5 Physical characteristics of ZnO nanorods via soft chemical
5.3 Post-annealing effects on ZnO nanorods
It is well known that both physical characterization and optoelectronic properties are strongly influenced by the defect concentration of native defect in ZnO and this can be modified via thermal treatment under different atmospheres and annealing conditions [118]. Therefore, it is important to investigate the effect of post-annealing on the crystallinity and optical properties of ZnO nanorods. The variation of structural and optical properties of ZnO nanorods with thermal annealing conditions will be detailed discussed in this work. Figure 5.5 plots the room-temperature photoluminescence spectra of the ZnO nanorods annealed under various conditions.
Only two emission peaks at 377nm (UV emission) and 595 nm (visible emission) were observed. It was found that UV emission intensities increase with annealing
suggesting that the native defects or non-radiative recombination can be reduced by post-annealing treatment in O2 and N2 atmospheres as shown in Figure 5.5(a) and (b).
However, the ZnO nanorods annealed in N2 show stronger visible emission peaks compared to that annealed in oxygen atmosphere because the oxygen vacancies become the predominant point defects in N2 atmosphere. In addition, a unique phenomenon was observed for the sample annealed in H2/N2 atmosphere, as presented in Figure 5.5(c). An optimal UV emission occurs at the 600oC. It was believed that H2/N2 treatment is able to passivate native defects or impurities that contribute to visible transition, because the hydrogen atoms can be situated in various lattice positions. However, when the sample was exposed to H2/N2 at 800oC, the UV emission almost disappeared. This marked change can be attributed to the fact that a high-temperature reduction environment of H2/N2 could damage the crystal structure of the ZnO nanorods by surface etching [119]. It was also observed that the peak position of the longer-wavelength visible emission band shifts with different treatments. The peak position of the visible (deep-level) emission is related to the predominant defects in the ZnO nanorods and both defects of zinc interstitials and oxygen vacancies are strongly modified by changing annealing temperature and using different atmospheres [120]. The relative PL ratios (IUV/IDLE) of the samples as a function of various atmospheres can be further summarized and presented in Figure 5.6, revealing that the improvement in the optical quality of post-annealed ZnO nanorods is not only dominated by the annealing temperature but also the annealing atmosphere.
Figure 5.5: Room-temperature PL spectra of ZnO nanorods annealed at various temperatures in (a) O2, (b) N2 and (c) H2/N2 atmospheres.
Figure 5.7(a) shows that the as-grown ZnO nanorods are perpendicular to the substrate with a uniform length of 900-950 nm. As annealed at 600oC in H2/N2 , the cross-sectional morphology of the annealed ZnO nanorods in Figure 5.7(b)is slightly
different from that of the as-grown ZnO nanorods. It was found that the diameter of the annealed ZnO nanorodswas locally necked, as shown in the set of Figure 5.7(b).
When the ZnO nanorods were annealed at 800˚C in H2/N2, the surface image in Figure 5.7(c) shows that all the ZnO nanorods were collapsed on the substrate, probably because of surface etching. These results may elucidate why the UV peak was rapidly and suddenly disappeared at 800˚C in Figure 5.5(c). In contrast, as the ZnO nanorods were annealed at 800oC in both O2 and N2 atmospheres, the morphology of the ZnO nanorods almost remain unchanged as compared to that of the as-grown ZnO nanorods (Figure 5.8(a) and (c)). At 1000oC in N2 and O2, the surface morphology of ZnO nanorods was changed from “rod-like” to irregular shape as shown in Figure 5.8(b) and (d), perhaps because of the melting and re-growth of the nanorods.
Figure 5.6: IUV/IDEL of the annealed ZnO nanorods dependent on various annealing conditions.
Figure 5.7: SEM images of (a) the as-grown ZnO and annealed ZnO nanorods at (b) 600 oC, (c) 800 oC in H2/N2. The inset in Figure 5.7(b) showing the ZnO nanorods become locally necking.
