B.S. Witkowski

Zinc oxide for electronic, photovoltaic and optoelectronic applications

We show that the atomic layer deposition (ALD) technique has great potential for widespread use in the production of ZnO films for applications in electronic, photovoltaic (PV), and optoelectronic devices. The low growth temperature makes ALD-grown ZnO films suitable for fabrication of various semiconductor/organic hybrid structures. This opens up the possibility of novel devices based on very cheap organic materials, including organic light emitting diodes and third-generation PV cells.

Capability for Fine Tuning of the Refractive Index Sensing Properties of Long-Period Gratings by Atomic Layer Deposited Al2O3 Overlays

This work presents an application of thin aluminum oxide (Al2O3) films obtained using atomic layer deposition (ALD) for fine tuning the spectral response and refractive-index (RI) sensitivity of long-period gratings (LPGs) induced in optical fibers. The technique allows for an efficient and well controlled deposition at monolayer level (resolution ∼ 0.12 nm) of excellent quality nano-films as required for optical sensors. The effect of Al2O3 deposition on the spectral properties of the LPGs is demonstrated experimentally and numerically. We correlated both the increase in Al2O3 thickness and changes in optical properties of the film with the shift of the LPG resonance wavelength and proved that similar films are deposited on fibers and oxidized silicon reference samples in the same process run. Since the thin overlay effectively changes the distribution of the cladding modes and thus also tunes the devices RI sensitivity, the tuning can be simply realized by varying number of cycles, which is proportional to thickness of the high-refractive-index (n > 1.6 in infrared spectral range) Al2O3 film. The advantage of this approach is the precision in determining the film properties resulting in RI sensitivity of the LPGs. To the best of our knowledge, this is the first time that an ultra-precise method for overlay deposition has been applied on LPGs for RI tuning purposes and the results have been compared with numerical simulations based on LP mode approximation.

The properties of tris "8-hydroxyquinoline… aluminum organic light emitting diode with undoped zinc oxide anode layer

Transparent and conductive undoped zinc oxide films were prepared by atomic layer deposition method for use in tris (8-hydroxyquinoline) aluminum (Alq3)-based organic light emitting diodes. The properties of the ZnO layers were investigated. The ZnO/CuI/Alq3/poly(ethylene glycol) dimethyl ether/Al device turned on at 7.9 V and demonstrated external quantum efficiency of 1.5% which is better comparing to the same structure but with indium tin oxide as anode layer.

Homogeneous and heterogeneous magnetism in (Zn,Co)O : From a random antiferromagnet to a dipolar superferromagnet by changing the growth temperature

For more than a decade ZnO doped with Mn and Co has remained as one of the most prospected diluted magnetic semiconductor for spintronic applications with conflicting outcome concerning the genuineness of its room temperature ferromagnetism. In order to clarify this issue we investigate (Zn,Co)O layers grown by atomic layer deposition at low temperatures. We employ and relay on wide range of extensive material characterization, which in combination with superconducting quantum interference device magnetometry allow us decisively exemplify the growth temperature as the key factor discriminating between paramagnetic (obtained at 160 °C) and various forms of ferromagnetic responses, seen when the grows is carried out at 200 °C and above.

Paramagnetism of cobalt-doped ZnO nanoparticles obtained by microwave solvothermal synthesis.

Summary Zinc oxide nanopowders doped with 1–15 mol % cobalt were produced by the microwave solvothermal synthesis (MSS) technique. The obtained nanoparticles were annealed at 800 °C in nitrogen (99.999%) and in synthetic air. The material nanostructure was investigated by means of the following techniques: X-ray diffraction (XRD), helium pycnometry density, specific surface area (SSA), inductively coupled plasma optical emission spectrometry (ICP-OES), extended X-ray absorption fine structure (EXAFS) spectroscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and with magnetometry using superconducting quantum interference device (SQUID). Irrespective of the Co content, nanoparticles in their initial state present a similar morphology. They are composed of loosely agglomerated spherical particles with wurtzite-type crystal structure with crystallites of a mean size of 30 nm. Annealing to temperatures of up to 800 °C induced the growth of crystallites up to a maximum of 2 μm in diameter. For samples annealed in high purity nitrogen, the precipitation of metallic α-Co was detected for a Co content of 5 mol % or more. For samples annealed in synthetic air, no change of phase structure was detected, except for precipitation of Co3O4 for a Co content of 15 mol %. The results of the magentometry investigation indicated that all as-synthesized samples displayed paramagnetic properties with a contribution of anti-ferromagnetic coupling of Co–Co pairs. After annealing in synthetic air, the samples remained paramagnetic and samples annealed under nitrogen flow showed a magnetic response under the influences of a magnetic field, likely related to the precipitation of metallic Co in nanoparticles.

