TCTA:Ir(ppy)3 Green Emissive Blends in Organic Light-Emitting Transistors (OLETs).

Organic light-emitting transistors are photonic devices combining the function of an electrical switch with the capability of generating light under appropriate bias conditions. Achieving high-performance light-emitting transistors requires high-mobility organic semiconductors, optimized device structures, and highly efficient emissive layers. In this work, we studied the optoelectronic response of green blends (TCTA:Ir(ppy)3) with varying doping concentrations in the limit of field-effect within a transistor device configuration. Increasing the dye concentration within the blend leads to a quenching of the photoluminescence signal; however, when implemented in a multilayer stack in a transistor, we observed an approximately 5-fold improvement in the light output for a 10% Ir(ppy)3 doping blend. We analyzed our results in terms of balanced charge transport in the emissive layer, which, in the limit of field-effect (horizontal component), leads to an improved exciton formation and decay process. While the performances of our devices are yet to achieve the state-of-the-art diode counterpart, this work demonstrates that engineering the emissive layer is a promising approach to enhance the light emission in field-effect devices. This opens the way for a broader exploitation of organic light-emitting transistors as alternative photonic devices in several fields, ranging from display technology to flexible and wearable electronics.

High-k Fluoropolymers Dielectrics for Low-Bias Ambipolar Organic Light Emitting Transistors (OLETs).

Organic light emitting transistors (OLETs) combine, in the same device, the function of an electrical switch with the capability of generating light under appropriate bias conditions. In this work, we demonstrate how engineering the dielectric layer based on high-k polyvinylidene fluoride (PVDF)-based polymers can lead to a drastic reduction of device driving voltages and the improvement of its optoelectronic properties. We first investigated the morphology and the dielectric response of these polymer dielectrics in terms of polymer (P(VDF-TrFE) and P(VDF-TrFE-CFE)) and solvent content (cyclopentanone, methylethylketone). Implementing these high-k PVDF-based dielectrics enabled low-bias ambipolar organic light emitting transistors, with reduced threshold voltages (<20 V) and enhanced light output (compared to conventional polymer reference), along with an overall improvement of the device efficiency. Further, we preliminary transferred these fluorinated high-k dielectric films onto a plastic substrate to enable flexible light emitting transistors. These findings hold potential for broader exploitation of the OLET platform, where the device can now be driven by commercially available electronics, thus enabling flexible low-bias organic electronic devices.

Engineering Dielectric Materials for High-Performance Organic Light Emitting Transistors (OLETs).

Organic light emitting transistors (OLETs) represent a relatively new technology platform in the field of optoelectronics. An OLET is a device with a two-fold functionality since it behaves as a thin-film transistor and at the same time can generate light under appropriate bias conditions. This Review focuses mainly on one of the building blocks of such device, namely the gate dielectrics, and how it is possible to engineer it to improve device properties and performances. While many findings on gate dielectrics can be easily applied to organic light emitting transistors, we here concentrate on how this layer can be exploited and engineered as an active tool for light manipulation in this novel class of optoelectronic devices.

3D Multi-Branched SnO2 Semiconductor Nanostructures as Optical Waveguides.

Nanostructures with complex geometry have gathered interest recently due to some unusual and exotic properties associated with both their shape and material. 3D multi-branched SnO2 one-dimensional nanostructrures, characterized by a "node"-i.e., the location where two or more branches originate, are the ideal platform to distribute signals of different natures. In this work, we study how this particular geometrical configuration affects light propagation when a light source (i.e., laser) is focused onto it. Combining scanning electron microscopy (SEM) and optical analysis along with Raman and Rayleigh scattering upon illumination, we were able to understand, in more detail, the mechanism behind the light-coupling occurring at the node. Our experimental findings show that multi-branched semiconductor 1D structures have great potential as optically active nanostructures with waveguiding properties, thus paving the way for their application as novel building blocks for optical communication networks.

Integration of ZnO and CuO nanowires into a thermoelectric module.

Zinc oxide (ZnO, n-type) and copper oxide (CuO, p-type) nanowires have been synthesized and preliminarily investigated as innovative materials for the fabrication of a proof-of-concept thermoelectric device. The Seebeck coefficients, electrical conductivity and thermoelectric power factors (TPF) of both semiconductor materials have been determined independently using a custom experimental set-up, leading to results in agreement with available literature with potential improvement. Combining bundles of ZnO and CuO nanowires in a series of five thermocouples on alumina leads to a macroscopic prototype of a planar thermoelectric generator (TEG) unit. This demonstrates the possibility of further integration of metal oxide nanostructures into efficient thermoelectric devices.

Tuning electronic transport in cobalt-filled carbon nanotubes using magnetic fields.

Metal-filled and decorated carbon nanotubes represent a class of quasi one-dimensional hybrid systems with enormous potential for applications in nanoelectronics and spintronics. Here we show that is possible to control the electrical conduction in ferromagnetic metal-filled carbon nanotubes by means of external magnetic fields, suggesting specific dimensionality-dependent conduction regimes. By increasing the magnetic field, we drive the charge flow from a positive to a negative magneto-conductance, revealing channel-selective conduction. Furthermore, the zero-field current temperature dependence shows different regimes, suggesting that the inter-shell hopping, assisted by the cobalt clusters, plays a key role in the dimensional crossover. The possibility of engineering and controlling the nature and size of the conducting shells and the filling with magnetic materials can allow the implementation of these systems in tunable hybrid nano-sensors and multifunctional magnetic devices.

