The discovery of graphene and its fascinating capabilities has triggered an unprecedented interest in inorganic two-dimensional (2D) materials. van der Waals layered materials such as graphene, hexagonal boron nitride, transition metal dichalcogenides, and the more recently re-discovered black phosphorus (BP) indeed display an exceptional technological potential for engineering nano-electronic and nano-photonic devices and components "by design,"offering a unique platform for developing new devices with a variety of "ad hoc"properties. In this Perspective article, we provide a vision on the key transformative applications of 2D nanomaterials for the development of nanoelectronic, nanophotonic, optical, and plasmonic devices at terahertz frequencies, highlighting how the rich physical phenomena enabled by their unique band structure engineering can allow them to boost the vibrant field of quantum science and quantum technologies.

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 I-on/I-off, 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 I-on/I-off > 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.

We demonstrate sub-harmonic and heterodyne mixing beyond cutoff in AlGaAs/InGaAs with integrated planar antenna connected to the channel ends.

The channel of a high electron mobility transistor can work as a resonant microcavity for plasma waves, provided that the plasmon decay length is much larger than the cavity length. We have performed a spectroscopic study in the 0.15-0.4 THz range of the power absorbed by the micrometric channel of a two-dimensional electron gas (2DEG) transistor, where the active layer is formed by a remotely doped AlGaAs/InGaAs/AlGaAs quantum well where the electron mobility increases with decreasing temperature. The radiation emitted by a tunable frequency-multiplied THz oscillator was coupled to the cavity by an integrated lens-antenna optical system. The rectified signal is measured as a function of frequency and a strong increase upon cooling to 10 K is found at specific radiation frequencies, indicating the formation of standing plasma waves in the microcavity formed by the channel.

Despite the variety of functional properties of molecular materials, which make them of interest for a number of technologies, their tendency to form inhomogeneous aggregates in thin films and to self-organize in polymorphs are considered drawbacks for practical applications. Here, we report on the use of polymorphic molecular fluorescent thin films as time temperature integrators, a class of devices that monitor the thermal history of a product. The device is fabricated by patterning the fluorescent model compound thieno(bis) imide-oligothiophene. The fluorescence colour of the pattern changes as a consequence of an irreversible phase variation driven by temperature, and reveals the temperature at which the pattern was exposed. The experimental results are quantitatively analysed in the range 20-200 degrees C and interpreted considering a polymorph recrystallization in the thin film. Noteworthy, the reported method is of general validity and can be extended to every compound featuring irreversible temperature-dependent change of fluorescence.

This perspective deals with the coupling of ionic and electronic transport in organic electronic devices, focusing on electrolyte-gated transistors. These include electrolyte-gated organic field-effect transistors (EG-OFETs) and organic electrochemical transistors (OECTs). EG-OFETs, based on molecules and polymers, can be operated at low electrical bias (about 1 V or below) and permit unprecedented charge carrier densities within the transistor channel. OECTs can be operated in aqueous environment as efficient ion-to-electron converters, thus providing an interface between the worlds of biology and electronics. The exploration and the exploitation of coupled ionic and electronic transport in organic materials brings together different disciplines such as materials science, physics, chemistry, electrochemistry, organic electronics and biology.

In this paper we investigate the distribution of the electrically available states near the band-edge in pentacene thin films of different thicknesses, aiming to the identification of the active thickness of pentacene layers in fully operational devices such as organic thin film transistors (OTFTs). The film structure has been studied by X-ray diffraction technique, while their relative electronic density of states distribution (DOS) around the band-edge has been investigated by photocurrent (PC) spectroscopy analyses. The effects of ion implantation on OTFTs have been investigated by PC analyses of OTFTs implanted with N+ ions of different energy and doses. We show how PC spectroscopy has the remarkable ability to detect modifications of the DOS distribution in a non invasive way, thus allowing the direct study of the active semiconductor film in fully operational OTFTs.

Here we report an investigation of the growth of picene by supersonic molecular beam deposition on thermal silicon oxide and on a self-assembled monolayer of hexamethyldisiloxane (HMDS). In both cases film morphology shows a structure with very sharp island edges and well-separated islands which size and height depend on the deposition conditions. Picene films growth on bare silicon covered with hydrophobic HDMS shows islands characterized by large regular crystallites of several micrometers; on the other hand, films growth on silicon oxide shows smaller and thicker islands. We analyzed the details of the growth model and describe it as a balancing mechanism involving the weak interaction between molecules and surface and the strong picene-picene interaction that leads to a different Schwoebel-Ehrlich barrier in the first layer with respect to the successive one Finally, we study. the charge transport properties of these films by fabricating field-effect transistors devices in both top and bottom contact configuration. We notice that substrate influences the electrical properties of the device and we obtained a maximum mobility value of 1.2 cm(2) V-1 s(-1) measured on top contact devices in air.

