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Resistive switching (RS) devices based on self-assembled nanowires (NWs) and nanorods (NRs) represent a fascinating alternative to conventional devices with thin film structure. The high surface-to-volume ratio may indeed provide the possibility of modulating their functionalities through surface effects. However, devices based on NWs usually suffer from low resistive switching performances in terms of operating voltages, endurance and retention capabilities. In this work, we report on the resistive switching behaviour of ZnO NW arrays, grown by hydrothermal synthesis, that exhibit stable, bipolar resistive switching characterized by SET/RESET voltages lower than 3 V, endurance higher than 1100 cycles and resistance state retention of more than 105 s. The physical mechanism underlying these RS performances can be ascribed to nanoionic processes involving the formation/rupture of conductive paths assisted by oxygen-related species in the ZnO active layer. The reported results represent, to the best of our knowledge, the best resistive switching performances observed in ZnO NW arrays in terms of endurance and retention.

In this work, we report on the metal–insulator transition and electronic transport properties of single crystalline ZnO nanowires synthetized by means of Chemical Vapor Deposition. After evaluating the effect of adsorbed species on transport properties, the thermally activated conduction mechanism was investigated by temperature-dependent measurements in the range 81.7–250 K revealing that the electronic transport mechanism in these nanostructures is in good agreement with the presence of two thermally activated conduction channels. More importantly, it was observed that the electrical properties of ZnO NWs can be tuned from semiconducting to metallic-like as a function of temperature with a metal-to-insulator transition (MIT) observed at a critical temperature above room temperature (T c ∼ 365 K). Charge density and mobility were investigated by means of field effect measurements in NW field-effect transistor configuration. Results evidenced that the peculiar electronic transport properties of ZnO NWs are related to the high intrinsic n-type doping of these nanostructures that is responsible, at room temperature, of a charge carrier density that lays just below the critical concentration for the MIT. This work shows that native defects, Coulomb interactions and surface states influenced by adsorbed species can significantly influence charge transport in NWs.

This work reports the fabrication of memristive devices based on iron oxide (Fe2O3) thin films grown by atomic layer deposition (ALD) using ferrocene as iron precursor and ozone as oxidant. An excellent control of the ALD process was achieved by using an experimental procedure based on a sequence of micro-pulses, which provided long residence time and homogeneous diffusion of precursors, allowing ALD of thin films with smooth morphology and crystallinity which was found to increase with layer thickness, at temperatures as low as 250 °C. The resistive switching of symmetric Pt/Fe2O3/Pt thin film devices exhibited bipolar mode with good stability and endurance. Multi-level switching was achieved via current and voltage control. It was proved that the ON state regime can be tuned by changing the current compliance while the OFF state can be changed to intermediate levels by decreasing the maximum voltage during RESET. The structural analysis of the switched oxide layer revealed the presence of nano-sized crystalline domains corresponding to different iron oxide phases, suggesting that Joule heating effects during I-V cycling are responsible for a crystallization process of the pristine amorphous layer.

Al2O3/TiO2 multilayer structures were fabricated by atomic layer deposition (ALD) to examine the effect of Al2O3 on the resistive switching behavior of TiO2 thin films. The doping process via ALD consisted in the fabrication of a multilayer structure, in which Al2O3 single layers were periodically inserted into TiO2 films during ALD. The presence of Al atoms induced localized structural and chemical variations that allowed tuning the electrical response of TiO2 devices. Multilayer and doped samples were deposited at low temperature (100 °C), using TiCl4 and TMA as metal precursor and H2O as oxidation source. The memristive behavior of Pt/TiOx:AlOy/Pt symmetric devices was tested in voltage sweep mode, showing a bipolar switching with stable high and low resistance states. The variation of doping concentration of Al2O3 in the TiO2 film obtained by ALD allowed to tune switching voltages, resistance values and ROFF/RON ratio. The fine control of these variables adds a degree of freedom in the control of MIM memristors, exploiting the combination of different binary oxides and producing devices with highly defined and tunable electrical properties.

Memristors are nanoscale devices able to generate intense fields by the application of relatively low voltages, which warrants peculiar properties such as fast, non-volatile and low-energy electrical switching, as well as the possibility of retaining their internal resistance state according to the history of applied voltage and current. Memristors are predicted to revolutionize the current approaches in computer electronics architecture with their application, for instance, as resistive random access memory. Moreover they are indicated as the first brick to create neuromorphic systems and artificial intelligence. The use of graphene oxide as active material for memristive switching systems offers an exciting alternative to other classes of materials, such as transition metal oxide and organic thin films. Graphene oxide is electrically insulating due to the presence of oxygen functionalities, with the advantage of being truly atomically-thin, which makes it the perfect candidate for the fabrication of memristive devices. Different mechanisms were recently proposed for graphene oxide memristive systems, but a definitive evidence in their support is still missing. This challenge has stimulated an extensive activity towards a robust and predictive understanding of the physical phenomena that lie behind this peculiar behavior. A comparative review of several graphene oxide memristive devices is here provided, with a distinction between two different mechanisms for resistance switching: oxygen ions drift and metal filament formation.

