Sonja Hartner

Electrical properties of aluminum-doped zinc oxide (AZO) nanoparticles synthesized by chemical vapor synthesis.

Aluminum-doped zinc oxide nanoparticles have been prepared by chemical vapor synthesis, which facilitates the incorporation of a higher percentage of dopant atoms, far above the thermodynamic solubility limit of aluminum. The electrical properties of aluminum-doped and undoped zinc oxide nanoparticles were investigated by impedance spectroscopy. The impedance is measured under hydrogen and synthetic air between 323 and 673 K. The measurements under hydrogen as well as under synthetic air show transport properties depending on temperature and doping level. Under hydrogen atmosphere, a decreasing conductivity with increasing dopant content is observed, which can be explained by enhanced scattering processes due to an increasing disorder in the nanocrystalline material. The temperature coefficient for the doped samples switches from positive temperature coefficient behavior to negative temperature coefficient behavior with increasing dopant concentration. In the presence of synthetic air, the conductivity firstly increases with increasing dopant content by six orders of magnitude. The origin of the increasing conductivity is the generation of free charge carriers upon dopant incorporation. It reaches its maximum at a concentration of 7.7% of aluminum, and drops for higher doping levels. In all cases, the conductivity under hydrogen is higher than under synthetic air and can be changed reversibly by changing the atmosphere.

Electrical Transport in Semiconductor Nanoparticle Arrays: Conductivity, Sensing and Modeling

Electrical properties of nanoparticle ensembles are dominated by interparticle transport processes, mainly due to particle–particle and particle-contact interactions. This makes their electrical properties dependent on the network properties such as porosity and particle size and is a main prerequisite for solid- state gas sensors, as the surrounding gas atmosphere influences the depletion layer surrounding each particle. Different kinds of nanoparticle arrays such as pressed pellets, printed layer, and thin films prepared by molecular beam-assisted deposition are characterized with respect to their electrical transport properties. Experimental results are shown for the electrical and sensing properties of several metal oxide nanoparticle ensembles and the influence of porosity is investigated during compaction of nanoparticle powders exposed to an external force. A model describing these properties is developed and it is shown that for a given material only porosity, geometry, and particle size influence the overall electrical properties. The model developed for the description of current transport in particulate matter can also be utilized to describe current-assisted sintering.

Optical and electrical properties of silicon nanoparticles

For the fabrication of optoelectronic devices based on silicon nanoparticles (Si-NPs), it is very important to understand their optical and electrical behavior. In this paper, we present the optical and electrical properties of Si-NPs. We demonstrate that the optical properties of Si-NPs depend on their size as well as their surface chemistry. The size of Si-NPs was finely tuned by etching them in a mixture of hydrofluoric acid (HF) and nitric acid (HNO3) for different times. The resulting Si- NPs exhibit bright luminescence across the visible spectrum. In order to stabilize the optical emission, the surface of freshly etched Si-NPs was successfully functionalized with organic molecules.As the surface chemistry is also expected to strongly influence the electrical transport between Si-NPs and therefore the electrical properties of Si-NP ensembles, the conductivity of pellets consisting of Si-NPs was measured using impedance spectroscopy. The surface oxide of Si-NPs was removed by etching them with HF acid. The freshly etched Si-NPs showed much higher conductivity compared to as-prepared samples. The surface functionalization of freshly etched Si-NPs slightly decreases their conductivity. However, it was observed that the conductivity was still much higher compared to as-prepared samples.

Stable aqueous dispersions of ZnO nanoparticles for ink-jet printed gas sensors

For the preparation of printable devices based on ZnO nanoparticles (ZnO NP), stable colloidal dispersions of these materials are highly desirable. ZnO NP have been synthesized by Chemical Vapor Synthesis. The particles have a spherical shape with a narrow size distribution. Stable aqueous dispersions of the ZnO NP have been successfully prepared after the addition of a polymeric stabilizer. The prepared dispersions are stable for at least 2 months without observable sedimentation. These stable dispersions are used to prepare ZnO NP films on different substrates by ink-jet printing. The viscosity and the surface tension of the dispersion as well as the printing parameters have been optimized for forming layers with high quality. Dense and low porosity layers of ZnO NP with a thickness between 100–250 nm have been prepared on different substrates. First measurements on ink-jet printed ZnO films are done on self fabricated inter digital capacitors (IDCs) at room temperature. The ZnO films show resistivity at room temperature of 7.76 kΩ.cm. For sensing measurements in hydrogen atmosphere, the sheet resistance decreases rapidly until it reaching metallic behavior. This behavior is reversible.