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Improvement of UV emission in ZnO thin film caused by a transition from polycrystalline to monocrystalline
Journal Pre-proof Improvement of UV emission in ZnO thin film caused by a transition from polycrystalline to monocrystalline
Linhua Xu, Wenjian Kuang, Zhanhui Liu, Fenglin Xian PII:
S0921-4526(20)30029-6
DOI:
https://doi.org/10.1016/j.physb.2020.412010
Reference:
PHYSB 412010
To appear in:
Physica B: Physics of Condensed Matter
Received Date:
19 September 2019
Accepted Date:
12 January 2020
Please cite this article as: Linhua Xu, Wenjian Kuang, Zhanhui Liu, Fenglin Xian, Improvement of UV emission in ZnO thin film caused by a transition from polycrystalline to monocrystalline, Physica B: Physics of Condensed Matter (2020), https://doi.org/10.1016/j.physb.2020.412010
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
Journal Pre-proof
Improvement of UV emission in ZnO thin film caused by a transition from polycrystalline to monocrystalline Linhua Xu a, b, c, *, Wenjian Kuang a, b, c, Zhanhui Liu a, Fenglin Xian a, b, c a
School of Physics and Optoelectronic Engineering, Nanjing University of Information Science & Technology, Nanjing 210044, China b Jiangsu Key Laboratory for Optoelectronic Detection of Atmosphere and Ocean，Nanjing University of Information Science & Technology, Nanjing 210044, China c Optics and Photonic Technology Laboratory, Nanjing University of Information Science & Technology, Nanjing 210044, China
The corresponding author is Dr. Linhua Xu and his address and other information is as follows: Postal address: School of Physics and Optoelectronic Engineering, Nanjing University of Information Science & Technology, Ningliu Road 219#, Nanjing, 210044, China E-mail: [email protected] Tel: +86-025-587311031 Fax: +86-025-58731031 Abstract: ZnO thin films and ZnO nanostructures have good UV emission properties, which are ideal materials for preparing short-wavelength light-emitting devices. In this study, Fe-doped ZnO thin films were prepared by sol-gel method using ferric chloride as the doping source. Compared with undoped ZnO thin films, the UV emission of low-concentration Fe-doped ZnO thin films did not show significant changes. However, the UV emission of high-concentration Fe-doped (≥4at.%) ZnO thin films was significantly enhanced and the refractive index had also increased largely. It is observed by a scanning electron microscope (SEM) that the high-concentration Fe-doped ZnO thin film has a two-layer structure: the upper layer is composed of the frustum of hexagonal pyramid (FHP) shaped nanostructures, and the bottom layer in contact with the substrate is composed of polycrystalline spherical grains. Selected area electron diffraction (SAED) and high resolution transmission electron microscopy (HRTEM) show that the upper FHP-shaped nanostructure is a
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Journal Pre-proof single crystal with high crystalline quality, while the bottom layer is polycrystalline. Therefore, the formation of single crystal ZnO nanostructures greatly improves the optical quality of ZnO thin films. At present, as far as we know, the experimental results of partially converting polycrystalline ZnO into single crystal ZnO to greatly improve the UV emission performance of ZnO thin films have rarely been reported. The ZnO thin film with a two-layer structure prepared here is a candidate material for preparing UV light-emitting devices. Key words: Photoluminescence; UV emission; ZnO thin films; sol-gel method; polycrystalline; single crystal
1. Introduction Zinc Oxide (ZnO) is a wide band gap semiconductor that crystallizes into a wurtzite structure at normal temperature and pressure. Its high exciton binding energy (~60meV) makes it have high efficient UV emission caused by free exciton recombination at room temperature and even higher temperature [1, 2]. This characteristic has made ZnO nanomaterial an ideal candidate for fabricating short-wavelength light-emitting devices. In addition, ZnO has the following advantages, such as abundant raw materials, diverse morphology, non-toxicity, stable physical properties and so on. In recent years, the luminescence behavior of ZnO nanomaterials has attracted great attention from researchers [3-6]. The research on the luminescence behavior of ZnO still focuses on two aspects: one is how to improve the UV emission efficiency of ZnO, the other is to reveal the visible emission mechanisms of ZnO. In order to improve the UV emission efficiency of ZnO
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Journal Pre-proof materials, researchers have adopted different strategies. For example, Norek et al. [7] improved the UV emission properties of ZnO thin films by the surface plasmon of Al and the passivation effect of the aluminum oxide coating. Al-Nafiey et al. [8] improved the UV emission efficiency by annealing ZnO nanorods in high pressure water vapor. Davood Raoufi [9] increased the UV emission efficiency of ZnO nanoparticles by using appropriate annealing temperatures. Zheng et al. [10] enhanced the UV emission performance of ZnO particles by Y doping. Baranowska-Korczyc et al. [11] found that depositing a layer of ZnS on the surface of ZnO thin films could passivate surface defects and improve their UV emission efficiency. Jayalakshmi et al. [12] improved the UV emission properties of ZnO by constructing a hybrid material using reduced graphene oxide and ZnO. It can be known from the above results that the improvement of the crystallization quality and the reduction of defects are the key factors to enhance the UV emission of ZnO. In this study, Fe-doped ZnO thin films were deposited by sol-gel method using FeCl3 as the doping source. When the Fe-doping concentration is higher than 2at.%, the FHP-shaped nanostructures are formed on the polycrystalline ZnO thin film surface. The selected area electron diffraction and high resolution TEM analysis indicate that these nanostructures are high-quality single crystals. Compared with undoped ZnO thin films, those with FHP-shaped nanostructures exhibit higher ultraviolet emission performance. As far as we know, the improvement of UV emission properties of ZnO thin films caused by the transformation from polycrystalline to monocrystalline is seldom reported. Such a method of obtaining
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Journal Pre-proof ZnO thin films with high optical quality by doping has the advantages of low cost and simple operation. The details are exhibited below.
