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The electrocatalytic reduction of CO2 into useful fuels, exploiting rationally designed, inexpensive, active, and selective catalysts, produced through easy, quick, and scalable routes, represents a promising approach to face today’s climate challenges and energy crisis. This work presents a facile strategy for the preparation of doped SnO2 as an efficient electrocatalyst for the CO2 reduction reaction to formic acid and carbon monoxide. Zn or Ti doping was introduced into a mesoporous SnO2 matrix via wet impregnation and atomic layer deposition. It was found that doping of SnO2 generates an increased amount of oxygen vacancies, which are believed to contribute to the CO2 conversion efficiency, and among others, Zn wet impregnation resulted the most efficient process, as confirmed by X-ray photoelectron spectroscopy analysis. Electrochemical characterization and active surface area evaluation show an increase of availability of surface active sites. In particular, the introduction of Zn elemental doping results in enhanced performance for formic acid formation, in comparison to un-doped SnO2 and other doped SnO2 catalysts. At −0.99 V versus reversible hydrogen electrode, the total faradaic efficiency for CO2 conversion reaches 80%, while the partial current density is 10.3 mA cm−2. These represent a 10% and a threefold increases for faradaic efficiency and current density, respectively, with respect to the reference un-doped sample. The enhancement of these characteristics relates to the improved charge transfer and conductivity with respect to bare SnO2.

The electrocatalytic reduction of CO2 into useful fuels, exploiting rationally designed, inexpensive, active, and selective catalysts, produced through easy, quick, and scalable routes, represents a promising approach to face today’s climate challenges and energy crisis. This work presents a facile strategy for the preparation of doped SnO2 as an efficient electrocatalyst for the CO2 reduction reaction to formic acid and carbon monoxide. Zn or Ti doping was introduced into a mesoporous SnO2 matrix via wet impregnation and atomic layer deposition. It was found that doping of SnO2 generates an increased amount of oxygen vacancies, which are believed to contribute to the CO2 conversion efficiency, and among others, Zn wet impregnation resulted the most efficient process, as confirmed by X-ray photoelectron spectroscopy analysis. Electrochemical characterization and active surface area evaluation show an increase of availability of surface active sites. In particular, the introduction of Zn elemental doping results in enhanced performance for formic acid formation, in comparison to un-doped SnO2 and other doped SnO2 catalysts. At −0.99 V versus reversible hydrogen electrode, the total faradaic efficiency for CO2 conversion reaches 80%, while the partial current density is 10.3 mA cm−2. These represent a 10% and a threefold increases for faradaic efficiency and current density, respectively, with respect to the reference un-doped sample. The enhancement of these characteristics relates to the improved charge transfer and conductivity with respect to bare SnO2.

Memristive devices based on a resistive switching mechanism are considered very promising for nonvolatile memory and unconventional computing applications, even though many details of the switching mechanisms are not yet fully understood. Here, we report a nanostructural study by means of high-resolution transmission electron microscopy and spectroscopy techniques of a Ag/ZnO/Pt memristive device. To ease the localization of the filament position for its characterization, we propose to use the guiding effect of regular perturbation arrays obtained by FIB technology to assist the filament formation. HRTEM and EDX were used to identify the composition and crystalline structure of the so-obtained conductive filaments and surrounding regions. It was determined that the conducting paths are composed mainly of monocrystalline Ag, which remains polycrystalline in some circumstances, including the zone where the switching occurs and at secondary filaments created at the grain boundaries of the polycrystalline ZnO matrix. We also observed that the ZnO matrix shows a degraded quality in the switching zone, while it remains unaltered in the rest of the memristive device.

The physical mechanism involved in resistive switching phenomena occurring in devices based on ZnO nanowire (NW) arrays may vary considerably, also depending on the structure of the switching layer. In particular, it is shown here that the formation of a ZnO base layer between the metallic catalyst substrate and the NW, which is typical of CVD-grown ZnO NW arrays, should not be neglected when explaining the switching physical mechanism. The structural and electrical properties of this layer are investigated after the mechanical removal of NWs. Electrical measurements were performed in the presence of NWs and, after their removal, showed that the base alone exhibits resistive switching properties. The proposed switching mechanism is based on the creation/rupture of an oxygen vacancies conductive path along grain boundaries of the polycrystalline base. The creation of the filament is facilitated by the high concentration of vacancies at the grain boundaries that are oriented perpendicularly to the electrode...

