Effect of thermal annealing on the structure and magnetism of Fe-doped ZnO nanocrystals synthesized by solid state reaction

High purity Fe₂O₃/ZnO nanocomposites were annealed in air at different temperatures between 100 and 1200 °C to get Fe-doped ZnO nanocrystals. The structure and grain size of the Fe₂O₃/ZnO nanocomposites were investigated by X-ray diffraction 2θ scans. Annealing induces an increase of the grain size from 25 to 195 nm and appearance of franklinite phase of ZnFe₂O₄. Positron annihilation measurements reveal large number of vacancy defects in the interface region of the Fe₂O₃/ZnO nanocomposites, and they are gradually recovered with increasing annealing temperature. After annealing at temperatures higher than 1000 °C, the number of vacancies decreases to the lower detection limit of positrons. Room temperature ferromagnetism can be observed in Fe-doped ZnO nanocrystals using physical properties measurement system. The ferromagnetism remains after annealing up to 1000 °C, suggesting that it is not related with the interfacial defects.     

ZnO nanorod arrays fabrication via chemical bath deposition: Ligand concentration effect study

A new ligand, N,N,N′,N′-tetramethylethylenediamine, has been used to grow ZnO nanorods on silicon substrates via a two steps approach. A preliminary seeding on silicon substrates has been combined with chemical bath deposition using a Zinc acetate–N,N,N′,N′-tetramethylethylenediamine aqueous solution. The used diamino ligand has been selected as Zn²⁺ complexing agent and the related hydrolysis generates the reacting ions (Zn²⁺ and OH⁻) responsible for the ZnO growth. The seed layer has been annealed at low temperature (<200 °C) and the ZnO nanorods have been grown on this ZnO amorphous layer. There is experimental evidence that the ligand concentration (ranging from 5 to 50 mM) strongly affects the alignment of ZnO nanorods on the substrate, their lateral dimension and the related surface density. Length and diameter of ZnO nanorods increase upon increasing the ligand concentration, while the nanorod density decreases. Even more important, it has been demonstrated, as proof of concept, that chemical bath deposition can be usefully combined with colloidal lithography for selective ZnO nanorod deposition. Thus, by patterning the ZnO seeded substrate with polystyrene microsphere colloidal lithography, regular Si hole arrays, spatially defined by hexagonal ZnO nanorods, have been successfully obtained.     

Thermo-electrochemical selective growth of ZnO nanorods on any noble metal electrodes

Selective growth of ZnO nanorods has been successfully performed on the patterned Au/Ti metal electrode regions on a glass substrate by using a seeded thermo-electrochemical method in an acidic growth solution. The selective growth mechanism of the thermo-electrochemical method was proposed by using a series of chemical reactions for the first time. The thermo-electrochemical selective ZnO growth was performed on the cathode electrode at a temperature below 90 °C. A ZnO seed layer was precoated and selectively etched away from the non-metal regions in order to create the patterned selective nucleation sites on which the precursors are transferred and crystallized into ZnO nanorods. Both the dimensions and the placements of the ZnO nanorods have been simultaneously controlled. Energy dispersive X-ray spectrometry showed that the selectively grown ZnO nanorods consist of only Zn and O, indicating that the selectively grown ZnO nanorods are pure and contamination free. XRD and electron diffraction patterns revealed that the obtained ZnO nanorods have a wurtzite single-crystal structure.     

Transparent conducting oxides for electrode applications in light emitting and absorbing devices

In both light emitting devices such as light emitting diodes (LEDs), and light absorbing devices such as solar cells (also photodetectors), which are gaining considerable interest for their energy saving and energy production capability, respectively, a compromise must be struck between the need to increase the light emitting/absorbing area/potential and the need for low series resistance of the metal contact grid. This undesirable compromise can be mitigated by using transparent conducting oxides (TCOs), which heretofore have been dominated by ITO (indium tin oxide—an In-rich alloy of indium oxide and tin oxide). Due to the expected scarcity of Indium used in ITO, efforts are underway to develop indium-free TCOs for the above-mentioned devices as well as flat panel displays. ZnO heavily doped with Ga or Al (GZO or AZO) is becoming a very attractive candidate for future generation TCOs. GZO and AZO as well as multilayer TCOs consisting of two TCO layers with a thin metal layer in between have been widely investigated for LEDs and solar cells to enhance device performance. This article succinctly reviews the latest developments in and properties of TCOs, particularly in relation to thin film transparent electrode applications for LEDs and solar cells. Pertinent critical issues and possible solutions are provided as well.     

Regrowth of ZnO thin film with high surface flatness and enhanced optical properties on annealed buffer layers by rf sputtering deposition

We report a study on improving the surface flatness, optical properties, and crystallinity of ZnO thin films by rf sputtering deposition. ZnO thin films grown on sapphire substrate were first exposed to post-growth annealing, and then used to regrow high-quality ZnO thin films on top. Under the same deposition conditions, the regrown ZnO layers showed much improved crystallinity, surface flatness and enhanced optical properties. The effect of the annealed layer in improving the quality of the ZnO thin film is discussed in terms of characterization results from crystal orientation, surface morphology, and photoluminescence. It was clearly observed that, during the annealing process, the ZnO grains coalesced to form larger grains and smoother surfaces, with better crystallinity and fewer defects, which resulted in the much improved quality of the regrown ZnO thin films.     

