Barth, M. Gempp, S. Rademacher an J. Wöllenstein, Low-cost roll-to-roll colorimetric gas sensor system for fire detection, Soli-State Sensors, Actuators and Microsystems, 2013 Transducers & Eurosensors [3] J. Courbat, D. Briand, J.Wöllenstein, N.F. de Rooij, Colorimetric gas sensors based on optical waveguides made on plastic foil, Procedia Chemistry (2009) 576-579