Carlos García Núñez

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.

Large-area self-assembly of silica microspheres/nanospheres by temperature-assisted dip-coating

This work reports a temperature-assisted dip-coating method for self-assembly of silica (SiO2) microspheres/nanospheres (SPs) as monolayers over large areas (∼cm2). The area over which self-assembled monolayers (SAMs) are formed can be controlled by tuning the suspension temperature (Ts), which allows precise control over the meniscus shape. Furthermore, the formation of periodic stripes of SAMs, with excellent dimensional control (stripe width and stripe-to-stripe spacing), is demonstrated using a suitable set of dip-coating parameters. These findings establish the role of Ts, and other parameters such as withdrawal speed (Vw), withdrawal angle (θw), and withdrawal step length (Lw). For Ts ranged between 25 and 80 °C, the morphological analysis of dip-coatings shows layered structures comprising of defective layers (25-60 °C), single layers (70 °C), and multilayers (>70 °C) owing to the variation of SP flux at the meniscus/substrate assembling interface. At Ts = 70 °C, there is an optimum Vw, approximately equal to the downshift speed of the meniscus (Vm = 1.3 μm/s), which allows the SAM formation over areas (2.25 cm2) roughly 10 times larger than reported in the literature using nanospheres. Finally, the large-area SAM is used to demonstrate the enhanced performance of antireflective coatings for photovoltaic cells and to create metal nanomesh for Si nanowire synthesis.

Investigation of surface plasmon resonance in Au nanoparticles deposited on ZnO:Al thin films

Spectroscopic ellipsometry in external reflection (ER) and total internal reflection (TIR) modes is used to characterize surface plasmon resonance in Au nanoparticles (AuNPs) deposited on Al-doped ZnO films via surface thiolation. ER ellipsometry exhibits high sensitivity to the alkanethiol layer as well as to localized surface plasmons at energies around 2.3 eV. TIR ellipsometry reveals resonances at higher energies (2.9–3.35 eV), which are dependent on the environment used: air or deionized water. Coupling between charge dipoles inside the AZO layer and surface plasmons may account for the existence of those resonances.

Photodetector fabrication by dielectrophoretic assembly of GaAs nanowires grown by a two-steps method

GaAs nanowires (NWs) are promising advanced materials for the development of high performance photodetectors in the visible and infrared range. In this work, we optimize the epitaxial growth of GaAs NWs compared to conventional procedures, by introducing a novel two-steps growth method that exhibits an improvement of the resulting NW aspectratio and an enhancement of the NW growth rate. Moreover, we investigate the contactless manipulation of NWs using non-uniform electric fields to assemble a single GaAs NW on conductive electrodes, resulting in assembly yields above 90%/site and an alignment yields of around 95%. The electrical characteristics of the dielectrophoretic contact formed between the NW and the electrode have been measured, observing that the use of n-type Al-doped ZnO (AZO) as electrode material for NW alignment produces Schottky barrier contacts with the GaAs NW body. Moreover, our results show the fast fabrication of diodes with rectifying characteristics due to the formation of a low-resistance contact between the Ga catalytic droplet at the tip of the NW and the AZO electrode. The current-voltage measurements of a single GaAs NW diode under different illumination conditions show a strong light responsivity of the forward bias characteristic mainly produced by a change on the series resistance.

Modelling of nanowire FETs based neural network for tactile pattern recognition in E-skin

This paper presents device, circuit and system modelling to validate the use of neural nanowire FETs (u-NWFETs) towards a hardware-realizable Neural Network. Hardware neural networks are promising for neuromorphic computing and have many prospective applications for bi-directional interface in prosthetics, and electroceuticals etc. Device simulation of a u-NWFET has been carried out followed by circuit implementation to validate the use of silicon nanowires (Si-NWs) as neuronal elements. A system level simulation of 258 neurons (225 sensor neurons, 50 hidden layer neurons and 3 output layer neurons) has been performed to demonstrate tactile pattern recognition. Training has been carried out and validation of the trained network gives an accurate classification of a database of 50 tactile images into 3 classifiers.

A novel growth method to improve the quality of GaAs nanowires grown by Ga-assisted chemical beam epitaxy

The successful synthesis of high crystalline quality and high aspect ratio GaAs nanowires (NWs) with a uniform diameter is needed to develop advanced applications beyond the limits established by thin film and bulk material properties. Vertically aligned GaAs NWs have been extensively grown by Ga-assisted vapor-liquid-solid (VLS) mechanism on Si(111) substrates, and they have been used as building blocks in photovoltaics, optoelectronics, electronics, and so forth. However, the nucleation of parasitic species such as traces and nanocrystals on the Si substrate surface during the NW growth could affect significantly the controlled nucleation of those NWs, and therefore the resulting performance of NW-based devices. Preventing the nucleation of parasitic species on the Si substrate is a matter of interest, because they could act as traps for gaseous precursors and/or chemical elements during VLS growth, drastically reducing the maximum length of grown NWs, affecting their morphology and structure, and reducing the NW density along the Si substrate surface. This work presents a novel and easy to develop growth method (i.e., without using advanced nanolithography techniques) to prevent the nucleation of parasitic species, while preserving the quality of GaAs NWs even for long duration growths. GaAs NWs are grown by Ga-assisted chemical beam epitaxy on oxidized Si(111) substrates using triethylgallium and tertiarybutylarsine precursors by a two-step-based growth method presented here; this method includes a growth interruption for an oxidation on air between both steps of growth, reducing the nucleation of parasitic crystals on the thicker SiO x capping layer during the second and longer growth step. VLS conditions are preserved overtime, resulting in a stable NW growth rate of around 6 μm/h for growth times up to 1 h. Resulting GaAs NWs have a high aspect ratio of 85 and average radius of 35 nm. We also report on the existence of characteristic reflection high-energy electron diffraction patterns associated with the epitaxial growth of GaAs NWs on Si(111) substrates, which have been analyzed and compared to the morphological characterization of GaAs NWs grown for different times under different conditions.

