Polymer functionalized nanostructured porous silicon for selective water vapor sensing at room temperature

https://doi.org/10.1016/j.spmi.2017.02.044Get rights and content

Highlights

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    Study of structural properties of porous silicon samples after surface modification.

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    Enhanced sensitivity was observed for functionalized samples.

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    Room temperature operated water vapor sensor using functionalized porous silicon.

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    Role of polymer functionalization in improving sensing performance.

Abstract

This paper highlights the surface treatment of porous silicon (PSi) for enhancing the sensitivity of water vapors at room temperature. A simple and low cost technique was used for fabrication and functionalization of PSi. Spin coated polyvinyl alcohol (PVA) was used for functionalizing PSi surface. Morphological and structural studies were conducted to analyze samples using SEM and XRD/Raman spectroscopy respectively. Contact angle measurements were performed for assessing the wettability of the surfaces. PSi and functionalized PSi samples were tested as sensors in presence of different analytes like ethanol, acetone, isopropyl alcohol (IPA) and water vapors in the range of 50–500 ppm. Electrical measurements were taken from parallel aluminium electrodes fabricated on the functionalized surface, using metal mask and thermal evaporation. Functionalized PSi sensors in comparison to non-functionalized sensors depicted selective and enhanced response to water vapor at room temperature. The results portray an efficient and selective water vapor detection at room temperature.

Introduction

Water vapor sensing is being studied and tested for the past three to four decades now. The water vapor measurement is done in growing plants, electrical devices, food industry, wireless sensors and environmental monitoring [1], [2], [3], [4]. The importance of water vapor measurement led to the development of sensors with higher sensitivity, response time and longer stability. The different types of humidity sensors are based on water phase protonic ceramic materials, polyvinyl alcohol dielectric films, polymers, multi wall carbon nano tubes, pristine carbon nano tubes, resistive and capacitive configuration [5], [6], [7], [8]. All these different materials are used in the development of sensors because of their hydrophobic/hydrophilic property. Functionalization plays a key role in improving selectivity, surface properties and stability of surface. Room temperature sensing remains a challenge as most of the sensors operate at higher temperatures [9], [10]. There is a great emphasis on preparation of nano-sensors which can function at room temperature. Nano-porous silicon is a network of nano crystallites with large number of nano pores and known for its tunable morphology, enhanced sensitivity and large surface area-to- volume ratio [11], [12], [13]. Functionalization of such a surface can make it selective for a particular gas/vapor and also improve its performance.

In the present work, the fuctionalization is done by poly vinyl alcohol (PVA), which is a water-soluble polymer was spin coated on PSi for two cycles at constant rpm. PSi and functionalized PSi samples have been analyzed using field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), Raman Spectroscopy, Photoluminence, and contact angle measurements. Sensors based on both PSi and its counterpart were tested in real time in presence of different analytes in the range of 50–500 ppm.

Section snippets

Fabrication process

Sensor fabrication process was done using p-type silicon (Si) wafer with resistivity 1–10 Ω-cm. Wafer cleaning was performed using degreasing followed by piranha cleaning. Since, native oxide gets formed on the surface after piranha cleaning therefore before PSi fabrication, the wafers were dipped in buffered hydrofluoric acid (BHF). PSi fabrication was done using electrochemical etching using electrolyte solution of HF and ethanol in 1:1 ratio. There are few important parameters which effect

Morphology analysis

The morphology information of any porous material can be obtained in detail by using SEM micrograph. Fig. 2(a) shows the SEM of as-anodized PSi. The homogeneous distribution of the pores on the surface of the samples is clearly visible with an average pore size of around 12 nm. Fig. 2(b) depicts the cross-sectional SEM of PSi with the approximate pore depth around 3 μm. Fig. 2(c) visualizes the morphology of PVA functionalized PSi. It shows a flaky structure on top of PSi surface. This confirms

Sensing studies

All samples were tested in presence of different analytes like ethanol, acetone, IPA and water vapor in wide range of concentration 50–500 ppm. A change in resistance was measured through parallel electrodes on the PSi and PVA functionalized PSi surface, upon exposure to analytes. The value of resistance Rair and Ranalyte are the resistances measured in air and analyte environment respectively. Sensor response, (S) is defined as the ratio of change in resistance in presence of analyte ΔR=R

Conclusion

Sensors based on PSi and PVA functionalized PSi were fabricated, characterized, and tested in presence of VOCs and water vapors at room temperature in wide range of ppm. The role of functionalization by PVA was explored on sensing properties of PSi. Functionalized PSi had shown highly sensitive and selective response for water vapors. A simple one post processing step of PVA spin coating modified the properties of porous surface and an enhanced response to water vapors was obtained. Such

Acknowledgement

Authors would like to acknowledge Nanoscale Research Facility (N.R.F) and Central Research Facility (C.R.F) IIT Delhi for providing characterization facilities. Authors would like to thank Sophisticated Analytical Instrumentation Facility (SAIF) at IIT Bombay for FE-SEM analysis. First author would like to acknowledge MHRD, Government of India for providing the financial assistantship for research. Third author would like to thank Department of Science and Technology (DST), Government of India

References (22)

  • A.A. Haidry et al.

    Hydrogen gas sensors based on nanocrystalline TiO2 thin films

    Cent. Eur. J. Phys.

    (2011)

    • Cited by (5)

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