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.

ZnO nanostructures are grown by a simple chemical vapour deposition method directly on a stainless steel disc current collector and successfully tested in lithium cells. The structural/morphological characterization points out the presence of well-defined polycrystalline nanostructures having different shapes and a preferential orientation along the c -axis direction. In addition, the high active surface of the ZnO nanostructures, which accounts for a large electrode/electrolyte contact area, and the complete wetting with the electrolyte solution are considered to be responsible for the good electrical transport properties and the adequate electrochemical behaviour, as confirmed by cyclic voltammetry and galvanostatic charge/discharge cycling. Indeed, despite no binder or conducting additives are used, when galvanostatically tested in lithium cells, after an initial decay, the ZnO nanostructures can provide a rather stable specific capacity approaching 70 μA h cm −2 (i.e., around 400 mA h g −1 ) after prolonged cycling at 1 C, with very high Coulombic efficiency and an overall capacity retention exceeding 62%.

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.

Microbial fuel cell (MFC) is an upcoming technology that allows oxidizing organic matter to generate current by microorganism's activity. To render MFCs a cost-effective and energy sustainable technology, low-cost materials can be employed as support for bacteria growth and proliferation. With this purpose in mind, ceramic Berl saddles were opportunely covered by a thin and conductive carbon layer, thus obtaining an innovative low-cost anode material able to efficiently recover the electrons released by bacteria metabolisms. The conductive layer was obtained by using α- d -glucose deposition process within the following steps: impregnation, caramelization, and pyrolysis. In this way, a homogenous coating of polycrystalline graphitic carbon was successfully obtained and characterized by several methods. The carbon-coated Berl saddles were then tested as anode material in a two-compartment MFC prototype, in batch mode and using Saccharomyces cerevisiae as active microorganisms. The MFC performances were evaluated using electrochemical techniques. The carbon-coated Berl saddles showed a maximum power density of 130 mW m−2 (29.6 mA L−1) which is about 2–3 times higher than the values reported in literature by using commercial anode materials. In particular, we have carefully estimated the production and process costs of these carbon-coated Berl saddles used in our MFC prototype, obtaining a value comparable to the commercial carbon felt employed in the same MFC apparatus. All these results confirm that our innovative carbon-coated Berl saddles not only satisfy the electrical requirements, but also favor an optimal bacteria adhesion and can be produced as a low-cost anode for scaling-up MFC.

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.

Nanocrystalline ZnO films with strong (0002) texture and fine grains were deposited onto ultra-nanocrystalline diamond (UNCD) layers on silicon using high target utilization sputtering technology. The unique characteristic of this sputtering technique allows room temperature growth of smooth ZnO films with a low roughness and low stress at high growth rates. Surface acoustic wave (SAW) devices were fabricated on ZnO/UNCD structure and exhibited good transmission signals with a low insertion loss and a strong side-lobe suppression for the Rayleigh mode SAW. Based on the optimization of the layered structure of the SAW device, a good performance with a coupling coefficient of 5.2% has been realized, promising for improving the microfluidic efficiency in droplet transportation comparing with that of the ZnO/Si SAW device. An optimized temperature coefficient of frequency of −23.4 ppm °C−1 was obtained for the SAW devices with the 2.72 µm-thick ZnO and 1.1 µm-thick UNCD film. Significant thermal effect due to the acoustic heating has been redcued which is related to the temperature stability of the ZnO/UNCD SAW device.

Ultra-smooth nanocrystalline diamond (UNCD) films with high-acoustic wave velocity were introduced into ZnO-based surface acoustic wave (SAW) devices to enhance their microfluidic efficiency by reducing the acoustic energy dissipation into the silicon substrate and improving the acoustic properties of the SAW devices. Microfluidic efficiency of the ZnO-based SAW devices with and without UNCD inter layers was investigated and compared. Results showed that the pumping velocities increase with the input power and those of the ZnO/UNCD/Si devices are much larger than those of the ZnO/Si devices at the same power. The jetting efficiency of the droplet was improved by introducing the UNCD interlayer into the ZnO/Si SAW device. Improvement in the microfluidic efficiency is mainly attributed to the diamond layer, which restrains the acoustic wave to propagate in the top layer rather than dissipating into the substrate.

