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Synthesis and photoluminescence characterizations of the Er3+-doped ZnO nanosheets with irregular porous microstructure
Materials Science in Semiconductor Processing 41 (2016) 32–37
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Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Synthesis and photoluminescence characterizations of the Er3 þ -doped ZnO nanosheets with irregular porous microstructure Jihui Lang, Jiaying Wang, Qi Zhang, Songsong Xu, Donglai Han, Jinghai Yang n, Qiang Han, Lili Yang, Yingrui Sui, Xiuyan Li, Xiaoyan Liu Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education, Jilin Normal University, Siping 136000, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 3 June 2015 Received in revised form 31 July 2015 Accepted 11 August 2015
Er-doped ZnO nanosheets with high quality were synthesized by the hydrothermal and post-annealing techniques, and the effect of erbium dopant on the structures, morphologies and photoluminescence properties of the as-synthesized samples were determined using XRD, SEM, TEM, EDS, PL and Raman spectroscopy. The results showed that Er3 þ ions were successfully incorporated into the crystal lattice of ZnO host, and some irregular porous microstructure with diameter of 3–10 nm could be seen on ZnO nanosheets as various doping concentrations. It was found that the crystallization and photoluminescence properties of ZnO nanosheets were strongly influenced by erbium doping concentration. The ultraviolet emission and deep level emission were both appeared in PL spectra, and the intensity of the whole deep level emission was enhanced with erbium doping, indicating the deep-level-defect luminescent centers were increased in the doped samples. Moreover, the crystallization of the samples became worse due to more defects by erbium doping. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Oxides Chemical synthesis Optical properties Defects
1. Introduction In recent years, considerable interest has been focused on metal oxide nanomaterials that are extensively used in a number of applications due to their possession of enthralling optical, electrical, magnetic and catalytic properties [1–3]. Among metal oxides, zinc oxide (ZnO) nanomaterials as a wide band gap semiconductor attracts a major intention in potential applications of science and technology such as nano-optoelectronic devices, ultraviolet lasers, gas sensors, solar-cells, light emitting diodes (LED) and excellent photo catalysts [4–8]. Importantly, it is economical, environmental friendly, and exhibits high thermal and chemical stability [9,10]. These properties make it a unique host material for doping with luminescence centers and it can exhibit efficient emission even at or above room-temperature (RT). Rare-earth (RE) ion doped ZnO nanomaterials have attracted intensive interests, since RE elements such as Eu, Nd, Sm, Ce and Er have great possibility to efficiently modulate the emission in the visible range due to their unique optical properties and excellent qualification to be radiative centers [11–18]. Up to now, the different synthesis route has been developed for the preparation of RE-doped ZnO nanomaterials, such as sol–gel, pulsed laser deposition, magnetron n
Corresponding author. Fax: þ86 434 3294566. E-mail address: [email protected] (J. Yang).
http://dx.doi.org/10.1016/j.mssp.2015.08.022 1369-8001/& 2015 Elsevier Ltd. All rights reserved.
sputtering, chemical vapor deposition, spray pyrolysis, solution combustion and vapor transport route [19–22]. Among these nanomaterials, Er3 þ doped ZnO nanomaterials can emit luminescence including the 1.54 μm infrared emission and the green emission via direct and indirect excitation, and the abundant 4forbital configurations of Er exhibit sharp emission lines and intense 4f–4f transitions even at low symmetry [17–19]. Meanwhile, Er3 þ ions can be also as the doped modifier due to its efficient modulation of deep-level emission. However, the reports mainly focus on the infrared emission of Er3 þ doped ZnO nanomaterials and about the modulation of deep-level emission is limited. Furthermore, because of the different chemical properties between trivalent RE ions and the cations of ZnO, it is rather difficult to incorporate RE ions into the lattice of semiconductors effectively via a simple method. The most important is that the synthesis and photoluminescence characterizations of the Er-doped ZnO nanosheets with irregular porous microstructure by the hydrothermal and post-annealing techniques have been rarely reported [23–25]. Spontaneously, the effects of Er doping on the structural and photoluminescence properties of ZnO nanosheets are not clear yet due to the limited investigation. Hence, more attention should be paid onto the Er-doped ZnO nanostructures systems. In this paper, we explore the hydrothermal method to prepare the Er3 þ -doped ZnO nanosheets with irregular porous microstructure and successfully control the morphology of the samples.
J. Lang et al. / Materials Science in Semiconductor Processing 41 (2016) 32–37
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The most important is that the structural and photoluminescence properties of the as-synthesized Er3 þ -doped ZnO nanosheets can be adjusted by changing the erbium doping concentration.
