Summary

We demonstrated that the growth of patterned ZnO nanorods can be controlled by changing the annealing conditions of the ZnOf/Si substrates. When the ZnOf/Si substrate was annealed above a critical temperature to promote the crystallization of ZnO phase, both ZNs and ZnOf

on Si substrate were found to become crystallographically matched. In this work, it reveals that the ZNs seem to preferentially nucleate from the cup tip near the grain boundary between two ZnO grains in the ZnO film. However, a higher annealing temperature may lead to the formation of a larger ZnO crystal due to coplanar coalescence behavior of several individual ZnO nanorods.

Fig. 4.1 SEM image of the ZNs synthesized in (a) 0.0025 M (b) 0.01 M (c) 0.04 M (d) 0.08 M

20 30 40 50 60 (100)

(002)

(101)

Intensity(a.u.)

2θ

0.0025M 0.01M 0.04M 0.08M

Fig. 4.2 XRD analysis of the ZNs synthesized in different concentrations.

Fig. 4.3 SEM image of the ZNs synthesized at (a) 55 ^oC (b) 65 ^oC (c) 75 ^oC (d) 85 ^oC.

Fig. 4.4 XRD analysis of the ZNs synthesized in different growth temperature.

20 25 30 35 40 45 50 55 60

(101) (100)

(002)

Intensity (a.u .)

2θ

55^oC 65^oC 75^oC 85^oC

Fig. 4.5 AFM images of (a) as-grown ZnO films and (b) annealed ZnO film at 800^oC.

(a)

(b) (a)

Fig. 4.6 SEM images of the ZnO nanorods (a) grown on the patterned ZnOf/Si, (b) grown at 1μm

1μm 1μm 1μm (c)

20μm 50nm

500nm 500nm

(d)

(a) (b)

(c)

0 100 200 300 400 500 600 700 800 900

Fig. 4.7 Both ZNs diameter and ZnOf grain size as a function of annealing temperature.

Fig 4.8 (a) TEM bright-field and (b) dark field images of the aligned ZNs grown on ZnOf/Si substrate annealed at 600^oC. A corresponding diffraction pattern is shown in the inset of Fig. (b) for the selected single nanorod. (c) A high-resolution TEM image of (a) showing the interface between ZNs and ZnOf.

(002) (110)

50nm

20nm (a)

(b)

(c) (b) (a)

(c)

Fig. 4.9 (a) Showing the low-magnification TEM images of ZnO nanorods grown on the annealed ZnOf/Si at 800^oC. (b) HRTEM images of the ZnO nanorods marked in the frame of (a) along with split diffraction pattern in the inset. HRTEM image of (c) left, (d) middle and (e) right side of the larger nanorod, showing the larger ZnO nanorod seems to be composed of three ZNs.

20nm

200nm

(c) (c) (d) (e)

(b) (a)

Chapter 5

Growth behavior and microstructure evolution of ZnO nanorods grown on Si in aqueous solution

5.1 Introduction

One-dimensional (1D) semiconductor nanostructures have been extensively studied for their potential applications in manufacturing electronicand optoelectronic devices [59]. Zinc oxide (ZnO) is an important electronic and photonic material because of its wide and direct band gap (ΔEg 3:37 eV) material [60]. The strong exciton binding energy of ZnO (60 meV) is much larger than that of GaN (25 meV), which can ensure an efficient exciton emission at room temperature. Recently, highly oriented nanorod arrays of ZnO nanostructures have demonstrated their potential applications in manufacturing electronic and optoelectronic devices [61,62]. Various methods, including chemical, electrochemical, and physical deposition techniques have been employed to synthesize 1D ZnO nanostructures such as catalytic growth via the vapor–liquid–solid (VLS) epitaxial method [63] and metalorganic-chemical vapor deposition (MOCVD) [64]. However, those methods are expensive and energy consuming processes since they are operated under extreme conditions.

On the other hand, many wet-chemical approaches have been used for large oriented arrays of ZnO nanorods on polycrystalline (or single-crystalline) substrates from aqueous solutions [65].

It is well conceived that the preparation of ZnO via solution chemical routes provides a promising option for large-scale production of these materials. Recently, Vayssieres et al. has proposed a novel theoretical concept, called ‘‘purpose-built materials’’, to grow arrayed

nanorods and nanowires of ZnO fromaqueo us solutions [66]. Such an approach does not require any template, membrane or applied external field to create anisotropic nanoparticles.

