Chapter 3: Controlled Growth of ZnO Nanowires and Nanorods
3.2. Structural Characterization
3.2.3. Raman Scattering Studies
The ZnO NWs, nanoribbons and NRs are further studied by micro–Raman scattering measurements, which are shown in Fig. 3.15. The Raman spectra of the ZnO NWs, nanoribbons and NRs show characteristic phonon modes of the good quality crystalline hexagonal wurtzite phase. The summary of the observed peak positions and corresponding intensities are presented in Table–3.1. The observed strong Raman mode at ~436.5 cm-1 corresponds to phonon mode of wurtzite ZnO.203 The occurrence of this peak is due to the vibration of oxygen atoms in ZnO lattice. Along with the strong mode five other Raman modes of wurtzite ZnO phase are observed. These modes are located at ~330, ~379, ~408, ~540 and ~579 cm-1 corresponding to the 2 , TO, TO,
300 400 500 600 700
379.0 580.0
436.5
330.7 378.8
436.5
579.8
Raman Shift (cm-1)
(c) (b) (a)
407.0 330.0
330.4
Raman Intensity (arb. units)
437.0 378.2
408.0 540.8
Figure 3.15: (a) The micro–Raman spectra of ZnO nanostructures: (a) nanowires, (b) nanoribbons and (c) nanorods grown by using ZnO nano powder.
2LA and LO phonon modes of wurtzite ZnO, respectively. Compared to the Raman modes of strain free wurtzite ZnO ( mode at 438.0 cm-1), the observed Raman peaks are found to be red shifted. This peak shift is due to the presence of a tensile strain in the as–grown ZnO nanostructures.222 Besides the local heating effect on the Raman shift, a major contribution to the red shift comes from the tensile strain present in the as–
grown ZnO nanostructures. In the present case, the quantum confinement effect is unlikely to affect the Raman modes due to the large size of the nanostructures as compared to exciton–Bohr radius of ZnO. Therefore, the Raman results of the as–grown ZnO nanostructures are in agreement with the XRD analysis.
Table 3.1: Summary of the observed peak positions and corresponding intensities (inside bracket) of the Raman modes of ZnO NWs, nanoribbons and NRs.
Sample
Position of the Raman peaks in cm-1 (Intensity in arb. units) Peak I Peak II Peak
III Peak IV Peak V
Peak VI Nanowires 330.0
(186)
378.2 (169)
408.0 (223)
436.5 (764)
540.8
(206) –
Nanoribbons 330.7
(144) 378.8
(161) – 436.5
(750) – 579.8
(284) Nanorods 330.4
(158) 379.0
(76) 407.0
(106) 437.0
(692) – 580.0
(116) Peak
Identity 2LA
Similar to the earlier mentioned Raman modes for the NWs, the Raman spectra of the combined seeded layer and Au catalyst assisted grown NWs show highly crystalline nature with very strong mode, which is shown in Fig. 3.16. Other Raman modes associated with the hexagonal phase are also observed with relatively weak intensities. It is found that the relative intensity of the mode in the NWs grown at 800°C is much stronger than the case of NWs grown at 750°C, which indicates higher crystallinity. Very low value of FWHM (~7.3 cm-1) of mode further indicates highly aligned structure due to small area of scattering cross–section.
The structural quality of the Al:ZnO NWs is similarly checked by micro–Raman analysis. Figure 3.17 shows the Raman spectra of the 3% and 6% Al:ZnO NWs. The Raman spectra show characteristic Raman modes of hexagonal ZnO, which are summarized in Table–3.2. The Raman peaks at ~332, ~381 and ~438 cm-1 correspond to
300 400 500 600 700
Intensity (arb. units)
Raman Shift (cm-1)
Ts=800oC Ts=750oC
2E2 ATO1 ETO
1
Ehigh2
2LA ALO
1
Figure 3.16: The micro–Raman spectra of the the combined seeded layer and Au catalyst assisted grown vertically aligned ZnO nanowires arrays, at substrate temperature (Ts) (a) 750°
and 800°C, respectively.
2 , TO and modes of hexagonal ZnO, respectively.203 The strong mode indicates the highly crystalline structure of the as–grown Al:ZnO NWs. It is also observed that with increase in Al doping concentration, the intensity of the mode decreases. The observed intensity reduction indicates the deterioration of crystal quality.
Along with the hexagonal modes, two additional modes are observed from both the samples, one at 620 cm−1and other at 680 cm−1. The intensity of the above two peaks are increases with the increase in Al concentration. In general, it has been observed that doping caused structural disorder in the host lattice. Therefore, above two peaks can be
300 400 500 600 700 800
2 4 6 8 10 12
ATO
1
Intensity (arb. units)
Raman Shift (cm-1)
Al:ZnO NW s(6%) Al:ZnO NW s(3%)
2E2
Ehigh
2
structural disorder
Figure 3.17: The micro–Raman spectra of the 3% and 6% Al doped ZnO nanowires, respectively.
attributed to the structural disorder induced modes, as both the peaks are not associated with the Raman modes of Al. Here, as the doping concentration is increased, the substitution of dopants into the Zn lattice can cause strain and the lattice oxygen can be shared by both Zn and Al. This will lead to neighboring disorder and local geometric disorientation, as a result increase in intensity of the disorder related peak. The strain–
free ZnO bulk crystal shows mode at 438 cm−1. In the Fig. 3.17, frequency of the mode for 3% and 6% Al:ZnO NWs are at 438.1 cm−1 and 438.7 cm−1, respectively.
From previous studies, it has been seen that mode is very sensitive to strain/stress state of ZnO crystal.223,224 Therefore, high frequency shift of mode is the result of increment of compressive biaxial strain. Earlier, similar compressive strain is detected from the Al doped ZnO thin films.225
Table 3.2: Summary of the observed peak positions and corresponding intensities (inside bracket) of the Raman modes of Al:ZnO NWs.
Sample
Position of the Raman peaks in cm-1 (Intensity in arb. units)
Peak I Peak II Peak III Peak IV Peak V 3% Al:ZnO
Nanowires
332.0 (231)
382.0 (200)
438.1 (772)
620.0 (550)
680.0 (450) 6% Al:ZnO
Nanowires
332.9 (229)
381.8 (201)
438.7 (551)
620.5 (665)
680.5 (640) Peak
Identity Structural Disorder