2. Experimental 2.1. Synthesis of the Er3 þ -doped ZnO nanosheets Er3 þ -doped ZnO nanosheets with irregular porous microstructure were synthesized by the hydrothermal method accompanied with the post-annealing treatment. All the starting materials were analytical grade and were used without further purification. Zinc acetate dihydrate (Zn(Ac)2 2H2O) and erbium oxide (Er2O3) were used as Zn and Er sources, respectively. Urea ((NH2)2CO) was used as OH source for forming the complexes with the above metal sources. The experiment process could be described briefly as follows. First, Er2O3 powder was dissolved in concentrated hydrochloric acid and then heated at 120 °C for a certain time. A certain amount of deionized water was added to the above mixture to form 0.1 mM of Er(Cl)3 solution. Second, the Zn(Ac)2 2H2O and (NH2)2CO was dissolved in the deionized water, respectively. Then the precursor solution was prepared by mixing the above aqueous solutions, while keeping the Er/Zn ratios of 0%, 2%, 3% and 4% in molarities. After stirring, the precursor solution was transferred into a 100 ml Teflonlined autoclave, which was filled to nearly 80% of its capacity. The autoclave was kept in a dry cabinet at 155 °C for 24 h, then the solution was cooled down to room temperature. White powder collected from the bottom of the container was washed with deionized water and ethanol, and then dried at 60 °C. Finally, the final products were annealed at 400 °C for 2 h in Ar gas atmosphere. 2.2. Characterization of the samples X-ray diffraction (XRD) (MAC Science, MXP18, Japan), scanning electron microscopy (SEM) (Hitachi, S-570), transmission electron microscopy (TEM) (JEM-2100HR, Japan), energy dispersive spectroscopy (EDS) (200 keV, JEM-2100HR, Japan), Raman spectroscopy (514.5 nm, argon ion laser, Renishaw-inVia) and photoluminescence (PL) (325 nm, He–Cd Laser, Renishaw-inVia) were used to characterize the structures, morphologies, chemical composition and photoluminescence properties of the Er3 þ -doped ZnO nanosheets.
Fig. 1. XRD patterns of ZnO:xEr nanosheets (a) 0%, (b) 2%, (c) 3%, and (d) 4% (inset shows the XRD pattern of Er2O3).
obtained by fitting the XRD data with the least square method (LSM). The lattice parameters of the ZnO:4%Er (a ¼0.32578 nm, c¼ 0.52200 nm), ZnO:3%Er (a ¼0.32566 nm, c¼ 0.52179 nm) and ZnO:2%Er (a ¼0.32559 nm, c ¼0.52170 nm) increase compared with the undoped one (a ¼0.32553 nm, c ¼0.52163 nm), which indicates that the Er3 þ ions have been incorporated into the ZnO lattice and substituted the Zn ion sites because the ionic radius of Er3 þ (0.088 nm) is much bigger than that of Zn2 þ (0.074 nm). In order to further research the microstructure of the ZnO:xEr samples (x ¼0%, 2%, 3% and 4%), the RT Raman spectra of the four samples are given in Fig. 2, which is excited by 514 nm line from an argon laser. As we know, the wurtzite-type ZnO belongs to the space group C64v with two formula units in the primitive cell. The zone-center optical phonons can be classified according to the following irreducible representations: Γopt = A1 + E1 + 2E2 + 2B1 [26–29]. The B1 modes are silent modes, the A1 and E1 modes are polar modes and both Raman and infrared active, and they split into transverse optical (TO) and longitudinal optical (LO) phonons, whereas the E2 modes are nonpolar and Raman active only. In Fig. 2, all samples show the strongest and sharpest peak centered at about 438.1 cm 1 which can be assigned to the E2H mode [26,30,31]. The appearance of this peak in all Raman spectra indicates that the samples have the good crystallization, and the decreased intensity of this peak with Er doping further indicates that the crystallization of the samples become worse, which shows a good agreement with XRD results. In addition, the other modes
3. Results and discussion Fig. 1 shows the typical XRD patterns of the as-synthesized ZnO:xEr samples (x¼0%, 2%, 3% and 4%). As shown in Fig. 1, no other diffraction peaks are detected except hexagonal wurtzite ZnO peaks, which indicate the presence of ZnO without any amorphous component, erbium metal and other additional Er2O3 crystalline phase (the inset shows the standard XRD pattern of Er2O3). It also illustrates the incorporation of Er3 þ ions into ZnO lattice. Moreover, the high intensity of the diffraction peaks is characteristic of the highly crystallized of the samples. Comparing the XRD patterns of undoped and Er-doped ZnO samples, the intensity of the diffraction peaks are decreased with the increase of the erbium doping concentration, which implies the worse crystallization of the doped samples. Furthermore, as shown in Fig. 1 (a–d), the positions of the main diffraction peaks shift to the smaller angle slightly, indicating that the lattice parameters are a little larger than those of undoped ZnO. In order to further prove it, we also calculate the lattice parameters of the four samples from XRD patterns, and the lattice parameters of these samples are
Fig. 2. Room-temperature Raman spectra of ZnO:xEr nanosheets (a) 0%, (b) 2%, (c) 3%, and (d) 4%.
