Direct Growth of Feather-Like ZnO Structures by a Facile Solution Technique for Photo-Detecting Application

Direct Growth of Feather-Like ZnO Structures by a Facile Solution Technique for Photo-Detecting... The feather-like hierarchical zinc oxide (ZnO) was synthesized via successive ionic layer adsorption and reaction without any seed layer or metal catalyst. A possible growth mechanism is proposed to explain the forming process of ZnO feather-like structures. Meanwhile, the photo-electronic performances of the feather-like ZnO have been investigated with the UV-vis-NIR spectroscopy, I-V and I-tmeasurements. The results indicate that feather-like ZnO hierarchical structures have good anti-reflection and excellent photo-sensitivity. All results suggest that the direct growth processing of novel feather-like ZnO is envisaged to have promising application in the field of photo-detector devices. Keywords: Photo-response, Nanostructures, Feather-like hierarchical structures, Successive ionic layer adsorption and reaction Background handy and large area preparation. However, these methods Zinc oxide (ZnO) is a very versatile material due to its often require a seed layer and metal catalysts. ZnO seed wide bandgap (~3.37 eV) and large exciton binding layer growth may already have a well control for the ZnO energy, up to 60 meV, which allow the fabrication of UV nanostructure growth, which normally needs to be [1, 2] and blue light-emitting diode [3]. In recent years, in- annealed with a high temperature or complicated vacuum tensive efforts have been put in the exploration of photo- equipments [16]. In addition, using a seed layer and metal detectors [4, 5] based on the three-dimensional (3D) ZnO catalysts could make the synthesis procedure more com- architectures with the micrometer- and nanometer-scale plex and introduce impurities which influence the proper- building blocks. Compared with mono-morphological ties of the ZnO structure. ZnO structures, 3D hierarchical ZnO structures possess a Therefore, it still remains an enormous challenge to large surface area which could facilitate the adsorption of develop a facile room-temperature method that needs light. Generally, 3D hierarchical ZnO structures such as not any seed layer or metal catalyst for producing hier- flower-like structures [6], texture [7], nanotubes [8], and archical ZnO structures. dendritic-like [9] and feather-like [10] structures exhibit Herein, in this work, a new attempt was made to pre- outstanding optical [11], electronic [12], catalytic proper- pare ZnO hierarchical structures, which was used with- ties [9] and thus have many potential applications in solar out any seed layer or metal catalyst based on successive cells, gas sensors, photo-catalysts, and other fields. To ionic layer adsorption and reaction (SILAR) processing. synthesize hierarchical ZnO structures, various physical, The novel and unusual feather-like ZnO hierarchical chemical [13], and electrochemical [14] methods have structures were obtained for the first time based on been employed. Among them, the hydrothermal/sol- SILAR at room temperature. A possible mechanism was vothermal method [15] is very popular because of its proposed to explain the growth process of the ZnO feather-like structures. In addition, the photoelectric properties of the feather-like ZnO/p-Si heterojunctions * Correspondence: jiangyurong@whut.edu.cn had been investigated, and the results indicate that Henan Key Laboratory of Photovoltaic Materials, College of Physics and Materials Science, Henan Normal University, Xinxiang 453007, China feather-like ZnO nanostructures have excellent anti- Full list of author information is available at the end of the article © The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Jiang et al. Nanoscale Research Letters (2017) 12:483 Page 2 of 6 reflection characteristics and good photosensitivity, diodes characteristics, the electrode of 12-nm semitrans- which suggests that these hierarchical structures have a parent Cu film was deposited on the ZnO/p-Si by the potential application in the photo-electronic devices. thermal evaporation masked with an area of 5 mm × 5 mm. The schematic of diode is shown in Fig. 4c. Methods First Si (100) substrates were ultrasonically cleaned for Results and discussion 10 min in ethanol. Second, 0.01 mol of zinc acetate Figure 1a shows that ZnO has feather-like morphology, (Zn(CH COO) ) was dissolved into 100 mL of deionized which is novel and unusual. The longitudinal length of 3 2 water, then ammonia hydroxide was added into the solu- feather-like structures varies between 300 and 800 nm, tion until its pH was around 11, to form a uniform and its lateral length is different from 200 to 400 nm. The transparent solution under stirring, which is the precur- magnified SEM image in Fig. 1b shows that the hierarch- sor solution of feather-like ZnO. Afterward, silicon wafer ical structures are obtained. Meanwhile, the branches of was dipped into the predecessor solution for 30 s, and feather-like 3D structures are interestingly assembled the ion complex was absorbed into the Si substrate, then perpendicularly to the nano-sheet trunks. Figure 1c shows the Si substrate was taken out and put into deionized the TEM image of an individual hierarchical structure. water for 20 s and was washed with ultrapure water for The dark dots and translucent