Uwe Klemradt

Facile Access to Large-Scale, Self-Assembled, Nacre-Inspired, High-Performance Materials with Tunable Nanoscale Periodicities

Although advances have been reported to mimic the mechanically excellent structure of natural nacre, larger-scale applications are still limited due to time and energy-intensive preparation pathways. Herein, we demonstrate that simple high-shear homogenization of dispersions containing biobased high molecular weight sodium carboxymethyl cellulose (700 kg/mol, CMC) and natural sodium montmorillonite (MTM), serving as the soft energy-dissipating phase and reinforcing platelets, respectively, can be used to prepare large-area and thick films with well-aligned hard/soft nacre-mimetic mesostructure. During this process, core-shell nanoplatelets with intrinsic hard/soft structure form, which then self-assemble into a layered nanocomposite during water removal. The nanoscale periodicities of the alternating hard/soft layers can be precisely tuned by changing the ratio of CMC to MTM, which allows studying the evolution of mechanical properties as a function of the lamellar nanoscale periodicity and fractions of hard to soft material. Remarkable mechanical stiffness (25 GPa) and strength (320 MPa) can be obtained placing these materials among the top end of nacre-inspired materials reported so far. Mechanical homogenization also allows direct preparation of concentrated, yet homogeneous, gel-like dispersions of high nanoclay content, suited to doctor-blade large-area and thick films with essentially the same properties as films cast from dilute dispersions. In terms of functional properties, we report high-transparency, shape-persistent fire-blocking and the ability to surface-pattern via inkjet printing. Considering the simple, fully scalable, waterborne preparation pathway, and the use of nature-based components, we foresee applications as ecofriendly, bioinspired materials to promote sustainable engineering materials and novel types of functional barrier coatings and substrates.

Polarization and lattice strains in epitaxial BaTiO3 films grown by high-pressure sputtering

High-quality BaTiO3 films with thicknesses ranging from 2.9to175nm were grown epitaxially on SrRuO3-covered (001)-oriented SrTiO3 substrates by high-pressure sputtering. The crystal structure of these films was studied by conventional and synchrotron x-ray diffraction. The in-plane and out-of-plane lattice parameters were determined as a function of film thickness by x-ray reciprocal space mapping around the asymmetric (1¯03) Bragg reflection. BaTiO3 films were found to be fully strained by the SrTiO3 substrate up to a thickness of about 30nm. Ferroelectric capacitors were then fabricated by depositing SrRuO3 top electrodes, and the polarization-voltage hysteresis loops were recorded at the frequencies 1–30kHz. The observed thickness effect on the lattice parameters and polarization in BaTiO3 films was analyzed in the light of strain and depolarizing-field effects using the nonlinear thermodynamics theory. The theoretical predictions are in reasonable agreement with the measured thickness dependences, alt...

Detection of filament formation in forming-free resistive switching SrTiO3 devices with Ti top electrodes

We investigated the influence of Ti top electrodes on the resistive switching properties of SrTiO3 thin film devices. Above a Ti layer thickness of 5 nm, the initial resistance is strongly reduced, giving rise to forming-free devices. Hard x-ray photoemission experiments reveal the Ti layer to be composed of several oxide phases, induced by the redox-reaction at the Ti/SrTiO3 interface. Grazing incidence small angle x-ray scattering measurements indicate that the reduction of the SrTiO3 thin film occurs in a filamentary way. We attribute this behavior to the preferential reduction of SrTiO3 thin films along highly defective areas.

Wedgelike ultrathin epitaxial BaTiO3 films for studies of scaling effects in ferroelectrics

To study ferroelectric size effects in heteroepitaxial SrRuO3∕BaTiO3∕SrRuO3 capacitors, ultrathin BaTiO3 layers were deposited in wedge form across SrTiO3 substrates. The wedgelike films were fabricated by using either an off-center substrate-target geometry or via a moveable shutter during high-pressure sputter deposition. The crystallinity, composition, and surface roughness along wedgelike BaTiO3 films were verified by x-ray diffraction, Rutherford backscattering spectrometry, and atomic force microscopy, respectively. The electrical measurements performed at 77K showed that, despite progressive reduction in remanent polarization as the film thickness decreases even the 3.5-nm-thick BaTiO3 film retains a large remanent polarization of 28μC∕cm2.

