Michael Kracker

Textures of Au, Pt and Pd/PdO nanoparticles thermally dewetted from thin metal layers on fused silica

This article presents results concerning the basic dewetting procedure of thin metal films on an amorphous substrate. Thin layers of gold, platinum and palladium are deposited on fused silica glass sheets by sputtering. The 14 to 18 nm thick layers of Au, Pt and Pd are dewetted into nanoparticles (NP) at temperatures from 424 to 1600 °C. The morphology of the resulting NP is characterized using scanning electron microscopy (SEM) while crystallographic analyses are performed using electron backscatter diffraction (EBSD). Multiple textures are described to occur and coexist in NP-layers of the respective materials and a temperature dependence of the texture formation in gold NP is proved. The transformation of Pd to PdO and its effects on the texture is also analyzed. It is shown that the cooling rate has an effect on the texture in the case of gold NP as well as on the physical structure of the Pd/PdO NP. Twinning relationships could be described to occur on all samples.

Oriented growth of a β-quartz solid solution from a MgO–Al2O3–SiO2 glass coated by a sol–gel ZrO2 layer

Glass sheets with the mol% composition 22.5MgO·22.5Al2O3·55SiO2 were coated with a thin ZrO2 layer on both sides via sol–gel dip coating. The samples were heated in a two-step process to 850 °C for 1 h and to 1000 °C for another 3 or 20 h. X-ray diffraction (XRD), optical microscopy and scanning electron microscopy (SEM), including electron backscatter diffraction (EBSD), were performed to investigate the crystallization of this surface modified glass. Two types of indialite and a β-quartz solid solution (β-QSS) crystallize at, or near, the glass surface. After a growth selection during the first ca. 500 μm of growth, only a highly oriented β-QSS layer aligned with the c-axis perpendicular to the initial surface continues to grow into the bulk. The β-QSS was not observed if the glass was not coated with ZrO2 before annealing. The influence of ZrO2 on the crystallization in this system is discussed.

Surface crystallization of low thermal expansion Ba0.5Sr0.5Zn2Si2O7 from an 8 BaO·8 SrO·34 ZnO·50 SiO2 glass

Thermal treatment of a glass with the composition 8 BaO·8 SrO·34 ZnO·50 SiO2 has led to the crystallization of a Ba1−xSrxZn2Si2O7 solid solution. This solid solution has a very low or even negative thermal expansion. The glass system possesses a strong tendency towards surface crystallization, while bulk nucleation is negligible. The surface crystallization behavior was characterized using differential scanning calorimetry, X-ray diffraction, optical microscopy, and scanning electron microscopy including electron backscatter diffraction. The observed morphology strongly depends on the type of surface pretreatment. Two different surface qualities were compared: a polished surface and a surface obtained by cutting. The as-cut surface shows a significantly enhanced nucleation rate, which leads to smaller crystals caused by a growth selection near to the surface. The crystal orientation related to the inward growth starting from the surface was successfully investigated by EBSD, for the first time, using a recently reported crystal structure. The crystals show a preferred orientation of the c-axis perpendicular to the surface; this has a significant effect on the crack formation.

Replica Extraction Method on Nanostructured Gold Coatings and Orientation Determination Combining SEM and TEM Techniques

In the field of electron microscopy the replica technique is known as an indirect method and also as an extraction method that is usually applied on metallurgical samples. This contribution describes a fast and simple transmission electron microscopic (TEM) sample preparation by complete removal of nanoparticles from a substrate surface that allows the study of growth mechanisms of nanostructured coatings. The comparison and combination of advanced diffraction techniques in the TEM and scanning electron microscopy (SEM) provide possibilities for operators with access to both facilities. The analysis of TEM-derived diffraction patterns (convergent beam electron diffraction) in the SEM/electron backscatter diffraction software simplifies the application, especially when the patterns are not aligned along a distinct zone axis. The study of the TEM sample directly by SEM and transmission Kikuchi diffraction allows cross-correlation with the TEM results.

Redox effects and formation of gold nanoparticles for the nucleation of low thermal expansion phases from BaO/SrO/ZnO/SiO2 glasses

Glasses in the system BaO/SrO/ZnO/SiO2 containing 0.01 and 0.1 mol% gold were used to study the formation of gold nanoparticles with the aim to use them as nucleation agents. In order to promote gold clustering, the glasses were additionally doped with 0.5 mol% Sb2O3. Depending on the heat treatment schedule, Au particle sizes were in the range from 6 to above 50 nm. In contrast to many other gold ruby glass systems, the clustering is completely prevented by the absence of antimony; then the glasses remain colorless. Surprisingly, at higher temperatures, a re-dissolution of gold clusters was also observed, which now allows the formulation of a more comprehensive model concerning the redox and clustering behavior. This growth model is completed by the fact that a high gold concentration enables the stabilization of much smaller Au clusters. Mie theory with the aid of quantum confined size-dependent dielectric functions was successfully used to describe the optical behavior of the gold nanoparticles also for sizes below 10 nm. These results were confirmed using high resolution scanning transmission electron microscopy, including energy dispersive X-ray spectroscopy. It could also be shown that small gold particles up to a size of 50 nm are not effective as nucleating agents.

The effect of different platinum concentrations as nucleation agent in the BaO/SrO/ZnO/SiO2 glass system

The recently reported solid solution of Ba1−xSrxZn2Si2O7 has a similar structure to the high-temperature phase of BaZn2Si2O7 and possesses a coefficient of thermal expansion close to zero or even negative. Without nucleating agents, glasses in this system show solely surface crystallization. In order to stimulate volume crystallization, different quantities of platinum were added to the glass. The characteristic temperatures of the glasses were obtained by differential scanning calorimetry. In order to achieve volume crystallization, a two-step thermal treatment (nucleation and crystal growth) above the glass transition temperature was carried out. The phase identification was performed by X-ray diffraction, and the microstructure was studied by scanning electron microscopy including energy-dispersive X-ray spectroscopy and electron backscatter diffraction.

Photoinduced formation of silver nanoparticles in a new Na_2O/K_2O/CaO/CaF_2/Al_2O_3/ZnO/SiO_2 photo thermal refractive glass: evidence of Ag - AgBr core shell structures

Photo thermal refractive glasses of the type Na2O-SiO2-Al2O3-K2O-CaO-CaF2-ZnO doped with CeO2, Ag2O, SnO2, Sb2O3 were prepared using different concentrations of KBr. A UV irradiation followed by a thermal treatment leads to the formation of Ag-nanoparticles, indicated by the appearance of a plasmon resonance peak. This optical resonance position shifts with increasing KBr concentrations to higher wavelengths caused by the formation of an AgBr shell. The Mie theory with the aid of the optical dispersion of AgBr together with the measured dispersion of the used glasses was successfully applied to describe the optical relation of particle size and AgBr core shell thickness within the given glass. The results were compared with UV-vis-NIR spectroscopy.