Tatiana Berzina

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.

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.

Pattern recognition approach to the study of the interactions between metalloporphyrin Langmuir–Blodgett films and volatile organic compounds

Abstract In recent years metalloporphyrins and their derivatives have been proposed as sensitive layer for mass variation based transducers for the detection of volatile compounds. Their sensitivity/selectivity properties have suggested their application in electronic nose systems. Despite the positive applications appeared so far, a systematic study of the kind of interactions ruling the sensing mechanism has not yet been conducted. The selectivity towards a variety of compounds suggests the presence of weak Van der Waals or hydrogen bond interactions, but also certain evidences about coordination interactions with the central metal ion coordinated to the porphyrin were reported. In this paper, a study oriented towards a deeper comprehension of such mechanisms is introduced. From the experimental point of view, a set of quartz microbalances, coated with Langmuir–Blodgett films of various metalloporphyrins, were exposed to a number of volatile compounds at various concentrations. Linear sorption energy relationship theory provides, in many cases, a satisfactory model for sorption sensors in which Van der Waals and hydrogen bonding are responsible for the interaction. Nevertheless, the application of such a model results inadequate in the case here discussed. A more satisfactory treatment of the data was obtained considering a pattern recognition approach based, in particular, on a self-organizing neural network. This type of analysis has shown the role played by the central metal in determining the sensor selectivity; furthermore, the results have allowed to hypotize the presence of a coordination mechanism, superimposed to the weak interactions, between volatile compounds and metalloporphyrin films.

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).

Stochastic hybrid 3D matrix: learning and adaptation of electrical properties

Memristive devices are electronic elements with memory properties. This feature marks them out as possible candidates for mimicking synapse properties. Development of systems capable of performing simple brain operations demands a high level of integration of elements and their 3D organization into networks. Here, we demonstrate the formation and electrical properties of stochastic polymeric matrices. Several features of the network revealed similarities with those of the nervous system. In particular, applying different training protocols, we obtained two kinds of learning comparable to the “baby” and “adult” learning in animals and humans. To mimic “adult” learning, multi-task training was applied simultaneously resulting in the formation of few parallel pathways for a given task, modifiable by successive training. To mimic “baby” learning (imprinting), single task training was applied at one time, resulting in the formation of multiple parallel signal pathways, scarcely influenced by successive training.

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.