Chapter 3: Controlled Growth of ZnO Nanowires and Nanorods
3.2. Structural Characterization
3.2.1. X–Ray Diffraction Studies
XRD is an ideal probe to characterize structure and orientation of the NWs/NR with respect to the substrate. The XRD measurement is performed with Cu K radiation at 3°
grazing angle and with higher integration time. The structural characterization of the ZnO NWs and NRs done by XRD shows characteristic peaks of pure hexagonal wurtzite phase of ZnO. Figure 3.9 shows the XRD patterns of the Au catalytically and combines Au and seed assisted grown ZnO NWs. XRD pattern of the ZnO nanoribbons is shown at Fig. 3.9 (d) as inset. All the patterns show one intense peak corresponding to the (002) plane and a weak peak corresponding to the (103) plane of hexagonal structure. Strong (002) peak and smaller full width at half maximum (FWHM) value indicates the c–axis orientation of the single crystalline ZnO NWs and the growth direction is perpendicular to the base surface. From Fig. 3.9 (a–d), comparatively higher values of peak intensity ratio of (002) peak to (103) peak is observed from Fig. 3.9 (b and c). This indicates well–aligned vertical growth of the NWs, which were grown by using combined seed layer and Au catalyst. The XRD results are in agreement with the SEM/FESEM results.
The lattice parameters for hexagonal ZnO NWs are estimated from the (002) plane using the equation:
…………. (3.1) Where, a and c are the lattice parameters and h, k, and l are the Miller indices and dhkl is the inter–planar spacing for the plane (hkl). This interplanar spacing can be calculated from the Bragg angle θ using the relation,
2 sin .………… (3.2)
Where λ is the wavelength of x–ray (1.5406 Å), θ is diffraction angle, and n is the order of diffraction (n=1). The calculated lattice parameters are a= 3.250Å, c= 5.208Å and a=
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(b)
(103) (002)
Intensity (arb. unit)
2(Degree) (103)
(002)
Intensity (arb. unit)
2(Degree)
(a)
(d) (c)
(103)
(002)
Intensity (arb. unit)
2(Degree)
(103) (002)
Intensity (arb. unit)
2(Degree)
2(Degree)
(112)(103)
(002)
Figure 3.9: XRD patterns of the ZnO nanowires grown by: (a) Au catalyst assisted, (b and c) combined seed layer and Au catalyst assisted at substrate temperature of 750° and 850°c, respectively. Above mentioned nanowires are grown by using ZnO bulk powder as the source material. (d) XRD pattern of the Au catalyst assisted grown ZnO nanowires by using ZnO nano powder as the source. Inset shows the XRD pattern of the ZnO nanoribbons.
3.249Å, c= 5.207Å for the Au catalyst assisted grown NWs by using bulk and nano powder, respectively. On the other hand, the lattice parameters for the combined seed and Au assisted grown NWs are a= 3.250Å, c= 5.206Å and a= 3.251Å, c= 5.208Å for the NWs grown at 750° and 800°C, respectively. Therefore, as compared to the lattice parameters of bulk ZnO (a=3.249, c=5.206, JCPDS Data, PDF#790206), the obtained lattice constants of the as–grown NWs are found to be larger. This indicates that the as–
grown ZnO NWs are weakly strained and the nature of strain is tensile. The calculated strain varies in the range 0.02–0.04% of the above samples.
The XRD result of the chemically grown ZnO NWs is shown in Fig. 3.10, which shows a strong peak at 34.45° corresponding to (002) planes of hexagonal ZnO. The observed strong peak indicates highly crystalline nature of the as–grown NWs with growth orientation along the c–axis of hexagonal structure of ZnO.
XRD pattern of the ZnO seed layer is shown in Fig. 3.11(a), which reveals (002) oriented growth of the ZnO film. Figure 3.11(b-d) shows the XRD patterns of the ZnO
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(002)
Intensity (arb. unit)
2(Degree)
Figure 3.10: XRD pattern of the chemically grown vertically aligned ZnO nanowires.
NRs grown at 900°, 850° and 700C, respectively. In this case, we observed a strong (002) peak of hexagonal ZnO, indicating the c–axis orientation. These patterns also indicate a well–aligned orientation and the growth direction is perpendicular to the basal plane. Identical growth orientations of the seed layer and the as–grown NRs suggest that
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(b)
(002)
2 (Deg.)
Intensity (arb. unit) Intensity (arb. unit) (101)
(002)
2 (Deg.)
Intensity (arb. unit) (d)
(c)
2 (Deg.)
Intensity (arb. unit) (002) 103)
(002)
2 (Deg.)
ZnO Seed Layer (a)
Figure 3.11: XRD patterns of ZnO seed layer and nanorods arrays: (a) sputter deposited ZnO seed layer on Si substrate; (b–d) vertically aligned ZnO nanorods grown at substrate temperature 900°, 850° and 700°C, respectively
Au layer transfer the same orientation of the ZnO seed layer to the ZnO NRs leading to the well–aligned growth. Therefore, structural quality and orientation of the ZnO seed layer are the key factors for the well–aligned growth of the NWs/NRs. Relative intensities of the XRD peaks show that NRs grown at higher temperature have higher value of peak intensity, which implies higher crystallinity. Because, highly crystalline sample contains larger number of perfect Bragg planes than the weakly crystalline sample, even if both the samples have same crystal volume. The as–grown NRs at 900°C show very small FWHM (0.0305°) and highest intensity peak in XRD, which is ~1.6 times higher than that of the NRs grown at 850°C. And the NRs grown at 700°C (Fig.
3.11(d)) show large FWHM (0.0987°) and weak (002) peak intensity, which is ~46 times smaller than the NRs grown at 900°C. As the nanorods contains (101) planes at the connecting face between top (002) and side (100) surface of the nanorods, it may possible that comparatively larger number of (101) planes take part in the scattering event than the (002) plane. This results in a weak (002) peak.
The structural characterization of the Al:ZnO NWs also performed and presented in Fig.
3.12. The XRD patterns of the 3% and 6% doped Al:ZnO NWs show characteristics peaks of crystalline hexagonal phase of ZnO with strong (002) planes related peak. No other peaks related to the Al or aluminium oxide are detected. Exact position of the (002) peak is calculated from the Lorentzian fitting to the experimental data. It is found that the peak position is slightly shifted towards higher 2θ. This up shift indicates that Al:ZnO NWs are weakly strained and the nature of strain is compressive, It is also noticed that the compressive strain increases with increase in Al concentration. In case of Al doping in ZnO, it is known that the Al atoms occupy the Zn lattice site due to
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(103)
Intensity (arb. units)
2 (Deg.)
Al:ZnO NWs(3%) Al:ZnO NWs(6%) (002)
Figure 3.12: XRD patterns of the Al doped ZnO NWs, where Mo (λ=0.7093Å) x-ray gun was used.
almost identical covalent radius of Al atom (121 pm)219 and the Zn atom (122 pm).
Formation of interstitial Al atoms is also possible, however the chances are lower compared to the possibility of substitutional doping. Therefore, incorporation of Al into ZnO leads to slight decrease of lattice constants in ZnO, resulting in the compressive strain in the Al:ZnO NWs. It is known that, strain has a strong affect on the optical properties, as it directly influences the band gap of the material. Compressive strain usually leads to increase in band gap and the tensile strain leads to decrease in band gap.
220,221 Therefore, a change in optical properties with band gap widening is expected from the Al:ZnO NWs and this is discussed in the next chapter.