Metal-filled carbon nanotubes as a novel class of photothermal nanomaterials.

Metal-filled carbon nanotubes represent a novel class of photothermal nanomaterials: when illuminated by visible light they exhibit a strong enhancement of the temperature at the metal sites, due to the enhanced plasmonic light absorption at the metal surface, which behaves as a heat radiator. Potential applications include nanomedicine, heat-assisted magnetic recording, and light-activated thermal gradient-driven devices.

Cobalt nanocluster-filled carbon nanotube arrays: engineered photonic bandgap and optical reflectivity.

Perfect vertically aligned and periodically arranged arrays of multidielectric heterostructures are ideal platforms both for photonic crystals and photonic bandgap materials. Carbon nanotubes grown inside anodic alumina templates form a novel class of heterostructured materials ideally suited for building such platforms. By engineering metallic (cobalt) nanoclusters inside the nanotubes, we present a novel method for tailoring the photonic bandgap as well as the magnitude of the reflectivity in these systems. We present spectroscopic ellipsometry (SE) and reflectivity measurements to investigate the effect of the presence of cobalt clusters on the optical response of multiwall carbon nanotubes (MWNT) grown in anodized alumina template. The real (ε(1)) and imaginary (ε(2)) part of the pseudodielectric function of the MWNT and Co-MWNT system have been studied in a wide energy range (1.4-5 eV). We found that the cobalt filling modifies the electronic structure of the nanotubes, suggesting that the insertion of the clusters leads to a semiconductor behavior. Angle-resolved reflectivity measurements further show that the metal filling drastically enhances the optical response up to 2 orders of magnitude.

Luttinger liquid to Al'tshuler-Aronov transition in disordered, many-channel carbon nanotubes.

We present a simple method to increase the conductance of individual template-grown carbon nanotubes contacted with platinum electrodes using a high-bias treatment process which decorates the nanotubes with Pt nanoclusters. The temperature dependence of conductance of the modified carbon nanotubes follows very closely a Luttinger-liquid to Al'tshuler-Aronov anomaly and reveals enhanced number of channels and decreased disorder-density compared to the pristine tubes. Low temperature magnetotransport shows evidence of Rashba spin-orbit coupling, usually absent in bare nanotubes, and a strong suppression of phase coherence at low electronic energies due to electron-electron interactions.

Detection of nanoscale magnetic activity using a single carbon nanotube.

The ultimate conductometric sensor for ferromagnetic activity of nanoscale magnetic materials could be a single carbon nanotube. We show that the electrical conductance of an individual carbon nanotube is sensitive to magnetic transitions of nanoscale magnets embedded inside it. To establish this, multiwall carbon nanotubes were impregnated with cobalt nanoclusters. Temperature dependence of conductance (5 K < T <300 K) of these nanotubes shows the usual Lüttinger-liquid power law behavior at higher temperatures and an onset of Coulomb blockade at lower temperatures. At the lowest temperature (T approximately 6 K), the differential conductance (dI/dV versus V) develops aperiodic fluctuations under an external magnetic field B, the rms amplitude of which grows with the magnitude of the field itself. Low-temperature magnetoconductance, studied as function of temperature and bias, can be interpreted in terms of weak antilocalization effects due to the presence of the magnetized clusters. The temperature dependence of magnetoconductance further presents a "peak"-like feature and slow dynamics around T =55 K, which depend on the magnitude and history of the applied B field. These observations indicate a sensitivity of electronic transport in the multiwall nanotubes to the dynamics of nanoscale magnets at low temperature.

Multifunctional composites using reinforced laminae with carbon-nanotube forests.

Traditional fibre-reinforced composite materials with excellent in-plane properties fare poorly when out-of-plane through-thickness properties are important. Composite architectures with fibres designed orthogonal to the two-dimensional (2D) layout in traditional composites could alleviate this weakness in the transverse direction, but all of the efforts so far have only produced limited success. Here, we unveil an approach to the 3D composite challenge, without altering the 2D stack design, on the basis of the concept of interlaminar carbon-nanotube forests that would provide enhanced multifunctional properties along the thickness direction. The carbon-nanotube forests allow the fastening of adjacent plies in the 3D composite. We grow multiwalled carbon nanotubes on the surface of micro-fibre fabric cloth layouts, normal to the fibre lengths, resulting in a 3D effect between plies under loading. These nanotube-coated fabric cloths serve as building blocks for the multilayered 3D composites, with the nanotube forests providing much-needed interlaminar strength and toughness under various loading conditions. For the fabricated 3D composites with nanotube forests, we demonstrate remarkable improvements in the interlaminar fracture toughness, hardness, delamination resistance, in-plane mechanical properties, damping, thermoelastic behaviour, and thermal and electrical conductivities making these structures truly multifunctional.

Aligned carbon nanotube-polymer hybrid architectures for diverse flexible electronic applications.

We present the fabrication and electrical characterization of a flexible hybrid composite structure using aligned multiwall carbon nanotube arrays in a poly(dimethylsiloxane) (PDMS) matrix. Using lithographically patterned nanotube arrays, one can make these structures at any length scale from submicrometer levels to bulk quantities. The PDMS matrix undergoes excellent conformal filling within the dense nanotube network, giving rise to extremely flexible conducting structures with unique electromechanical properties. We demonstrate its robustness against high stress conditions, under which the composite is found to retain its conducting nature. We also demonstrate that these structures can be utilized directly as flexible field-emission devices. Our devices show some of the best field-enhancement factors and turn-on electric fields reported so far.