Electrical properties of gas-phase synthesized nanoparticles (English shop)

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Abstract

Zinc oxide (ZnO) doped with aluminum receives increasing attention for being an alternative material for the established but much more expensive indium tin oxide (ITO) due to the fact, that it has comparable electrical and optical properties. The electrical properties of mechanically compacted pellets prepared from nanosized ZnO powders are investigated using impedance spectroscopy. The impedance of the samples is measured in hydrogen and in synthetic air between room temperature and 400◦C. In both atmospheres, the measurements show two different electrical transport processes depending on the temperature and the doping level. In synthetic air, the conductivity increases for doping concentrations up to 7.74% of aluminum and collapses for higher doping levels. In hydrogen atmosphere, the conductivity decreases with rising doping level of Al. This behavior can be explained by generation of free charge carriers due to the incorporation of hydrogen and doping with aluminum, respectively. At higher temperatures and at high doping concentrations, scattering processes at grain boundaries as well as lattice defects increasingly affect the charge carrier transport processes leading to a decreasing overall conductivity. The electrical conductivity shows reversible behavior when the atmosphere is changed from hydrogen to ambient conditions and back. To replace ITO in applications, transparent conductive layers with good electrical and optical properties are required. ZnO dispersions are prepared and printed on pre-structured substrates by ink-jet printing to investigate the electrical and sensing properties of printed films. The properties are measured without any annealing steps from room temperature up to 200◦C in ambient conditions and in hydrogen atmosphere using impedance spectroscopy. Compared to the measurements in air, the resistance in hydrogen decreases by a factor of five even at room temperature. The ink-jet printed ZnO films with nanosized particles can be used as sensor without any annealing or post-processing for sensing.

Silicon (Si) nanoparticles are envisioned for a broad range of applications, ranging from electroluminescent devices over biomarkers and lithium ion batteries to solar cells. One of the major challenges with respect to these applications is to effectively stabilize the silicon particles against oxidation. Electrical properties of as-prepared as well as functionalized silicon nanoparticles are investigated. The native oxide shell of the as-prepared silicon nanoparticles is removed and the electrical properties are measured and a re-oxidation of the silicon nanoparticles can be observed after a few hours. A fast and efficient process to functionalize silicon nanoparticles with n-alkenes is introduced. The freshly etched particles are subsequently grafted with even-numbered n-alkenes from C6 to C18 in order to prevent the particles from re-oxidation. FTIR spectra are used to confirm the successful attachment of the organic molecules and provide insight into the binding mechanism. Electrical properties are investigated by impedance spectroscopy showing the effect of surface functionalization on the conductivity of compacted nanoparticle ensembles. It is observed that particles covered with alkenes from C6 to C12 exhibit higher conductivity than the as-prepared materials, while surface functionalization with C14 and higher leads to almost insulating nanoparticle arrays. Despite freshly etched silicon nanoparticles, dodecene-terminated particles showed the best conductivity as well as a very good long-term stability against oxidation. FTIR spectroscopy indicated that particles stabilized with C6 to C10 are less stable due to a creeping re-oxidation. The most promising results is the functionalization of silicon nanoparticles with alkenes with twelve carbon atoms (C12). A variable range hopping transport mechanism or a charge carrier limited transport mechanism exist for all silicon nanoparticles. Especially for the functionalized particles, a dependence of the hopping distances can be observed. This does not mean that the charge carriers use the shortest way but the energetically most advantageous.

Short summary

Zinc oxide (ZnO) is currently regarded as a promising candidate for replacing the very expensive indium tin oxide (ITO), since it possesses electrical and optical properties comparable to those of ITO. The electrical properties of mechanically compacted pellets made from ZnO nanoparticles are investigated by means of impedance spectroscopy. The impedance of the samples is examined in hydrogen atmosphere and in synthetic air up to 400◦C. The measurements in hydrogen environment and in synthetic air show different results, depending on temperature and doping concentration. In synthetic air, the conductivity increases up to an aluminum (Al) doping concentration of 7.7% and collapses for higher Al concentrations. Under hydrogen atmosphere, the conductivity decreases with increasing Al doping concentration. This behavior can be explained by the formation of free charge carriers due to hydrogen doping from the surrounding atmosphere and due to the correspondingly increasing Al concentration. At higher temperatures and doping concentrations, scattering processes at grain boundaries and lattice defects occur more frequently, interrupting the transport of the charge carriers. The differing conductivity shows reversible behavior when switching from hydrogen atmosphere to synthetic air and back.

For the application of ZnO in transparent conductive films and sensors made from low-cost materials, stable dispersions with very small (semi-)conducting particles in the nanometer range are required. Stable ZnO dispersions were prepared and deposited onto pre-structured substrates by ink-jet printing. The electrical sensing properties of the printed layers were measured up to 200◦C in ambient air. Prior to the impedance measurements, the layers were not annealed. Compared to the measurements in air, it was observed that the resistance of the layer under hydrogen atmosphere at room temperature is reduced by a factor of five. The prepared, printed ZnO layers show a good response behavior towards hydrogen and can be used without any prior annealing processes.

Silicon particles are currently used for a large variety of applications. These applications range from electroluminescent devices over biomarkers and lithium ion batteries to solar cells. One of the tasks in improving the properties is to find a method for stabilizing the silicon surface against oxidation. The oxide shell present around the silicon nanoparticle after synthesis is removed. The electrical properties of the freshly etched particles are measured and a rapid re-oxidation of the particles is observed within hours. A fast and effective process for functionalization with n-alkenes is presented. The particles are prepared with alkenes of different chain lengths from C6 to C18 in order to prevent re-oxidation of the particles after the etching process. With the help of FTIR measurements, the successful attachment of the organic molecules to the silicon surface is demonstrated. Electrical measurements are carried out by means of impedance spectroscopy in order to investigate the effect of functionalization with the different alkenes on the electrical conductivity. A higher conductivity is found for all silicon nanoparticles functionalized with the alkenes from C6 to C12. The particles functionalized with alkenes of a chain length of C14 and above show a very low conductivity, which is in part worse than that of the silicon particles in their as-prepared state. Functionalization with the alkene C12 shows the best results and remains stable in its electrical properties even after more than half a year. FTIR measurements show that the particles functionalized with C6 and C10 do not possess complete stability against re-oxidation. The functionalization of silicon with dodecene is the most stable against re-oxidation and shows the best electrical properties. A “variable range hopping” transport mechanism is observed for the charge carriers between the particles. Especially for the functionalized particles, the distance covered by the charge carriers depends on the type of functionalization. However, this does not mean that the charge carrier takes the shortest path, but rather the energetically most favorable one.