Current researches

Principle of local switching current spectroscopy: Induced switching current by reversal of the polarization.

Schematics of polarization switching event. The upward polarization of the central zone in the initial state is reversed by the application of voltage via AFM tip in contact with the zone. The switching results in the flow of charges in the circuit to compensate the new polarization charges that appear at the surfaces of the switched zone after the polarization reversal. This generates current peaks whose shape depends on the kinetics of the switching process and the associated compensation process.

Thin films of organic ferroelectrics

The croconic acid is an organic ferroelectric with a large polarisation (ref PhD Ferroelectric properties of organic croconic acid: http://www.theses.fr/2020STRAE021 or direct download the pdf file: https://publication-theses.unistra.fr/public/theses_doctorat/2020/MOHAPATRA_Sambit_2020_ED182.pdf.)

Ferroelectric polarization estimation in nanoscopic systems

Local polarisation loop determined by switching current

The nanoscopic characterization of ferroelectric thin films is crucial from their device application point of view. Standard characterization techniques are based on detecting the nanoscopic charge compensation current (switching current) caused by the polarization reversal in the ferroelectric. Owing to various surface and bulk limited mechanisms, leakage currents commonly appear during such measurements, which are frequently subtracted using the device I–V characteristic by employing a positive-up-negative-down measurement scheme. By performing nanoscopic switching current measurements on a commonly used ferroelectric, BiFeO3, we show that such characterization methods may be prone to large errors in polarization estimation on ferro-resistive samples due to current background subtraction issues. Specifically, when the resistance of the ferroelectric thin film is associated with polarization reversal, background current subtraction is not accurate due to the mismatch of the I–V characteristics for two polarization states. We show instead that removing the background current by an asymmetric least-squares subtraction method, though not perfect, gives a much better estimation of the ferroelectric properties of the sample under study.

Mohapatra, S., Weber, W., Bowen, M., Boukari, S., Da Costa, V., 2022. Toward accurate ferroelectric polarization estimation in nanoscopic systems. Journal of Applied Physics 132, 134101. https://doi.org/10.1063/5.0102920

Mohapatra, S., Beaurepaire, E., Weber, W., Bowen, M., Boukari, S., Da Costa, V., 2021. Nanoscale reversal of stable room temperature ferroelectric polarization in organic croconic acid thin films. arXiv:2104.03084 [cond-mat, physics:physics].

*****

Organic ferroelectric Croconic Acid

Ferroelectric domain structure of croconic acid film

Owing to prospective energy-efficient and environmentally benign applications, organic ferroelectric materials are useful and necessary alternative to inorganic ferroelectrics. Although the first discovered ferroelectric, Rochelle salt, was a salt of an organic compound, organic ferroelectrics have not been as abundant as the inorganic ones. Further, the small polarization values in the organic systems discovered so far have been a demotivating factor for their applications. However, scientific interest and activities surrounding such materials, for the purpose of fundamental understanding and practical applications, have significantly risen lately, especially after the discovery of above-room-temperature ferroelectricity in croconic acid (4,5-dihydroxy-4-cyclopentene-1,2,3-trione, H2C5O5) crystals with polarization values rivalling those found in inorganic ferroelectrics. Its large polarization, organic nature, and vacuum sublimability make croconic acid an ideal candidate for non-toxic and lead-free device applications. In this review article, we survey the scientific activities carried out so far involving ferroelectricity in this novel material, paying equal attention to its bulk single crystal and thin film forms. While we discuss about the origin of ferroelectric order and the reversal of polarization in the bulk form, we also summarize the directions toward applications of the thin films.

Mohapatra, S., Cherifi-Hertel, S., Kuppusamy, S.K., Schmerber, G., Arabski, J., Gobaut, B., Weber, W., Bowen, M., Costa, V.D., Boukari, S., 2022. Organic ferroelectric Croconic Acid: A concise survey from bulk single crystals to thin films. J. Mater. Chem. C. https://doi.org/10.1039/D1TC05310H

Mohapatra, S., Cherifi-Hertel, S., Kuppusamy, S.K., Schmerber, G., Arabski, J., Gobaut, B., Weber, W., Bowen, M., Da Costa, V., Boukari, S., 2021. Organic ferroelectric Croconic Acid: A concise survey from bulk single crystals to thin films. arXiv:2110.12519 [cond-mat].

