Ilias Katsouras

The negative piezoelectric effect of the ferroelectric polymer poly(vinylidene fluoride)

Piezoelectricity describes interconversion between electrical charge and mechanical strain. As expected for lattice ions displaced in an electric field, the proportionality constant is positive for all piezoelectric materials. The exceptions are poly(vinylidene fluoride) (PVDF) and its copolymers with trifluoroethylene (P(VDF-TrFE)), which exhibit a negative longitudinal piezoelectric coefficient. Reported explanations exclusively consider contraction with applied electric field of either the crystalline or the amorphous part of these semi-crystalline polymers. To distinguish between these conflicting interpretations, we have performed in situ dynamic X-ray diffraction measurements on P(VDF-TrFE) capacitors. We find that the piezoelectric effect is dominated by the change in lattice constant but, surprisingly, it cannot be accounted for by the polarization-biased electrostrictive contribution of the crystalline part alone. Our quantitative analysis shows that an additional contribution is operative, which we argue is due to an electromechanical coupling between the intermixed crystalline lamellae and amorphous regions. Our findings tie the counterintuitive negative piezoelectric response of PVDF and its copolymers to the dynamics of their composite microstructure.

Polarization fatigue of organic ferroelectric capacitors

The polarization of the ferroelectric polymer P(VDF-TrFE) decreases upon prolonged cycling. Understanding of this fatigue behavior is of great technological importance for the implementation of P(VDF-TrFE) in random-access memories. However, the origin of fatigue is still ambiguous. Here we investigate fatigue in thin-film capacitors by systematically varying the frequency and amplitude of the driving waveform. We show that the fatigue is due to delamination of the top electrode. The origin is accumulation of gases, expelled from the capacitor, under the impermeable top electrode. The gases are formed by electron-induced phase decomposition of P(VDF-TrFE), similar as reported for inorganic ferroelectric materials. When the gas barrier is removed and the waveform is adapted, a fatigue-free ferroelectric capacitor based on P(VDF-TrFE) is realized. The capacitor can be cycled for more than 108 times, approaching the programming cycle endurance of its inorganic ferroelectric counterparts.

Universal Scaling of the Charge Transport in Large-Area Molecular Junctions

Charge transport through alkanes and para-phenylene oligomers is investigated in large-area molecular junctions. The molecules are self-assembled in a monolayer and contacted with a top electrode consisting of poly(3,4-ethylenedioxythiophene)-poly(4-styrenesulfonic acid) (PEDOT:PSS). The complete set of J(V,T) characteristics of both saturated and π-conjugated molecules can be described quantitatively by a single equation with only two fit parameters. The derived parameters, in combination with a variation of the bulk conductivity of PEDOT:PSS, demonstrate that the absolute junction resistance is factorized with that of PEDOT:PSS.

Low voltage extrinsic switching of ferroelectric delta-PVDF ultra-thin films

Non-volatile memories operating at low voltage are indispensable for flexible micro-electronic applications. To that end, ferroelectric δ-PVDF films were investigated as a function of layer thickness down to 10 nm ultra-thin films. Capacitors were fabricated using PEDOT:PSS as non-reactive electrode. Full polarization reversal was obtained at an unprecedented voltage below 5 V. The remanent polarization of 7 μC/cm2 and coercive field of 120 MV/m are independent of layer thickness, demonstrating that ferroelectric switching in δ-PVDF is extrinsic, dominated by inhomogeneous nucleation and growth. The ease of processing of δ-PVDF allowed to determine a lower limit of the critical ferroelectric thickness.

Retention of intermediate polarization states in ferroelectric materials enabling memories for multi-bit data storage

A homogeneous ferroelectric single crystal exhibits only two remanent polarization states that are stable over time, whereas intermediate, or unsaturated, polarization states are thermodynamically instable. Commonly used ferroelectric materials however, are inhomogeneous polycrystalline thin films or ceramics. To investigate the stability of intermediate polarization states, formed upon incomplete, or partial, switching, we have systematically studied their retention in capacitors comprising two classic ferroelectric materials, viz. random copolymer of vinylidene fluoride with trifluoroethylene, P(VDF-TrFE), and Pb(Zr,Ti)O3. Each experiment started from a discharged and electrically depolarized ferroelectric capacitor. Voltage pulses were applied to set the given polarization states. The retention was measured as a function of time at various temperatures. The intermediate polarization states are stable over time, up to the Curie temperature. We argue that the remarkable stability originates from the coexistence of effectively independent domains, with different values of polarization and coercive field. A domain growth model is derived quantitatively describing deterministic switching between the intermediate polarization states. We show that by using well-defined voltage pulses, the polarization can be set to any arbitrary value, allowing arithmetic programming. The feasibility of arithmetic programming along with the inherent stability of intermediate polarization states makes ferroelectric materials ideal candidates for multibit data storage.

