Influence of polaron doping and concentration dependent FRET on luminescence of PAni-PMMA blends for application in PLEDs

Effect of surface ligands on the photoinduced electron transfer rate and efficiency in ZnO quantum dots and graphene oxide assemblies

Apart from biocompatibility, ZnO quantum dots (QDs) are considered to be an efficient luminescence material due to their low cost and high redox potential. Here, we report the synthesis of ZnO QDs by using five different functionalizing ligands like mercaptoacetic acid (MAA), 3-mercaptopropionic acid (MPA), octadecene (ODE), ethylene glycol (EG), and oleyl amine (OLA) and fabricate their assemblies with graphene oxide (GO). We investigate the role of functionalizing ligands as a surface modifier of ZnO QDs for their attachment to GO. The steady-state photoluminescence (SSPL) and time-resolved photoluminescence (TRPL) analyses demonstrate the photoluminescence (PL) quenching of ZnO QDs in ZnO QDs-GO assembly. The highest reduction in PL intensity is observed with ZnO QDs-GO assembly with EG as a surface functionalizing ligand. Cyclic voltammetry (CV) analysis confirms the feasibility of charge transfer from ZnO QDs to the GO. The maximum (79.43%) charge transfer efficiency (ECT) is observed in the case of ZnO-MAA-GO as compared to other assemblies. This means the thiol group-containing ligands facilitate charge transfer as compared to hydroxyl and amine group ligands. This leads to the conclusion that charge transfer in ZnO QDs-GO assemblies depends strongly on the nature of surface ligands.

Structure-correlated excitation wavelength-dependent optical properties of ZnO nanostructures for multifunctional applications

Excitation wavelength-dependent visible emissions from ZnO nanostructures demonstrate that defect states are insufficient to explain their optical properties.

Synergistic influence of FRET, bulk recombination centers, and charge separation in enhancing the visible-light-driven photocatalytic activity of Cu2+-ion-doped ZnO nanoflowers

Pure ZnO and a group of Cu²⁺-ion-doped (4, 6, and 8 wt%) ZnO nanomaterials are synthesized using the co-precipitation technique. X-ray diffraction and Fourier transform infrared spectroscopy confirm both the substitution of Zn²⁺ ions by Cu²⁺ ions in the ZnO lattice and formation of the ZnO/CuO composite. The divalent oxidation state of Cu is confirmed using X-ray photoelectron spectroscopy. A suppression in the oxygen vacancy density is observed up to a doping level of 6 wt%, but beyond that it increases. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) show a cross-linked nanoflower-like structure. The presence of a separate CuO phase is also confirmed via TEM. Absorption spectroscopy yields a reduction in the bandgap up to 6 wt%, after which it is increased for 8 wt%. An enhanced plasmon band in the spectra reveals the presence of CuO. The photoluminescence is quenched for doping up to 6 wt%, and with further doping the emission is enhanced. These observations are explained by the doping-concentration-dependent Förster resonance energy transfer (FRET) phenomenon between the ZnO (donor) and the CuO (acceptor). For the highest doping concentration, the emission profile shows a sudden enhancement resulting from the simultaneous competition of two FRET mechanisms (the intra-acceptor mechanism and the inter-donor-acceptor mechanism). By contrast, for other doped nanomaterials, the inter-donor-acceptor FRET mechanism with doping-concentration dependence is able to explain the suppression of the emission intensity. All doped nanomaterials show an improved visible-light-driven photocatalytic efficiency compared with pure ZnO for methylene blue, which results from the synergistic effects of a reduction in the concentration of bulk defects, enhanced charge separation, and FRET. The highest photocatalytic performance is demonstrated by the 6 wt% nanomaterial due to its optimum doping concentration. However, beyond this concentration, the formation of excessive CuO on the surface of ZnO increases the concentration of bulk defects, and the simultaneous occurrence of the inter-donor-acceptor FRET and intra-acceptor FRET mechanisms takes place leading to the rapid recombination of electron-hole pairs and reduced photocatalytic activity.

Insights into the impact of photophysical processes and defect state evolution on the emission properties of surface-modified ZnO nanoplates for application in photocatalysis and hybrid LEDs

The effects of surface modification on the defect state densities, optical properties, and photocatalytic and quantum efficiencies of zinc oxide (ZnO) nanoplates were studied in this work. The aim of this study is to identify the photophysical processes that dictate the quenching of emission from defect states upon surface modification and the role of different defects such as zinc interstitials (Zn_i) or oxygen vacancies (V_O) beside the photophysical processes in determining the photocatalytic efficiency of plate-like ZnO nanostructures. For controlling the intrinsic defect state densities of ZnO nanoplates, which is difficult to achieve, their surface was modified using different polymers such as PMMA and PVA. X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) emission spectroscopy were employed to identify and quantify the defect states. The analysis of relative defect state densities of Zn_i or V_O showed that Zn_i significantly impacts the photocatalytic activity (PCA) besides V_O, but it has a lower influence than V_O because of the difference in the accessibility and intrinsic nature of these two defects. Synchronous quenching of emission from different defect states with different formation energies and its correlation with the photocatalytic activity led us to conclude that photophysical processes such as concentration-dependent Förster resonance energy transfer (FRET), charge transfer (CT) and Zn_i defects play a significant role behind PCA, which has been previously reported to be influenced by V_O only. FRET and CT also play a critical role behind emission quenching upon surface modification. Upon the surface modification of nanoplates, a drop in the quantum efficiency from 12.14% to 4.44% was observed with the fine-tuning of emission colour from bluish-white to blue. Besides the defect states, FRET and CT phenomena are dominant in reducing the quantum efficiency of hybrid light-emitting diodes (HyLEDs) and photocatalytic efficiency. Therefore, the work outlines the reason behind the suppression of luminescence and photocatalytic efficiency of ZnO nanoparticles after surface modification and how to optimise them for their applications as an emissive layer in HyLEDs and efficient photocatalysts.

Synthesis and Conductivity Studies of Poly(Methyl Methacrylate) (PMMA) by Co-Polymerization and Blending with Polyaniline (PANi)

Poly(methyl methacrylate) (PMMA) is a lightweight insulating polymer that possesses good mechanical stability. On the other hand, polyaniline (PANi) is one of the most favorable conducting materials to be used, as it is easily synthesized, cost-effective, and has good conductivity. However, most organic solvents have restricted potential applications due to poor mechanical properties and dispersibility. Compared to PANi, PMMA has more outstanding physical and chemical properties, such as good dimensional stability and better molecular interactions between the monomers. To date, many research studies have focused on incorporating PANi into PMMA. In this review, the properties and suitability of PANi as a conducting material are briefly reviewed. The major parts of this paper reviewed different approaches to incorporating PANi into PMMA, as well as evaluating the modifications to improve its conductivity. Finally, the polymerization condition to prepare PMMA/PANi copolymer to improve its conductivity is also discussed.

Compositional variation dependent colour tuning and observation of Förster resonant energy transfer in Cd(1−x)ZnxS nanomaterials

Energy level diagram of FRET process in Cd_(1−x)Zn_xS nanomaterials between donor (ZnS) and acceptor (CdS): the smaller ZnS materials transfer energy nonradiatively to the larger CdS materials when there is sufficient spectral overlap between them.