2. Experiments The sol-gel method was used to prepare Fe-doped ZnO thin films. First, a ZnO sol was prepared: a certain amount of zinc acetate (Zn(CH3COO)2·2H2O) and ferric chloride (FeCl3·6H2O) were put in absolute ethanol, and the above mixed solution was stirred at 60 ℃ for 1 h while monoethanolamine was added dropwise to the above solution. Finally, a transparent and homogeneous sol was formed. In the ZnO precursor solution, the molar ratio of zinc acetate to monoethanolamine was 1:1, and the concentration of zinc ion was 0.3 mol/L. In order to study the effect of different Fe-doping concentration on ZnO thin films, five different ZnO sols were prepared, in which the molar ratios of ferric chloride to zinc acetate were 0, 0.02, 0.04, 0.06 and 0.08, respectively. After these sols were aged at room temperature for 24 h, they were coated on Si substrates by dip-coating method to form ZnO thin films. Every time one layer was coated, it would be preheated. The preheating temperature and time were 280 ℃ and 4 min, respectively. The process from sol coating to preheating treatment was repeated eight times. Eventually, the Fe-doped ZnO thin films were annealed at 500 ℃ for 60 min. The annealing was carried out in an air atmosphere. An X-ray diffractometer (Bruker D8 Advance) was used to analyze the crystal phase of the samples; a field emission scanning electron microscope (S4800) was adopted to observe the microscopic morphology of the films; the composition was analyzed by X-ray photoelectron spectroscopy (XPS). The photoluminescence
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Journal Pre-proof behavior of the samples was investigated by a fluorescence detection equipment (LabRAM HR800). An ellipsometer was applied to measure the refractive index of the ZnO thin films using a He-Ne laser as the light source.
3. Results and discussion 3.1 Evolution of morphology and microstructure of ZnO thin films with the rise of Fe doping concentration The surface and cross-sectional morphology images of the samples are shown in Fig.1. For the undoped ZnO thin films, the structure is compact and the surface is smooth. When 2% FeCl3 is doped into ZnO thin film, it can be clearly seen from the cross-sectional morphology that the crystal grains become smaller. When the doping concentration rises to 4%, FHP-shaped nanostructures appear on the surface of the ZnO thin film. At this time, the ZnO thin film consists of two layers: the upper layer is composed of many FHP-shaped nanostructures, while the bottom layer is composed of many spherical nanocrystals. It is apparent that these FHP-shaped nanostructures are grown perpendicular to the surface of the substrate. When the doping concentration of FeCl3 is further increased, the areal density and uniformity of the FHP-shaped nanostructures in the upper layer are further improved, and the grain size in the bottom layer is further reduced. The reduction of ZnO grains indicates that the rise of Fe-doping concentration increases the internal stress in the film and affects the normal growth of ZnO grains [13]. It can be known from the above results that the appearance and evolution of the special morphology on the surface of ZnO thin films are related to the doping of FeCl3. The special morphologies of ZnO thin films caused
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Journal Pre-proof by doping have also been reported by other researchers [14, 15]. It should be noted that the doping source of Fe (such as ferric nitrate, iron acetate, ferric chloride, etc.) has a great influence on the formation of special morphologies, and the related research is ongoing in our laboratory. In this work, we mainly discuss the improvement of optical properties caused by the morphology evolution of ZnO thin films. The growth mechanism of FHP-shaped ZnO nanostructures caused by FeCl3 doping is still in-depth research and will be reported elsewhere. Fig.2 shows the XRD pattern of the samples. A strong (002) diffraction peak is observed for all the films, indicating that the ZnO thin films have a wurtzite structure and a preferred orientation along the c-axis perpendicular to the substrate surface. When Fe doping concentration is relatively high, (002) peak is intensified and (004) diffraction peak belonging to wurtzite ZnO also appears. This indicates that the FHP-shaped ZnO nanostructures on the films surface have better c-axis orientation than undoped ZnO thin films. XPS spectra of 6% Fe-doped ZnO thin film were measured to obtain information on the composition of the sample and the chemical states of the surface elements, as shown in Fig. 3. From the full spectrum of Fig. 3 (a), it can be seen that the surface of the sample is very pure and contains no impurities except Zn, O, Fe and Cl elements. Fig. 3 (b) displays the high-resolution spectrum of Zn 2p. The two highly geometrically symmetrical peaks at 1021.98 eV and 1044.98 eV correspond to Zn 2p3/2 and Zn 2p1/2 respectively, indicating that Zn ion in the sample is in the valence state of + 2 [16, 17]. Fig. 3 (c) displays the high resolution O 1s peak, which contains
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Journal Pre-proof two sub-peaks with the center at 530.1 and 531.5 eV, respectively. The former comes from lattice oxygen bonded to Zn ions, and the latter comes from oxygen vacancies [16, 17]. Fig. 3 (d) gives the XPS spectrum of high resolution Fe 2p. The two signal peaks at 710.58 and 724.28 eV belong to Fe 2p3/2 and Fe 2p1/2, respectively. These two binding energies imply that Fe is in the + 3 valence state [18]. Fig. 3(e) exhibits a high resolution spectrum of Cl 2p centered at 198.3 eV, indicating that Cl is present in the +7 valence state; its intensity is very low, suggesting a very low content in the ZnO thin film. Our quantitative calculation shows that the doping concentration of Cl in this sample is 2.4 at.% relative to O. In order to further determine the distribution of the elements in ZnO thin films, we took the element mapping images for the sample doped with 6% Fe ions, as shown in Fig. 4. It can be seen that the Fe and Cl elements are uniformly distributed in the film. From the above results, it can be known that these doped ZnO thin films contain not only Fe but also Cl. Therefore, to be precise, the samples prepared here are Fe and Cl co-doped ZnO thin films. Fig. 5 shows the microstructure observed by high-resolution TEM and selected area electron diffraction patterns. The sample to be tested was a fragment peeled off from the Si substrate. It can be seen from the images of Fig. 5 (a, b) that the ZnO thin film contains FHP-shaped nanostructures and granular crystal grains. The selected area electron diffraction was performed on the region ① in Fig. 5(b), and the diffraction pattern is shown in the inset; the bright diffraction spots indicate that the FHP-shaped nanostructure grown on the ZnO thin film surface is single crystal. Fig. 5 (c) and 5 (d) show the high resolution images of regions ① and ② in Fig. 5 (b), respectively. It is