HR-TEM was further used to study the ZnO nanorods annealed at 1000oC in N2 to elucidate the variation in the local structure. Figure 5.9(a) reveals that the ZnO nanorods were shortened to 300-400 nm, and became partially joined to their neighbors. The split diffraction spots in the central region of the SAED pattern suggest that the merged ZnO nanorods are not perfectly aligned in either the a (or b) and c directions, as presented in the inset of Figure 5.9(b). Moreover, HR-TEM images of the ZnO nanorods in Figure 5.9(c) demonstrate that partial amorphous was formed in the single-crystal ZnO nanorod. In addition, several lattice fringes (marked with arrows) appear in the edge of the ZnO nanorod in Figure 5.9(d). Furthermore,
according to X-ray photoelectron spectroscopy (XPS) analysis, the atomic ratio of O to Zn was approximately to 0.9 for the nanorods and the single of O2- ions in the oxygen-deficient regions were almost covered by the background signal, this indicating these amorphous regions appeared to contain some structural defects. The primary defect type in the region may be considered as oxygen vacancies as evidenced by PL spectra.
Figure 5.8: SEM images of the annealed ZnO nanorods at (a) 800oC, (b) 1000oC in O2, and (c) 800oC, (d) 1000oC in N2.
Figure 5.9: (a) High-magnification SEM and (b) low-magnification TEM images of ZnO nanorods annealed at 1000oC in N2 ambient with a corresponding diffraction pattern in the inset. A high-resolution TEM image of (b), showing the selected area in the (c) neck and (d) top of the ZnO nanorod.
5.4 Summary
We have developed a low-temperature synthetic route to prepare well-aligned arrays of oriented ZnO nanorods in the diameter of ~60 nm on the organic substrates without
any extra buffered layer in aqueous solution. HRTEM analysis demonstrates that well-aligned ZnO nanorods are preferentially nucleated from the ZnO monolayer under the concave regions of PS layers. The optical quality of ZnO nanorods grown on a flexible substrate could be improved by controlling growth conditions. This simple approach shows great potential for optoelectronic devices because it can produce large-scale highly well-aligned ZnO nanorods on flexible organic substrates.
In addition, photoluminescence spectra indicate that the optical quality of the ZnO nanorods can be changed and controlled by annealing the ZnO nanorods in various atmospheres at different temperatures. For the sample annealed in both O2 and N2 atmospheres, the room-temperature UV emission of the ZnO nanorods increases with the increase of temperature due to the reduction of structure defects. In contrast, in H2/N2 atmosphere, a stronger UV emission occurs at 600oC, above that the ZnO nanorods would be collapsed on the substrate and their optical property would be deteriorated by H2 etching process during a higher temperature annealing treatment.
The above-mentioned results may suggest that the crystallinity and optical properties of the ZnO nanorods can be improved by post-annealing treatment.
Chapter 6
Plasma treatment on ZnO nanorods
6.1 Introduction
Although ZnO was reported to be the most potential material to realize the next generation in UV semiconductor laser, most of the ZnO crystal is an n-type because it contains significant concentrations of shallow donors and native defects (Zn interstitials, O vacancies). Therefore, the control of defect states becomes an important issue for improvement of emission efficiency. However, unfortunately, most 1D nanomaterials (nanowires or nanorods) have been paraded as “perfect-like”
or “defects-free” nanocrystals so that there are rarely efficient methods to fabricate high-quality ZnO nano-scale nanostructures via doping the grown ZnO nanorods at lower temperatures because of no suitable diffusion paths except for in-situ growth of doped-ZnO via vapor-liquid-solid (VLS) process which usually takes place at high temperatures [121,122]. But, in our previous studies reveal that the single crystalline ZnO nanorods grown from aqueous solution contain many defects which can provide for the doping or incorporation of impurities into ZnO nanorods [123]. Therefore, in this chapter, the reaction between the plasma gas and ZnO nanorods will be discussed.