However, it is worth noting that without suitable treatment on the substrate, highly oriented ZnO nanorods grown on a Si wafer have been rarely achieved due to the larger mismatch (~40%) between the substrate and the ZnO nanorods. Yamabi et al. reported that without undercoats on the surface, clusters of spindle-shaped hexagonal crystallite with diameters of 0.5–1.0 mm were scattered over the surface of a silicon wafer, but highly aligned arrays of ZnO nanorods with diameters of 20–100nm can be developed on Si substrate undercoated with Zn(Ac)2 and annealed at 500 ^oC [67]. Recently, Choy et al. also found that high-quality ZnO nanorods were successfully grown on a Si wafer by a wet-chemical process at 95 ^oC for 6 h, where the Si wafer was dip-coated with 4 nm sized ZnO nanoparticles as a buffer and seed layer prior to crystal growth [68]. These observations reveal the importance of surface characteristics or treatment on the growth of well-oriented ZnO nanorods in aqueous solution.

However, little investigation has paid attention to the growth behavior of such ZnO nanorods, in particular to tailor their orientation onto the substrates that can be used to control the structure morphology and optoelectronic properties. Therefore, in this work, a simple wet-chemical method was used to study the growth behavior of both scattered and aligned ZnO nanorods on Si by monitoring the average aspect ratio of ZnO nanorods as a function of growth time in aqueous solution. By controlling the experimental conditions, scattered or well-aligned ZnO nanorods with different aspect ratios can be obtained. Moreover, high-resolution transmission electron microscopy (HRTEM) was performed to investigate the microstructure evolution of both ZnO nanorods.

5.2 Phase and morphology of ZnO nanorods grown on different substrates

Two substrates, i.e., Si and ZnO-coated Si, were used in this work to study the growth

behavior of ZNs in the precursor solution at different temperatures of 55–95 ^oC for several hours. No ZNs were observed below 50 ^oC, indicating that a thermal barrier for the growth of ZNs cannot be overcome. Above 95 ^oC, ZNs cannot be synthesized because the reaction temperature is near the melting point of the aqueous solution. Fig. 5.1 shows the SEM images of the as-synthesized ZNs grown on these two kinds of substrates at 75 ^oC for 10 h in the 0.02M solution. As shown in Fig. 5.1(a), ZNs with dimensions of 7~10 mm in length and 0.5~1 mm in diameter were scattered on the Si substrate. On the other hand, high densities of well-aligned ZNs with dimensions of 20–200nm in diameter and up to several mm in length were obtained when ZNs were grown on ZnOf/Si as shown in Fig. 5.1(b). It seems to imply that the formation of well-aligned ZNs on a pure Si substrate is difficult because a large mismatch (~40%) exists between ZnO and Si. Hence, this would retard the nucleation of ZnO nanorods on Si substrate. This observation strongly reveals the importance of the substrate characterization on the growth behavior of ZNs in the aqueous solution.

The X-ray diffraction patterns (XRD) in Fig. 5.2 show that the ZnO crystal phase starts to appear at 50 ^oC and is well crystallized above 65 ^oC. A strongly oriented peak can be indexed as the wurtzite ZnO at 75 ^oC in 0.02M aqueous solutions for the ZNs grown on Si and ZnOf/Si substrate. However, it should be pointed out that a remarkable difference in XRD patterns exists between these two substrates. As shown in Fig. 5.2(a), the diffraction peaks were indexed as (1 0 0), (0 1 0) and (1 0 1) planes of ZnO for the ZNs grown on Si substrate.

On the other hand, Fig. 5.2(b) illustrates that the strongest diffraction peak corresponding to the (0 0 2) plane of ZnO was detected when the ZNs were grown on ZnOf/Si substrate. These observations obviously reveal that there exists different growth behavior between (1 0 0)-oriented (called scattered) ZNs on Si and (0 0 2)-oriented (called aligned) ZNs on ZnOf/Si.

