Ferroelectric materials have applications in next-generation electronics devices from optoelectronic modulators and random access memory to piezoelectric transducers and tunnel junctions. Now researchers at Tokyo Institute of Technology report insights into the properties of epitaxial hafnium-oxide-based (HfO2-based) thin films, confirming a stable ferroelectric phase up to 450°C ("Growth of epitaxial orthorhombic YO1.5-substituted HfO2 thin film"). As they point out, "This temperature is sufficiently high for HfO2-based ferroelectric materials to be used in stable device operation and processing as this temperature is comparable to those of other conventional ferroelectric materials." Reports of ferroelectric properties in thin films of substituted hafnium-oxide—where some ions were replaced with other metals—have attracted particular interest because these films are already used in electronics and are compatible with the silicon fabrication techniques that dominate the industry. However attempts to study the crystal structure of HfO2-based thin films in detail to understand these ferroelectric properties have met with challenges due to the random orientation of the polycrystalline films. In order to obtain thin films with a well-defined crystal orientation, Takao Shimizu, Hiroshi Funakubo and colleagues at Tokyo Institute of Technology turned to a growth approach that had not been tried with HfO2-based materials before—epitaxial film growth. They then used a range of characterisation techniques—including x-ray diffraction analysis and wide-area reciprocal space mapping—to identify changes in the crystal structure as the yttrium content increased. They found a change from a low- to a high-symmetry phase via an interim orthorhombic phase with increasing yttrium from -15% substituted yttrium oxide. 
The x-ray diffraction patterns with inclination angle of 45° observed for 0.07YO1.5-0.93HfO2 film measured from room temperature to 600°C. (b) The integrated intensity of the 111 super-spot of 0.07YO1.5-0.93HfO2 film as a function of temperature. Further studies confirmed that this orthorhombic phase is ferroelectric and stable for temperatures up to 450°C. They conclude, "The present results help to clarify the nature of ferroelectricity in HfO2-based ferroelectric materials and also its potential application in various devices." Background Hafnium oxide thin films The dielectric constant (high-κ) of HfO2 has previously attracted interest for use in electronics components such as dynamic random-access memory (DRAM) capacitors and is already used for high-κ gates in devices. As a result its compatibility with the CMOS processing that dominates current electronics fabrication is already known. Ferroelectric properties have been reported in HfO2 thin films with some hafnium ions substituted by different types of ions including yttrium, aluminium and lanthanum, as well as silicon and zirconium. The researchers studied HfO2 films substituted with the yttrium oxide YO1.5 as ferroelectric properties have already been reported in films of this material. Epitaxial growth Well-defined crystal orientation with respect to the substrate can be obtained in epitaxially grown films but the process usually requires high temperatures. Due to the tendency to decompose into non-ferroelectric phases HfO2 are usually prepared by crystallization of amorphous films. The researchers used pulsed laser deposition to prepare epitaxially grown HfO2-based films without destroying the ferroelectric phase. The films were grown on yttria-stabilised zirconia and were around 20 nm thick. Crystal phases and characterization HfO2 exists in a stable low-symmetry monoclinic phase, where the structure resembles rectangular prism with a parallelogram base. This structure changes to a high-symmetry cubic or tetragonal structured phase through a metastable orthorhombic phase. Monoclinic, cubic and tetragonal crystalline structures have inversion centres, which rule out ferroelectric properties. Therefore the researchers focused on the orthorhombic. The coexistence of several phases in HfO2 further complicates studies of crystal structure, making it yet more desirable to obtain films with well-defined crystal orientations. Prior to the current work it was still unclear whether epitaxial growth of HfO2-based films was possible. Previous work had used transmission electron microscopy and simultaneous convergent beam electron diffraction to confirm the existence of the orthorhombic phase, but more detailed analysis of the crystalline structure proved difficult due to the random polycrystalline orientation. With the epitaxially grown thin films the researchers were able to use x-ray diffraction analysis and wide-area reciprocal space mapping measurements to identify the orthorhombic phase. They then used aberration-corrected annular bright-field and high angle annular dark field scanning transmission electron microscopy to confirm that the orthorhombic phase was ferroelectric.
