Ken Elen

Hydrothermal synthesis of ZnO nanorods: a statistical determination of the significant parameters in view of reducing the diameter

In this paper a 2(8-4) fractional factorial design of experiments is applied to identify the important parameters that affect the average diameter of ZnO rods, synthesized by means of a hydrothermal procedure. A water-based Zn(2+) precursor is used for the formation of one-dimensional ZnO particles, without the presence of an organic additive. Results indicate that, at the investigated levels, four of the parameters have a significant effect on the mean diameter. These are the temperature, the heating rate, stirring and an ultrasonic pre-treatment of the precursor solution. Experiments carried out with zinc acetate and zinc chloride do not show a significant difference in rod diameter. Other parameters that do not show a significant effect are the concentration of Zn(2+), the molar ratio between the hydroxyl and the zinc ions, and the reaction time. Interactions are observed between stirring and an ultrasonic pre-treatment and between the zinc concentration and the OH:Zn ratio. By fixing the significant factors at their optimal value it is possible to decrease the mean diameter. The particles are characterized by means of x-ray diffraction (XRD) and transmission electron microscopy (TEM).

Relation between synthesis conditions, dopant position and charge carriers in aluminium-doped ZnO nanoparticles

The ability to achieve an understanding of the correlations between chemical synthesis, doping mechanism and properties of aluminium-doped zinc oxide (ZnO:Al) nanocrystals is of great importance to evaluate the potential of ZnO:Al nanocrystals as optimal building blocks for solution deposited transparent conductive oxide layers. Two series of Al-doped ZnO nanoparticles were synthesized by solution-based methods under different conditions yielding phase-pure wurtzite ZnO:Al nanocrystals with different morphologies (quasispheres and rods). In both series 80% of the input Al is incorporated in the ZnO:Al crystals. Furthermore, 27Al nuclear magnetic resonance demonstrated a 5 times higher tetrahedral Al content for the quasispheres, which can be directly linked to the presence of free charge carriers, as showed with Fourier transform infrared spectroscopy. XPS data show the Zn2+ and O2− chemical state of the constituting ions. It is demonstrated that an efficient Al doping, creating charge carriers in the ZnO, can be achieved by controlling the chemical synthesis parameters of the nanoparticles.

Comparison of two novel solution-based routes for the synthesis of equiaxed ZnO nanoparticles

Due to a dominant one-dimensional growth rate, nanoparticles of zinc oxide often show a rodlike morphology. As a result, the synthesis of small spherical nanoparticles of undoped ZnO remains challenging. This paper presents two procedures that successfully produce a powder consisting of equiaxed zinc oxide nanoparticles: one using a polyethylene glycol- (PEG-) assisted solvothermal method and the other by calcination of zinc oxalate obtained from a microemulsion-mediated method. In the latter, zinc-substituted aerosol OT (AOT) is used as a surfactant. The samples are characterized by inductively coupled plasma-atomic emission spectroscopy (ICP-AES), thermogravimetric analysis (TGA), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), photon correlation spectroscopy (PCS), and photoluminescence (PL) spectroscopy. Both synthesis techniques produce nanoparticles with similar sizes in the range of 10 to 20 nm. Dense aggregates observed in the calcined powder are infrequent in the case of the solvothermal method.

Thermal decomposition synthesis of Al-doped ZnO nanoparticles: an in-depth study

Al-doped ZnO nanoparticles are synthesized by means of a heating up solution based thermal decomposition method. The synthesis involves a reaction of zinc acetylacetonate hydrate, aluminium acetylacetonate and 1,2-hexadecanediol in the presence of oleic acid and oleyl amine. A proposed reaction mechanism from reagents to monomers is corroborated by analysis of the evolving gases using headspace GC-MS analysis. The Al-doped ZnO nanoparticles synthesized are dynamically stabilized by adsorbed oleate ions, after deprotonation of oleic acid by oleyl amine, as was found by NOESY proton NMR and complementary FTIR spectroscopy. Precession electron diffraction shows a simultaneous increase in lattice parameters with Al concentration. This, together with HAADF-STEM and EDX maps, indicates the incorporation of Al into the ZnO nanoparticles. By the combination of complementary characterization methods during all stages of the synthesis, it is concluded that Al is incorporated into the ZnO wurtzite lattice as a dopant.

