Growth dynamics of ZnO nanowire on a fiber-tip air bubble

We demonstrate a novel method of online monitoring of ZnO nanowire growth by an optical method. Nanowires were grown on the surface of an optical fiber tip and their effect on the spectral characteristics of a fiber-tip Fabry-Pérot interferometer (FPI) was investigated. The interference fringe of the fiber tip yielded a linear red shift against the ZnO nanowire growth. Results also indicated that the ZnO thin layer has a stronger correlation with the reflection spectra variation of the FPI than the ZnO nanowires. The growth rate of the ZnO thin layer was measured by monitoring the free spectral range (FSR) of the reflection spectra, which achieved growth rates of 2 nm/min from 40 to 100 min and approximately 1.1 nm/min from 100 to 120 min in a 0.01-M growth solution, and 6.8 nm/min from 40 to 90 min in a 0.05-M growth solution. The proposed method is simple and cost effective, and has potential for wider applications in nanostructure manufacturing. © 2017 Optical Society of America OCIS codes: (160.4236) Nanomaterials; (120.2230) Fabry-Pérot; (240.6490) Spectroscopy, surface; (060.2370) Fiber optics sensors. References and links 1. X. W. Sun, J. Z. Huang, J. X. Wang, and Z. Xu, “A ZnO Nanorod Inorganic/Organic Heterostructure LightEmitting Diode Emitting at 342 nm,” Nano Lett. 8(4), 1219–1223 (2008). 2. M. H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, and P. Yang, “RoomTemperature Ultraviolet Nanowire Nanolasers,” Science 292(5523), 1897–1899 (2001). 3. T. Y. Liu, H. C. Liao, C. C. Lin, S. H. Hu, and S. Y. Chen, “Biofunctional ZnO Nanorod Arrays Grown on Flexible Substrates,” Langmuir 22(13), 5804–5809 (2006). 4. H. Y. Yue, S. Huang, J. Chang, C. Heo, F. Yao, S. Adhikari, F. Gunes, L. C. Liu, T. H. Lee, E. S. Oh, B. Li, J. J. Zhang, T. Q. Huy, N. V. Luan, and Y. H. 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Introduction
ZnO nanowires (NWs) comprise a very important class of nanomaterial that has been widely applied in the fabrication of different optoelectric devices, such as light-emitting diodes [1], ultraviolet lasers [2], biophotonic sensors [3,4], field-effect transistors [5], solar cells [6], and piezo-photonic devices [7].Owing to easy operation, low cost, and low growth temperature, ZnO NW arrays have been finding wide applications in the field of optical fiber biophotonic and biochemical sensors.For example, ZnO NW arrays on an optical fiber can be used to measure the concentrations of different types of gases, e.g., aqueous vapor [8][9][10], ethanol [11,12], and H 2 S [13].Vsyssieres reviewed a promising hydrothermal method that could be used to grow ZnO NW arrays on the surface of an optical fiber [14,15].The characteristic behavior of ZnO NWs on an optical surface is an important factor that can affect the device properties, and should be accurately monitored during the growth process.Precise control of the growing of ZnO NWs will help to achieve better performance, as well as wider applications of such devices.Several methods have been proposed for in situ monitoring of the growth of nanostructures, such as scanning tunneling microscopy, reflection high-energy electron diffraction, environmental scanning electron microscopy [16], and X-ray diffraction [17].Optical methods for monitoring nanostructure growth are simple and precise.Heurlin et al. reported a method of characterizing nanowire growth using optical reflectance spectra for analysis [18].Kim et al. described an in situ method of monitoring the growth of aligned carbon nanotubes based on an optical interference technique [19].However, few studies focused on real-time monitoring of ZnO NW growth on an optical fiber surface have been reported to date.One method has been proposed to investigate the growth of ZnO NWs by studying the spectral properties of the long-period fiber grating (LPFG) [12].However, the shifts of the LPFG resonance dips were nonlinear with the growth rate of ZnO NWs, which makes taking the measurements inconvenient.
In this work, we monitored the growth of ZnO NWs online by monitoring the spectral evolution of a fiber-tip Fabry-Pérot interferometer (FPI) coated with ZnO.The FPI was formed by introducing an air bubble at the fiber end using an electric arc discharge technique.The interference fringe of the FPI exhibited a red shift as the ZnO coating on the fiber tip grew thicker.A linear response of the free spectral range (FSR) against the ZnO NW thickness variation was obtained during the growing process.The growth rate of the ZnO thin layer was measured by monitoring the FSR shift of the reflection spectra, which achieved growth rates of 2 nm/min from 40 to 100 min and of approximately 1.1 nm/min from 100 to 120 min in a 0.01-M growth solution.

