Pablo Stoliar

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.

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. The wires exhibit ohmic conductivity with a value of 0.2 S/cm that increases, after thermal decomposition of the platinum-carbonyl cluster precursor to Pt, to 35 S/cm.

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. Electrical characteristics on the dried DNA/pentacene FETs were studied as a function of DNA concentration in the solution. Shift of the pinch-off voltage across a wide range of DNA concentration, from very diluted to highly concentrated, is observed. It can be ascribed to additional positive charges in the semiconductor induced by DNA at a rate of one charge for every 200 base pairs. The sensitivity 74 ng/cm(2), corresponding to 650 ng/ml, is limited by the distribution of FET parameters upon repeated cycles, and is subjected to substantial improvement upon standardization. Our work demonstrates the possibility to develop label-free transducers suitable to operate in regimes of high molecular entanglement.

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.

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). At early incubation times, cell adhesion exhibits a transition from higher to lower values at Theta approximately 10 ML. This is explained using a model of conformal adhesion of the cell membrane onto the growing pentacene islands. From the model fitting of the data, we show that the cell explores the surface with a deformation of the membrane whose minimum curvature radius is 90 (+/- 45) nm. The transition in the adhesion at approximately 10 ML arises from the saturation of xi accompanied by the monotonic increase of sigma, which leads to a progressive decrease of the pentacene local radius of curvature and hence to the surface area accessible to the cell. Cell proliferation is also enhanced for Theta < 10 ML, and the optimum morphology parameter ranges for cell deployment and growth are sigma <or= 6 nm, xi > 500 nm, and d(f) > 2.45. The characteristic time of cell proliferation is tau approximately 10 +/- 2 h.

Resistive switching dependence on atomic layer deposition parameters in HfO2-based memory devices

Resistance random access memory (ReRAM) is considered a promising candidate for the next generation of non-volatile memory. In this work, we fabricate Co/HfO2/Ti devices incorporating atomic-layer-deposited HfO2 thin films as the active material grown under different atomic layer deposition (ALD) conditions. We focus on analyzing the effect of ALD conditions on the resistive switching behaviour of the devices. Electrical characterization reveals a particular non-crossing current–voltage curve and bipolar resistive switching behaviour. Device memory properties were confirmed by stability and retention measurements as well as voltage pulses, by which logical computational processes were conducted. X-ray photoelectron spectroscopy combined with electrical measurements demonstrates that the presence of Hf sub-oxides at the interface with the underlying Ti layer is required in order to achieve a stable switching device. The ability of Ti to scavenge oxygen from the HfO2 is shown to be affected by the ALD conditions.

Carbon nanotube networks patterned from aqueous solutions of latex bead carriers

Patterning arrays of single walled carbon nanotubes (SWCNTs) is an important technological step for a range of applications of these materials in electronics, optics, electrochemistry and bio-sensing. Here, we demonstrate the fabrication of conductive networks of SWCNTs with controlled porosity on the mesoscopic scale by two dimensional assembly, from aqueous solutions, of monodisperse polystyrene (PS) beads carrying SWCNTs at their surface. SWCNTs are adsorbed onto the bead surface by non covalent hydrophobic interactions. Once the PS beads self-organize into ordered arrays, they are removed by an organic solvent, yielding a connected SWCNTs network that retains the geometrical structure of the bead assembly. The electrical properties of these ordered SWCNTs arrays exhibit a Mott-conducting behavior with RT conductivity of 100 S cm−1.

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. Dewetted isotropic material accumulates to the negative electrode by applying a longitudinal electric field of about 1 V/microm. Dewetting and electromigration expose an ultrathin film, a few monolayers thick, exhibiting columnar liquid crystal order. The layers of this ultrathin film melt progressively above T(C) and can be individually exfoliated by electromigration, starting from the ninth down to the first monolayer. The analysis of the current flowing through the junction as a function of the temperature, together with the comparative imaging of the evolution of morphology, yields a detailed picture of the changes in the dimensionality of the conductive phthalocyanine film and allows us to extract the behavior of the order parameter. The phenomenon of electromigration opens interesting questions on the technological control of individual monolayers on device patterns.

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. The device is fabricated by irradiation with an electron beam of regions of a PIB thin film to different doses of electrons. A different dewetting behavior occurs at these regions upon thermal exposure, depending on the dose. The experimental results are quantified by means of a model of dewetting based on nucleation and growth of holes in a strong slippage regime.

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. Applying a diffusion based model in which xi scales as xi approximately D(-0.25).C(-0.5), we extracted the DNA diffusion coefficient D approximately 2 x 10(-7) cm(2)/s. This value is consistent with a high-molecular-weight plasmid DNA supercoiled in the solution.