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J. Lang et al. / Materials Science in Semiconductor Processing 41 (2016) 32–37
4800 4400 4000 3600 Counts
3200 2800 2400 2000 1600 1200 800 400 0 0.00
1.00
2.00
3.00
4.00
5.00 keV
6.00
7.00
8.00
9.00
10.00
Fig. 3. EDS spectrum of ZnO:3%Er nanosheets.
of the E2 and A1 of wurtzite ZnO are allowed and observed, in agreement with the XRD results regarding the dominant characters of the wurtzite phase in all samples. Combined with the XRD analysis, it indicates a substitutional incorporation of Er ions on the Zn sites of the host lattice. In order to confirm the presence of Er3 þ and determine its chemical composition, EDS analysis of ZnO:3%Er sample are given in Fig. 3. For accuracy, EDS measurement has been carried out at a number of locations throughout the sample. From Fig. 3, three elements of Zn, O and Er are existed in the obtained sample, and it can be deduced that the Er ions have a þ3 oxidation valance state combined with the XRD and Raman analysis in our case [14–16]. According to the EDS analysis, the ratio of Er and Zn is about 0.021:0.979, which is close to the proposed doping concentration. All the above results including XRD, Raman and EDS indicate that Er3 þ ions are successfully doped into the crystal lattice of ZnO
matrix without forming erbium oxides or any other impurities at the surface of ZnO. Fig. 4 shows the SEM images of the as-synthesized ZnO:xEr samples (x ¼0%, 2%, 3% and 4%). From Fig. 4(a–d), it can be observed that all the samples are composed of nanosheets, and the nanosheets are stacked each other. In order to see the nanosheets clearly, the TEM images of these samples are shown in Fig. 5. Undoped and Er-doped ZnO samples with various doping concentrations are all the nanosheets with irregular porous microstructures, and the diameters of the porous microstructures are about 3–10 nm. Compared with the four images, it can be seen that the image contrasts of undoped and Er-doped nanosheets are different. With the increase of the Er doping concentration, the image contrast is darkened, indicating that Er ions have been incorporated into the ZnO nanosheets. Fig. 6 shows the RT PL spectra of the as-synthesized ZnO:xEr samples (x¼ 0%, 2%, 3% and 4%). In Fig. 6(a), it exhibits a strong UV emission attributed to near-band-edge (NBE) exciton recombination [32,33], and a weak deep level emission (DLE) ascribed to defects level related transition [34–36]. Compared with the four spectra, we observe the ratio of the above two emissions is different, and the intensity of DLE enhances with the Er doping concentration increasing. In order to see the result clearly, the integrated intensity ratios of IUV/IDLE versus Er3 þ ions doping concentration are given in Fig. 7. It can be seen that the ratios are dramatically decreased with Er doping as shown in this figure. As we know, the ratio of IUV/IDLE is related to the crystallization of a sample. So we believe that the crystallization of the samples may became worse with the increase of doping concentration due to more defects introduced by the existence of Er impurities in ZnO nanosheets. In addition, the DLE band is very broad and asymmetric for all the samples. So we magnify the PL spectra ranged
Fig. 4. SEM images of ZnO:xEr nanosheets (a) 0%, (b) 2%, (c) 3%, and (d) 4%.
J. Lang et al. / Materials Science in Semiconductor Processing 41 (2016) 32–37
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Fig. 5. TEM images of ZnO:xEr nanosheets (a) 0%, (b) 2%, (c) 3%, and (d) 4%.
