Single fiber UV detector based on hydrothermally synthesized ZnO nanorods for wearable computing devices

Applied Surface Science 428 (2018) 233–241

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Single fiber UV detector based on hydrothermally synthesized ZnO nanorods for wearable computing devices Tae Hoon Eom, Jeong In Han ∗ Department of Chemical and Biochemical Engineering, Dongguk University-Seoul, 04620, Seoul, South Korea

a r t i c l e

i n f o

Article history: Received 27 June 2017 Received in revised form 7 September 2017 Accepted 15 September 2017 Available online 18 September 2017 Keywords: ZnO nanorod Single fiber Wearable computing device UV detector RF magnetron sputtering

a b s t r a c t There has been increasing interest in zinc oxide (ZnO) based ultraviolet (UV) sensing devices over the last several decades owing to their diverse range of applications. ZnO has extraordinary properties, such as a wide band gap and high exciton binding energy, which make it a beneficial material for UV sensing device. Herein, we show a ZnO UV sensing device fabricated on a cylindrical Polyethylene terephthalate (PET) monofilament. The ZnO active layer was synthesized by hydrothermal synthesis and the Cu electrodes were deposited by radio frequency (RF) magnetron sputtering. Cu thin film was deposited uniformly on a single PET fiber by rotating it inside the sputtering chamber. Various characteristics were investigated by changing the concentration of the seed solution and the growth solution. The growth of ZnO nanorods was confirmed by Field Emission Scanning Electron Microscopy (FESEM) to see the surface state and structure, followed by X–ray Diffraction (XRD) and X–ray photoelectron spectroscopy (XPS) analysis. Also, current–voltage (I–V) curves were obtained to measure photocurrent and conductance. Furthermore, falling response time, rising response time, and responsivity were calculated by analyzing current-time (I–t) curves. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Due to the tremendous growth of the internet and electronic devices, various technologies have been combined and integrated. Internet of Things (IOT) technology has emerged as the best application of combined high–tech industries because it collects enormous quantity of data through the internet, sensor networks, and computing devices [1,2]. By collecting data from our everyday life through sensor networks, especially through smart textiles, big data is being created in numerous ways [3–6]. Smart textiles are fabrics that respond to environmental changes thanks to embedded electronic materials [7]. The responding features contribute smart textiles perform as various types of sensors and actuators. Plenty of devices have been fabricated which react to temperature [8,9], humidity [10,11], pressure [12,13] and ultraviolet (UV) [14]. Among these diverse devices, UV sensing devices have been actively developed and have appeared in markets over the past decades. Nitride based materials such as GaN, AlGaN, InN Inx Ga1-x N have been widely researched for UV detectors [15]. Recently, zinc oxide (ZnO) has become one of the most promising and ideal metal oxides for UV sensing device owing to the large surface area to volume

∗ Corresponding author. E-mail addresses: [email protected] (T.H. Eom), [email protected] (J.I. Han). http://dx.doi.org/10.1016/j.apsusc.2017.09.127 0169-4332/© 2017 Elsevier B.V. All rights reserved.

ratio, wide bandgap of 3.37 eV, high exciton binding energy of 60 eV, and photo–absorption in UV range [16–19]. Due to these advantages, nano–scaled ZnO particles have wide applications in such devices as solar cells [20,21], gas sensors [22], nanogenerators [23], and light emitting diodes [24]. Furthermore, ZnO is well suited to being a semiconducting layer due to the low cost and numerous different synthesis methods, including the sol–gel method, solvothermal method, and hydrothermal method. Different kinds of ZnO nanostructures are created based on the synthesis methods, such as nanoparticle, nanorod, nanoneedle, nanowire, nanobelt, nanotube, and nanoflower. This present work focused on ZnO nanorods and nanowires, which were synthesized in situ via hydrothermal synthesis using seed solution and growth solution. Plenty of resources exist which detail this two–step ZnO synthesis. First, a substrate is coated by a seed layer consisting of ZnO precursors, commonly by spin coating on a Si substrate, and then ZnO nanorods are grown hydrothermally. Zinc acetate dehydrate can be used as a precursor for the seed solution while Zinc nitrate hexahydrate and HMT can be used for the growth solution precursor [25–27]. This method is appropriate for synthesizing ZnO on a planar substrate; therefore, our research group used dip–coating to create the seed layer coating on a non–planar PET monofilament. Hexamethylenetetramine (HMT) was also used in the growth solution in this work to grow ZnO into a rod–shape.

