Contents lists available at ScienceDirect Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp Fast response relative humidity clad-modified multimode optical fiber sensor with hydrothermally dimension controlled ZnO nanorods S. Azad a , E. Sadeghi a, ⁎ , R. Parvizi a , A. Mazaheri b a Department of Physics, College of Sciences, Yasouj University, Yasouj 75914-353, Iran b University institute of applied science/optic and laser research center, Isfahan 8315713115, Iran ARTICLE INFO Keywords: Humidity sensor Hydrothermal synthesis Morphology Multimode optical fiber Optical fiber sensor ZnO nanorods ABSTRACT This work investigates the growth of dense and well-aligned Zinc Oxide (ZnO) nanostructure on multimode optical fiber through low temperature hydrothermal technique and its application as Relative Humidity (RH) sensor. Exponentially decaying evanescent fields were exploited for developing intensity modulated fiber optic sensors. To maximize evanescent fields in the sensing region, a 125 μm diameter optical fiber was chemically etched to 28 μm. It accordingly gives rise to a significantly enhanced interaction between the device and ambient environment. ZnO nanorods were grown on the 28 μm diameter optical fiber with different precursor concentrations and pH values. The nanorods with high density and uniformity were obtained for 0.01 M concentration. The developing samples were characterized by X-Ray Diffraction (XRD) pattern, Scanning Electron Microscopy (SEM) and Photoluminescence (PL) spectroscopy. Experimental results revealed a significant increase in the height and width of nanorods by reducing pH, which led to red-shift of the PL peaks. Sensitivity was investigated in two different intervals; lower and higher than 55% RH such that further variation of the output light intensity was observed in the former case. Comparative sensing experiments demonstrated response/recovery time decreased to 2.3 s/5.2 s for sample with 0.01 M concentration. Significant features, such as fast response and recovery time, make the developed device a promising candidate in the deployment of future networks particularly for long-haul telecommunications. 1. Introduction Monitoring humidity is essential in various chemical industries, intelligent control of the living environment in buildings and biological products [1–4]. Depending on the applications, a humidity sensor requires high sensitivity, accuracy, low noise, small dimension and low cost [5–7]. Various types of humidity sensor such as resistive [8], capacitive [5] and optical sensors [9–11] have been deliberated and reported in the literature. Optical sensors exhibited high sensitivity, wide detection range and fast response. One way to fabricate optical sensors is employing optical fibers coated with nanostructures. Nanos- tructures (e.g. metal oxides) coating on the etched optical fiber induce light intensity variation in response to an external stimulus. Zinc oxide (ZnO) is one of the most studied oxide semiconductors that has a wide band gap of approximately 3.3 eV, along with a large exciton binding energy of 60 meV (at room temperature). These nanostructures might find applications in electronic, optoelectronic, electrochemical and energy harvesting devices [12–17], such as solar cells [18], photocatalysts [19], nanogenerators [20] and nanosensors [21]. A variety of ZnO nanostructures, such as nanowires, nanorods, nanocombs, nanobelts, and nanorings have been reported in the literature [22]. Among different morphologies of ZnO, one-dimensional structure has unique property of high surface-to-volume ratio and is relatively simple to grow in aqueous solutions due to the polar nature of the (002) crystal plane [15,17]. Various techniques have been devoted to synthesize ZnO nanorods, including physical and chemical vapor deposition [23,24] and vapor transport [25]. These techniques involve high temperature and complex vacuum environment during the growth process. A solution-based hydrothermal method of growing ZnO nanorods is energy-efficient, simple and environmentally friendly [26,27]. In recent decades, hydrothermally growing nanorods on the planer substrates have been studied intensively [28–32]. However, few publications investigated the well-arrayed dense nanorods on the curved surfaces such as optical fiber platform [33,34]. ZnO nanorods, with relative larger refractive index than silica, coated optical fiber induced more interaction between fiber-guided light and the ambient environment. This hybrid structure opens up enormous possibilities for various types of sensing application owing to special features such as http://dx.doi.org/10.1016/j.mssp.2017.04.024 Received 17 November 2016; Received in revised form 10 April 2017; Accepted 24 April 2017 ⁎ Corresponding author. E-mail address: sadeghi@yu.ac.ir (E. Sadeghi). Materials Science in Semiconductor Processing 66 (2017) 200–206 1369-8001/ © 2017 Elsevier Ltd. All rights reserved. MARK