R. Zamboni

Electrical characteristics of field-effect transistors formed with ordered α-sexithienyl

Abstract X-ray observations on α-sexithienyl allowed us to reveal that this material, in the form of evaporated film, has an ordered crystalline structure, with an average dimension of the grains of 30 nm. Field-effect transistors, fabricated by using evaporated α-sexithienyl films as active element of the devices, showed good modulation characteristics, but low charge carrier mobilities. This fact is attributed to the small grain dimensions and it is suggested that better results could be obtained by using opportune evaporation substrates.

A transparent organic transistor structure for bidirectional stimulation and recording of primary neurons

Real-time stimulation and recording of neural cell bioelectrical activity could provide an unprecedented insight in understanding the functions of the nervous system, and it is crucial for developing advanced in vitro drug screening approaches. Among organic materials, suitable candidates for cell interfacing can be found that combine long-term biocompatibility and mechanical flexibility. Here, we report on transparent organic cell stimulating and sensing transistors (O-CSTs), which provide bidirectional stimulation and recording of primary neurons. We demonstrate that the device enables depolarization and hyperpolarization of the primary neuron membrane potential. The transparency of the device also allows the optical imaging of the modulation of the neuron bioelectrical activity. The maximal amplitude-to-noise ratio of the extracellular recording achieved by the O-CST device exceeds that of a microelectrode array system on the same neuronal preparation by a factor of 16. Our organic cell stimulating and sensing device paves the way to a new generation of devices for stimulation, manipulation and recording of cell bioelectrical activity in vitro and in vivo.

Tetracene light-emitting transistors on flexible plastic substrates

We report on organic light-emitting (field-effect) transistors (LETs) fabricated on a flexible and transparent plastic foil (Mylar), acting both as substrate and gate dielectric. The foil is patterned on one side with bottom-contact gold source and drain electrodes, while a thin film of gold is evaporated on the opposite side of the foil to form the gate electrode. A vacuum sublimed tetracene film is employed as an active layer for charge transport and light emission. Atomic force microscopy shows that tetracene films have a good adhesion on Mylar and exhibit a granular structure. The transistor shows unipolar p-type behavior with mobilities typically of 5×10−4cm2∕Vs. Drain-source current and electroluminescence have been simultaneously measured. Provided a suitable gate bias is applied, light emission occurs at drain-source voltages (Vds) above saturation. LETs on plastic substrates could open the way to flexible devices combining the switching function of a transistor and the light emission.

Low-threshold blue lasing from silk fibroin thin films

Silk is a natural biocompatible material that can be integrated in a variety of photonic systems and optoelectronic devices. The silk replication of patterned substrates with features down to tens of nanometers is exploited to realize highly transparent, mechanically stable, and free-standing structures with optical wavelength size. We demonstrate organic lasing from a blue-emitting stilbene-doped silk film spin-coated onto a one-dimensional distributed feedback grating (DFB). The lasing threshold is lower than that of organic DFB lasers based on the same active dye. These findings pave the way to the development of an optically active biocompatible technological platform based on silk.

Wave-dispersed third-order nonlinear optical properties of C60 thin films

Abstract Results of wave-dispersed third harmonic generation measurements in sublimed C60 thin films are reported and discussed within a three-level model. Two strong resonant enhancements in cubic susceptibility χ(3)(−3ω; ω, ω, ω) are observed. The first one, occurring at a fundamental wavelength of 1.3 μm with a χ(3)(−3ω; ω, ω, ω) maximum value of 6.1×10−11 esu, is interpreted in terms of a two-photon resonance with the one-photon forbidden electronic T1g level. The second resonance at 1.064 μm, with a maximum value of χ(3)(−3ω; ω, ω, ω)=8.2×10−11 esu is interpreted as a three-photon resonance with the lowest one-photon allowed T1u electronic level.

Optical, electrical and structural comparative study of polycondensed thiophene based polymers

Abstract An exhaustive characterization of an homologous series of polycondensed thiophene based polymers has been carried out in order to provide a comparative and consistent survey of some of the crucial properties that are generally related to the electrical properties of poly-heterocycles.

