Antonio Cassinese

Stem cell-compatible eumelanin biointerface fabricated by chemically controlled solid state polymerization†

Relying on the water-dependent hybrid conductor properties of eumelanin biopolymers, a structurally controlled melanin thin film that can be reversibly switched-on and off by hydration–dehydration cycles and used as a biocompatible platform for stem cell growth and differentiation up to 11 days is reported. A key feature of the new eumelanin-based bioelectronic interface is its remarkable structural regularity and integrity, as the result of an ad hoc fabrication protocol involving ammonia-induced solid state polymerization (AISSP) technology of a 5,6-dihydroxyindole thin film. The AISSP technology is operationally simple and versatile, enabling the preparation of device-quality thin films (AFM and MALDI-MS evidence) on various substrates with efficient chemical control over molecular complexity. Overall the results of this study pave the way for the implementation of tailored eumelanin thin films for bioelectronic devices, e.g., in organic electrochemical transistors (OECTs).

Very low bias stress in n-type organic single-crystal transistors

Bias stress effects in n-channel organic field-effect transistors (OFETs) are investigated using N,N′-bis(n-alkyl)-(1,7 and 1,6)-dicyanoperylene-3,4:9,10-bis(dicarboximide)s (PDIF-CN2) single-crystal devices with Cytop gate dielectric, both under vacuum and in ambient. We find that the amount of bias stress is very small as compared to all (p-channel) OFETs reported in the literature. Stressing the PDIF-CN2 devices by applying 80 V to the gate for up to a week results in a decrease of the source drain current of only ∼1% under vacuum and ∼10% in air. This remarkable stability of the devices leads to characteristic time constants τ, extracted by fitting the data with a stretched exponential—that are τ ∼ 2 × 109 s in air and τ ∼ 5 × 109 s in vacuum—approximately two orders of magnitude larger than the best values reported previously for p-channel OFETs.

Matrix assisted pulsed laser deposition of melanin thin films

Melanins constitute a very important class of organic pigments, recently emerging as a potential material for a new generation of bioinspired biocompatible electrically active devices. In this paper, we report about the deposition of synthetic melanin films starting from aqueous suspensions by matrix assisted pulsed laser evaporation (MAPLE). In particular, we demonstrate that it is possible to deposit melanin films by MAPLE even if melanin (a) is not soluble in water and (b) absorbs light from UV to IR. AFM images reveal that the film surface features are highly depending on the deposition parameters. UV-VIS and FTIR spectra show both the optical properties and the molecular structure typical of melanins are preserved.

Photoinduced long-term memory effects in n-type organic perylene transistors

In this paper, the photoexcitation response of high mobility n-type organic field-effect transistors is analyzed. White light exposure of N,N′-dioctyl-3,4,9,10-perylene tetracarboxylic diimide (PTCDI-C8H) transistors is demonstrated to promote the occurrence of metastable conductance states with very long retention times, similar to what has been previously reported for p-type compounds. Even in the absence of a gate-source voltage VGS, the complete recovery of the initial electrical condition can take up to 20 days. However, the initial state restoring is electrically controllable by the application of a positive VGS. These effects suggest that PTCDI-C8H is an interesting n-type material for the development of light-sensitive organic circuitry.

Current distribution effects in organic sexithiophene field effect transistors investigated by lock-in thermography: Mobility evaluation issues

In this Letter, a lock-in thermography technique has been used to investigate the actual current distribution profile in the active channel region of organic field effect transistors. The high accuracy of the setup shows an evidence of nonuniformity in the current flow over the device area. The physical origin of this experimental occurrence is tentatively ascribed to a not uniform contact resistance distribution along the channel width or to inhomogeneities in the interface traps distribution. The subsequent implications on the carrier mobility evaluation are discussed too.

Field-effect tuning of carrier density in Nd1.2Ba1.8Cu3Oy thin films

Using a field effect device we modified the number of holes in the surface layers of 4 to 10 unit cell Nd1.2Ba1.8Cu3Oy (NBCO) epitaxial films grown on (100) SrTiO3 substrates. The results obtained on a set of 12 devices demonstrate that it is possible to induce reversible changes of the hole density of NBCO films by field effect. It is found that the field effect becomes less pronounced increasing the film thickness. Insulating–superconducting transition was observed in one 8 unit cell NBCO field effect device.

