Browsing by Author "Stallinga, Peter"
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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.
- An ultra-low-cost RCL-meterPublication . Inácio, Pedro; Guerra, Rui; Stallinga, PeterAn ultra-low-cost RCL meter, aimed at IoT applications, was developed, and was used to measure electrical components based on standard techniques without the need of additional electronics beyond the AVR® micro-controller hardware itself and high-level routines. The models and pseudo-routines required to measure admittance parameters are described, and a benchmark between the ATmega328P and ATmega32U4 AVR® micro-controllers was performed to validate the resistance and capacitance measurements. Both ATmega328P and ATmega32U4 micro-controllers could measure isolated resistances from 0.5 Ω to 80 MΩ and capacitances from 100 fF to 4.7 mF. Inductance measurements are estimated at between 0.2 mH to 1.5 H. The accuracy and range of the measurements of series and parallel RC networks are demonstrated. The relative accuracy (ar) and relative precision (pr) of the measurements were quantified. For the resistance measurements, typically ar, pr < 10% in the interval 100 Ω–100 MΩ. For the capacitance, measured in one of the modes (fast mode), ar < 20% and pr < 5% in the range 100 fF–10 nF, while for the other mode (transient mode), typically ar < 20% in the range 10 nF–10 mF and pr < 5% for 100 pF–10 mF. ar falls below 5% in some sub-ranges. The combination of the two capacitance modes allows for measurements in the range 100 fF–10 mF (11 orders of magnitude) with ar < 20%. Possible applications include the sensing of impedimetric sensor arrays targeted for wearable and in-body bioelectronics, smart agriculture, and smart cities, while complying with small form factor and low cost.
- 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].
- 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.
- Carrier multiplication in germanium nanocrystalsPublication . Saeed, Saba; de Weerd, Chris; Stallinga, Peter; Spoor, Frank C. M.; Houtepen, Arjan J.; Siebbeles, Laurens D. A.; Gregorkiewicz, TomCarrier multiplication is demonstrated in a solid-state dispersion of germanium nanocrystals in a silicon-dioxide matrix. This is performed by comparing ultrafast photo-induced absorption transients at different pump photon energies below and above the threshold energy for this process. The average germanium nanocrystal size is approximately 5-6 nm, as inferred from photoluminescence and Raman spectra. A carrier multiplication efficiency of approximately 190% is measured for photo-excitation at 2.8 times the optical bandgap of germanium nanocrystals, deduced from their photoluminescence spectra.
- Detection of explosive vapors using organic thin-film transistorsPublication . Bentes, E.; Gomes, Henrique L.; Stallinga, Peter; Moura, L.Field effect transistors (FETs) based on organic materials were investigated as sensors for detecting 2,4,6-trinitrotoluene (TNT) vapors. Several FET devices were fabricated using two types of semiconducting organic materials, solution processed polymers deposited by spin coating and, oligomers (or small molecules) deposited by vacuum sublimation. When vapors of nitroaromatic compounds bind to thin films of organic materials which form the transistor channel, the conductivity of the thin film increases and changes the transistor electrical characteristic. The use of the amplifying properties of the transistor represents a major advantage over conventional techniques based on simple changes of resistance in polymers frequently used in electronic noses.
- Determination of deep and shallow levels in conjugated polymers by electrical methodsPublication . Stallinga, Peter; Gomes, Henrique L.; Rost, H.; Holmes, A. B.; Harrison, M. G.; Friend, R. H.; Biscarini, F.; Taliani, C.; Jones, G. W.; Taylor, D. M.Conjugated organic semiconductors have been submitted to various electrical measurement techniques in order to reveal information about shallow levels and deep traps in the forbidden gap. The materials consisted of poly[2-methoxy, 5 ethyl (2' hexyloxy) paraphenylenevinylene] (MEH-PPV), poly(3-methylthiophene) (PMeT), and alpha-sexithienyl (alpha T6) and the employed techniques were IV, CV, admittance spectroscopy, TSC, capacitance and current transients. (C) 1999 Elsevier Science B.V. All rights reserved.
- Determining carrier mobility with a metal–insulator–semiconductor structurePublication . Stallinga, Peter; Benvenho, A. R. V.; Smits, E. C. P.; Mathijssen, S. G. J.; Cölle, M.; Gomes, Henrique L.; De Leeuw, Dago M.The electron and hole mobility of nickel-bis(dithiolene) (NiDT) are determined in a metal– insulator–semiconductor (MIS)structure using admittance spectroscopy. The relaxation times found in the admittance spectra are attributed to the diffusion time of carriers to reach the insulator interface and via Einstein’s relation this yields the mobility values. In this way, an electron mobility of 1:9 104 cm2=Vs and a hole mobility of 3:9 106 cm2=Vs were found. It is argued that the low mobility is caused by an amphoteric mid-gap trap level. The activation energy for electrons and holes from these traps is found to be 0.46 eV and 0.40 eV, respectively.
- Education today: 12 + 5 < 4 - lessons of education reforms in Portugal and beyondPublication . Khmelinskii, Igor; Fonseca, Custódia; Stallinga, PeterSince the adoption of the ‘Lei de Bases…’ of 1984, the quality of education in Portugal is declining, undermined by ‘critical, creative and independent thinking’, implemented by neglecting memorization as a learning tool, as supposedly students should understand things without knowing them. As a consequence, vast majority of students can’t retain any abstract knowledge. They prepare from scratch for their tests and forget everything afterwards. The students never acquire essential primary-school skills such as capacity to do mental calculations, hence the title of this report, comparing contemporary school + university education to pre-1984 primary school of 4 years. The quality of education is further degraded by ‘evaluation’ of teachers at school and university, judged by academic success and degree of satisfaction of their students. With the students objectively incapable to learn, understand or remember, the teachers have a dilemma of either letting such students pass without retained knowledge, skills and competences, or else have their own ‘evaluation’ suffer. As the generations change, students who were ‘passed’ become teachers themselves, still with no retained knowledge and thus no moral authority to fail their own students. Thus, the level of requirements monotonously degrades, with the educational fraud perpetuated in the new generations.
- Electrical characterization of CVD diamond-n(+) silicon junctionsPublication . Rodrigues, A. M.; Gomes, Henrique L.; Stallinga, Peter; Pereira, L.; Pereira, E.The electrical characteristics of CVD-diamond/n(+)-Si heterojunction devices are reported. Below 250 K the diodes show an unusual inversion of their rectification properties. This behavior is attributed to an enhanced tunneling component due to interface states, which change their occupation with the applied bias. The temperature dependence of the loss tangent shows two relaxation processes with different activation energies. These processes are likely related with two parallel charge transport mechanisms, one through the diamond grain, and the other through the grain boundary. (C) 2001 Elsevier Science B.V. Ah rights reserved.