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Fig. 1. (a) Schematic diagram of ZnO nanorod based UV sensing device (b) absorbance spectrum of ZnO nanorod (c) effect of [S]:[G] ratio on atomic percentage of Zn based on EDS spectrum.

Due to the variety of synthesis methods, ZnO has been synthesized on diverse substrates in the literature. Several researchers have fabricated flexible ZnO UV detectors on polymer and fiber substrates. Zhang et al. reported a ZnO UV detector based on flexible poly urethane (PU) fibers; however, the PU fibers were woven together, resulting in a rough surface and low photocurrent [28]. Athauda et al. introduced ZnO nanowires grown hydrothermally on nylon fibers [29]. Similarly, these nylon fibers were also woven together. Liu et al. synthesized ZnO on Kevlar fibers for UV sensing devices. ZnO nanowires were synthesized vertically in a furnace; however, the temperature of the furnace was extremely high, ranging from 380 ◦ C to 510 ◦ C [30]. These are the constraints of woven fiber devices. Single fibers, such as a PET monofilament, are required for wearable devices. However, deposition of thin films on cylindrical fibers remains a problem. Several methods, such as electroless plating, have been used to deposit an electrode layer on a PET fiber, but the surface was less smooth than sputtering deposition [31]. Jang et al. deposited ZnO seed layer by RF magnetron sputtering on a carbon fiber [32]. Also, Ag nanowires were coated by spray coating method which were used as electrode. However, without any rotation device, the uniformity of ZnO seed layer could not

be fine. In addition, the characterization of the device was performed after placing bundle of fibers laterally. The deposition of Cu thin films in our work was performed by a process that our research group designed to apply thin metal films to various sensors [33–36]. Various metal thin films were deposited uniformly on a PET monofilament by rotating the filament sequentially during the sputtering process. Also, the measurement of electrical features was held by a single fiber which is appropriate for wearable computing devices. In this work, our research group successfully synthesized ZnO nanorods grown on a flexible PET monofilament. The PET fiber was maintained as a single fiber with high thermal stability. A Cu thin film electrode was deposited by radio frequency (RF) magnetron sputtering. The whole PET monofilament was immersed in a seed solution followed by vertical growth in a growth solution. The molecular concentration of both the seed and growth solutions varied between 4 different values, expressed as [S]:[G], and the results demonstrated the effect of the molecular ratio of seed solution and growth solution. The surface of the ZnO nanorods was investigated by FESEM, and various UV sensing properties were studied using I–V and I–t characteristic curves.

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Fig. 2. ×5000 Magnification FESEM image of crystal structure of synthesized ZnO. (a) for [S]:[G] = 1:10 (b) for [S]:[G] = 4:1 (c) for [S]:[G] = 10:1 (d) for [S]:[G] = 20:1. X20000 magnification FESEM image of crystal structure of synthesized ZnO (e) for [S]:[G] = 1:10 (f) for [S]:[G] = 4:1 (g) for [S]:[G] = 10:1 (h) for [S]:[G] = 20:1.

Fig. 3. ×200 Magnification FESEM image of surface morphology of ZnO layer. (a) for [S]:[G] = 1:10 (b) for [S]:[G] = 4:1 (c) for [S]:[G] = 10:1 (d) for [S]:[G] = 20:1. X20000 magnification FESEM image of ZnO nanorods and nanowires (e) for [S]:[G] = 1:10 (f) for [S]:[G] = 4:1 (g) for [S]:[G] = 10:1 (h) for [S]:[G] = 20:1.

