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Microscopic model for switching kinetics in organic ferroelectrics following the Merz law
Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering. Heidelberg Univ, Germany.ORCID iD: 0000-0003-4728-8446
Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering.
Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering. Heidelberg Univ, Germany.ORCID iD: 0000-0002-7104-7127
2020 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 101, no 21, article id 214301Article in journal (Refereed) Published
Abstract [en]

From an application perspective, one of the most important parameters of a ferroelectric is its switching time, and understanding its limiting factors is key to improve device performance. While there is a variety of competing models for switching kinetics in realistic (disordered) ferroelectrics, they are often merely descriptive and provide little insight into the underlying microscopic mechanisms. This holds in particular for the classical Merz law, which describes the commonly observed exponential field dependence of the switching time. Here, we investigate the switching kinetics in the archetypical molecular ferroelectric trialkylbenzene-1,3,5-tricarboxamide using an electrostatic kinetic Monte Carlo model. The simulated field dependence follows the Merz law, which shows that a simple system of interacting dipoles is sufficient to obtain this behavior, even without explicitly considering domain walls or defects that are commonly thought to be involved in the emergence of the Merz law. Through a detailed analysis of the nucleation process, we can relate the macroscopic switching time to the microscopic nucleation energy barrier, which in turn is related to a field-dependent nucleus size. Finally, we use the acquired insight into the nucleation process to derive the Merz law from the theory of thermally activated nucleation-limited switching. This analytical model provides a physically transparent description of the switching kinetics in both experiments and simulations.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC , 2020. Vol. 101, no 21, article id 214301
National Category
Other Physics Topics
Identifiers
URN: urn:nbn:se:liu:diva-166472DOI: 10.1103/PhysRevB.101.214301ISI: 000537145900003OAI: oai:DiVA.org:liu-166472DiVA, id: diva2:1444142
Note

Funding Agencies|Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO Mat LiU) [2009 00971]; VetenskapsradetSwedish Research Council

Available from: 2020-06-20 Created: 2020-06-20 Last updated: 2020-07-01
In thesis
1. Switching Kinetics and Charge Transport in Organic Ferroelectrics
Open this publication in new window or tab >>Switching Kinetics and Charge Transport in Organic Ferroelectrics
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The continued digitalization of our society means that more and more things are getting connected electronically. Since currently used inorganic electronics are not well suited for these new applications because of costs and environmental issues, organic electronics can play an important role here. These essentially plastic materials are cheap to produce and relatively easy to recycle. Unfortunately, their poor performance has so far hindered widespread application beyond displays.

One key component of any electronic device is the memory. For organic electronics several technologies are being investigated that could serve as memories. One of these are the ferroelectrics, materials that have a spontaneous electrical polarization that can be reversed with an electric field. This bistable polarization which shows hysteresis makes these materials excellent candidates for use as memories.

This thesis focuses on a specific type of organic ferroelectric, the supramolecular discotics. These materials consist of disk‐like molecules that form columns in which all dipolar groups are aligned, giving a macroscopic ferroelectric polarization. Of particular interest are the benzenetricarboxamides (BTA), which are used as a model system for the whole class of discotic ferroelectrics. BTA uses a core‐shell architecture which allows for easy modification of the molecular structure and thereby the ferroelectric properties. To gain a deeper understanding of the switching processes in this organic ferroelectric BTA, both microscopic and analytical modeling are used. This is supported by experimental data obtained through electrical characterization.

The microscopic model reduces the material to a collection of dipoles and uses electrostatics to calculate the probability that these dipoles flip. These flipping rates are the input for a kinetic Monte Carlo simulation (kMC), which simulates the behavior of the dipoles over time. With this model we simulated three different switching processes on experimental time and length scales: hysteresis loops, spontaneous depolarization, and switching transients. The results of these simulations showed a good agreement with experiments and we can rationalize the obtained parameter dependencies in the framework of thermally activated nucleation limited switching (TA‐NLS).

The microscopic character of the model allows for a unique insight into the nucleation process of the polarization switching. We found that nucleation happens at different locations for field driven polarization switching as compared to spontaneous polarization switching. Field‐driven nucleation happens at the contacts, whereas spontaneous depolarization starts at defects. This means that retention times in disordered ferroelectrics could be improved by reducing the disorder, without affecting the coercive field. Detailed analysis of the nucleation process also revealed a critical nucleation volume that decreases with applied field, which explains the Merz‐like field‐dependence of the switching time observed in experiments.

In parallel to these microscopic simulations we developed an analytical framework based on the theory of TA‐NLS. This framework is mainly focused on describing the switching transients of disordered ferroelectrics. It can be combined with concepts of the Preisach model, which considers a non‐ideal ferroelectric as a collection of ideal hysterons. We were able to relate these hysterons and the distribution in their up‐ and down‐switching fields to the microscopic structure of the material and use the combined models to explain experimentally observed dispersive switching kinetics.

Whereas ferroelectrics on their own could potentially serve as memories, the readout of ferroelectric memories becomes easier if they are combined with semiconductors. We have introduced several molecular materials following the same design principle of a core‐shell structure, which uniquely combine ferroelectricity and semiconductivity in one material. The experimental IV‐curves of these materials could be described using an asymmetric Marcus hopping model and show their potential as memories. The combination of modeling and experimental work in this thesis thereby provides an increased understanding of organic ferroelectrics, which is crucial for their application as memories.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2020. p. 94
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 2080
Keywords
Organic Electronics, Ferroelectrics, Organic Ferroelectrics
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-167271 (URN)10.3384/diss.diva-167271 (DOI)9789179298289 (ISBN)
Public defence
2020-09-25, Online (contact martijn.kemerink@cam.uni-heidelberg.de) and Schrödinger (E324), F Building, Campus Valla, Linköping, 13:15 (English)
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Available from: 2020-08-26 Created: 2020-07-01 Last updated: 2020-09-07Bibliographically approved

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