Differential Entropy: An Appropriate Analysis to Interpret the Shape Complexity of Self-Similar Organic Islands.

Differential entropy, along with fractal dimension, is herein employed to describe and interpret the shape complexity of self-similar organic islands. The islands are imaged with in situ Atomic Force Microscopy, following, step-by-step, the evolution of their shape while deposition proceeds. The fractal dimension shows a linear correlation with the film thickness, whereas the differential entropy presents an exponential plateau.

A Functional Spiking Neural Network of Ultra Compact Neurons.

We demonstrate that recently introduced ultra-compact neurons (UCN) with a minimal number of components can be interconnected to implement a functional spiking neural network. For concreteness we focus on the Jeffress model, which is a classic neuro-computational model proposed in the 40's to explain the sound directionality detection by animals and humans. In addition, we introduce a , whose architecture is inspired by the Hodgkin-Huxley axon delay-line and where the UCNs implement the nodes of Ranvier.

Source-measuring unit for characterizing resistive switching devices.

This manuscript presents a home-made source-measuring unit (SMU) that was developed to characterize Resistive Switching (RS) devices. It can apply voltage up to ±10 V (setting time <1 µs) and measure the current at the same time. The particularity of this SMU is that it can rapidly switch between high current measurements (up to ±25 mA, settling time <25 µs) and low current measurements (typically ∼100 nA, noise level with digital filtering <1 nA, settling time <2 ms).

Biologically Relevant Dynamical Behaviors Realized in an Ultra-Compact Neuron Model.

We demonstrate a variety of biologically relevant dynamical behaviors building on a recently introduced ultra-compact neuron (UCN) model. We provide the detailed circuits which all share a common basic block that realizes the leaky-integrate-and-fire (LIF) spiking behavior. All circuits have a small number of active components and the basic block has only three, two transistors and a (SCR).

Quantitative phase-mode electrostatic force microscopy on silicon oxide nanostructures.

Phase-mode electrostatic force microscopy (EFM-Phase) is a viable technique to image surface electrostatic potential of silicon oxide stripes fabricated by oxidation scanning probe lithography, exhibiting an inhomogeneous distribution of localized charges trapped within the stripes during the electrochemical reaction. We show here that these nanopatterns are useful benchmark samples for assessing the spatial/voltage resolution of EFM-phase. To quantitatively extract the relevant observables, we developed and applied an analytical model of the electrostatic interactions in which the tip and the surface are modelled in a prolate spheroidal coordinates system, fitting accurately experimental data.

Spike-shape dependence of the spike-timing dependent synaptic plasticity in ferroelectric-tunnel-junction synapses.

Resistive switching (RS) devices have attracted increasing attention for artificial synapse applications in neural networks because of their nonvolatile and analogue resistance changes. Among the neural networks, a spiking neural network (SNN) based on spike-timing-dependent plasticity (STDP) is highly energy efficient. To implement STDP in resistive switching devices, several types of voltage spikes have been proposed to date, but there have been few reports on the relationship between the STDP characteristics and spike types.

An ultra-compact leaky-integrate-and-fire model for building spiking neural networks.

We introduce an ultra-compact electronic circuit that realizes the leaky-integrate-and-fire model of artificial neurons. Our circuit has only three active devices, two transistors and a silicon controlled rectifier (SCR). We demonstrate the implementation of biologically realistic features, such as spike-frequency adaptation, a refractory period and voltage modulation of spiking rate.

Multifrequency Force Microscopy of Helical Protein Assembly on a Virus.

High-resolution microscopy techniques have been extensively used to investigate the structure of soft, biological matter at the nanoscale, from very thin membranes to small objects, like viruses. Electron microscopy techniques allow for obtaining extraordinary resolution by averaging signals from multiple identical structures. In contrast, atomic force microscopy (AFM) collects data from single entities.

Universal electric-field-driven resistive transition in narrow-gap Mott insulators.

A striking universality in the electric-field-driven resistive switching is shown in three prototypical narrow-gap Mott systems. This model, based on key theoretical features of the Mott phenomenon, reproduces the general behavior of this resistive switching and demonstrates that it can be associated with a dynamically directed avalanche. This model predicts non-trivial accumulation and relaxation times that are verified experimentally.

Avalanche breakdown in GaTa4Se(8-x)Te(x) narrow-gap Mott insulators.

Mott transitions induced by strong electric fields are receiving growing interest. Recent theoretical proposals have focused on the Zener dielectric breakdown in Mott insulators. However, experimental studies are still too scarce to conclude about the mechanism.

Non-Hebbian learning implementation in light-controlled resistive memory devices.

Non-Hebbian learning is often encountered in different bio-organisms. In these processes, the strength of a synapse connecting two neurons is controlled not only by the signals exchanged between the neurons, but also by an additional factor external to the synaptic structure. Here we show the implementation of non-Hebbian learning in a single solid-state resistive memory device.

A light-controlled resistive switching memory.

Sketch of the configuration of a light-controlled resistive switching memory. Light enters through the Al(2) O(3) uncovered surface and reaches the optically active p-Si substrate, where carriers are photogenerated and subsequently injected in the Al(2) O(3) layer when a suitable voltage pulse is applied. The resistance of the Al(2) O(3) can be switched between different non-volatile states, depending on the applied voltage pulse and on the illumination conditions.