plate correspond to the 20 times to remove impurities such as unconsolidated branches and the nano-sheet trunk. Because the size of zinc hydroxide (Zn(OH) ). Finally, the samples were put feather-like ZnO is beyond 200 nm, the lattice fringe into deionized water with 90 °C for 1 min; in this step, could not be revealed. Figure 2 show the typical TEM the unreacted ion complex and zinc hydroxide which images of a nanorod segment form the ZnO feathers, it had been absorbed can be resolved into pure ZnO. In a proves the nanorod is a single crystal. typical SILAR experiment, we circulated the above steps Figure 1e shows the peaks of EDS in which only Zn, O, for 20 times. The crystal structures of feather-like ZnO C, and Si were found in our sample, which indicates that were characterized by X-ray diffraction (XRD) and energy the process of SILAR is successful to deposit pure ZnO disperse spectrometer (EDS). The surface morphology onto silicon. The XRD (Fig. 1e) reveals the crystal struc- was investigated by scanning electron microscopy (SEM) ture and phase purity of the ZnO hierarchical structures. and transporting electron microscopy (TEM). Further- All the diffraction peaks of the products match very well more, we also analyzed I-V and I-t characteristics of with those of wurtzite ZnO (JCPDS file 36-1451), as well feather-like ZnO/p-Si. In order to measure the photo- as a dominant diffraction peak corresponding to the p-Si (a) (b) () c 200nm 1um 200nm (f) (d) () e (c) ZnO P-Si 200nm Fig. 1 a, b The SEM images of feather-like ZnO grown on silicon. c The TEM image of individual feather-like ZnO. d The cross-sectional SEM image of feather-like ZnO/p-Si. e The EDS analysis of the ZnO/p-Si, indicating that the predominant composition is Zn. f XRD patterns of feather-like ZnO/p-Si Jiang et al. Nanoscale Research Letters (2017) 12:483 Page 3 of 6 Fig. 2 TEM images of a hierarchical ZnO structure segment (400). No diffraction peaks from other impurities are On the basis of the above results, it can be speculated found in the spectrum; the result indicates that the struc- that the feather-like ZnO hierarchical structures were syn- ture is pure hexagonal wurtzite ZnO. Moreover, the inten- thesized via a two-stage nucleation-growth process. Figure sity of peak (002) is rather higher than peaks (100) and 4 shows the schematic diagram describing the formation (101); this shows that the crystalline is along the (002) axis processes of ZnO hierarchical structures. First, ammonia preferred orientation. The sharp diffraction peaks reveal hydroxide is used to provide hybroxyl anions (OH )which that ZnO have high crystal structure of pure quality. increases the pH of reaction solution and the alkalinity of 2− It should be mentioned here that no ZnO hierarchical the reaction solution, then the Zn(OH) ions are obtained. 2− 2− structures are found even though the reaction is carried Upon the dehydration of Zn(OH) ions, Zn(OH) ions 4 4 out under the same environment when using Si nano- are adsorbed onto the Si substrate and subsequently dis- wires with all crystal directions replacing Si (100) sub- solved to form homogeneous ZnO nuclei followed by the strates (shown as Fig. 3). The results indicate that the water bath at 90 °C [17]. During this process, the trunk for- crystal direction plays a key role in the nucleation and mation of ZnO nano-sheets with {110} planar surface at growth of ZnO hierarchical structure. the initial stage can be ascribed to the excess OH ions (a) (b) Fig. 3 SEM images of ZnO grown on silicon nanowires: a morphology and b cross section Jiang et al. Nanoscale Research Letters (2017) 12:483 Page 4 of 6 (a) (b) (c) Fig. 4 The schematic diagram of formation processes for ZnO hierarchical structures: a the trunk formation of ZnO nano-sheets; b secondary heterogeneous nucleation and growth of branches; c the continuously growth of primary nano-sheets and secondary nano-branches constructs the feather-like ZnO hierarchical structures 2− and abundant Zn(OH) ions (shown as Fig. 4a), which nucleation and growth of branches (shown as Fig. 4b). can stabilize the surface charge and the structure of Zn Finally, the continuously growth of primary nano-sheets (001) surface to some extent, allowing fast growth along and secondary nano-branches constructs the feather-like the [100] direction [18]. Second, the surface of the primary ZnO hierarchical structures (shown as Fig. 4c). ZnO nano-sheets trunk formed during the initial growth To investigate the optical properties of feather-like stage has many crystalline boundaries which contain more ZnO, the room-temperature PL was obtained by using a defects than other regions. These defects on the surface of He–Cd laser (λ = 325 nm) as the excitation source as trunk provide active sites for secondary heterogeneous shown in Fig. 5a. Two emission peaks are apparently (a) (b) (c) (d) Fig. 5 a PL spectrum of feather-like ZnO. b Reflection spectra of ZnO/Si and Si planar. c The schematic of feather-like ZnO/Si photo-diodes. d I-V curves of feather-like ZnO/Si; the inset of d is the lnI-V curves Jiang et al. Nanoscale Research Letters (2017) 12:483 Page 5 of 6 (a) (b) (c) Fig. 6 a I-V curves of feather-like ZnO/Si and nano-dot ZnO/p-Si; the insert is the reflection spectra. b The energy band diagram of ZnO/p-Si heterojunction. c I-t curves of feather-like ZnO/p-Si and p-Si planar