2. Experimental section 2.1. Cu electrode deposition Cu thin films were deposited on bare cylindrical PET monofilaments by RF magnetron sputtering. Bare PET monofilaments had

diameters of 300 ␮m and lengths of 60 mm. To obtain clean and linear monofilament surfaces, PET monofilaments were baked on a 100 ◦ C hot plate for 30 min, followed by cleaning. PET monofilaments were cleaned in a sonication tube using methanol for 10 min and acetone for 10 min. Due to the cylindrical shape of PET monofilaments, they were rotated in the sputtering chamber to obtain a

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highly uniform surface [33]. The sputtering power was 150 W, the working pressure was 3.15 × 10−2 Torr, and the working distance was 85 mm. Also, the flow rate of argon gas was 20 SCCM, and Cu was deposited for 10 min. The length of each Cu electrode was 10 mm and the distance between them was 1 mm. 2.2. Preparation of the seed solution and the growth solution The ZnO layer was synthesized by a simple hydrothermal method and our research group synthesized various nanoparticles through hydrothermal method [37]. For the synthesis of ZnO nanorods, a ZnO seed solution and a ZnO growth solution were prepared [29]. The ratio of ZnO seed solution to ZnO growth solution ([S]:[G]) was varied with four different values (0.1, 4, 10, and 20). The molarity of the seed solution was fixed to 100 mM while that of the growth solution was changed (1000 mM, 25 mM, 10 mM, and 5 mM). To prepare the 100 mM seed solution, 2.2 g of Zn (CH3 COO)2 ·2H2 O was dissolved in 100 ml of Isopropyl alcohol (IPA). Then, 40 ml of NaOH (20 mM) was added dropwise to the above solution. The process was stirred continuous using magnetic stirring and incubated at room temperature for an additional 3 h [29,38]. Meanwhile, ZnO growth solution was made by mixing Zn(NO3 )2 ·6H2 O and C6 H12 N4 . To create the 1000 mM growth solution, 28.04 g of HMT and 59.50 g of zinc nitrate hexahydrate were dissolved and mixed with 200 ml of deionized water. The solution was stirred for 24 h. 2.3. Growth of ZnO nanorod on a PET monofilament The PET monofilament was immersed into the ZnO seed solution via dip–coating method followed by baking at 150 ◦ C. The dip–coating procedure and the post–baking procedure were repeated 3 times until a uniform ZnO surface was obtained. The ZnO growth solution was transferred to the Teflon autoclave and ZnO seed solution coated PET monofilament was dipped into the ZnO growth solution. ZnO nanorods were grown in the autoclave at 95 ◦ C for 8 h in a dry oven. Afterwards, ZnO nanorods were grown larger by holding at room temperature for 10 h. Finally, the white product was collected and washed with deionized water several times. 2.4. Characterization and measurement of the electrical properties I–V characteristic curves of different [S]:[G] ratios were measured with varying UV powers (195 ␮W cm−2 , 360 ␮W cm−2 , 580 ␮W cm−2 , 770 ␮W cm−2 , 1080 ␮W cm−2 ) by the Keithley 6517A. The voltage ranged from −2 V to 2 V and the voltage step was 0.05 V. For precise measurement, samples were put into a black box during UV curing. I–t curves were also measured by the Keithley 6517A, and the rising and falling response times were calculated. To demonstrate the surface morphology of the Cu electrodes, ZnO nanorods, and nanoneedles, FESEM (JEOL, JSM–7100F) was used. Impurities and structures of ZnO nanorods were evaluated by X–ray Diffraction Spectroscopy (XRD) and X–ray photoelectron spectroscopy (XPS). 3. Results and discussion The schematic diagram of ZnO UV sensing device is depicted in Fig. 1(a). Bare cylindrical PET monofilament was selected as a flexible substrate. A 10 mm long Cu thin film was deposited on the PET monofilament at room temperature by RF magnetron sputtering. Between the Cu electrodes, a 1 mm long ZnO layer was grown hydrothermally. The seed layer, containing zinc acetate dihydrate and NaOH, was coated on a PET monofilament to stimulate the

