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Kinetic Monte Carlo simulations of organic ferroelectrics
Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.ORCID iD: 0000-0003-4728-8446
KTH Royal Inst Technol, Sweden.
Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
KTH Royal Inst Technol, Sweden.
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2019 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 21, no 3, p. 1375-1383Article in journal (Refereed) Published
Abstract [en]

Ferroelectrics find broad applications, e.g. in non-volatile memories, but the switching kinetics in real, disordered, materials is still incompletely understood. Here, we develop an electrostatic model to study ferroelectric switching using 3D Monte Carlo simulations. We apply this model to the prototypical small molecular ferroelectric trialkylbenzene-1,3,5-tricarboxamide (BTA) and find good agreement between the Monte Carlo simulations, experiments, and molecular dynamics studies. Since the model lacks any explicit steric effects, we conclude that these are of minor importance. While the material is shown to have a frustrated antiferroelectric ground state, it behaves as a normal ferroelectric under practical conditions due to the large energy barrier for switching that prevents the material from reaching its ground state after poling. We find that field-driven polarization reversal and spontaneous depolarization have orders of magnitude different switching kinetics. For the former, which determines the coercive field and is relevant for data writing, nucleation occurs at the electrodes, whereas for the latter, which governs data retention, nucleation occurs at disorder-induced defects. As a result, by reducing the disorder in the system, the polarization retention time can be increased dramatically while the coercive field remains unchanged.

Place, publisher, year, edition, pages
ROYAL SOC CHEMISTRY , 2019. Vol. 21, no 3, p. 1375-1383
National Category
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Identifiers
URN: urn:nbn:se:liu:diva-154324DOI: 10.1039/c8cp06716cISI: 000456147000040PubMedID: 30601493OAI: oai:DiVA.org:liu-154324DiVA, id: diva2:1285532
Note

Funding Agencies|Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009 00971]; Vetenskapsradet; SeRC (Swedish e-Science Research Center)

Available from: 2019-02-04 Created: 2019-02-04 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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