Switchable tribology of ferroelectrics.

Switchable tribological properties of ferroelectrics offer an alternative route to visualize and control ferroelectric domains. Here, we observe the switchable friction and wear behavior of ferroelectrics using a nanoscale scanning probe-down domains have lower friction coefficients and show slower wear rates than up domains and can be used as smart masks. This asymmetry is enabled by flexoelectrically coupled polarization in the up and down domains under a sufficiently high contact force. Moreover, we determine that this polarization-sensitive tribological asymmetry is widely applicable across various ferroelectrics with different chemical compositions and crystalline symmetry. Finally, using this switchable tribology and multi-pass patterning with a domain-based dynamic smart mask, we demonstrate three-dimensional nanostructuring exploiting the asymmetric wear rates of up and down domains, which can, furthermore, be scaled up to technologically relevant (mm-cm) size. These findings demonstrate that ferroelectrics are electrically tunable tribological materials at the nanoscale for versatile applications.

Thickness-Dependent Perovskite Octahedral Distortions at Heterointerfaces.

In this study, we analyze how the octahedral tilts and rotations of thin films of LaNiO3 and LaAlO3 grown on different substrates, determined using synchrotron X-ray diffraction-measured half-integer Bragg peaks, depend upon the total film thickness. We find a striking difference between films grown on SrTiO3 and LaAlO3 substrates which appears to stem not only from the difference in epitaxial strain state but also from the level of continuity at the heterointerface. In particular, the chemically and structurally discontinuous LaNiO3/SrTiO3 and LaAlO3/SrTiO3 interfaces cause a large variation in the octahedral network as a function of film thickness whereas the rather continuous LaNiO3/LaAlO3 interface seems to allow from just a few unit cells the formation of a stable octahedral pattern corresponding to that expected only given the applied biaxial strain.

InteractiveXRDFit: a new tool to simulate and fit X-ray diffractograms of oxide thin films and heterostructures.

InteractiveXRDFit is a custom-made MATLAB program that calculates the X-ray diffracted intensity for oxide thin films and heterostructures based on a library of inorganic materials.

Tuning of the depolarization field and nanodomain structure in ferroelectric thin films.

The screening efficiency of a metal-ferroelectric interface plays a critical role in determining the polarization stability and hence the functional properties of ferroelectric thin films. Imperfect screening leads to strong depolarization fields that reduce the spontaneous polarization or drive the formation of ferroelectric domains. We demonstrate that by modifying the screening at the metal-ferroelectric interface through insertion of ultrathin dielectric spacers, the strength of the depolarization field can be tuned and thus used to control the formation of nanoscale domains. Using piezoresponse force microscopy, we follow the evolution of the domain configurations as well as polarization stability as a function of depolarization field strength.

Improper ferroelectricity in perovskite oxide artificial superlattices.

Ferroelectric thin films and superlattices are currently the subject of intensive research because of the interest they raise for technological applications and also because their properties are of fundamental scientific importance. Ferroelectric superlattices allow the tuning of the ferroelectric properties while maintaining perfect crystal structure and a coherent strain, even throughout relatively thick samples. This tuning is achieved in practice by adjusting both the strain, to enhance the polarization, and the composition, to interpolate between the properties of the combined compounds. Here we show that superlattices with very short periods possess a new form of interface coupling, based on rotational distortions, which gives rise to 'improper' ferroelectricity. These observations suggest an approach, based on interface engineering, to produce artificial materials with unique properties. By considering ferroelectric/paraelectric PbTiO3/SrTiO3 multilayers, we first show from first principles that the ground-state of the system is not purely ferroelectric but also primarily involves antiferrodistortive rotations of the oxygen atoms in a way compatible with improper ferroelectricity. We then demonstrate experimentally that, in contrast to pure PbTiO3 and SrTiO3 compounds, the multilayer system indeed behaves like a prototypical improper ferroelectric and exhibits a very large dielectric constant of epsilon(r) approximately 600, which is also fairly temperature-independent. This behaviour, of practical interest for technological applications, is distinct from that of normal ferroelectrics, for which the dielectric constant is typically large but strongly evolves around the phase transition temperature and also differs from that of previously known improper ferroelectrics that exhibit a temperature-independent but small dielectric constant only.

Advanced fabrication and characterization of epitaxial ferroelectric thin films and multilayers.

Understanding the behavior of ferroelectrics on the nanoscale level requires the production of materials of the highest quality and advanced characterization techniques for probing the fascinating properties of these systems with reduced dimensions. Here we give an overview of our recent achievements in this area, which includes the detailed study of the suppression of ferroelectricity in PbTiO3 thin films, the fabrication of PbTiO3/SrTiO3 superlattices in which ferroelectricity shows some surprising behavior, and finally the manipulation of nanoscale ferroelectric domains using the atomic force microscope which leads to the precise analysis of domain wall creep and roughness in Pb(Zr,Ti)O3 thin films.

Ferroelectricity and tetragonality in ultrathin PbTiO3 films.

The evolution of tetragonality with thickness has been probed in epitaxial c-axis oriented PbTiO3 films with thicknesses ranging from 500 down to 24 A. High resolution x ray pointed out a systematic decrease of the c-axis lattice parameter with decreasing film thickness below 200 A. Using a first-principles model Hamiltonian approach, the decrease in tetragonality is related to a reduction of the polarization attributed to the presence of a residual unscreened depolarizing field. It is shown that films below 50 A display a significantly reduced polarization but still remain ferroelectric.