Fig. 4. X–Ray diffraction pattern of synthesized ZnO at distinct [S]:[G] ratio and bare PET.

growth of ZnO nanorods [39]. Due to the low viscosity of the seed solution, post–baking was performed repeatedly at 150 ◦ C. Once the PET monofilament was covered successfully by the seed layer, it was immersed in the prepared growth solution. The growth solution was composed of equimolar amounts of zinc nitrate hexahydrate and hexamethylenetetramine (HMT). The formation of ZnO nanorods is described by the following equations. (CH2 )6 N4 + 6H2 O → 6HCHO + 4NH3

(1)

NH3 + H2 O → NH+ + OH− 4

(2)

−

2+

2OH + Zn

→ ZnO (s) + H2 O

(3)

By reacting with deionized water, HMT decomposed to produce ammonia, which then reacts with water to generate a hydroxyl ion. Thus, HMT acted as a source of hydroxyl ions [40]. Zinc ion, expressed in equation (3), was derived from zinc nitrate hexahydrate. In this work, the [S]:[G] ratio was varied by changing the amount of these precursors. The shape and structure of synthesized ZnO varied with the amount of HMT used. The explanation of the morphology of ZnO is explained further later on in this manuscript. Fig. 1(b) shows the UV–vis spectra of ZnO nanorods. The electrons were transported through the ZnO nanorod when illuminated with UV light [41]. The absorbance of ZnO steadily increased as the wavelength was reduced to 400 nm. The highest absorbance was observed at a 360 nm wavelength. This demonstrates that the synthesized ZnO responds well to UV illumination. This is due to the adsorption and desorption of oxygen molecules to the ZnO surface. In the dark, the adsorbed oxygen molecules react with electrons to generate a depletion layer which reduces the conductivity of the ZnO UV sensor. In contrast, under UV illumination, electron–hole pairs are created by light absorption and can move through the depletion layer. Also, surface trapped O2− ions combine with holes,

1/2

Zn 2p

Zn 2p

Zn LMM c

Zn LMMd

Zn LMM b

1:10 4:1 10:1 20:1

C 1s

Zn 3s

Zn 3d

Zn 3p

[S]:[G]= [S]:[G]= [S]:[G]= [S]:[G]=

Zn LMM a O1s

(a)

237 3/2

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0

300

600

900

1200

Binding Energy (eV) [S]:[G]= 1:10 [S]:[G]= 4:1 [S]:[G]= 10:1 [S]:[G]= 20:1

Intensity (a.u.)

Zn2p

1014

1017

1020

1023

1026

1029

(c)

O1S

[S]:[G]= 1:10 [S]:[G]= 4:1 [S]:[G]= 10:1 [S]:[G]= 20:1

Intensity (a.u.)

(b)

528.0

530.4

532.8

535.2

537.6

Fig. 5. (a) XPS analysis of ZnO layer (b) Zn 2p (c) O 1s of the core level XPS spectra for the ZnO film.

