Ragnhild Sæterli

Transparent and conducting ITO thin films by spin coating of an aqueous precursor solution

An environmentally friendly aqueous sol–gel process has been developed to fabricate thin films of indium tin oxide (ITO). A stable sol was prepared from indium nitrate and tin acetate precursors together with acetic acid and ethylene glycol. The sol transformed into an amorphous gel during heating, which decomposed and crystallized further to nano-crystalline ITO at ∼300 °C. The nano-crystalline ITO powders prepared from the precursor gel were homogeneous and single phase with particle sizes around 15 nm. The aqueous sol was applied for spin coating of ITO films on glass substrates. The deposited thin films were homogeneous and continuous with no cracks or pin-holes and exhibited very good and reproducible optical transparency and electrical conductivity, with a specific resistance of 4.59 × 10−3 Ω cm, thereby demonstrating the potential of this Pechini related sol–gel process. Experiments to determine the robustness of the process with respect to the concentration of the precursors and substitution of the organic components with other alcohols or acids were also performed, and some important aspects of the chemistry of the sol–gel process are addressed.

Influence of Volatile Chlorides on the Molten Salt Synthesis of Ternary Oxide Nanorods and Nanoparticles

A molten salt synthesis route, previously reported to yield BaTiO3, PbTiO3, and Na2Ti6O13 nanorods, has been re-examined to elucidate the role of volatile chlorides. A precursor mixture containing barium (or lead) and titanium was annealed in the presence of NaCl at 760 or 820 degrees C. The main products were respectively isometric nanocrystalline BaTiO3 and PbTiO3. Nanorods were also detected, but electron diffraction revealed that the composition of the nanorods was respectively BaTi2O5/BaTi5O11 and Na2Ti6O13 for the two different systems, in contradiction to the previous studies. It was shown that NaCl reacted with BaO (PbO) resulting in loss of volatile BaCl2 (PbCl2) and formation and preferential growth of titanium oxide-rich nanorods instead of the target phase BaTiO3 (or PbTiO3). The molten salt synthesis route may therefore not necessarily yield nanorods of the target ternary oxide as reported previously. In addition, the importance of NaCl(g) for the growth of nanorods below the melting point of NaCl was demonstrated in a special experimental setup, where NaCl and the precursors were physically separated.

Molten salt synthesis of K4Nb6O17, K2Nb4O11 and KNb3O8 crystals with needle- or plate-like morphology

Here we report on molten salt synthesis of four different compounds in the K2O–Na2O–Nb2O5 system. The three compounds K4Nb6O17, K2Nb4O11 and KNb3O8, with non-cubic crystal structures, were prepared as single crystalline particles with large aspect ratios. K4Nb6O17 was prepared with a plate-like morphology, K2Nb4O11 a fibre-like and finally KNb3O8 had a plate like morphology. KxNa1−xNbO3, with a cubic perovskite crystal structure at the synthesis conditions, was only obtained as cube-shaped crystals, which became larger with increasing synthesis temperature. A systematic study of the influence of the processing conditions on the morphology of the product compounds was conducted. The study demonstrated that the morphology of the product phase was controlled by the crystal structure and the connectivity of the NbO6-octahedra of the crystal structures. Finally, the crystal structure of K2Nb4O11 was determined to be a tungsten bronze type structure with space groupP4/mbm.

Polarization control in ferroelectric PbTiO3 nanorods

In this work we demonstrate by transmission electron microscopy and piezoresponse force spectroscopy that the polarization direction in hydrothermally synthesized lead titanate (PbTiO3) nanorods can be changed from parallel to the nanorod axis to perpendicular to it by a simple heat treatment above the Curie temperature. The heat treatment also introduced 90° domains, caused a rearrangement of the surface and a reduction in the amounts of defects. The polarization of the heat-treated nanorods could be successively switched in the direction perpendicular to the nanorod axis. This control of the polarization in PbTiO3 nanorods opens up possibilities of tailoring the ferroelectric properties and is therefore highly relevant for the use of ferroelectric nanorods in devices.

Experimental and theoretical study of electron density and structure factors in CoSb3

We refine two low-order structure factors of the skutterudite CoSb₃ using convergent beam electron diffraction. The relatively large unit cell of this material causes the disks to overlap and introduces a series of challenges in the refinement procedure. These challenges and future work-arounds are discussed. The refined structure factors F₂₀₀ and F₆₀₀ are compared to X-ray diffraction and density functional calculated values, the latter calculated using two different functionals. Both relaxed and experimental lattice parameters are tested to explicitly highlight the impact of the lattice geometry and atomic position on the structure factors.

Detailed TEM characterization of PbTiO3 nanorods

1D functional oxides at nm-scale are interesting for fundamental reasons and promising for future applications. Here, ferroelectric PbTiO3 nanorods, produced through a hydrothermal process, have been studied in detail by transmission electron microscopy. The length (up to one μm) and the diameter (30–100 nm) as well as the growth direction ([001]) of the nanorods could easily be determined using conventional imaging and electron diffraction techniques. However, variations along the length of the rods were clearly visible in the bright field images. Steps on the outer surfaces of the rods could be identified using energy filtered transmission electron microscopy and spectrum imaging thickness maps. The thickness variation parallel to the electron beam affected the bright field contrast and energy dispersive spectroscopy of the nanorods. From cross-sectional specimens, it was determined that the outer surfaces of the rods were dominantly {110} type, leading to a rectangular cross-section. The cross section diameter of the rods was reduced by the introduction of {100} surfaces. In addition, the cross-sectioned specimen revealed the presence of internal channels in the growth direction, especially in the bottom part of the rods. Such a detailed structural description of the nanorods was necessary to study the possible ferroelectric domain structure and to reveal the growth mechanism of the rods.