M.K. Kennedy

Tailored nanoparticle films from monosized tin oxide nanocrystals: Particle synthesis, film formation, and size-dependent gas-sensing properties

In order to investigate the change of gas-sensitive properties of undoped tin oxide nanoparticle films depending on particle size, a thin film synthesis technique has been developed. Well-defined tin oxide nanoparticles have been prepared using a gas-phase condensation method. Pure SnO was used as starting material and was evaporated at T=820 °C. The resulting particles were sintered and crystallized in-flight at T=650 °C. Size-selected nanoparticles ranging from 10 to 35 nm were produced to form a nanoparticle film by means of electrostatic precipitation or low pressure impaction. The effect of in-flight oxidation, sintering, and crystallization on the structure, size, and size distribution of nanoparticles have been studied in detail. The samples show n-type semiconductors’ behavior like bulk SnO2. The influence of particle size on gas sensitivity and response behavior is investigated for C2H5OH at operating temperatures 200–300 °C using silicon substrates having an interdigitated contact pattern and an...

Nanoparticles from the Gas Phase as Building Blocks for Electrical Devices

Electrical device development is driven by miniaturization and possibilities to use new chemical and physical effects. Nanotechnology offers both aspects. The structural dimensions of materials and devices are small and because of that large exchange surfaces are provided but also effects like quantum effects may occur and be used to get new or at least improved properties of nanostructured materials and devices.Nanoparticles are of special interest because of their nanodimensions in all three directions, so that nanoeffects become most prominent. They can be synthesized in solid materials, in liquids and in gases. Gas synthesis has several advantages compared to the other phases, especially the high cleanliness which can be achieved. In case of electrical devices the particles have to be deposited onto substrates in a structured way.The substrate may consist out of microelectronic devices in which the deposited nanoparticles are introduced for the basic function. In case of a transistor this would be the gate function, in case of a sensor this would be the sensing layer, where the contact with the measurement object takes place. For two kinds of particles SnO2 and PbS, synthesized in the gas phase, we demonstrate the way how to create devices with improved sensor properties.

Quantum size effect of valence band plasmon energies in Si and SnOx nanoparticles

Spherical Si and SnOx nanoparticles in the size range between 3 and 30nm have been synthesized by microwave induced decomposition of silane and gas phase condensation, respectively. They are deposited on thin metal films and investigated by electron microscopy, Auger electron, and electron energy loss spectroscopy. An analysis of the surface composition and stoichiometry reveals that the Si particles are covered with a native oxide of less than 1nm. The energy loss spectra show features corresponding to electronic excitations in the nanoparticles due to valence band plasmons, interband transitions, and core-level ionizations. The plasmon energies are found to increase with decreasing particle diameter d as d−1.17 for Si and d−0.83 for SnOx. These energy shifts are related to the change of the dielectric band gap energy of the semiconductor due to quantum size effects.

Vibrational and defect states in SnOx nanoparticles

We have studied SnOx nanoparticles fabricated by gas-phase condensation and in-flight sintering using Raman and photoluminescence (PL) spectroscopy. We are able to identify various vibrational states of the rutile phase of the SnOx crystal. By thorough analysis of the vibrational modes, we are able to determine the bond lengths of the O–O and Sn–O bonds for the substoichiometric SnO1.5, leading, together with x-ray diffraction data, to a full characterization of the SnO1.5 lattice. In absorption and photoluminescence spectra, we observe a finite density of states inside the band gap due to oxygen vacancies, giving rise to a midgap luminescence peak. Our results suggest that the defect related luminescence efficiency is limited by nonradiative recombination processes and by the oxygen vacancy density. We therefore conclude that the PL intensity has a maximum around a stoichiometry of SnO1.7.

Fully automated, gas sensing, and electronic parameter measurement setup for miniaturized nanoparticle gas sensors

In this study, a measurement setup has been designed and fabricated for the measurement of gas sensor characteristics and electronic parameters of nanostructured thin layers in the temperature range from room temperature to 450 °C in controlled gas environments. The setup consists of: (i) a gas environment chamber, (ii) a specially designed substrate and substrate holder, and (iii) control, supply, and measurement electronics. The buried geometry of the contacts is specially designed for the deposition of nanoparticles from the gas phase to guarantee uniform thin layers, and the setup can be used to make measurement on high resistivity (1010 Ω cm) nanoparticle samples. The gas inlet, operating temperature, and electronic control of the measurement system are automated by means of a personal computer. Coupling the measurements of interdependent gas sensing and electronic parameters at identical conditions, in a single setup encompassing a wide range of sensing gas levels and substrate temperatures, makes t...

Synthesis and characterization of monosized SnOx nanoparticles for gas sensing applications

In this paper we present an evaporation-condensation route to produce single sized SnOx nanoparticles using a Differential Mobility Analyser (DMA). The nanostructure and size distribution of the particles were studied as a function of the evaporation and sintering temperature. Transmission Electron Microscope (TEM) micrographs were taken to study the morphology of the nanoparticles. X-ray diffraction (X-ray), Rutherford backscattering (RBS), Auger electron microscopy (AES) and electron diffraction measurements were used to study the structure and stoichiometry of the deposited material. After an annealing process at Tequals300 degree(s)C in synthetic air particle size dependent conductivity measurements of monosized 10nm, 20nm and 30nm particles, such as Sensitivity S and dynamic behavior of the thin films acting as a gas sensor, were done.