From liquid to thin film: colloidal suspensions for tungsten oxide as an electrode material for Li-ion batteries

Using a colloidal suspension, tungsten oxide thin films (150 nm) have been prepared via ultrasonic spray deposition using two different current collectors, namely TiN and Pt. First, the precursor chemistry was studied, revealing that the tungsten present is reduced due to the formation of chlorine gas. Due to a dehydrogenation 1,1-diethoxyethane (DEE) and hydrogen chloride (HCl) evolve from the precursor, reducing the chloride content of the precursor. The thin films were annealed at 400 and 500 °C, yielding tetragonal tungsten oxide without the presence of chlorides. Electrochemical analysis indicated that the TiN current collector has a pronounced positive effect on cycling behavior of the WO3 thin film. A higher annealing temperature yields an improved performance, but annealing at temperatures as low as 400 °C also yielded electrochemically active WO3. The current study presents a versatile method to produce electrochemically active tungsten oxide thin films with a high volumetric capacity (640 mA h cm−3) at relatively low temperature to be applied in all-solid-state Li-ion batteries.

Ultrasonically spray coated silver layers from designed precursor inks for flexible electronics

Integration of electronic circuit components onto flexible materials such as plastic foils, paper and textiles is a key challenge for the development of future smart applications. Therefore, conductive metal features need to be deposited on temperature sensitive substrates in a fast and straightforward way. The feasibility of these emerging (nano-) electronic technologies depends on the availability of well-designed deposition techniques and on novel functional metal inks. As ultrasonic spray coating (USSC) is one of the most promising techniques to meet the above requirements, innovative metal organic decomposition (MOD) inks are designed to deposit silver features on plastic foils. Various amine ligands were screened and their influence on the ink stability and the characteristics of the resulting metal depositions were evaluated to determine the optimal formulation. Eventually, silver layers with excellent performance in terms of conductivity (15% bulk silver conductivity), stability, morphology and adhesion could be obtained, while operating in a very low temperature window of 70 °C-120 °C. Moreover, the optimal deposition conditions were determined via an in-depth analysis of the ultrasonically sprayed silver layers. Applying these tailored MOD inks, the USSC technique enabled smooth, semi-transparent silver layers with a tunable thickness on large areas without time-consuming additional sintering steps after deposition. Therefore, this novel combination of nanoparticle-free Ag-inks and the USSC process holds promise for high throughput deposition of highly conductive silver features on heat sensitive substrates and even 3D objects.

Increasing the solubility limit for tetrahedral aluminium in ZnO: Al nanorods by variation in synthesis parameters

Nanocrystalline ZnO:Al nanoparticles are suitable building blocks for transparent conductive layers. As the concentration of substitutional tetrahedral Al is an important factor for improving conductivity, here we aim to increase the fraction of substitutional Al. To this end, synthesis parameters of a solvothermal reaction yielding ZnO:Al nanorods were varied. A unique set of complementary techniques was combined to reveal the exact position of the aluminium ions in the ZnO lattice and demonstrated its importance in order to evaluate the potential of ZnO:Al nanocrystals as optimal building blocks for solution deposited transparent conductive oxide layers. Both an extension of the solvothermal reaction time and stirring during solvothermal treatment result in a higher total tetrahedral aluminium content in the ZnO lattice. However, only the longer solvothermal treatment effectively results in an increase of the substitutional positions aimed for.

