3.1 AFM and Phase mode of F8BT/ZnONR/AgNP
The in-depth investigation in Fig. 1 utilized Atomic Force Microscopy (AFM) to analyze the F8BT polymer and its nanocomposites with silver nanoparticles (Ag) and zinc oxide nanorods (ZnONR). This analysis offers valuable insight into the morphological and compositional changes resulting from the inclusion of these nanomaterials. The AFM topography image of the pristine F8BT polymer in Fig. 1 (a) displays a surface with uniformly distributed features of varying heights between 4nm to 11 nm and a surface roughness Ra = 4.8 nm. The accompanying phase image in Fig. 1 (b) reveals uniform phase contrast, indicating homogeneity in the polymer's properties. Line profiles extracted from these images in Fig. 1 (c) show minor variations in surface topography and consistent phase shift, suggesting uniform compositional properties. The phase shift along the A-B line ranges from 141.5° to 148° for pristine F8BT polymer. When Ag nanoparticles are introduced, the AFM topography image in Fig. 1 (d) shows significant changes in surface morphology, with distinct features and increased surface roughness Ra = 7.12 nm. The phase image in Fig. 1 (e) displays varying phase contrast, reflecting differences in composition between the Ag nanoparticles and the surrounding polymer matrix. Line profiles in Fig. 1 (f) provide a detailed analysis of the Ag nanoparticle's influence, showing significant peaks and sharp changes in phase shift. The presence of an Ag nanoparticle and F8BT polymer is highlighted by the phase peak. The phase shift along the A-B profile ranges from 141° to 145°, passing through a region with a dark spot corresponding to smaller Ag nanoparticles, as indicated by the reduced phase shift. In the case of the F8BT/ZnONR nanocomposite, the AFM topography image in Fig. 1 (h) shows even more pronounced surface features and higher surface roughness Ra = 10.52 nm compared to the F8BT/Ag composite, with the ZnO nanorods appearing as prominent structures and creating distinct topographical features. The phase image in Fig. 1 (i) also exhibits varied phase contrast around the ZnONR, indicating a substantial difference in composition between the ZnO nanorods and the polymer. Line profiles in Fig. 1 (j) further highlight the influence of ZnONRs on the polymer surface, showing substantial elevations and corresponding shifts in the phase profile from 114 to 122°, indicating the significant impact of ZnONRs on the polymer's local composition. The AFM analysis demonstrates how the incorporation of Ag nanoparticles and ZnO nanorods into the F8BT polymer matrix alters the surface shape and composition. The detailed images and line profiles provide a thorough understanding of the structural changes caused by these nanocomposites.
3.2 Kelvin Probe Force Microscopy (KPFM)
Figure 2 illustrates the use of Atomic Force Microscopy (AFM) and Kelvin Probe Force Microscopy (KPFM) to examine pristine F8BT on a silicon substrate. In Fig. 2(a), a 2D AFM topography image shows the surface features of the F8BT at a scan size of 2 µm × 2 µm. Figure 2(b) presents a 3D projection of the topography, displaying granular features with varying heights. The line profile in Fig. 2(c) reveals a substantial height of 8 nm. Figure 2(d) presents the surface potential, indicating regions of higher potential in red and lower potential in blue. The graph demonstrates fluctuations in voltage between point A and point B. Figure 2(e) offers a 3D projection of the surface potential, showing changes throughout the space. Lastly, Fig. 2(f) shows the line profile of the contact potential difference (CPD) of the pristine F8BT, which is measured at 44 mV.
