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Role of ZnO thin film in the vertically aligned growth of ZnO nanorods by chemical bath deposition
Applied Surface Science 379 (2016) 440–445
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Role of ZnO thin film in the vertically aligned growth of ZnO nanorods by chemical bath deposition Nguyen Thanh Son a , Jin-Seo Noh a,∗ , Sungho Park b,∗∗ a b
Department of Nano-Physics, Gachon University, 1342 Seongnamdaero, Sujeong-gu, Seongnam-si, Gyeonggi-do 461-701, South Korea Department of Chemistry, Daejin University, Sundan-dong, Phocheon-si, Gyeonggi-do 487-711, South Korea
a r t i c l e
i n f o
Article history: Received 26 January 2016 Received in revised form 29 March 2016 Accepted 17 April 2016 Available online 19 April 2016 Keywords: Vertically aligned ZnO nanorods ZnO seed layers Chemical bath deposition RF sputtering
a b s t r a c t The effect of ZnO thin film on the growth of ZnO nanorods was investigated. ZnO thin films were sputterdeposited on Si substrate with varying the thickness. ZnO nanorods were grown on the thin film using a chemical bath deposition (CBD) method at 90 ◦ C. The ZnO thin films showed granular structure and vertical roughness on the surface, which facilitated the vertical growth of ZnO nanorods. The average grain size and the surface roughness of ZnO film increased with an increase in film thickness, and this led to the increase in both the average diameter and the average length of vertically grown ZnO nanorods. In particular, it was found that the average diameter of ZnO nanorods was very close to the average grain size of ZnO thin film, confirming the role of ZnO film as a seed layer for the vertical growth of ZnO nanorods. The CBD growth on ZnO seed layers may provide a facile route to engineering vertically aligned ZnO nanorod arrays. © 2016 Published by Elsevier B.V.
1. Introduction Zinc oxide (ZnO) nanostructures have long been gaining attention due to their intriguing properties such as wide and direct band gap (3.37 eV) at room temperature and simultaneous revelation of high electrical, optical, and piezoelectric performance. ZnO nanostructures have found applications in the fields of piezoelectric transducers, photovoltaic devices, gas sensors, biosensors, transistors, and optoelectronic devices [1–7]. In particular, highly-oriented ZnO nanowire or nanorod arrays are required for high-performance optoelectronic devices, and their growth method are of critical importance. Several groups have grown vertically aligned ZnO nanowire/nanorod arrays on silicon or glass substrates without the use of any textured thin film as a seed layer. These arrays were synthesized at a temperature range of 400–600 ◦ C, using metal-organic chemical vapor deposition (MOCVD) [8–10], pulsed laser deposition (PLD) [11], or chemical vapor transport (CVT) [12]. However, all of these methods employ complex and expensive processes at high temperatures. As an alternative of these physical vapor methods, solution-based ZnO nanorod growth methods
∗ Corresponding author. ∗∗ Corresponding author. E-mail addresses: [email protected] (J.-S. Noh), [email protected] (S. Park). http://dx.doi.org/10.1016/j.apsusc.2016.04.107 0169-4332/© 2016 Published by Elsevier B.V.
were emerged, which do not need high temperature or vacuum [13–15]. Chemical bath deposition (CBD) has particularly received a great interest because of its simple experimental setup, low cost, and good potential for scaling up. As far as the CBD growth of ZnO nanorods is concerned, using hexamethylenetetramine (HMTA) ensures high crystallinity and good morphological property of ZnO nanorods, as compared with other reducing agents [16,17]. Vertically aligned ZnO nanorod arrays have also been grown on various substrates with the support of textured ZnO seed layers such as ZnO colloids, ZnO nanocrystal layers, and ZnO thin films [18–20]. For instance, Greene et al. prepared ZnO colloids and nanocrystals in aqueous solution by the hydrolysis of zinc salts on Si substrate at 200–350 ◦ C [19]. Solis-Pomar et al. deposited ZnO thin films with different thicknesses by atomic layer deposition [20]. However, these methods also require high temperature or costly processes. As previously demonstrated for various applications [21–27], sputtering ZnO thin films on a substrate may be a much simpler way to prepare a ZnO seed layer for the subsequent growth of vertically aligned ZnO nanorods. Till now, no systematic study has been reported on a correlation between the property of sputtered ZnO seed film and the features of ZnO nanorods grown from that. In this work, ZnO seed layers are deposited on Si substrate by RF magnetron sputtering at room temperature, with varying the layer thickness. Thickness-dependent surface morphology of the seed layer and its effect on the vertical growth of ZnO nanorod
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Fig. 1. AFM images of ZnO seed layers sputtered for (a), (b) 10 min, (c), (d) 15 min, (e), (f) 20 min, and (g), (h) 25 min. (a), (c), (e), (g) Plan view images. (b), (d), (f), (h) tilted view images. Image size 1 × 1 m2 .
arrays are investigated. The CBD technique is employed to grow the ZnO nanorod arrays using HMTA as a reducing agent.
adjusted from 10 to 25 min to control the thickness of ZnO film in the range of 80–200 nm.
2. Experimental
2.2. Synthesis of ZnO nanorod arrays
The fabrication process of vertically aligned ZnO nanorod arrays includes two steps: preparation of textured ZnO thin films using RF sputtering and ZnO nanorod growth using CBD method.