Contact effects have been analyzed in fully printed p-channel OTFTs based on a pentacene derivative as organic semiconductor and with Au source-drain contacts. In these devices, contact effects lead to an apparent decrease of the field effect mobility with decreasing L and to a failure of the gradual channel approximation (GCA) in reproducing the output characteristics. Experimental data have been reproduced by two-dimensional numerical simulations that included a Schottky barrier (Phi(b) = 0.46 eV) at both source and drain contacts and the effects of field-induced barrier lowering. The barrier lowering was found to be controlled by the Schottky effect for an electric field E < 10(5) V/cm, while for higher electric fields we found a stronger barrier lowering presumably due to other field-enhanced mechanisms. The analysis of numerical simulation results showed that three different operating regimes of the device can be identified: (1) low vertical bar V-ds vertical bar, where the channel and the Schottky diodes at both source and drain behave as gate voltage dependent resistors and the partition between channel resistance and contact resistance depends upon the gate bias; (2) intermediate V-ds, where the device characteristics are dominated by the reverse biased diode at the source contact, and (3) high vertical bar V-ds vertical bar, where pinch-off of the channel occurs at the drain end and the transistor takes control of the current. We show that these three regimes are a general feature of the device characteristics when Schottky source and drain contacts are present, and therefore the same analysis could be extended to TFTs with different semiconductor active layers.

Metal-semiconductor field effect transistors (MESFETs) based on hydrogen terminated diamond were fabricated according to different layouts. Aluminum gates were used on single crystal and low-roughness polycrystalline diamond substrates while gold was used for ohmic contacts. Hydrogen terminated layers were deeply investigated by means of Hall bars and transfer length structures. Room temperature Hall and field effect mobility values in excess of 100 cm(2) V(-1) s(-1) were measured on commercial and single crystal epitaxial growth (100) plates by using the same hydrogenation process. Hydrogen induced two-dimensional hole gas resulted in sheet resistances essentially stable and repeatable depending on the substrate quality. Self-aligned 400 nm gate length FETs on single crystal substrates showed current density and transconductance values >100 mA mm(-1) and >40 mS mm(-1), respectively. Devices with gate length L(G) = 200 nm showed f(Max) = 26.4 GHz and f(T) = 13.2 GHz whereas those fabricated on polycrystalline diamond, with the same gate geometry, exceeded f(Max) = 23 GHz and f(T) = 7 GHz. This work focused on the optimization of a self-aligned gate structure with respect to the fixed drain-to-source structure with which we observed higher frequency values; the new structure resulted in improvement of DC characteristics, better impedance matching and a reduction in the f(Max)/f(T) ratio.

We explore the charge transport mechanism in organic semiconductors based on a model that accounts for the thermal intermolecular disorder at work in pure crystalline compounds, as well as extrinsic sources of disorder that are present in current experimental devices. Starting from the Kubo formula, we describe a theoretical framework that relates the time-dependent quantum dynamics of electrons to the frequency-dependent conductivity. The electron mobility is then calculated through a relaxation time approximation that accounts for quantum localization corrections beyond Boltzmann theory, and allows us to efficiently address the interplay between highly conducting states in the band range and localized states induced by disorder in the band tails. The emergence of a "transient localization" phenomenon is shown to be a general feature of organic semiconductors that is compatible with the bandlike temperature dependence of the mobility observed in pure compounds. Carrier trapping by extrinsic disorder causes a crossover to a thermally activated behavior at low temperature, which is progressively suppressed upon increasing the carrier concentration, as is commonly observed in organic field-effect transistors. Our results establish a direct connection between the localization of the electronic states and their conductive properties, formalizing phenomenological considerations that are commonly used in the literature.

We report the hot photoexcited electron transfer across the coaxial interface of a cylindrical core-shell nanowire. Modulation of the transfer rates, manifested as a large tunability of the voltage onset of negative differential resistance and of voltage-current phase, is achieved using three different modes. The coupling of electrostatic gating, incident photon energy, and the incident photon intensity to transfer rates is facilitated by the combined influences of geometric confinement and heterojunction shape on hot-electron transfer, and by electron-electron scattering rates that can be altered by varying the incident photon flux, with evidence of weak electron-phonon scattering. Dynamic manipulation of this transfer rate permits the introduction and control of a continuously adjustable phase delay of up to about 130 within a single nanometer-scale device element.

In this Article, we present a comprehensive study of organic field-effect transistors (OFETs) made of thin films of methyl, n-butyl, and n-hexyl end-substituted quaterthiophenes on a transparent substrate platform. This particular platform has been already used for organic light-emitting diodes (OLEDs) but rarely employed for OFETs. In perspective, this is a very promising route for the development of field-effect photonic applications such as organic light-emitting transistors (OLETs). A systematic characterization of the organic films has been made by means of atomic force microscopy (AFM) and X-ray diffraction (XRD) to correlate morphology, crystallinity and charge mobility to the alkyl chain length. In particular, a charge mobility value of 0.09 cm(2)/(V s) has been obtained in transparent OFETs with a large area channel for DH4T grown at room temperature. This mobility exceeds the one obtained on silicon-oxide substrates and is likely due to a more favorable interaction of the DH4T molecules with the PMMA layer employed as gate dielectric.