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Journal Pre-proof further confirmed that the FHP-shaped nanostructures on the ZnO thin film surface are single crystal and the bottom layer is polycrystalline zinc oxide. 3.2 The photoluminescence behavior and refractive index of the samples The
luminescence
behavior
of
these
samples
has
been
studied
by
photoluminescence with an excitation wavelength of 325 nm. Fig. 6 (a) shows the photoluminescence spectra of the ZnO thin films doped with Fe ions. All the samples show a strong UV emission and a weak visible emission. The UV emission of ZnO originates from the recombination of free excitons, while the visible emission is related to defects such as oxygen vacancies and zinc interstitials [19-22]. Compared with pure ZnO thin films, Fe-doped ZnO thin films (doping concentration is higher than 2%) exhibit stronger ultraviolet emission, while visible emission decreases. The enhanced ultraviolet emission should be attributed to FHP-shaped nanostructures with high crystalline quality. In comparison with the undoped ZnO thin films, the ultraviolet emission intensity of 8% Fe-doped ZnO thin films is increased by 5 times. For 2% Fe-doped ZnO thin film, the UV emission intensity is almost the same as that of the pure ZnO thin film, but the visible emission intensity is weakened. The inset of Fig. 6(a) shows the PL spectrum of 8% Fe-doped ZnO thin film which is not annealed. Its ultraviolet emission is very weak, and the main luminous region is located at the range of 450-550 nm. This is because the Fe-doped ZnO thin film without post-annealing has poor crystalline quality and contains a large number of point defects such as zinc interstitials and oxygen vacancies which resulted in the dominant deep-level emission. Therefore, after annealing, the formation of highly crystalline
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Journal Pre-proof FHP-shaped nanostructures is responsible for the enhanced UV emission. The luminescent quality of ZnO materials can generally be evaluated by the ratio of ultraviolet emission intensity to visible emission intensity. Fig. 6(b) exhibits the ratios of the UV emission intensity to the visible emission intensity for different samples. It can be seen that with the rise of Fe-doping concentration, the ratio of ultraviolet to visible emission intensity increases gradually, indicating that the ultraviolet emission performance of ZnO thin films is gradually improved. It should be pointed out that the significant improvement of ultraviolet emission performance of ZnO thin films is not related to Fe ions doped in ZnO lattices, but comes from the highly crystalline nanostructures on the films surface. Previously, Fe-doped ZnO materials have been also studied by many people. For example, Santhosh et al. [18] prepared Fe3+ ions doped ZnO thin films using ferric nitrate as doping source. Compared with undoped ZnO thin films, the ultraviolet emission of Fe-doped ZnO thin films was not significantly enhanced, but the blue emission at 443.14 nm was largely enhanced. Ariyakkani et al. [23] also prepared Fe-doped ZnO thin films using ferric nitrate as doping source. Those ZnO thin films exhibited co-emission characteristics of ultraviolet (392 nm) and violet (412 nm). When the Fe-doping concentration was higher than 3 at.%, both the ultraviolet and violet emissions of the ZnO thin film were gradually weakened. Saha et al. [24] deposited Fe-doped ZnO thin films using Fe2O3 as a doping source. In comparison with undoped ZnO thin films, UV emission was attenuated and green emission was enhanced. The above results show that Fe-doping reduces the crystallization quality of ZnO to a certain extent, and at the same time
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Journal Pre-proof leads to an increase in point defects or non-radiative recombination centers, which is not conducive to the improvement of UV emission performance. In order to further verify that the ZnO thin films with FHP-shaped nanostructures have excellent UV emission properties, we compare the luminescence performance of ZnO materials prepared by different methods under the same test conditions. The photoluminescence spectra are shown in Fig. 7. The inset in Fig. 7 is the morphology images of the ZnO materials prepared by different methods. In order to improve the crystal quality of these ZnO materials, they have been annealed at 450 ℃ for half an hour. For the ZnO thin films prepared by electron beam evaporation and magnetron sputtering, it can be seen from the insets that they are dense in structure, uniform in grain size, and smooth on film surface. They also show green emission with higher intensity than ultraviolet emission. Green emission of ZnO is generally considered to be related to oxygen vacancy defects [25, 26]. The above results imply that ZnO thin films prepared by electron beam evaporation and magnetron sputtering contain more oxygen vacancies. In addition, for the ZnO thin films deposited by electron beam evaporation, there is red emission at 690 nm. At present, the mechanism of red emission with this wavelength is still not clear. As for ZnO nanorod arrays grown by hydrothermal method, a strong red-orange emission and a weak UV emission are exhibited (this is consistent with the luminescence behavior of many hydrothermally grown ZnO materials [27, 28]). By comparison of luminescence behavior, it can be known that the ZnO thin films with a double-layer structure prepared here have better ultraviolet emission performance.