In situ x-ray reflectivity study of the oxidation kinetics of liquid gallium and the liquid alloy

The low-temperature oxidation kinetics of liquid Ga and the liquid alloy has been investigated by means of in situ x-ray specular reflectivity. A combination of angle-dispersive and energy-dispersive measurements was used to obtain a time resolution of 20 minutes with a sealed-tube reflectometer, allowing us to follow the growth of a natural oxide layer for 1 to 170 hours following preparation of a fresh surface. Oxide layer thicknesses between 10 and 30 A were measured for Ga at and the liquid alloy at , and . The oxidation was found to follow a logarithmic growth law in all cases.

High-resolution x-ray reflectivity study of thin layered Pt-electrodes for integrated ferroelectric devices

The structural interface properties of layered Pt/Ti/SiO2/Si electrodes have been investigated using high-resolution specular and diffuse x-ray reflectivity under grazing angles. Currently this multilayer system represents a technological standard as bottom electrodes for ferroelectric thin-film applications. For the electronic and ferroelectric properties of integrated devices, the film-electrode interface is of crucial importance. We focused on Pt 1000 A/Ti 100 A/SiO2/Si electrodes prepared under annealing conditions as employed in industrial processing, prior to the deposition of ferroelectric films. The comparison between annealed and non-annealed electrodes clearly revealed strong interfacial effects due to interdiffusion and oxidation of Ti, especially at the Pt-Ti interface. Migration of Ti into the Pt layer results in a shift of the critical angle due to the enclosure of TiO2-x within the Pt layer. The heterogeneous distribution of TiO2-x suggests a diffusion mechanism mainly along the Pt grain boundaries. At the SiO2 interface a relatively weakly oxidized Ti layer of 20 A remained, which is most probably correlated with the remaining adhesion to the substrate.

Structural investigations of Pt/TiOx electrode stacks for ferroelectric thin film devices

Effects of the thermal treatment and the fabrication process of Pb(Zr0.3Ti0.7)O3 (PZT) thin films using chemical solution deposition on Pt∕TiOx electrode stacks were investigated using complementary analytical techniques including atomic force microscopy (AFM), x-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and grazing incidence x-ray reflectivity of synchrotron radiation. The surface and interface structures of the Pt∕TiOx electrode stacks with different thermal treatments, and the PZT∕Pt∕TiOx sample were examined. The propagation of Pt hillocks on the bare Pt∕TiOx electrode stacks upon the annealing was observed. AFM observations also revealed that the upper surface of the Pt bottom electrode under PZT thin film became rougher than that of the bare Pt electrode with the same thermal history. Global structural information including the density, surface or interface root-mean-square roughness, and thickness of each constituent layer in the samples were determined using...

Probing fatigue in ferroelectric thin films with subnanometer depth resolution

The authors report the study of polarization fatigue in Pb(Zr,Ti)O3 (PZT) ferroelectric thin films using in situ high-resolution grazing incidence x-ray specular reflectivity of synchrotron radiation. The results demonstrate that there is no formation of a region of different electron densities in the film growth direction with subnanometer depth resolution during fatigue. The upper bounds on the theoretically predicted interfacial accumulation of oxygen vacancies at the interfaces between PZT and Pt electrodes are determined by the comparison of experimental results and theoretical simulations.

An acoustic emission study of the effect of a magnetic field on the martensitic transition in Ni2MnGa

Acoustic emission measurements during the martensitic transition of a Ni–Mn–Ga magnetic shape-memory alloy reveal the intermittent and jerky character of the transition. The distribution of the amplitude of the acoustic emission events shows power law behavior, which reflects the absence of characteristic scales in the process. In this paper we show that the distribution is affected by an applied magnetic field, which proves that the transition dynamics is strongly influenced by magnetostructural coupling taking place at multiple length scales. The martensitic start temperature and the power law exponent of the amplitude distribution are measured in dependence of the applied field.

Growth-induced interface roughness of GaAs/AlAs-layers studied by X-ray scattering under grazing angles

Abstract We have studied GaAs/AlGaAs- and GaAs/AlAs-samples grown by molecular beam epitaxy by X-ray scattering under grazing angles to investigate surface and interface roughness. We discuss the reliability of buried interface roughnesses derived from fits to specular reflectivity by means of a sample series grown at substrate temperatures from 605 to 685°C. Even for samples of high perfection no abrupt interfaces were found. We have applied this method to the problem of growth-induced roughening of GaAs- and AlAs-layers. Sample series with increasing film thickness between 50 and 2000 A for AlAs and 5000 A for GaAs have been investigated. We observe roughening which in the case of AlAs does not follow a power law expected within the framework of kinetic roughening theory. However, for GaAs the data can be described by a very small growth exponent β = 0.11 ± 0.03.