Fig. 7. Integrated intensity ratios of IUV/IDLE versus Er3 þ ions doping concentration. Fig. 6. Room-temperature PL spectra of ZnO:xEr nanosheets (a) 0%, (b) 2%, (c) 3%, and (d) 4%.
are quite different, and the value-added intensity of P1 is much higher than that of P2. That is to say, Ip /Ip value increases step by 1
from 450 nm to 750 nm for these samples. As shown in Fig. 8(a–d), the good fitting to the experimental data could be only obtained when deconvolutions of two Gaussians, centers at about 520 nm and 600 nm, are used for the spectra, which indicates that two kinds of deep-level-defect luminescent centers exist in the samples. For the sake of discussion, we name the two peaks as P1 and P2, respectively. In the Fig. 8, it is found that the variations in intensities of the two peaks are obviously different. With the increase of Er doping concentration, the intensities of P1 and P2 are both enhanced. But the value-added intensities of the two peaks
2
step as the Er doping concentration increases from 0 to 4%. As mentioned above, P1 centered at about 520 nm is the green emission, and P2 centered at about 600 nm is the yellow–orange emission. The green emission is commonly associated with the presence of the oxygen vacancies (VO ) and zinc interstitials ( Zni ) in the ZnO matrix [37–39]. As we know, the oxygen vacancies can occur in three different charge states. They are the neutral oxygen vacancy ( VO0 ), the singly ionized oxygen vacancy ( VO*) and the doubly ionized oxygen vacancy ( VO**). Among the three kinds of oxygen vacancies, only VO* can act as luminescent centers in ZnO
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J. Lang et al. / Materials Science in Semiconductor Processing 41 (2016) 32–37
on the structural and photoluminescence properties of ZnO nanosheets with irregular porous microstructures. The results of structural characterization indicate that the Er ions are incorporated into ZnO crystal lattice. PL spectra show that the broad DLE can be fitted with Gaussian functions to identify two emissions, and the two emissions are both strongly enhanced with Er doping concentration increasing, which indicates that the more deep-level-defect luminescent centers in the doped samples. Moreover, the crystallization of the nanosheets become worse with the increase of Er doping concentration due to more defects introduced by the existence of Er impurities in the nanosheets.
Acknowledgments
Fig. 8. Zoom to area of the DLE in PL spectra of Fig. 6, (a) 0%, (b) 2%, (c) 3%, and (d) 4%.
matrix. So the VO* center should be the reason for green emission of ZnO crystal among the three kinds of oxygen vacancies, and it is suggest that the green emission results from the recombination of a photogenerated hole with the single ionized charge state of the defect [29,40]. Above all, it indicates that the deep-level-defect luminescent centers of VO* and Zni are both the main origins for the green emission according to the recent reports. As for the orange– yellow emission of ZnO crystal, it has been recently identified with different optical characteristics that at least two different defect origins, i.e., the oxygen interstitials ( Oi ) and/or Zn vacancies ( VZn ), create the luminescence centers [41,42]. It is well known that the stronger the intensity of DLE, the more deep-level-defect luminescent centers there are. In our case, the intensity of DLE composed of P1 and P2 is enhanced with the increase of Er doping concentration, which indicates that the deep-level-defect luminescent centers are increased by Er doping. The result has a good agreement with our previous reports, wherein we testified that the intensity of DLE could be enhanced with the RE and transitionmetal (TM) ions doping concentration increases by experimentally [43–45]. We deduce that the modulation of the lattice internal defects is the main reason for it. Usually, the impurities doping into semiconductors will lead more vacancies, such as VO and VZn , due to the lattice atom displacement, and make the atomic arrangement disordered in the implanted region or the amorphous region formed. And the Er ions exist in the ZnO lattice with þ 3 valence and they are apt to take the position of Zn ions which has þ2 valence, so Er3 þ is inevitable to attract more excess oxygen to be present in the Oi position. In addition, a part of Er3 þ ions will occupy the VZn positions to lower the VZn concentration and the others may be substituted the Zn ion sites to raise the Zni concentration when the Er3 þ ions are incorporated into ZnO lattice. According to the above analysis, the intensities of P1 and P2 will be increased due to more luminescence centers mentioned above in our samples. And the lower value-added intensity of P2 is related with the decrease of the VZn concentration to a certain extent, which finally results in the increase of Ip /Ip value as the Er doping 1 2 concentration enhances.
4. Conclusion We have synthesized the Er3 þ -doped ZnO nanosheets via a two-step hydrothermal method accompanied with the post-annealing treatment. Based on the XRD, Raman, EDS, SEM, TEM and PL spectral analyses, we have revealed the effect of erbium doping
This work is supported by the National Natural Science Foundation of China (Grant nos. 11204104 and 11254001), National Programs for High Technology Research and Development of China (863) (Item no. 2013AA032202), Program for New Century Excellent Talents in University (No. NCET-13-0824) and Program for the development of Science and Technology of Jilin province (Item no. 20140101205JC and 201215222).
