Showing papers by "Diego P. Morales published in 2019"

Laser-Fabricated Reduced Graphene Oxide Memristors.

Abstract: Finding an inexpensive and scalable method for the mass production of memristors will be one of the key aspects for their implementation in end-user computing applications. Herein, we report pioneering research on the fabrication of laser-lithographed graphene oxide memristors. The devices have been surface-fabricated through a graphene oxide coating on a polyethylene terephthalate substrate followed by a localized laser-assisted photo-thermal partial reduction. When the laser fluence is appropriately tuned during the fabrication process, the devices present a characteristic pinched closed-loop in the current-voltage relation revealing the unique fingerprint of the memristive hysteresis. Combined structural and electrical experiments have been conducted to characterize the raw material and the devices that aim to establish a path for optimization. Electrical measurements have demonstrated a clear distinction between the resistive states, as well as stable memory performance, indicating the potential of laser-fabricated graphene oxide memristors in resistive switching applications.

Design, fabrication and characterization of capacitive humidity sensors based on emerging flexible technologies

Abstract: This work presents a case-based comparison between two emerging fabrication techniques for the development of conductive patterns for flexible electronics: inkjet-printing and nanographene production by laser-scribing. In particular, these two methods are used to fabricate planar interdigitated electrode (IDE) capacitors with Kapton® HN polyimide as supporting flexible substrate. Silver-based electrodes are manufactured by inkjet-printing, while a laser-scribing technique is used to obtain laser-reduced graphene oxide (laser-rGO) patterns from deposited graphene oxide (GO) and laser-induced graphene (LIG) layouts from the bare polyimide substrate. The comparison is focused on the application of these IDE capacitors as relative humidity (RH) sensors. The different sensors are benchmarked in terms of sensitivities to RH as well as thermal drift and linearity considering frequency dependencies. The results show that the manufactured capacitors exhibit a very competitive performance as capacitive structures when compared with other similar capacitive sensors from the literature. Furthermore, inkjet-printed and LIG-based capacitors stand out for its thermal stability and linearity.

Inexpensive and flexible nanographene-based electrodes for ubiquitous electrocardiogram monitoring

Abstract: Flexible electronics is one of the fundamental technologies for the development of electronic skin, implant wearables, or ubiquitous biosensing. In this context, graphene-derived materials have attracted great interest due to their unique properties to fulfill the demands of these applications. Here we report a simple one-step method for the fabrication of electrophysical electrodes based on the photothermal production of porous nanographene structures on the surface of flexible polyimide substrates. This approach constitutes an inexpensive alternative to the commercial medical electrodes, leading to a lower and much more stable skin–electrode contact resistance and providing comparable signal transduction. This technology has been framed inside the IoT paradigm through the development of a denoising and signal classification clustering algorithm suitable for its implementation in wearable devices. The experiments have shown promising achievements regarding noise reduction, increasing the crest factor ~3.7 dB, as well as for the over 90% heart rate-monitoring accuracy. A cheap graphene foam electrode has been shown to deliver both accurate acquisition and efficient processing of biosignal to enable next generation medical and wearable devices. A team led by Dr Francisco Romero from University of Granada, Spain develops a one-step and inexpensive method to make electrophysical electrodes for biocompatible signal transduction. They employ low diode lasers to selectively induce highly porous structures in the graphene foam on a flexible substrate. When combined with a clustering algorithm, the graphene foam electrodes can effectively suppress the artifact and noise signals and extract heart beat pattern with more than 90% accuracy. These results present delicate balance between the high accuracy data acquisition and efficient data processing, which are both important for the elemental devices in the internet-of-things paradigm.

Wearable System for Biosignal Acquisition and Monitoring Based on Reconfigurable Technologies.

Abstract: Wearable monitoring devices are now a usual commodity in the market, especially for the monitoring of sports and physical activity. However, specialized wearable devices remain an open field for high-risk professionals, such as military personnel, fire and rescue, law enforcement, etc. In this work, a prototype wearable instrument, based on reconfigurable technologies and capable of monitoring electrocardiogram, oxygen saturation, and motion, is presented. This reconfigurable device allows a wide range of applications in conjunction with mobile devices. As a proof-of-concept, the reconfigurable instrument was been integrated into ad hoc glasses, in order to illustrate the non-invasive monitoring of the user. The performance of the presented prototype was validated against a commercial pulse oximeter, while several alternatives for QRS-complex detection were tested. For this type of scenario, clustering-based classification was found to be a very robust option.

Inexpensive Graphene Oxide Heaters Lithographed by Laser.