***

Nanoscopic polarization reversal processes in an organic ferroelectric thin film. Nanoscale

Switching current spectroscopy (current probed by AFM tip)

Towards eliminating toxic substances from electronic devices, Croconic Acid (CA) has great potential as a sublimable organic ferroelectric material. While studies on CA thin films are just beginning to emerge, as the organic ferroelectric with the highest saturation polarization, its capability to be integrated in nanodevices remains unexplored owing to the difficulties in laterally nanoscopic ferroelectric characterization. We demonstrate at the laterally nanoscopic scale robust switching of a stable enduring ferroelectric polarization at room temperature in CA thin films, without leakage. The challenging ferroelectric characterization at the nanoscale is performed using a unique combination of piezoresponse force microscopy, polarization switching current spectroscopy and concurrent strain response. This helps rationalize the otherwise asymmetric polarization-voltage hysteresis due to background noise limited undetectable switching currents, which are statistically averaged in macrojunctions but become prevalent at the nanoscale. Apart from successfully estimating the nanoscopic polarization in CA thin films, we show that CA may be a promising lead-free organic ferroelectric towards nanoscale device integration. Our results, being valid irrespective of the ferroelectrics’ nature; organic or inorganic, pave the way for fundamental understandings and technological applications of nanoscopic polarization reversal mechanisms.

Mohapatra, S., Beaurepaire, E., Weber, W., Bowen, M., Boukari, S., Da Costa, V., 2021. Accessing nanoscopic polarization reversal processes in an organic ferroelectric thin film. Nanoscale. https://doi.org/10.1039/D1NR05957B

Mohapatra, S., Beaurepaire, E., Weber, W., Bowen, M., Boukari, S., Da Costa, V., 2021. Nanoscale reversal of stable room temperature ferroelectric polarization in organic croconic acid thin films. arXiv:2104.03084 [cond-mat, physics:physics].

Ferroelectric properties of organic croconic acid films grown on spintronically relevant substrates

Control of a ferroelectric state of a nano domain thanks to AFM tip (croconic acid molecular film deposeted on Cobalt)

The discovery of stable room temperature ferroelectricity in croconic acid, an organic ferroelectric material, with polarization values on par with those found in inorganic ferroelectric materials and highest among organic ferroelectric materials, has opened up possibilities to realize myriads of nano-electronic and spintronic devices based on organic ferroelectrics. Such possibilities require an adequate understanding of the ferroelectric properties of croconic acid grown on surfaces that are commonly employed in device fabrication. While several macroscopic studies on relatively larger crystals of croconic acid have been performed, studies on thin films are only in their early stages. We have grown thin films of croconic acid on gold and cobalt surfaces, which are commonly used in spintronic devices as metallic electrodes, and studied the films’ ferroelectric response using ex situ piezoresponse force microscopy at room temperature. We show that the polarization reversal in croconic acid domains is sensitive to the substrate surface. Using the same experimental protocol, we observe the robust polarization reversal of a single, mostly in-plane electrical domain for a cobalt substrate, whereas polarization reversal is hardly observed for a gold substrate. We attribute this difference to the substrate’s influence on the croconic acid molecular networks. Our study suggests that to realize devices one has to take care about the substrate on which croconic acid will be deposited. The fact that polarization switching is robust on the cobalt surface can be used to fabricate multifunctional devices that utilize the cobalt/croconic acid interface.

Mohapatra, S., Da Costa, V., Avedissian, G., Arabski, J., Weber, W., Bowen, M., Boukari, S., 2020. Robust ferroelectric properties of organic croconic acid films grown on spintronically relevant substrates. Mater. Adv. https://doi.org/10.1039/D0MA00147C

Mohapatra, S., Da Costa, V., Avedissian, G., Arabski, J., Weber, W., Bowen, M., Boukari, S., 2020. Robust ferroelectric properties of organic Croconic Acid films grown on spintronically relevant substrates. arXiv:2004.04592 [cond-mat].

Academic background

University of Strasbourg

Research background

Ferroélectriques organiques en films minces.
ferroelectric, antiferroelectric, ferroelectric organic, ferroelectrique organique, local polarisation of ferroelectrics (piezo force microscopy PFM), switching current
Effet tunnel : événement rares, fluctuation des courants tunnel et paramètres statistiques, cartographie de la barrière tunnel (jonctions métal/oxyde) http://af.bardou.free.fr/.
Transport polarisé en spin (vanne de spin, jonctions tunnel magnétorésistives).
Nanostructures magnétiques, domaines magnétiques, multicouches magnétiques, MFM.

Publications

Connecter la recherche et les chercheurs

Victor Da Costa

https://orcid.org/0000-0002-2889-7871