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Switching Kinetics and Charge Transport in Organic Ferroelectrics
Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering. (Complex Materials and Devices)ORCID iD: 0000-0003-4728-8446
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 [en]
Organic Electronics, Ferroelectrics, Organic Ferroelectrics
National Category
Condensed Matter Physics
Identifiers
URN: urn:nbn:se:liu:diva-167271DOI: 10.3384/diss.diva-167271ISBN: 9789179298289 (print)OAI: oai:DiVA.org:liu-167271DiVA, id: diva2:1450350
Public defence
2020-09-25, Online (contact martijn.kemerink@cam.uni-heidelberg.de) and Schrödinger (E324), F Building, Campus Valla, Linköping, 13:15 (English)
Opponent
Supervisors
Available from: 2020-08-26 Created: 2020-07-01 Last updated: 2020-09-07Bibliographically approved
List of papers
1. Ferroelectric self-assembled molecular materials showing both rectifying and switchable conductivity
Open this publication in new window or tab >>Ferroelectric self-assembled molecular materials showing both rectifying and switchable conductivity
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2017 (English)In: Science Advances, E-ISSN 2375-2548, Vol. 3, no 9, article id e1701017Article in journal (Refereed) Published
Abstract [en]

Advanced molecular materials that combine two or more physical properties are typically constructed by combining different molecules, each being responsible for one of the properties required. Ideally, single molecules could take care of this combined functionality, provided they are self-assembled correctly and endowed with different functional subunits whose strong electronic coupling may lead to the emergence of unprecedented and exciting properties. We present a class of disc-like semiconducting organic molecules that are functionalized with strong dipolar side groups. Supramolecular organization of these materials provides long-range polar order that supports collective ferroelectric behavior of the side groups as well as charge transport through the stacked semiconducting cores. The ferroelectric polarization in these supramolecular polymers is found to couple to the charge transport and leads to a bulk conductivity that is both switchable and rectifying. An intuitive model is developed and found to quantitatively reproduce the experimental observations. In a larger perspective, these results highlight the possibility of modulating material properties using the large electric fields associated with ferroelectric polarization.

Place, publisher, year, edition, pages
AMER ASSOC ADVANCEMENT SCIENCE, 2017
National Category
Textile, Rubber and Polymeric Materials
Identifiers
urn:nbn:se:liu:diva-145259 (URN)10.1126/sciadv.1701017 (DOI)000423949400008 ()28975150 (PubMedID)
Note

Funding Agencies|Netherlands Organization for Scientific Research (NWO) Nano program; Ministerio de Educacion, Culture y Deporte (MECD) (FPU fellowship); MINECO, Spain [CTQ-2014-52869-P, CTQ2014-57729-P]; Comunidad de Madrid [S2013/MIT-2841 FOTOCARBON]; European Research Council [StG-279548]; Dutch Polymer Institute; Dutch Ministry of Education, Culture and Science [024.001.035]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009 00971]

Available from: 2018-02-21 Created: 2018-02-21 Last updated: 2020-07-01
2. Physical reality of the Preisach model for organic ferroelectrics
Open this publication in new window or tab >>Physical reality of the Preisach model for organic ferroelectrics
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2018 (English)In: Nature Communications, E-ISSN 2041-1723, Vol. 9, article id 4409Article in journal (Refereed) Published
Abstract [en]

The Preisach model has been a cornerstone in the fields of ferromagnetism and ferroelectricity since its inception. It describes a real, non-ideal, ferroic material as the sum of a distribution of ideal hysterons. However, the physical reality of the model in ferroelectrics has been hard to establish. Here, we experimentally determine the Preisach (hysteron) distribution for two ferroelectric systems and show how its broadening directly relates to the materials morphology. We connect the Preisach distribution to measured microscopic switching kinetics that underlay the macroscopic dispersive switching kinetics as commonly observed for practical ferroelectrics. The presented results reveal that the in principle mathematical construct of the Preisach model has a strong physical basis and is a powerful tool to explain polarization switching at all time scales in different types of ferroelectrics. These insights lead to guidelines for further advancement of the ferroelectric materials both for conventional and multi-bit data storage applications.

Place, publisher, year, edition, pages
NATURE PUBLISHING GROUP, 2018
National Category
Other Physics Topics
Identifiers
urn:nbn:se:liu:diva-152613 (URN)10.1038/s41467-018-06717-w (DOI)000448044300007 ()30352995 (PubMedID)
Note

Funding Agencies|Vetenskapsradet; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [200900971]

Available from: 2018-11-09 Created: 2018-11-09 Last updated: 2023-03-28
3. Kinetic Monte Carlo simulations of organic ferroelectrics
Open this publication in new window or tab >>Kinetic Monte Carlo simulations of organic ferroelectrics
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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
National Category
Other Physics Topics
Identifiers
urn:nbn:se:liu:diva-154324 (URN)10.1039/c8cp06716c (DOI)000456147000040 ()30601493 (PubMedID)
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
4. Microscopic model for switching kinetics in organic ferroelectrics following the Merz law
Open this publication in new window or tab >>Microscopic model for switching kinetics in organic ferroelectrics following the Merz law
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
National Category
Other Physics Topics
Identifiers
urn:nbn:se:liu:diva-166472 (URN)10.1103/PhysRevB.101.214301 (DOI)000537145900003 ()
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

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