structures observed. The first emission band at 384 nm is obviously resistance of the diode, and I is the reverse bias satur- caused by the excitations, which can be attributed to the ation current represented. The behavior of the I-V curve UV near-band edge emission [18]. Meanwhile, it is visu- can be partly explained by a band diagram based on the alized that the weaker visible emission appeared by a Anderson model [21]. Moreover, the ratio of photo broad emission band at 443 nm in the green region, re- current to dark current is ~90.24 under the reverse bias vealing their collective optical properties. The irradiative at −2 V bias, which suggests that this structure has an recombination of a photo-generated non-equilibrium obvious photo-response behavior. carriers occupying the oxygen vacancy may give rise to To confirm further that the present feather-like hierarch- the green peak would be the existence of oxygen vacan- ical structures offer the beneficial effect on rectifying char- cies in the films [19]. acteristics, we have also measured the I-V characteristics of Figure 5b shows reflection of the feather-like ZnO/Si nano-dot-like ZnO/Si (Fig. 6a). The results indicate that and planar Si measured by UV-vis-NIR spectroscopy. It feather-like hierarchical ZnO/Si had a better rectifying ef- shows that reflection of feather-like ZnO/Si is obviously fect than nano-dot-like ZnO/Si. Therefore, the feather-like reduced compared with p-Si planar (from 40 to 10%), hierarchical ZnO could effectively suppress the charges and a relatively low reflection in the range of 300 to recombination activity and enhance the rectifying effect. 400 nm resulting from band-to-band absorption. The The energy band diagram of ZnO/p-Si heterojunction superior anti-reflection characteristics with an average was constructed at equilibrium shown as Fig. 6b. In this reflection of less than 10% are observed for ZnO/Si in diagram, the electron affinities for ZnO and Si are taken wavelengths shorter than 400 nm which is the optical as 4.35 and 4.05 eV, respectively. bandgap of ZnO materials [20]. This result indicates that The conduction band offset is ΔE = 0.3 eV, while the feather-like ZnO structures act as an excellent anti- valence band offset is ΔE = 2.54 eV; thus, the conduc- reflection. Therefore, it has a potential application as the tion of holes dominates the forward I-V characteristic of anti-reflection in solar cell. the junction. The valence band offset is very large, there Figure 5d shows the I-V curve of feather-like ZnO/p-Si is a diffusion of electrons from n-ZnO to p-Si and diffu- heterojunction, which is measured in dark and under sion of holes from p-Si to n-ZnO because electrons are AM 1.5 sunlight respectively at room temperature. It minority carriers and holes are majority carriers in p-Si shows rectifying behavior for the junctions indicating and electron are majority carriers and holes are minority formation of a diode between ZnO and Si. The rectifica- carriers in n-ZnO. At low forward voltage, the current tion ratio is as high as 535 at −1 V (1695 at −2V)ina increases exponentially. Therefore, the forward I-V char- dark condition. This indicates that the rectifying behav- acteristics in Fig. 4d can be explained. ior of ZnO/Si is quite excellent. Theoretically, the I-V Figure 6c is the I-t curve of the feather-like ZnO/p-Si relation for a heterojunction could be described as and p-Si planar structure when irradiated with 365-nm UV light at 1-V bias voltage. The response current qvðÞ −IR (I = I − I ) in the ZnO/p-Si device is 0.10 mA, light UV dark I ¼ I exp −1 ð1Þ which is 90% enhancement as compared to Si planar nKT device having a response current of 0.01 mA. The en- where K is the Boltzmann’s constant, T is the absolute hancement in the response current of ZnO/p-Si as com- temperature in Kelvin, q is the unit charge of a single pared to p-Si planar mainly could be due to the presence electron, and n is the ideality factor. R is the series of ZnO/p-Si heterojuncton, which could fastly separate s Jiang et al. Nanoscale Research Letters (2017) 12:483 Page 6 of 6 Table 1 The photo-response parameters of the feather-like Received: 30 May 2017 Accepted: 26 July 2017 ZnO/p-Si and p-Si planar structures Samples I (mA) I (mA) Sensitivity τ (s) τ (s) dark light g d References 1. Teng F, Zheng L, Hu K, Chen H, Li Y, Zhang Z, Fang X (2016) A surface Feather-like ZnO/p-Si 0.048 0.159 2.3 11 9 oxide thin layer of copper nanowires enhanced the UV selective response p-Si 0.051 0.0612 0.22 2.2 2.1 of a ZnO film photodetector. J Mater Chem C 4(36):8416–8421 2. Hu K, Teng F, Zheng L, Yu P, Zhang Z, Chen H, Fang X (2017) Binary response Se/ZnO p-n heterojunction UV photodetector with high on/off ratio and fast speed. Laser Photonics Rev 11(1) the generated carriers and reduce the recombination rate 3. Chen H, Liu H, Zhang Z, Hu K, Fang X (2016) Nanostructured of photogenerated free charge carriers. The feather-like photodetectors: from ultraviolet to terahertz. Adv Mater 28(3):403–433 ZnO/p-Si device show a single exponential rise under il- 4. Yin B, Qiu Y, Zhang H, Luo Y, Zhao Y, Yang D, Hu L (2017) Improved photoresponse performance of a self-powered Si/ZnO heterojunction lumination which can be attributed to the recombination ultraviolet and visible photodetector by the piezo-phototronic effect. of the electron-hole pairs. In