ASBTRACTZinc Oxide (ZnO) thin films have been addressed as promising candidates for the fabrication of Resistive Random Access Memory devices, which are alternative to conventional charge-based flash memories. According to the filamentary conducting model and charge trapping/detrapping theory developed in the last decade, the memristive behavior of ZnO thin films is explained in terms of conducting filaments formed by metallic ions and/or oxygen vacancies, and their breaking through electrochemical redox reactions and/or recombination of oxygen vacancies/ions. A comparative review of the memristive properties of ZnO thin films grown by sputtering, atomic layer deposition (ALD), pulsed laser deposition (PLD), and sol-gel methods is here proposed. Sputtered ZnO thin films show promising resistive switching behaviors, showing high on/off ratios (10–104), good endurance, and low operating voltages. ALD is also indicated to be useful for growing conformal ZnO layers with atomic thickness control, resulting in ...

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.

This work investigates titanium dioxide nanotube arrays (TiO

Nowadays, graphene oxide based materials and their derivatives can find application in many technological fields and devices. Among them, field emission transistor devices are certainly one of the most promising applications that exploit the unique electronic properties of this material. In addition, countless types of sensing devices based on graphene oxide materials were recently developed, due to the outstanding electrical conductivity and superior mechanical strength, as well as the exceptionally large amount of possibilities for environmental interaction due to the high number of reactive groups present on the large surface area available. Moreover, reduced graphene oxide is considered one of the best candidates for replacing the materials currently used as transparent conductive films for many applications, due to its electrical and mechanical properties, high carrier mobility and optical transparency in the visible range. This chapter summarizes the enormous potential for application of graphene oxide in these fields, with several examples from the current scientific literature. Finally, the possibility of fabricating memristor devices based on graphene oxide is also discussed.

This work reports the formulation of water-based graphene oxide/acrylic nanocomposite inks, and the structural and electrical characterization of test patterns obtained by inkjet direct printing through a commercial piezoelectric micro-fabrication device. Due to the presence of heavily oxygenated functional groups, graphene oxide is strongly hydrophilic and can be readily dispersed in water. Through a process driven by UV irradiation, graphene oxide contained in the inks was reduced to graphene during photo-curing of the polymeric matrix. Printed samples of the nanocomposite material showed a decrease of resistivity with respect to the polymeric matrix. The analysis of the influence of printed layer thickness on resistivity showed that thin layers were less resistive than thick layers. This was explained by the reduced UV penetration depth in thick layers due to shielding effect, resulting in a less effective photo-reduction of graphene oxide.

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.

Micro and nanocrystalline diamond layers have been deposited on molybdenum substrates by hot-filament CVD, and tested in the Mega Amp Spherical Tokamak as a protective coating of fusion plasma-facing materials. The modification of surface properties induced by high density plasma was investigated by SEM, X-ray photoelectron spectroscopy and Raman spectroscopy. Although some modifications of the coating, amorphization and some traces of arcing, diamond proved to be a viable protection as most of the samples were still coated after the plasma exposure. Fuel retention measurements, evaluated by nuclear reaction analysis, showed that a small amount of deuterium was trapped in the surface of the coatings. The chemical erosion of the layers was 50% lower than graphite, as evaluated by dedicated experiments in Pilot-PSI, a linear plasma simulator device.

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.

Atomic layer deposition (ALD) represents one of the most fundamental techniques capable of satisfying the strict technological requirements imposed by the rapidly evolving electronic components industry. The actual scaling trend is rapidly leading to the fabrication of nanoscaled devices able to overcome limits of the present microelectronic technology, of which the memristor is one of the principal candidates. Since their development in 2008, TiO2 thin film memristors have been identified as the future technology for resistive random access memories because of their numerous advantages in producing dense, low power-consuming, three-dimensional memory stacks. The typical features of ALD, such as self-limiting and conformal deposition without line-of-sight requirements, are strong assets for fabricating these nanosized devices. This work focuses on the realization of memristors based on low-temperature ALD TiO2 thin films. In this process, the oxide layer was directly grown on a polymeric photoresist, thus...