6.2 High conductivity ZnO nanorods in hydrogen-plasma
Numerous prior investigations have reported that hydrogen ions not only could be
more easily diffused into ZnO in the plasma-treated process and combine with other defects to form a shallow donor in ZnO, but also can increase the free electron concentration of ZnO to improve conductivity and strongly passivate the deep level emission to enhance the band edge luminescence. Therefore, in this work, a simple method by combining the aqueous solution process with hydrogen plasma treatment was proposed to develop high-quality well-aligned arrays of ZnO nanorods on organic substrates. Figure 6.1 shows the SEM images taken from several samples with highly uniform and densely packed arrays of ZnO nanorods grown on flexible four-inch PC substrate (Figure 6.1(a)). The ZnO nanorods in Figure 6.1(b) present a well-defined hexagonal shape with a homogeneous diameter of approximately ~70 nm. The cross-sectional SEM image in Figure 6.1(c) demonstrates that the highly oriented ZnO nanorods with a uniform length of 500-520 nm are perpendicularly grown to the PC substrate. Figure 6.1(d) shows the high resolution TEM (HRTEM) image of the well-aligned ZnO nanorods grown on PC substrates with the corresponding selected area electron diffraction pattern shown in the inset. The fringe spacing between two adjacent lattice planes is about 0.52 nm which is nearly consistent with the c-axis parameter in hexagonal ZnO structure (c = 0.521 nm in ZnO wurtzite structure), indicating that <001> is the preferred growth direction for the ZnO nanorods. In addition, some stacking faults are also observed (marked with arrows) and these stacking faults seem not to affect the crystal quality of ZnO nanorods because the selected-area electron diffraction pattern in the inset of Figure 6.1(d) reveals that the ZnO nanorods still exhibit single crystalline structure.
Figure 6.1: SEM images of large arrays of oriented ZnO nanorods grown on polycarbonate (PC) substrates for 8 h. (a) Photograph of the flexible PC grown with arrayed ZnO nanorods. (b) Low magnified top view image of ZnO nanorods. (c) Cross-sectional SEM image of ZnO nanorods grown on PC substrates. (d) High-resolution electron micrograph of ZnO nanorods with the SAED pattern shown in the inset.
Figure 6.2: (a) PL spectra and (b) IUV/IDEL of ZnO nanorods with and without hydrogen plasma treatment at room temperature. The inset image of (b) shows the FESEM surface morphology of ZnO nanorods with hydrogen plasma treatment of t
=900 sec.
Figure 6.2 illustrates the photoluminescence (PL) property of ZnO nanorods at room temperature. As shown in Figure 6.2(a), the PL spectrum of non-plasma ZnO nanorods presents a weak ultraviolet (UV) emission peak at 3.28 eV and a relatively strong deep-level emission peak at 2.10 eV. Similar phenomena are also reported in the related literature [109,110]. The UV emission peak of ZnO is generally attributed to the exciton-related activity, and the deep level emission may be due to the transitions of native defects such as oxygen vacancies and zinc interstitials [54,55]. In addition, the imperfect boundaries and stacking faults of ZnO nanorods would cause the unstable surface status to trap impurities and further damage the optical property, especially as the diameter of ZnO nanorods was down to nano-scale. It was believed that those induced defects are probably related to the fast growth of the ZnO nanorods in the aqueous solution. In contrast, with the hydrogen plasma treatment, all the ZnO nanorods show much better PL property than that of non-plasma sample and the deep level emission was almost covered by the background signal in all plasma-treated samples. It was believed that the native defects or impurities contributing to visible transition can be passivated by H2 plasma treatment, because the hydrogen atoms can be situated in various lattices positions and the most presumably stable position is the H configured at Zn-O bond center, which acts as a shallow donor.[ref-22] Similar observations for the passivation of hydrogen plasma treatment on the visible emission of ZnO are also reported [124]. Figure 6.2(b) illustrates the ratio of peak intensity of Ultraviolet (UV) emission (IUV) to that of deep level emission (IDLE). The value of the relative PL ratio increases with the increase of plasma treatment duration up to 300 sec in all the samples and then becomes slightly decreased. A higher PL ratio implies that the plasma-treated ZnO nanorods exhibit higher optical quality. Moreover, it can be clearly observed that the crystal morphology of vertically well-aligned ZnO nanorods seems not to be affected by a lengthy plasma treatment even at 900 sec as
revealed by the SEM images in the set of Figure 6.2(b).
Figure 6.3: (a) Zn 2p spectra obtained from the ZnO nanorods with and without hydrogen plasma treatment. (b) Dependence of relative intensity ratio (O1s) of three fitted components centered at 530.15, 531.25, and 532.40 eV for the ZnO nanorods with and without hydrogen plasma treatment.