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Journal Pre-proof Refractive index is an important parameter for designing optoelectronic devices and also an important parameter for reflecting the crystalline quality of zinc oxide [29]. The refractive indexes of these Fe-doped ZnO films are shown in Fig. 8. At the time of measurement, data were obtained at five different spots of each sample, and the average of the five measured values was taken as the refractive index of the ZnO thin film. It can be seen that when the doping concentration of Fe is higher than 2 at.%, the refractive index of the ZnO thin film is gradually increased with the rise of Fe-doping concentration and finally close to the refractive index of bulk ZnO of 2.0 [30]. This also reflects from one side that the FHP-shaped nanostructure caused by high concentration of Fe-doping improves the overall optical quality of the ZnO thin film.
4. Conclusion In this study, Fe-doped ZnO thin films were fabricated by sol-gel method using ferric chloride as the doping source. When the doping concentration is higher than 2%, the FHP-shaped nanostructures appear on the ZnO thin film surface. High-resolution TEM and selected area electron diffraction indicate that the FHP-shaped nanostructures are single crystals. As the Fe-doping concentration rises, the areal density and uniformity of the FHP-shaped nanostructures gradually improve. Compared with pure ZnO thin films, high-concentration Fe-doped ZnO thin films exhibit improved UV emission performance, which is attributed to the formation of highly crystalline FHP-shaped nanostructures. Such a double-layered ZnO thin film has potential applications in short-wavelength optoelectronic devices and photonic devices.
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Acknowledgements This work is supported by the Natural Science Foundation of the Jiangsu Higher Education Institutions in China (Grant no. 16KJB430021, 17KJB430022) and the Natural Science Foundation of Jiangsu Province in China (Grant no. BK 20180784).
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Figure captions 1. Fig.1 SEM images of ZnO thin films with Fe-doping concentration of 0% (a), 2% (b), 4% (c), 6% (d) and 8% (e) 2. Fig.2 XRD patterns of the samples with various Fe-doping concentration 3. Fig. 3 Typical XPS spectra of 6% Fe-doped ZnO thin film: (a) survey spectrum, (b) high-resolution spectrum of Zn 2p, (c) high-resolution spectrum of O 1s, (d) high-resolution spectrum of Fe 2p and (e) high-resolution spectrum of Cl 2p 4. Fig. 4 Element mapping images of Fe-doped ZnO thin film 5. Fig. 5 (a, b) Low resolution TEM images and (c, d) high resolution TEM images 6. Fig. 6 Photoluminescence spectra of the samples with different Fe-doping levels 7. Fig. 7 Photoluminescence spectra and morphology images of ZnO materials deposited by: (a) sol-gel method (Fe-doping), (b) magnetron sputtering, (c) electron beam evaporation and (d) hydrothermal method 8. Fig. 8 The refractive index of ZnO thin films as a function of Fe doping concentration
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Fig.1 SEM images of ZnO thin films with Fe-doping concentration of 0% (a), 2% (b), 4% (c), 6% (d) and 8% (e)
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Fig.2 XRD patterns of the samples with various Fe-doping concentration
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Fig. 3 Typical XPS spectra of 6% Fe-doped ZnO thin film: (a) survey spectrum, (b) high-resolution spectrum of Zn 2p, (c) high-resolution spectrum of O 1s, (d) high-resolution spectrum of Fe 2p and (e) high-resolution spectrum of Cl 2p
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Fig. 4 Element mapping images of Fe-doped ZnO thin film
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Fig. 5 (a, b) Low resolution TEM images and (c, d) high resolution TEM images
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Fig. 6 Photoluminescence spectra of the samples with different Fe-doping levels
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Fig. 7 Photoluminescence spectra and morphology images of ZnO materials deposited by: (a) sol-gel method (Fe-doping), (b) magnetron sputtering, (c) electron beam evaporation and (d) hydrothermal method
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Fig. 8 The refractive index of ZnO thin films as a function of Fe doping concentration
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Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Author contributions Linhua Xu: Conceptualization, Methodology, Experiment, Data analysis, Investigation, Writing Original Draft. Wenjian Kuang: Resources, Data analysis, Review & Editing Zhanhui Liu: Resources, Investigation. Fenglin Xian: Resources, Writing - Review & Editing
Linhua Xu, Wenjian Kuang, Zhanhui Liu, Fenglin Xian PII:
S0921-4526(20)30029-6
DOI:
https://doi.org/10.1016/j.physb.2020.412010
Reference:
PHYSB 412010
To appear in:
Physica B: Physics of Condensed Matter
Received Date:
19 September 2019
Accepted Date:
12 January 2020
Please cite this article as: Linhua Xu, Wenjian Kuang, Zhanhui Liu, Fenglin Xian, Improvement of UV emission in ZnO thin film caused by a transition from polycrystalline to monocrystalline, Physica B: Physics of Condensed Matter (2020), https://doi.org/10.1016/j.physb.2020.412010
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
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Improvement of UV emission in ZnO thin film caused by a transition from polycrystalline to monocrystalline Linhua Xu a, b, c, *, Wenjian Kuang a, b, c, Zhanhui Liu a, Fenglin Xian a, b, c a
