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Layer-by-layer Nano-assembled Polypyrrole Humidity Sensor

R. Nohria, R.K. Khillan, Y. Su, Y. Lvov and K. Varahramyan
Louisiana Tech University, US

Keywords:
nano-assembly, humidity sensor, polypyrrole

Abstract:
In recent years the influence of humidity has been given top priority in moisture sensitive areas such as high voltage engineering systems, storage areas, computer rooms etc. Conducting or semiconducting conjugated polymers show significant change in their electrical properties after exposure to humidity. A number of different techniques such as electrochemical polymerization, chemical and electrochemical deposition and spin coating have been applied for the fabrication of polymer humidity sensors[1, 2]. In this paper we report fabrication and comparison of highly sensitive and fast response humidity sensors of conducting polymer Polypyrrole (PPy) using Layer-by-Layer nano-assembly. PPy-based humidity sensors were also fabricated by spin coating and Drop-on-demand inkjet (IJ) printing for comparison. Layer-by-layer nano-assembly of poly(styrenesulfonate) (PSS), and PPy which act as polyanion and polycation respectively, was done on glass substrate. Five alternating precursor layers of (poly(allylamine)(PAH)/PSS) were deposited prior to self-assembly of ten polymer sensing layers. The overall thickness of the sensing layers is around 20 nm. A 5% wt. PPy solution was also spin coated at 3000 rpm for 25 seconds on glass substrate. The thickness of layer was measured to be 120nm.Water soluble conductive polymer PPy was used as an ink to be inkjet printed on glass substrate. Two layers of PPy (Fig. 1) were printed at the substrate temperature of 55 0C. The thickness of IJ printed sensor layer is about 150 nm as measured by the Tencor Alpha-step profilometer. The change of electrical resistance (Fig. 2) in the sensing polymer was monitored when the sensing element was exposed to humidity. These LbL assembled nano-films produced a fast change in resistance when exposed to humidity. The change in resistance was monitored for humidity ranging from 45 % to 90%. The experimental results show 10% decrease in resistance for the LbL-based PPy and 8% resistance change for the spin coated PPy and 11 % change in resistance for IJ printed PPy for every 5% increase in humidity. A comparison was made in terms of sensitivity and response time for the LbL, IJ printed and spin coated techniques. The response time is tested to be 25 seconds for the LbL-based sensor, 35 seconds for the IJ printed sensor and 60 seconds for spin coated sensor. The degradation was noted when the sensors were exposed to air environment for 7 days. Both LbL and IJ printed humidity sensor showed less drift in sensor baseline resistance as compared to spin coated humidity sensor. Moreover the degradation of the polymer sensor is being studied by using Fourier Transform Infra-Red spectroscopy. Such PPy humidity sensors find application in disposable handheld humidity detectors, due to low cost and ease of fabrication. References: 1. K. I. Arshak, et al. “Investigation into a novel humidity sensor operating at room temperature”, Microelectronics Journal, Vol 33, 2002, . 2. Shilpa Jain, et al. “Humidity sensing with weak acid-doped polyaniline and its composites”, Sensors and Actuators B: Chemical, Vol96, 2003

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