Direct Growth of Feather-Like ZnO Structures by a Facile Solution Technique for Photo-Detecting Application
© The Author(s). 2017
Received: 30 May 2017
Accepted: 26 July 2017
Published: 10 August 2017
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.
Zinc oxide (ZnO) is a very versatile material due to its wide bandgap (~3.37 eV) and large exciton binding energy, up to 60 meV, which allow the fabrication of UV [1, 2] and blue light-emitting diode . In recent years, intensive efforts have been put in the exploration of photodetectors [4, 5] based on the three-dimensional (3D) ZnO architectures with the micrometer- and nanometer-scale building blocks. Compared with mono-morphological ZnO structures, 3D hierarchical ZnO structures possess a large surface area which could facilitate the adsorption of light. Generally, 3D hierarchical ZnO structures such as flower-like structures , texture , nanotubes , and dendritic-like  and feather-like  structures exhibit outstanding optical , electronic , catalytic properties  and thus have many potential applications in solar cells, gas sensors, photo-catalysts, and other fields. To synthesize hierarchical ZnO structures, various physical, chemical , and electrochemical  methods have been employed. Among them, the hydrothermal/solvothermal method  is very popular because of its handy and large area preparation. However, these methods often require a seed layer and metal catalysts. ZnO seed layer growth may already have a well control for the ZnO nanostructure growth, which normally needs to be annealed with a high temperature or complicated vacuum equipments . In addition, using a seed layer and metal catalysts could make the synthesis procedure more complex and introduce impurities which influence the properties of the ZnO structure.
Therefore, it still remains an enormous challenge to develop a facile room-temperature method that needs not any seed layer or metal catalyst for producing hierarchical ZnO structures.
Herein, in this work, a new attempt was made to prepare ZnO hierarchical structures, which was used without any seed layer or metal catalyst based on successive ionic layer adsorption and reaction (SILAR) processing. The novel and unusual feather-like ZnO hierarchical structures were obtained for the first time based on SILAR at room temperature. A possible mechanism was 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 had been investigated, and the results indicate that feather-like ZnO nanostructures have excellent anti-reflection characteristics and good photosensitivity, which suggests that these hierarchical structures have a potential application in the photo-electronic devices.
First Si (100) substrates were ultrasonically cleaned for 10 min in ethanol. Second, 0.01 mol of zinc acetate (Zn(CH3COO)2) was dissolved into 100 mL of deionized water, then ammonia hydroxide was added into the solution until its pH was around 11, to form a uniform transparent solution under stirring, which is the precursor solution of feather-like ZnO. Afterward, silicon wafer was dipped into the predecessor solution for 30 s, and the ion complex was absorbed into the Si substrate, then the Si substrate was taken out and put into deionized water for 20 s and was washed with ultrapure water for 20 times to remove impurities such as unconsolidated zinc hydroxide (Zn(OH)2). Finally, the samples were put into deionized water with 90 °C for 1 min; in this step, the unreacted ion complex and zinc hydroxide which had been absorbed can be resolved into pure ZnO. In a typical SILAR experiment, we circulated the above steps for 20 times. The crystal structures of feather-like ZnO were characterized by X-ray diffraction (XRD) and energy disperse spectrometer (EDS). The surface morphology was investigated by scanning electron microscopy (SEM) and transporting electron microscopy (TEM). Furthermore, we also analyzed I-V and I-t characteristics of feather-like ZnO/p-Si. In order to measure the photo-diodes characteristics, the electrode of 12-nm semitransparent Cu film was deposited on the ZnO/p-Si by the thermal evaporation masked with an area of 5 mm × 5 mm. The schematic of diode is shown in Fig. 4c.
Results and discussion
Figure 1e shows the peaks of EDS in which only Zn, O, C, and Si were found in our sample, which indicates that the process of SILAR is successful to deposit pure ZnO onto silicon. The XRD (Fig. 1e) reveals the crystal structure and phase purity of the ZnO hierarchical structures. All the diffraction peaks of the products match very well with those of wurtzite ZnO (JCPDS file 36-1451), as well as a dominant diffraction peak corresponding to the p-Si (400). No diffraction peaks from other impurities are found in the spectrum; the result indicates that the structure is pure hexagonal wurtzite ZnO. Moreover, the intensity of peak (002) is rather higher than peaks (100) and (101); this shows that the crystalline is along the (002) axis preferred orientation. The sharp diffraction peaks reveal that ZnO have high crystal structure of pure quality.
Figure 5b shows reflection of the feather-like ZnO/Si and planar Si measured by UV-vis-NIR spectroscopy. It shows that reflection of feather-like ZnO/Si is obviously reduced compared with p-Si planar (from 40 to 10%), and a relatively low reflection in the range of 300 to 400 nm resulting from band-to-band absorption. The superior anti-reflection characteristics with an average reflection of less than 10% are observed for ZnO/Si in wavelengths shorter than 400 nm which is the optical bandgap of ZnO materials . This result indicates that feather-like ZnO structures act as an excellent anti-reflection. Therefore, it has a potential application as the anti-reflection in solar cell.
where K is the Boltzmann’s constant, T is the absolute temperature in Kelvin, q is the unit charge of a single electron, and n is the ideality factor. R s is the series resistance of the diode, and I 0 is the reverse bias saturation current represented. The behavior of the I-V curve can be partly explained by a band diagram based on the Anderson model . Moreover, the ratio of photo current to dark current is ~90.24 under the reverse bias at −2 V bias, which suggests that this structure has an obvious photo-response behavior.
The energy band diagram of ZnO/p-Si heterojunction was constructed at equilibrium shown as Fig. 6b. In this diagram, the electron affinities for ZnO and Si are taken as 4.35 and 4.05 eV, respectively.
The conduction band offset is ∆E c = 0.3 eV, while the valence band offset is ∆E v = 2.54 eV; thus, the conduction of holes dominates the forward I-V characteristic of the junction. The valence band offset is very large, there is a diffusion of electrons from n-ZnO to p-Si and diffusion of holes from p-Si to n-ZnO because electrons are minority carriers and holes are majority carriers in p-Si and electron are majority carriers and holes are minority carriers in n-ZnO. At low forward voltage, the current increases exponentially. Therefore, the forward I-V characteristics in Fig. 4d can be explained.
The photo-response parameters of the feather-like ZnO/p-Si and p-Si planar structures
I dark (mA)
I light (mA)
τ g (s)
τ d (s)
Feather-like hierarchical ZnO structures were successfully synthesized without any seed layer or metal catalyst by a facile SILAR technique at room temperature. The probable mechanism of a two-stage nucleation-growth process had been proposed. Meanwhile, the feather-like ZnO possesses excellent anti-reflection, good photo-response, and enhanced UV photocurrent. All enhanced characteristics are attributed to the presence of novel feather-like ZnO; this hierarchical ZnO structures probably have potential application in photo-detector devices.
This work is supported by the Chinese Nature Science Foundation Committee (No. 61640406), colleges and universities in Henan Province Key Scientific Research Project Funding Scheme (No. 17A140020), and Henan Normal University Youth Backbone Teachers (No. 5101029470611).
JY carried out the fabrication of the ZnO and analysis of the photo-response properties and drafted the manuscript. LX participated in the experimental design and the sequence alignment of the manuscript. CF participated in the SEM characterization. HL carried out the TEM characterization. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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