Zinc nitrate hexahydrate [Zn(NO3 )2 ·6H2 O] and HMTA [C6 H12 N4 ] were purchased from Sigma-Aldrich and used as reagents without further treatment. The aqueous solutions of 50 mM Zn(NO3 )2 and 50 mM HMTA were prepared in deionized (DI) water, and the equal amount of zinc salt solution and HMTA solution were mixed in a 100 ml beaker. Substrates with pre-deposited ZnO seed layers were vertically immersed into the mixed solution. Then, the beaker was sealed and has been placed in a water bath at 90 ◦ C for an hour. Finally, the samples were rinsed with DI water several times and dried at 90 ◦ C for several hours.
2.1. Preparation of ZnO films ZnO seed layers were deposited on Si (100) substrates by RF magnetron sputtering at room temperature with a power of 120 W. Si substrates were sequentially washed in acetone and isopropyl alcohol (IPA) in an ultrasonic bath, and dried in convention oven at 60 ◦ C before the deposition of seed layers. Sputtering time was
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Fig. 2. Top view SEM images of ZnO nanorod arrays grown on the ZnO seed layers pre-sputtered for (a) 10 min, (b) 15 min, (c) 20 min, and (d) 25 min. ZnO nanorods were synthesized at the same conditions for all samples.
2.3. Characterization The surface morphology of ZnO thin films was analyzed by atomic force microscopy (AFM, Brucker MultiMode). The shapes and dimensions of vertically grown ZnO nanorod arrays were characterized using a field emission scanning electron microscope (FE-SEM, JEOL JSM-7500F). The crystal structure and the crystal quality of ZnO nanorods were examined by X-ray diffraction (XRD, X’Pert PW3040). Additionally, ZnO nanorods were collected for the transmittance test and the UV absorption test. For this, the as-grown ZnO nanorod array samples were immersed in a small beaker filled with a 5 ml of ethanol, then scraped with tweezers to take ZnO nanorods apart from the substrate. Finally, the beaker was put into an ultrasonic bath for dispersing ZnO nanorods in solution. The UV absorption test was performed for ZnO nanorods dispersed in ethanol using a UV–vis Spectrophotometer (Varian Cary 50 Bio). ZnO nanorods were spin-coated on a polyethylene terephthalate (PET) film for the optical transmittance test.
3. Results and discussion Fig. 1 shows the AFM images of ZnO seed layers prepared by RF sputtering. The growth rate was kept constant at 8 nm/min for all samples, and the sputtering time was altered to obtain ZnO seed layers of 80, 120, 160, and 200 nm, respectively, in thickness. It is found from the AFM images that ZnO thin films are composed of small grains and the grain size depends on the sputtering time. For the 10 min-sputtered ZnO thin film, small ZnO grains are observed on the surface (Fig. 1(a) and (b)). The typical rootmean-square (RMS) roughness and the average grain size of the ZnO film are estimated to be 0.912 nm and 70 nm, respectively. Prolonging the deposition time to 15 and 20 min, both the RMS roughness and the average grain size become larger: 1.145/80 nm for a 15 min-sputtered sample and 1.299/100 nm for a 20 min sample (Fig. 1(c)–(f)). The increase of the surface roughness and the average grain size with sputtering time can also be confirmed from AFM line-scan profiles, as shown in Fig. S1. ZnO thin films sputtered for 10–20 min show the relatively uniform grain structures
on the surface. In contrast, a 25 min-sputtered ZnO film is comprised of disparate sizes of grains, as shown in Fig. 1(g) and (h). The smaller and larger grains have the average sizes of 50 nm and 150 nm, respectively. This separation of grain size is attributable to the Ostwald ripening [28,29]. The surface of 25 min-sputtered ZnO film is rougher than other films with a RMS roughness of 2.438 nm. The rough grain structure of ZnO thin film with a certain extent of vertical protrusion is highly likely to induce the vertical growth of ZnO nanorods in the subsequent solution growth step. This possibility was checked out using a CBD method. In this study, ZnO nanorods were grown by the CBD method using an aqueous solution of HMTA and zinc nitrate. HMTA plays important roles to initiate and accelerate the nanorod growth through multiple chemical reactions [20]: (CH2 )6 N4 + 6H2 O ↔ 6CH2 O + 4NH3 (CH2 )6 N4 + Zn
2+
↔ [Zn(CH2 )6 N4 ] +
Zn Zn
2+
+ 4NH3 ↔ Zn(NH3 )4 −
+ 4OH ↔ Zn(OH)4
(1) (2)
−
(3)
2+
(4)
NH3 + H2 O ↔ NH4 + OH 2+
2+
2−
(5)
Zn(NH3 )4 2+ + 2OH− ↔ ZnO + 4NH3 + H2 O
(6)
Zn(OH)4 2− ↔ ZnO + H2 O + 2OH−
(7)
[Zn(CH2 )6 N4 ]2+ + 2OH− ↔ ZnO + H2 O + (CH2 )6 N4
(8)
In the solution, HMTA is first decomposed to NH3 , which generates OH− ions by the reaction with H2 O. These OH− ions dominantly involve in the growth of ZnO nanorods. Furthermore, HMTA also forms the [Zn(CH2 )6 N4 ]2+ complex through the reaction with Zn2+ ions, and this complex accelerates the growth process of ZnO nanorods. Fig. 2 shows the top view SEM images of ZnO nanorod arrays grown on the pre-deposited ZnO seed layers. Indeed, it is found that dense ZnO nanorods are almost vertically grown on the seed layers. These vertically grown nanorods are clearly distinguished from ZnO nanoflowers synthesized in HMTA solution without any substrate (Fig. S2) or from ZnO dendrites formed on Si substrate (Fig. S3). For
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Fig. 3. (a)–(d) 45◦ tilted SEM images of the ZnO nanorod arrays as those in Fig. 2. ZnO seed layers were prepared by sputtering for (a) 10 min, (b) 15 min, (c) 20 min, and (d) 25 min. 90◦ tilted SEM images for the interface between the seed layer and the nanorods for (e) 20 min sample and (f) 25 min sample.
