Browsing by Author "Gomes, Henrique L."
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- A microelectrode impedance method to measure interaction of cellsPublication . Gomes, Henrique L.; Leite, Ricardo; Afonso, R.; Stallinga, Peter; Cancela, M. LeonorAn impedance method was developed to determine how immune system cells (hemocyte) interact with intruder cells (parasites). When the hemocyte cells interact with the parasites, they cause a defensive reaction and the parasites start to aggregate in clusters. The level of aggregation is a measure of the host-parasite interaction, and provides information about the efficiency of the immune system response. The cell aggregation is monitored using a set of microelectrodes. The impedance spectrum is measured between each individual microelectrode and a large reference electrode. As the cells starts to aggregate and settle down towards the microelectrode array the impedance of the system is changed. It is shown that the system impedance is very sensitive to the level of cell aggregation and can be used to monitor in real time the interaction between hemocyte cells and parasites.
- All-inkjet printed organic transistors: Dielectric surface passivation techniques for improved operational stability and lifetimePublication . Gomes, Henrique L.; Medeiros, M. C. R.; Villani, F.; Canudo, J. M.; Loffredo, F.; Miscioscia, R.; Martinez-Domingo, Carme; Ramon, Eloi; Sowade, Enrico; Mitra, K. Y.We report about the use of a printed pentafluorothiophenol layer on top of the dielectric surface as a passivation coating to improve the operational stability of all-ink-jet printed transistors. Transistors with bottom-gate structure were fabricated using cross-linked poly-4-vinylphenol (c-PVP) as dielectric layer and an ink formulation of an amorphous triarylamine polymer as semiconductor. The resulting TFTs had low turn-on voltage (Vth < |5 V|) and a mobility ≈0.1 cm2/(V s). A comparison of identically fabricated transistors shows that devices with coated dielectric have a higher operational stability than those using bare c-PVP. This conclusion is supported by a quantitative study of the threshold voltage shift with time under continuous operation. Long exposure to the ambient atmosphere causes an increase in the threshold voltage strongly dependent on the used semiconducting ink formulation.
- All-inkjet-printed thin-film transistors: manufacturing process reliability by root cause analysisPublication . Sowade, Enrico; Ramon, Eloi; Mitra, Kalyan Yoti; Martinez-Domingo, Carme; Pedro, Marta; Pallares, Jofre; Loffredo, Fausta; Villani, Fulvia; Gomes, Henrique L.; Teres, Lluis; Baumann, Reinhard R.We report on the detailed electrical investigation of all-inkjet-printed thin-film transistor (TFT) arrays focusing on TFT failures and their origins. The TFT arrays were manufactured on flexible polymer substrates in ambient condition without the need for cleanroom environment or inert atmosphere and at a maximum temperature of 150 degrees C. Alternative manufacturing processes for electronic devices such as inkjet printing suffer from lower accuracy compared to traditional microelectronic manufacturing methods. Furthermore, usually printing methods do not allow the manufacturing of electronic devices with high yield (high number of functional devices). In general, the manufacturing yield is much lower compared to the established conventional manufacturing methods based on lithography. Thus, the focus of this contribution is set on a comprehensive analysis of defective TFTs printed by inkjet technology. Based on root cause analysis, we present the defects by developing failure categories and discuss the reasons for the defects. This procedure identifies failure origins and allows the optimization of the manufacturing resulting finally to a yield improvement.
- Analysis of deep levels in a phenylenevinylene polymer by transient capacitance methodsPublication . Gomes, Henrique L.; Stallinga, Peter; Rost, H.; Holmes, A. B.; Harrison, M. G.; Friend, R. H.Transient capacitance methods were applied to the depletion region of an abrupt asymmetric n(+) -p junction of silicon and unintentionally doped poly[2-methoxy, 5 ethyl (2' hexyloxy) paraphenylenevinylene] (MEH-PPV). Studies in the temperature range 100-300 K show the presence of a majority-carrier trap at 1.0 eV and two minority traps at 0.7 and 1.3 eV, respectively. There is an indication for more levels for which the activation energy could not be determined. Furthermore, admittance data reveal a bulk activation energy for conduction of 0.12 eV, suggesting the presence of an additional shallow acceptor state. (C) 1999 American Institute of Physics. [S0003-6951(99)02308-6].
- Anomalous temperature dependence of the current in a metal-oxide-polymer resistive switching diodePublication . Gomes, Henrique L.; Rocha, Paulo R. F.; Kiazadeh, Asal; De Leeuw, Dago M.; Meskers, S. C. J.Metal-oxide polymer diodes exhibit non-volatile resistive switching. The current–voltage characteristics have been studied as a function of temperature. The low-conductance state follows a thermally activated behaviour. The high-conductance state shows a multistep-like behaviour and below 300K an enormous positive temperature coefficient. This anomalous behaviour contradicts the widely held view that switching is due to filaments that are formed reversibly by the diffusion of metal atoms. Instead, these findings together with small-signal impedance measurements indicate that creation and annihilation of filaments is controlled by filling of shallow traps localized in the oxide or at the oxide/polymer interface.