Understanding the mechanisms acting behind the functionalization of graphene is of paramount importance for the application of functionalized graphene in polymeric nano-composite materials. This work reports the study of the influence of benzophenone in a UV-mediated grafting process on graphene oxide, carried out by unconventional spectroscopic techniques, such as electron spin resonance and thermogravimetric analysis coupled with in situ infra-red spectroscopy. Using these techniques, a direct investigation of the grafting process was achieved for the first time, while up to now only indirect evidence was provided, opening new perspectives for the study of small molecule grafting on graphene sheets. The presence of benzophenone grafted onto the reduced graphene oxide surface was demonstrated, and in particular an unstable radical species attributable to the semipinacol radical of benzophenone was revealed, which is a key step of the functionalization process. Moreover, X-ray photoelectron spectroscopy demonstrated that the grafting process effectively reduced graphene oxide recovering the properties of graphene, contemporarily leaving active sites for further polymer functionalization.

The growth of hydrogenated sp3-phase of diamond on the sp2-phase of graphite by Microwave Plasma Enhanced Chemical Vapour Deposition (MPECVD) is a challenge, primarily because hydrogen etches graphite much faster than the growth rate of diamond. To enhance nucleation of diamond on graphite, we used a plethora of techniques such as plasma etching, ion bombardment, manual scratching, and scratching by ultrasonic agitation. Nanocrystalline and polycrystalline diamond thin-films were grown by MPECVD on the surface of pre-treated or pristine graphite using 1.5, 3.0, and 3.6 kW microwave power. Samples were characterised by Scanning Electron Microscopy, Raman Spectroscopy, and X-ray Photoelectron Spectroscopy. Species in the gas phase during film deposition were monitored by Optical Emission Spectroscopy. We have found that the surface area covered and the morphology of the diamond films are dependent on the surface pre-treatment. The crystallite size of the films depends on the microwave power used during MPECVD growth. The results of this study establish the protocols for diamond deposition by MPECVD on graphite substrates with a desired crystalline quality based on the pre-treatment of the substrate and the microwave power used during MPECVD. These results are important to modern applications, such as plasma facing materials, in which diamond has shown outstanding performance in contrast to that of graphite.

This work reports the electrical characterization of a water-based graphene oxide/acrylic composite material, which was directly inkjet printed to fabricate dissipative patterns. The graphene oxide filler, which is strongly hydrophilic due to its heavily oxygenated surface and can be readily dispersed in water, was reduced by UV irradiation during photo-curing of the polymeric matrix. The concurrent polymerization of the acrylic matrix and reduction of graphene oxide filler was demonstrated by real-time resistance measurements during UV light irradiation. The presence of graphene filler allowed decreasing the resistance of the pure polymeric matrix by nearly five orders of magnitude. This was explained by the fact that clusters of reduced graphene oxide inside the polymer matrix act as preferential pathways for the mobility of charge carriers, thus leading to an overall decrease of the material’s resistance.

Zinc oxide nanowires (ZnO NWs) were grown by a two-step growth method, involving the deposition of a patterned ZnO thin seeding layer and the chemical vapor deposition (CVD) of ZnO NWs. Two ways of patterning the seed layer were performed. The seeding solution containing ZnO precursors was deposited by sol–gel/spin-coating technique and patterned by photolithography. In the other case, the seeding solution was directly printed by inkjet printing only on selected portion of the substrate areas. In both cases, crystallization of the seed layer was achieved by thermal annealing in ambient air. Vertically aligned ZnO NWs were then grown by CVD on patterned, seeded substrates. The structure and morphology of ZnO NWs was analyzed by means of X-ray diffraction and field emission scanning electron microscopy measurements, respectively, while the vibrational properties were evaluated through Raman spectroscopy. Results showed that less-defective, vertically aligned, c-axis oriented ZnO NWs were grown on substrates patterned by photolithography while more defective nanostructures were grown on printed seed layer. A feature size of 30 µm was transferred into the patterned seed layer, and a good selectivity in growing ZnO NWs was obtained.