An easy method for the fabrication of a completely tunable capacitor based on inkjet-printed hybrid organic systems is here reported. The quantum relativistic properties of graphene-induce electronic resonances between the polyaniline polymeric matrix and the graphene filler, with extremely long transfer rates. These events induce peculiar physical phenomena due to impedance hyperbolicity, such as voltage-controlled phase shifting. The apparent capacitance is shown to diverge to infinity having a sign dependent on the frequency sweep direction. Hence devices may be geometrically tuned to operate with desired capacitance (either positive or negative) at the desired frequency. Among various possibilities of theoretical models that can explain the negative capacitance found, the standard space charge accumulation theory is considered and compared to other quantum relativity effects. The discussion of experimental results based on those theories suggests that a new paradigm is required for a comprehensive explanation of the observed phenomena, which include the observation of high-frequency (MHz) negative capacitance, voltage-controlled phase shifting, and diverging negative/positive capacitance in correspondence of the characteristic resonance frequency (kHz). The various possibilities of exploiting these peculiar properties in advanced electronic devices are also discussed.

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.

Carbon nanotubes (CNTs) are obtained by chemical vapor deposition (CVD) on uncoated silicon and quartz substrates. The process of synthesis involves the co-evaporation of a carbon precursor and a metal catalyst in a nitrogen atmosphere in a high temperature furnace. Beside the formation of CNTs, by varying few parameters, other carbon structures can be deposited, such as a nanographite layer. In particular we believe that our version of thermal CVD is an economic and efficient process in alternative to other methods for the growth of nanographite. The morphology and structure of our samples were characterized by multi-wavelength Micro-Raman spectroscopy, SEM and HR-TEM analyses. We found that our CNTs have an average diameter of 80 nm, with length between few and hundreds of micrometers. Brunauer–Emmett–Teller (BET) analysis was used to calculate the specific surface area and porosity. Furthermore, we have performed an hydrogen storage experiment on our CNTs samples, finding an adsorption capacity of about 1.7 wt%, at 14 bar and 190 °C.

Specific on-resistance Ron estimated from current density-voltage characteristics of Schottky diodes on thick layers exhibits variations from tens of mW.cm2 to tens of W.cm2 for different doping levels. In order to understand the occurrence of high on-state resistance, Schottky barrier heights were first estimated for both forward and reverse bias with the application of thermionic emission theory and were in agreement with a literature reported values. Decrease in mobility with the temperature was observed and its dependencies of T–1.3 and T–2.0 for moderately doped and low doped samples respectively were estimated. From deep level measurements by Minority Carrier Transient Spectroscopy, an influence of shallow boron related levels and D-center on dependence of on-state resistance was observed, being more pronounced in low doped samples. Similar tendency was observed in depth profiling of Ron. This suggests a major role of boron in a compensation mechanism thus resulting in high Ron.

We present a theoretical and experimental study on the design, fabrication and characterization of Schottky Barrier Diodes (SBD) on commercial 4H-SiC epitaxial layers. Numerical simulations were performed with a commercial tool on different edge termination structures, with the aim of optimizing the device behavior. For each termination design, SBD were fabricated and characterized by means of electrical measurements vs. temperature. Simulations provided also useful data for the assessment of the device process technology.

Thin films of nickel were grown on highly n-doped 4H-SiC bulk wafers by thermal evaporation in vacuum. Thermal treatments between 673 and 1323 K have been performed in order to study the structure and microstructure of the films. By XRD2 analysis, it was found that the Ni2Si phase, responsible of the ohmic behaviour of the contact, is oriented and large crystallite grows when the temperature of the treatment exceeds 1123 K. Moreover, by sample rotation, the preferred orientation was overcome and the Ni2Si quantity was evaluated. The structure and microstructure results were correlated to the evolution of Schottky barrier height obtained by electrical measurements.

The presence of defects on 4H-SiC wafers was carefully evidenced by different kinds of techniques such as optical microscopy and scanning electron microscopy. Highlighted defects were also analyzed by atomic force microscopy and profilometer technique and the presence of different SiC polytypes inclusions was found by Raman spectroscopy. Schottky diodes were realized on investigated wafers in order to obtain informations about the correlation between defects and electrical properties of the devices. Electrical characterization has shown the influence of defects in voltage reverse breakdown and Deep Level Transient Spectroscopy has evidenced the presence of two main centers of recombination.

