Calogero Sciascia

Dielectrophoretic Assembly of High-Density Arrays of Individual Graphene Devices for Rapid Screening

We establish the use of dielectrophoresis for the directed parallel assembly of individual flakes and nanoribbons of few-layer graphene into electronic devices. This is a bottom-up approach where source and drain electrodes are prefabricated and the flakes are deposited from a solution using an alternating electric field applied between the electrodes. These devices are characterized by scanning electron microscopy, atomic force microscopy, Raman spectroscopy, and electron transport measurements. They are electrically active and their current carrying capacity and subsequent failure mechanism is revealed. Akin to carbon nanotubes, we show that the dielectrophoretic deposition is self-limiting to one flake per device and is scalable to ultralarge-scale integration densities, thereby enabling the rapid screening of a large number of devices.

Sub-Micrometer Charge Modulation Microscopy of a High Mobility Polymeric n-Channel Field-Effect Transistor

After two decades of fundamental research and steady development, solution processable organic fi eld-effect transistors (OFETs) have recently reached a mature stage that preludes their adoption in a variety of commercial applications from light-weight, stretchable, large-area sensors to low-cost, fl exible electronic circuits. [ 1–5 ] Despite the fact that fi eld-effect mobilities ( μ fe ) exceeding 1 cm 2 V − 1 s − 1 for both pand n-channel OFETs are now achievable with commercial, off-the-shelf conjugated organic small molecules and polymers, [ 4 ] fundamental studies are still strongly needed because of an incomplete understanding of the main mechanisms governing charge injection and transport in such devices. [ 7–9 ] To this extent, probing techniques capable of providing local information regarding mobility, fi eld, and charge distribution along the channel of a working device would be greatly benefi cial. Of particular interest for organic semiconductors is the relationship between microstructure and charge transport properties. This is even more evident in the case of recently developed high-mobility and stable n-channel OFETs, [ 6 ] in which the unusual face-on orientation of molecules has confounded expectations of what is required for effective charge transport. Scanning Kelvin-probe miscroscopy (SKPM) [ 7 ] is a powerful technique that allowed to map the potential profi le along the channels of OFETs with resolution better than 50 nm. [ 8 , 9 ]

Mapping Orientational Order of Charge-Probed Domains in a Semiconducting Polymer

Structure-property relationships are of fundamental importance to develop quantitative models describing charge transport in organic semiconductor based electronic devices, which are among the best candidates for future portable and lightweight electronic applications. While microstructural investigations, such as those based on X-rays, electron microscopy, or polarized optical probes, provide necessary information for the rationalization of transport in macromolecular solids, a general model predicting how charge accommodates within structural maps is not yet available. Therefore, techniques capable of directly monitoring how charge is distributed when injected into a polymer film and how it correlates to structural domains can help fill this gap. Supported by density functional theory calculations, here we show that polarized charge modulation microscopy (p-CMM) can unambiguously and selectively map the orientational order of the only conjugated segments that are probed by mobile charge in the few nanometer thick accumulation layer of a high-mobility polymer-based field-effect transistor . Depending on the specific solvent-induced microstructure within the accumulation layer, we show that p-CMM can image charge-probed domains that extend from submicrometer to tens of micrometers size, with markedly different degrees of alignment. Wider and more ordered p-CMM domains are associated with improved carrier mobility, as extracted from device characteristics. This observation evidences the unprecedented opportunity to correlate, directly in a working device, electronic properties with structural information on those conjugated segments involved in charge transport at the buried semiconductor-dielectric interface of a field-effect device.

Electric field and charge distribution imaging with sub-micron resolution in an organic Thin-Film Transistor

Theoretically, the electric field becomes infinite at corners of two and three dimensions and edges of three dimensions. Conventional finite-element and boundary element methods do not yield satisfactory results at close proximity to these singular locations. In this paper, we describe the application of a fast and accurate BEM solver (which uses exact analytic expressions to compute the effect of source distributions on flat surfaces) to compute the electric field near three-dimensional corners and edges. Results have been obtained for distances as close as 1µm near the corner / edge and good agreement has been observed between the present results and existing analytical solutions.

Carbon nanotubes-photochromic polymer blends: Light-triggered conductance switching device

We demonstrate an easy and scalable way to get devices in which the electrical resistance can be tuned by UV/visible light exposure. In particular, such a feature is based on photochromic diarylethene polymer - carbon nanotubes blend where reversible light controlled switching of the photochromic component induces a change in the electrical conductivity of the material. Differently from the single molecule approach, we use a simple ‘wet-chemistry method’ to obtain a net of carbon nanotubes embedded in a photochromic polymer. When the device is illuminated with UV light, a decreasing in the electric resistance of the active layer occurs. A resistance modulation up to 300% of the initial value is achieved, which is completely reversible with visible light illumination. The process is repeatable many times with a good fatigue resistance at room conditions. Supported by electrical and spectroscopic evidences, we show that the light-triggered electrocyclization of the polymer affects the inter-tube charge mobility, resulting in large overall change in the resistance of the device.

Light-triggered conducting properties of a random carbon nanotubes network in a photochromic polymer matrix

Photochromic materials reversibly change their colour due to a photochemical reaction that takes place when the material is irradiated with photons of suitable energy. This peculiar feature has been extensively exploited to develop smart sunglasses, filters and inks. With a proper molecular design it is possible to enable modulation not only of colour but also of other properties such as refractive index, dipole moment, nonlinear optical properties or conductivity by a photoswitching of the molecular structure. The approach herein developed consists in modifying, upon irradiation, the properties of a molecular component coupled with the photochromic molecule. In particular, the switching features of photochromic systems are matched with the intriguing peculiar properties of carbon nanotubes (CNTs). A photochromic polyester has been properly synthesised to be used as switching polymer matrix coupled with a network of CNTs. Irradiation of the polymer/CNTs blend results into a light-triggered conductance switching. The reversible electrocyclization of the polymer under UV-vis illumination results into a modification of the inter-tube charge mobility, and accordingly, of the overall resistance of the blend. Solution techniques allow us to obtain blended films with sheet resistance modulation larger than 150%, good thermal stability and fatigue resistance at room conditions, in an easier, faster and scalable way as respect to the single-molecule approach.ÿ

High-resolution mapping of electric field inside organic optoelectronic devices

By combining confocal microscopy with electroreflectance spectroscopy we directly map electric field amplitude distribution between electrodes in a prototypical organic semiconductor device. We demonstrate this approach on a copper phthalocyanine photodetector.