Confined Franz–Keldysh effect in ZnO quantum dots

Within the framework of the effective mass approximation, the confined Franz–Keldysh effect is investigated theoretically in a cylindrical ZnO quantum dot (QD). Numerical results show that the application of an electric field can decrease the strength and the threshold energy of the optical absorption coefficient in ZnO QD. There are additional oscillations in the absorption above the effective band gap, which are due to the Franz–Keldysh effect which occurs in the presence of the electric field. Our results also show that the electric field has a more obviously influence on the optical absorption in cylindrical ZnO QD with larger dot height.     

Filtering performance improvement in V-doped ZnO/diamond surface acoustic wave filters

High-frequency surface acoustic wave (SAW) filters using undoped and V-doped ZnO films were fabricated on diamond. Compared with their counterparts, the SAW filters using V-doped ZnO films have higher electromechanical coupling coefficient of ∼2.9% and lower insertion loss. The filtering performance improvement is considered to be due to the ferroelectricity in V-doped ZnO films and the resultant high piezoresponse (∼110 pm/V), which is one order of magnitude larger than that of undoped ZnO films. In addition, more perfect (0 0 2) preferred orientation, better uniform grains and smoother surface of V-doped ZnO films also contribute to the filtering performance improvement.     

Room-temperature ferromagnetism of the amorphous Cu-doped ZnO thin films

Amorphous copper-doped ZnO thin films (ZnO:Cu) prepared on glass substrates by the radio-frequency magnetron co-sputtering have been investigated. Magnetic measurements indicated that the amorphous ZnO:Cu thin films were ferromagnetic at room temperature and the saturation magnetization was much higher than that of the polycrystalline films. X-ray diffraction results showed there was no Cu₂O phase in amorphous ZnO:Cu films, which might be the reason for the high magnetic moment of the films. On the other hand, the high saturation magnetization of the amorphous ZnO:Cu films could also attribute to that there was no limit of solid solubility of Cu in amorphous ZnO solvent. The X-ray photoelectron spectroscopy study of the amorphous ZnO:Cu thin films reveal that copper was in Cu²⁺ chemical state.     

In situ preparation and fluorescence quenching properties of polythiophene/ZnO nanocrystals hybrids through atom-transfer radical polymerization and hydrolysis

In this paper, a new approach for in situ preparing nanocomposites of conjugated polymers (CPs) and semiconductor nanocrystals was developed. Polythiophene grafted poly(zinc methacrylate) (PTh-g-PZMA) copolymer was synthesized by atom-transfer radical polymerization (ATRP) of zinc methacrylate (ZMA) initiated from the macroinitiator poly(2,5-(3-(bromoisopropyl-carbonyl-oxymethylene) thiophene)) (PTh-Br) with pendant initiator groups. Subsequently, the polythiophene grafted poly(methacrylate)/ZnO (PTh-g-PMA/ZnO) hybrid heterojunction nanocomposites were successfully prepared by in situ hydrolysis of PTh-g-PZMA casting films in alkaline aqueous solution. The structures of PTh-Br, PTh-g-PZMA and PTh-g-PMA/ZnO were confirmed by the proton nuclear magnetic resonance (¹H NMR) spectra, Fourier transform infrared (FTIR) spectra and X-ray photoelectron spectroscopy (XPS). The morphologies of PTh-g-PMA/ZnO films prepared for different hydrolysis time were observed in the cross-sections by scanning electron microscope (SEM). The SEM images revealed that ZnO nanocrystals were uniformly dispersed in polymers without any aggregation and the appearances of ZnO nanocrystals changed from nanoparticles to nanorods with the hydrolysis treatment time increasing. The optical properties of these nanocomposites were studied by ultraviolet-visible (UV-vis) absorption and fluorescence spectroscopy. UV-vis absorption spectroscopy showed that the adsorption band of PTh-g-PMA/ZnO hybrids was broader than that of PTh-Br, implying that the existence of ZnO nanocrystals increased the optical absorption region of hybrids. The photoluminescence (PL) spectra of the hybrids showed that fluorescence quenching occurred in PTh-g-PMA/ZnO blends and a maximum of 85% of the fluorescence intensity quenched in the PTh-g-PMA/ZnO obtained from treatment in NaOH aqueous solution for 2 h, which revealed the existence of photo-induced charge transfer between the polythiophene chains and ZnO. These results indicated that the hybrid heterojunction nanocomposites could be promising candidates for photovoltaic applications.     

Influence of growth temperature of TiO2 buffer on structure and PL properties of ZnO films

A series of ZnO films with TiO₂ buffer on Si (1 0 0) substrates were prepared by DC reactive sputtering. Growth temperature of TiO₂ buffer changed from 100 °C to 400 °C, and the influence on the crystal structures and optical properties of ZnO films have been investigated. The XRD results show that the ZnO films with TiO₂ buffer have a hexagonal wurtzite structure with random orientation, and with the increase of growth temperature of TiO₂ buffer, the residual stresses were released gradually. Specially, the UV emission enhanced distinctly and FWHMs (full width half maximum) decreased linearly with the increasing TiO₂ growth temperature. The results all come from the improvement of crystal quality of ZnO films.