Table 1, we reviewed all pa- Semicond Sci Technol 32(6):064002 rameters from the two devices. As compared with bare Si 5. Lee SH, Kim SH, Yu JS (2016) Metal-semiconductor-metal near-ultraviolet (~380 nm) photodetectors by selective area growth of ZnO nanorods and planar, the sensitivity of the feather-like ZnO/Si structure SiO 2 passivation. Nanoscale Res Lett 11(1):333 has been improved nearly 10 times. Furthermore, as 6. Dalvand R, Mahmud S, Rouhi J (2015) Direct growth of flower-like ZnO shown in Fig. 5c, their rise and decay times have been nanostructures on porous silicon substrate using a facile low-temperature technique. Mater Lett 160:444–447. greatly increased for the feather-like ZnO/Si device which 7. Hong J-I, Bae J, Wang ZL, Snyder RL (2009) Room-temperature, texture- can be attributed to the recombination of holes-electrons. controlled growth of ZnO thin films and their application for growing The results suggest that the feather-like hierarchical ZnO aligned ZnO nanowire arrays. Nanotechnology 20(8):085609 8. Stassi S, Cauda V, Ottone C, Chiodoni A, Pirri CF, Canavese G (2015) Flexible structures exhibit excellent sensitivity to UV light. These piezoelectric energy nanogenerator based on ZnO nanotubes hosted in a cyclic behaviors also reveal that both devices show highly polycarbonate membrane. Nano Energy 13:474–481 repeatable photo-response with UV illumination. 9. Changdong G, Cheng C, Huang H, Wong T, Wang N, Zhang T-Y (2009) Growth and photocatalytic activity of dendrite-like ZnO@ Ag heterostructure nanocrystals. Crystal Growth Design 9(7):3278–3285 Conclusions 10. Zhang N, Yu K, Zhu Z, Jiang D (2008) Synthesis and humidity sensing Feather-like hierarchical ZnO structures were successfully properties of feather-like ZnO nanostructures with macroscale in shape. Sensors Actuators A Phys 143(2):245–250 synthesized without any seed layer or metal catalyst by a 11. Djurišić AB, Leung YH (2006) Optical properties of ZnO nanostructures. facile SILAR technique at room temperature. The prob- Small 2(8-9):944–961 able mechanism of a two-stage nucleation-growth process 12. Hewlett RM, McLachlan MA (2016) Surface structure modification of ZnO and the impact on electronic properties. Adv Mater 28(20):3893–3921 had been proposed. Meanwhile, the feather-like ZnO pos- 13. Cheng A-J, Tzeng Y, Zhou Y, Park M, Wu T-h, Shannon C, Wang D, Lee W sesses excellent anti-reflection, good photo-response, and (2008) Thermal chemical vapor deposition growth of zinc oxide nanostructures enhanced UV photocurrent. All enhanced characteristics for dye-sensitized solar cell fabrication. Appl Phys Lett 92(9):092113 14. Dalvand R, Mahmud S, Rouhi J, Raymond Ooi CH (2015) Well-aligned ZnO are attributed to the presence of novel feather-like ZnO; nanoneedle arrays grown on polycarbonate substrates via electric field- this hierarchical ZnO structures probably have potential assisted chemical method. Mater Lett 146:65–68 application in photo-detector devices. 15. Wahid KA, Lee WY, Lee HW, Teh AS, Bien DCS, Azid IA (2013) Effect of seed annealing temperature and growth duration on hydrothermal ZnO nanorod Acknowledgements structures and their electrical characteristics. Appl Surf Sci 283:629–635 This work is supported by the Chinese Nature Science Foundation 16. Kathalingam A, Kim H-S (2017) Annealing induced p-type conversion and Committee (No. 61640406), colleges and universities in Henan Province Key substrate dependent effect of n-ZnO/p-Si heterostructure. Mater Lett 196:30–32 Scientific Research Project Funding Scheme (No. 17A140020), and Henan 17. Liu H, Li M, Wei Y, Liu Z, Hu Y, Ma H (2014) A facile surfactant-free synthesis Normal University Youth Backbone Teachers (No. 5101029470611). of flower-like ZnO hierarchical structure at room temperature. Mater Lett 137:300–303 Authors’ contributions 18. Zhang D-F, Sun L-D, Zhang J, Yan Z-G, Yan C-H (2008) Hierarchical JY carried out the fabrication of the ZnO and analysis of the photo-response construction of ZnO architectures promoted by heterogeneous nucleation. properties and drafted the manuscript. LX participated in the experimental Crystal Growth Design 8(10):3609–3615 design and the sequence alignment of the manuscript. CF participated in 19. Maosong Mo, Jimmy C Yu, Lizhi Zhang, and S-KA Li (2005) Self-assembly of the SEM characterization. HL carried out the TEM characterization. All authors ZnO nanorods and nanosheets into hollow microhemispheres and read and approved the final manuscript. microspheres. Adv Mater 17(6):756–760 20. Liu C, Meng D, Wu X, Wang Y, Yu X, Zhang Z, Liu X (2011) Synthesis, Competing interests characterization and optical properties of sheet-like ZnO. Mater Res Bull The authors declare that they have no competing interests. 46(9):1414–1416 21. Mridha S, Dutta M, Basak D (2009) Photoresponse of n-ZnO/p-Si heterojunction towards ultraviolet/visible lights: thickness dependent Publisher’sNote behavior. J Mater Sci Mater Electron 20:376–379 Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Author details Henan Key Laboratory of Photovoltaic Materials, College of Physics and Materials Science, Henan Normal University, Xinxiang 453007, China. School of Computer and Information Engineering, Henan Normal University, Xinxiang, China. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Nanoscale Research Letters Springer Journals