In spite of the high potentiality of silicon carbide (SiC), its technology shows at the moment some limitations, due to the defects present in the crystalline structure. We have focused our analysis on commercial 4H–SiC epitaxial layers. A preliminary investigation has been performed by Optical and Scanning Electron microscopies with the aim to evidence the defect morphology on a large scale. An insight on the defect structure has been obtained by Atomic Force Microscopy, profilometer technique, Micro-Raman and Micro-Photoluminescence spectroscopies. Different types of defects such as comets, super dislocations, etch pits and so on, have been characterized finding interesting peculiarities such as different polytypes inclusions. Moreover, the influence of such defects on the SiC electrical performance has been deeply analyzed through the realization of Schottky barriers onto SiC regions including specific kinds of defects, then performing electrical characterization such as current–voltage (I–V) analysis. Deep Level Transient Spectroscopy (DLTS) yielded the energy position in the SiC gap, the concentration and the capture cross section of two center of recombination.

The reversible resistive switching in ZnO nanowire-based memristive devices is here discussed. Four configurations were studied: top electrode in Pt, with and without polymer coating (polyacrylic acid); top electrode in Cu, with and without the same polymer coating. All the tested devices showed a bipolar resistive switching behavior, with enhanced I-V non-linearity when coated with the polymer. While well-known physical mechanisms such as vacancy and ion-dominated conduction are clearly involved in resistive switching of ZnO nanowires, the coating with the polymer induces new features in the I-V characteristics, probably thanks to the modification of the surface states of ZnO nanowires. Thus, new memristive nanodevices with enhanced electrical features can be developed with such approach.

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.

Engineering applications for printed electronics demand solution processable electrically conductive materials, in the form of inks, to realize interconnections, piezoresistive pressure sensors, thermoresistive temperature sensors, and many other devices. Polyaniline is an intrinsically conductive polymer with modest electrical properties but clear advantages in terms of solubility and stability with temperature and in time. A comprehensive study, starting from its synthesis, primary doping, inkjet printing and secondary doping is presented, with the aim of elucidating the doping agent effects on its morphology, printability and electronic performance.

Memristive nanoscale devices can generate intense fields by the application of relatively low voltages. This peculiar property allows fast, nonvolatile, and low-energy electrical switching as well as the possibility of retaining the internal resistance state according to the history of applied voltage and current. Memristors are predicted to revolutionize the current approaches in computer electronic architecture with their application, for instance, as resistive random access memory and for neuromorphic artificial intelligence. The use of graphene nanostructures for memristive switching systems offers an exciting alternative to other classes of materials, such as transition metal oxide and organic thin films

Electrochemical double-layer capacitor (EDLC) electrodes are conventionally based on carbon materials, which provide increased chemical stability and the possibility of a large number of charge and discharge cycles. However, their specific capacitance is generally much lower than pseudo-capacitors based on metal oxide or conductive polymer electrodes. Carbon-based electrodes for electrochemical devices can be hybridized with metal oxide functionalities in order to provide catalytic activity that increases their electrochemical performances. We report the preparation of a conductive hybrid electrode of reduced graphene oxide and molybdenum oxide by a facile one-pot hydrothermal synthesis. A three-dimensional network structure comprising graphene and molybdenum oxide was obtained when phosphomolybdic acid was used as a precursor for molybdenum oxide. The hybrid material contains polycrystalline nanoparticles of molybdenum (IV) oxide (MoO 2 ) that covers the surface of reduced graphene oxide. Compared to graphene electrodes, the hybrid electrodes showed significantly improved specific capacitance, as good as three times higher (381 vs. 140 F/g), and considerable reduction of the equivalent series resistance (by about half) when they were used in a supercapacitor with sodium containing aqueous electrolyte.

Silicon carbide is a semiconductor of choice for the fabrication of high-power, high-temperature and high-frequency electronic devices. Nevertheless, such a material still presents many problems as regards the crystallographic quality and the presence of defects, which influence the device performance. We have investigated 4H-SiC wafers and 4H-SiC epitaxial layers by microscopy and structural techniques in order to obtain information about the defect morphology. The goal of this analysis will be to correlate them with the electrical properties of SiC for power electronic device applications.