Multifunctional Supramolecular Organic Ferroelectrics

Abstract

Ferroelectric materials are known and valued for their multifunctionality arising from the possibility to perturb the remnant ferroelectric polarization by electric field, temperature and/or mechanical stimuli. While inorganic ferroelectrics dominate the current market, their organic counterparts may provide highly desired properties like eco-friendliness, easy-processability and flexibility, concomitantly opening unique opportunities to combine multiple functionalities into a single compound that facilitates unprecedented device concepts and designs. Supramolecular organic ferroelectrics of columnar discotic type, that are the topic of this thesis, offer additional advantages related to their strong hierarchical self-assembly and easy tunability by molecular structure modifications, allowing optimization of ferroelectric characteristics and their hybridization with, e.g., semiconductivity. This not only leads to textbook ferroelectric materials that can be used as model systems to understand the general behaviour of ferroics, but also gives rise to previously unobserved effects stemming from the interplay of different functionalities.

Switching Behavior of Dip‐Coated Organic Ferroelectric Trialkylbenzene‐1,3,5‐tricarboxamide (BTA)

“…The presented intensity spectrum of drop-casted BTA-C 8 is in good agreement with previous XRD investigations of BTA-C n . [2,10] The intensity peaks at 2θ ≈ 4.5° and its multiples represent the intercolumnar distance. The peaks representing π-π stacking distance are hardly distinguishable since the aligned molecule bundles are in-plane, as will be discussed further below.…”

“…Typically, this requires a field annealing procedure, during which the device is heated and subsequently cooled down under the application of a strong, alternating electric field. [ 2,9,15 ] Apart from being technologically highly undesired—not only does the procedure take time but it also comes with a non‐negligible risk of short‐circuiting the device—it becomes less effective or impossible for larger (semiconducting) compounds due to a reduction in the required molecular mobility.…”

“…The latter is the source of ferroelectric polarization and stabilizes the columnar structure through dipolar hydrogen bonding interactions. [2,4] BTA columns organize themselves in a hexagonal packing when going from the solution to a more solid state, which is mostly driven by the steric repulsion between the alkyl chains. [4] In recent works, we have shown that changing the peripheral substitution pattern allows to tune and improve the ferroelectric properties over a very wide range, leading to, amongst others, a polarization retention time in excess of 10 years, even at elevated temperatures.…”

“…[9] Around and above room temperature, BTAs typically form liquid crystals of the columnar discotic type. [2,4,[10][11][12][13] Molecular stability is provided by both the flat core through π-π stacking and the dipolar (amide) moieties. The latter is the source of ferroelectric polarization and stabilizes the columnar structure through dipolar hydrogen bonding interactions.…”

“…Typically, this requires a field annealing procedure, during which the device is heated and subsequently cooled down under the application of a strong, alternating electric field. [2,9,15] Apart from being technologically highly undesired-not only does the procedure take time but it also comes with a non-negligible risk of short-circuiting the device-it Supramolecular organic ferroelectrics have unique properties related to plasticity and freedom of design but the extensive post-deposition treatments that are typically required to achieve saturation polarization limit their applicability. A possible solution is to pre-align the molecules.…”