Direct Growth of Feather-Like ZnO Structures by a Facile Solution Technique for Photo-Detecting Application

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Abstract

The feather-like hierarchical zinc oxide (ZnO) was synthesized via successive ionic layer adsorption and reaction without any seed layer or metal catalyst. A possible growth mechanism is proposed to explain the forming process of ZnO feather-like structures. Meanwhile, the photo-electronic performances of the feather-like ZnO have been investigated with the UV-vis-NIR spectroscopy, I-V and I-tmeasurements. The results indicate that feather-like ZnO hierarchical structures have good anti-reflection and excellent photo-sensitivity. All results suggest that the direct growth processing of novel feather-like ZnO is envisaged to have promising application in the field of photo-detector devices. Keywords: Photo-response, Nanostructures, Feather-like hierarchical structures, Successive ionic layer adsorption and reaction Background handy and large area preparation. However, these methods Zinc oxide (ZnO) is a very versatile material due to its often require a seed layer and metal catalysts. ZnO seed wide bandgap (~3.37 eV) and large exciton binding layer growth may already have a well control for the ZnO energy, up to 60 meV, which allow the fabrication of UV nanostructure growth, which normally needs to be [1, 2] and blue light-emitting diode [3]. In recent years, in- annealed with a high temperature or complicated vacuum tensive efforts have been put in the exploration of photo- equipments [16]. In addition, using a seed layer and metal detectors [4, 5] based on the three-dimensional (3D) ZnO catalysts could make the synthesis procedure more com- architectures with the micrometer- and nanometer-scale plex and introduce impurities which influence the proper- building blocks. Compared with mono-morphological ties of the ZnO structure. ZnO structures, 3D hierarchical ZnO structures possess a Therefore, it still remains an enormous challenge to large surface area which could facilitate the adsorption of develop a facile room-temperature method that needs light. Generally, 3D hierarchical ZnO structures such as not any seed layer or metal catalyst for producing hier- flower-like structures [6], texture [7], nanotubes [8], and archical ZnO structures. dendritic-like [9] and feather-like [10] structures exhibit Herein, in this work, a new attempt was made to pre- outstanding optical [11], electronic [12], catalytic proper- pare ZnO hierarchical structures, which was used with- ties [9] and thus have many potential applications in solar out any seed layer or metal catalyst based on successive cells, gas sensors, photo-catalysts, and other fields. To ionic layer adsorption and reaction (SILAR) processing. synthesize hierarchical ZnO structures, various physical, The novel and unusual feather-like ZnO hierarchical chemical [13], and electrochemical [14] methods have structures were obtained for the first time based on been employed. Among them, the hydrothermal/sol- SILAR at room temperature. A possible mechanism was vothermal method [15] is very popular because of its proposed to explain the growth process of the ZnO feather-like structures. In addition, the photoelectric properties of the feather-like ZnO/p-Si heterojunctions * Correspondence: jiangyurong@whut.edu.cn had been investigated, and the results indicate that Henan Key Laboratory of Photovoltaic Materials, College of Physics and Materials Science, Henan Normal University, Xinxiang 453007, China feather-like ZnO nanostructures have excellent anti- Full list of author information is available at the end of the article © The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Jiang et al. Nanoscale Research Letters (2017) 12:483 Page 2 of 6 reflection characteristics and good photosensitivity, diodes characteristics, the electrode of 12-nm semitrans- which suggests that these hierarchical structures have a parent Cu film was deposited on the ZnO/p-Si by the potential application in the photo-electronic devices. thermal evaporation masked with an area of 5 mm × 5 mm. The schematic of diode is shown in Fig. 4c. Methods First Si (100) substrates were ultrasonically cleaned for Results and discussion 10 min in ethanol. Second, 0.01 mol of zinc acetate Figure 1a shows that ZnO has feather-like morphology, (Zn(CH COO) ) was dissolved into 100 mL of deionized which is novel and unusual. The longitudinal length of 3 2 water, then ammonia hydroxide was added into the solu- feather-like structures varies between 300 and 800 nm, tion until its pH was around 11, to form a uniform and its lateral length is different from 200 to 400 nm. The transparent solution under stirring, which is the precur- magnified SEM image in Fig. 1b shows that the hierarch- sor solution of feather-like ZnO. Afterward, silicon wafer ical structures are obtained. Meanwhile, the branches of was dipped into the predecessor solution for 30 s, and feather-like 3D structures are interestingly assembled the ion complex was absorbed into the Si substrate, then perpendicularly to the nano-sheet trunks. Figure 1c shows the Si substrate was taken out and put into deionized the TEM image of an individual hierarchical structure. water for 20 s and was washed with ultrapure water for The dark dots and translucent plate correspond to the 20 times to remove impurities