Controlling the on/off current ratio of ferroelectric field-effect transistors

The on/off current ratio in organic ferroelectric field-effect transistors (FeFETs) is largely determined by the position of the threshold voltage, the value of which can show large device-to-device variations. Here we show that by employing a dual-gate layout for the FeFET, we can gain full control over the on/off ratio. In the resulting dual-gate FeFET the ferroelectric gate provides the memory functionality and the second, non-ferroelectric, control gate is advantageously used to set the threshold voltage. The on/off ratio can thus be maximized at the readout bias. The operation is explained by the quantitative analysis of charge transport in a dual-gate FeFET.

Extending the voltage window in the characterization of electrical transport of large-area molecular junctions

A large bias window is required to discriminate between different transport models in large-area molecular junctions. Under continuous DC bias, the junctions irreversibly break down at fields over 9 MV/cm. We show that, by using pulse measurements, we can reach electrical fields of 35 MV/cm before degradation. The breakdown voltage is shown to depend logarithmically on both duty cycle and pulse width. A tentative interpretation is presented based on electrolysis in the polymeric top electrode. Expanding the bias window using pulse measurements unambiguously shows that the electrical transport exhibits not an exponential but a power-law dependence on bias.

Transverse charge transport through DNA oligomers in large-area molecular junctions

We investigate the nature of charge transport in deoxyribonucleic acid (DNA) using self-assembled layers of DNA in large-area molecular junctions. A protocol was developed that yields dense monolayers where the DNA molecules are not standing upright, but are lying flat on the substrate. As a result the charge transport is measured not along the DNA molecules but in the transverse direction, across their diameter. The electrical transport data are consistent with the derived morphology. We demonstrate that the charge transport mechanism through DNA is identical to non-resonant tunneling through alkanethiols with identical length, classifying DNA as a dielectric.

Solid-state-processing of δ-PVDF

Poly(vinylidene fluoride) (PVDF) has long been regarded as an ideal piezoelectric ‘plastic’ because it exhibits a large piezoelectric response and a high thermal stability. However, the realization of piezoelectric PVDF elements has proven to be problematic due to, amongst other reasons, the lack of industrially scalable methods to process PVDF into the appropriate polar crystalline forms. Here, we show that fully piezoelectric PVDF films can be produced via a single-step process that exploits the fact that PVDF can be molded at temperatures below its melting temperature, i.e. via solid-state-processing. We demonstrate that we thereby produce δ-PVDF, the piezoelectric charge coefficient of which is comparable to that of biaxially stretched β-PVDF. We expect that the simplicity and scalability of solid-state processing combined with the excellent piezoelectric properties of our PVDF structures will provide new opportunities for this commodity polymer and will open a range of possibilities for future, large-scale, industrial production of plastic piezoelectric films.

Ferroelectricity and piezoelectricity in soft biological tissue: Porcine aortic walls revisited

Recently reported piezoresponse force microscopy (PFM) measurements have proposed that porcine aortic walls are ferroelectric. This finding may have great implications for understanding biophysical properties of cardiovascular diseases such as arteriosclerosis. However, the complex anatomical structure of the aortic wall with different extracellular matrices appears unlikely to be ferroelectric. The reason is that a prerequisite for ferroelectricity, which is the spontaneous switching of the polarization, is a polar crystal structure of the material. Although the PFM measurements were performed locally, the phase-voltage hysteresis loops could be reproduced at different positions on the tissue, suggesting that the whole aorta is ferroelectric. To corroborate this hypothesis, we analyzed entire pieces of porcine aorta globally, both with electrical and electromechanical measurements. We show that there is no hysteresis in the electric displacement as well as in the longitudinal strain as a function of applied electric field and that the strain depends on the electric field squared. By using the experimentally determined quasi-static permittivity and Young’s modulus of the fixated aorta, we show that the strain can quantitatively be explained by Maxwell stress and electrostriction, meaning that the aortic wall is neither piezoelectric nor ferroelectric, but behaves as a regular dielectric material.