all of the samples, ZnO nanorods have a perfect hexagonal shape, representing the manifestation of wurtzite crystal structure of ZnO. The diameters of ZnO nanorods appear to increase in the order of sputtering time, indicating the formation of thicker nanorods on the larger and rougher grains. In the aspect of nanorod size uniformity, ZnO nanorods grown on the 25 min-grown seed layer seemingly consist of different size groups, which is consistent with the observation in the grain structure of the ZnO seed layer. ZnO nanorods from a 20 min seed layer exhibit very good vertical alignment, whereas nanorods from 10 min and 25 min samples show some fraction of slanted nanorods. This difference in vertical alignment is speculated to arise from the difference in grain shapes and grain size distributions of the seed layers. To examine the vertical alignment of ZnO nanorods more closely, the samples were broken into pieces and SEM images were collected from the broken pieces tilted by 45◦ . Fig. 3 shows the 45◦ tilted SEM images of ZnO nanorod arrays. As previously witnessed from Fig. 2, most ZnO nanorods stand vertically although significant numbers of slanted nanorods are observed in the arrays formed on 10 min- and 25 min-sputtered seed layers (Fig. 3(a) and (d)). For the ZnO nanorods synthesized on 15 min and 20 min samples, in particular, both the vertical alignment and the size uniformity are good. In order to investigate more direct correlation between ZnO seed layers and ZnO nanorods, we measured 90◦ tilted SEM images
for the bisected samples with being focused on the interface. Both ZnO seed layers sputtered respectively for 20 min (Fig. 3(e)) and 25 min (Fig. 3(f)) show columnar structures, and ZnO nanorods seem to grow from the tips of the columns. Although there is a slight mismatch between nanorod growth locations and the cut plane, ZnO nanorods seemingly grow from the surface grains of ZnO seed layers, which are surface morphologies of columnar ZnO seed layers. The columnar structure of ZnO film is generally oriented in c-axis, and its top surface tends to be polar and have high surface energy [20]. To reduce the surface energy, ionic reactants in the solution preferentially react on the surface, leading to the vertical growth of ZnO nanorods. As described above, the diameter of ZnO nanorods looks bigger as a function of sputtering time (in other words, seed layer thickness), except for nanorods on 25 min seed layer (200 nm in thickness). Interestingly, not only the diameter, but also the length of ZnO nanorods seems to increase as the seed layer thickness increases. To see more reliable correlations between the nanorod dimensions and sputtering time, the thicknesses and the lengths of ZnO nanorods were carefully measured for multiple nanorods on the respective samples. Fig. 4(a) presents the variations in the average diameter (davg ), average length (lavg ), and their distributions (d, l) of ZnO nanorods depending on sputtering time. The davg ’s (d’s) of ZnO nanorods grown on 10 min-, 15 min-, 20 min-, and 25 min-
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Fig. 5. XRD patterns of ZnO nanorod arrays grown on ZnO seed layers with three different thicknesses. ZnO seed layers were sputter-deposited for 10 min, 15 min, and 25 min.
Fig. 4. (a) The average diameters and the average lengths of ZnO nanorods as a function of sputtering time. The average grain sizes of ZnO seed layers dependent on the sputtering time are also provided for comparison. (b) A schematic illustration showing a correlation between the dimensions of ZnO nanorods and the surface features of ZnO seed layers.