Figure 6.3 shows the XPS core level spectra taken from the ZnO nanorods surface after the plasma treatment in H2. As compared with the non-plasma sample, Figure 6.3 (a) illustrates that the peak shift of 0.2-0.3 eV toward higher binding energies was observed in Zn 2p spectrum for H2 plasma-treated ZnO nanorods. This peak shift indicates the reduction of the surface band bending and this may be related to the doping or incorporation of H ions into ZnO nanorods [125]. Thus, the H2
plasma treatment seems to partially recover or reduce surface defects, leading to the decrease of densities of surface states on ZnO nanorods. However, as the ZnO nanorods were treated with H2-plasma duration more than 300 sec, the Zn 2p peak is shifted toward a higher binding energy around 0.5-0.8 eV. This phenomenon reveals that the surface band bending of ZnO nanorods was slightly changed by H2 plasma treatment. Moreover, in comparison with non-plasma sample, the intensity of Zn 2p peak would decrease with the increase of the plasma duration. It might imply that
were induced to raise the intensity of deep level emission. However, this phenomenon was not observed in our study. Therefore, it can be inferred that hydrogen plasma treatment might passivate these defect centers to suppress the deep level emission.
In addition, the O1s peak presents different feature for the samples treated with various duration of hydrogen plasma as one can see in Figure 6.3 (b). The typical O1s peak can be consistently fitted by three nearly Gaussian, centered at 530.15, 531.25, and 532.40 eV, respectively. Generally, the high binding energy component located at 532.40 is usually attributed to O-H bonds [126] which could be attributed to the absorbed hydrogen ions during the plasma treatment of ZnO nanorods. Furthermore, it can be observed that the peak intensity at high binding energy would increase as increasing plasma duration up to 300s and then it gets saturated. It indicates that hydrogen plasma treatment could efficiently modify the surface states of ZnO nanorods by absorbing the hydrogen ions to reduce the unstable dangling bonds on the surface region of ZnO nanorods. Figure 6.3(b) also shows the variation of the medium binding energy component of O1s peak (centered at 531.25 eV) with plasma duration. The peak is generally associated with O2- ions in oxygen deficient regions within the matrix of ZnO. It was observed that the peak intensity of this component does not obviously change with hydrogen plasma treatment. This indicates that the oxygen vacancies can not be reduced by H2-plasma treatment to improve the optical properties in ZnO nanorods. To further understand the importance of hydrogen plasma treatment on the passivation effects of ZnO nanorods, the H2-plasma samples were further annealed at various temperatures in nitrogen atmosphere. (Note: the post-annealed specimen were grown on Si substrate and then treated by hydrogen plasma at the same conditions as that grown on the flexible substrates.) It was found that the relative PL ratio would decrease drastically while the samples were annealed more than 400oC, as shown in Figure 6.4. Furthermore, the PL spectrum of the
post-annealed ZnO nanorods was almost restored to that of original ZnO nanorods (ZnO nanorods without H2 plasma treated). This implies that surface absorption and doping effects of hydrogen ions on ZnO nanorods can be recovered by thermal annealing process. Therefore, it can be concluded that both defect passivation and modification of surface state on hydrogen-plasma ZnO nanorods are responsible for the enhanced optical properties.
Figure 6.4: IUV/IDEL of ZnO nanorods (grown on ZnO film/ Si substrate) with hydrogen-plasma for 300 sec and then annealed at 400, and 600oC (PA@400 and PA@600 oC). The corresponding PL spectra of ZnO nanorods were shown in the inset.
Figure 6.5: Comparison of I-V curves for the ZnO nanorods with and without
Figure 6.5 shows the I-V curve for a homojunction of n-type ZnO nanorods (treated by H2 plasma duration of 900 sec) on n-type ZnO films. The structure of In metal/ZnO nanorods/ZnO films/In metal/PC substrate was used for the I-V measurement of ZnO nanorods. It was found that the non-plasma ZnO nanorods present a higher resistivity about hundreds of MΩ, which is about 2 orders of magnitude larger than that reported in the literature for the naked single ZnO nanowire (above 3.5 MΩ). In contrast, when the ZnO nanorods undergo hydrogen plasma treatment over than 300 sec, the resistivity of ZnO nanorods decrease by typically 5-6 orders of magnitude. It suggests that hydrogen plasma can efficiently raise the free electron concentration and increase the conductivity of ZnO nanorods.