ZnO NWs growth on optical fibers
Figure 1 illustrates the growing process of well-arrayed ZnO NWs on optical fibers, which involves the following steps.In step 1, shown in Fig. 1(1), an optical fiber was cleaned in an ultrasonic bath with deionized water, ethanol, and acetone for 15 min each, consecutively, followed by a baking process at 80 °C for 30 min.A scanning electron microscope (SEM) image of the optical fiber taken following step 1 is shown in Fig. 2(a).In step 2, shown in Fig. 1(2), ZnO seed particles were synthesized in a colloidal solution.A 0.03-M sodium hydroxide [NaOH, 96% purity] solution was prepared in 100 ml of ethanol [CH 3 CH 2 OH, 99.7% purity] under magnetic stirring.A 0.01-M zinc acetate [Zn(CH 3 COO) 2 •2H 2 O, 99% purity, Sigma-Aldrich Corp., St. Louis, MO, USA] solution was prepared in 50 ml of ethanol [CH 3 CH 2 OH, 99.7% purity] under magnetic stirring.Sodium hydroxide solution was added dropwise to the zinc acetate solution under continuous stirring.The mixture of the two solutions was then added into a temperaturecontrolled water bath under continuous stirring at 60 °C for 2 h.A colloidal solution with a large amount of white precipitate was synthesized; however, the precipitate suspended in the solution impeded the uniform growth of ZnO NWs.The transparent colloidal solution was prepared by means of stewing for several hours; the colloidal solution was tested by measuring the Tyndall effect, and a distinct light path was observed in the solution upon exposure to a handheld laser.In step 3, shown in Fig. 1(3), the ZnO seed particles were securely coated on the optical fiber using the Czochralski method and then annealed five times at 150 °C for 30 min.6), all fibers covered by high-density ZnO NWs were rinsed with deionized water for 10 min, followed by baking at 80 °C for 1.5 h. Figure 2(c) is a SEM image of the optical fiber taken following step 6, and the details of the ZnO NWs coated on the fiber tip are shown in the inset.

Online monitoring of ZnO NWs growth
The fiber-tip FPI used to monitor the growth of the ZnO NWs online was fabricated using the electric arc discharge method detailed in Ref [20].As a micro-bubble is formed in the end of the fiber, interference fringes resulting from the resonance within the air bubble are observed in the reflection spectrum, as seen in Fig. 3.The inset of Fig. 3 shows the detail of the micro-bubble in the end of the fiber.The envelope of the fringe is contributed by the FP resonance in the thin silica diaphragm, the spacing of which can be expressed as λ 2 ⁄ 2n silica t s , where t s is the diaphragm thickness of the FPI.The measured envelope spacing and the calculated t s values are listed in Table 1.  Figure 4(a) illustrates the experimental setup, in which nine well-cut ends of singlemode fibers (SMFs) and the fiber-tip FPI were mounted on a Teflon holder.The samples were rinsed as depicted in Fig. 1(1).Following the deposition of ZnO seed particles, the samples were chemically treated in an aqueous environment, 0.01-M zinc nitrate hydrate (Zn(NO 3 ) 2 •6H 2 O, 99.0% purity, Sigma-Aldrich), and 0.01-M hexamethylenetetramine (C 6 H 12 N 4 , 99.5% purity, Sigma-Aldrich).The solution was heated from 20 °C to the optimum temperature of 95 °C over a period of 40 min, after which the originally transparent and colorless solution exhibited a "milky appearance," signaling the onset of ZnO NW growth, and remained so until termination of the procedure.Optical characterization was performed using a superluminescent source and an optical spectrum analyzer (OSA) for monitoring the spectral response of the fiber-tip FPI in real time.The reflection spectra of the fiber-tip FPI were recorded at 10-min intervals.The reflection spectra at different growth times are illustrated in Fig. 4(b), from which it is seen that as growth time increased, the envelopes shifted toward longer wavelength.The relationship between growth time and the wavelength shifts of the envelopes is shown by the plotline of black squares in Fig. 4(c).The envelopes did not exhibit an obvious shift within the first 40 min, but started to exhibit a red shift soon after.The wavelength shift stopped approximately 120 min after the growth process started, indicating that the growth of the ZnO NWs had reached a saturation plateau.
As the spectrum envelope spacing is determined by the thickness of the silica diaphragm and the thickness of the ZnO layer, the growth process of the ZnO NWs can be monitored online by observing the spectral characteristics of the overlaid fiber-tip FPI.To investigate the ZnO characterization at different times during the growth process, nine well-cut fiber ends deposited with ZnO seed particles were positioned in the growth solution, which allowed their selective removal from the solution at predetermined times, namely at 50, 60, 70, 80, 90, 100, 110, 120, and 150 min after immersion.The ZnO NWs coating the samples were then washed and baked using the process depicted in Fig. 1(6) to remove excess ZnO.SEM images of the nine samples are presented as Fig. 5(a)-5(i).The short and sparse NWs grown by immersion in the growth solution for 50 min seem to have undergone a gradual increase in height and density with increasing immersion time.The ZnO NWs exhibit a very dense layer structure before 120 min, as can be seen in Figs.5(a)-5(h), followed by the blossoming of rod structures of ZnO NWs from the dense layer, which can be seen in Fig. 5(i).These results indicate that the growth of the ZnO layer has stopped after 120 min, while the growth of ZnO NWs has continued.The surface of the fiber-tip FPI before immersion into the growth solution and after immersion for 150 min is presented in Figs.6(a) and 6(b), respectively.Figure 6(c) is a magnified view of the structure of the ZnO NWs, which shows no dangling NWs or deposition of other debris.