Layered distribution of charge carriers in organic thin film transistors.

Drain-source current in organic thin-film transistors has been monitored in situ and in real time during the deposition of pentacene. The current starts to flow when percolation of the first monolayer (ML) occurs and, depending on the deposition rate, saturates at a coverage in the range 2-7 MLs. The number of active layers contributing to the current and the spatial distribution of charge carriers are modulated by the growth mode.

Multiscale morphology of organic semiconductor thin films controls the adhesion and viability of human neural cells.

We investigate how multiscale morphology of functional thin films affects the in vitro behavior of human neural astrocytoma 1321N1 cells. Pentacene thin film morphology is precisely controlled by means of the film thickness, Theta (here expressed in monolayers (ML)). Fluorescence and atomic force microscopy allow us to correlate the shape, adhesion, and proliferation of cells to the morphological properties of pentacene films controlled by saturated roughness, sigma, correlation length, xi, and fractal dimension, d(f).

Synchronized optical and electrical characterization of discotic liquid crystals thin films.

We describe a setup suitable for simultaneously measuring optical and electrical properties of a liquid crystal mesophase upon temperature variation, and the difference in the order parameters between the bulk and the interface with the substrate. It integrates high-resolution polarized light optical microscopy, temperature regulation, and electrical measurements in a controlled atmosphere with a software kernel that controls the instruments and synchronizes the data streams. A user-friendly interface allows us to program multistep experiments controlling all the instruments and data acquisition by a specifically designed scheduler.

Time-temperature integrator based on the dewetting of polyisobutylene thin films.

This work reports the application of a patterned thin film of polyisobutylene (PIB) irradiated with an electron beam as a time-temperature integrator, i.e., a device that is able to record the thermal history of a product.

Measurement of DNA morphological parameters at highly entangled regime on surfaces.

The morphology of circular DNA deposited from a solution on the mica surface is analyzed from the power spectrum density (PSD) of the atomic force microscopy (AFM) images. Sample morphology is modulated in a broad range of concentration C from isolated molecules to highly entangled networks. DNA exhibits a multiaffine behavior with two correlation length scales: the persistence length P which remains constant ( approximately 50 nm) within the C range and the intermolecular distance xi which exhibits a decay with increasing C.

DNA adsorption measured with ultra-thin film organic field effect transistors.

Organic ultra-thin film field effect transistors (FET) are operated as label-free sensors of deoxyribonucleic acid (DNA) adsorption. Linearized plasmid DNA molecules (4361 base pairs) are deposited from a solution on two monolayers thick pentacene FET. The amount of adsorbed DNA is measured by AFM and correlated to the concentration of the solution.

Monolayer control of discotic liquid crystal by electromigration of dewetted layers in thin film devices.

We show that ultrathin films of a semiconductive discotic liquid crystal, viz. phthalocyanines, can be organized to form a conductive channel tens of microns long between Au electrodes with thickness control over a single monolayer. Our approach exploits the electromigration of the isotropic phase formed starting from the pretransitional region of the columnar-isotropic phase transition.

Conductive sub-micrometric wires of platinum-carbonyl clusters fabricated by soft-lithography.

Conductive wires of sub-micrometer width made from platinum-carbonyl clusters have been fabricated by solution-infilling of microchannels as in microinject molding in capillaries (MIMIC). The process is driven by the liquid surface tension within the micrometric channels followed by the precipitation of the solute. Orientation of supramolecular crystalline domains is imparted by the solution confinement combined with unidirectional flow.

High-resolution mapping of the electrostatic potential in organic thin-film transistors by phase electrostatic force microscopy.

We investigate by a scanning probe technique termed phase-electrostatic force microscopy the local electrostatic potential and its correlation to the morphology of the organic semiconductor layer in operating ultra-thin film pentacene field effect transistors. This technique yields a lateral resolution of about 60 nm, allowing us to visualize that the voltage drop across the transistor channel is step-wise. Spatially localized voltage drops, adding up to about 75% of the potential difference between source and drain, are clearly correlated to the morphological domain boundaries in the pentacene film.

Charge injection across self-assembly monolayers in organic field-effect transistors: odd-even effects.

We investigate the role of self-assembly monolayers in modulating the response of organic field-effect transistors. Alkanethiol monolayers of chain length n are self-assembled on the source and drain electrodes of pentacene field-effect transistors. The charge carrier mobility mu exhibits large fluctuations correlated with odd-even n.

Field effect transistors with organic semiconductor layers assembled from aqueous colloidal nanocomposites.

We demonstrate field effect transistors based on organic semiconductor molecules dispersed in a self-organized polystyrene (PS) latex bead matrix. An aqueous colloidal composite made of PS and tetrahexylsexithiophene (H4T6) is deposited with a micropipet into the channel of a bottom-contact field effect transistor. The beads self-organize into a network whose characteristic distances are governed by their packing.

Field-effect transistors based on self-organized molecular nanostripes.

Charge transport properties in organic semiconductors depend strongly on molecular order. Here we demonstrate field-effect transistors where drain current flows through a precisely defined array of nanostripes made of crystalline and highly ordered molecules. The molecular stripes are fabricated across the channel of the transistor by a stamp-assisted deposition of the molecular semiconductors from a solution.