Microconfined flow behavior of red blood cells

Red blood cells (RBCs) perform essential functions in human body, such as gas exchange between blood and tissues, thanks to their ability to deform and flow in the microvascular network. The high RBC deformability is mainly due to the viscoelastic properties of the cell membrane. Since an impaired RBC deformability could be found in some diseases, such as malaria, sickle cell anemia, diabetes and hereditary disorders, there is the need to provide further insight into measurement of RBC deformability in a physiologically relevant flow field. Here, RBCs deformability has been studied in terms of the minimum apparent plasma-layer thickness by using high-speed video microscopy of RBCs flowing in cylindrical glass capillaries. An in vitro systematic microfluidic investigation of RBCs in micro-confined conditions has been performed, resulting in the determination of the RBCs time recovery constant, RBC volume and surface area and RBC membrane shear elastic modulus and surface viscosity. It has been noticed that the deformability of RBCs induces cells aggregation during flow in microcapillaries, allowing the formation of clusters of cells. Overall, our results provide a novel technique to estimate RBC deformability and also RBCs collective behavior, which can be used for the analysis of pathological RBCs, for which reliable quantitative methods are still lacking.

Direct current and alternating current electrical transport properties of regioregular poly[3-(4-alkoxyphenyl)-thiophenes]

In this paper, the direct current and alternating current (ac) electrical transport properties of doped and undoped regioregular poly[3-(4-alkoxyphenyl)-thiophenes], where the alkoxy groups are O-(CH2)n−1CH3 with n=1,4,6, and 8, have been investigated. The films have been synthesized by an experimental procedure based on the oxidation of 3-(4-alkoxylphenyl)-thiophenes with molecular oxygen in presence of VO(acac)2, as the catalyst. Unlike other examples reported in the literature, this approach allows obtaining well structured spin-coated films without the necessity of further processes, such as annealing or exposition to solvent vapors. Direct current-voltage measurements, performed in planar and transverse configuration on 1 μm thick films, show both ohmic and space charge limited current behavior, at low and high applied fields, respectively. Due to the film ordered structure, a significant electrical anisotropy was found. In order to deeply investigate the basic conduction mechanisms, ac measurements ...

Chemisorption, morphology, and structure of a n-type perylene diimide derivative at the interface with gold: influence on devices from thin films to single molecules.

We have investigated thin films of a perylene diimide derivative with a cyano-functionalized core (PDI-8CN2) deposited on Au(111) single crystals from the monolayer to the multilayer regime. We found that PDI-8CN2 is chemisorbed on gold. The molecules experience a thickness-dependent reorientation, and a 2D growth mode with molecular stepped terraces is achieved adopting low deposition rates. The obtained results are discussed in terms of their impact on field effect devices, also clarifying why the use of substrate/contact treatments, decoupling PDI-8CN2 molecules from the substrate/contacts, is beneficial for such devices. Our results also suggest that perylene diimide derivatives with CN bay-functionalization are very promising candidates for single-molecule electronic devices.

Bias stress instability in organic transistors investigated by ac admittance measurements

In this paper, the bias stress effect (BSE) in organic field-effect transistors has been analyzed by an alternative experimental approach based on ac admittance (Y=G+jωC) measurements. conductance (C) and capacitance (G) curves have been recorded as a function of frequency at different times of the bias stress experiments and simultaneously fitted through a transmission line circuit, able to separately model the conducting properties of the channel and contact regions. The determination of the time behavior of the model fitting parameters is assumed as the starting point for a quantitative analysis of the BSE occurrence. This experimental procedure clarifies that both channel resistance (Rch) and contact resistance (Rc) are largely affected by the BSE, while the channel capacitance (Cch), related to the charge accumulation sheet, and the contact capacitance (Cc) result almost unchanged.