Slanted Co/Cu multilayer nanocolumns were grown on Si and gold-coated Si substrates by two-source oblique-angle vapour deposition. The respective column lengths and layer thicknesses were varied from ∼400 to ∼700 nm and ∼6 to ∼16 nm. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images show the column diameters to be less than 65 nm. The ferromagnetic nature of the nanocolumns was verified by vibrating sample magnetometry. Compositional analysis using electron energy loss spectroscopic elemental mapping and energy dispersive x-ray spectrometry of a single nanocolumn showed alternating bands of Co and Cu, indicating a multilayer structure. X-ray diffraction (XRD) results showed that the Co and Cu layers are predominantly face-centred cubic (fcc) and a minor hexagonal close packed (hcp) Co phase was also detected. The hcp phase may be due to a high density of stacking faults in the fcc Co. TEM studies have shown an epitaxial relationship between the Co and Cu layers within the individual columns. By comparing the XRD and TEM results, we conclude that the (020) planes are parallel to the column axis, and the column growth is nearly parallel to the [101] direction.

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In this work, pure ZnO, pure polyaniline (PANI), and a set of PANI-capped ZnO samples of various PANI concentrations are synthesized using the coprecipitation, oxidative polymerization, and ex situ methods, respectively. The X-ray diffraction results exhibit suppression in directional growth and shifting in ZnO-related peak positions in the capped samples with increasing PANI content. Interactions between ZnO and PANI chains are confirmed by Fourier transform infrared (FTIR) spectroscopy. The morphological characterizations (scanning electron microscopy (SEM) and transmission electron microscopy (TEM)) reveal the presence of an agglomerated spherelike structure of PANI on top of ZnO nanosheets. The X-ray photoelectron spectra (XPS) reveal a reduction in the density of surface as well as deep oxygen vacancy defects of ZnO after capping with PANI. The absorption spectra reveal an enhancement in the bipolaron band absorption for capped samples, which reinforces the presence of PANI chains. The photoluminescence spectra show a quenching in the emission intensity of ZnO after the addition of PANI; the overall quenching is discussed through concentration-dependent Förster resonance energy transfer (FRET) theory between ZnO (donor) and PANI (acceptor) between ZnO (donor) and PANI (acceptor). The sample capped with the least PANI content shows maximum quenching in the emission profile due to the encounter among the intradonor and the inter-donor–acceptor energy transfer mechanism. For rest of the capped samples, emissions are explained using the concentration-dependent inter-donor–acceptor FRET theory. Finally, the catalytic study of all capped samples and pure ZnO is performed through the degradation of methylene blue (MB) dye by irradiating UV light. Despite having quenched photoluminescence intensity, the results show a reduction in degradation efficiency for PANI-capped ZnO compared to pure ZnO contrary to the common trend. This is associated with the change in the density of oxygen vacancy sites of ZnO. Of the two oxygen vacancy sites, the surface oxygen vacancy sites play a major role as carrier trap centers and the deep oxygen vacancy sites delay the recombination process. The mechanism behind the effect of change in the density of these two vacancy types on photocatalysis is explained. Thus, by concentration-dependent FRET between ZnO and PANI and the oxygen vacancy defect density, the emission intensity and the photocatalytic activity of PANI-capped ZnO can be tuned.

The role of quantum mechanical phenomena such as polaron-exciton quenching interaction and concentration-dependent FRET in determining the luminescence efficiency of PAni-PMMA polymer blends has been investigated. PAni samples prepared in different environments using different acids and bases show different absorbance and emission profiles indicating a direct relation between generated polarons in PAni by acid-base doping-dedoping and photoluminescence spectra of PAni. The observed low luminescence in PAni has been modeled using exciton quenching by polarons through charge transfer. The investigation also reveals that the effect of exciton quenching by polarons becomes pronounced when the polaron concentration in PAni reaches a density of ∼1017-1018 polarons cm-3. To overcome the low emission efficiency of PAni, poly(methyl methacrylate) (PMMA) has been blended with PAni. The blending of donor PMMA (D) with acceptor PAni (A) gives rise to another quantum phenomenon - donor PMMA concentration dependent FRET between PAni (A) and PMMA (D). It is experimentally observed from the photoluminescence measurements of blends that at high donor PMMA concentration above a critical value in the PAni-PMMA polymer blend the emission profile of blends drops sharply. Donor concentration dependent FRET is a contradictory observation with respect to standard concentration independent FRET theory due to competition between inter-layer donor-acceptor and donor-donor intra-layer energy transfer within the donor layer. At high donor concentration intra-donor interaction gradually overtakes inter-layer donor-acceptor FRET which modifies the lifetime of the donor. The modification decreases the quantum yield of the donor and hence emission efficiency of blends above a critical concentration of PMMA by reducing inter donor-acceptor FRET. Thus, polaron exciton quenching and concentration dependent FRET are two dominant physical phenomena controlling luminescence in PAni-PMMA polymer blends. Therefore, optimization of luminescence of PAni-PMMA should be achieved by tuning the factors like reduction of spectral overlap between polarons and excitons in PAni, the density of PAni, diffusion of excitons in blends, and intra donor FRET within the PMMA layer before consideration of the blend being used as an emissive layer in PLEDs.

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