Figure 3 illustrates the characteristics of the KPFM of the F8BT/ZnONR nanocomposites on a silicon substrate. The topography image from an AFM in Fig. 3(a) shows the surface appearance of the nanocomposite, with ZnO nanorods embedded in the F8BT matrix, with the polymer material indicated by the darker areas. Figure 3(b) showcases a three-dimensional view of the surface topography, highlighting variations in height. ZnO nanorods are evident in the 3D surface plot, reaching a maximum height of about 42.73 nm, while the polymer areas have a lower profile. Figure 3(c) presents a line profile extracted from image 3(a), displaying differences in height across the nanocomposite surface. ZnO nanorods have a height of approximately 13 nm and a lateral length of around 155 nm, while the surrounding polymer matrix shows a lower height of about 3 nm. These findings suggest that the ZnO nanorods are effectively embedded within the polymer matrix, impacting the nanocomposite's optical characteristics. In Fig. 3(d), a potential map of the F8BT/ZnONR nanocomposite is shown, acquired through Kelvin probe force microscopy (KPFM), revealing variations in surface potential. ZnO nanorods have different contact potential differences (CPD) compared to the F8BT polymer. The color scale on the map indicates the uneven distribution of surface potential, ranging from red (higher potential) to blue (lower potential). Figure 3(e) provides a three-dimensional view of the surface potential map, offering a clearer visualization of potential differences. The regions with ZnO nanorods exhibit slightly different potential compared to the surrounding F8BT regions. Lastly, in Fig. 3(f), the line profile of CPD along the A-B line from image 3(d) provides quantitative information about surface potential variations, with a CPD difference between ZnO nanorods and F8BT of approximately 14 mV and 38 mV, indicating a distinction of the two materials.
The KPFM for the Highly Ordered Pyrolytic Graphite (HOPG) reference sample, grade ZYA, is shown in Fig. 4. The sample measures 10 mm by 10 mm with a thickness of 1 mm and has a mosaic spread of 0.4 ± 0.1°. According to the Fig. 4(b), the average Contact Potential Difference (CPD) for this sample is around 28 mV. This sample was supplied by MikroMasch.
Kelvin Probe Force Microscopy (KPFM) is a technique used to precisely measure the work function of materials. The surface potential is a key factor in KPFM and is responsible for the difference in work function values between the sample surface and the AFM probe tip. This difference is calculated using Eq. (1) that includes the electron charge and the Contact Potential Difference[22, 23].
$$\:{{\upvarphi\:}}_{\text{s}\text{a}\text{m}\text{p}\text{l}\text{e}}={{\upvarphi\:}}_{\text{t}\text{i}\text{p}}-\text{e}{\text{V}}_{\text{C}\text{P}\text{D}}\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:1\:$$
To ensure accurate measurements of the work function of pristine F8PT and F8BT/ZnONR nanocomposites, it is important to determine the work function of the cantilever in the equipment. Calibration is necessary to obtain reliable measurements and commonly involves using a reference material with a known work function for standardization. In this study, Highly Oriented Pyrolytic Graphite (HOPG) with a work function range of 4.5 to 5 eV is used as the reference material for calibration and measurement accuracy[24, 25].
Using Eq. (1), we can derive similar equations for the F8BT and HOPG substrates. The work function of the F8BT substrate is represented as\(\:{\:\varphi\:}_{F8BT}\), and its specific Contact Potential Difference is denoted as V(CPD, F8PT). Similarly, the work function of the HOPG substrate is \(\:{\varphi\:}_{HOPG}\:\)and its distinct Contact Potential Difference is V(CPD,HOPG). The difference between the work functions of these two substrates can be determined using the following equations.
$$\:{{\upvarphi\:}}_{\text{F}8\text{B}\text{T}}={{\upvarphi\:}}_{\text{H}\text{O}\text{P}\text{G}}+\text{e}\left({\text{V}}_{\left(\text{C}\text{P}\text{D},\:\text{H}\text{O}\text{P}\text{G}\right)}-{\text{V}}_{\left(\text{C}\text{P}\text{D},\:\text{F}8\text{B}\text{T}\right)\:}\:\right)\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:2$$
Upon analyzing the CPD values displayed in Fig. 2(f) and Fig. 4(b), which indicate a measurement of 44mV and 28mV for V(CPD,F8BT) and V(CPD,HOPG) respectively, and considering the work function of HOPG at 4.5 eV, it was determined using Eq. 2 that the work function of the F8BT polymer is 4.484 eV. The work function represents the energy difference between the Fermi level and the vacuum level. By utilizing the HOMO and LUMO levels of F8BT at -5.9 eV and − 3.3 eV[26], relative to the vacuum level, the work function can be computed as the absolute value of the Fermi level, giving an equivalent of 4.6 eV. This calculated value closely matches the experimental value that was obtained.
