Effects of free electrons and quantum confinement in ultrathin ZnO films: a comparison between undoped and Al-doped ZnO

Enhanced Sol-Gel Route to Obtain a Highly Transparent and Conductive Aluminum-Doped Zinc Oxide Thin Film

The electrical and optical properties of sol–gel derived aluminum-doped zinc oxide thin films containing 2 at.% Al were investigated considering the modifying effects of (1) increasing the sol H₂O content and (2) a thermal treatment procedure with a high-temperature approach followed by an additional heat-treatment step under a reducing atmosphere. According to the results obtained via the TG-DTA analysis, FT-IR spectroscopy, X-ray diffraction technique, and four-point probe resistivity measurements, it is argued that in the modified sample, the sol hydrolysis, decomposition of the deposited gel, and crystallization of grains result in grains of larger crystallite size in the range of 20 to 30 nm and a stronger c-axis preferred orientation with slightly less microstrain. The obtained morphology and grain-boundary characteristics result in improved conductivity considering the resistivity value below 6 mΩ·cm. A detailed investigation of the samples’ optical properties, in terms of analyzing their absorption and dispersion behaviors through UV-Vis-NIR spectroscopy, support our reasoning for the increase of the mobility, and to a lesser extent the concentration of charge carriers, while causing only a slight degradation of optical transmittance down to nearly 80%. Hence, an enhanced performance as a transparent conducting film is claimed for the modified sample by comparing the figure-of-merit values.

Non-monotonous size-dependent photoluminescence and excitonic relaxations in nanostructured ZnO thin films

The size dependence of room-temperature photoluminescence (PL) accompanied with near-band-edge emission (NBE) and defect-associated green emission (GE) was investigated using high-quality crystalline nanostructured ZnO thin films with grain sizes varying from 29 nm to 2 nm. The size dependence of correlated intensities of the PL bands was pursued in correlation with structural and defect evolution revealed by X-ray photoelectron spectroscopy (XPS) and previous studies of XRD and Raman scattering. In contrast to the influence of thermally activated reconstruction and changes in defect densities, quantum size effects emerging at grain sizes below a critical value, i.e., d_c ∼ 10 nm were inspected in relation to the observed blueshift in the bandgap and correlated variations in the size dependence of the intensity of NBE and GE. Taking into account the geometry-modelled relative emission efficiency, (i) the observed overall linear size dependence of the relative intensity I_NBE/I_GE was consistent with assuming a 1.05 nm-thick GE-active surface layer, and (ii) a local maximum of I_NBE/I_GE emerging near grain radius R ≈ 4 nm was identified in relation to the theoretically predicted local minimum in size-dependent exciton radiative lifetime due to the intrinsic quantum nature of excitons confined in ZnO. Our results have provided new insights into non-monotonous size-dependent PL of ZnO, which can benefit future photoelectronic device design by taking advantage of the size-controlled stability of confined excitons in nanostructured thin films and luminescent quantum dots.

To comprehensively understand the behaviors of the near-band-edge emission and green emission (NBE, GE), the volume-weighting (VW) model is adapted to take into account a dead layer of confined excitons.

Optical Properties of Al-Doped ZnO Films in the Infrared Region and Their Absorption Applications

The optical properties of aluminum-doped zinc oxide (AZO) thin films were calculated rapidly and accurately by point-by-point analysis from spectroscopic ellipsometry (SE) data. It was demonstrated that there were two different physical mechanisms, i.e., the interfacial effect and crystallinity, for the thickness-dependent permittivity in the visible and infrared regions. In addition, there was a blue shift for the effective plasma frequency of AZO when the thickness increased, and the effective plasma frequency did not exist for AZO ultrathin films (< 25 nm) in the infrared region, which demonstrated that AZO ultrathin films could not be used as a negative index metamaterial. Based on detailed permittivity research, we designed a near-perfect absorber at 2-5 μm by etching AZO-ZnO alternative layers. The alternative layers matched the phase of reflected light, and the void cylinder arrays extended the high absorption range. Moreover, the AZO absorber demonstrated feasibility and applicability on different substrates.

Diatom frustules decorated with zinc oxide nanoparticles for enhanced optical properties

Zinc oxide (ZnO) nanoparticles were synthesized on diatomite (DE) surface by a low temperature sol gel technique, starting from zinc acetate dihydrate (Zn(CH₃COO)₂ · 2H₂O) solution in water/ethyl alcohol, in presence of triethanolamine (TEA) with functions of Zn²⁺ chelating agent, catalyst and mediator of nanoparticle growth on DE surface. Microstructural features were investigated by field emission scanning electron microscopy and x-ray diffraction. ZnO crystalline nanoparticles, well distributed both on the surface and into the porous architecture of diatomite, were obtained just after the synthesis carried out at 80 °C without the need of calcination treatments. The optical properties of ZnO/DE hybrid powders were measured for the first time by means of photoacoustic spectroscopy (PAS). A new method to retrieve both the optical absorption and scattering coefficients from PAS is here discussed for powder aggregates. The fingerprint of the zinc oxide nanoparticles has been highlighted in the Mie scattering resonance in the UV-Vis range, and in the enhancement of the optical absorption with respect to diatomite.

Evolution of dielectric function of Al-doped ZnO thin films with thermal annealing: effect of band gap expansion and free-electron absorption

Evolution of dielectric function of Al-doped ZnO (AZO) thin films with annealing temperature is observed. It is shown that the evolution is due to the changes in both the band gap and the free-electron absorption as a result of the change of free-electron concentration of the AZO thin films. The change of the electron concentration could be attributed to the activation of Al dopant and the creation/annihilation of the donor-like defects like oxygen vacancy in the thin films caused by annealing.

Sustainable synthesis of metals-doped ZnO nanoparticles from zinc-bearing dust for photodegradation of phenol

A novel strategy of waste-cleaning-waste is proposed in the present work. A metals-doped ZnO (M-ZnO, M = Fe, Mg, Ca and Al) nanomaterial has been prepared from a metallurgical zinc-containing solid waste "fabric filter dust" by combining sulfolysis and co-precipitation processes, and is found to be a favorable photocatalyst for photodegradation of organic substances in wastewater under visible light irradiation. All the zinc and dopants (Fe, Mg, Ca and Al) for preparing M-ZnO are recovered from the fabric filter dust, without any addition of chemical as elemental source. The dust-derived M-ZnO samples deliver single phase indexed as the hexagonal ZnO crystal, with controllable dopants species. The photocatalytic activity of the dust-derived M-ZnO samples is characterized by photodegradation of phenol aqueous solution under visible light irradiation, giving more prominent photocatalytic behaviors than undoped ZnO. Such enhancements may be attributed to incorporation of the dust-derived metal elements (Fe, Mg, Ca and Al) into ZnO structure, which lead to the modification of band gap and refinement of grain size. The results show a feasibility to utilize the industrial waste as a resource of photodegradating organic substances in wastewater treatments.