Discussion
As shown in Fig. 4(c), the growth of ZnO NWs on the fiber-tip FPI largely modifies the spectral response of the FPI.This phenomenon can be explained as follows.The ZnO nanostructure, combined with the fiber-tip FPI, has formed a multilayer FP resonator, as illustrated in Fig. 7. Before ZnO NW growth, three reflected waves were collected and guided back to the SMF: one is from the bubble bottom, i.e., surface-I, and the other two are from the inner and outer surfaces of the silica diaphragm, i.e., surface-II and surface-III.The envelope of the spectrum shown in Fig. 3 is attributed to the FP resonance within the silica diaphragm.As the ZnO nanostructure was grown on the surface of the fiber-tip FPI, the original FP etalon was extended to a multilayer etalon, and the envelope in Fig. 4(b) is considered the result of the FP interference between surface-II and surface-IV.For such a FPI, the envelope spacing Λ can be expressed as (1) where n s and n ZnO are the refractive indices of silica and ZnO, respectively, and t s and t ZnO are the thicknesses of the silica diaphragm and the ZnO nanostructure, respectively.The envelope spacing decreases as the ZnO layer becomes thicker according to Eq. ( 1), which leads to the red shift of the interference fringe.The red shift of the interference fringe slowly stopped after 120 min according to a line of black squares in Fig. 4(c), for a 0.01-M growth solution, while Fig. 5 shows that the growth of the ZnO thin layer stopped after approximately 120 min.These results lead us to conclude that the ZnO thin layer may be the component making an effective contribution to the extension of the FP etalon.Figures 8(a) and 8(b) clearly show the structure of the ZnO NWs and the ZnO thin layer covering the surface of the fiber-tip FPI.A dense growth of ZnO NWs, and finally a film-like layer is formed at the bottom, which makes it forming an optical resonator attached to the original silica one.This phenomenon indicates that the spectra have been more influenced by the thickness of the dense ZnO thin layer rather than by the entire length of the ZnO NWs.The different durations for growing the ZnO thin layer to different thicknesses, t ZnO , can be calculated using Eq.(1).Table 2 shows that the ZnO thin-layer thickness after immersion in a 0.01-M growth solution for 120 min is approximately 160 nm.The growth rate of the ZnO thin layer is plotted by a line of blue circles in Fig. 4(c).The results show that the growth rate is approximately 2.3 nm/min from 40 to 100 min and approximately 1.1 nm/min from 100 to 120 min for a growth solution concentration of 0.01 M. We also plot the growth-rate curve of the ZnO thin layer in a growth solution concentration of 0.05-M by a line of blue squares in Fig. 4(c); the growth rate for this case is approximately 6.8 nm/min from 40 to 90 min.The wavelength response plotted against ZnO growth is linear with good repeatability, which makes this method a reliable scheme for online monitoring of ZnO nanostructure growth.