increasing the electron concentration. Thus, by increasing electron concentration, the photoconductivity of ZnO UV sensor increases [42,43]. As stated above, ZnO is widely used in UV detecting devices due to its wide band gap and large binding energy. Furthermore, ZnO can be synthesized using various method and can have diverse chemical structures [39]. The mechanism of ZnO growth is divided into two stages which are nucleation and crystal growth. By adjusting the molecular concentration of ZnO seed solution, the behavior of the nucleation stage varies. This affects the surface of the ZnO thin film and consequently results in the photoconductive behavior of ZnO UV sensors [44]. Meanwhile, the atomic percentage of ZnO was investigated by Energy–dispersive X–ray spectroscopy (EDS) spectroscopy, as shown by the FESEM images in Fig. 2. The atomic percentage of Zn was 52.4% for [S]:[G] = 1:10, 48% for [S]:[G] = 4:1, 46.6% for [S]:[G] = 10:1, and 41% for [S]:[G] = 20:1. This result was plotted in Fig. 1(c) and the purity of ZnO was excellent [45]. It showed fine linearity and Zn% tended to decrease as the [S]:[G] ratio increased. In this work, the ZnO layer was synthesized hydrothermally using different [S]:[G] ratios. Fig. 2 shows the shape and the structure of synthesized ZnO nanorods. Synthesized ZnO with hexagonal cross sections, flat surfaces, and low aspect ratios were defined as ZnO nanorods [29]. In contrast, those which had sharp tips and high aspect ratios were defined as ZnO nanowires or ZnO nanoneedles. For [S]:[G] = 1:10, the length of the diagonal line was about 1.8 ␮m and the length of the ZnO nanorod was 4.5 ␮m. The length of diagonal line was 0.2 ␮m for [S]:[G] = 4:1, 0.07 ␮m for [S]:[G] = 10:1, and 0.03 ␮m for [S]:[G] = 20:1. Also, the length of ZnO nanorods was 6 ␮m for [S]:[G] = 4:1, 3 ␮m for [S]:[G] = 10:1, and 1.43 ␮m for [S]:[G] = 20:1. ZnO nanorods tended to grow longer and more

hexagonally with higher molecular concentrations of ZnO in the growth solution. By comparing the structures of each ZnO nanorod, it seemed that the ZnO nanorods for [S]:[G] = 1:10 has the best crystallinity and wurzite structure. As the [S]:[G] ratio increased, the shape of the ZnO structures changed to nanowires. In Fig. 2, as the [S]:[G] ratio increases, the crystallinity of ZnO nanorods is poorer and the size of ZnO crystalline is smaller. As the concentration of seed solution increase, the nucleation site for ZnO increases as well. Thus, as the nucleation of ZnO increases, ZnO growth is limited and the crystallinity of ZnO degrades. On the other hand, the less the nucleation of ZnO, the bigger ZnO growth and the better the crystallinity of ZnO. These result in the formation of ZnO nanorods and nanowires in Fig. 2 with various [S]:[G] ratio. Cylindrical PET monofilament substrate with 300 ␮m diameter could be observed by FESEM images in Fig. 3(a)–(d). Moreover, ZnO nanorods were grown vertically on PET monofilament with high orientation which is shown in Fig. 3(e)–(g). However, no orientation was found in grown ZnO nanowire in Fig. 3(h). This confirms that the crystallinity and orientation of ZnO nanorods is drastically affected by the ratio of precursors when the nanorods are synthesized hydrothermally. Comprehensive information about the structure of ZnO nanorods was obtained using XRD spectroscopy. Five different XRD patterns are plotted in Fig. 4. The XRD pattern of bare PET monofilament was measured initially and then XRD patterns were investigated for the four different [S]:[G] ratios. The shape and peak position of the bare PET spectra showed similar patterns in every spectrum, proving that PET monofilament was a good choice of substrate. Also, XRD patterns of ZnO parts had identical 2␪ values, but different intensities. All spectra showed six peaks, which occurred at 2␪ values of 31.5, 34, 36, 47.5, 56.5 and 62.5, demonstrating the

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Fig. 6. I–V characteristic curve of ZnO UV sensor (a) for [S]:[G] = 1:10 (b) for [S]:[G] = 4:1 (c) for [S]:[G] = 10:1 (d) Correlation between conductance and UV illumination power.

