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ZnO nanorod arrays were synthesized by hydrothermal method with two different zinc salts as precursors: zinc acetate and zinc nitrate. Different anions in solution distinctly influence the intrinsic defects in ZnO nanostructures, resulting in different photoluminescence properties. The defects induced by precursors were systematically studied by photoluminescence spectroscopy, X-ray photoelectron spectrometer and electron paramagnetic resonance. The results show that zinc acetate precursor mainly introduces zinc vacancy to the lattice while ZnO nanorods obtained from zinc nitrate contain more interstitial oxygen.
© 2016 Optical Society of America
1. Introduction
Zinc oxide is considered to be a promising candidate for optoelectronic applications such as photo detectors, light emitting diodes (LED), UV laser diodes and solar cells due to its wide band-gap and large exciton binding energy of 60 meV at room temperature [1–5]. Besides, other properties of ZnO such as chemical-sensing effects and piezoelectricity enable its applications in physical chemistry fields such as photocatalysts, chemical sensors and electric generators [6–10]. Among these applications, defects and impurities are decisive in determining its physical and chemical properties, which are crucial to the functionality of ZnO made devices [6, 11]. Other applications such as memristors using the resistive switching character of ZnO and spintronic devices using room temperature ferromagnetism of ZnO also rely on ZnO’s defect properties [12–14]. Since the intrinsic defects play such a significant role in all applications of ZnO, it is of paramount importance to understand the defects and their forming conditions in ZnO. However, controversial debates have continued for decades on intrinsic defects and related issues such as the origin of visible emission band in photoluminescence spectrum, signals in electron paramagnetic resonance and the origin of n-type conductivity in ZnO nanostructures [15–19].
Vapor transport and hydrothermal process are two major methods to grow ZnO due to their scalability and low cost. Nonetheless, compared to vapor phase grown process, synthetic method through liquid-phase is much more controllable in the preparation process of nanostructures [20]. Most previous work studied the effect of post annealing, while few were focused on the effect of precursors in aqueous growth process ending up with contradictory results and the discussions went no further than the different morphology obtained [21, 22]. Zinc acetate and zinc nitrate are two most frequently used zinc salts by researchers in hydrothermal growth of ZnO, however we found that ZnO obtained from these two precursors present distinctly different morphology and optical properties, and there has been no systematical study on the reason of the differences.
Herein, we compare hydrothermally grown ZnO using zinc acetate and zinc nitrate as precursors to unveil the influence of different anions on defects in ZnO. The grown and post annealed ZnO samples are investigated in detail by photoluminescence spectrum (PL), X-ray photoelectron spectrometer (XPS) and electron paramagnetic resonance (EPR), and a comprehensive understanding of the defects and their recombination routes in ZnO is deduced. We found that ZnO obtained from these two precursors possess apparently different morphology and optical properties, which implies that the defects inside are distinct.
2. Experimental section
ZnO thin films were synthesized by hydrothermal method on a n-type (001) silicon wafer. The silicon substrate was cleaned by acetone and ethyl alcohol in an ultrasonic bath for 15 min, then a thin layer of ZnO acting as a seed layer was sputtered by magnetron sputtering under the same setting with post annealing at 550 °C for 1 h. Since substrates were prepared under the same condition, differences induced by different substrates can be eliminated. The substrate with ZnO seed layer was then leaned upside down against the wall of a container filled with precursor solutions. Zinc acetate dehydrate (Zn(CH3COO)2⋅2H2O) and zinc nitrate hexahydrate (Zn(NO3)2⋅6H2O) were used as two different zinc sources. Methenamine (C6H12N4), also called HMT, which supplied hydroxyl ions to the solution, was added in equal concentration with zinc source to form a 0.1 M solution. After stirring, the container was kept in an oven for 2 h at 95 °C. ZnO grown in zinc acetate and zinc nitrate solution were denoted as A-ZnO and N-ZnO, respectively. The reactions proceeded as the formulas shown below:
The as-grown samples were then annealed for 1 h in different atmospheres (air, nitrogen, and hydrogen/nitrogen (10% hydrogen) gas mixture) at different temperatures (400 – 900 °C) in a horizontal tube furnace. X-ray diffraction (XRD) data of the samples were collected from a PANalytical X’Pert PRO diffractometer using Cu Kα radiation. The morphology of the samples was characterized by a field-emission scanning electron microscope (SEM, Hitachi S-4800). PL measurement was performed on a luminescence spectrometer (Edinburgh Instruments FLS 920) with Xe lamp emitting variable excitation wavelength and He-Cd laser at 325nm as excitation sources. EPR data of the samples were taken using an ESRA-300 spectrometer.