The work function of ZnO nanorods in F8BT/ZnONR nanocomposites was calculated using the contact potential difference (CPD) of ZnONR, which was found to be 38mV for lengths of approximately 13 nm and 155 nm, resulting in a work function of 4.49 eV. The CPD of F8BT was determined to be 14 mV with a work function of 4.514 eV. It is worth noting that the CPD of F8BT decreases in hybrid systems due to various factors such as charge transfer, work function alignment, and morphological effects. In ZnO NRs/F8BT nanocomposites, charge transfer at the material interface reduces the number of free charges at the surface, leading to a lower CPD. Additionally; differences in work function between the materials can cause charges to redistribute, resulting in a reduced CPD. The introduction of F8BT can also impact the CPD by altering the surface roughness and nanostructure of ZnO NRs in the hybrid structure[27, 28].
3.3 Optical properties of F8BT and F8BT/ZnONR/AgNP nanocomposites
An analysis of the absorbance and energy gap of F8BT, as well as its combinations with ZnO nanorods and silver nanoparticles, indicates how these nanomaterials affect the optical properties of the polymer. In the absorbance spectra Fig. 5 (a), F8BT shows two absorption peaks at 320 nm and 464 nm, representing π-π* transitions and electronic transitions within the polymer backbone[2, 3]. When ZnO nanorods are introduced, absorbance at both wavelengths increases, potentially improving light absorption through enhanced scattering and light-harvesting efficiency[29]. Furthermore, the semiconducting nature of ZnO nanorods can promote charge transfer interactions with the polymer, further impacting light absorption. The inclusion of silver nanoparticles also increases absorbance, in the 320 nm and 446 nm regions, due to the plasmonic properties of AgNP. The combined effects of ZnO nanorods and AgNP result in the highest overall absorbance, indicating synergistic interactions. Additionally, the charge transfer interactions facilitated by the semiconducting nature of ZnO nanorods can enhance charge separation and transport within the polymer, further improving the absorption performance. Moreover, the plasmonic properties of silver nanoparticles allow for effective trapping and localization of light, leading to increased photon absorption and potential enhancement of the charge generation process[30, 31]. Overall, the synergistic effects of ZnO nanorods and AgNP on absorbance suggest their potential as crucial components in the development of high-performance photovoltaic devices.