wurzite structure of ZnO nanorods [31,46,47]. The intensity of the (002) peak was lower than that of the (100) and (101) peaks, thus ZnO growth was not much preferred to c axis. Furthermore, it was confirmed that ZnO nanorods had a low aspect ratio, proving the definition of ZnO nanorods. Moreover, the overall intensity was higher in ZnO nanorods made using low [S]:[G] ratios. This indicates that the crystallinity and growth were nicer when using low [S]:[G] ratios, which is also shown in Fig. 2. No other peaks were observed except ZnO, so it was proven that no impurities existed in the ZnO layer. XPS spectroscopy was used for further investigation of ZnO nanorod formation, as shown in Fig. 5(a). The C, Zn, and O peaks were observed in every XPS spectra, demonstrating that ZnO was well synthesized. Among these many peaks, the Zn2p3/2 peak and O1s peak were individually plotted in Fig. 5(b) and (c). The binding energy of Zn2p3/2 peak was 1020.25 eV for [S]:[G] = 1:10, 1019.875 eV for [S]:[G] = 4:1, 1020.125 eV for [S]:[G] = 10:1, and 1020.375 eV for [S]:[G] = 20:1. The binding energy of the O1s peak was 529.125 eV for [S]:[G] = 1:10, 528.75 eV for [S]:[G] = 4:1, 528.875 eV for [S]:[G] = 10:1, and 529.25 eV for [S]:[G] = 20:1. Previously reported papers suggested that the binding energy of the Zn2p3/2 peak ranged from 1021.2 eV to 1022.7 eV while that of O1s peak ranged from 529.6 eV to 531.820 eV, [48–51]. The small shift in binding energy and weak intensity shown for the ZnO nanorod made with [S]:[G] = 1:10 was a result of abundant Zn–O bonding.

This result agrees well with the crystallinity of ZnO nanorods and XPS spectra. The crystallinity of ZnO was due to distinct wurzite structures and can be explained by a high atomic percentage of Zn. The tendency of photoelectric performance versus [S]:[G] ratio was equal to that of crystallinity of ZnO nanorods versus [S]:[G] ratio. The I–V curve of the ZnO UV sensor was characterized in Fig. 6. For wearable device applications, low voltage is required; therefore, the voltage range was set from −0.5 V to 0.5 V. The UV illumination intensity ranged from 195 ␮W cm−2 to 1080 ␮W cm−2 and the wavelength of UV light was 365 nm. The conventional UV illumination intensity value of previously reported UV sensors was typically around 1 mW cm−2 [41,52]. Our range was set according to this literature. All of the ZnO nanorods responded to UV illumination, as shown in Fig. 6, except ZnO nanorods made using the [S]:[G] = 20:1 ratio. As discussed above in Figs. 2 and 3, highly orientation growth and good crystallinity observed in the samples of [S]:[G] = 1:10, 4:1 and 10:1 were appeared in ZnO nanorods while no orientation growth and poor crystallinity in the sample of [S]:[G] = 20:1 ratio were appeared in ZnO nanowire. The formation of these highly oriented ZnO nanorods in the sample of high growth solution concentrations was observed in the literature [29]. Also, the higher the concentration of growth solution, the more the degree of the crystallinity of ZnO nanorods and the bigger the size of ZnO crystalline, as can be seen in Figs. 2 and 3. That is, the highly oriented and more

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Fig. 7. Photocurrent versus time in 5 distinct UV illumination intensity (a) for [S]:[G] = 1:10 (b) for [S]:[G] = 4:1 (c) for [S]:[G] = 10:1. Photocurrent versus UV power in diverse applied voltage range (d) for [S]:[G] = 1:10 (e) for [S]:[G] = 4:1 (f) for [S]:[G] = 10:1.