read more "Thin films offer promise for ferroelectric devices"
The x-ray diffraction patterns with inclination angle of 45° observed for 0.07YO1.5-0.93HfO2 film measured from room temperature to 600°C. (b) The integrated intensity of the 111 super-spot of 0.07YO1.5-0.93HfO2 film as a function of temperature. Further studies confirmed that this orthorhombic phase is ferroelectric and stable for temperatures up to 450°C. They conclude, "The present results help to clarify the nature of ferroelectricity in HfO2-based ferroelectric materials and also its potential application in various devices." Background Hafnium oxide thin films The dielectric constant (high-κ) of HfO2 has previously attracted interest for use in electronics components such as dynamic random-access memory (DRAM) capacitors and is already used for high-κ gates in devices. As a result its compatibility with the CMOS processing that dominates current electronics fabrication is already known. Ferroelectric properties have been reported in HfO2 thin films with some hafnium ions substituted by different types of ions including yttrium, aluminium and lanthanum, as well as silicon and zirconium. The researchers studied HfO2 films substituted with the yttrium oxide YO1.5 as ferroelectric properties have already been reported in films of this material. Epitaxial growth Well-defined crystal orientation with respect to the substrate can be obtained in epitaxially grown films but the process usually requires high temperatures. Due to the tendency to decompose into non-ferroelectric phases HfO2 are usually prepared by crystallization of amorphous films. The researchers used pulsed laser deposition to prepare epitaxially grown HfO2-based films without destroying the ferroelectric phase. The films were grown on yttria-stabilised zirconia and were around 20 nm thick. Crystal phases and characterization HfO2 exists in a stable low-symmetry monoclinic phase, where the structure resembles rectangular prism with a parallelogram base. This structure changes to a high-symmetry cubic or tetragonal structured phase through a metastable orthorhombic phase. Monoclinic, cubic and tetragonal crystalline structures have inversion centres, which rule out ferroelectric properties. Therefore the researchers focused on the orthorhombic. The coexistence of several phases in HfO2 further complicates studies of crystal structure, making it yet more desirable to obtain films with well-defined crystal orientations. Prior to the current work it was still unclear whether epitaxial growth of HfO2-based films was possible. Previous work had used transmission electron microscopy and simultaneous convergent beam electron diffraction to confirm the existence of the orthorhombic phase, but more detailed analysis of the crystalline structure proved difficult due to the random polycrystalline orientation. With the epitaxially grown thin films the researchers were able to use x-ray diffraction analysis and wide-area reciprocal space mapping measurements to identify the orthorhombic phase. They then used aberration-corrected annular bright-field and high angle annular dark field scanning transmission electron microscopy to confirm that the orthorhombic phase was ferroelectric.
Martina Abrigo Using a technique called electrospinning – in which polymer filaments 100 times thinner than a human hair are squeezed out of an electrified nozzle – Ms Abrigo and her colleagues have made nanofibre meshes that can draw bacteria from a wound. In the first phase of research polymer nanofibres were placed over the top of films of Staphylococcus aureus, a bacterium involved in chronic wound infection. The researchers found the bacteria quickly attached to the fibres. When the fibres were smaller than the individual bacteria, fewer cells attached and those that did attach died as they attempted to wrap around the fibre. In the second phase, the tiny nanofibres were coated with different compounds and tested on the bacteria Escherichia coli, also commonly found in chronic wounds. The researchers found these bacteria rapidly transferred onto fibres coated with allylamine, independent of the fibre size, but did not attach to fibres coated with acrylic acid. In the third phase of research, the nanofibre meshes have been tested on tissue-engineered skin models in a partnership with researchers at the University of Sheffield in the UK. The results of this research are yet to be published, but indicate that similar effects could be seen in living tissue. “For most people, wounds heal quickly. But for some people, the repair process gets stuck and so wounds take much longer to heal. This makes them vulnerable to infection,” Mas Abrigo said. “We hope this work will lead to smart wound dressings that could prevent infections. Doctors could put a nanomesh dressing on a wound and simply peel it off to get rid of the germs.” A paper on bacterial response to meshes with different fibre diameters was published in ("Electrospun Polystyrene Fiber Diameter Influencing Bacterial Attachment, Proliferation, and Growth"). A paper investigating the effect of fibre surface chemistry on bacterial behaviour was published in ("Bacterial response to different surface chemistries fabricated by plasma polymerization on electrospun nanofibers").
Supramolecular nanofiber consisting of more than 10,000 perfectly ordered building blocks, which enables an energy transport over a distance of more than 4 micrometers at room temperature. (Image: A. T. Haedler) The research groups of Richard Hildner in Experimental Physics and Hans-Werner Schmidt in Macromolecular Chemistry at the University of Bayreuth prepared supramolecular nanofibers, which can consist of more than 10,000 identical building blocks. The core of the building block is a so-called carbonyl-bridged triarylamine. This triarylamine derivative was synthesized by the research group of Milan Kivala in Organic Chemistry at the University of Erlangen-Nuremberg and chemically modified at the University of Bayreuth. Three naphthalimidbithiophene chromophores are linked to this central unit. Under specific conditions, the building blocks spontaneously self-assemble and form nanofibers with lengths of more than 4 micrometers and diameters of only 0.005 micrometer. For comparison: a human hair has a thickness of 50 to 100 micrometers. With a combination of different microscopy techniques the scientists at the University of Bayreuth were able to visualize the transport of excitation energy along these nanofibers. In order to achieve this long-range energy transport, the triarylamine cores of the building blocks, that are perfectly arranged face to face, act in concert. Thus, the energy can be transferred in a wave-like manner from one building block to the next: This phenomenon is called quantum coherence. “These highly promising nanostructures demonstrate that carefully tailoring materials for the efficient transport of light energy is an emerging research area” says Dr. Richard Hildner, an expert in the field of light harvesting at the University of Bayreuth. The research area light harvesting aims at a precise description of the transport processes in natural photosynthetic machineries to use this knowledge for building novel nanostructures for power generation from sunlight. In this field interdisciplinary groups of researchers work together in the Bavarian initiative Solar Technologies Go Hybrid and in the Research Training Group Photophysics of synthetic and biological multichromophoric systems (GRK 1640) funded by the German Research Foundation (DFG).