such as unconsolidated branches and the nano-sheet trunk. Because the size of zinc hydroxide (Zn(OH) ). Finally, the samples were put feather-like ZnO is beyond 200 nm, the lattice fringe into deionized water with 90 °C for 1 min; in this step, could not be revealed. Figure 2 show the typical TEM the unreacted ion complex and zinc hydroxide which images of a nanorod segment form the ZnO feathers, it had been absorbed can be resolved into pure ZnO. In a proves the nanorod is a single crystal. typical SILAR experiment, we circulated the above steps Figure 1e shows the peaks of EDS in which only Zn, O, for 20 times. The crystal structures of feather-like ZnO C, and Si were found in our sample, which indicates that were characterized by X-ray diffraction (XRD) and energy the process of SILAR is successful to deposit pure ZnO disperse spectrometer (EDS). The surface morphology onto silicon. The XRD (Fig. 1e) reveals the crystal struc- was investigated by scanning electron microscopy (SEM) ture and phase purity of the ZnO hierarchical structures. and transporting electron microscopy (TEM). Further- All the diffraction peaks of the products match very well more, we also analyzed I-V and I-t characteristics of with those of wurtzite ZnO (JCPDS file 36-1451), as well feather-like ZnO/p-Si. In order to measure the photo- as a dominant diffraction peak corresponding to the p-Si (a) (b) () c 200nm 1um 200nm (f) (d) () e (c) ZnO P-Si 200nm Fig. 1 a, b The SEM images of feather-like ZnO grown on silicon. c The TEM image of individual feather-like ZnO. d The cross-sectional SEM image of feather-like ZnO/p-Si. e The EDS analysis of the ZnO/p-Si, indicating that the predominant composition is Zn. f XRD patterns of feather-like ZnO/p-Si Jiang et al. Nanoscale Research Letters (2017) 12:483 Page 3 of 6 Fig. 2 TEM images of a hierarchical ZnO structure segment (400). No diffraction peaks from other impurities are On the basis of the above results, it can be speculated found in the spectrum; the result indicates that the struc- that the feather-like ZnO hierarchical structures were syn- ture is pure hexagonal wurtzite ZnO. Moreover, the inten- thesized via a two-stage nucleation-growth process. Figure sity of peak (002) is rather higher than peaks (100) and 4 shows the schematic diagram describing the formation (101); this shows that the crystalline is along the (002) axis processes of ZnO hierarchical structures. First, ammonia preferred orientation. The sharp diffraction peaks reveal hydroxide is used to provide hybroxyl anions (OH )which that ZnO have high crystal structure of pure quality. increases the pH of reaction solution and the alkalinity of 2− It should be mentioned here that no ZnO hierarchical the reaction solution, then the Zn(OH) ions are obtained. 2− 2− structures are found even though the reaction is carried Upon the dehydration of Zn(OH) ions, Zn(OH) ions 4 4 out under the same environment when using Si nano- are adsorbed onto the Si substrate and subsequently dis- wires with all crystal directions replacing Si (100) sub- solved to form homogeneous ZnO nuclei followed by the strates (shown as Fig. 3). The results indicate that the water bath at 90 °C [17]. During this process, the trunk for- crystal direction plays a key role in the nucleation and mation of ZnO nano-sheets with {110} planar surface at growth of ZnO hierarchical structure. the initial stage can be ascribed to the excess OH ions (a) (b) Fig. 3 SEM images of ZnO grown on silicon nanowires: a morphology and b cross section Jiang et al. Nanoscale Research Letters (2017) 12:483 Page 4 of 6 (a) (b) (c) Fig. 4 The schematic diagram of formation processes for ZnO hierarchical structures: a the trunk formation of ZnO nano-sheets; b secondary heterogeneous nucleation and growth of branches; c the continuously growth of primary nano-sheets and secondary nano-branches constructs the feather-like ZnO hierarchical structures 2− and abundant Zn(OH) ions (shown as Fig. 4a), which nucleation and growth of branches (shown as Fig. 4b). can stabilize the surface charge and the structure of Zn Finally, the continuously growth of primary nano-sheets (001) surface to some extent, allowing fast growth along and secondary nano-branches constructs the feather-like the [100] direction [18]. Second, the surface of the primary ZnO hierarchical structures (shown as Fig. 4c). ZnO nano-sheets trunk formed during the initial growth To investigate the optical properties of feather-like stage has many crystalline boundaries which contain more ZnO, the room-temperature PL was obtained by using a defects than other regions. These defects on the surface of He–Cd laser (λ = 325 nm) as the excitation source as trunk provide active sites for secondary heterogeneous shown in Fig. 5a. Two emission peaks are apparently (a) (b) (c) (d) Fig. 5 a PL spectrum of feather-like ZnO. b Reflection spectra of ZnO/Si and Si planar. c The schematic of feather-like ZnO/Si photo-diodes. d I-V curves of feather-like ZnO/Si; the inset of d is the lnI-V curves Jiang et al. Nanoscale Research Letters (2017) 12:483 Page 5 of 6 (a) (b) (c) Fig. 6 a I-V curves of feather-like ZnO/Si and nano-dot ZnO/p-Si; the insert is the reflection spectra. b The energy band diagram of ZnO/p-Si heterojunction. c I-t curves of feather-like ZnO/p-Si and p-Si planar structures observed. The first emission band at 384 nm is