3. Results and discussions
3.1. Morphology
Top-view and cross-sectional SEM images of A-ZnO and N-ZnO films are shown in Figs. 1(a)-1(d), respectively. They clearly indicate that these films are composed of dense hexagonally shaped ZnO columns, which grow vertically on silicon substrate and merge with each other forming a columnar film. However, ZnO samples prepared from different precursors show different morphology: A-ZnO possesses a closely overlapping flake-like configuration while N-ZnO has a straight-standing rod-like morphology whose mean diameter is much smaller than A-ZnO as shown in Figs. 1(a) and 1(b). Films with different shapes also possess different thicknesses: A-ZnO is 1μm while N-ZnO is 1.6μm, indicating that N-ZnO sample grows faster in length while A-ZnO shows more conspicuous lateral growth tendency. The morphology of ZnO nanorods undergoes a significant change after thermal treatment at 900 °C in air: the expanded columns fill in the inter-spaces among pillars as shown in Figs. 1(e) and 1(f). The diameter of these columns in N-ZnO sample expands almost twofold: the mean diameter increases from 189 nm (400°C) to 357 nm (900 °C), as shown in the insets of Figs. 1(b) and 1(f). Meanwhile, the high temperature enables oxygen atoms to have enough kinetic energy to diffuse out of the crystal, leaving a mass amount of oxygen vacancies, which is verified by XRD, PL and EPR results in the following.
Fig. 1 Top-view SEM images of (a) A-ZnO, (b) N-ZnO, cross-sectional images of (c) A-ZnO, (d) N-ZnO, and top-view images of (e) A-ZnO, (f) N-ZnO annealed in air at 900 °C, respectively. Scale bar is 1μm. Diameter distribution and the mean diameter of the rods is shown in insets.
Since A-ZnO possesses a smaller thickness and a larger mean diameter than N-ZnO, the aspect ratio (length/diameter) of A-ZnO is smaller than that of N-ZnO. The concentration ofsolution and the preparation time are the same, the only difference between these two precursors is the anions in solution, therefore it can be inferred that anions have a significant impact on the nucleation rate and assembling site resulting in different morphology. As a polar crystal, ZnO is constituted by tetrahedrally coordinated Zn2+ and O2-ions stacking alternately along c axis [23]. During hydrothermal growth, acetate molecules may preferentially bound to Zn2+ on polar surface (0001) through –COO- and -OH functions. Such surface interaction affects hydroxyl ions released by HMT from contacting and reacting with Zn2+, which postpones the longitudinal growth and benefits the lateral growth of ZnO nanorods, resulting in the larger diameter and smaller film thickness. During the hydrothermal process, acetate ions may take a similar role as citrate which is always used as a surface modifier during the formation of nanostructures [24, 25]. As to N-ZnO, without the modification in the longitudinal direction, atoms would preferentially stack in c-axis due to the lower surface energy on polar faces, giving rise to the straight-standing rod-like morphology. While for A-ZnO, the hindered longitudinal growth rate forces the nucleation sites to take place in lateral directions, resulting in the closely overlapping configuration composed of shorter and fatter ZnO nanorods. Acetate ions absorbed on Zn2+ polar surface not only mediates the longitudinal growth rate but also postpones zinc species in solution transporting to the proper crystalline lattice site, thus leading to the formation of zinc vacancy. Therefore it is not surprised to find that N-ZnO shows a better c axis orientation while A-ZnO possesses a VZn-induced defect emission which will be discussed in detail below.
3.2. Structure
As-grown and post annealed ZnO at 400 °C and 900 °C in air are investigated by XRD, and the results are shown in Fig. 2. The strong (0002) Bragg peak of wurtzite ZnO in XRD pattern demonstrates a strong preferential c-axis orientation. Since (0001) polar faces of the wurtzite structure have a significantly lower surface energy than non-polar planes, atoms would prefer to stack in c-axis, giving rise to the anisotropic growth of ZnO with perpendicular orientation to the substrate [26]. The XRD peak intensity of N-ZnO is larger than that of A-ZnO, indicating a better orientation of c -axis direction in N-ZnO which is in consistent with SEM cross-section images. The reduction of full width at half maximum (FWHM) of (0002) peak after 400 °C thermal treatment implies the increase of crystallization and the average size of ZnO grains. Besides, the peak detected at 33.16° can be assigned to the (112) orientation of Zn2SiO4 due to the fusion of bottom ZnO with Si substrate. The exact (0002) peak positions in Fig. 2 are shown in Table 1. The shift of the (0002) peak to higher site after thermal treatment is due to the compressive strain along c-axis. During annealing at high temperature, oxygen atoms preferentially diffuse out of ZnO crystal site leaving oxygen vacancies (Vo) in the lattice, thus give rise to the compression stress along (0002) direction. For samples annealed at 900 °C, other than the out diffusion of atoms from the lattice, the nanorods expand as shown in Fig. 1, and the wurtzite structure severely degrades, resulting in the large decrease of (0002) peak intensity for 900 °C-annealed ZnO.