Nanostructured polymeric composites are promising materials for the fabrication of piezoresistive devices because they show a huge variation in electrical resistance when subjected to mechanical deformation. Quantum tunneling composites feature a conduction mechanism occurring between the metallic filler and copper particles embedded in a polydimethylsiloxane (PDMS) insulating matrix, and the mechanism is enhanced by the spiky morphology of the particles. PEDOT:PSS electrodes are patterned on either side of the composite by inkjet printing, a technology that allows one-step fabrication processes. The adhesion and spreading of conductive printed ink drops are controlled and enhanced by pre-treating the samples surface in an atmospheric pressure plasma customized system. Because of an extremely high metal to polymer ratio, which results in the different surface and dielectric properties of the composite, conventional plasma conditions are not suitable to allow the control of spreading. The optimal plasma conditions for ink/surface compatibility were found using quantitative comparison based on image analysis and numerical interpretation of the adhesion/roughness properties such as bulging and spread.

Surface and quantum confinement effects in one-dimensional systems such as ZnO nanowires are responsible for novel electrical properties, and can be exploited to tune electrical transport on the nanoscale. The investigation of new physical mechanisms for resistive switching can be fulfilled by studying metal/insulator/metal memristive devices that take advantage of the unique properties of one-dimensional nanoscale metal oxides. In particular, the mechanisms of resistive switching between multiple resistance states in such nanostructures can be associated with the variation of internal physical states. Here we demonstrate both experimentally and theoretically that core–shell structures based on polyacrylic acid coated ZnO nanowires exhibit a resistive switching behavior characterized by internal multiple resistance states, owing to the changes in surface states induced by redox reactions occurring at their surfaces. The introduction of a thin layer of polymer coating resulted in a resistive switching between more than two states. Specifically, the existence of two intermediate states in addition to the high and low resistance states was revealed during DC measurements in voltage sweep mode. All resistive states showed low variability over cycling. The mechanism of switching between multiple steps, as probed by density functional theory calculations, was associated with redox reactions involving species at the interface (e.g. methanal or hydroxyl groups), each characterized by a given redox potential. Therefore, multiple resistance states were induced by specific and stable threshold voltages, as shown experimentally.

Poly- and nanocrystalline diamond films have been deposited using microwave plasma enhanced CVD with gas mixtures of x%CH4/15%H2/Ar (x = 0.5, 1, 3, and 5). After deposition the resulting films were exposed to a hydrogen plasma etching for 30 min. The hydrogen plasma produced preferential etching of non-diamond carbon on the surface of the samples and the development of steps and pits. Raman spectroscopy and X-ray photoelectron spectroscopy analyses on the etched films showed increased sp3/sp2 ratio and decreased surface oxygen. The etch mechanism proposed is regression of pre-existing steps and step flow.

Resistive switching (RS) devices are considered as the most promising alternative to conventional random access memories. They interestingly offer effective properties in terms of device scalability, low power-consumption, fast read/write operations, high endurance and state retention. Moreover, neuromorphic circuits and synapse-like devices are envisaged with RS modeled as memristors, opening the route toward beyond-Von Neumann computing architectures and intelligent systems. This work investigates how the RS properties of zinc oxide thin films are related to both sputtering deposition process and device configuration, i.e. valence change memory and electrochemical metallization memory (ECM). Different devices, with an oxide thickness ranging from 50-250 nm, are fabricated and deeply characterized. The electrical characterization evidences that, differently from typical nanoscale amorphous oxides employed for resistive RAMs (HfO x , WO x , etc), sub-micrometric thicknesses of polycrystalline ZnO layers with ECM configuration are needed to achieve the most reliable devices. The obtained results are deeply discussed, correlating the RS mechanism to material nanostructure.

Diamond thin films have showed outstanding performance when exposed to extreme conditions such as high power plasmas. However there are always concerns about the stability of the diamond structure in the presence of other materials deposited on the film surface by plasma diffusion. It is known that diamond films are etched by hydrogen plasma but in the presence of Si, carbon fibres are formed. In this report we show for the first time the effect of Si on the production of fibres under etching conditions and propose growth mechanisms based on the results of characterisation techniques. Carbon fibres have been synthesised on diamond films and characterised by scanning electron microscopy, X-ray photoelectron spectroscopy and Raman spectroscopy. In situ optical emission spectroscopy was performed during the experiments showing different concentration of growing species which may result in the observed variability of fibres growth rate and morphology. Furthermore, fibres varied in size and shape depending on the structure of the diamond films. The surfaces of the fibres contain silicon and are oxidised having COO and CO groups as seen by XPS analysis. Raman analyses revealed a spectrum typical for graphite combined with that from diamond that remains on the surface after hydrogen bombardment. The results of this study show the experimental conditions in which carbon fibres are produced under high hydrogen etching of diamond films and opens the possibility to other applications such as catalysis, sensors and the production of electrodes, since they combine the unmatchable properties of a diamond supporting substrate with the unique properties of carbon fibres.

