Controlling Ambipolar Transport and Voltage Inversion in Solution-Processed Thin-Film Devices through Polymer Blending

Ambipolar semiconductors are attracting a great interest as building blocks for photovoltaics and logic applications. Field-effect transistors built on solution-processable ambipolar materials hold strong promise for the engineering of large-area low-cost logic circuits with a reduced number of devices components. Such devices still suffer from a number of obstacles including the challenging processing, the low Ion/Ioff, the unbalanced mobility, and the low gain in complementary metal–oxide–semiconductor (CMOS)-like circuits. Here, we demonstrate that the simple approach of blending commercially available n- and p-type polymers such as P(NDI2OD-T2), P3HT, PCD-TPT, PDVT-8, and IIDDT-C3 can yield high-performing ambipolar field-effect transistors with balanced mobilities and Ion/Ioff > 10^7. Each single component was studied separately and upon blending by means of electrical characterization, ambient ultraviolet photoelectron spectroscopy, atomic force microscopy, and grazing incidence wide angle X-ray scattering to unravel the correlation between the morphology/structure of the semiconducting films and their functions. Blends of n- and p-type semiconductors were used to fabricate CMOS-like inverter circuits with state-of-the-art gains over 160 in the case of P(NDI2OD-T2) blended with PDVT-8. Significantly, our blending approach was successful in producing semiconducting films with balanced mobilities for each of the four tested semiconductor blends, although the films displayed different structural and morphological features. Our strategy, which relies on establishing a correlation between ambipolar performances, film morphology, molecular structure, and blending ratio, is extremely efficient and versatile; thus it could be applied to a wide range of polymers or solution processable small molecules

Fast-Response Photonic Device Based on Organic-Crystal Heterojunctions Assembled into a Vertical-Yet-Open Asymmetric Architecture

Crystalline dioctyl-3,4,9,10-perylenedicarboximide nanowires and 6,13-bis(triisopropylsilylethynyl) pentacene microplates are integrated into a vertical-yet-open asymmetrical heterojunction for the realization of a high-performance organic photovoltaic detector, which shows fast photoresponse, ultrahigh signal-to-noise ratio, and high sensitivity to weak light

Multi-modal sensing in spin crossover compounds

We exploited the solvatochromic spin-state switching in a spin crossover (SCO) compound based on the Fe-II complex and the simultaneous change of spectroscopic properties for selective multimodal sensing of methanol and ethanol. We demonstrate that sensing capabilities are due to the inclusion of methanol or ethanol molecules into the crystalline structure, which tailors simultaneously the transition temperature, colour, birefringence and vibrational modes. We exploited this capability by integrating a neutral compound, switchable at room temperature, into a micrometric TAG sensitive to the colour and birefringence. The system was characterised by optical microscopy, magnetic susceptibility, Raman spectroscopy and X-ray diffraction

Graphene exfoliation in the presence of semiconducting polymers for improved film homogeneity and electrical performances

We report on the production of hybrid graphene/semiconducting polymer films in one step procedure by making use of ultrasound-assisted liquid-phase exfoliation of graphite powder in the presence of π-conjugated polymers, i.e. poly(3-hexylthiophene) (P3HT) or poly[4-(4,4-dihexadecyl-4H-cyclopenta[1,2-b:5,4-b']dithiophen-2-yl)-alt-[1,2,5]thiadiazolo-[3,4-c]pyridine] (PCDTPT). The polymers were chosen in view of their different propensity to form crystalline structures, their decoration with alkyl chains that are known to possess high affinity for the basal plane of graphene, the energy levels of their frontier orbitals which are extremely similar to the work function of graphene, and their high electrical performance when integrated in field-effect transistors (FETs). The polymers act as a dispersion-stabilizing agent and prevent the re-aggregation of the exfoliated graphene flakes, ultimately enabling the production of homogeneous bi-component dispersions. The electrical characterization of few-layer graphene/PCDTPT hybrids, when integrated as active layer in bottom-contact bottom-gate FETs, revealed an increase of the field-effect mobility compared to the π-conjugated-based pristine devices, a result which can be attributed to the joint effect of the few-layer graphene sheets and semiconducting polymers improving the charge-transport in the channel of the field-effect transistor. In particular, few-layer graphene/PCDTPT films displayed a 30-fold increase of PCDTPT's mobility if compared to pristine polymer samples. Such findings represent a step forward towards the optimization of graphene exfoliation and processing into electronic devices, as well as towards improved electrical performance in organic-based field-effect transistors

Local structure and magnetotransport in Sr2FeMoO6 double perovskite compounds: an EXAFS study