Table 1. (0002) peak position of as-grown and 400 °C, 900 °C air-annealed A-ZnO and N-ZnO samples.
3.3. Defect-related properties
In addition to differences in morphology and aspect ratio, A-ZnO and N-ZnO also differs in optical properties. Figure 3 is the comparison of PL spectra for A-ZnO and N-ZnO annealed under different atmospheres. A-ZnO always possesses an obvious blue-green band despite of the annealing atmosphere, however, which can hardly be detected in N-ZnO. In addition, a near-band-edge emission (NBE) around 380 nm and a yellow-orange band centered at 620 nm can be observed in all samples. Visible emission bands in ZnO are usually assigned as the defect-related recombinations, however defect types resulting in these emission bands are in debate for decades [15, 16, 27].
Fig. 3 PL spectra of A-ZnO, N-ZnO annealed at 500 °C in (a) air, (b) nitrogen and (c) vacuum, respectively.
In order to identify the defects leading to the visible bands, PL spectra with variable excitation energy is conducted. We adjust the incidence wavelength from 300 nm to 420 nm, covering the band-gap of ZnO. Filters with proper cut-off wavelengths are used according tothe varying excitation energy and the corresponding reception wavelength. Figures 4(a) and 4(b) are PL spectra of N-ZnO annealed in H2 and O2, respectively. The yellow-orange emission disappears after annealing in reduced atmosphere while enhances after O2 annealing, which proves that Oi is the related origin. Yellow-orange band can be excited under excitation energy higher than the band-gap, besides, the closer the excitation energy is to the band-gap the larger the emission intensity is. This emission band vanishes as soon as the excitation wavelength is below band-gap, as seen in rosy line of Fig. 4(b) with incidence wavelength of 390 nm, confirming that it originates from the transition of conduction band electrons to Oi level. Theoretical calculation also presents that Oi can accept two electrons acting as an acceptor in n-type ZnO [25] in consistent with our results.
Fig. 4 PL spectra of N-ZnO annealed in (a) reduced and (b) oxidizing atmospheres measured with various excitation wavelengths.
Figure 5 shows PL spectra of both A-ZnO and N-ZnO annealed at 900 °C with variable excitation wavelength, indicating that green emission with wavelength above 500 nm behaves the same way as yellow-orange emission mentioned above. The more the excitation energy approaches the band-gap, the more emission it gives off, and the emission cannot be excited by further lower excitation energy. Therefore, the green emission comes from the recombination of electrons from conduction band to Vo defects at deep level. When samples were excited by incidence energy higher than the band-gap, photoelectrons were pumped through the band-gap, relaxed to conduction band edge and recombined with deeply trapped holes in oxygen vacancies, giving off the broad green light above 500 nm. More importantly, we also use the variable excitation PL spectra to identify the additional blue-green band of A-ZnO. The blue-green band around 470 - 500 nm can easily be excited under all kinds of excitation energy as shown in Figs. 4 and 5. The fact that this emission band intensity becomes higher with lower excitation energy is due to the emergence of an additional excitation path. As the excitation energy becomes smaller than the band-gap, electrons in valence band no longer have enough energy to be excited to conduction band through the band-gap, thus electrons trapped in VZn- are pumped to conduction band and recombine with VZn -related defects, giving off the blue-green emission. The energy levels match perfectly with first principle calculations given by Janotti et al [28], which shows that the transition levels of VZn ε (2−/−) and ε (-/0) are 0.87 and 0.18 eV above the valence band, respectively. Therefore, when illuminated with lower excitation energy, especially energy less than the band-gap of ZnO, the intensity of the blue-green band emission becomes larger.
Fig. 5 PL spectra of (a) A-ZnO and (b) N-ZnO annealed at high temperature measured with various excitation wavelength, respectively.