Steering the Properties of MoOx Hole Transporting Layers in OPVs and OLEDs: Interface Morphology vs. Electronic Structure

The identification, fine-tuning, and process optimization of appropriate hole transporting layers (HTLs) for organic solar cells is indispensable for the production of efficient and sustainable functional devices. In this study, the optimization of a solution-processed molybdenum oxide (MoOx) layer fabricated from a combustion precursor is carried out via the introduction of zirconium and tin additives. The evaluation of the output characteristics of both organic photovoltaic (OPV) and organic light emitting diode (OLED) devices demonstrates the beneficial influence upon the addition of the Zr and Sn ions compared to the generic MoOx precursor. A dopant effect in which the heteroatoms and the molybdenum oxide form a chemical identity with fundamentally different structural properties could not be observed, as the additives do not affect the molybdenum oxide composition or electronic band structure. An improved surface roughness due to a reduced crystallinity was found to be a key parameter leading to the superior performance of the devices employing modified HTLs.

Screen-printing of flexible semi-transparent electrodes and devices based on silver nanowire networks

Silver nanowire networks have demonstrated significant potential as semi-transparent electrodes for various applications. However, for their widespread utilisation in devices, upscaled coating technologies such as screen-printing need to be explored and related to this, the formulation of suitable inks is indispensable. This work contributes to this effort by the synthesis of Ag-NW based formulations. The rheological characteristics that are essential for screen-printing are obtained by the addition of hydrophobically modified cellulose. The electrical and optical characteristics of screen-printed features on PET are compared by a Van der Pauw method and UV-vis spectroscopy. Despite the presence of the cellulose additive, the screen-printed electrodes exhibit a transmittance from 92.8% to 57.3% and a sheet resistance down to 27 Ohm sq-1. Based on the percolation theory in composites, a mathematical expression is presented, which allows the in-depth analysis of the resulting opto-electrical properties. The application potential of the nanowire-containing formulations is finally demonstrated by screen-printing functional, flexible electroluminescent devices.

Aqueous solution-based synthesis and deposition of crystalline In-Ga-Zn-oxide films with an enhanced mobility

AbstractIn-Ga-Zn-oxide (IGZO), in its amorphous state, is known to have a high electron mobility and low off-state current inside transistor devices, which may even be further improved by crystallization. Thin films of the IGZO superlattice structure require an optimal layer homogeneity in addition to precise control of the stoichiometry, which can be achieved by using a solution-based process. In this work, an aqueous precursor system is developed, starting from the respective metal (hydr)oxides. A stable multimetal precursor is obtained, which is ideally suited for solution-deposition via spin-coating. Through an optimized multi-step thermal treatment, crystalline thin films of IGZO are obtained that show a preferential c-axis orientation after rapid-thermal annealing at 1000 °C in inert conditions. The resulting film shows a good optical transparency (>70%) and an improved carrier mobility (27.2 cm²/Vs) compared to typical solution-processed amorphous IGZO films, and is therefore promising for further application. Top: Overview of the precursor synthesis, which consists of the metal (hydr)oxides as a starting product, citric acid (ligand) and ammonia (pH adjustment and bridging ion). Middle: Schematic route of the film processing. The precursor is applied on a substrate via spincoating. Intermediate hotplate steps (200–400–600 °C) decompose the precursor into an oxide film. Finally, a thermal treatment (1000 °C) inside an RTP (double-substrate setup) crystallizes the oxide films. Bottom: Thermogravimetric plot of the thermal decomposition of the precursor solution // Plane view SEM micrograph of the film morphology after annealing at 1000 °C for 500 s // XRD diffractogram of the film after annealing at 1000 °C for 500 s.HighlightsSolution-based deposition starting from an aqueous multimetal citrate precursor systemCrystalline IGZO films that show a preferential c-axis orientation after annealing at 1000 °CAn improved carrier mobility compared to typical solution-processed amorphous IGZO filmsA good optical transparency has been observed in the visible-light range