Abstract: In this paper, we present a simple and inexpensive method for the fabrication of high-performance graphene-based heaters on different large-scale substrates through the laser photothermal reduction of graphene oxide (laser-reduced graphene-oxide, LrGO). This method allows an efficient and localized high level of reduction and therefore a good electrical conductivity of the treated films. The performance of the heaters is studied in terms of steady-state temperature, power consumption, and time response for different substrates and sizes. The results show that the LrGO heaters can achieve stable steady-state temperatures higher than 200 °C when a voltage of 15 V is applied, featuring a time constant of around 4 s and a heat transfer coefficient of ~200 °C cm2/W. These characteristics are compared with other technologies in this field, demonstrating that the fabrication approach described in this work is competitive and promising to fabricate large-scale flexible heaters with a very fast response and high steady-state temperatures in a cost-effective way. This technology can be easily combined with other fabrication methods, such as screen printing or spray-deposition, for the manufacturing of complete sensing systems where the temperature control is required to adjust functionalities or to tune sensitivity or selectivity.

A Practical Approach to the Design of a Highly Efficient PSFB DC-DC Converter for Server Applications

Abstract: The phase shift full bridge (PSFB) is a widely known isolated DC-DC converter topology commonly used in medium to high power applications, and one of the best candidates for the front-end DC-DC converter in server power supplies. Since the server power supplies consume an enormous amount of power, the most critical issue is to achieve high efficiency. Several organizations promoting electrical energy efficiency, like the 80 PLUS, keep introducing higher efficiency certifications with growing requirements extending also to light loads. The design of a high efficiency PSFB converter is a complex problem with many degrees of freedom which requires of a sufficiently accurate modeling of the losses and of efficient design criteria. In this work a losses model of the converter is proposed as well as design guidelines for the efficiency optimization of PSFB converter. The model and the criteria are tested with the redesign of an existing reference PSFB converter of 1400 W for server applications, with wide input voltage range, nominal 400 V input and 12 V output; achieving 95.85% of efficiency at 50% of the load. A new optimized prototype of PSFB was built with the same specifications, achieving a peak efficiency of 96.68% at 50% of the load.

Assessment of three electrolyte–molecule electrostatic interaction models for 2D material based BioFETs

Abstract: BioFETs based on two-dimensional materials (2DMs) offer a unique opportunity to enhance, at a low cost, the sensitivity of current biosensors enabling the design of compact devices compatible with standard CMOS technology. The unique combination of large exposed surface areas and minimal thicknesses of 2DMs is an outstanding feature for these devices, and the assessment of their behaviour requires combined experimental and theoretical efforts. In this work we present a 2D-material based BioFET simulator including complex electrolyte reactions and analysing different models for the electrolyte–molecule interaction. These models describe how the molecular charge is screened by the electrolyte ions when their distributions are modified. The electrolyte simulation is validated against experimental results as well as against the analytical predictions of the Debye–Huckel approximation. The role of the electrolyte charge screening as well as the impact of the interaction model on the device responsivity are analysed in detail. The results are discussed in order to conclude about the consequences of employing different interaction approximations for the simulation of BioFETs and more generally on the correct modelling of biomolecule-device interaction in BioFETs.

Acoustic characterization of laser-induced graphene film thermoacoustic loudspeakers

Abstract: We report on the superior performance of laser-induced graphene (LIG) film thermoacoustic (TA) loudspeakers. LIG films are fabricated by laser-ablation of polyimide foil and contacted by screen-printing of silver paste, allowing for a facile, ambient condition fabrication process. The LIG film TA loudspeakers achieve up to 50 dB SPL at 1m distance, showing a 10 dB higher efficiency with respect to the nanostructured TA loudspeakers with polyimide substrate reported in literature.

An Optimized Measurement Algorithm for Gas Sensors Based on Carbon Nanotubes: Optimizing Sensor Performance and Hardware Resources

Abstract: This paper presents a novel algorithm for the measurement of resistive-type gas sensors with carbon nanotubes (CNTs) as sensitive layer. Contrary to conventional strategies, which extract the sensor information from the normalized resistance, the proposed algorithm is based on the variation in resistance over time. The results have demonstrated that the time necessary to get the maximum performance of these sensors is reduced around a 25% when compared with the conventional approach for any of the recovery strategies analyzed (passive desorption, external heating, or dc voltage). The hardware implementation of the proposed algorithm in a field-programmable gate array (FPGA) has also demonstrated that, in addition to optimizing the sensor performance in terms of time response and sensitivity, this measurement algorithm yields a significant minimization of the sensor readout circuit resources at both software and hardware levels paving the way for future development of smart sensors for the Internet of Things (IoT) applications.