School of Physics and Optoelectronic Engineering, Nanjing University of Information Science & Technology, Nanjing 210044, China b Jiangsu Key Laboratory for Optoelectronic Detection of Atmosphere and Ocean，Nanjing University of Information Science & Technology, Nanjing 210044, China c Optics and Photonic Technology Laboratory, Nanjing University of Information Science & Technology, Nanjing 210044, China
The corresponding author is Dr. Linhua Xu and his address and other information is as follows: Postal address: School of Physics and Optoelectronic Engineering, Nanjing University of Information Science & Technology, Ningliu Road 219#, Nanjing, 210044, China E-mail: [email protected] Tel: +86-025-587311031 Fax: +86-025-58731031 Abstract: ZnO thin films and ZnO nanostructures have good UV emission properties, which are ideal materials for preparing short-wavelength light-emitting devices. In this study, Fe-doped ZnO thin films were prepared by sol-gel method using ferric chloride as the doping source. Compared with undoped ZnO thin films, the UV emission of low-concentration Fe-doped ZnO thin films did not show significant changes. However, the UV emission of high-concentration Fe-doped (≥4at.%) ZnO thin films was significantly enhanced and the refractive index had also increased largely. It is observed by a scanning electron microscope (SEM) that the high-concentration Fe-doped ZnO thin film has a two-layer structure: the upper layer is composed of the frustum of hexagonal pyramid (FHP) shaped nanostructures, and the bottom layer in contact with the substrate is composed of polycrystalline spherical grains. Selected area electron diffraction (SAED) and high resolution transmission electron microscopy (HRTEM) show that the upper FHP-shaped nanostructure is a
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Journal Pre-proof single crystal with high crystalline quality, while the bottom layer is polycrystalline. Therefore, the formation of single crystal ZnO nanostructures greatly improves the optical quality of ZnO thin films. At present, as far as we know, the experimental results of partially converting polycrystalline ZnO into single crystal ZnO to greatly improve the UV emission performance of ZnO thin films have rarely been reported. The ZnO thin film with a two-layer structure prepared here is a candidate material for preparing UV light-emitting devices. Key words: Photoluminescence; UV emission; ZnO thin films; sol-gel method; polycrystalline; single crystal
1. Introduction Zinc Oxide (ZnO) is a wide band gap semiconductor that crystallizes into a wurtzite structure at normal temperature and pressure. Its high exciton binding energy (~60meV) makes it have high efficient UV emission caused by free exciton recombination at room temperature and even higher temperature [1, 2]. This characteristic has made ZnO nanomaterial an ideal candidate for fabricating short-wavelength light-emitting devices. In addition, ZnO has the following advantages, such as abundant raw materials, diverse morphology, non-toxicity, stable physical properties and so on. In recent years, the luminescence behavior of ZnO nanomaterials has attracted great attention from researchers [3-6]. The research on the luminescence behavior of ZnO still focuses on two aspects: one is how to improve the UV emission efficiency of ZnO, the other is to reveal the visible emission mechanisms of ZnO. In order to improve the UV emission efficiency of ZnO
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Journal Pre-proof materials, researchers have adopted different strategies. For example, Norek et al. [7] improved the UV emission properties of ZnO thin films by the surface plasmon of Al and the passivation effect of the aluminum oxide coating. Al-Nafiey et al. [8] improved the UV emission efficiency by annealing ZnO nanorods in high pressure water vapor. Davood Raoufi [9] increased the UV emission efficiency of ZnO nanoparticles by using appropriate annealing temperatures. Zheng et al. [10] enhanced the UV emission performance of ZnO particles by Y doping. Baranowska-Korczyc et al. [11] found that depositing a layer of ZnS on the surface of ZnO thin films could passivate surface defects and improve their UV emission efficiency. Jayalakshmi et al. [12] improved the UV emission properties of ZnO by constructing a hybrid material using reduced graphene oxide and ZnO. It can be known from the above results that the improvement of the crystallization quality and the reduction of defects are the key factors to enhance the UV emission of ZnO. In this study, Fe-doped ZnO thin films were deposited by sol-gel method using FeCl3 as the doping source. When the Fe-doping concentration is higher than 2at.%, the FHP-shaped nanostructures are formed on the polycrystalline ZnO thin film surface. The selected area electron diffraction and high resolution TEM analysis indicate that these nanostructures are high-quality single crystals. Compared with undoped ZnO thin films, those with FHP-shaped nanostructures exhibit higher ultraviolet emission performance. As far as we know, the improvement of UV emission properties of ZnO thin films caused by the transformation from polycrystalline to monocrystalline is seldom reported. Such a method of obtaining
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Journal Pre-proof ZnO thin films with high optical quality by doping has the advantages of low cost and simple operation. The details are exhibited below.