The above results reveal that with the H2–plasma treatment on ZnO nanorods, both optical and electrical properties could be substantially improved and increased. This implies that incorporating hydrogen into ZnO nanorods not only passivates the native defects but also acts as a shallow donor to improve the conductivity.
6.3 Rectifying behavior of ZnO nanorods by ammonia-plasma
The ZnO nanorods present single crystalline structure and a well-defined hexagonal plane with a homogeneous diameter of approximately 60-70 nm. The cross-sectional scanning electron microcopy (SEM) image in Figure 6.6(a) shows that the highly oriented ZnO nanorods with a uniform length of 500-520 nm are perpendicularly grown to the substrate. In addition, it was found that by controlling the experimental conditions, highly arrayed ZnO nanorods or nanowires with different aspect ratios can be grown from the chemical aqueous solution [116]. A close observation on the
microstructure of the ZnO nanorods, as show in Figure 6.6(b), reveals that the surface morphology of ZnO nanorods shows a waved shape structure with 0.5 ~ 1 nm roughnesses. In addition, some stacking faults are also observed (marked with arrows).
However, these small surface roughness and stacking faults seem not to affect the crystal quality of ZnO nanorods because the selected-area electron diffraction pattern in the inset of Figure 6.6(b) reveals that the ZnO nanorods still exhibit a single crystalline structure. Figure 6.6(b) clearly describes the perpendicular directional growth of the ZnO nanorods where the singular fringes spacing is about 0.51nm, which is nearly consistent with the c-axis parameter in hexagonal ZnO structure (c = 0.521 nm in ZnO wurtzite structure), indicating that <001> is the preferred growth direction for the ZnO nanorods, in consistence with XRD patterns that shows a single strong ZnO (002) peak at 2θ =34.4o (not shown here).
Figure 6.6: Cross-sectional scanning electron microcopy (SEM) micrograph of (a) ZnO nanorods/ZnO film/Si, (b) surface microstructure and stacking faults (marked with arrows) of ZnO nanorods. The inset image of (b) displays SAED pattern.
Figure 6.7(a) illustrates the photoluminescence (PL) property of ZnO nanorods at room temperature. The PL spectrum of non-plasma ZnO nanorods presents a weak ultraviolet (UV) emission peak at 3.28 eV and a relatively strong deep-level emission peak at 2.10 eV. The UV emission peak of ZnO is generally attributed to an exciton-related activity [1], and the deep level emission may be due to the transitions of native defects such as oxygen vacancies and zinc interstitials. In addition, the imperfect boundaries in ZnO nanorods would cause the unstable surface status to trap impurities and further damage the optical property, especially for the nano-scale ZnO nanorods. In contrast, it was found that the peak intensity of the UV emission increases with the plasma duration up to 90 sec but the deep level emission in all plasma-treated samples tends to disappear, indicating that the native defects or impurities, contributing to visible transition, can be much reduced by NH3 plasma
Figure 6.7(a) illustrates the photoluminescence (PL) property of ZnO nanorods at room temperature. The PL spectrum of non-plasma ZnO nanorods presents a weak ultraviolet (UV) emission peak at 3.28 eV and a relatively strong deep-level emission peak at 2.10 eV. The UV emission peak of ZnO is generally attributed to an exciton-related activity [1], and the deep level emission may be due to the transitions of native defects such as oxygen vacancies and zinc interstitials. In addition, the imperfect boundaries in ZnO nanorods would cause the unstable surface status to trap impurities and further damage the optical property, especially for the nano-scale ZnO nanorods. In contrast, it was found that the peak intensity of the UV emission increases with the plasma duration up to 90 sec but the deep level emission in all plasma-treated samples tends to disappear, indicating that the native defects or impurities, contributing to visible transition, can be much reduced by NH3 plasma