This work investigates titanium dioxide nanotube arrays (TiO2-NTA) grown by anodic oxidation as an active material for memristive applications. In particular, metal-insulator-metal structures made of vertically oriented amorphous TiO2-NTA grown on titanium foil were exploited in Ti/TiO2-NTA/Pt devices. The deposition of a polymeric thin film between NTA and top electrodes significantly improved the stability of the devices and increased by more than double the off/on resistance ratio. The resistive switching of TiO2-NTA samples crystallised by thermal annealing was also studied. Such devices displayed nonlinear I-V curves characterised by a smooth rectifying behaviour, without any evident resistive switching (RS). Also in this case, the interposition of the polymeric layer enhanced the RS behaviour of TiO2-NTA samples, remarkably increasing the devices' off/on ratio and endurance. The rise of high resistance states can be simply related to the addition of the polymer as resistance in series, while the variation of the low resistance states is here attributed to the occurrence of surface chemical reactions between polymer functional groups and the metal oxide, which increase the charge carriers available for conduction.

In the present work multi-wall carbon nanotubes (MWNTs), which were synthesized on uncoated silicon by an easily scaled-up catalytical chemical vapor deposition (CVD), were subjected to three different functionalization treatments. Both acid treatment, by means of a HNO 3 –H 2 SO 4 1:3 mixture or chlorosulfuric acid (HSO 3 Cl) solution, and high temperature basic digestion with sodium hydroxide (NaOH) were involved. The efficiency of these methods were investigated by thermo-gravimetric analyses (TGA), Raman spectroscopy, Brunauer–Emmett–Teller (BET) analysis, X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray (EDX) and electron microscopy (SEM, TEM) techniques, and the results were compared. Among the various treatments that have been tested, the 1:3 solution of nitric and sulphuric acid was the most effective in modifying the CNT surface, inducing the formation of functional groups that may be used in biological applications.

[3] Huang JF, Zeng XR, Li HJ, Xiong XB, Huang M. Influence of thepreparing temperature on phase, microstructure and anti-oxidationproperty of SiC coating for C/C composites. Carbon 2004;42:1517–21.[4] Fu QG, Li HJ, Shi XH, Li KZ, Sun GD. Silicon carbide coating toprotect carbon/carbon composites against oxidation. Scripta Mater2005;52:923–7.[5] Kowbei W, Withers JC. CVD and CVR silicon-based functionallygradient coatings on C–C composites. Carbon 1995;33:415–26.[6] Fang HT, Jing CZ, Zhang DY, Jae HJ, Yoo DH. A Si–Mo fusedslurry coating for oxidation protection of carbon–carbon composites.J Mater Sci Lett 2001;20:175–7.[7] Zhang YL, Li HJ, Fu QG, Li KZ, W J, Wang PY. A C/SiC gradientoxidation protective coating for carbon/carbon composites. Surf CoatTechnol 2006;201(6):3491–5.

We report about the electron paramagnetic resonance and nuclear magnetic resonance signals in bulk and thin film-type ultrananocrystalline diamond with and without nitrogen. The localization and nature of defects for powder and compact film samples were analyzed. From the analysis of spin–lattice and spin–spin relaxation times, we have found that spin states sit in sp2 enriched region belonging to the grain boundaries.

Memristors are passive two-terminal memory devices predicted to have a tremendous impact on many research fields and common applications, paving the way to adaptive electronics and high computing systems. We report on a metal/insulator/metal memristor based on a graphene oxide layer, deposited by inkjet printing at room temperature. The electrical characterization of devices, showing hysteretic characteristics typical of bipolar memristive switching, are discussed and correlated to the structural and compositional analysis of the materials. The electroforming process is ascribed to a lowering in contact resistance due to carbon diffusion in Ag electrode, while the oxygen ion drift is identified as the main physical mechanism for Ag/GO/ITO resistive switching.