sputtered seed layers are 70 (50–100), 76.5 (60–100), 97 (70–120), and 83 (50–150) nm, respectively, while lavg ’s are 542, 542, 653, and 695 nm for the same samples. The rather wide distributions of the nanorod dimensions are attributed to non-uniform surface morphologies of ZnO seed layers, as shown in Fig. 1 and Fig. S1. Surprisingly, the average diameters of ZnO nanorods are very close to the average grain sizes of the respective ZnO seeds, convincing that ZnO nanorods have grown from the individual grains in the seed layer (see the sputtering-time-dependent average grain sizes in the same figure). Also, the average length of ZnO nanorods generally increases with an increase in the sputtering time. Two factors are supposed to contribute to this correlation: surface roughness of ZnO seed layer and the density of ZnO nanorods. ZnO nanorods are most likely easier to grow from rougher surface at the initial stage, which is confirmed by our experimental data that show longer nanorods from rougher ZnO seed layer with a larger RMS roughness. Moreover, it is natural that the nanorod density is lower for the thicker ZnO nanorods because identical amounts of Zn precursor and HMTA were used for every synthesis. Zn ions and Zn complexes, which are essential for the ZnO nanorod growth, can be more easily delivered to both the front and the side surface of
growing nanorod when the nanorod density is low, contributing to the faster growth of ZnO nanorods. These correlations between the dimensions of ZnO nanorods and the grain features of ZnO seed layers are schematically drawn in Fig. 4(b). Crystal structures and crystal quality of ZnO nanorods were analyzed by XRD. Fig. 5 shows the XRD patterns of ZnO nanorod arrays grown on the ZnO seed layers sputtered for 10, 15, and 25 min. For all samples, strong peaks appear at 2 ∼ 34.5◦ , which are indexed to (002) plane of hexagonal ZnO crystal. This is in good agreement with the hexagonal shape of ZnO nanords (Fig. 2), and indicates that ZnO nanorods have a strong c-axis orientation. Unlike ZnO nanorods grown on thinner seed layers, nanorods on 25 minsputtered seed layer show additional peaks near the strongest peak: the peaks at 2 ∼ 34.5◦ and ∼61.8◦ are assigned to (100) and (101) planes. This is presumed to be associated with the co-existence of thin and thick nanorods in the sample on the thickest seed layer. Optical properties of our vertically grown ZnO nanorods were tested using UV absorption and optical transmittance methods. The UV absorption spectrum for a sample shows a clear absorption peak centered at 366 nm, and the band gap of the vertically grown ZnO nanorods is estimated at 3.39 eV (see Fig. S4). The transmittance is near 90% in the visible light region (400–800 nm) for all tested samples (Fig. S5(a)). The high transmittance to the visible light reflects that our ZnO nanorods have good optical quality. To further analyze the optical band gaps of ZnO nanorods, the transmittance spectra were converted to (␣h)2 versus h plots (Fig. S5(b)). Here, h is the incident photon energy and ␣ is the absorption coefficient [30]. The band gap energy extracted from these plots is 3.33 eV for all of the samples, which is close to the value (3.39 eV) calculated from the UV absorption spectrum above and agrees well with the ideal band gap energy (3.37 eV) of ZnO crystal. 4. Conclusions ZnO thin films were deposited on Si substrate at room temperature by RF sputtering. The film thickness was adjusted in the range of 80–200 nm by controlling the sputtering time. Both the surface roughness and the average grain size of ZnO film turned out to increase with an increase in the film thickness. ZnO nanorods were grown on these ZnO thin films on Si substrate, using a CBD technique. It was disclosed that dense, hexagonal-shaped ZnO nanorods were almost vertically aligned, and their average diameters were very close to the average grain sizes of ZnO films. Furthermore, the
N.T. Son et al. / Applied Surface Science 379 (2016) 440–445
average diameter and the average length of ZnO nanorods generally increased as ZnO film became thicker, which is related to the variations in the average grain size and RMS roughness of ZnO film depending on the film thickness. These results indicate that ZnO thin film well-functions as a seed layer for the vertically aligned growth of ZnO nanorods in the subsequent solution synthesis step. The vertically grown ZnO nanorods have shown good crystalline and optical qualities. Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2013R1A1A2010630). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2016.04. 107. References [1] Z.L. Wang, J. Song, Piezoelectric nanogenerators based on zinc oxide nanowire arrays, Science 312 (2006) 242–246. [2] J. Zhou, Y. Gu, P. Fei, W. Mai, Y. Gao, R. Yang, G. Bao, Z.L. Wang, Flexible piezotronic strain sensor, Nano Lett. 8 (2008) 3035–3040. [3] M. Law, L.E. Greene, J.C. Johnson, R. Saykally, P. Yang, Nanowire dye-sensitized solar cells, Nat. Mater. 4 (2005) 455–459. [4] T. Shinagawa, K. Shibata, O. Shimomura, M. Chigane, R. Nomura, M. Izaki, Solution-processed high-haze ZnO pyramidal textures directly grown on a TCO substrate and the light-trapping effect in Cu2O solar cells, J. Mater. Chem. C 2 (2014) 2908–2917. [5] B. Pradhan, S.K. Batabyal, A.J. Pal, Vertically aligned ZnO nanowire arrays in Rose Bengal-based dye-sensitized solar cells, Sol. Energy Mater. Sol. Cells 91 (2007) 769–773. [6] L. Sang, M. Liao, M. Sumiya, A comprehensive review of semiconductor ultraviolet photodetectors: from thin film to one-dimensional nanostructures, Sensors 13 (2013) 10482–10518. [7] J.J. Hassan, M.A. Mahdi, S.J. Kasim, N.M. Ahmed, H.A. Hassan, Z. Hassan, High sensitivity and fast response and recovery times in a ZnO nanorod array/p-Si self-powered ultraviolet detector, Appl. Phys. Lett. 101 (2012) 261108. [8] W.I. Park, G.C. Yi, M. Kim, S.J. Pennycook, ZnO nanoneedles grown vertically on Si substrates by non-catalytic vapor-phase epitaxy, Adv. Mater. 14 (2002) 1841–1843. [9] J.J. Wu, S.C. Liu, Low-temperature growth of well-aligned ZnO nanorods by chemical vapor deposition, Adv. Mater. 14 (2002) 215–218. [10] H. Yuan, Y. Zhang, Preparation of well-aligned ZnO whiskers on glass substrate by atmospheric MOCVD, J. Cryst. Growth 263 (2004) 119–124.
