Bernd Reichenberg

Piezoresponse in the light of surface adsorbates: Relevance of defined surface conditions for perovskite materials

We report on the influence of a surface layer prevailing on perovskites on the piezoelectricity measured by piezoresponse force microscopy. Surface sensitive measurements show that this layer consists of chemisorbates and physisorbates. The surface layer can be removed to a large extent by heating the sample under ultrahigh vacuum conditions. It is shown that the effect of this treatment on the piezoresponse of the material is significant as the potential difference applied to the sample is no longer reduced by a voltage drop across the adsorbate layer. As a consequence the internal electric field is higher in comparison to the presence of a layer and so the piezoresponse is higher as well.

Comparison of in-plane and out-of-plane optical amplification in AFM measurements

The in-plane image of piezoresponse force microscopy (PFM) generally exhibits a higher resolution and less noise than the out-of-plane image. Geometrical considerations indicate that the optical in-plane amplification is ≈40 times larger than the out-of-plane amplification. We experimentally confirm this explanation in a dedicated setup.

Analysis of shape effects on the piezoresponse in ferroelectric nanograins with and without adsorbates

Using BaTiO3 as a piezoelectric model system we compare a finite element model with experimental data to demonstrate the impact of grain topography on the in-plane piezoelectric response at the perimeter. Our findings emphasize the need for a careful consideration of both electric field and piezoelectric tensor orientation. An analysis is given showing that the in-plane piezoresponse is a function of two directions of the electric field, whereas the out-of-plane response is a function of all three directions of the applied field. The effect of an adsorbate layer on the piezoelectric response is quantified with typical material parameters.

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

Contributions to in-plane piezoresponse on axially symmetrical samples

We report on the influence of system-immanent asymmetries on the interpretation of in-plane piezoresponse force microscopy (PFM). As PFM is a surface scanning method, the electromechanical interaction of probe tip and sample is a key aspect of all experiments. An initial characterization of topography, surface state, and conductivity is mandatory to separate their signal from the response due to an in-plane polarization state. Our findings underline that any reduction of radial symmetry in the tip-sample system creates an otherwise symmetry-prohibited in-plane signal.

Tungsten coatings by chemical solution deposition for ceramic electrodes in fluorescent tubes

In the lighting industry, the concept of ceramic cold cathodes provided with an inside tungsten coating is under consideration to be applied in mercury-free fluorescent lamps for advertising purposes. In this regard, the applicability of chemical solution deposition to fabricate metallic films on the inner surface of alumina tubules was established. The technique involves the preparation of tungsten oxide layers from a peroxotungstic acid precursor solution and their subsequent reduction to tungsten in the presence of hydrogen. Diverse characterisation of the precursor and the intermediates was carried out in order to determine the influence of their properties on the quality of the final coatings. By acquiring the optimum process parameters, highly-conducting tungsten films were obtained. The temperatures allowing for conversion of the oxide to the metal as well as for sintering of the derived tungsten films were observed to be substantially lower than in bulk processes. Electrode prototypes of the described type were accordingly realised and operated successfully in a fluorescent tube test setup, thus proving the feasibility of the hitherto theoretical idea.

Extrinsic Contributions to Piezoresponse Force Microscopy

Piezoresponse force microscopy (PFM) is the method of choice to investigate piezoactivity on a nanometer scale. A careful distinction between intrinsic and extrinsic effects are mandatory, especially when measuring ferroelectric nanostructures. We focus on two omnipresent extrinsic contributions with a substantial impact: firstly adsorbates on the surface of perovsike materials and secondly the dependence of the lateral piezoresponse on the topography. A thorough understanding of these extrinsic contributions is essential in order to avoid ambiguities in the analysis of PFM measurements.