Juan Carlos Gonzalez‐Rosillo, Rafael Ortega‐Hernandez, Benedikt Arndt, Mariona Coll, Regina Dittmann, Xavier Obradors, Anna Palau, Jordi Suñe, Teresa Puig. Adv. Electron. Mater. 2019, 1800629.

Hailin Wang, Jaume Gazquez, Carlos Frontera, Matthew F. Chisholm, Alberto Pomar, Benjamin Martinez & Narcis Mestres. NPG Asia Materials volume 11, Article number: 44 (2019). 

Ferroelectric perovskite oxides are emerging as a promising photoactive layer for photovoltaic applications because of their very high stability and their alternative ferroelectricity-related mechanism for solar energy conversion that could lead to extraordinarily high efficiencies. One of the biggest challenges so far is to reduce their band gap toward the visible region while simultaneously retaining ferroelectricity. To address these two issues, herein an elemental composition engineering of BiFeO₃ is performed by substituting Fe by Co cations, as a means to tune the characteristics of the transition metal–oxygen bond. We demonstrate by solution processing the formation of epitaxial, pure phase, and stable BiFe_1–xCo_xO₃ thin films for x ≤ 0.3 and film thickness up to 100 nm. Importantly, the band gap can be tuned from 2.7 to 2.3 eV upon cobalt substitution while simultaneously enhancing ferroelectricity. As a proof of concept, nonoptimized vertical devices have been fabricated and, reassuringly, the electrical photoresponse in the visible region of the Co-substituted phase is improved with respect to the unsubstituted oxide.

José M. Vila-Fungueiriño,  Andres Gomez,  Jordi Antoja-Lleonart,  Jaume Gazquez,  César Magén,  Beatriz Noheda  and  Adrian Carretero-Genevrier. Nanoscale, 2018  

DOI:10.1039/C8NR05737K

We use an original water-based chemical method, to grow pure epitaxial BiFeO₃ (BFO) ultra-thin films with excellent piezoelectric properties. Particularly, we show that this novel chemical route produces a higher natural ferroelectric domain size distribution and coercive field compared to similar BFO films grown by physical methods. Moreover, we measured the d₃₃ piezoelectric coefficient of 60 nm thick BFO films with a direct approach, using Direct Piezoelectric Force Microscopy (DPFM). As a result, first piezo-generated charge maps of a very thin BFO layer were obtained applying this novel technology. We also performed a comparative study of the d₃₃ coefficients between standard PFM analysis and the DPFM microscopy showing similar values i.e. 17 pm/V and 22 pC/N respectively. Finally, we proved that the directionality of the piezoelectric effect in BFO ferroelectric thin films is preserved at low thickness dimensions demonstrating the potential of chemical processes for the development of low cost functional ferroelectric and piezoelectric devices.

B. Mundet, J. Jareño, J. Gazquez, M. Varela, X. Obradors, and T. Puig. Phys. Rev. Materials 2, 063607 

DOI:https://doi.org/10.1103/PhysRevMaterials.2.063607

In this work we evaluate the defects and the associated distortions present in tensile and compressive-strained chemical solution deposition–derived $\mathrm{NdNi}{O}_3$ (NNO) and $\mathrm{LaNi}{O}_3$ (LNO) thin films by means of aberration corrected scanning transmission electron microscopy. We elucidate a fundamental link between strain and the most common defect observed in nickelate films, the Ruddlesden-Popper fault (RPF), which will ultimately impinge on the electrical properties of the films. Overall, the concentration of RPF defects increases with the lattice mismatch. More specifically, LNO films are always metallic, although transitioning from compressive to tensile strain results in the appearance of RPFs and an increase of the resistivity. On the other hand, NNO films always behave as insulators under tensile strain, whereas under compressive strain the increase of the thickness makes the onset of the metal-to-insulator transition shift to higher temperatures.

Piezo-generated charge mapping revealed through direct piezoelectric force microscopy

A. Gomez, M. Gich, A. Carretero-Genevrier, T. Puig & X. Obradors Nature Communications 8, Article number: 1113 (2017) doi:10.1038/s41467-017-01361-2 

While piezoelectric and ferroelectric materials play a key role in many everyday applications, there are still a number of open questions related to their physics. To enhance our understanding of piezoelectrics and ferroelectrics, nanoscale characterization is essential. Here, we develop an atomic force microscopy based mode that obtains a direct quantitative analysis of the piezoelectric coefficient d₃₃. We report nanoscale images of piezogenerated charge in a thick single crystal of periodically poled lithium niobate (PPLN), a bismuth ferrite (BiFO₃) thin film, and lead zirconate titanate (PZT) by applying a force and recording the current produced by these materials. The quantification of d₃₃coefficients for PPLN (14 ± 3 pC per N) and BFO (43 ± 6 pC per N) is in agreement with the values reported in the literature. Even stronger evidence of the reliability of the method is provided by an equally accurate measurement of the significantly larger d₃₃ of PZT.