Tullio Toccoli

Nanostructured TiO2 thin films prepared by supersonic beams and their application in a sensor array for the discrimination of VOC

Abstract In this paper we present the study on gas sensing properties of TiO2 thin films grown by seeded supersonic beam of cluster oxides. Supersonic cluster beams, produced by a pulsed micro-plasma cluster source have been successfully used to prepare nanocrystalline thin films of TiO2. The obtained gas sensor devices, prepared by using different deposition parameters, were suitable for the use in a gas sensor array for electronic nose. Sensors showed a quite good response to ethanol, methanol and propanol. Principal component analysis was used as method for testing the array capability in discrimination and recognition of the volatile compounds.

Highly ordered films of quaterthiophene grown by seeded supersonic beams

Seeded supersonic beams of oligothiophenes have been developed and used to grow very high quality films on CaF2 and SiO2. Optical characterizations show features previously observed only on single crystals and very thin films. In particular, the vibronic progression of the 0–0 transition is well resolved at 10 K. Highly supersonic beams give best results. High quality films are produced over a range of thicknesses ⩾500 nm. The correlation between properties of the films and beam parameters are discussed. The energy state of oligomers is envisaged to play a key role in the growth.

Growth of titanium dioxide films by cluster supersonic beams for VOC sensing applications

We developed and tested gas sensing devices based on TiO/sub 2/ nano-crystalline films produced at room temperature by the novel growth method of cluster beam deposition. The devices show a very good response to ethanol, methanol, and propanol and an overall performance comparable or better to the best devices reported in literature, based on films grown with other techniques. The major advantage of our growth method is that there is no need of the thermal annealing or doping processes usually required to improve sensitivity and reliability of gas sensing devices based on nanostructured thin film. The sensors dynamic response gives a maximum sensitivity for ethanol at about 250/spl deg/C. As the morphological (AFM) and structural (XRD) characterizations of the films show, the high performance of our sensors could only be achieved because their nano-crystalline structure was well controlled by the properties of the cluster precursors in the supersonic beam. We envisage possible further developments in terms of sensitivity and selectivity of gas sensing devices based on films grown by cluster beam deposition.

High mobility n-type organic thin-film transistors deposited at room temperature by supersonic molecular beam deposition

In this paper, we report on the fabrication of N,N′-1H,1H-perfluorobutil dicyanoperylenediimide (PDIF-CN2) organic thin-film transistors by Supersonic Molecular Beam Deposition. The devices exhibit mobility up to 0.2 cm2/V s even if the substrate is kept at room temperature during the organic film growth, exceeding by three orders of magnitude the electrical performance of those grown at the same temperature by conventional Organic Molecular Beam Deposition. The possibility to get high-mobility n-type transistors avoiding thermal treatments during or after the deposition could significantly extend the number of substrates suitable to the fabrication of flexible high-performance complementary circuits by using this compound.

3D reconstruction of pentacene structural organization in top-contact OTFTs via resonant soft X-ray reflectivity

Herein, we describe the use of soft X-ray reflectivity at the carbon K-edge to study the molecular organization (orientation, structure, and morphology) of pentacene active films in a top-contact transistor geometry. This technique is not affected by sample charging, and it can be applied in the case of insulating substrates. In addition, the sampling depth is not limited to the near-surface region, giving access to buried device interfaces (metal/organic and dielectric/organic). Spectral lineshape simulations, based on ab-initio calculations using a realistic 3D layer-by-layer model, allow us to unravel the details of the molecular organization in all the specific and crucial areas of the active film, overcoming the limitations of conventional approaches. The tilt angle of the long molecular axis in the whole film is found to progressively decrease with respect to the substrate normal from 25° to 0° with the increasing film thickness. A full vertical alignment, optimal for in-plane charge hopping, is reached only after the complete formation of the first five monolayers. Remarkably, starting from the first one in contact with the dielectric substrate, all the monolayers in the stack show a change in orientation with the increasing thickness. On the other hand, at the buried interface with a gold top-contact, the molecules assume a flat orientation that only propagates for two or three monolayers into the organic film. Top-contact devices with the highest performances can thus be obtained using films of at least ten monolayers. This explains the observed thickness dependence of charge mobility in pentacene transistors.

Versatile and Scalable Strategy To Grow Sol–Gel Derived 2H-MoS2 Thin Films with Superior Electronic Properties: A Memristive Case

Transition metal dichalcogenides, such as molybdenum disulfide (MoS2), show peculiar chemical/physical properties that enable their use in applications ranging from micro- and nano-optoelectronics to surface catalysis, gas and light detection, and energy harvesting/production. One main limitation to fully harness the potential of MoS2 is given by the lack of scalable and low environmental impact synthesis of MoS2 films with high uniformity, hence setting a significant challenge for industrial applications. In this work, we develop a versatile and scalable sol-gel-derived MoS2 film fabrication by spin coating deposition of an aqueous sol on different technologically relevant, flexible substrates with annealing at low temperatures (300 °C) and without the need of sulfurization and/or supply of hydrogen as compared to cutting-edge techniques. The electronic and physical properties of the MoS2 thin films were extensively investigated by means of surface spectroscopy and structural characterization techniques. Spatially homogenous nanocrystalline 2H-MoS2 thin films were obtained exhibiting high chemical purity and excellent electronic properties such as an energy band gap of 1.35 eV in agreement with the 2H phase of the MoS2, and a density of states that corresponds to the n-type character expected for high-quality 2H-MoS2. The potential use of sol-gel-grown MoS2 as the candidate material for electronic applications was tested via electrical characterization and demonstrated via the reversible switching in resistivity typical for memristors with a measured ON-OFF ratio ≥102. The obtained results highlight that the novel low-cost fabrication method has a great potential to promote the use of high-quality MoS2 in technological and industrial-relevant scalable applications.

Thin films devices of organic materials by supersonic molecular beams

The possibility to control the energetic properties of the molecules before their deposition can be a great advantage in the growth of ordered films of organic materials. To this end we have developed a new deposition method based on supersonic molecular beam. With this method the kinetic energy, the momentum and the internal energy can be controlled varying the properties of the supersonic expansion. Very interesting results are achieved with oligothiophene where the thin flims several hundred on nm thick grown with this method show optical, morphological and structural characteristics similar to the single crystal. Supersonic molecular beam deposition (SuMBE) is therefore proposed as an interesting method to study and to improve the performance of organic based devices.

Preparation of high-quality organic films by deposition and co-deposition via supersonic seeded beams

Recent results obtained in the growth of organic materials by supersonic molecular beams (SuMBE) are presented. Compared to other vacuum deposition methods, it allows an accurate control on the initial state (kinetic energy, momentum, flux, etc.) of the molecules during the deposition process. We show that such feature can be efficiently exploited to prepare films, the structure and morphology of which resemble those single crystals, therefore achieving unprecedented control on their optical and electronic properties even for relatively thick samples (> 500 nm). We also report on the use of SuMBE for processes of co-deposition with the aim of functionalizing or doping organic semiconductors. We achieved co-depositions of a p-type semiconductor, as metal phthalocyanines, with an n- type semiconductor, as fullerenes, without phase segregation between the two compounds and with a high control on the deposition parameters. This aspect is very attractive for the growth of organic p-n junction in view of improving photovoltaic cells and gas sensors.