3.3.1 Linear optical studies
Figure 3 illustrates the optical absorbance (A) and transmittance (T) spectra of bare and different ZCO PNCs in the range of 200–1800 nm. According to A spectra, one can notice that at any incident λ, A increases due to ZCO content increase to 10 wt%. This behavior refers to additional defects and localized energy states created in the host blend [44]. Besides, obvious absorption redshifts of ZCO PNCs to larger wavelength (inset of Fig. 3a) with respect to the bare blend are observed, which nominate shrinkages in the optical band gaps. Moreover, all films possess absorption peaks in the wavelength range 210–220 nm. These peaks attribute to PVA π→π* electronic transitions [7, 13, 21]. Whereas, according to the transmittance spectra (Fig. 3b), T reduces as a result of increasing ZCO content. For example, T reduces from 70% (bare) to 15% (10 wt% of ZCO PNCs). Moreover, clear shifts in the cut-off wavelength to longer wavelengths are observed upon doping, which suggest their uses in optical filters.
The direct/indirect (Egd/Egi) band gap of ZCO nanofillers, bare and different ZCO PNCs have been determined based on Tauc’s method (Eq. 4). α values of PNCs at different are calculated by Eq. 2. (αhν)2 and (αhν)0.5 curves of the bare and different ZCO PNCs vs. hν are plotted (Fig. 4 (a) and (b)) to deduce the Egd and Egi values, respectively. Whereas Fig. 4 (a) inset illustrates Tauc’s plot of ZCO NPs. Eg values meet hν-axis intercepts of elongated linear portions of the plotted curves (Table 1). First, the inset reveals that Eg of ZCO NPs is 2.19 eV. Besides, Egd and Egi of the bare PVA/PVP/graphene blend decrease from 5.20 eV and 4.94 eV to 4.36 eV and 4.29 eV (10 wt% ZCO PNCs). The shrinkage in host’s Eg due to ZCO doping is caused by defects and localized energy states created between its HOMO and LUMO [21, 65]. The same behavior is reported in the literature [13, 66]. Heiba et al. found that PVA/PEG’ Eg reduced from 4.70 eV to 3.98 eV via doping with 5 wt% of Gd2O3 [44]. Also, the Eg value of PVA/PVP blend reduced from 5.15 eV to 3.05 eV by 10 wt% of Co0.9Cu0.1S incorporating as reported in Ref. [20]. The role of the defects in the host’s band gap has been investigated by Eu determination (Eq. 5). Figure 5 shows plots of ln α of bare and different ZCO PNCs as a function of hν. Eu values are also included in Table 1. Noticeably, Eu rises from 0.62 eV to 1.28 eV (10 wt% ZCO PNCs), that indicates defects’ density increase [18, 48] due to ZCO doping. The created defects and localized energy states play a significant role in recombination and trapping sites in Eg host. This novel result of the ability to tailor Eg of PVA/PVP/graphene by ZCO doping as an environmentally friendly material is highly appreciated in various optical applications.
Table 1
Optical band gap, Urbach energy and refractive index of ZCO PNCs
wt.% | Direct Egd (eV) | Indirect Egi (eV) | Eu (eV) | n @ hν = 2.0 eV |
Bare blend | 5.20 | 4.94 | 0.62 | 1.29 |
0.1 | 5.17 | 4.91 | 0.68 | 1.30 31 |
0.5 | 5.15 | 4.88 | 0.73 | 1.33 |
1.0 | 5.13 | 4.86 | 0.94 | 1.41 |
5.0 | 5.09 | 4.69 | 1.19 | 1.60 |
10.0 | 4.36 | 4.29 | 1.28 | 1.82 |
Extinction coefficient (K) and refractive index (n) of bare and ZCO PVA/PVP/graphene blend have been investigated to validate their optical devices utilization. At different incident photons’ energy, K and n (Eq.’s 2 and 6) are illustrated in Fig. 6 (a) and (b). It is noted that the extinction coefficient K tends to remain quasi-steady in the non-absorption region, whereas it arises sharply in the absorption region. Furthermore, K increases via ZCO increasing to 10 wt.%, which attributes to the absorption dispersion [24]. According to Fig. 6 (b), the refractive index increases slowly upon increasing hν in the visible-NIR regions. Although it increases significantly as hν increased in UV region. In instance, n rises from 1.29 (bare) to 1.82 (10.0 wt.% ZCO PNC) at hν = 2.0 eV. This valuable improvement in n of host blend is caused by packing density increase, which affects the reflectance of the prepared PNCs [24, 25, 56]. Moreover, the increase in the intermolecular bonds of the host blend due to ZCO doping leads to clear changes in the polarization within the prepared PNCs [67, 68]. It is worth mentioning that enrichment of refractive index of PVA/PVP/graphene using ZCO NPs nominates it for new optical devices and optoelectronic uses.