5.3 Microstructural analysis of ZnO nanorods

ZnO has a hexagonal lattice with a c/a axial ratio of 1.602 [69]. The most commonly observed morphologies of ZnO are either rod-like or needlelike crystals, especially the prepared ZnO in aqueous solution because of anisotropic crystal growth. The TEM bright-field (BF) image and corresponding selected-area diffraction pattern (SADP) of the scattered ZNs are shown in Figs. 5.3(a) and (b), respectively. The SADP in Fig. 5.3(b) clearly indicates that the scattered

ZNs were grown along the ٛ[0110] direction. This result is also consistent with the XRD ⁻ analysis that the ZNs are preferentially oriented along the ٛ[011 0] direction. A close ⁻ examination of the SADP shows the split of diffraction spots and extra diffraction spots, located halfwaybetween the central (0 0 0) spot, the 0002 spots and f {0110} spots. However, ⁻ no Morie’ fringes were observed in the corresponding HRTEM in Fig. 5.3(c) so that double diffraction can be neglected in this study. Since the crystallographic phase of these ZNs belongs to wurtzite structure with a space group of P63mc, it easily appears as a characteristic

‘‘zigzag’’ structure [70,71], so-called ‘‘superlattice diffraction’’, corresponding to the observed extra spots in the SADP. It seems to imply that the crystal structure of the scattered ZNs was constituted from many tiny areas such as domains or mosaic texture according to the disoriented few degrees in SADP. It is well known that a crystal with mosaic structure does not have its atoms arranged on a perfectly regular lattice extending from one side of the crystal to the other; instead, the lattice is broken up into a number of tiny blocks, each slightly disoriented one froman other. Within these tiny blocks, there exists a structural mismatch at the interface between these domains [72], as in this case with 1–21 difference in the SADP.

Furthermore, the HRTEM image in Fig. 5.3(c) recorded from the scattered ZNs reveals that there seemto be two types of lattice fringes in this HRTEM image: one with wave-like fringes (as arrowed in Fig. 5.3(c)) and the other with straight fringes. This variation of lattice fringes as marked with arrows seems to be caused by these two types of crystal arrangements. Fig.

5.4(a) shows the TEM BF image of the aligned ZNs grown on ZnOf/Si substrate. In contrast

to the scattered ZNs, most of the ZNs were grown along the direction perpendicular to the ZnOf/Si substrate. A higher magnification in Fig. 5.4(b) indicates that the lattice fringes are perpendicular to the longitude direction of the ZNs, and the singular fringe spacing is about 0.51 nm, which is nearly consistent with the c-axis parameter in hexagonal ZnO structure (c=0.521nm in ZnO structure). This demonstrates that the [0 0 0 2] direction is the preferred growth direction for the well-aligned ZNs. In addition, as one pays attention to the interface between ZNs and ZnOf/Si shown in Fig. 5.4(c), it was observed that the well-aligned ZNs seemto be nucleated from the cup tip near the grain boundary between two ZnO grains in ZnO film. The lattice image at the joint of the interface, shown in the inset of Fig. 5.4(c), clearly indicates that the lattice fringes near the interface of ZnO grains in ZnO film are continuous without any cracks. The formation of the cup shape around grain boundaries between two ZnO grains is supposed to be correlated with the solution reaction during the nucleation and growth of ZNs. This implies that the cup tip probably becomes a preferential nucleation position for the well-aligned ZNs grown on ZnOf/Si. However, in addition to (0 0 0 2) fringes, some ZNs with white lines dissecting the ZNs can be identified as marked with arrows in Fig.

5.5 that is probably due to faster stacking and the instability of the polar (0 0 2) plane in ZnO.

These kinds of planar defects are easily developed at a higher growth temperature, i.e. 90 ^oC, and can be considered as extrinsic stacking faults with insertion of an extra Zn-O layer parallel to the basal plane. It was worth noting that the presence of (0 0 0 1) stacking faults provides a possible diffusion path to modify its optoelectronic properties according to our study [73]. However, wellaligned ZnO nanorods without any planar defects can also be obtained in aqueous solution by controlling the experimental conditions.