Hafnium dioxide as a passivating layer and diffusive barrier in ZnO/Ag Schottky junctions obtained by atomic layer deposition

This paper reports on ZnO/Ag Schottky junctions obtained by the low temperature atomic layer deposition process. Introducing the thin (from 1.25 to 7.5 nm) layer of hafnium dioxide between the ZnO layer and evaporated Ag Schottky contact improves the rectification ratio to about 105 at 2V. For the ZnO/Ag junctions without the HfO2 interlayer, the rectification ratio is only 102. We assign this effect to the passivation of ZnO surface accumulation layer that is reported for ZnO thin films.

Biodegradation of the ZnO:Eu nanoparticles in the tissues of adult mouse after alimentary application

Biodegradable zinc oxide nanoparticles (ZnO NPs) are considered promising materials for future biomedical applications. To fulfil this potential, biodistribution and elimination patterns of ZnO NPs in the living organism need to be resolved. In order to investigate gastrointestinal absorption of ZnO NPs and their intra-organism distribution, water suspension of ZnO or fluorescent ZnO:Eu (Europium-doped zinc oxide) NPs (10mg/ml; 0.3ml/mouse) was alimentary-administered (IG: intra-gastric) to adult mice. Internal organs collected at key time-points after IG were evaluated by AAS for Zn concentration and analysed by cytometric techniques. We found that Zn-based NPs were readily absorbed and distributed (3 h post IG) in the nanoparticle form throughout the organism. Results suggest, that liver and kidneys were key organs responsible for NPs elimination, while accumulation was observed in the spleen and adipose tissues. We also showed that ZnO/ZnO:Eu NPs were able to cross majority of biological barriers in the organism (including blood-brain-barrier).

Chlorination of carbon nanotubes obtained on the different metal catalysts

In this paper, a chlorination method is proposed for simultaneous purification and functionalization of carbon nanotubes, thus increasing their ability to use. Carbon nanotubes were obtained by CVD method through ethylene decomposition on the nanocrystalline iron or cobalt or bimetallic iron-cobalt catalysts. The effects of temperature (50, 250, and 450°C) in the case of carbon nanotubes obtained on the Fe-Co catalyst and type of catalyst (Fe, Co, Fe/Co) on the effectiveness of the treatment and functionalization were tested. The phase composition of the samples was determined using the X-ray diffraction method. The quantitative analysis of metal impurity content was validated by means of the thermo gravimetric analysis. Using X-ray Photoelectron Spectroscopy (XPS), Energy Dispersive Spectroscopy (EDS) analysis, and also Mohr titration method, the presence of chlorine species on the surface of chlorinated samples was confirmed.

Si/ZnO nanorods/Ag/AZO structures as promising photovoltaic plasmonic cells

The test structures for photovoltaic (PV) applications based on zinc oxide nanorods (NRs) that were grown using a low-temperature hydrothermal method on p-type silicon substrates (100) covered with Ag nanoparticles (NPs) were studied. The NPs of three different diameters, i.e., 5–10 nm, 20-30 nm, and 50–60 nm, were deposited using a sputtering method. The morphology and crystallinity of the structures were confirmed by scanning electron microscopy and Raman spectroscopy. It was found that the nanorods have a hexagonal wurtzite structure. An analysis of the Raman and photoluminescence spectra permitted the identification of the surface modes at 476 cm−1 and 561 cm−1. The presence of these modes is evidence of nanorods oriented along the wurtzite c-axis. The NRs with Ag NPs were covered with a ZnO:Al (AZO) layer that was grown using the low-temperature atomic layer deposition technique. The AZO layer served as a transparent ohmic contact to the ZnO nanorods. The applicability of the AZO layer for this purpo...

Photovoltaic properties of ZnO nanorods/p-type Si heterojunction structures

Summary Selected properties of photovoltaic (PV) structures based on n-type zinc oxide nanorods grown by a low temperature hydrothermal method on p-type silicon substrates (100) are investigated. PV structures were covered with thin films of Al doped ZnO grown by atomic layer deposition acting as transparent electrodes. The investigated PV structures differ in terms of the shapes and densities of their nanorods. The best response is observed for the structure containing closely-spaced nanorods, which show light conversion efficiency of 3.6%.