Elsevier

Materials Chemistry and Physics

Volume 181, 15 September 2016, Pages 7-11
Materials Chemistry and Physics

Materials science communication
ZnO nanorods/polyaniline heterojunctions for low-power flexible light sensors

https://doi.org/10.1016/j.matchemphys.2016.06.061Get rights and content

Highlights

  • ZnO NRs/polyaniline p-n junction photodetectors were fabricated on flexible substrates.

  • The fabricated ZnO NRs grew along the (002) direction.

  • The fabricated ZnO NRs have low compressive strain.

  • The ZnO NRs/PAni junction showed a high sensitivity of 85%.

  • The photodetectors showed quantum efficiency as high as 12%.

Abstract

Zinc oxide nanorods (ZnO NRs) were directly grown on p-type polyaniline (PAni)/polyethylene terephthalate (PET) using chemical bath deposition method at low temperature. Field emission scanning electron microscopy and X-ray diffraction techniques were used to study the morphology and structure of the fabricated films. The resulted ZnO NRs are hexagonal and grew vertically on the PAni surface in the (002) direction along the c-axis. The compressive strain, Raman and photoluminescence measurements confirmed the high-quality crystal structure of the formed ZnO NRs with no damage of the PAni surface. The photodetector made using ZnO NRs/PAni junction showed a sensitivity of 85% and a quantum efficiency of 12.3% at 5 V.

Introduction

Flexible sensors are playing an increasingly paramount role in optoelectronics applications [1]. While the most sizably voluminous market is currently for glucose sensors utilized by diabetics, other types of flexible sensors are emerging [1]. IDTechEx forecasts that the market for flexible sensors will have incremented by more than $1 billion by 2020 [2]. To this end, several types of flexible polymeric materials, such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN), are currently considered due to their exceptional dielectric properties, corrosion resistance, high processing temperature between 196 and 260 C, low coefficient of friction, and low cost among other credentials. In particular, PET is more advantageous over indium-doped tin oxide (ITO) and fluorine-doped tin oxide (FTO) due to its high transparency, good flexibility, availability on industrial scale, and low cost [3].

On the other hand, ZnO nanorods have recently attracted consequential attention due to their unique shape and structure resulting in remarkable piezoelectric, magnetic, and optoelectronic properties. Specifically, the high optical gain of ZnO [4] makes it the material of focus in optoelectronics and sensing applications. However, the difficulty of doping ZnO to p-type polarity has led the researchers to seek to create heterojunctions with other p-type semiconductors to enable the use of ZnO in a plethora of electronic devices [1]. In particular, the combination of ZnO with polymers is a current hot research focus [1]. In this regard, most of ZnO NRs/polymer-based heterostructures are synthesized through surface coating of the polymer layer, with very limited reports on the direct growth of ZnO NRs on conducting polymeric substrates. To this end, aqueous chemical growth method has been greatly employed to assemble ZnO nanorods on a variety of substrates including amorphous, polycrystalline, single crystalline, and flexible substrates [2], [3], [4], [5], [6].

Herein, we report the successful fabrication of high quality, vertically aligned ZnO nanorods/polyaniline heterojunctions on PET flexible substrates and investigate their applicability as flexible light sensors. FESEM, XRD, PL and Raman characteristics of the fabricated heterojunctions are discussed.

Section snippets

Materials and methods

PET substrates were cleaned using 2-propanol and acetone to remove any contamination on the surface. Polyaniline emeraldine salt powder (Sigma Aldrich) was dissolved in formic acid under stirring for 30 min. Then, thin PAni films were fabricated by coating polyaniline solution onto the PET substrates, and kept in air for 1 h at room temperature. In the second stage, a radio frequency (RF) magnetron system with a ZnO target (99.999% purity) was used to deposit ZnO seed layer onto the PAni thin

Results and discussion

Fig. 2 shows the morphology of the fabricated ZnO nanorods grown on the ZnO seed layer/PAni substrates. Vertically oriented, high-density, hexagonal ZnO nanorods with diameters ranging from 73 nm to 87 nm and lengths of 1.4 ± 0.1 μm were successfully obtained. No delimitation was observed, highlighting the importance of the ZnO seed layer. In order to grow the best ZnO (NRs) by chemical bath deposition method, ZnO seeds were needed as nuclei sites for the growth of the ZnO (NRs) [7], [8], [9].

Conclusion

Low cost, high quality ZnO nanorods were grown on p-type Anipolymeric films via chemical bath deposition and coating methods. Phase analysis studies showed that ZnO nanorods/PAni exhibited a strong (002) diffraction peak, confirming the preferential growth along the c-axis. The high PL intensity of the peak appeared in the UV region revealed the high quality of ZnO NRs on PAni, as also confirmed by Raman spectroscopy. The ZnO NRs/PAni-based photodetectors offered excellent stability over time.

Acknowledgments

The authors would like to thank the Nano-Optoelectronics Research and Technology (N.O.R.) of the School of Physics, Universiti Sains Malaysia.

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    Permanent address: Polymer Research Center, University of Basra, Iraq.

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