## 3 Results and Discussion  
_CuBi\({}_{2}\)O\({}_{4}\) NP characterization._ The XRD pattern of the powdered CuBi\({}_{2}\)O\({}_{4}\) NPs corresponds to that reported in the crystallographic card (International Center for Diffraction Data) ICDD 01-079-1810, for CuBi\({}_{2}\)O\({}_{4}\) in a tetragonal crystalline structure, with spatial group _P4/ncc_ and crystal lattice parameters a = b = 8.48 A, c = 5.79 A (Figure 2a). All peaks in the pattern correspond to the single crystalline phase [13; 15; 16], and there are no other diffraction peaks attributed to impurities. An average crystallite size of 24 \(\pm\) 2 nm was estimated using [PERSON]'s equation on the (200), (211) and (202) diffraction peaks. Furthermore, the HR-TEM micrographs corroborate the formation of CuBi\({}_{2}\)O\({}_{4}\) nanocrystals. In Figure 2b, a smaller isolated CuBi\({}_{2}\)O\({}_{4}\) NP with 10.3 nm for each side is presented. The measured interplanar spacing of 4.2 A corresponds to the (200) plane, and in the corresponding FFT (fast Fourier transform), the indexed planes belong to the tetragonal crystal structure (ICDD 01-079-1810) of CuBi\({}_{2}\)O\({}_{4}\) (Figure 2c), confirming the nanoparticle composition. The importance of the crystalline structure in these NPs lies in the fact that the charge transport is strongly conditioned by the order of the structure of the material:  
Figure 1: Device structure made of glass/ITO/PEDOT:PSS-CuBi\({}_{2}\)O\({}_{4}\) NP/Ag. when the order is of a higher level, the more favored overlap of the structures, which may define the energy bands and the spacing between them, will exist. On the other hand, six Raman bands centered at 88, 122, 182, 255, 392 and 573 cm\({}^{-1}\), characteristic of CuBi\({}_{2}\)O\({}_{4}\) NPs, are observed (Figure 3a). These bands are associated with the bending vibration of Bi rhombohedra (B\({}_{\text{2g}}\)), the translational vibration of the CuO\({}_{4}\) plane along the Z-axis (A\({}_{\text{1g}}\)), the Cu-Cu vibration (E\({}_{\text{g}}\)), the rotation of two stacked CuO\({}_{4}\) squares in opposite directions (A\({}_{\text{1g}}\)), the Bi-O stretching vibration (A\({}_{\text{1g}}\)) and the in-plane breathing of CuO\({}_{4}\) squares (A\({}_{\text{1g}}\)), respectively. All these peaks correspond to those reported by other authors [41; 42; 43; 44; 45], whereas in the FTIR spectrum of the CuBi\({}_{2}\)O\({}_{4}\) NPs (Figure 3b), two strong signals are seen at 517 and 411 cm\({}^{-1}\), which are, respectively, assigned to the stretching mode of the Bi-O bonds of BiO\({}_{6}\) and the stretching vibration of the Cu-O bond. From diffuse reflectance spectroscopy (DRS), the UV-visible spectrum of the powdered CuBi\({}_{2}\)O\({}_{4}\) NPs (Figure 4a) was obtained. This spectrum shows a wide absorption band between 300 and 790 nm, centered at 527 nm. The optical bandgap energy of the CuBi\({}_{2}\)O\({}_{4}\) NPs was estimated at 1.7 eV from Tauc's plot obtained from their DRS spectrum (Figure 4b). According to [PERSON]'s equation for a direct bandgap: (\(\alpha\)hv)\({}^{2}\) = A\({}^{2}\)(hv - Eg), where \(\alpha\) is the absorption coefficient, hv is the photon energy, Eg is the bandgap energy and A depends on the type of transition, when \(\alpha\)hv = 0, then Eg = hv. The bandgap energy is determined by plotting (\(\alpha\)hv)\({}^{2}\) versus hv and finding the intercept on the hv axis by extrapolating the plot to (\(\alpha\)hv)\({}^{2}\) = 0. Due to the size of the NPs obtained here, it is not expected that the bandgap energy would have shifted to the ultraviolet region. Figure 3: Spectra of (**a**) Raman and (**b**) FTIR of the CuBi\({}_{2}\)O\({}_{4}\) NPs. Figure 2: (**a**) Powder XRD pattern and (**b**) HR-TEM micrograph of CuBi\({}_{2}\)O\({}_{4}\) NPs. In the corresponding FFT pattern (**c**), the indexed planes belong to the tetragonal crystal structure of CuBi\({}_{2}\)O\({}_{4}\).