Sr2FeMoO6 oxides with double perovskite structure1 are half metallic ferromagnetic with elevated Curie temperature (Tc > 400 K) and present large magnetoresistance at room temperature2. In the last few years these compounds stimulated large interest as potential electrodes in magnetic tunnel junction and innovative spintronic materials. On the other hand these peculiar magnetic and electronic properties attracted the fundamental research in the field of heavily correlated electron systems. Their crystallographic structure derives from the ideal cubic perovskites: it is made of weakly tetragonal distorted cubic units with the Sr ions located at the center and the Fe(Mo)O6 sharing the cube edges. The regular alternation of Fe and Mo along the lattice edges (chemical order) strengthens the magnetoresistance while increasing the miss-site defects (chemical disorder) weakens the magnetoresistance. A kind of double exchange interaction (DE) between Fe ions along the Fe-O-Mo-O-Fe chains mediated by Mo, has been suggested to explain the metallic state. Chemical disorder reduces the DE coupling and gives rise to tunnel type conductibility. The most striking features in this picture is the high Curie temperature, well above that observed in the well known Mn-perovskites (La1-xCaxMnO3, La1-xSrxMnO3 and related systems), which imply huge exchange interactions despite the very long distance among Fe ions (~8 Å)3. This work proposes a detailed EXAFS study on two Sr2FeMoO6, a fully chemically ordered and a fully chemically disordered sample, in order to get insights on the micro-structural origin of their magnetotransport properties. X-ray absorption experiments were performed at the Italian beamline (GILDA-BM8) at the ESRF. In order to have a complete and exhaustive description of local order Fe K-edge (~7112 eV), Mo-K edge (~20000 eV) and W-LIII (~10200 eV) edge EXAFS data were analysed. The complete EXAFS spectra were refined taking into account for single as well multiple scattering contributions till about 6 Å. This permitted to probe the relative arrangement of Fe and Mo ions. The coherence of the structural models, as resulting from the three edges analysis strengthens and gives confidence on the results. The main result is that either ordered and disordered compounds show very similar chemically ordered local structure, i.e. the presence of Fe-Fe and Mo-Mo coordination is weak (almost negligible) in both the samples. These finding contrasts with the information from X-ray diffraction analysis, i.e. a high ordered (order parameter ~ 90%) and a completely disordered sample. Our results suggest that the long-range chemically disordered sample is made of small (20-30 Å) chemically ordered clusters. Such a small size would produce weak and broad superlattice reflections easily confused with the background in standard diffraction patterns

Real-time Structural and Electrical Investigation of PDI8-CN2 based OFET

Semiconductor thin-film devices based on organic molecules are of great interest for the development of high performance organic field effect transistors (OFETs) and organic light emitting diodes (OLEDs), as well as to underscore fundamental charge transport effects in molecular solids. Among the n-type organic molecules, perylene derivatives are very promising. In particular PDI-8CN2, N,N’-bis(n-octyl)-dicyanoperylene-3,4:9,10-bis (dicarb-oximide), has been reported to allow the fabrication of OFETs with excellent electrical performance (high-mobility: 0.16-0.6 cm2 V-1s-1) and remarkably high stability in air. In these systems, the charge mobility depends on the overlap between π-π orbitals of vicinal molecules, which is mainly influenced by the structure and morphology of the first layers of organic film at the interface with the dielectric. Since the structure of these first layers may significantly differ from that of the bulk, the determination of the molecular orientation and packing of organic molecules at the substrate interface is a crucial input for modelling the electronic band structure and the associated charge-transport properties. For this reason we have performed Grazing Incidence X-ray Diffraction (GIXD) and X-Ray Reflectivity (XRR) measurements, in situ and real time during the UHV deposition of PDI-8CN2. Moreover, in situ and real time electrical measurements were performed on FET structures during the semiconductor deposition. Thanks to these time resolved measurements we could describe i) the thin-film growth dynamics, ii) the molecular packing and microstructure of the organic thin film, iii) the influence of the substrate temperature and the deposition flux, and iv) the relation between the charge transport properties and the growth mechanism of the thin film

Ultrafast and Highly Sensitive Chemically Functionalized Graphene Oxide-Based Humidity Sensors: Harnessing Device Performances via the Supramolecular Approach

Humidity sensors have been gaining increasing attention because of their relevance for well-being. To meet the ever-growing demand for new cost-efficient materials with superior performances, graphene oxide (GO)-based relative humidity sensors have emerged recently as low-cost and highly sensitive devices. However, current GO-based sensors suffer from important drawbacks including slow response and recovery, as well as poor stability. Interestingly, reduced GO (rGO) exhibits higher stability, yet accompanied by a lower sensitivity to humidity due to its hydrophobic nature. With the aim of improving the sensing performances of rGO, here we report on a novel generation of humidity sensors based on a simple chemical modification of rGO with hydrophilic moieties, i.e., triethylene glycol chains. Such a hybrid material exhibits an outstandingly improved sensing performance compared to pristine rGO such as high sensitivity (31% increase in electrical resistance when humidity is shifted from 2 to 97%), an ultrafast response (25 ms) and recovery in the subsecond timescale, low hysteresis (1.1%), excellent repeatability and stability, as well as high selectivity toward moisture. Such highest-key-performance indicators demonstrate the full potential of two-dimensional (2D) materials when decorated with suitably designed supramolecular receptors to develop the next generation of chemical sensors of any analyte of interest

Quantitative analysis of scanning tunneling microscopy images of mixed-ligand-functionalized nanoparticles

Ligand-protected gold nanoparticles exhibit large local curvatures, features rapidly varying over small scales, and chemical heterogeneity. Their imaging by scanning tunneling microscopy (STM) can, in principle, provide direct information on the architecture of their ligand shell, yet STM images require laborious analysis and are challenging to interpret. Here, we report a straightforward, robust, and rigorous method for the quantitative analysis of the multiscale features contained in STM images of samples consisting of functionalized Au nanoparticles deposited onto Au/mica. The method relies on the analysis of the topographical power spectral density (PSD) and allows us to extract the characteristic length scales of the features exhibited by nanoparticles in STM images. For the mixed-ligand-protected Au nanoparticles analyzed here, the characteristic length scale is 1.2 \ub1 0.1 nm, whereas for the homoligand Au NPs this scale is 0.75 \ub1 0.05 nm. These length scales represent spatial correlations independent of scanning parameters, and hence the features in the PSD can be ascribed to a fingerprint of the STM contrast of ligand-protected nanoparticles. PSD spectra from images recorded at different laboratories using different microscopes and operators can be overlapped across most of the frequency range, proving that the features in the STM images of nanoparticles can be compared and reproduced