Matthias Dietze

Effects of rare earth manganites on structural, ferroelectric, and magnetic properties of BiFeO3 thin films

The results of structural studies of pure-phase perovskite thin films of BiFeO3–10% RMnO3 (R=La, Eu, Gd, Tb, and Dy) are presented. Raman scattering studies show line broadening similar to what is reported for RMnO3; they were attributed to Jahn–Teller distortion and orthorhombic structural change. Evidence of an anomaly of the orthorhombic distortion at GdMnO3 could be obtained. Interestingly the addition of GdMnO3 leads to a substantial increase in magnetization, combined with moderate ferroelectric polarization. All other compositions are characterized by higher ferroelectric polarization but almost no magnetization.

Thick film polymer-ceramic composites for pyroelectric applications

Thick films of 0-3 composites of lead-zirconate-titanate ceramic and polyvinylidene-trifluorethylene copolymer have been produced by spin coating on gold-coated silicon wafers. The dielectric properties were investigated as a function of ceramic volume fraction and temperature. Pyroelectric measurements were undertaken by temperature modulation with a Peltier element. Additionally, the pyroelectric response has been investigated up to 3000Hz using a modulated laser. The piezoelectric response of the composites obtained by using a laser vibrometer are also reported. It is shown that the dielectric constant increases with increasing volume fraction of ceramic and that it reaches a maximum at a temperature in the range of 65–70°C due to the ferroelectric-paraelectric phase transition of the polymer matrix. The pyroelectric coefficient increases to 92μCm−2K−1 at a ceramic volume fraction of 20%. Furthermore the effective piezoelectric charge coefficient d33 of the composite almost vanishes at this composition...

Orientation and phase transition dependence of the electrocaloric effect in 0.71PbMg1/3Nb2/3O3-0.29PbTiO3 single crystal

In this letter, the electrocaloric effect (ECE) of 〈111〉- and 〈001〉-oriented 0.71PbMg1/3Nb2/3O3-0.29PbTiO3 (0.71PMN-0.29PT) single crystals is investigated with emphasis on the effects of phase transitions and their dependence on electric field. We show that crystal orientation and more specifically the complex phase transitions taking place in the specific composition above have large effects on the maximum ECE temperature, ΔTmax, its peak temperature, TEC, and the width of the ECE peak of the ΔT(T) curve. The investigation shows that ECE may be tuned by a proper choice of 0.71PMN-0.29PT crystal orientations.

Multiferroic properties of Bi0.9Gd0.1Fe0.9Mn0.1O3 thin film

Monolithic oxides whose lattice structures exhibit both ferroelectric and magnetic ordering and eventually intrinsic coupling between them are known as multiferroics. The possibility to switch magnetization by an electric field and reciprocally polarization by a magnetic field may allow new applications spanning the fields from new memory devices to medical measuring technology. Until now, the only monolithic oxide that has been shown to exhibit both (weak) magnetism and ferroelectric polarization at room temperature is BiFeO3 (BFO). However, as this material is a line compound, i.e., with sharply defined stoichiometry, its properties largely depend on defects and parasitic phases that could arise during processing. In the present work, we report on a new compound based on the solid solution GdMnO3–BiFeO3 (GdMBFO) that exhibit a fairly high magnetization coupled with moderate ferroelectric polarization at room temperature. Using conductive-tip atomic force microscopy, we show the highly electrically homog...

Macro–meso-porous TiO2, ZnO and ZnO–TiO2-composite thick films. Properties and application to photocatalysis

We report on a new and versatile method for preparing homogenous, crack-free and macro–meso-porous thick films of TiO2, ZnO and ZnO–TiO2 composites using a single step coating procedure and fairly low annealing temperatures of a maximum of 400 °C. The method relies on an oxide nanopowder filler in an optimized precursor solution. The films are well adherent to the substrate, show homogeneously distributed open porosity, and are hydrophilic. The photocatalytic activity of these films was characterized using methyl orange as a model dye. We show that the TiO2 films have better photocatalytic activity than ZnO and ZnO–TiO2 composite films. The advantages of these composite films over particulate photocatalysts lie in their robustness and ease of application as no filtration is needed. Furthermore it is possible to apply them on suitable large area membranes.