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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Role of ZnO thin film in the vertically aligned growth of ZnO nanorods by chemical bath deposition Nguyen Thanh Son a , Jin-Seo Noh a,∗ , Sungho Park b,∗∗ a b
Department of Nano-Physics, Gachon University, 1342 Seongnamdaero, Sujeong-gu, Seongnam-si, Gyeonggi-do 461-701, South Korea Department of Chemistry, Daejin University, Sundan-dong, Phocheon-si, Gyeonggi-do 487-711, South Korea
a r t i c l e
i n f o
Article history: Received 26 January 2016 Received in revised form 29 March 2016 Accepted 17 April 2016 Available online 19 April 2016 Keywords: Vertically aligned ZnO nanorods ZnO seed layers Chemical bath deposition RF sputtering
a b s t r a c t The effect of ZnO thin film on the growth of ZnO nanorods was investigated. ZnO thin films were sputterdeposited on Si substrate with varying the thickness. ZnO nanorods were grown on the thin film using a chemical bath deposition (CBD) method at 90 ◦ C. The ZnO thin films showed granular structure and vertical roughness on the surface, which facilitated the vertical growth of ZnO nanorods. The average grain size and the surface roughness of ZnO film increased with an increase in film thickness, and this led to the increase in both the average diameter and the average length of vertically grown ZnO nanorods. In particular, it was found that the average diameter of ZnO nanorods was very close to the average grain size of ZnO thin film, confirming the role of ZnO film as a seed layer for the vertical growth of ZnO nanorods. The CBD growth on ZnO seed layers may provide a facile route to engineering vertically aligned ZnO nanorod arrays. © 2016 Published by Elsevier B.V.
1. Introduction Zinc oxide (ZnO) nanostructures have long been gaining attention due to their intriguing properties such as wide and direct band gap (3.37 eV) at room temperature and simultaneous revelation of high electrical, optical, and piezoelectric performance. ZnO nanostructures have found applications in the fields of piezoelectric transducers, photovoltaic devices, gas sensors, biosensors, transistors, and optoelectronic devices [1–7]. In particular, highly-oriented ZnO nanowire or nanorod arrays are required for high-performance optoelectronic devices, and their growth method are of critical importance. Several groups have grown vertically aligned ZnO nanowire/nanorod arrays on silicon or glass substrates without the use of any textured thin film as a seed layer. These arrays were synthesized at a temperature range of 400–600 ◦ C, using metal-organic chemical vapor deposition (MOCVD) [8–10], pulsed laser deposition (PLD) [11], or chemical vapor transport (CVT) [12]. However, all of these methods employ complex and expensive processes at high temperatures. As an alternative of these physical vapor methods, solution-based ZnO nanorod growth methods
∗ Corresponding author. ∗∗ Corresponding author. E-mail addresses: [email protected] (J.-S. Noh), [email protected] (S. Park). http://dx.doi.org/10.1016/j.apsusc.2016.04.107 0169-4332/© 2016 Published by Elsevier B.V.
were emerged, which do not need high temperature or vacuum [13–15]. Chemical bath deposition (CBD) has particularly received a great interest because of its simple experimental setup, low cost, and good potential for scaling up. As far as the CBD growth of ZnO nanorods is concerned, using hexamethylenetetramine (HMTA) ensures high crystallinity and good morphological property of ZnO nanorods, as compared with other reducing agents [16,17]. Vertically aligned ZnO nanorod arrays have also been grown on various substrates with the support of textured ZnO seed layers such as ZnO colloids, ZnO nanocrystal layers, and ZnO thin films [18–20]. For instance, Greene et al. prepared ZnO colloids and nanocrystals in aqueous solution by the hydrolysis of zinc salts on Si substrate at 200–350 ◦ C [19]. Solis-Pomar et al. deposited ZnO thin films with different thicknesses by atomic layer deposition [20]. However, these methods also require high temperature or costly processes. As previously demonstrated for various applications [21–27], sputtering ZnO thin films on a substrate may be a much simpler way to prepare a ZnO seed layer for the subsequent growth of vertically aligned ZnO nanorods. Till now, no systematic study has been reported on a correlation between the property of sputtered ZnO seed film and the features of ZnO nanorods grown from that. In this work, ZnO seed layers are deposited on Si substrate by RF magnetron sputtering at room temperature, with varying the layer thickness. Thickness-dependent surface morphology of the seed layer and its effect on the vertical growth of ZnO nanorod
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Fig. 1. AFM images of ZnO seed layers sputtered for (a), (b) 10 min, (c), (d) 15 min, (e), (f) 20 min, and (g), (h) 25 min. (a), (c), (e), (g) Plan view images. (b), (d), (f), (h) tilted view images. Image size 1 × 1 m2 .
arrays are investigated. The CBD technique is employed to grow the ZnO nanorod arrays using HMTA as a reducing agent.
adjusted from 10 to 25 min to control the thickness of ZnO film in the range of 80–200 nm.
2. Experimental
2.2. Synthesis of ZnO nanorod arrays
The fabrication process of vertically aligned ZnO nanorod arrays includes two steps: preparation of textured ZnO thin films using RF sputtering and ZnO nanorod growth using CBD method.