Conclusions
We have presented a novel online scheme for monitoring the growth of ZnO NWs on an optical fiber surface by means of analyzing the reflection spectra of a fiber-tip FPI.By monitoring the reflection spectra of the fiber-tip FPI, the growth rate of ZnO NWs can be determined.The interference fringe of the FPI shifted toward longer wavelengths linearly with a rate of 2.3 nm/min from 40 to 100 min and of 1.1 nm/min from 100 to 120 min in a 0.01-M growth solution, and a growth rate of 6.8 nm/min from 40 to 90 min in a 0.05-M growth solution, respectively.In both cases, the wavelength shifts could be saturated to further increase the growth time.The results indicate that the shifts of the reflection spectra of the FPI are mainly driven by the growth of the ZnO thin layer, but not by the NWs grown on the fiber-tip surface.The proposed method is simple, reliable, and cost effective, and is promising for potential applications in nanostructure fabrication.

Figure 2 (
b) is a SEM image of the optical fiber taken following step 3, and the details of the ZnO seed particles coated on the fiber tip are shown in the inset.In step 4, shown in Fig. 1(4), the growth solution was prepared by mixing zinc nitrate hydrate [Zn(NO 3 ) 2 •6H 2 O, 99.0% purity, Sigma-Aldrich] solution with hexamethylenetetramine [C 6 H 12 N 4 , 99.5% purity, Sigma-Aldrich] solution, 0.01 M, in 100 mL of deionized water.In step 5, shown in Fig. 1(5), after uniformly coating the fibers with ZnO seed particles, hydrothermal ZnO NW growth was carried out by suspending the pretreated fibers in the growth solution at 95 °C for several hours.In step 6, shown in Fig. 1(

Fig. 1 .
Fig. 1.Flowchart of process of growing ZnO NWs on an optical fiber: (1) Cleaning of the fiber in an ultrasonic washing machine with acetone, ethanol, isopropyl alcohol (IPA), and deionized water for 10 min each, consecutively; (2) preparation of the seed solution under stirring and heating at 60 °C for 2 h; (3) deposition of the seed particles on the fiber using the Czochralski method and annealing in the thermotank at 150 °C for 30 min five times; (4) preparation of the growth solution under stirring for 30 min at room temperature; (5) growth of the ZnO NWs on the fiber in the thermotank for several hours at 95 °C; (6) cleaning of the sample with deionized water for 10 min followed by baking at 80 °C for 1.5 h.

Fig. 2 .
Fig. 2. SEM images of the process of growing ZnO NWs on an optical fiber: (a) optical fiber after consecutive ultrasonic washings in acetone, ethanol, isopropyl alcohol (IPA), and deionized water for 10 min each; (b) ZnO seed particles deposited on the fiber tip and the detailed view of the ZnO seed layer at the tip of the fiber (see inset); (c) ZnO NWs grown on the fiber and detailed view of the ZnO NWs at the tip of the fiber (see inset).

Fig. 3 .
Fig. 3. Reflection spectrum and optical microscope images of the fiber-tip FPI.Table1.Envelope spacing, central wavelength of envelope, and thickness of fiber-tip FPI diaphragm before ZnO NW growth.

Fig. 4 .
Fig. 4. (a) Experimental setup for online monitoring of ZnO NW growth; (b) reflection spectra of the fiber-tip FPI at different growth-time durations; (c) envelope shifts of the spectra (left-hand vertical axis) and growth rate of ZnO thin-layer thickness (right-hand vertical axis).

Fig. 6 .
Fig. 6.SEM images of ZnO NWs covering the fiber-tip FPI immersed in the growth solution for 150 min: (a) Top view of FPI before ZnO NW growth; (b) top view of FPI after ZnO NW growth; (c) magnified view of ZnO NWs grown on FPI.

Fig. 7 .
Fig. 7. Schematic of the fiber tip FPI with ZnO thin layer.

Fig. 8 .
Fig. 8. (a) SEM image (lateral view) of ZnO NWs and the ZnO thin film covering the fiber-tip FPI surface; (b) SEM image (bottom view) of ZnO NWs and ZnO thin layer.