Vincenzo Vinciguerra

Growth mechanisms in chemical vapour deposited carbon nanotubes

We present a model for the process of the growth of carbon nanotubes (CNTs) obtained by chemical vapour deposition in the presence of transition metal nanoparticles (Me-NPs) which act as a catalyst. We have deduced that the growth of a CNT occurs in the presence of two forces: (i) a viscous force, due to the surrounding hot gas, which opposes and slows down the growth of the CNT, and (ii) an extrusive force that causes the growth and that in the steady-state stage of the growth is completely balanced by the viscous force. We believe that it is the great decrease in free energy in the assembling reaction that occurs at the interface of the Me-NP catalyst that causes the extrusive force for the growth of a CNT. Moreover, the process of chemisorption of a C2 fragment, through the interaction of the C2?? ?system with the 3d metal orbitals, has been considered as well as the coordination action of the Fe, Ni and Co metal surfaces. The structural properties of the Fe, Co and Ni surfaces show that the (1, ? 1, 0) planes of Fe and the (1, 1, 1) planes of Co and Ni exhibit the symmetry and distances required to overlap with the lattice of a graphene sheet. This gives us information about the coordination mechanism responsible for assembling the CNTs. In fact, we show that it is possible to cleave an Me-NP in such a way as to match the correct symmetry and dimension of the armchair structure of a single-walled nanotube. The mechanism of C2 addition at the edge of the growing CNT has also been considered in relation to the highest occupied molecular orbital?lowest unoccupied molecular orbital (HOMO?LUMO) symmetry. We demonstrate that the action of d orbitals of the metal atoms forming the Me-NP makes possible the thermally forbidden reaction, which involves the C2?? system.

pH sensing properties of graphene solution-gated field-effect transistors

The use of graphene grown by chemical vapor deposition to fabricate solution-gated field-effect transistors (SGFET) on different substrates is reported. SGFETs were fabricated using graphene transferred on poly(ethylene 2,6-naphthalenedicarboxylate) substrate in order to study the influence of using a flexible substrate for pH sensing. Furthermore, in order to understand the influence of fabrication-related residues on top of the graphene surface, a fabrication method was developed for graphene-on-SiO2 SGFETs that enables to keep a graphene surface completely clean of any residues at the end of the fabrication. We were then able to demonstrate that the electrical response of the SGFET devices to pH does not depend either on the specific substrate on which graphene is transferred or on the existence of a moderate amount of fabrication-related residues on top of the graphene surface. These considerations simplify and ease the design and fabrication of graphene pH sensors, paving the way for developing low c...

AlN texturing and piezoelectricity on flexible substrates for sensor applications

We show that AlN-based piezocapacitors with relatively high piezoelectric coefficient (d33) values (3–4 pC/N) can be fabricated on polyimide (PI) substrates at 160 °C or even at room temperature by sputtering processes. With respect to PI, a reduction of the piezoelectric performances was observed on polyethylene naphthalate (PEN). With the same approach, a d33 value as high as 8 pC/N was achieved on rigid substrates (SiO2/Si). In all cases, a thin Al buffer layer was deposited, immediately before AlN, without breaking the vacuum in the deposition chamber, in order to preserve the interface from contaminations that would obstruct the optimal atomic stratification with the desired [0001] growth axis. The piezoelectric behavior was thus correlated to the degree of texturing of the AlN layer through the evaluation of the XRD texturing coefficients and to the morphology by means of AFM analyses. We show that a high level of roughness introduced by the PEN substrate, coupled with the effect of the substrate fl...

Stretchable wireless system for sweat pH monitoring.

Sensor-laden wearable systems that are capable of providing continuous measurement of key physiological parameters coupled with data storage, drug delivery and feedback therapy have attracted huge interest. Here we report a stretchable wireless system for sweat pH monitoring, which is able to withstand up to 53% uniaxial strain and more than 500 cycles to 30% strain. The stretchability of the pH sensor patch is provided by a pair of serpentine-shaped stretchable interconnects. The pH sensing electrode is made of graphite-polyurethane composite, which is suitable for biosensor application. The sensing patch validated through in-depth electrochemical studies, exhibits a pH sensitivity of 11.13 ± 5.8 mV/pH with a maximum response time of 8 s. Interference study of ions and analyte (Na+, K+ and glucose) in test solutions shows negligible influence on the pH sensor performance. The pH data can be wirelessly and continuously transmitted to smartphone through a stretchable radio-frequency-identification antenna, of which the radiating performance is stable under 20% strain, as proved by vector network analyzer measurement. To evaluate the full system, the pH value of a human sweat equivalent solution has been measured and wirelessly transmitted to a custom-developed smart phone App.