Zinc nitrate hexahydrate [Zn(NO3 )2 ·6H2 O] and HMTA [C6 H12 N4 ] were purchased from Sigma-Aldrich and used as reagents without further treatment. The aqueous solutions of 50 mM Zn(NO3 )2 and 50 mM HMTA were prepared in deionized (DI) water, and the equal amount of zinc salt solution and HMTA solution were mixed in a 100 ml beaker. Substrates with pre-deposited ZnO seed layers were vertically immersed into the mixed solution. Then, the beaker was sealed and has been placed in a water bath at 90 ◦ C for an hour. Finally, the samples were rinsed with DI water several times and dried at 90 ◦ C for several hours.
2.1. Preparation of ZnO films ZnO seed layers were deposited on Si (100) substrates by RF magnetron sputtering at room temperature with a power of 120 W. Si substrates were sequentially washed in acetone and isopropyl alcohol (IPA) in an ultrasonic bath, and dried in convention oven at 60 ◦ C before the deposition of seed layers. Sputtering time was
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Fig. 2. Top view SEM images of ZnO nanorod arrays grown on the ZnO seed layers pre-sputtered for (a) 10 min, (b) 15 min, (c) 20 min, and (d) 25 min. ZnO nanorods were synthesized at the same conditions for all samples.
2.3. Characterization The surface morphology of ZnO thin films was analyzed by atomic force microscopy (AFM, Brucker MultiMode). The shapes and dimensions of vertically grown ZnO nanorod arrays were characterized using a field emission scanning electron microscope (FE-SEM, JEOL JSM-7500F). The crystal structure and the crystal quality of ZnO nanorods were examined by X-ray diffraction (XRD, X’Pert PW3040). Additionally, ZnO nanorods were collected for the transmittance test and the UV absorption test. For this, the as-grown ZnO nanorod array samples were immersed in a small beaker filled with a 5 ml of ethanol, then scraped with tweezers to take ZnO nanorods apart from the substrate. Finally, the beaker was put into an ultrasonic bath for dispersing ZnO nanorods in solution. The UV absorption test was performed for ZnO nanorods dispersed in ethanol using a UV–vis Spectrophotometer (Varian Cary 50 Bio). ZnO nanorods were spin-coated on a polyethylene terephthalate (PET) film for the optical transmittance test.
3. Results and discussion Fig. 1 shows the AFM images of ZnO seed layers prepared by RF sputtering. The growth rate was kept constant at 8 nm/min for all samples, and the sputtering time was altered to obtain ZnO seed layers of 80, 120, 160, and 200 nm, respectively, in thickness. It is found from the AFM images that ZnO thin films are composed of small grains and the grain size depends on the sputtering time. For the 10 min-sputtered ZnO thin film, small ZnO grains are observed on the surface (Fig. 1(a) and (b)). The typical rootmean-square (RMS) roughness and the average grain size of the ZnO film are estimated to be 0.912 nm and 70 nm, respectively. Prolonging the deposition time to 15 and 20 min, both the RMS roughness and the average grain size become larger: 1.145/80 nm for a 15 min-sputtered sample and 1.299/100 nm for a 20 min sample (Fig. 1(c)–(f)). The increase of the surface roughness and the average grain size with sputtering time can also be confirmed from AFM line-scan profiles, as shown in Fig. S1. ZnO thin films sputtered for 10–20 min show the relatively uniform grain structures
on the surface. In contrast, a 25 min-sputtered ZnO film is comprised of disparate sizes of grains, as shown in Fig. 1(g) and (h). The smaller and larger grains have the average sizes of 50 nm and 150 nm, respectively. This separation of grain size is attributable to the Ostwald ripening [28,29]. The surface of 25 min-sputtered ZnO film is rougher than other films with a RMS roughness of 2.438 nm. The rough grain structure of ZnO thin film with a certain extent of vertical protrusion is highly likely to induce the vertical growth of ZnO nanorods in the subsequent solution growth step. This possibility was checked out using a CBD method. In this study, ZnO nanorods were grown by the CBD method using an aqueous solution of HMTA and zinc nitrate. HMTA plays important roles to initiate and accelerate the nanorod growth through multiple chemical reactions [20]: (CH2 )6 N4 + 6H2 O ↔ 6CH2 O + 4NH3 (CH2 )6 N4 + Zn
2+
↔ [Zn(CH2 )6 N4 ] +
Zn Zn
2+
+ 4NH3 ↔ Zn(NH3 )4 −
+ 4OH ↔ Zn(OH)4
(1) (2)
−
(3)
2+
(4)
NH3 + H2 O ↔ NH4 + OH 2+
2+
2−
(5)
Zn(NH3 )4 2+ + 2OH− ↔ ZnO + 4NH3 + H2 O
(6)
Zn(OH)4 2− ↔ ZnO + H2 O + 2OH−
(7)
[Zn(CH2 )6 N4 ]2+ + 2OH− ↔ ZnO + H2 O + (CH2 )6 N4
(8)
In the solution, HMTA is first decomposed to NH3 , which generates OH− ions by the reaction with H2 O. These OH− ions dominantly involve in the growth of ZnO nanorods. Furthermore, HMTA also forms the [Zn(CH2 )6 N4 ]2+ complex through the reaction with Zn2+ ions, and this complex accelerates the growth process of ZnO nanorods. Fig. 2 shows the top view SEM images of ZnO nanorod arrays grown on the pre-deposited ZnO seed layers. Indeed, it is found that dense ZnO nanorods are almost vertically grown on the seed layers. These vertically grown nanorods are clearly distinguished from ZnO nanoflowers synthesized in HMTA solution without any substrate (Fig. S2) or from ZnO dendrites formed on Si substrate (Fig. S3). For
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Fig. 3. (a)–(d) 45◦ tilted SEM images of the ZnO nanorod arrays as those in Fig. 2. ZnO seed layers were prepared by sputtering for (a) 10 min, (b) 15 min, (c) 20 min, and (d) 25 min. 90◦ tilted SEM images for the interface between the seed layer and the nanorods for (e) 20 min sample and (f) 25 min sample.