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Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Synthesis and photoluminescence characterizations of the Er3 þ -doped ZnO nanosheets with irregular porous microstructure Jihui Lang, Jiaying Wang, Qi Zhang, Songsong Xu, Donglai Han, Jinghai Yang n, Qiang Han, Lili Yang, Yingrui Sui, Xiuyan Li, Xiaoyan Liu Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education, Jilin Normal University, Siping 136000, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 3 June 2015 Received in revised form 31 July 2015 Accepted 11 August 2015
Er-doped ZnO nanosheets with high quality were synthesized by the hydrothermal and post-annealing techniques, and the effect of erbium dopant on the structures, morphologies and photoluminescence properties of the as-synthesized samples were determined using XRD, SEM, TEM, EDS, PL and Raman spectroscopy. The results showed that Er3 þ ions were successfully incorporated into the crystal lattice of ZnO host, and some irregular porous microstructure with diameter of 3–10 nm could be seen on ZnO nanosheets as various doping concentrations. It was found that the crystallization and photoluminescence properties of ZnO nanosheets were strongly influenced by erbium doping concentration. The ultraviolet emission and deep level emission were both appeared in PL spectra, and the intensity of the whole deep level emission was enhanced with erbium doping, indicating the deep-level-defect luminescent centers were increased in the doped samples. Moreover, the crystallization of the samples became worse due to more defects by erbium doping. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Oxides Chemical synthesis Optical properties Defects
1. Introduction In recent years, considerable interest has been focused on metal oxide nanomaterials that are extensively used in a number of applications due to their possession of enthralling optical, electrical, magnetic and catalytic properties [1–3]. Among metal oxides, zinc oxide (ZnO) nanomaterials as a wide band gap semiconductor attracts a major intention in potential applications of science and technology such as nano-optoelectronic devices, ultraviolet lasers, gas sensors, solar-cells, light emitting diodes (LED) and excellent photo catalysts [4–8]. Importantly, it is economical, environmental friendly, and exhibits high thermal and chemical stability [9,10]. These properties make it a unique host material for doping with luminescence centers and it can exhibit efficient emission even at or above room-temperature (RT). Rare-earth (RE) ion doped ZnO nanomaterials have attracted intensive interests, since RE elements such as Eu, Nd, Sm, Ce and Er have great possibility to efficiently modulate the emission in the visible range due to their unique optical properties and excellent qualification to be radiative centers [11–18]. Up to now, the different synthesis route has been developed for the preparation of RE-doped ZnO nanomaterials, such as sol–gel, pulsed laser deposition, magnetron n
Corresponding author. Fax: þ86 434 3294566. E-mail address: [email protected] (J. Yang).
http://dx.doi.org/10.1016/j.mssp.2015.08.022 1369-8001/& 2015 Elsevier Ltd. All rights reserved.
sputtering, chemical vapor deposition, spray pyrolysis, solution combustion and vapor transport route [19–22]. Among these nanomaterials, Er3 þ doped ZnO nanomaterials can emit luminescence including the 1.54 μm infrared emission and the green emission via direct and indirect excitation, and the abundant 4forbital configurations of Er exhibit sharp emission lines and intense 4f–4f transitions even at low symmetry [17–19]. Meanwhile, Er3 þ ions can be also as the doped modifier due to its efficient modulation of deep-level emission. However, the reports mainly focus on the infrared emission of Er3 þ doped ZnO nanomaterials and about the modulation of deep-level emission is limited. Furthermore, because of the different chemical properties between trivalent RE ions and the cations of ZnO, it is rather difficult to incorporate RE ions into the lattice of semiconductors effectively via a simple method. The most important is that the synthesis and photoluminescence characterizations of the Er-doped ZnO nanosheets with irregular porous microstructure by the hydrothermal and post-annealing techniques have been rarely reported [23–25]. Spontaneously, the effects of Er doping on the structural and photoluminescence properties of ZnO nanosheets are not clear yet due to the limited investigation. Hence, more attention should be paid onto the Er-doped ZnO nanostructures systems. In this paper, we explore the hydrothermal method to prepare the Er3 þ -doped ZnO nanosheets with irregular porous microstructure and successfully control the morphology of the samples.
