ZnO characteristics

Optoelectronics

4.3. Results and discussion

4.3.1. ZnO characteristics

Figure 4.1a represents the FESEM image of the ZnO nanorods. The hexagonal shape of the ZnO nanorod is quite evident in this figure. Figure 4.1b shows the proof of the growth of ZnO on the graphite layer, which is coated on a paper substrate. The paper substrate and the graphite layer are indicated separately in this figure to show them in a distinguishable manner. The marked portion in figure 4.1c was selected for EDX analysis and elemental mapping of the synthesized nanorods. The peaks in figure 4.1d are confirming the presence of elements Zn and O. The inset shows the weight percentages of these two elements, whereas figure 4.1e,f shows the uniform scattering of Zn and O elements in the area of scanning. Figure 4.2a represents a typical FETEM image of the CD-decorated ZnO nanorods. In figure 4.2b, an enlarged section of figure 4.2a is shown.

Figure 4.1. (a) FESEM image to show the synthesized ZnO nanorods. (b) FESEM image to show the growth of ZnO nanorods on graphite-coated paper substrate. (c) Scanning area for EDX analysis. (d) EDX spectrum of the synthesized ZnO nanorods. (e) Zn and (f) O mapping images.

The image unveils several tiny spots on the surface of the nanorod as pointed with the arrows, which can be ascribed as the attachment of CDs onto the surface of the ZnO nanorod. The HRTEM image of a section of the nanorod is represented in figure 4.2c.

Although the lattice spacing of mono crystallinity is shown in this figure, the polycrystallinity of the synthesized ZnO nanorods can be confirmed by the SAED pattern shown in figure 4.2d. The presence of multiple circular orbits of the bright dots is the evidence of the polycrystallinity of the nanorods. This statement can be confirmed from the XRD pattern of the synthesized nanorods, which is shown in figure 4.3a. In the figure,

Figure 4.2. FETEM image of the CD decorated ZnO nanorods at 100 nm (a), and 50 nm (b) scales, the HRTEM image (c), and the SAED pattern (d) of the ZnO nanorod.

the formation of polycrystalline ZnO nanorods can be verified by comparing the crystal planes (100), (002), (101), (102), (110), (103), (112), and (201) at 2θ values of 31.86, 34.54, 36.36, 47.64, 56.68, 62.98,68.06, and 69.17° respectively. Notably, these values are in good agreement with previously reported XRD data of ZnO nanorods [30]. The presence of the planes (100), (002), and (101) suggests the formation of hexagonal wurtzite ZnO nanorods [31], and these peaks are sharp, narrow, and of quite high intensity, which tells us about the better crystallinity of the sample. However, the lower

intensities of the other diffraction peaks correspond to some defects present in the crystalline structure of the nanorods. The defect in the crystalline structure creates a strain in the nanorods, which ultimately causes a change in the overall crystallite size. The Williamson–Hall (W-H) technique [32] has been implemented to calculate the average crystallite size (𝐿) and strain (𝜀) of the nanorods. The equation is given below,

𝛽𝑐𝑜𝑠𝜃 𝜆 =¹

𝐿+^{𝜀𝑠𝑖𝑛𝜃}

𝜆 (4.1)

where β stands for full width at half-maxima (FWHM), θ is the Bragg angle, and λ corresponds to the wavelength used in XRD (λ = 0.15418 nm). The intercept and the slope of the plot between ^{𝛽𝑐𝑜𝑠𝜃}

𝜆 and ^{𝑠𝑖𝑛𝜃}

𝜆 shown in figure 4.3b, helps to get the average crystallite size (𝐿) and the strain (𝜀), respectively. The obtained values of average crystallite size and the strain are 135.13 nm and 0.0067, respectively. The information about the amount of defect present in the crystal structure can be calculated from the parameter termed as dislocation density (δ). The dislocation density was calculated using the following equation [33].

𝛿 = ¹

𝐿² (4.2)

The calculated value of dislocation density is 5.476 × 10^–5 nm^–2. Considering the high intensities of the peaks present at 31.86, 34.54, and 36.36°, we have calculated the lattice constants of the synthesized ZnO nanorods for the crystalline planes (100), (002), and (101), respectively. The following equations [34] were used to calculate the constants.

𝑐 = ^𝜆

𝑠𝑖𝑛𝜃 (4.3)

Figure 4.3. (a) XRD pattern of the sample, (b) ^{𝛽𝑐𝑜𝑠𝜃}

𝜆 vs.^{𝑠𝑖𝑛𝜃}

𝜆 plot.