We report on organic thin film transistors (OTFTs) based on copper phthalocyanine (CuPc) having electrodes consisting of isolated carbon nanotube (CNT) arrays embedded in the organic layer. CuPc OTFT with CNT array electrodes show p-type behavior with Ohmic hole injection, high hole mobility, and enhanced switching characteristics at low voltage. The p-type devices are converted to ambipolar OTFT by vacuum annealing. Despite the large offset between the CNT work function and the CuPc energy levels, electron injection characteristics are also Ohmic. The extension of CNT electrodes to the phthalocyanine family confirms the validity of this contact approach for organic electronic devices.

We investigate the photoresponse of field-effect transistors based on conjugated polymer electrospun fibers. The electrical performances of single fiber transistors are controlled by modulating the channel conductivity under white light illumination. We demonstrate a photoresponsivity up to 100 mA/W for a 500-nm channel width fiber phototransistor illuminated by an intensity of 9.6 mW/cm(2). Studying the photoresponse switching cycles evidences that the photocurrent relaxation time can be reduced down to about 40 s by increasing the fiber surface-to-volume ratio

Oligothiophene-substituted 1,1,4,4-tetracyanobutadienes (TCBDs) have been synthesized by [2þ 2] cycloaddition reactions between tetracyanoethylene and oligothiophene alkynes. The TCBD moiety is compared to other electron acceptors attached to dibutylterthiophene including dicyanovinyl (DCV) and tricyanovinyl (TCV).These donor-acceptor molecules (TCBD-3T, DCV-3T, andTCV-3T) show red-shifted absorption spectra relative to the unsubstituted oligothiophene as a result of intramolecular charge-transfer (ICT). Monosubstituted terthiophenes bearing the electron acceptors show both oxidation and reduction processes as characterized by cyclic voltammetry. Density functional theory (DFT) calculations are used to explain the electronic and redox properties of the materials. Electrochemical oxidation of a bis(terthienyl)-substitutedTCBDmolecule (3T-TCBD-3T) yields a conducting polymer exhibiting balanced ambipolar redox conduction with similar values for the oxidized and reduced states of the polymer (1 _ 10-3 S cm-1). Raman spectra of the asymmetric donor-acceptor materials are characterized by two intense bands characteristic of the aromatic and quinoidal regions in the conjugated À-system of the oligothiophene.

We have developed novel organic photonic devices which are capable of all-optical ultrafast gain switching for use in data communication networks and lab on chip type applications. We have exploited different kinds of devices such as a solid state film, a doped plastic optical fibre and an optofluidic microchannel. We have achieved ultrafast switching (<150 fs) over a broad wavelength range (~100 nm) with a switch on/off ratio of at least 70%. © The Royal Society of Chemistry 2010.

The polymorphism of titanyl phthalocyanine films, grown on atomically flat mica substrates, has been systematically studied by micro-Raman spectroscopy, correlating structure and optical properties. Different growth regimes, using hyperthermal seeded supersonic beams, have been explored as a function of the substrate temperature. Specific signatures in micro-Raman spectra, correlated to different phases, are identified and discussed. We demonstrate the unprecedented ability to grow crystalline films at low temperature, with improved structural order, and we show that different regimes lead to grain dimensions in a range from the nanometric to the micrometric scale. The local micro-Raman analysis, carried out on crystallites with regular shapes, allows discriminating different structural phases of the single crystalline grains. We provide evidence that different growth regimes are achieved and controlled, paving the way to phase selection, which is envisaged as a key feature to improve device performance.

The polymorphism of titanyl phthalocyanine films, grown on atomically flat mica substrates, has been systematically studied by micro-Raman spectroscopy, correlating structure and optical properties. Different growth regimes, using hyperthermal seeded supersonic beams, have been explored as a function of the substrate temperature. Specific signatures in micro-Raman spectra, correlated to different phases, are identified and discussed. We demonstrate the unprecedented ability to grow crystalline films at low temperature, with improved structural order, and we show that different regimes lead to grain dimensions in a range from the nanometric to the micrometric scale. The local micro-Raman analysis, carried out on crystallites with regular shapes, allows discriminating different structural phases of the single crystalline grains. We provide evidence that different growth regimes are achieved and controlled, paving the way to phase selection, which is envisaged as a key feature to improve device performance.

Newly synthesized thiophene (T) and benzothiadiazole (B) co-oligomers of different size, alternation motifs, and alkyl substitution types are reported. Combined spectroscopic data, electrochemical analysis, and theoretical calculations show that the insertion of a single electron-deficient B unit into the aromatic backbone strongly affects the LUMO energy level. The insertion of additional B units has only a minor effect on the electronic properties. Cast films of oligomers with two alternated B rings (B-T-B inner core) display crystalline order. Bottom-contact FETs based on films cast on bare SiO2 show hole-charge mobilities of 1 x 10(-3)-5 x 10(-3) cm(2) V-1 s(-1) and I-on/I-off ratios of 10(5)-10(6). Solution-cast films of cyclohexyl-substituted compounds are amorphous and do not show FET behavior. However, the lack of order observed in these films can be overcome by nanorubbing and unconventional wet lithography, which allow for fine control of structural order in thin deposits.