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The most important is that the structural and photoluminescence properties of the as-synthesized Er3 þ -doped ZnO nanosheets can be adjusted by changing the erbium doping concentration.
2. Experimental 2.1. Synthesis of the Er3 þ -doped ZnO nanosheets Er3 þ -doped ZnO nanosheets with irregular porous microstructure were synthesized by the hydrothermal method accompanied with the post-annealing treatment. All the starting materials were analytical grade and were used without further purification. Zinc acetate dihydrate (Zn(Ac)2 2H2O) and erbium oxide (Er2O3) were used as Zn and Er sources, respectively. Urea ((NH2)2CO) was used as OH source for forming the complexes with the above metal sources. The experiment process could be described briefly as follows. First, Er2O3 powder was dissolved in concentrated hydrochloric acid and then heated at 120 °C for a certain time. A certain amount of deionized water was added to the above mixture to form 0.1 mM of Er(Cl)3 solution. Second, the Zn(Ac)2 2H2O and (NH2)2CO was dissolved in the deionized water, respectively. Then the precursor solution was prepared by mixing the above aqueous solutions, while keeping the Er/Zn ratios of 0%, 2%, 3% and 4% in molarities. After stirring, the precursor solution was transferred into a 100 ml Teflonlined autoclave, which was filled to nearly 80% of its capacity. The autoclave was kept in a dry cabinet at 155 °C for 24 h, then the solution was cooled down to room temperature. White powder collected from the bottom of the container was washed with deionized water and ethanol, and then dried at 60 °C. Finally, the final products were annealed at 400 °C for 2 h in Ar gas atmosphere. 2.2. Characterization of the samples X-ray diffraction (XRD) (MAC Science, MXP18, Japan), scanning electron microscopy (SEM) (Hitachi, S-570), transmission electron microscopy (TEM) (JEM-2100HR, Japan), energy dispersive spectroscopy (EDS) (200 keV, JEM-2100HR, Japan), Raman spectroscopy (514.5 nm, argon ion laser, Renishaw-inVia) and photoluminescence (PL) (325 nm, He–Cd Laser, Renishaw-inVia) were used to characterize the structures, morphologies, chemical composition and photoluminescence properties of the Er3 þ -doped ZnO nanosheets.
Fig. 1. XRD patterns of ZnO:xEr nanosheets (a) 0%, (b) 2%, (c) 3%, and (d) 4% (inset shows the XRD pattern of Er2O3).
obtained by fitting the XRD data with the least square method (LSM). The lattice parameters of the ZnO:4%Er (a ¼0.32578 nm, c¼ 0.52200 nm), ZnO:3%Er (a ¼0.32566 nm, c¼ 0.52179 nm) and ZnO:2%Er (a ¼0.32559 nm, c ¼0.52170 nm) increase compared with the undoped one (a ¼0.32553 nm, c ¼0.52163 nm), which indicates that the Er3 þ ions have been incorporated into the ZnO lattice and substituted the Zn ion sites because the ionic radius of Er3 þ (0.088 nm) is much bigger than that of Zn2 þ (0.074 nm). In order to further research the microstructure of the ZnO:xEr samples (x ¼0%, 2%, 3% and 4%), the RT Raman spectra of the four samples are given in Fig. 2, which is excited by 514 nm line from an argon laser. As we know, the wurtzite-type ZnO belongs to the space group C64v with two formula units in the primitive cell. The zone-center optical phonons can be classified according to the following irreducible representations: Γopt = A1 + E1 + 2E2 + 2B1 [26–29]. The B1 modes are silent modes, the A1 and E1 modes are polar modes and both Raman and infrared active, and they split into transverse optical (TO) and longitudinal optical (LO) phonons, whereas the E2 modes are nonpolar and Raman active only. In Fig. 2, all samples show the strongest and sharpest peak centered at about 438.1 cm 1 which can be assigned to the E2H mode [26,30,31]. The appearance of this peak in all Raman spectra indicates that the samples have the good crystallization, and the decreased intensity of this peak with Er doping further indicates that the crystallization of the samples become worse, which shows a good agreement with XRD results. In addition, the other modes
3. Results and discussion Fig. 1 shows the typical XRD patterns of the as-synthesized ZnO:xEr samples (x¼0%, 2%, 3% and 4%). As shown in Fig. 1, no other diffraction peaks are detected except hexagonal wurtzite ZnO peaks, which indicate the presence of ZnO without any amorphous component, erbium metal and other additional Er2O3 crystalline phase (the inset shows the standard XRD pattern of Er2O3). It also illustrates the incorporation of Er3 þ ions into ZnO lattice. Moreover, the high intensity of the diffraction peaks is characteristic of the highly crystallized of the samples. Comparing the XRD patterns of undoped and Er-doped ZnO samples, the intensity of the diffraction peaks are decreased with the increase of the erbium doping concentration, which implies the worse crystallization of the doped samples. Furthermore, as shown in Fig. 1 (a–d), the positions of the main diffraction peaks shift to the smaller angle slightly, indicating that the lattice parameters are a little larger than those of undoped ZnO. In order to further prove it, we also calculate the lattice parameters of the four samples from XRD patterns, and the lattice parameters of these samples are
Fig. 2. Room-temperature Raman spectra of ZnO:xEr nanosheets (a) 0%, (b) 2%, (c) 3%, and (d) 4%.