crystallized ZnO nanorods have the tendency of much more electric current generation with UV irradiation. This means that the randomly grown ZnO nanowire was not acceptable for electrons to flow through them. These factors such as the degree of orientation arrangement, crystallinity and size of ZnO nanorod influenced the electrical properties which resulted in barely no current flow in ZnO nanowires of randomly grown, poor crystalline and small size. Also, the I–V curves showed excellent linearity, indicating that ZnO UV sensors had Ohmic contact rather than Schottky contact [53]. Among them, ZnO nanorods made using high [S]:[G] ratios showed a significant current change, indicating good UV responsiveness. When UV intensity was 1080 ␮W cm−2 , the resistance was at its lowest, resulting in high current and good responsiveness to UV light. Also, current grew as UV light intensity increased for all ZnO nanorods. Resistance was calculated as the reciprocal of slope in I–V curves and conductance was defined as the reciprocal of resistance, as shown in Fig. 6(d). The conductance of ZnO nanorods made using [S]:[G] = 20:1 was not plotted in the figures due to their high resistance. The conductance varied from 0.016 ␮S to 0.23 ␮S for [S]:[G] = 1:10, 0.032 ␮S to 0.36 ␮S for [S]:[G] = 4:1, and 0.053 ␮S to 0.6 ␮S for [S]:[G] = 1:10. We confirmed that with a high [S]:[G] ratio, UV sensing devices with high conductance could be fabricated. To further study the electrical properties of ZnO UV sensing devices, current change was measured as a function of time. Under specific UV illumination intensity, maximum current was examined under identical voltage (4 V), as plotted in Fig. 7(a), (b), and (c).

For [S]:[G] = 1:10, the time needed to approach maximum current was drastically shorter than that of [S]:[G] = 4:1 and 10:1. This was shown in Fig. 8(a) as average rising response time and in Fig. 8(b) as average falling response time. The response time of ZnO UV sensing devices was shorter when UV illumination intensity was stronger, and the linearity between response time and UV illumination intensity was excellent. The short response time was due to the high concentration of photo–generated electron–hole pairs when under high UV illumination intensity [41]. In addition, ZnO nanorods with high [S]:[G] ratios showed higher current than that of ZnO nanorods with low [S]:[G] ratios. This result is in line with the I–V curves. In Fig. 7(d), (e), and (f), maximum current increases with higher applied voltages due to a higher electron escape energy from the ZnO surface [53]. Also, the maximum current for [S]:[G] = 1:10 was much higher than that of [S]:[G] = 4:1 and 10:1, and relatively higher than that from previously reported papers [48,54,55]. High maximum current and responsiveness were caused by good crystallinity of ZnO nanorods which acted well as an electron transport layer. Overall, ZnO nanorods made with [S]:[G] = 1:10 showed the best electrical properties, response time, conductance, and UV responsiveness. 4. Conclusions ZnO nanorods were hydrothermally synthesized at low temperature on a single PET fiber to create a UV sensing device.

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Fig. 8. (a) Rising response time of ZnO nanorods (b) Falling response time of ZnO nanorods (c) Responsivity of ZnO UV sensing device in diverse voltage range under 1080 ␮W cm−2 .

Hydrothermal synthesis was conducted using seed solution and growth solution, and the Cu electrode was deposited by RF magnetron sputtering. Deposition and growth were verified by FESEM image. Thanks to the rotating procedure during the sputtering, the surface state and uniformity was excellent. Various characteristic changed when the ratio of seed solution to growth solution was varied; therefore, four distinct ratios were selected to investigate this ratio’s effect on the electrical properties, UV responsiveness, and growth mechanism. ZnO was grown in the shape of a nanorod and displayed apparent wurzite structure in the case of [S]:[G] = 1:10, as proven by XPS and XRD spectra. This condition also had the highest photocurrent, highest responsiveness, and shortest UV response time. However, by increasing [S]:[G] ratio, ZnO was grown in the shape of nanowires rather than nanorods. Moreover, electrical features worsened, including photocurrent, responsiveness, and UV response time. This indicates that the growth mechanism was dramatically affected by [S]:[G] ratio, and [S]:[G] = 1:10 led to the best properties for UV sensing devices and wearable computing devices.

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