2. Experiments The sol-gel method was used to prepare Fe-doped ZnO thin films. First, a ZnO sol was prepared: a certain amount of zinc acetate (Zn(CH3COO)2·2H2O) and ferric chloride (FeCl3·6H2O) were put in absolute ethanol, and the above mixed solution was stirred at 60 ℃ for 1 h while monoethanolamine was added dropwise to the above solution. Finally, a transparent and homogeneous sol was formed. In the ZnO precursor solution, the molar ratio of zinc acetate to monoethanolamine was 1:1, and the concentration of zinc ion was 0.3 mol/L. In order to study the effect of different Fe-doping concentration on ZnO thin films, five different ZnO sols were prepared, in which the molar ratios of ferric chloride to zinc acetate were 0, 0.02, 0.04, 0.06 and 0.08, respectively. After these sols were aged at room temperature for 24 h, they were coated on Si substrates by dip-coating method to form ZnO thin films. Every time one layer was coated, it would be preheated. The preheating temperature and time were 280 ℃ and 4 min, respectively. The process from sol coating to preheating treatment was repeated eight times. Eventually, the Fe-doped ZnO thin films were annealed at 500 ℃ for 60 min. The annealing was carried out in an air atmosphere. An X-ray diffractometer (Bruker D8 Advance) was used to analyze the crystal phase of the samples; a field emission scanning electron microscope (S4800) was adopted to observe the microscopic morphology of the films; the composition was analyzed by X-ray photoelectron spectroscopy (XPS). The photoluminescence
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Journal Pre-proof behavior of the samples was investigated by a fluorescence detection equipment (LabRAM HR800). An ellipsometer was applied to measure the refractive index of the ZnO thin films using a He-Ne laser as the light source.
3. Results and discussion 3.1 Evolution of morphology and microstructure of ZnO thin films with the rise of Fe doping concentration The surface and cross-sectional morphology images of the samples are shown in Fig.1. For the undoped ZnO thin films, the structure is compact and the surface is smooth. When 2% FeCl3 is doped into ZnO thin film, it can be clearly seen from the cross-sectional morphology that the crystal grains become smaller. When the doping concentration rises to 4%, FHP-shaped nanostructures appear on the surface of the ZnO thin film. At this time, the ZnO thin film consists of two layers: the upper layer is composed of many FHP-shaped nanostructures, while the bottom layer is composed of many spherical nanocrystals. It is apparent that these FHP-shaped nanostructures are grown perpendicular to the surface of the substrate. When the doping concentration of FeCl3 is further increased, the areal density and uniformity of the FHP-shaped nanostructures in the upper layer are further improved, and the grain size in the bottom layer is further reduced. The reduction of ZnO grains indicates that the rise of Fe-doping concentration increases the internal stress in the film and affects the normal growth of ZnO grains [13]. It can be known from the above results that the appearance and evolution of the special morphology on the surface of ZnO thin films are related to the doping of FeCl3. The special morphologies of ZnO thin films caused
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Journal Pre-proof by doping have also been reported by other researchers [14, 15]. It should be noted that the doping source of Fe (such as ferric nitrate, iron acetate, ferric chloride, etc.) has a great influence on the formation of special morphologies, and the related research is ongoing in our laboratory. In this work, we mainly discuss the improvement of optical properties caused by the morphology evolution of ZnO thin films. The growth mechanism of FHP-shaped ZnO nanostructures caused by FeCl3 doping is still in-depth research and will be reported elsewhere. Fig.2 shows the XRD pattern of the samples. A strong (002) diffraction peak is observed for all the films, indicating that the ZnO thin films have a wurtzite structure and a preferred orientation along the c-axis perpendicular to the substrate surface. When Fe doping concentration is relatively high, (002) peak is intensified and (004) diffraction peak belonging to wurtzite ZnO also appears. This indicates that the FHP-shaped ZnO nanostructures on the films surface have better c-axis orientation than undoped ZnO thin films. XPS spectra of 6% Fe-doped ZnO thin film were measured to obtain information on the composition of the sample and the chemical states of the surface elements, as shown in Fig. 3. From the full spectrum of Fig. 3 (a), it can be seen that the surface of the sample is very pure and contains no impurities except Zn, O, Fe and Cl elements. Fig. 3 (b) displays the high-resolution spectrum of Zn 2p. The two highly geometrically symmetrical peaks at 1021.98 eV and 1044.98 eV correspond to Zn 2p3/2 and Zn 2p1/2 respectively, indicating that Zn ion in the sample is in the valence state of + 2 [16, 17]. Fig. 3 (c) displays the high resolution O 1s peak, which contains
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Journal Pre-proof two sub-peaks with the center at 530.1 and 531.5 eV, respectively. The former comes from lattice oxygen bonded to Zn ions, and the latter comes from oxygen vacancies [16, 17]. Fig. 3 (d) gives the XPS spectrum of high resolution Fe 2p. The two signal peaks at 710.58 and 724.28 eV belong to Fe 2p3/2 and Fe 2p1/2, respectively. These two binding energies imply that Fe is in the + 3 valence state [18]. Fig. 3(e) exhibits a high resolution spectrum of Cl 2p centered at 198.3 eV, indicating that Cl is present in the +7 valence state; its intensity is very low, suggesting a very low content in the ZnO thin film. Our quantitative calculation shows that the doping concentration of Cl in this sample is 2.4 at.% relative to O. In order to further determine the distribution of the elements in ZnO thin films, we took the element mapping images for the sample doped with 6% Fe ions, as shown in Fig. 4. It can be seen that the Fe and Cl elements are uniformly distributed in the film. From the above results, it can be known that these doped ZnO thin films contain not only Fe but also Cl. Therefore, to be precise, the samples prepared here are Fe and Cl co-doped ZnO thin films. Fig. 5 shows the microstructure observed by high-resolution TEM and selected area electron diffraction patterns. The sample to be tested was a fragment peeled off from the Si substrate. It can be seen from the images of Fig. 5 (a, b) that the ZnO thin film contains FHP-shaped nanostructures and granular crystal grains. The selected area electron diffraction was performed on the region ① in Fig. 5(b), and the diffraction pattern is shown in the inset; the bright diffraction spots indicate that the FHP-shaped nanostructure grown on the ZnO thin film surface is single crystal. Fig. 5 (c) and 5 (d) show the high resolution images of regions ① and ② in Fig. 5 (b), respectively. It is