all of the samples, ZnO nanorods have a perfect hexagonal shape, representing the manifestation of wurtzite crystal structure of ZnO. The diameters of ZnO nanorods appear to increase in the order of sputtering time, indicating the formation of thicker nanorods on the larger and rougher grains. In the aspect of nanorod size uniformity, ZnO nanorods grown on the 25 min-grown seed layer seemingly consist of different size groups, which is consistent with the observation in the grain structure of the ZnO seed layer. ZnO nanorods from a 20 min seed layer exhibit very good vertical alignment, whereas nanorods from 10 min and 25 min samples show some fraction of slanted nanorods. This difference in vertical alignment is speculated to arise from the difference in grain shapes and grain size distributions of the seed layers. To examine the vertical alignment of ZnO nanorods more closely, the samples were broken into pieces and SEM images were collected from the broken pieces tilted by 45◦ . Fig. 3 shows the 45◦ tilted SEM images of ZnO nanorod arrays. As previously witnessed from Fig. 2, most ZnO nanorods stand vertically although significant numbers of slanted nanorods are observed in the arrays formed on 10 min- and 25 min-sputtered seed layers (Fig. 3(a) and (d)). For the ZnO nanorods synthesized on 15 min and 20 min samples, in particular, both the vertical alignment and the size uniformity are good. In order to investigate more direct correlation between ZnO seed layers and ZnO nanorods, we measured 90◦ tilted SEM images
for the bisected samples with being focused on the interface. Both ZnO seed layers sputtered respectively for 20 min (Fig. 3(e)) and 25 min (Fig. 3(f)) show columnar structures, and ZnO nanorods seem to grow from the tips of the columns. Although there is a slight mismatch between nanorod growth locations and the cut plane, ZnO nanorods seemingly grow from the surface grains of ZnO seed layers, which are surface morphologies of columnar ZnO seed layers. The columnar structure of ZnO film is generally oriented in c-axis, and its top surface tends to be polar and have high surface energy [20]. To reduce the surface energy, ionic reactants in the solution preferentially react on the surface, leading to the vertical growth of ZnO nanorods. As described above, the diameter of ZnO nanorods looks bigger as a function of sputtering time (in other words, seed layer thickness), except for nanorods on 25 min seed layer (200 nm in thickness). Interestingly, not only the diameter, but also the length of ZnO nanorods seems to increase as the seed layer thickness increases. To see more reliable correlations between the nanorod dimensions and sputtering time, the thicknesses and the lengths of ZnO nanorods were carefully measured for multiple nanorods on the respective samples. Fig. 4(a) presents the variations in the average diameter (davg ), average length (lavg ), and their distributions (d, l) of ZnO nanorods depending on sputtering time. The davg ’s (d’s) of ZnO nanorods grown on 10 min-, 15 min-, 20 min-, and 25 min-
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Fig. 5. XRD patterns of ZnO nanorod arrays grown on ZnO seed layers with three different thicknesses. ZnO seed layers were sputter-deposited for 10 min, 15 min, and 25 min.
Fig. 4. (a) The average diameters and the average lengths of ZnO nanorods as a function of sputtering time. The average grain sizes of ZnO seed layers dependent on the sputtering time are also provided for comparison. (b) A schematic illustration showing a correlation between the dimensions of ZnO nanorods and the surface features of ZnO seed layers.
