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Rainer Waser

Publications and source records attributed to Rainer Waser.

11 recordsLinked to original sources
PubMed2006

Unit-cell scale mapping of ferroelectricity and tetragonality in epitaxial ultrathin ferroelectric films.

Typically, polarization and strain in ferroelectric materials are coupled, leading to the generally accepted direct relation between polarization and unit-cell tetragonality. Here, by means of high-resolution transmission electron microscopy we map, on the unit-cell scale, the degree of tetragonality and the displacements of cations away from the centrosymmetry positions in an ultrathin epitaxial PbZr(0.2)Ti(0.8)O(3) film on a SrRuO(3) electrode layer deposited on a SrTiO(3) substrate. The lattice is highly tetragonal at the centre of the film, whereas it shows reduced tetragonality close to the interfaces. Most strikingly, we find that the maximum off-centre displacements for the central area of the film do not scale with the tetragonality. This challenges the fundamental belief in a strong polarization-tetragonality coupling in PbTiO(3)-based ferroelectrics, at such thicknesses. Furthermore, a systematic reduction of the atomic displacements is measured at the interfaces, suggesting that interface-induced suppression of the ferroelectric polarization plays a critical role in the size effect of nanoscale ferroelectrics.

Journal Article↗
PubMed2006

STM study of mixed alkanethiol/biphenylthiol self-assembled monolayers on Au(111).

A method is presented for depositing mixed self-assembled monolayers (SAMs) of dodecanethiol (C12) and 4'-methyl-1,1'-biphenyl-4-butane (H3C-C6H4-C6H4-(CH2)4-SH, BP4) by insertion of BP4 into a closely packed SAM of dodecanethiol on Au(111). Insertion takes place at defect sites such as domain boundaries or etch pits in the gold surface that are characteristic of C12 monolayers on gold. With a lower probability, insertion also occurs beside defect sites inside dodecanethiol domains. Insertion at defect sites results in domains of BP4, whereas insertion into C12 domains leads to isolated BP4 molecules. The isolated BP4 molecules are shown not to move at room temperature. By comparing the apparent height of the isolated BP4 molecules and BP4 domains, it is proposed that the isolated molecules have the same conformation as in the full-coverage phase. A simple two-layer model is proposed to characterize the current transport through BP4. The decay constant beta for the phenylene groups is deduced from the apparent STM heights of the inserted BP4 islands compared to the STM heights of the C12 closely packed monolayers.

Journal Article↗
PubMed2006

Switching the electrical resistance of individual dislocations in single-crystalline SrTiO3.

The great variability in the electrical properties of multinary oxide materials, ranging from insulating, through semiconducting to metallic behaviour, has given rise to the idea of modulating the electronic properties on a nanometre scale for high-density electronic memory devices. A particularly promising aspect seems to be the ability of perovskites to provide bistable switching of the conductance between non-metallic and metallic behaviour by the application of an appropriate electric field. Here we demonstrate that the switching behaviour is an intrinsic feature of naturally occurring dislocations in single crystals of a prototypical ternary oxide, SrTiO(3). The phenomenon is shown to originate from local modulations of the oxygen content and to be related to the self-doping capability of the early transition metal oxides. Our results show that extended defects, such as dislocations, can act as bistable nanowires and hold technological promise for terabit memory devices.

Aluminum Oxide↗
PubMed2006

Sample-tip interaction of piezoresponse force microscopy in ferroelectric nanostructures.

We report on qualitative and quantitative implications of the sample-tip interaction in piezoresponse force microscopy. Our finite-element analysis of adsorbate effects, sample heterogeneities, and tip asymmetries is in agreement with experimental observation of ferroelectric nanostructures. Qualitative discrepancies arise from locally asymmetric tip-sample interaction. Any quantitative determination of field-related material parameters as required for the verification of semiempirical models of the ferroelectric limit typically relies on an overestimated field across the sample. Our findings indicate that adsorbates reduce the actual field across the nanograin by roughly one order of magnitude.

Computer Simulation↗
PubMed2005

Polymorphism in micro-, submicro-, and nanocrystalline NaNbO3.

NaNbO(3) powders with various particle sizes (ranging from 30 nm to several microns) and well-controlled stoichiometry were obtained through microemulsion-mediated synthesis. The effect of particle size on the phase transformation of the prepared NaNbO(3) powders was studied using X-ray powder diffraction, Raman spectroscopy, and nuclear site group analysis based on these spectroscopic data. Coarsened particles exhibit an orthorhombic Pbcm (D(2h)(11), no. 57) structure corresponding to the bulk structure, as observed for single crystals or powders prepared by conventional solid-state reaction. The crystal symmetry of submicron powders was refined with the space group Pmc2(1) (C(2v)(2), no. 26). The reduced perovskite cell volumes of these submicron powders were most expanded compared to all the other structures. Fine particles with a diameter of less than 70 nm as measured from SEM observations showed an orthorhombic Pmma (D(2h)(5), no. 51) crystal symmetry. The perovskite formula cell of this structure was pseudocubic and was the most compact one. A possible mechanism of the phase transformation is suggested.

Journal Article↗
PubMed2005

Rectangular (3 x 2 square root of 3) superlattice of a dodecanethiol self-assembled monolayer on Au(111) observed by ultra-high-vacuum scanning tunneling microscopy.

A rectangular (3 x 2 radical3) surface lattice for long-term-annealed dodecanethiol self-assembled monolayers on Au(111) is observed by ultra-high-vacuum scanning tunneling microscopy. The new lattice has the same density and a unit cell of the same size as the well-known c(4 x 2) reconstruction. In contrast, it does not show hexagonal symmetry but rather a sort of thiol pairing, leading to a shift in the binding position of every second molecule. The described structure is believed to be an intermediate phase close to desorption.

Letter↗
PubMed2004

Mononuclear precursor for MOCVD of HfO2 thin films.

We report the precursor characteristics of a novel mononuclear mixed alkoxide compound [Hf(O(i)Pr)2(tbaoac)2] and its application towards MOCVD of HfO2 thin films in a production tool CVD reactor.

Journal Article↗
PubMed2003

Observation of vacancy defect migration in the cation sublattice of complex oxides by 18O tracer experiments.

We report on 18O tracer diffusion experiments and model calculations for the study of cation vacancy migration in oxide crystals. The model takes advantage of the electrostatic coupling forces between anion and cation defects that allow the evolution of the cation vacancy profile to be observed by anion tracer experiments. Applied to SrTiO3, the ambipolar diffusion of strontium vacancies with H(A)=3.5 eV was found to be the dominant reequilibration mechanism of the cation sublattice. This result is in contrast to earlier studies proposing the formation of SrO intergrowth phases.

Journal Article↗
PubMed2003

Theoretical calculations and performance results of a PZT thin film actuator.

High piezoelectric coupling coefficients of PZT-based material systems can be employed for actuator functions in micro-electro-mechanical systems (MEMS) offering displacements and forces which outperform standard solutions. This paper presents simulation, fabrication, and development results of a stress-compensated, PZT-coated cantilever concept in which a silicon bulk micromachining process is used in combination with a chemical solution deposition (CSD) technique. Due to an analytical approach and a finite element method (FEM) simulation for a tip displacement of 10 microm, the actuator was designed with a cantilever length of 300 microm to 1000 microm. Special attention was given to the Zr/Ti ratio of the PZT thin films to obtain a high piezoelectric coefficient. For first characterizations X-ray diffraction (XRD), scanning electron microscopy (SEM), hysteresis-, current-voltage I(V)- and capacitance-voltage C(V)-measurements were carried out.

Journal Article↗