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Journal Pre-proof further confirmed that the FHP-shaped nanostructures on the ZnO thin film surface are single crystal and the bottom layer is polycrystalline zinc oxide. 3.2 The photoluminescence behavior and refractive index of the samples The
luminescence
behavior
of
these
samples
has
been
studied
by
photoluminescence with an excitation wavelength of 325 nm. Fig. 6 (a) shows the photoluminescence spectra of the ZnO thin films doped with Fe ions. All the samples show a strong UV emission and a weak visible emission. The UV emission of ZnO originates from the recombination of free excitons, while the visible emission is related to defects such as oxygen vacancies and zinc interstitials [19-22]. Compared with pure ZnO thin films, Fe-doped ZnO thin films (doping concentration is higher than 2%) exhibit stronger ultraviolet emission, while visible emission decreases. The enhanced ultraviolet emission should be attributed to FHP-shaped nanostructures with high crystalline quality. In comparison with the undoped ZnO thin films, the ultraviolet emission intensity of 8% Fe-doped ZnO thin films is increased by 5 times. For 2% Fe-doped ZnO thin film, the UV emission intensity is almost the same as that of the pure ZnO thin film, but the visible emission intensity is weakened. The inset of Fig. 6(a) shows the PL spectrum of 8% Fe-doped ZnO thin film which is not annealed. Its ultraviolet emission is very weak, and the main luminous region is located at the range of 450-550 nm. This is because the Fe-doped ZnO thin film without post-annealing has poor crystalline quality and contains a large number of point defects such as zinc interstitials and oxygen vacancies which resulted in the dominant deep-level emission. Therefore, after annealing, the formation of highly crystalline
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Journal Pre-proof FHP-shaped nanostructures is responsible for the enhanced UV emission. The luminescent quality of ZnO materials can generally be evaluated by the ratio of ultraviolet emission intensity to visible emission intensity. Fig. 6(b) exhibits the ratios of the UV emission intensity to the visible emission intensity for different samples. It can be seen that with the rise of Fe-doping concentration, the ratio of ultraviolet to visible emission intensity increases gradually, indicating that the ultraviolet emission performance of ZnO thin films is gradually improved. It should be pointed out that the significant improvement of ultraviolet emission performance of ZnO thin films is not related to Fe ions doped in ZnO lattices, but comes from the highly crystalline nanostructures on the films surface. Previously, Fe-doped ZnO materials have been also studied by many people. For example, Santhosh et al. [18] prepared Fe3+ ions doped ZnO thin films using ferric nitrate as doping source. Compared with undoped ZnO thin films, the ultraviolet emission of Fe-doped ZnO thin films was not significantly enhanced, but the blue emission at 443.14 nm was largely enhanced. Ariyakkani et al. [23] also prepared Fe-doped ZnO thin films using ferric nitrate as doping source. Those ZnO thin films exhibited co-emission characteristics of ultraviolet (392 nm) and violet (412 nm). When the Fe-doping concentration was higher than 3 at.%, both the ultraviolet and violet emissions of the ZnO thin film were gradually weakened. Saha et al. [24] deposited Fe-doped ZnO thin films using Fe2O3 as a doping source. In comparison with undoped ZnO thin films, UV emission was attenuated and green emission was enhanced. The above results show that Fe-doping reduces the crystallization quality of ZnO to a certain extent, and at the same time
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Journal Pre-proof leads to an increase in point defects or non-radiative recombination centers, which is not conducive to the improvement of UV emission performance. In order to further verify that the ZnO thin films with FHP-shaped nanostructures have excellent UV emission properties, we compare the luminescence performance of ZnO materials prepared by different methods under the same test conditions. The photoluminescence spectra are shown in Fig. 7. The inset in Fig. 7 is the morphology images of the ZnO materials prepared by different methods. In order to improve the crystal quality of these ZnO materials, they have been annealed at 450 ℃ for half an hour. For the ZnO thin films prepared by electron beam evaporation and magnetron sputtering, it can be seen from the insets that they are dense in structure, uniform in grain size, and smooth on film surface. They also show green emission with higher intensity than ultraviolet emission. Green emission of ZnO is generally considered to be related to oxygen vacancy defects [25, 26]. The above results imply that ZnO thin films prepared by electron beam evaporation and magnetron sputtering contain more oxygen vacancies. In addition, for the ZnO thin films deposited by electron beam evaporation, there is red emission at 690 nm. At present, the mechanism of red emission with this wavelength is still not clear. As for ZnO nanorod arrays grown by hydrothermal method, a strong red-orange emission and a weak UV emission are exhibited (this is consistent with the luminescence behavior of many hydrothermally grown ZnO materials [27, 28]). By comparison of luminescence behavior, it can be known that the ZnO thin films with a double-layer structure prepared here have better ultraviolet emission performance.
