M. P. Fontana

Hybrid electronic device based on polyaniline-polyethyleneoxide junction

A three-electrode hybrid molecular electronic element based on polyaniline (PANI) emeraldine base–polyethylene oxide∕LiCl was fabricated and tested. Source and drain electrodes were connected to the conducting polymer layer. Solid electrolyte was deposited as a narrow stripe over the PANI film, and the third electrode (gate), maintained at ground potential, was attached to it. Drain and gate currents were measured during a drain voltage sweep. Drain voltage–current characteristics revealed a rectifying behavior, while gate characteristics were similar to those for cyclic voltammograms. Such behavior was attributed to the electrochemical control of the redox state (and hence of conductivity) of PANI area under the solid electrolyte. The device was stable and reproducible with robust electrical characteristics. In particular, an asymmetry in time relaxation due to ion diffusion was found; a possible application of this to the use of this device in adaptive conducting networks is proposed.

Optimization of an organic memristor as an adaptive memory element

The combination of memory and signal handling characteristics of a memristor makes it a promising candidate for adaptive bioinspired information processing systems. This poses stringent requirements on the basic device, such as stability and reproducibility over a large number of training/learning cycles, and a large anisotropy in the fundamental control material parameter, in our case the electrical conductivity. In this work we report results on the improved performance of electrochemically controlled polymeric memristors, where optimization of a conducting polymer (polyaniline) in the active channel and better environmental control of fabrication methods led to a large increase both in the absolute values of the conductivity in the partially oxydized state of polyaniline and of the on-off conductivity ratio. These improvements are crucial for the application of the organic memristor to adaptive complex signal handling networks.

Electrochemical control of the conductivity in an organic memristor: a time-resolved X-ray fluorescence study of ionic drift as a function of the applied voltage.

Grazing-incidence X-ray fluorescence measurements were applied for a time-resolved study of an organic memristor conductivity variation mechanism. A comparison of these results with electrical measurements has allowed us to conclude that the variation of the fluorescence intensity of Rb ions is directly connected to the ionic charge transferred between the conducting polymer and the solid electrolyte, which made up the device. In addition, the conductivity of the memristor was shown to be a function of the transferred ionic charge.

Spectroscopic investigation of an electrochemically controlled conducting polymer-solid electrolyte junction

We have recently reported a hybrid conducting polymer-solid electrolyte heterojunction in which electronic channel current is controlled by ionic diffusion and redox reactions involving doped polyethyleneoxide and doped polyaniline (PANI). In this paper we demonstrate on the microscopic level the validity of the model we proposed to interpret the electronic behavior of the device we fabricated. In particular, we used resonance and vicinity (surface-enhanced-Raman-scattering like) enhanced micro-Raman spectroscopy to map out the redox state of PANI along the conducting channel, simultaneously with the determination of the voltamperometric characteristics in a potentiostat configuration. The Raman data were complemented as appropriate by the optical and Fourier transform infrared absorption spectroscopies.

Light-Driven Release from Polymeric Microcapsules Functionalized with Bacteriorhodopsin

Bacteriorhodopsin was incorporated into the shell of polymeric capsules. Light-driven variation of intercapsule volume pH with successive pore opening was demonstrated by scanning electron microscopy. Release of the encapsulated dye molecules was studied by confocal fluorescence microscopy.

Polymeric elements for adaptive networks

Polymeric electrochemical elements of adaptive networks are considered. The main features of these elements are compared with the elements of the nervous system of the snail Lymnaea stagnalis. In particular, synthetic analogues of neurons and synapses are fabricated. The capability of the system for learning is demonstrated with a model of the simplest network composed of eight electrochemical elements. An alternative approach based on the formation of fiber networks is proposed. This approach will make it possible to fabricate more complex systems with a random distribution of mutual contacts between elements.

Polymeric electrochemical element for adaptive networks: Pulse mode

An electrochemically controlled polymeric heterojunction working as a memristor, i.e., having memory properties, was investigated in pulse mode, mimicking synaptic behavior of signal transmission in biological systems. Influence of parameters such as pulse duration, interval between pulses, and value of potential base level was analyzed. Learning capabilities were shown to be reversible and repeatable for both potentiation and inhibition of signal transmission. The adaptive behavior of the element was investigated and was shown to be more efficient than the dc mode.

Non-equilibrium electrical behaviour of polymeric electrochemical junctions

Current oscillations were observed at constant applied voltage in electrochemical heterostructures, composed of conducting polymer, solid electrolyte and a charge accumulating system (graphite electrode or capacitor). These oscillations were attributed to the periodic modulation of the conductivity in the active zone of conducting polymer (polyaniline) due to ionic flow in the solid electrolyte (polyethylene oxide with LiClO4).

Origin of current oscillations in a polymeric electrochemically controlled element

We present a model for describing electrical properties of an electrochemically controlled heterojunction between a conducting polymer (polyaniline) and a solid electrolyte (lithium salt doped polyethylene oxide). In particular, the difference in the kinetics of the conductivity variation for different polarities of the bias voltage and appearance of current oscillations at constant applied voltage are considered. The results of our simulation are in good agreement with published experimental data.

Deposition of uniform conductive polyaniline films and approach for their patterning

Abstract Conductive polyaniline Langmuir–Blodgett (LB) films are considered to be candidates for numerous applications. However, the quality of their deposition is usually non-satisfactory. In addition, the possibilities of many applications are restricted by the intrinsic low speed of deposition with the LB technique. We have found a solution to these problems and obtained uniform ultra thin polyaniline films using the Langmuir–Schaefer (LS) technique, which allows very fast monolayer transfer onto the substrate. A well known approach of deposition of mixed monolayers was used to obtain uniform films, but in our case the matrix compound was chosen in such a way as to provide a possibility of its selective dissolution from the deposited multilayers without the deterioration of uniformity of remaining polyaniline layer. Finally, we demonstrate an approach to polyaniline film patterning with an electron beam, which is based on the possibility to cross-link the polyaniline molecules without considerable loss of film conductivity and on further dissolution of polyaniline from non-irradiated areas.