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4800 4400 4000 3600 Counts
3200 2800 2400 2000 1600 1200 800 400 0 0.00
1.00
2.00
3.00
4.00
5.00 keV
6.00
7.00
8.00
9.00
10.00
Fig. 3. EDS spectrum of ZnO:3%Er nanosheets.
of the E2 and A1 of wurtzite ZnO are allowed and observed, in agreement with the XRD results regarding the dominant characters of the wurtzite phase in all samples. Combined with the XRD analysis, it indicates a substitutional incorporation of Er ions on the Zn sites of the host lattice. In order to confirm the presence of Er3 þ and determine its chemical composition, EDS analysis of ZnO:3%Er sample are given in Fig. 3. For accuracy, EDS measurement has been carried out at a number of locations throughout the sample. From Fig. 3, three elements of Zn, O and Er are existed in the obtained sample, and it can be deduced that the Er ions have a þ3 oxidation valance state combined with the XRD and Raman analysis in our case [14–16]. According to the EDS analysis, the ratio of Er and Zn is about 0.021:0.979, which is close to the proposed doping concentration. All the above results including XRD, Raman and EDS indicate that Er3 þ ions are successfully doped into the crystal lattice of ZnO
matrix without forming erbium oxides or any other impurities at the surface of ZnO. Fig. 4 shows the SEM images of the as-synthesized ZnO:xEr samples (x ¼0%, 2%, 3% and 4%). From Fig. 4(a–d), it can be observed that all the samples are composed of nanosheets, and the nanosheets are stacked each other. In order to see the nanosheets clearly, the TEM images of these samples are shown in Fig. 5. Undoped and Er-doped ZnO samples with various doping concentrations are all the nanosheets with irregular porous microstructures, and the diameters of the porous microstructures are about 3–10 nm. Compared with the four images, it can be seen that the image contrasts of undoped and Er-doped nanosheets are different. With the increase of the Er doping concentration, the image contrast is darkened, indicating that Er ions have been incorporated into the ZnO nanosheets. Fig. 6 shows the RT PL spectra of the as-synthesized ZnO:xEr samples (x¼ 0%, 2%, 3% and 4%). In Fig. 6(a), it exhibits a strong UV emission attributed to near-band-edge (NBE) exciton recombination [32,33], and a weak deep level emission (DLE) ascribed to defects level related transition [34–36]. Compared with the four spectra, we observe the ratio of the above two emissions is different, and the intensity of DLE enhances with the Er doping concentration increasing. In order to see the result clearly, the integrated intensity ratios of IUV/IDLE versus Er3 þ ions doping concentration are given in Fig. 7. It can be seen that the ratios are dramatically decreased with Er doping as shown in this figure. As we know, the ratio of IUV/IDLE is related to the crystallization of a sample. So we believe that the crystallization of the samples may became worse with the increase of doping concentration due to more defects introduced by the existence of Er impurities in ZnO nanosheets. In addition, the DLE band is very broad and asymmetric for all the samples. So we magnify the PL spectra ranged
Fig. 4. SEM images of ZnO:xEr nanosheets (a) 0%, (b) 2%, (c) 3%, and (d) 4%.
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Fig. 5. TEM images of ZnO:xEr nanosheets (a) 0%, (b) 2%, (c) 3%, and (d) 4%.