To further verify the conclusion, electron paramagnetic resonance (EPR) was measured on A-ZnO and N-ZnO at room temperature. PL spectrums together with EPR give us a more complete insight into defect states in ZnO. Figure 6 shows the comparison of EPR spectrum of A-ZnO and N-ZnO annealed under the same condition. EPR spectrum with the corresponding PL spectra beneath conspicuously revealed the relation among defect types, their optical property and defect electron states. Two EPR signals can be observed. Signal g = 2.0052 detected around 3500 G on the low-field site originates from VZn- [29–31]. A higher intensity of the VZn-related signal of A-ZnO also exhibits that A-ZnO contains more zinc vacancies than N-ZnO, which is in agreement with the discussion above. First principle calculation also suggests that the formation energy of acceptor-like VZn decreases with increasing Fermi level, therefore VZn has low formation energy in n-type ZnO and serves as compensating centers [28, 32]. Although the ZnO film is not intentionally doped, H is unavoidably present during the growth procedure giving rise to a donor bound exciton state which is also the origin of ZnO’s n-type conductivity [27, 33]. The existence of VZn then allowed the compensation of H to take place. The amount of VZn in A-ZnO is larger thus can compensate more unintentionally introduced H.
Fig. 6 EPR spectra of A-ZnO, N-ZnO annealed at (a) 500 °C, (b) 700 °C and (c) 900 °C, respectively. Corresponding PL spectra of the same samples are plotted beneath, respectively.
Another signal g = 1.96 on the high-energy site around 3590 G is observed. Many researchers assigned g = 1.96 signal to the singly ionized Vo (Vo+) while first principle calculation suggests that Vo has a high formation energy in n-type ZnO with the most modest state being Vo2+ and neutral Vo [28]. EPR signal due to Vo+ can only be detected at g = 1.995 under low enough temperature with excitation, in which case Vo+ does not immediately decay into 2 + or 0 charge state [26]. Other than Vo, it is widely accepted that hydrogen acting as a shallow donor is the origin of signal g = 1.96 [30, 33], especially that hydrogen is abundant in solution phase during the hydrothermal growth process. Previous researchers have found that hydrogen atoms are easily incorporated into ZnO lattice by forming O-H bonds near zinc vacancies, existing in bond-centered position (HBC) or even in the form of H2 [34–36]. With increasing annealing temperatures, H atoms in ZnO begin to diffuse out and get trapped at high-temperature-induced defect VO, forming a new type of strong multicenter bond in Vo [36]. Though H in ZnO is highly mobile under thermal treatment, H now substitutes O position in Vo and exists in an energetically stable form of Ho [37] which could be detected by ESR and give rise to the observation of signal g = 1.96 in 700 °C -annealed samples. As annealing temperature further increases to the out diffusion temperature of Ho, the H-related g = 1.96 signal is no longer detected in 900 °C -annealed samples, as shown in Fig. 6(c). Under the annealing temperature of 900 °C, in addition to H and O atoms, Zn atoms also begin to diffuse out, thus PL spectra of A-ZnO and N-ZnO tend to have similar features exhibiting a massive enhancement of visible-to-NBE emission ratio, as shown in Fig. 6(f). The broad visible band can be divided into the Vo-related green band above 500 nm and VZn–related blue-green band below 500 nm as discussed above.
Zn/O lattice atom ratio calculated by XPS data also indicates that A-ZnO possess more VZn than N-ZnO as shown in Table 2. The area ratio of peak Zn 2p3/2 to O 1s1 represents the atom ratio of lattice Zn versus lattice O and that value of A-ZnO is always smaller than N-ZnO regardless of the annealing atmosphere, which indicates that A-ZnO samples intrinsically contain more VZn than N-ZnO ones.
Table 2. Lattice Zn/O ratio measured from Zn 2p3/2 /O 1s1 peak area ratio from XPS spectra of A-ZnO and N-ZnO post annealed at 600°C under the same condition in different atmosphere.
4. Conclusion
Different intrinsic defects induced by the two most widely used precursors: zinc acetate and zinc nitrate in hydrothermal growth process of ZnO are discussed and clarified for the first time. By considering both the growth process in solution where the adsorption effect of acetate anions take place and the evidence from post annealing treatment, we come to a conclusion that A-ZnO has more intrinsic VZn, resulting in the blue-green emission below 500 nm. In contrast, N-ZnO possesses more intrinsic Oi defects. The concentration of Oi in both A-ZnO and N-ZnO can be adjusted by post annealing treatment in reduced or oxidizing atmosphere. High temperature thermal treatment induced Vo results in the green emission band above 500 nm. Although thermal treatment is always used to adjust defects, the choice of precursors has as much influence on intrinsic defect types. With the thoroughly understanding of the defect formation process in ZnO, defects can be controlled and utilized to design better devices.
Funding
National Natural Science Foundation of China (No. 51272232); Program for New Century Excellent Talents in University; and the Fundamental Research Funds for the Central Universities (2015QNA3004, 2016XZZX002-01).
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