Heterogeneous integration of contact-printed semiconductor nanowires for high-performance devices on large areas

In this work, we have developed a contact-printing system to efficiently transfer the bottom-up and top-down semiconductor nanowires (NWs), preserving their as-grown features with a good control over their electronic properties. In the close-loop configuration, the printing system is controlled with parameters such as contact pressure and sliding speed/stroke. Combined with the dry pre-treatment of the receiver substrate, the system prints electronic layers with high NW density (7 NWs/μm for bottom-up ZnO and 3 NWs/μm for top-down Si NWs), NW transfer yield and reproducibility. We observed compactly packed (~115 nm average diameters of NWs, with NW-to-NW spacing ~165 nm) and well-aligned NWs (90% with respect to the printing direction). We have theoretically and experimentally analysed the role of contact force on NW print dynamics to investigate the heterogeneous integration of ZnO and Si NWs over pre-selected areas. Moreover, the contact-printing system was used to fabricate ZnO and Si NW-based ultraviolet (UV) photodetectors (PDs) with Wheatstone bridge (WB) configuration on rigid and flexible substrates. The UV PDs based on the printed ensemble of NWs demonstrate high efficiency, a high photocurrent to dark current ratio (>104) and reduced thermal variations as a result of inherent self-compensation of WB arrangement. Due to statistically lesser dimensional variations in the ensemble of NWs, the UV PDs made from them have exhibited uniform response.Nanosystems: Contact-Printing System Developed for Semiconductor NanowiresA contact-printing system has been developed that allows the printing of semiconductor nanowires (NWs), such that electronic layers made from printed NWs can lead to devices with uniform response over large areas. With many attractive features, the NWs have gained attention in meeting demands to integrate low-power miniaturized devices over large areas and with flexible substrates. However, achieving response uniformity with nanoscale devices made from NWs has proved challenging. The team from Bendable Electronics and Sensing Technologies (BEST) group headed by Professor Ravinder Dahiya at the University of Glasgow, United Kingdom has developed a contact-printing system that overcome this challenge and efficiently prints NWs to various substrates while preserving the key characteristics and good control over their electronic properties. The technology has high reproducibility and reliability. The team believes its system holds promise for heterogenous integration of different semiconductor and metal NWs, allowing large-area scalability and and possibly the 3-D integration on flexible substrates.

Live demonstration: Energy autonomous electronic skin for robotics

An electronic skin (e-skin) is an artificial smart skin that can provide similar sense of touch to robots and artificial prostheses by mimicking some of the features of human skin. In this regard, tactile e-skin is needed for accurate haptic perception in robots, amputees, as well as, wearable electronics. For example, a flexible e-skin provided with touch/pressure sensors will allow robots to detect the strength and location of the pressure exerted on the skin surface by surrounding objects. Energy autonomy, or also called self-powering, is also a critical feature for an e-skin, enabling portability and longer operation times without human intervention. Further, making the e-skin transparent adds an extra dimension in the functional design space of e-skin, allowing the integration of a solar cell underneath the skin while preserving light energy harvesting. Recent advances in photovoltaics are oriented towards the development of solar cells on stretchable/flexible substrates which will benefit the realization of suggested self-powered technology. Accordingly, the novel approach presented in this demo consists in a vertical layered stack structure, comprising a solar cell attached to the back plane of a transparent tactile skin, where e-skin transparency being a crucial feature that allows light pass through, making the building-block unique, and opening a new promising line of energy autonomous devices for portable flexible electronics.

Metal oxide nanowires as building blocks for light detectors, gas sensors and biosensors

Metal oxide nanowires are promising structures for the development of novel electronic and optoelectronic devices. In this work, we describe the synthesis of ZnO and CuO nanowires by vapor phase transport and Cu oxidation, respectively. After removal from the substrate, the nanowires are deposited on pairs of conductive electrodes previously evaporated on SiO2/Si substrates by dielectrophoresis. Al-doped ZnO (AZO) electrodes present good stability during the process and enable the fabrication of ultraviolet photoconductors and sensors along with ZnO nanowires. On the other hand, the deposition of CuO on AZO electrodes yields rectifying behavior related to the p-type conduction in CuO and the formation a diode-to-diode structure with the n-type electrodes. In contrast to ZnO photoconductors, these structures present optical response under illumination with visible and near-infrared light and short turn-on and recovery times.

Optical sensors based on metal oxide nanowires for UV/IR detection

Metal oxide nanowires (NWs) present high stability and excellent optical, electrical and mechanical properties. Their synthesis is cost-effective since they can be produced by means of conventional ovens using vapor phase transport or direct metal oxidation. In this work, n-type ZnO and p-type CuO NWs are deposited on pre-patterned electrodes of Aldoped ZnO (AZO) by dielectrophoresis. Performance of devices fabricated from single and multiple NWs are compared. Highly selective UV detection is demonstrated in n-type ZnO NW photoconductors with high external gains in the 0.09-1×108 range and slow time responses, both effects induced by surface effects. In contrast, n-p-n AZO/ CuO NW/AZO heterostructures show lower gains but faster optical responses, mainly limited by device parasitics. Given the CuO bandgap (1.2 eV), the results are quite promising for the development of hybrid metal oxide detection structures in imaging and photovoltaic applications.