obviously resistance of the diode, and I is the reverse bias satur- caused by the excitations, which can be attributed to the ation current represented. The behavior of the I-V curve UV near-band edge emission [18]. Meanwhile, it is visu- can be partly explained by a band diagram based on the alized that the weaker visible emission appeared by a Anderson model [21]. Moreover, the ratio of photo broad emission band at 443 nm in the green region, re- current to dark current is ~90.24 under the reverse bias vealing their collective optical properties. The irradiative at −2 V bias, which suggests that this structure has an recombination of a photo-generated non-equilibrium obvious photo-response behavior. carriers occupying the oxygen vacancy may give rise to To confirm further that the present feather-like hierarch- the green peak would be the existence of oxygen vacan- ical structures offer the beneficial effect on rectifying char- cies in the films [19]. acteristics, we have also measured the I-V characteristics of Figure 5b shows reflection of the feather-like ZnO/Si nano-dot-like ZnO/Si (Fig. 6a). The results indicate that and planar Si measured by UV-vis-NIR spectroscopy. It feather-like hierarchical ZnO/Si had a better rectifying ef- shows that reflection of feather-like ZnO/Si is obviously fect than nano-dot-like ZnO/Si. Therefore, the feather-like reduced compared with p-Si planar (from 40 to 10%), hierarchical ZnO could effectively suppress the charges and a relatively low reflection in the range of 300 to recombination activity and enhance the rectifying effect. 400 nm resulting from band-to-band absorption. The The energy band diagram of ZnO/p-Si heterojunction superior anti-reflection characteristics with an average was constructed at equilibrium shown as Fig. 6b. In this reflection of less than 10% are observed for ZnO/Si in diagram, the electron affinities for ZnO and Si are taken wavelengths shorter than 400 nm which is the optical as 4.35 and 4.05 eV, respectively. bandgap of ZnO materials [20]. This result indicates that The conduction band offset is ΔE = 0.3 eV, while the feather-like ZnO structures act as an excellent anti- valence band offset is ΔE = 2.54 eV; thus, the conduc- reflection. Therefore, it has a potential application as the tion of holes dominates the forward I-V characteristic of anti-reflection in solar cell. the junction. The valence band offset is very large, there Figure 5d shows the I-V curve of feather-like ZnO/p-Si is a diffusion of electrons from n-ZnO to p-Si and diffu- heterojunction, which is measured in dark and under sion of holes from p-Si to n-ZnO because electrons are AM 1.5 sunlight respectively at room temperature. It minority carriers and holes are majority carriers in p-Si shows rectifying behavior for the junctions indicating and electron are majority carriers and holes are minority formation of a diode between ZnO and Si. The rectifica- carriers in n-ZnO. At low forward voltage, the current tion ratio is as high as 535 at −1 V (1695 at −2V)ina increases exponentially. Therefore, the forward I-V char- dark condition. This indicates that the rectifying behav- acteristics in Fig. 4d can be explained. ior of ZnO/Si is quite excellent. Theoretically, the I-V Figure 6c is the I-t curve of the feather-like ZnO/p-Si relation for a heterojunction could be described as and p-Si planar structure when irradiated with 365-nm UV light at 1-V bias voltage. The response current qvðÞ −IR (I = I − I ) in the ZnO/p-Si device is 0.10 mA, light UV dark I ¼ I exp −1 ð1Þ which is 90% enhancement as compared to Si planar nKT device having a response current of 0.01 mA. The en- where K is the Boltzmann’s constant, T is the absolute hancement in the response current of ZnO/p-Si as com- temperature in Kelvin, q is the unit charge of a single pared to p-Si planar mainly could be due to the presence electron, and n is the ideality factor. R is the series of ZnO/p-Si heterojuncton, which could fastly separate s Jiang et al. Nanoscale Research Letters (2017) 12:483 Page 6 of 6 Table 1 The photo-response parameters of the feather-like Received: 30 May 2017 Accepted: 26 July 2017 ZnO/p-Si and p-Si planar structures Samples I (mA) I (mA) Sensitivity τ (s) τ (s) dark light g d References 1. Teng F, Zheng L, Hu K, Chen H, Li Y, Zhang Z, Fang X (2016) A surface Feather-like ZnO/p-Si 0.048 0.159 2.3 11 9 oxide thin layer of copper nanowires enhanced the UV selective response p-Si 0.051 0.0612 0.22 2.2 2.1 of a ZnO film photodetector. J Mater Chem C 4(36):8416–8421 2. Hu K, Teng F, Zheng L, Yu P, Zhang Z, Chen H, Fang X (2017) Binary response Se/ZnO p-n heterojunction UV photodetector with high on/off ratio and fast speed. Laser Photonics Rev 11(1) the generated carriers and reduce the recombination rate 3. Chen H, Liu H, Zhang Z, Hu K, Fang X (2016) Nanostructured of photogenerated free charge carriers. The feather-like photodetectors: from ultraviolet to terahertz. Adv Mater 28(3):403–433 ZnO/p-Si device show a single exponential rise under il- 4. Yin B, Qiu Y, Zhang H, Luo Y, Zhao Y, Yang D, Hu L (2017) Improved photoresponse performance of a self-powered Si/ZnO heterojunction lumination which can be attributed to the recombination ultraviolet and visible photodetector by the piezo-phototronic effect. of the electron-hole pairs. In Table 1, we reviewed all pa- Semicond Sci Technol 32(6):064002 