5.4 Growth behavior of ZnO nanorods

It is well known that the growth behavior of ZNs was strongly influenced by growth

conditions such as ion concentration in the solutions, reaction temperature and time, which can be elucidated by the change of the aspect ratio (AR=length/width) of ZNs. As illustrated in Fig. 5.6, when the pure Si substrate was placed in the Zn-containing aqueous solution at 75

oC, it was found that in an initial growth stage of ZNs, prior to 0.2 h, no obvious ZNs can be detected. However, above that (0.2–0.3 h), SEM images (not shown here) show a small amount of ZNs with ~20 nm in diameter and ~60 nm in length that were randomly scattered on Si substrate. Although the SiO2 is probably formed on the Si substrate, a similar growth phenomenon is also observed for SiO2/Si substrate. This implies the importance of ZnO film coating on the nucleation and growth of aligned ZnO nanorods. Above 0.5 h, the aligned ZNs can be clearly obtained but the scattered ZNs start to preferentially grow along the longitude (c-axis) direction. With an increase of growth time, it can be observed that the growth of scattered ZNs becomes slower compared to that of the aligned ZNs. Especially above 5 h, it was found that the AR of the scattered ZNs changes little (9–11). Furthermore, with increasing growth time up to 15 h, the AR slightly decreases. This implies that the scattered ZNs do not further grow as expected despite extending growth time. As evidenced from the TEM BF image and corresponding SADP of ZNs shown in Fig. 5.7, it seems to reveal that more than two ZNs are self-assembled together to for a bundle in a coplanar manner using their side planes to forma large ZN as shown in Fig. 5.7(c) of the TEM dark-field (DF) image.

This bundle of ZNs can be further transformed into another ZN with a larger dimension in diameter. In other words, the side crystal planes of the ZNs are able to glue together to forma larger crystal. This behavior is also called ‘‘oriented attachment’’ [74]. This observation may be used to explain the slight decrease in AR in the later growth stage (5–15 h). Consequently, the growth behavior of the scattered ZNs can be considered as a two-step growth mechanism.

In contrast, as the ZnOf/Si substrate was used and immersed into the precursor solution for the growth of ZNs, it was found that in the initial growth stage (to 0:5), no aligned ZNs can be clearly observed fromthe ZnOf/Si substrates, but at 1h, a smaller AR was obtained for the

aligned ZNs compared to that of scattered ZNs. This phenomenon reveals that the aligned ZNs need incubation time to nucleate from ZnO film (in a nucleation stage) but the scattered ZNs are under both stages of nucleation and growth. It was believed that the ZnO nuclei will be preferentially grown on the ZnOf/Si substrate in this stage due to the high affinity of the nuclei of ZnO nanorods to the surface of ZnO film, especially at the cup sites as shown in Fig.

5.4(c). In this condition, although an inherent asymmetry along the c-axis allows the anisotropic growth of the crystal along the [0 0 0 2] direction, the lateral growth along different directions is possible for the nucleus to reduce the surface energy effect. When the favorable nucleation positions of ZNs were formed on the ZnO film, ZnO growth unit (or coordination polyhedron) will stack in order and grow along the [0 0 0 2] direction.

Subsequently, a fast growth along the longitudinal direction (c-axis) for the aligned ZNs was expected because the growth in width direction is suppressed due to the size of the cup shape on ZnOf/Si substrate. Finally well aligned and highly oriented ZNs with an aspect ratio up to 25–30 can be obtained in 15 h. These results can be used to explain the observation that the well-aligned ZNs with a smaller AR were developed in an early stage, but later, the ZNs were rapidly grown along the longitudinal direction.

5.5 Summary

Single-crystal ZnO nanorods (ZNs) can be synthesized on both Si and ZnOf/Si substrates in an aqueous solution at 75 ^oC, but they present different growth behavior and direction. On pure Si substrate, ZNs were scattered over the entire Si substrate with a preferred orientation in the (1 0 0) plane or grown along the [0110] direction. HRTEM observation demonstrates ⁻ that the scattered ZNs present a two-stage growth mechanism with a self-assembly process of ZNs in the later growth stage. In contrast, on Si wafer with ZnO film coated, aligned ZNs

were directly grown along the [0 0 0 2] direction from the ZnO film on Si. In comparison with the AR of the scattered ZNs, a larger AR up to 25–30 was obtained. This simple approach should promise us a future large scale synthesis of the patterned growth of the highly well-aligned ZNs on various kinds of substrates buffered with a controlled morphology and roughness layer in an aqueous solution at low temperatures.

Fig. 5.1. SEM micrographs of ZnO nanorods grown on (a) Si and (b) ZnOf/Si substrates.

