Upper critical magnetic field in NbRe and NbReN micrometric strips.

Non-centrosymmetric superconductors have recently received significant interest due to their intriguing physical properties such as multigap and nodal superconductivity, helical vortex states, as well as non-trivial topological effects. Moreover, large values of the upper critical magnetic field have been reported in these materials. Here, we focus on the study of the temperature dependence of the perpendicular magnetic field of NbRe and NbReN films patterned in micrometric strips. The experimental data are studied within the Werthamer-Helfand-Hohenberg theory, which considers both orbital and Zeeman pair breaking. The analysis of the results shows different behavior for the two materials with a Pauli contribution relevant only in the case of NbReN.

Molecular Doping of CVD-Graphene Surfaces by Perfluoroalkyl-Substituted Perylene Diimides Derivatives.

Non-covalent π-π and dipolar interactions with small aromatic molecules have been widely demonstrated to be a valid option to tune graphene work functions without adding extrinsic scattering centers for charge carriers. In this work, we investigated the interaction between a CVD-graphene monolayer and a thermally evaporated sub-monolayer and the following few-layer thin films of similar perylene diimide derivatives: PDI8-CN2 and PDIF-CN2. The molecular influence on the graphene work function was estimated by XPS and UPS analysis and by investigating the surface potentials via scanning Kelvin probe force microscopy. The perfluorinated decoration and the steric interaction in the early stages of the film growth determined a positive work function shift as high as 0.7 eV in the case of PDIF-CN2, with respect to the value of 4.41 eV for the intrinsic graphene. Our results unambiguously highlight the absence of valence band shifts in the UPS analysis, indicating the prevalence of dipolar interactions between the graphene surface and the organic species enhanced by the presence of the fluorine-enriched moieties.

Novel Thienyl DPP derivatives Functionalized with Terminal Electron-Acceptor Groups: Synthesis, Optical Properties and OFET Performance.

Three novel diketopyrrolopyrrole (DPP) based small molecules have been synthesized and characterized in terms of their chemical-physical, electrochemical and electrical properties. All the molecules consist of a central DPP electron acceptor core symmetrically functionalized with donor bi-thienyl moieties and flanked in the terminal positions by three different auxiliary electron-acceptor groups. This kind of molecular structure, characterized by an alternation of electron acceptor and donor groups, was purposely designed to provide a significant absorption at the longer wavelengths of the visible spectrum: when analysed as thin films, in fact, the dyes absorb well over 800 nm and exhibit a narrow optical bandgap down to 1.28 eV. A detailed DFT analysis provides useful information on the electronic structure of the dyes and on the features of the main optical transitions. Organic field-effect transistors (OFETs) have been fabricated by depositing the DPP dyes as active layers from solution: the different end-functionalization of the dyes had an effect on the charge-transport properties with two of the dyes acting as n-type semiconductors (electron mobility up to 4.4 ⋅ 10-2  cm2 /V ⋅ s) and the third one as a p-type semiconductor (hole mobility up to 2.3 ⋅ 10-3  cm2 /V ⋅ s). Interestingly, well-balanced ambipolar transistors were achieved by blending the most performant n-type and p-type dyes with hole and electron mobility in the order of 10-3  cm2 /V ⋅ s.

Homocysteine Solution-Induced Response in Aerosol Jet Printed OECTs by Means of Gold and Platinum Gate Electrodes.

Homocysteine (Hcy) is a non-protein, sulfur-containing amino acid, which is recognized as a possible risk factor for coronary artery and other pathologies when its levels in the blood exceed the normal range of between 5 and 12 µmol/L (hyperhomocysteinemia). At present, standard procedures in laboratory medicine, such as high-performance liquid chromatography (HPLC), are commonly employed for the quantitation of total Hcy (tHcy), i.e., the sum of the protein-bound (oxidized) and free (homocystine plus reduced Hcy) forms, in biological fluids (particularly, serum or plasma). Here, the response of Aerosol Jet-printed organic electrochemical transistors (OECTs), in the presence of either reduced (free) and oxidized Hcy-based solutions, was analyzed. Two different experimental protocols were followed to this end: the former consisting of gold (Au) electrodes' biothiol-induced thiolation, while the latter simply used bare platinum (Pt) electrodes. Electrochemical impedance spectroscopy (EIS) analysis was performed both to validate the gold thiolation protocol and to gain insights into the reduced Hcy sensing mechanism by the Au-gated OECTs, which provided a final limit of detection (LoD) of 80 nM. For the OECT response based on Platinum gate electrodes, on the other hand, a LoD of 180 nM was found in the presence of albumin-bound Hcy, with this being the most abundant oxidized Hcy-form (i.e., the protein-bound form) in physiological fluids. Despite the lack of any biochemical functionalization supporting the response selectivity, the findings discussed in this work highlight the potential role of OECT in the development of low-cost point-of-care (POC) electronic platforms that are suitable for the evaluation, in humans, of Hcy levels within the physiological range and in cases of hyperhomocysteinemia.

Balanced Ambipolar Charge Transport in Phenacene/Perylene Heterojunction-Based Organic Field-Effect Transistors.

Electronic devices relying on the combination of different conjugated organic materials are considerably appealing for their potential use in many applications such as photovoltaics, light emission, and digital/analog circuitry. In this study, the electrical response of field-effect transistors achieved through the evaporation of picene and PDIF-CN2 molecules, two well-known organic semiconductors with remarkable charge transport properties, was investigated. With the main goal to get a balanced ambipolar response, various device configurations bearing double-layer, triple-layer, and codeposited active channels were analyzed. The most suitable choices for the layer deposition processes, the related characteristic parameters, and the electrode position were identified to this purpose. In this way, ambipolar organic field-effect transistors exhibiting balanced mobility values exceeding 0.1 cm2 V-1 s-1 for both electrons and holes were obtained. These experimental results highlight also how the combination between picene and PDIF-CN2 layers allows tuning the threshold voltages of the p-type response. Scanning Kelvin probe microscopy (SKPM) images acquired on picene/PDIF-CN2 heterojunctions suggest the presence of an interface dipole between the two organic layers. This feature is related to the partial accumulation of space charge at the interface being enhanced when the electrons are depleted in the underlayer.

Post-Deposition Wetting and Instabilities in Organic Thin Films by Supersonic Molecular Beam Deposition.

We discuss the formation and post-deposition instability of nanodrop-like structures in thin films of PDIF-CN2 (a perylene derivative) deposited via supersonic molecular beam deposition technique on highly hydrophobic substrates at room temperature. The role of the deposition rate on the characteristic lengths of the organic nanodrops has been investigated by a systematic analysis of atomic force microscope images of the thin films and through the use of the height-height correlation function. The nanodrops appear to be a metastable configuration for the freshly-deposited films. For this reason, post-deposition wetting effect has been examined with unprecedented accuracy throughout a year of experimental observations. The observed time scales, from few hours to months, are related to the growth rate, and characterize the thin films morphological reordering from three-dimensional nanodrops to a well-connected terraced film. While the interplay between adhesion and cohesion energies favors the formation of 3D-mounted structures during the growth, wetting phenomenon following the switching off of the molecular flux is found to be driven by an instability. A slow rate downhill process survives at the molecular flux shutdown and it is accompanied and maybe favored by the formation of a precursor layer composed of more lying molecules. These results are supported by simulations based on a non-linear stochastic model. The instability has been simulated, for both the growth and the post-growth evolution. To better reproduce the experimental data it is needed to introduce a surface equalizer term characterized by a relaxation time taking into account the presence of a local mechanism of molecular correlation.