sputtered seed layers are 70 (50–100), 76.5 (60–100), 97 (70–120), and 83 (50–150) nm, respectively, while lavg ’s are 542, 542, 653, and 695 nm for the same samples. The rather wide distributions of the nanorod dimensions are attributed to non-uniform surface morphologies of ZnO seed layers, as shown in Fig. 1 and Fig. S1. Surprisingly, the average diameters of ZnO nanorods are very close to the average grain sizes of the respective ZnO seeds, convincing that ZnO nanorods have grown from the individual grains in the seed layer (see the sputtering-time-dependent average grain sizes in the same figure). Also, the average length of ZnO nanorods generally increases with an increase in the sputtering time. Two factors are supposed to contribute to this correlation: surface roughness of ZnO seed layer and the density of ZnO nanorods. ZnO nanorods are most likely easier to grow from rougher surface at the initial stage, which is confirmed by our experimental data that show longer nanorods from rougher ZnO seed layer with a larger RMS roughness. Moreover, it is natural that the nanorod density is lower for the thicker ZnO nanorods because identical amounts of Zn precursor and HMTA were used for every synthesis. Zn ions and Zn complexes, which are essential for the ZnO nanorod growth, can be more easily delivered to both the front and the side surface of
growing nanorod when the nanorod density is low, contributing to the faster growth of ZnO nanorods. These correlations between the dimensions of ZnO nanorods and the grain features of ZnO seed layers are schematically drawn in Fig. 4(b). Crystal structures and crystal quality of ZnO nanorods were analyzed by XRD. Fig. 5 shows the XRD patterns of ZnO nanorod arrays grown on the ZnO seed layers sputtered for 10, 15, and 25 min. For all samples, strong peaks appear at 2 ∼ 34.5◦ , which are indexed to (002) plane of hexagonal ZnO crystal. This is in good agreement with the hexagonal shape of ZnO nanords (Fig. 2), and indicates that ZnO nanorods have a strong c-axis orientation. Unlike ZnO nanorods grown on thinner seed layers, nanorods on 25 minsputtered seed layer show additional peaks near the strongest peak: the peaks at 2 ∼ 34.5◦ and ∼61.8◦ are assigned to (100) and (101) planes. This is presumed to be associated with the co-existence of thin and thick nanorods in the sample on the thickest seed layer. Optical properties of our vertically grown ZnO nanorods were tested using UV absorption and optical transmittance methods. The UV absorption spectrum for a sample shows a clear absorption peak centered at 366 nm, and the band gap of the vertically grown ZnO nanorods is estimated at 3.39 eV (see Fig. S4). The transmittance is near 90% in the visible light region (400–800 nm) for all tested samples (Fig. S5(a)). The high transmittance to the visible light reflects that our ZnO nanorods have good optical quality. To further analyze the optical band gaps of ZnO nanorods, the transmittance spectra were converted to (␣h)2 versus h plots (Fig. S5(b)). Here, h is the incident photon energy and ␣ is the absorption coefficient [30]. The band gap energy extracted from these plots is 3.33 eV for all of the samples, which is close to the value (3.39 eV) calculated from the UV absorption spectrum above and agrees well with the ideal band gap energy (3.37 eV) of ZnO crystal. 4. Conclusions ZnO thin films were deposited on Si substrate at room temperature by RF sputtering. The film thickness was adjusted in the range of 80–200 nm by controlling the sputtering time. Both the surface roughness and the average grain size of ZnO film turned out to increase with an increase in the film thickness. ZnO nanorods were grown on these ZnO thin films on Si substrate, using a CBD technique. It was disclosed that dense, hexagonal-shaped ZnO nanorods were almost vertically aligned, and their average diameters were very close to the average grain sizes of ZnO films. Furthermore, the
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average diameter and the average length of ZnO nanorods generally increased as ZnO film became thicker, which is related to the variations in the average grain size and RMS roughness of ZnO film depending on the film thickness. These results indicate that ZnO thin film well-functions as a seed layer for the vertically aligned growth of ZnO nanorods in the subsequent solution synthesis step. The vertically grown ZnO nanorods have shown good crystalline and optical qualities. Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2013R1A1A2010630). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2016.04. 107. References [1] Z.L. Wang, J. Song, Piezoelectric nanogenerators based on zinc oxide nanowire arrays, Science 312 (2006) 242–246. [2] J. Zhou, Y. Gu, P. Fei, W. Mai, Y. Gao, R. Yang, G. Bao, Z.L. Wang, Flexible piezotronic strain sensor, Nano Lett. 8 (2008) 3035–3040. [3] M. Law, L.E. Greene, J.C. Johnson, R. Saykally, P. Yang, Nanowire dye-sensitized solar cells, Nat. Mater. 4 (2005) 455–459. [4] T. Shinagawa, K. Shibata, O. Shimomura, M. Chigane, R. Nomura, M. Izaki, Solution-processed high-haze ZnO pyramidal textures directly grown on a TCO substrate and the light-trapping effect in Cu2O solar cells, J. Mater. Chem. C 2 (2014) 2908–2917. [5] B. Pradhan, S.K. Batabyal, A.J. Pal, Vertically aligned ZnO nanowire arrays in Rose Bengal-based dye-sensitized solar cells, Sol. Energy Mater. Sol. Cells 91 (2007) 769–773. [6] L. Sang, M. Liao, M. Sumiya, A comprehensive review of semiconductor ultraviolet photodetectors: from thin film to one-dimensional nanostructures, Sensors 13 (2013) 10482–10518. [7] J.J. Hassan, M.A. Mahdi, S.J. Kasim, N.M. Ahmed, H.A. Hassan, Z. Hassan, High sensitivity and fast response and recovery times in a ZnO nanorod array/p-Si self-powered ultraviolet detector, Appl. Phys. Lett. 101 (2012) 261108. [8] W.I. Park, G.C. Yi, M. Kim, S.J. Pennycook, ZnO nanoneedles grown vertically on Si substrates by non-catalytic vapor-phase epitaxy, Adv. Mater. 14 (2002) 1841–1843. [9] J.J. Wu, S.C. Liu, Low-temperature growth of well-aligned ZnO nanorods by chemical vapor deposition, Adv. Mater. 14 (2002) 215–218. [10] H. Yuan, Y. Zhang, Preparation of well-aligned ZnO whiskers on glass substrate by atmospheric MOCVD, J. Cryst. Growth 263 (2004) 119–124.
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