rameters from the two devices. As compared with bare Si 5. Lee SH, Kim SH, Yu JS (2016) Metal-semiconductor-metal near-ultraviolet (~380 nm) photodetectors by selective area growth of ZnO nanorods and planar, the sensitivity of the feather-like ZnO/Si structure SiO 2 passivation. Nanoscale Res Lett 11(1):333 has been improved nearly 10 times. Furthermore, as 6. Dalvand R, Mahmud S, Rouhi J (2015) Direct growth of flower-like ZnO shown in Fig. 5c, their rise and decay times have been nanostructures on porous silicon substrate using a facile low-temperature technique. Mater Lett 160:444–447. greatly increased for the feather-like ZnO/Si device which 7. Hong J-I, Bae J, Wang ZL, Snyder RL (2009) Room-temperature, texture- can be attributed to the recombination of holes-electrons. controlled growth of ZnO thin films and their application for growing The results suggest that the feather-like hierarchical ZnO aligned ZnO nanowire arrays. Nanotechnology 20(8):085609 8. Stassi S, Cauda V, Ottone C, Chiodoni A, Pirri CF, Canavese G (2015) Flexible structures exhibit excellent sensitivity to UV light. These piezoelectric energy nanogenerator based on ZnO nanotubes hosted in a cyclic behaviors also reveal that both devices show highly polycarbonate membrane. Nano Energy 13:474–481 repeatable photo-response with UV illumination. 9. Changdong G, Cheng C, Huang H, Wong T, Wang N, Zhang T-Y (2009) Growth and photocatalytic activity of dendrite-like ZnO@ Ag heterostructure nanocrystals. Crystal Growth Design 9(7):3278–3285 Conclusions 10. Zhang N, Yu K, Zhu Z, Jiang D (2008) Synthesis and humidity sensing Feather-like hierarchical ZnO structures were successfully properties of feather-like ZnO nanostructures with macroscale in shape. Sensors Actuators A Phys 143(2):245–250 synthesized without any seed layer or metal catalyst by a 11. Djurišić AB, Leung YH (2006) Optical properties of ZnO nanostructures. facile SILAR technique at room temperature. The prob- Small 2(8-9):944–961 able mechanism of a two-stage nucleation-growth process 12. Hewlett RM, McLachlan MA (2016) Surface structure modification of ZnO and the impact on electronic properties. Adv Mater 28(20):3893–3921 had been proposed. Meanwhile, the feather-like ZnO pos- 13. Cheng A-J, Tzeng Y, Zhou Y, Park M, Wu T-h, Shannon C, Wang D, Lee W sesses excellent anti-reflection, good photo-response, and (2008) Thermal chemical vapor deposition growth of zinc oxide nanostructures enhanced UV photocurrent. All enhanced characteristics for dye-sensitized solar cell fabrication. Appl Phys Lett 92(9):092113 14. Dalvand R, Mahmud S, Rouhi J, Raymond Ooi CH (2015) Well-aligned ZnO are attributed to the presence of novel feather-like ZnO; nanoneedle arrays grown on polycarbonate substrates via electric field- this hierarchical ZnO structures probably have potential assisted chemical method. Mater Lett 146:65–68 application in photo-detector devices. 15. Wahid KA, Lee WY, Lee HW, Teh AS, Bien DCS, Azid IA (2013) Effect of seed annealing temperature and growth duration on hydrothermal ZnO nanorod Acknowledgements structures and their electrical characteristics. Appl Surf Sci 283:629–635 This work is supported by the Chinese Nature Science Foundation 16. Kathalingam A, Kim H-S (2017) Annealing induced p-type conversion and Committee (No. 61640406), colleges and universities in Henan Province Key substrate dependent effect of n-ZnO/p-Si heterostructure. Mater Lett 196:30–32 Scientific Research Project Funding Scheme (No. 17A140020), and Henan 17. Liu H, Li M, Wei Y, Liu Z, Hu Y, Ma H (2014) A facile surfactant-free synthesis Normal University Youth Backbone Teachers (No. 5101029470611). of flower-like ZnO hierarchical structure at room temperature. Mater Lett 137:300–303 Authors’ contributions 18. Zhang D-F, Sun L-D, Zhang J, Yan Z-G, Yan C-H (2008) Hierarchical JY carried out the fabrication of the ZnO and analysis of the photo-response construction of ZnO architectures promoted by heterogeneous nucleation. properties and drafted the manuscript. LX participated in the experimental Crystal Growth Design 8(10):3609–3615 design and the sequence alignment of the manuscript. CF participated in 19. Maosong Mo, Jimmy C Yu, Lizhi Zhang, and S-KA Li (2005) Self-assembly of the SEM characterization. HL carried out the TEM characterization. All authors ZnO nanorods and nanosheets into hollow microhemispheres and read and approved the final manuscript. microspheres. Adv Mater 17(6):756–760 20. Liu C, Meng D, Wu X, Wang Y, Yu X, Zhang Z, Liu X (2011) Synthesis, Competing interests characterization and optical properties of sheet-like ZnO. Mater Res Bull The authors declare that they have no competing interests. 46(9):1414–1416 21. Mridha S, Dutta M, Basak D (2009) Photoresponse of n-ZnO/p-Si heterojunction towards ultraviolet/visible lights: thickness dependent Publisher’sNote behavior. J Mater Sci Mater Electron 20:376–379 Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Author details Henan Key Laboratory of Photovoltaic Materials, College of Physics and Materials Science, Henan Normal University, Xinxiang 453007, China. School of Computer and Information Engineering, Henan Normal University, Xinxiang, China.

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Nanoscale Research LettersSpringer Journals

Published: Aug 10, 2017

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