Microcrystalline undoped and heavily boron-doped polycrystalline diamond layers have been deposited on various substrates by hot filament CVD and exposed to hydrogen plasma in a linear plasma reactor (Pilot-PSI, The Netherlands) that simulates the high flux and high density plasma conditions of tokamak divertors, as well as in the DIII-D tokamak (US). Pre- and post-exposure analysis by SEM and Raman spectroscopy characterised the surface appearance and the sp3 and sp2 components of the diamond films respectively. Surface roughness variation was measured by AFM. Hardness and Young's modulus were assessed by nanoindentation in order to characterise the effect of the plasma on the mechanical properties. HRTEM and EELS have been used to evaluate the nature of the modification induced at the diamond surface by the plasma exposure. The measurements have shown that, despite some surface amorphisation of the exposed layers, further long-term exposure studies are warranted as neither delamination, dramatic film failure nor entire erosion of the film were observed. © 2010 Elsevier B.V. All rights reserved.

Nanocrystalline diamond layers were deposited by hot-filament CVD on several substrates including molybdenum tiles and foils, graphite blocks and silicon wafers. The structural characterisation of the deposited layers, achieved by SEM, AFM, Raman and X-ray photoelectron spectroscopies, showed the typical features of nanocrystalline diamond. A nanocrystalline diamond coated molybdenum tile was exposed to a fusion grade deuterium plasma in the MAST tokamak, with a plasma current of 700 kA, core electron temperature of about 0.6 keV, and total exposure time of about 1 s. The diamond coating showed good resistance to the extremely harsh conditions of the fusion plasma edge.

We report a study of hydrogen adsorption experiment on SWNT samples deposited by chemical vapor deposition (CVD) and investigated with Electron Spin Resonance spectroscopy (ESR) before and after hydrogen treatment. The ESR investigation was performed at room and low temperature (4–30 K). We found that ESR technique is very useful to investigate paramagnetic centers and their interaction with hydrogen. The morphology and structure of the samples were characterized by Micro-Raman spectroscopy. Subsequently the thermal stability was investigated by thermogravimetric analysis (TGA) testing. Brunauer Emmett Teller (BET) analysis was used to calculate the specific surface area and porosity of the as grown samples. All characterizations showed a small presence of residual Fe impurity (3.3 wt.%).

Silicon carbide (SiC) is a wide band gap semiconductor, interesting for its physical properties such as high breakdown field, high saturated drift velocity and high thermal conductivity, which has been intensively studied in the last years. Although the high potentiality of this material, the SiC technology shows at the moment some limitations, indeed, the reliability of SiC-based devices is strictly correlated to the defects present in the crystalline structure. 4H-SiC epilayers were grown by Hot Wall Chemical Vapor Deposition (at 1600 degrees C) and by Sublimation techniques (at 2000 degrees C). A surface investigation of the epilayers has been performed finding particular physical finger-prints correlated with several kind of defects aimed at giving an important feedback to the epitaxial growth processes. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

In this work we show a plasma-assisted process for the deposition of good quality a-SiO 2 layers on 4H–SiC as a possible alternative to thermal oxidation. We used the plasma enhanced chemical vapor deposition (PECVD) technique for the growth of a-SiO 2 layers on 4H–SiC using SiH 4 and CO 2 as precursor gases in H 2 dilution. We showed that good quality oxide layers could be obtained by this method, with a growth rate varying from 1.3 to 2.1 A/s, depending on the RF power. An estimation of the interface charge was obtained by high frequency capacitance voltage (HFCV) characteristic, obtaining values comparable to the ones typical of thermally grown oxides. This process was used for the growth of a-SiO 2 insulating and protecting layers in the fabrication of Schottky diodes based on 4H–SiC, obtaining a breakdown voltage higher than 600 V.

4H–SiC wafers and epitaxial layers were analysed by optical microscopy, profilometer technique and scanning electron microscopy with the aim to evidence the defect morphology on large scale and to determine in both cases the different types of defects. A more detailed analysis has been performed by atomic force microscopy. Different types of defects such as micropipes, comets, super dislocations, etch pits and so on, have been characterized finding particular physical finger-prints. Electrical characterization performed on Schottky diodes realized on 4H–SiC wafers gave information about the correlation between defects and electrical performances of devices.

Polyaniline (PANI) was synthesised by oxidation polymerization starting from aniline dimer, thus avoiding the use of a toxic/mutagenic substance such as aniline. In order to work in an aqueous phase, polystyrene sulphonate (PSS) was used as an emulsioning/doping agent. The fine tuning of oxidant quantity allowed the production of PANI in the mixed leucoemeraldine/emeraldine oxidation states. PANI/PSS was solubilised in dimethylsulphoxide and used as ink for the fabrication of inkjet printed tracks, which showed an interesting negative capacitance effect that can be attributed to the presence of different oxidation states and the partial doping obtained by the use of PSS. Negative capacitance devices are extremely attractive in high frequency applications where positive parasitic capacitances have to be compensated.