Morphology control of 1D noble metal nano/heterostructures towards multi-functionality

We synthesize Pt, Pd, Au and layered multimaterial nanotubes via electrodeposition in anodic aluminium oxide (AAO) templates. We show that time controlled growth at constant electrolyte concentration allows the morphology and chemistry of the nanostructure to be controlled to a high degree. In particular the wall thickness of the nanotubes can be tuned from thin wall nanotubes to nanorods. The applications targeted are electrocatalysis, plasmon resonance and molecular detection via surface enhanced Raman spectroscopy (SERS). The electro-oxidation of methanol on Pt and that of formic acid on Pd are investigated and compared with state of the art Pt/C and Pd/C catalysts and some of the recent similar work from the literature. Nanotubes of both metals (Pt, Pd) show very high electrochemical surface area (ECSA) and enhanced electrocatalytic activity. Our electrocatalysis results show great promise towards direct liquid fuel cells. In terms of molecular detection using SERS we show that the SERS activity of Au-nanostructures is strongly dependent on morphology with very high SERS enhancements being related to partly filled Au-nanotubes.

Tuning the functional properties of PMN-PT single crystals via doping and thermoelectrical treatments

Single crystals based on solid solutions of lead-magnesium-niobate (PMN) and lead titanate (PT) have emerged as highly promising multifunctional systems combining piezoelectric, pyroelectric, and electro-optic properties that surpass by far those of the best known lead-zirkonium-titanate ceramics. In this paper we present new findings on how the phase transition temperature and the dielectric and ferroelectric properties can be tuned depending on crystal composition, orientation, and thermoelectrical treatment. Mn-doped and pure 0.72PbMg1/3Nb2/3O3-0.28PbTiO3 (0.72PMN-0.28PT) single crystals with ⟨111⟩ and ⟨001⟩ orientations were investigated. A special attention was devoted to field cooling (FC), i.e., cooling under electric field from different temperatures. The results illustrate different findings that were not reported before: the Curie temperature, i.e., ferroelectric-paraelectric transition temperature, is enhanced after field cooling of the Mn-doped, ⟨001⟩-oriented crystal while such a shift is not...

A universal, template-free approach to porous oxide and polymer film processing

We have developed a unique, template-free method for the processing of porous oxide and polymer films with a high degree of ordered porosity, taking advantage of micro-detonation. When mixing an alcoholic solution of ammonium or silver nitrate with an alcoxide precursor solution, a phenomenon of self-organization of immiscible organic solvent (Fuel)–nitrate(oxidant) micro-droplets occurs during film formation. Thermal activation leads to self-ignition reactions of the droplets resulting in the exothermic formation of gas species that upon expansion leave a porous structure behind. We demonstrate the method on thin TiO2 films and show that hierarchical porosity may be obtained by increasing the film thickness. In order to prove the universality of the method we apply it to a polymer solution, and achieve a nicely ordered hierarchical porosity in a 20 μm thick polymer film.

Template-assisted generation of three-dimensionally branched titania nanotubes on a substrate

Due to their high aspect ratio titania nanotubes have more surface area that enhances their ability towards harnessing light. Herein, we report on a template-assisted method to synthesize branched titania nanotubes anchored on a substrate. The branched structure consists of units of six or more tubes connected at their base; each tube has a dimension of approximately 6 μm long, 600 nm wide and 65 nm thick wall. Our approach takes advantage of hydrothermally processed branched ZnO nanorod (NR) arrays that act as sacrificial templates. After chemical dissolution of the ZnO film that inherently forms during ZnO–NR processing, the titania overlayer is coated atop of the template and annealed at 350 °C to produce the anatase nanostructure. Removal of the template under mild acidic conditions reveals the anatase tubes. The processes employed are well suited for large-scale application. The highly textured surface of the nanotubes also exhibits low reflectivity when compared to its thin film counterpart.

Single domain vs. polydomain [111] 0.72Pb(Mg1/3Nb2/3)O3-0.28PbTiO3 single crystal. Polarization switching, dielectric and pyroelectric properties

We introduce a poling treatment to induce a single domain structure in [111] single crystals of 0.72Pb(Mg1/3Nb2/3)O3-0.28PbTiO3 (PMN-PT). The properties obtained differ from those known for domain-engineered [111] PMN-PT single crystals. The dielectric, ferroelectric, and pyroelectric properties of this single domain structure are assessed in comparison to polydomain single crystals that were poled by conventional poling treatment. The attributes of the single domain state are a single switching event that reflects in a discontinuity of polarization at the coercive field, a substantially lower dielectric loss (than the multidomain state), and a higher detectivity for pyroelectric applications