Nanowire FET Based Neural Element for Robotic Tactile Sensing Skin

This paper presents novel Neural Nanowire Field Effect Transistors (υ-NWFETs) based hardware-implementable neural network (HNN) approach for tactile data processing in electronic skin (e-skin). The viability of Si nanowires (NWs) as the active material for υ-NWFETs in HNN is explored through modeling and demonstrated by fabricating the first device. Using υ-NWFETs to realize HNNs is an interesting approach as by printing NWs on large area flexible substrates it will be possible to develop a bendable tactile skin with distributed neural elements (for local data processing, as in biological skin) in the backplane. The modeling and simulation of υ-NWFET based devices show that the overlapping areas between individual gates and the floating gate determines the initial synaptic weights of the neural network - thus validating the working of υ-NWFETs as the building block for HNN. The simulation has been further extended to υ-NWFET based circuits and neuronal computation system and this has been validated by interfacing it with a transparent tactile skin prototype (comprising of 6 × 6 ITO based capacitive tactile sensors array) integrated on the palm of a 3D printed robotic hand. In this regard, a tactile data coding system is presented to detect touch gesture and the direction of touch. Following these simulation studies, a four-gated υ-NWFET is fabricated with Pt/Ti metal stack for gates, source and drain, Ni floating gate, and Al2O3 high-k dielectric layer. The current-voltage characteristics of fabricated υ-NWFET devices confirm the dependence of turn-off voltages on the (synaptic) weight of each gate. The presented υ-NWFET approach is promising for a neuro-robotic tactile sensory system with distributed computing as well as numerous futuristic applications such as prosthetics, and electroceuticals.

Stretchable resistive pressure sensor based on CNT-PDMS nanocomposites

Flexible pressure sensors attached conformably to skin are of great interest for wearable electronics and robotic applications. However, effective utilization of such devices often requires them to be stretchable. Herein we report a stretchable pressure sensor based on carbon nanotube - polydimethylsiloxane (CNT-PDMS) nanocomposite. The sensors are based on interdigitated silver (Ag) patterns as bottom electrodes which are connected by a top conductive polymer made of CNT-PDMS composite. The sensors are developed on a PDMS substrate to achieve the required elasticity. The performance of the sensors is assessed by measuring change in the resistance of the device for applied mechanical stimuli. The minimal detectable pressure by our sensor is 500Pa. It is noted that the conductivity of CNT-PDMS composites and Ag electrode spacing are the two critical factors significantly influencing the performance of the sensors.

Stretchable interconnects using screen printed nanocomposites of MWCNTs with PDMS and P(VDF-TrFE)

This paper presents the methodology developed for implementation of stretchable interconnects from nanocomposites of multi-walled carbon nanotubes (MWCNTs) with Polydimethylsiloxane (PDMS) and Polyvinylidene Fluoride Trifluoroethylene (P(VDF-TrFE)). The cost-effective manual screen printing technique has been adopted to realize the stretchable interconnects with different weight percentages (wt. %) of nanocomposite solutions. To improve the elongation or strechability interconnects have been patterned into a serpentine shape. Both the printed interconnects and bulk pellets of the composites have been evaluated through optical means and electrical measurements. It is found that the 6 wt % of MWCNTs-PDMS and 5 wt % of MWCNTs-P(VDF-TrFE) composites have the electrical resistance which is acceptable for many applications. Unlike the common metal interconnects, the nanocomposites based interconnects are highly flexible, stretchable and have good electrical properties. The stretchable interconnects can accommodate significant deformations while maintaining the electrical conductivities and hence they prove to be promising candidates for electronics over large areas.

A Compact SPICE Model for Organic TFTs and Applications to Logic Circuit Design

This work presents Spice implementation of a compact DC model developed for Organic Thin Film Transistors (OTFTs) that uses small molecules in binder matrix as active layer, and the simulation results of an inverter and a ring oscillator circuit which are built with this type of transistors. The DC model relies on a modified version of the Gradual Channel Approximation (GCA) that takes into account the contact effects occurring at non-ohmic metal/organic semiconductor junctions. The model also comprises channel length modulation and scalability of drain current with respect to channel length of the device. The experimental data of the fabricated device and Spice simulation results match well with each other. As an application example, an inverter and a ring oscillator circuit are simulated with Spice. Circuit simulation results show that it is possible to design further logic circuits using the presented model.

Water-gated organic transistors on polyethylene naphthalate films

Water-gated organic transistors have been successfully exploited as potentiometric transducers in a variety of sensing applications. The device response does not depend exclusively on the intrinsic properties of the active materials, as the substrate and the device interfaces play a central role. It is therefore important to fine-tune the choice of materials and layout in order to optimize the final device performance. Here, polyethylene naphthalate (PEN) has been chosen as the reference substrate to fabricate and test flexible transistors as bioelectronic transducers in liquid. PEN is a biocompatible substrate that fulfills the requirements for both bio-applications and micro-fabrication technology. Three different semiconducting or conducting polymer thin films employing pentacene, poly(3-hexylthiophene) or poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) were compared in terms of transconductance, potentiometric sensitivity and response time. The different results allow us to identify material properties crucial for the optimization of organic transistor-based transducers operating in water.

A compact Spice model for organic TFTs and applications to logic circuit design

This work introduces a compact DC model developed for organic thin film transistors (OTFTs) and its SPICE implementation. The model relies on a modified version of the gradual channel approximation that takes into account the contact effects, occurring at nonohmic metal/organic semiconductor junctions, modeling them as reverse biased Schottky diodes. The model also comprises channel length modulation and scalability of drain current with respect to channel length. To show the suitability of the model, we used it to design an inverter and a ring oscillator circuit. Furthermore, an experimental validation of the OTFTs has been done at the level of the single device as well as with a discrete-component setup based on two OTFTs connected into an inverter configuration. The experimental tests were based on OTFTs that use small molecules in binder matrix as an active layer. The experimental data on the fabricated devices have been found in good agreement with SPICE simulation results, paving the way to the use of the model and the device for the design of OTFT-based integrated circuits.