A silk platform that enables electrophysiology and targeted drug delivery in brain astroglial cells

Astroglial cell survival and ion channel activity are relevant molecular targets for the mechanistic study of neural cell interactions with biomaterials and/or electronic interfaces. Astrogliosis is the most typical reaction to in vivo brain implants and needs to be avoided by developing biomaterials that preserve astroglial cell physiological function. This cellular phenomenon is characterized by a proliferative state and altered expression of astroglial potassium (K(+)) channels. Silk is a natural polymer with potential for new biomedical applications due to its ability to support in vitro growth and differentiation of many cell types. We report on silk interactions with cultured neocortical astroglial cells. Astrocytes survival is similar when plated on silk-coated glass and on poly-D-lysine (PDL), a well known polyionic substrate used to promote astroglial cell adhesion to glass surfaces. Comparative analyses of whole-cell patch-clamp experiments reveal that silk- and PDL-coated cells display depolarized resting membrane potentials (-40 mV), very high input resistance, and low specific conductance, with values similar to those of undifferentiated glial cells. Analysis of K(+) channel conductance reveals that silk-astrocytes express large outwardly delayed rectifying K(+) current (K(DR)). The magnitude of K(DR) in PDL- and silk-coated astrocytes is similar, indicating that silk does not alter the resting K(+) current. We also demonstrate that guanosine- (GUO) embedded silk enables the direct modulation of astroglial K(+) conductance in vitro. Astrocytes plated on GUO-embedded silk are more hyperpolarized and express inward rectifying K(+) conductance (K(ir)). The K(+) inward current increases and this is paralleled by upregulation and membrane polarization of K(ir)4.1 protein signal. Collectively these results indicate that silk is a suitable biomaterial platform for the in vitro studies of astroglial ion channel responses and related physiology.

Disorder influenced optical properties of α-sexithiophene single crystals and thin evaporated films

Abstract We report measurements on steady state and time resolved photoluminescence (PL) and PL-excitation (PLE) of sexithiophene single crystals and evaporated thin films. In contrast to the broad spectral features usually observed even in crystals, we have observed for the first time sharp emission peaks from the single crystal superimposed on a broader background. The broad background is due to emission from defect states, which are observable in PLE measurements and site-selective PL. These defect states are visible both in films and crystals, but their density and nature are strongly dependent on the growth conditions. Additionally in the film a fast energy transfer from the primary excited molecular states to defect states is observed on a sub 100 ps timescale whereas the fluorescence lifetime of these defects is around 700 ps.

Polarised Electroluminescence from Vacuum-Grown Organic Light-Emitting Diodes

Sublimed organic thin films are usually amorphous, and light-emitting diodes that utilise such films as the emissive layer emit isotropically distributed light with random polarisation. However, α-sexithiophene (αT6) forms highly ordered thin films, and we show here that, by controlling the morphology of the emissive layer in a LED based on αT6, we can achieve polarised electroluminescence. Vacuum-sublimed films of αT6 contain highly ordered grains with the long axes of the molecules standing almost perpendicular to the substrate surface. Fluorescence arises from the decay of excited states localised on individual molecules (Frenkel excitons), and is polarised along the long molecular axis. Excitons diffuse within the film, and can migrate into defects such as grain boundaries and be trapped. In films containing small grains we observe isotropic emission from randomly distributed defects, but when we increase the grain size (to reduce the defect density) emission occurs from ordered regions and we measure polarised electroluminescence consistent with the bulk morphology.

Electronic levels ordering in α-sexithienyl

Abstract The electronic level ordering in a polycrystalline thin film of α-sexithienyl (T 6 ) has been determined by photoluminescence, one- and two-photon excitation, and photoconductivity measurements. The lowest 1 1 B u Frenkel exciton is located at 17 452 cm −1 . From site-selective photoluminescence we locate an extended inhomogeneous distribution of localized isolated T 6 levels (X-traps) below the 1 1 B u Frenkel exciton extending for 2000 cm −1 . The origin of the lowest 2 1 A g Frenkel exciton is located at 18 350 cm −1 , i.e. 898 cm −1 above the 1 1 B u . The photoconductivity action spectrum shows an onset at about the same energy of the 2 1 A g exciton band and reaches a maximum 1450 cm −1 above the 1 1 B u exciton band. T 6 is proposed as a good model compound for real polythiophene. The difference in energetics and photophysical properties due to different electronic correlation effects between polyenes and polythiophene is discussed.