Fig. 7. Integrated intensity ratios of IUV/IDLE versus Er3 þ ions doping concentration. Fig. 6. Room-temperature PL spectra of ZnO:xEr nanosheets (a) 0%, (b) 2%, (c) 3%, and (d) 4%.
are quite different, and the value-added intensity of P1 is much higher than that of P2. That is to say, Ip /Ip value increases step by 1
from 450 nm to 750 nm for these samples. As shown in Fig. 8(a–d), the good fitting to the experimental data could be only obtained when deconvolutions of two Gaussians, centers at about 520 nm and 600 nm, are used for the spectra, which indicates that two kinds of deep-level-defect luminescent centers exist in the samples. For the sake of discussion, we name the two peaks as P1 and P2, respectively. In the Fig. 8, it is found that the variations in intensities of the two peaks are obviously different. With the increase of Er doping concentration, the intensities of P1 and P2 are both enhanced. But the value-added intensities of the two peaks
2
step as the Er doping concentration increases from 0 to 4%. As mentioned above, P1 centered at about 520 nm is the green emission, and P2 centered at about 600 nm is the yellow–orange emission. The green emission is commonly associated with the presence of the oxygen vacancies (VO ) and zinc interstitials ( Zni ) in the ZnO matrix [37–39]. As we know, the oxygen vacancies can occur in three different charge states. They are the neutral oxygen vacancy ( VO0 ), the singly ionized oxygen vacancy ( VO*) and the doubly ionized oxygen vacancy ( VO**). Among the three kinds of oxygen vacancies, only VO* can act as luminescent centers in ZnO
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on the structural and photoluminescence properties of ZnO nanosheets with irregular porous microstructures. The results of structural characterization indicate that the Er ions are incorporated into ZnO crystal lattice. PL spectra show that the broad DLE can be fitted with Gaussian functions to identify two emissions, and the two emissions are both strongly enhanced with Er doping concentration increasing, which indicates that the more deep-level-defect luminescent centers in the doped samples. Moreover, the crystallization of the nanosheets become worse with the increase of Er doping concentration due to more defects introduced by the existence of Er impurities in the nanosheets.
Acknowledgments
Fig. 8. Zoom to area of the DLE in PL spectra of Fig. 6, (a) 0%, (b) 2%, (c) 3%, and (d) 4%.
matrix. So the VO* center should be the reason for green emission of ZnO crystal among the three kinds of oxygen vacancies, and it is suggest that the green emission results from the recombination of a photogenerated hole with the single ionized charge state of the defect [29,40]. Above all, it indicates that the deep-level-defect luminescent centers of VO* and Zni are both the main origins for the green emission according to the recent reports. As for the orange– yellow emission of ZnO crystal, it has been recently identified with different optical characteristics that at least two different defect origins, i.e., the oxygen interstitials ( Oi ) and/or Zn vacancies ( VZn ), create the luminescence centers [41,42]. It is well known that the stronger the intensity of DLE, the more deep-level-defect luminescent centers there are. In our case, the intensity of DLE composed of P1 and P2 is enhanced with the increase of Er doping concentration, which indicates that the deep-level-defect luminescent centers are increased by Er doping. The result has a good agreement with our previous reports, wherein we testified that the intensity of DLE could be enhanced with the RE and transitionmetal (TM) ions doping concentration increases by experimentally [43–45]. We deduce that the modulation of the lattice internal defects is the main reason for it. Usually, the impurities doping into semiconductors will lead more vacancies, such as VO and VZn , due to the lattice atom displacement, and make the atomic arrangement disordered in the implanted region or the amorphous region formed. And the Er ions exist in the ZnO lattice with þ 3 valence and they are apt to take the position of Zn ions which has þ2 valence, so Er3 þ is inevitable to attract more excess oxygen to be present in the Oi position. In addition, a part of Er3 þ ions will occupy the VZn positions to lower the VZn concentration and the others may be substituted the Zn ion sites to raise the Zni concentration when the Er3 þ ions are incorporated into ZnO lattice. According to the above analysis, the intensities of P1 and P2 will be increased due to more luminescence centers mentioned above in our samples. And the lower value-added intensity of P2 is related with the decrease of the VZn concentration to a certain extent, which finally results in the increase of Ip /Ip value as the Er doping 1 2 concentration enhances.
4. Conclusion We have synthesized the Er3 þ -doped ZnO nanosheets via a two-step hydrothermal method accompanied with the post-annealing treatment. Based on the XRD, Raman, EDS, SEM, TEM and PL spectral analyses, we have revealed the effect of erbium doping
This work is supported by the National Natural Science Foundation of China (Grant nos. 11204104 and 11254001), National Programs for High Technology Research and Development of China (863) (Item no. 2013AA032202), Program for New Century Excellent Talents in University (No. NCET-13-0824) and Program for the development of Science and Technology of Jilin province (Item no. 20140101205JC and 201215222).
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