The analysis of energy gaps Fig. 5(b) reveals the direct optical bandgap of the materials through the Tauc plot. Pure F8BT typically has a bandgap of 2.50 eV, a common characteristic for conjugated polymers used in optoelectronic applications. When ZnO nanorods are introduced to the polymer matrix, the bandgap increases to 2.63 eV. This shift is likely due to the interaction between F8BT and the ZnO nanorods, causing a slight adjustment in the electronic structure of the polymer, resulting in an increased energy required for electronic transitions. ZnO, as a wide-bandgap semiconductor, may also induce changes in the energy levels of the composite material through hybridization effects[32]. The addition of AgNPs to the F8BT matrix results in a further increase in the bandgap to 2.66 eV. The presence of AgNPs may be affecting the electronic environment of the polymer due to strong plasmonic interactions at the nanoparticle surface, potentially altering the density of states in the composite material. The higher bandgap seen when adding silver nanoparticles suggests the modification of electronic properties within the polymer, possibly due to the creation of new electronic states or the influence of nanoparticle-induced polarization effects[33, 34]. Surprisingly, when both AgNPs and ZnO nanorods are combined with F8BT, the bandgap remains at 2.63 eV, similar to the F8BT + ZnONR composite. This indicates that the simultaneous presence of both nanomaterials leads to a balancing effect on the bandgap, with the electronic effects of ZnO nanorods and AgNPs neutralizing each other to stabilize the bandgap at this value. This suggests that the composite reaches a state of electronic equilibrium, wherein the influences of the two nanomaterials on the polymer’s electronic structure are complementary. The addition of ZnO nanorods and silver nanoparticles to the F8BT polymer not only improves the absorption of light, but also changes the electronic structure of the material, resulting in adjustable optical properties[8, 35, 36]. These nanocomposites, with their increased absorbance and shifts in bandgap due to the plasmonic effects of AgNPs, show potential for use in advanced optoelectronic applications like solar cells or light-emitting devices, where controlling optical absorption and electronic transitions is crucial. The photoluminescence spectra depicted in Fig. 6 demonstrate the emission characteristics of the F8BT polymer in its pristine state and when incorporated with ZnO nanorods (ZnONR), silver nanoparticles (AgNP), and a combination of both ZnONR and AgNP. The peak emission for the pure F8BT polymer is observed at 558 nm; however, its intensity is lower compared to the other samples, suggesting a reduced emission efficiency in its pure form. Incorporating ZnO nanorods into F8BT (F8BT/ZnONR) substantially enhances PL intensity, with the emission peak shifting to 547 nm, indicating a blue shift. This significant increase in PL intensity suggests that ZnO nanorods promote improved exciton recombination, possibly due to the coupling of excitons in F8BT with surface states on the ZnO nanorods. The interaction between F8BT and ZnO nanorods likely leads to enhanced charge separation and reduced non-radiative losses, thereby increasing the radiative recombination rate. The blue shift might be attributed to changes in the local environment of F8BT molecules, as ZnO nanorods can modify the polymer’s electronic structure by inducing strain or altering molecular packing[37–40]. Adding silver nanoparticles (AgNP) to F8BT (F8BT/AgNP) results in a higher PL intensity, with the emission peak remaining at 547 nm, similar to F8BT/ZnONR. This enhancement is likely due to localized surface plasmon resonance (LSPR) effects from the AgNPs, which amplify the local electromagnetic field around the F8BT molecules, leading to increased radiative decay rates. However, the PL intensity is lower than F8BT/ZnONR, suggesting that while LSPR can boost emission, it may not be as effective as the exciton-surface state interaction seen with ZnO nanorods. Additionally, the nanoparticles may introduce non-radiative decay pathways or cause partial quenching of the excitons[41–43].The nanocomposite F8BT/ZnONR/AgNP, which includes ZnO nanorods and Ag nanoparticles, shows a moderate PL intensity. The peak moves slightly to 550 nm, indicating a small shift toward red compared to the separate ZnONR or AgNP composites. The increase in emissions in systems containing F8BT, ZnO nanoparticles, and silver nanoparticles can be attributed to several factors related to their interactions and properties. One significant factor is energy transfer mechanisms like Förster Resonance Energy Transfer (FRET), where non-radiative energy transfer occurs between a donor (F8BT) and an acceptor (ZnO or Ag NPs) when they are nearby and their energy levels align. This results in enhanced emission from the acceptor due to efficient energy transfer[44, 45]. Additionally, improved charge separation and reduced recombination in hybrid systems play a role, where excited electrons in F8BT can be efficiently transferred to the conduction band minimum of ZnO or the Fermi level of Ag NPs. This efficient charge transfer minimizes non-radiative recombination and increases the radiative recombination rate, resulting in higher emission intensity. Furthermore, the presence of ZnO and Ag NPs can help passivate surface states and defects in F8BT, reducing non-radiative decay paths and enhancing photon emission[46, 47].