- Bias-induced threshold voltages shifts in thin-film organic transistorsPublication . Gomes, Henrique L.; Stallinga, Peter; Dinelli, F.; Murgia, M.; Biscarini, F.; De Leeuw, D. M.; Muck, T.; Geurts, J.; Molenkamp, L. W.; Wagner, V.An investigation into the stability of metal-insulator-semiconductor (MIS) transistors based on alpha-sexithiophene is reported. In particular, the kinetics of the threshold voltage shift upon application of a gate bias has been determined. The kinetics follow stretched-hyperbola-type behavior, in agreement with the formalism developed to explain metastability in amorphous-silicon thin-film transistors. Using this model, quantification of device stability is possible. Temperature-dependent measurements show that there are two processes involved in the threshold voltage shift, one occurring at Tapproximate to220 K and the other at Tapproximate to300 K. The latter process is found to be sample dependent. This suggests a relation between device stability and processing parameters. (C) 2004 American Institute of Physics.
- Bioelectrical signal detection using conducting polymer electrodes and the displacement current methodPublication . Inácio, Pedro; Mestre, Ana L G; Medeiros, C.R.; Asgarifar, Sanaz; ELAMINE, Youssef; Canudo, Joana; Santos, João; Bragança, José; Morgado, Jorge; Biscarini, Fabio; Gomes, Henrique L.Conducting polymer electrodes based on poly (3, 4 ethylenedioxythiophene): polystyrene sulfonate were used to record electrophysiological signals from autonomous cardiac contractile cells present in embryoid bodies. Signal detection was carried out by measuring the displacement current across the polymer/electrolyte double-layer capacitance, and compared with voltage detection. While for relatively low capacitance electrodes, the voltage amplification provides higher signal quality, and for high capacitive electrodes, the displacement current method exhibits a higher signal-to-noise ratio. It is proposed that the displacement current method combined with high capacitive polymer-based electrodes is adequate to measure clusters of cells and whole organs. Our approach has a great potential in fundamental studies of drug discovery and safety pharmacology.
- Charge transport in poly(3-methylthiophene) schottky barrier diodesPublication . Gomes, Henrique L.; Taylor, D. M.; Underhill, A. E.Schottky-barrier devices were formed from electropolymerised films of poly (3-methylthiophene) (PMeT). Thermal annealing of a partially undoped film led to diodes with rectification ratios as high as 5900 at 1 V and 50,000 at 2.5 V and ideality factors slightly above 2. The temperature dependence of ac loss tangent and forward currents are identical suggesting that bulk effects dominate device behaviour event at very low forward voltages. Below 250 K forward currents are essentially independent of temperature. Preliminary TSC measurements show the presence of at least two trapping levels in the devices. © 1993.
- Confocal scanning raman spectroscopy (CSRS) of an operating organic light-emitting diodePublication . Paez-Sierra, B. A.; Gomes, Henrique L.Organic molecules with semiconducting properties are becoming nowadays core of the organic-based electronic era. Although organic light emitting diodes (OLEDs) have already matured for commercial applications, they still require longer device lifetimes. Some of the long-standing challenges in OLED technology relay on degradation and failure mechanisms. Several authors observed that degradation and subsequent damage of OLEDs is accompanied by formation of dark non-emissive spots [1-2]. Implementation of the confocal scanning Raman spectroscopy (CSRS) measurements helps to understand the chemistry, physics of OLEDs and moreover to have better confidence on their quality assurance.
- Controlling the crack formation in inkjet-printed silver nanoparticle thin-films for high resolution patterning using intense pulsed light treatmentPublication . Gokhale, Pritesh; Mitra, Dana; Sowade, Enrico; Mitra, Kalyan Yoti; Gomes, Henrique L.; Ramon, Eloi; Al-Hamry, Ammar; Kanoun, Olfa; Baumann, Reinhard R.During the last years, intense pulsed light (IPL) processing has been employed and studied intensively for the drying and sintering of metal nanoparticle layers deposited by means of printing methods on flexible polymer substrates. IPL was found to be a very fast and substrate-gentle approach qualified for the field of flexible and large-area printed electronics, i.e. manufactured via roll-to-roll processing. In this contribution, IPL is used for the fine-patterning of printed silver nanoparticle layers. The patterning is obtained by induced and controlled crack formation in the thin silver layer due to the intense exposure of IPL. The crack formation is controlled by selection of the substrate material, the fine-tuning of the morphology of the silver layer and an application of a dielectric layer on top of the silver layer that acts as a stress concentrator. Careful optimization of the IPL parameters allowed to adjust the lateral width of the crack. This novel approach turned out to be a fast and reproducible high-resolution patterning process for multiple applications, e.g. to pattern the source-drain electrodes for all-inkjet-printed thin-film transistors.