Fig. 5.2. X-ray diffraction patterns of ZnO nanorods grown on (a) Si and (b) ZnOf/Si substrates.

Fig. 5.3. (a) TEM bright-field (BF) image and (b) corresponding selected area diffraction pattern of ZnO nanorods grown on Si substrate. (c) Lattice fringes of ZnO nanorods in (a).

Fig. 5.4. (a)TEM BF image of aligned ZnO nanorods grown on ZnOf/Si substrate. (b) High-resolution TEM images of the aligned ZNs and (c) interface region between ZNs and ZnOf/Si.

Fig. 5.5. High-resolution TEM (HRTEM) image of ZnO nanorods showing the presence of stacking faults (SFs) as marked with arrows.

Fig. 5.6. Dependence of aspect raio (AR) on growth time for scattered and aligned ZnO nanorods.

Fig. 5.7. (a) TEM bright-field image, (b) corresponding selected area diffraction pattern, and (c) dark-field image of self-assembled ZnO bundles.

Chapter 6

Synthesis and Luminescent Properties of Strong Blue Light-Emitting Al

O

/ZnO Nanocables

6.1 Introduction

Al2O3 has been used for capacitor dielectrics and gate oxides in memory devices due to its high dielectric constant, very low permeability and high thermal conductivity.[75] However, the photoluminescence property of alumina film or nanoparticles has not been studied in detail. Yoldas et al. studied alumina–silica powders and stated that the presence of pentahedrally coordinated aluminum appears to be strongly correlated to the occurrence of photoluminescence. [38] Suga et al. studied alumina gel from an inorganic salt and alkoxide and mentioned that the photoluminescence is closely related to oxygen defects and the development of Al^V site. [39] However,no data exists for determining the dependence of the properties of a defect centre on the structure of its coordinate sites and the presence of luminescence. Recently,Li et al. reported that a broad band located around 422 nm could be detected from nano-sized γ-Al2O3 powder.[40] It is suggested that the produced defect level could induce γ-Al2O3 nanopowder to emit blue photoluminescence (PL) bands. However, most of those studies have been focused on nanopowders or gel films. In past years, ZnO/Al2O3 core/shell nanofibers have been prepared from the Al2O3 deposition of ZnO nanowires with an atomic layer deposition technique, but no photoluminescence properties have been reported.[45] These findings indicate that so far, the photoluminescence property of nano-scale alumina film has been not investigated. In addition, our previous study found that

when an alumina film is deposited on a ZnO-coated silicon substrate by a wet chemical process, the alumina film not only emits blue, but the blue emission can also be much enhanced compared to that of coatings on pure silicon. This indicates that ZnO plays an important role on the photoluminescence properties of nano-sized alumina films.

Wet chemical approaches are widely used for the fabrication of large oriented arrays of ZnO nanorods on Si or polymer organic substrates. ZnO has been recognized as one of the promising nanomaterials in optoelectronic device applications. Therefore, it is possible to develop the Al2O3/ZnO nanostructure into white phosphor if blue emission from alumina can be incorporated into the ZnO nano-structure. So far, to the best of our knowledge, there have been no further systematic investigations on the light-emitting properties of alumina-coated ZnO nanorods grown in aqueous solutions at lower temperatures. Furthermore, it has been challenging to develop Al2O3/ZnO or ZnO-based nanostructures with strong light emission by simple wet chemical processing. Therefore, a simple method of combining the aqueous solution process with a thermal treatment is proposed to develop Al2O3/ZnO nanocables with

Wet chemical approaches are widely used for the fabrication of large oriented arrays of ZnO nanorods on Si or polymer organic substrates. ZnO has been recognized as one of the promising nanomaterials in optoelectronic device applications. Therefore, it is possible to develop the Al2O3/ZnO nanostructure into white phosphor if blue emission from alumina can be incorporated into the ZnO nano-structure. So far, to the best of our knowledge, there have been no further systematic investigations on the light-emitting properties of alumina-coated ZnO nanorods grown in aqueous solutions at lower temperatures. Furthermore, it has been challenging to develop Al2O3/ZnO or ZnO-based nanostructures with strong light emission by simple wet chemical processing. Therefore, a simple method of combining the aqueous solution process with a thermal treatment is proposed to develop Al2O3/ZnO nanocables with