𝑎 = √¹

3 𝜆

𝑠𝑖𝑛𝜃 (4.4)

The calculated values are reported in table 4.1. A little deviation in the values of these lattice constants from the standard values can be attributed to the thermal as well as the lattice mismatch between the ZnO nanorods and the substrate on which the rods were grown. Apart from the peaks corresponding to ZnO, the peaks for the paper substrate (cellulose) can also be observed in the XRD plot shown in figure 4.3a. The broad diffraction peaks from 13 to 15 and 21 to 25° suggest the presence of cellulose, which is in good agreement with the previous reports [35, 36]. The peak at 26.5° confirms the existence of graphitic carbon in the substrate material [37].

Table 4.1. Lattice constants for different crystalline planes.

Crystalline plane 2θ c(nm) a(nm)

100 31.86° 5.061 3.239

002 34.54° 5.187 2.994

101 36.36° 4.936 2.849

The information about the defects in the synthesized ZnO was obtained from the PL spectra of the ZnO nanorods. Figure 4.4a shows the PL peaks of the nanorods at two different excitation wavelengths (340 and 380 nm). The peaks in this figure convey that the ZnO nanorods emit at a wide range of wavelengths. The presence of UV emission (370–385 nm) in the PL can be attributed to the recombination of the excitons, which were created near the bandgap region of the ZnO nanorods. The emissions at zones of green (485–525 nm), blue (455–485 nm), and violet (385–455) correspond to the deep level emissions (DLE) [38, 39]. The DLEs occur because of the presence of structural defects in ZnO nanorod crystals. These structural defects can be of several types, such as zinc vacancies, Zn interstitials intrinsic defects, oxygen antisites, oxygen vacancies, etc.

[38] The presence of green emission gets prominent when the excitation wavelength gets increased, as shown in figure 4.4a. This phenomenon can well be explained with the term

Figure 4.4. (a) PL peaks of ZnO at excitation wavelengths of 340 nm and 380 nm. (b) Deconvoluted PL peak of 340 nm excitation wavelength.

“red-edge-effect” (REE), where the emission maxima get red-shifted with the increase in excitation wavelength [40]. This red shift of the peak with the increase in excitation

wavelength can be explained as follows. The ZnO sample gets exposed to higher energy when it is excited at a lower wavelength, compared to the case when it is excited at a higher wavelength. Therefore, when it gives the emission spectra corresponding to its excitation wavelength, we can observe the peak at a lower emission wavelength for a lower excitation wavelength and vice versa. That is why, when the excitation wavelength gets increased, the presence of the green emission also gets prominent, whereas, in the case of a lower excitation wavelength, the other peaks (such as violet, blue, etc.) were more prominent. A similar sort of characteristic was generally recorded for ionic liquid (IL) with a dipolar probe molecule [41, 42]. In our system also, the ZnO nanorods were dispersed in water (polar solvent) to obtain the PL spectra, which emphasized getting a prominent green emission as the excitation light gets increased (from 340 to 380 nm).

The emission peak (excitation: 340 nm) has been deconvoluted in figure 4.4b to show the presence of individual components separately. The emission at ∼385 nm in this figure is corresponding to the exciton-based emission. In this case, the electron-hole recombination can occur at the valance band or at the traps present nearer to the valance band. The peak at ∼405 nm is due to the electronic transition to the single ionized oxygen vacancy (𝑉_𝑂⁺)from the conduction band. The peaks at ∼415, ∼432, and ∼456 nm can be attributed to the presence of defects such as interstitial Zn (𝑍𝑛_𝑖), ionized interstitial Zn (𝑍𝑛_𝑖⁻), and doubly ionized interstitial Zn (𝑍𝑛_𝑖²⁻) defects, respectively. The oxygen antisite (𝑂_𝑍𝑛), and the deep level oxygen vacancy (𝑉_𝑂) can be the reason behind the emission peaks in the green zone [38, 39]. The PL of the ZnO attached with CD is also shown in figure 4.5. In this figure, it is observed that the 𝑉_𝑂, 𝑍𝑛_𝑖, and 𝑍𝑛_𝑖²⁻ defects have remained intact into the composite as there are peaks in the green and violet zones.

Figure 4.5. Deconvolute PL peak of CD enhanced ZnO nanorods at an excitation wavelength of 340 nm.

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