Furthermore, the optical conductivity (σopt.) of bare and ZCO PNCs was determined using Eq. 9 and illustrated in Fig. 7. It is seen that σopt. increases gradually upon increasing hν in the non-absorption region, while it increases sharply in the absorption one. Also, σopt. of doped samples is more than that of bare one and increases upon increasing ZCO content at any specific photons’ energy. In instance, σopt. increases from 5.7 x 1010 sec.−1 (bare blend) to 4.4 x 1011 sec.−1 (10 wt% ZCO PNC) at hν = 2 eV. The valuable improvement in σopt. refers to the absorption increment and molecular excitations and hence carriers’ production [56, 69]. The increase in σopt. of PVA/PVP/graphene blend via ZCO doping nominates them for UV filters and solar cells applications.
3.3.2 Dielectric studies
Furthermore, the dielectric parameters (real (εr) and imaginary (εi)) of bare and ZCO PNCs have been investigated to propose their probable applications in related fields. Equations 8 and 9 were used to calculate εr and εi as displayed in Fig. 8. It is obvious that εr follows n performance, while εi performs in a similar manner K. In other words, εr increases slowly upon increasing hν in non-absorption region, whereas it increases profoundly in the absorption region. Whereas, εi stays quasi-constant as increasing hν in non-absorption region, and it rises greatly in the absorption region. Moreover, both εr and εi of the doped blends increase as ZCO concentration is raised to 10 wt.%. For example, at hν = 2 eV, εr of the bare blend (= 1.66) is duplicated due to 10 wt.% of ZCO doping. While εi increases from 2.35 x 10− 4 (bare blend) to 1.8 x 10− 3 (10 wt.% ZCO PNCs). This great enhancement in εr attributes to the energy states’ increment, while εr improvement refers to the dipole motion fluctuations [5, 69]. The same behavior is reported by Aslam et al. [18]. They proved that PVA dielectric parameters might be improved via CuO loading. It is noted that εr enhancement of PVA/PVP/graphene blends via ZCO doping recommends their uses in the energy storage devices and supercapacitors.
3.3.3 Nonlinear optical studies
Furthermore, investigating the nonlinear optical constants of the bare and ZCO PNCs is an important issue for photonic applications. The nonlinear optical performance occurs when such a material is exposed to a high intense UV illumination as well as pulsed lasers, where the polarization performs nonlinearly [70]. In this work, the first-order susceptibility χ(1), third-order susceptibility χ(3) and nonlinear refractive index n2 were calculated at hν by Eq.’s 10, 11 and 12 and revealed in Fig. 9 (a), (b) and (c). It is noticed that χ(1) behaves in similar way to n. Furthermore, χ(1) of the PVA/PVP/graphene blend increases upon increasing ZCO concentration. Besides, it increases sharply as hν increased in UV region. While χ(3) and n2 increase slowly in the non-absorption region. Although, both of them increase sharply in UV region. For instance, at hν = 5.0 eV, χ(1) and χ(3) rise from 0.19 and 1.30 x 10− 13 esu (bare blend) to 0.86 and 9.23 x 10− 11 esu (10 wt.% ZCO PNC). Our findings agree with the published data [48, 49, 71]. The valuable enrichment in PVA/PVP/graphene nonlinear optical parameters via incorporating with ZCO doping proposes it for optical signal devices and communication systems [70].