Alessio Marrani

Nanoscale Design of Multifunctional Organic Layers for Low-Power High-Density Memory Devices

We demonstrate the design of a multifunctional organic layer by the rational combination of nanosized regions of two functional polymers. Instead of relying on a spontaneous and random phase separation process or on the tedious synthesis of block copolymers, the method involves the nanomolding of a first component, followed by the filling of the resulting open spaces by a second component. We apply this methodology to fabricate organic nonvolatile memory diodes of high density. These are built by first creating a regular array of ferroelectric nanodots by nanoimprint lithography, followed by the filling of the trenches separating the ferroelectric nanodots with a semiconducting polymer. The modulation of the current in the semiconductor by the polarization state of the ferroelectric material is demonstrated both at the scale of a single semiconductor channel and in a microscopic device measuring about 80,000 channels in parallel, for voltages below ca. 2 V. The fabrication process, which combines synergetically orthogonal functional properties with a fine control over their spatial distribution, is thus demonstrated to be efficient over large areas.

An organic ferroelectric field effect transistor with poly(vinylidene fluoride-co-trifluoroethylene) nanostripes as gate dielectric

We demonstrate the fabrication of an organic Ferroelectric Field Effect Transistor (FeFET) incorporating a ferroelectric gate dielectric made of nanostripes obtained by nanoimprinting poly(vinylidene fluoride-co-trifluoroethylene) over a layer of semiconducting poly(triarylamine). The imprinting process results in a decreased switching voltage for the ferroelectric, by a factor of ca. 1.5, resulting in a decreased operating voltage compared to a reference FeFET with a continuous ferroelectric layer. The transistor consists of a large number of nanostripe-gated transistors placed in parallel, which also offers interesting possibilities for a strong size reduction of organic FeFETs.

Photostrictive effect in a polyvinylidene fluoride-trifluoroethylene copolymer

We present an extensive characterization and modeling of the use of a polyvinylidene fluoride trifluoroethylene copolymer P(VDF-TrFE) as a photo activated actuator. P(VDF-TrFE) copolymers are very appealing for applications because, on the opposite of the PVDF homopolymer, which requires a drawing or stretching followed by annealing and poling process to show ferroelectric properties, they spontaneously crystallize to beta phase. Based on the measurement of the bending of P(VDF-TrFE) strips under illumination, we extrapolated a model of the role of temperature, piezoelectric effect, and photostrictive components.

Organic ferroelectric/semiconducting nanowire hybrid layer for memory storage

Ferroelectric materials are important components of sensors, actuators and non-volatile memories. However, possible device configurations are limited due to the need to provide screening charges to ferroelectric interfaces to avoid depolarization. Here we show that, by alternating ferroelectric and semiconducting nanowires over an insulating substrate, the ferroelectric dipole moment can be stabilized by injected free charge carriers accumulating laterally in the neighboring semiconducting nanowires. This lateral electrostatic coupling between ferroelectric and semiconducting nanowires offers new opportunities to design new device architectures. As an example, we demonstrate the fabrication of an elementary non-volatile memory device in a transistor-like configuration, of which the source-drain current exhibits a typical hysteretic behavior with respect to the poling voltage. The potential for size reduction intrinsic to the nanostructured hybrid layer offers opportunities for the development of strongly miniaturized ferroelectric and piezoelectric devices.

Controlling dielectric loss and ionic conductivity through processing optimization of electrostrictive polymers

Electro-active polymers (EAPs) such as P(VDF-TrFE-CTFE) was demonstrated to be greatly promising in the field of flexible sensors and actuators[1], but their low dielectric strength driven by ionic conductivity is main concern for achieving high electrostrictive performance. The well-known quadratic dependence of applied electric field on strain response as well as mechanical energy density highlights the importance of improving EAPs electrical breakdown while reducing the leakage current. This paper demonstrates that by controlling processing parameters of polymer synthesis and fabrication procedure, it is possible to drastically increase the electrical breakdown and decrease the ionic conductivity, giving rise to an enhancement in breakdown voltage of around 64% and a reduction in leakage current intensity of 73% at 30V/μm. Effect of polymer crystallinity, molecular mass, as well as crystallization temperature on leakage current were also investigated..

Field-effect memory transistors based on arrays of nanowires of a ferroelectric polymer

Ferroelectric poly(vinylidene fluoride-co-trifluoroethylene), P(VDF-TrFE), is increasingly used in organic non-volatile memory devices, e.g., in ferroelectric field effect transistors (FeFETs). Here, we report on FeFETs integrating nanoimprinted arrays of P(VDF-TrFE) nanowires. Two previously-unreported architectures are tested, the first one consisting of stacked P(VDF-TrFE) nanowires placed over a continuous semiconducting polymer film; the second one consisting of a nanostriped blend layer wherein the semiconducting and ferroelectric components alternate regularly. The devices exhibit significant reversible memory effects, with operating voltages reduced compared to their continuous film equivalent, and with different possible geometries of the channels of free charge carriers accumulating in the semiconductor.