Production and characterisation of novel kinds of cluster matter and their behaviour under the influence of hydrogen and oxygen

With the decrease of the number of atoms in a solid state and hence its size (0.5 nm - 100 nm), new and exciting properties can be discovered. These novel materials fill the gap between homogeneous bulk material and single atoms or molecules. Physics, chemistry and materials science can all profit from this new matter. Many methods have been applied to create such materials. During the present thesis a novel UHV cluster source was used, which allows all measurements to be performed in-situ in an UHV environment. This cluster machine is called LUCAS, which stands for Laser Universal Cluster Ablation Source. It utilizes a high power laser system to ablate or evaporate atoms from a target under a 1 bar Ar atmosphere, which then condensate into nanoparticles using adiabatic expansion. The choice of possible materials is therefore quite large. The fabrication of different nanoparticle systems is an important aspect of this thesis. Apart from in-situ optical measurements, it was necessary to integrate the possibility to do very sensitive electrical measurements on percolated nanoparticles in-situ as well. We are now capable of measuring as low as 50 fA ± 10 fA. This is particularly useful, when only a few percolated nanoparticles are part of the conduction pathway. The particles were gathered on a substrate, which was either a quartz plate with gold contacts on the edges or a lithographically designed chip. Optical measurements were combined with electrical ones to obtain data on the clusters. Ex-situ electron microscopy, which was performed on a regular basis, delivers additional information. Optics can show the behavior of single, well-separated particles to some extent, even if the number of particles is quite high. Electrical measurements can only show the behavior of an entire coupled ensemble (with a percolated conduction pathway). If this ensemble is large, it is difficult to distinguish size effects. In this thesis first large particle ensembles of different materials were investigated by both optical and electrical means. If necessary, an effort was made to decrease the number of particles. To study few-particle-effects a chip geometry was designed in conjunction with Lund technical university. The particles were deposited on top of and between these contacts. Optical measurements are no longer possible on the chip. The electrical measurements revealed important information about a single nanoparticle. The measurements and their interpretation form the second important aspect of this thesis. The first materials group is that of yttrium and gadolinium. Both have many interesting properties. We looked primarily at their reaction with hydrogen (which is partly reversible at room temperature) and with oxygen. They undergo two electronic phase changes during hydrogenation. It was unclear, whether these electronic phase transitions occur abrupt or gradual. Our investigations answered this question. It is most likely abrupt. During the present thesis it was also revealed, that yttrium reacts very sensitively to even small hydrogen concentrations. The experimental substrate geometry could therefore be used as a blueprint for a hydrogen sensor prototype. Similar hydrogenation experiments were performed on Gd nanoparticles 12 nm in size, which also possess reversible optical features under the influence of hydrogen due to an electronic phase transition, which was previously unknown. The second materials group included indium oxide, tin oxide and tin doped indium oxide. Remarkably, these materials have good conductive properties, as well as being transparent in the visible range of the spectrum, which has interesting applications (for example in solar cells). Nanoparticles created from these materials have similar properties, such as good conductivity and high transmission in the VIS. During previous work in our group on ITO with a specific chemical composition it was found, that ITO predominantly condensates into rods, i.e. particles with different axis. It was shown in the present thesis, that the geometry of the particles depends heavily on the parameters used during production. It can either be more rod-like or more cube-like. This result might also have importance for the ITO thin film industry. Furthermore, it was found, that ITO, indium oxide and tin oxide nanoparticles show extraordinary behaviour under an oxygen atmosphere, and can be used as oxygen sensors. The third material, which was investigated, is Ni, for which the controlled oxidation was examined. Our experiments proved, that it is not straight-forward to produce nickel nanoparticles, without oxygen impairing the results. However, it was shown, that the effects of oxygen can be treated in a controlled manner.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2003

Author dc:creator

• Bour, Giorah

Contributors dc:contributor

• Kreibig, Uwe

• info:eu-repo/classification/ddc/530
• Physik
• nanoparticle
• optical spectroscopy
• electrical resistance
• ITO
• indium oxide
• tin oxide
• yttrium
• gadolinium
• nickel
• hydrogenation
• oxidation

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

eng

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-123528%22

OAI identifier oai:identifier

oai:publications.rwth-aachen.de:61916