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Article Dans Une Revue Scientific Reports Année : 2019

Crystalline heterogeneity in single ferroelectric nanocrystals revealed by polarized nonlinear microscopy

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Ferroelectric nanocrystals have considerable interest for applications in nanophotonics, optical memories and bio-imaging. Their crystalline nature at the nanoscale remains however poorly known, mostly because structural investigation tools on single nanocrystals are lacking. In this work we apply polarization resolved second harmonic generation (p-sHG) imaging on isolated Barium titanate (Batio 3) nanocrystals to unravel their crystalline nature, exploiting the sensitivity of polarized SHG to local non-centrosymmetry and nanocrystals surface responses. We evidence crystalline heterogeneities in Batio 3 nanocrystals manifested by a centrosymmetric shell around the tetragonal core of the crystals, corroborating hypotheses from previous ensemble structural investigations. This study shows that in contrast to bulk materials, nanocrystals exhibit a complex composition, which is seen to be reproducible among nanocrystals. p-sHG appears furthermore as a powerful methodology that reports structural behaviors in nanoscale dielectrics materials, at the individual nanoparticle scale. Understanding the optical and physical processes occurring at the nanoscale in nanomaterials is fundamental for various applications that cover their use as nano-emitters in imaging, as ferroelectric nano-objects in non-volatile memories and piezo-electric devices, but also for electro-optics and more generally nanophotonics. One of the most attractive property of dielectric and ferroelectric nanocrystals is their ability to generate efficient nonlinear optical interactions even though their size scale down below hundreds of nanometers, which opens to an extensive range of functions that rely on optical frequency mixing. Barium Titanate (BaTiO 3) nanocrystals are particularly interesting in this prospect, being ferroelectric oxides that exhibit a stable tetragonal phase at room temperature. BaTiO 3 nanocrystals are thus efficient non-centrosymmetric second harmonic generation (SHG) emitters, which signal is detectable down to 20 nm sizes 1 , making them suitable biomarkers for nonlinear background-free imaging in cells or tissues 2,3. Recently the SHG efficiency of BaTiO 3 nanocrystals has been even further improved by plasmonic enhancement 4 or benefiting from Mie resonances 5. BaTiO 3 nanocrystals are thus promising candidates for nonlinear imaging, along with other oxides and dielectric crystals 6-8. Despite their recognized efficiency, BaTiO 3 nanocrystals exhibit complex structural behaviors that are still under debate 9,10. Bulk BaTiO 3 crystals are known to undergo a transformation from a cubic into a tetragonal structure below 130 °C, further followed by an orthorhombic form below 0 °C 11. Under different constraint such as varied chemical preparation conditions, strain, or at small sizes, BaTiO 3 is however prone to morphotropic phase boundary, which allows the coexistence of competing crystalline phases. In particular, transition from tetragonal to cubic phases below a critical particle size of about 50 nm has been observed 12,13 , as well as the formation of a disordered shell around a tetragonal core 14,15. Studies on powder X-ray diffraction have confirmed this view by revealing that the surface of BaTiO 3 nanocrystals relaxes to the cubic paraelectric phase, with an increasing contribution at small nanocrystal sizes 16. Such phases have however never been directly imaged at the single nano-object scale by lack of measurable nanoscale properties, leaving many unknowns on the composition of individual nanocrystals and its heterogeneity among nanocrystals. As a consequence, there is today a poor knowledge on how the structure of such nanocrystals influences their optical functions, which are averaged over the scale of optical diffraction limit (e.g. a few hundreds of nanome-ters). This knowledge is yet a key element for the design of future nanophotonics devices. In this work, we address
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hal-02410714 , version 1 (13-12-2019)

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Carolina Rendón-Barraza, Flavia Timpu, Rachel Grange, Sophie Brasselet. Crystalline heterogeneity in single ferroelectric nanocrystals revealed by polarized nonlinear microscopy. Scientific Reports, 2019, 9 (1), ⟨10.1038/s41598-018-38229-4⟩. ⟨hal-02410714⟩
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