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Journal Pre-proof Refractive index is an important parameter for designing optoelectronic devices and also an important parameter for reflecting the crystalline quality of zinc oxide [29]. The refractive indexes of these Fe-doped ZnO films are shown in Fig. 8. At the time of measurement, data were obtained at five different spots of each sample, and the average of the five measured values was taken as the refractive index of the ZnO thin film. It can be seen that when the doping concentration of Fe is higher than 2 at.%, the refractive index of the ZnO thin film is gradually increased with the rise of Fe-doping concentration and finally close to the refractive index of bulk ZnO of 2.0 [30]. This also reflects from one side that the FHP-shaped nanostructure caused by high concentration of Fe-doping improves the overall optical quality of the ZnO thin film.
4. Conclusion In this study, Fe-doped ZnO thin films were fabricated by sol-gel method using ferric chloride as the doping source. When the doping concentration is higher than 2%, the FHP-shaped nanostructures appear on the ZnO thin film surface. High-resolution TEM and selected area electron diffraction indicate that the FHP-shaped nanostructures are single crystals. As the Fe-doping concentration rises, the areal density and uniformity of the FHP-shaped nanostructures gradually improve. Compared with pure ZnO thin films, high-concentration Fe-doped ZnO thin films exhibit improved UV emission performance, which is attributed to the formation of highly crystalline FHP-shaped nanostructures. Such a double-layered ZnO thin film has potential applications in short-wavelength optoelectronic devices and photonic devices.
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Acknowledgements This work is supported by the Natural Science Foundation of the Jiangsu Higher Education Institutions in China (Grant no. 16KJB430021, 17KJB430022) and the Natural Science Foundation of Jiangsu Province in China (Grant no. BK 20180784).
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Figure captions 1. Fig.1 SEM images of ZnO thin films with Fe-doping concentration of 0% (a), 2% (b), 4% (c), 6% (d) and 8% (e) 2. Fig.2 XRD patterns of the samples with various Fe-doping concentration 3. Fig. 3 Typical XPS spectra of 6% Fe-doped ZnO thin film: (a) survey spectrum, (b) high-resolution spectrum of Zn 2p, (c) high-resolution spectrum of O 1s, (d) high-resolution spectrum of Fe 2p and (e) high-resolution spectrum of Cl 2p 4. Fig. 4 Element mapping images of Fe-doped ZnO thin film 5. Fig. 5 (a, b) Low resolution TEM images and (c, d) high resolution TEM images 6. Fig. 6 Photoluminescence spectra of the samples with different Fe-doping levels 7. Fig. 7 Photoluminescence spectra and morphology images of ZnO materials deposited by: (a) sol-gel method (Fe-doping), (b) magnetron sputtering, (c) electron beam evaporation and (d) hydrothermal method 8. Fig. 8 The refractive index of ZnO thin films as a function of Fe doping concentration
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Fig.1 SEM images of ZnO thin films with Fe-doping concentration of 0% (a), 2% (b), 4% (c), 6% (d) and 8% (e)
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Fig.2 XRD patterns of the samples with various Fe-doping concentration
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Fig. 3 Typical XPS spectra of 6% Fe-doped ZnO thin film: (a) survey spectrum, (b) high-resolution spectrum of Zn 2p, (c) high-resolution spectrum of O 1s, (d) high-resolution spectrum of Fe 2p and (e) high-resolution spectrum of Cl 2p
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Fig. 4 Element mapping images of Fe-doped ZnO thin film
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Fig. 5 (a, b) Low resolution TEM images and (c, d) high resolution TEM images
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Fig. 6 Photoluminescence spectra of the samples with different Fe-doping levels
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Fig. 7 Photoluminescence spectra and morphology images of ZnO materials deposited by: (a) sol-gel method (Fe-doping), (b) magnetron sputtering, (c) electron beam evaporation and (d) hydrothermal method
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Fig. 8 The refractive index of ZnO thin films as a function of Fe doping concentration
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Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Author contributions Linhua Xu: Conceptualization, Methodology, Experiment, Data analysis, Investigation, Writing Original Draft. Wenjian Kuang: Resources, Data analysis, Review & Editing Zhanhui Liu: Resources, Investigation. Fenglin Xian: Resources, Writing - Review & Editing