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Rodrigues, J, Cerqueira AFR, Sousa MG, Santos NF, Pimentel A, Fortunato E, da Cunha AF, Monteiro T, Costa FM.  Submitted.  {Exploring the potential of laser assisted flow deposition grown ZnO for photovoltaic applications}. Materials Chemistry and Physics. AbstractWebsite

Zinc oxide (ZnO) is a widely studied wide band gap semiconductor with applications in several fields, namely to enhance solar cells efficiency. Its ability to be grown in a wide variety of nanostructured morphologies, allowing the designing of the surface area architecture constitutes an important advantage over other semiconductors. Laser assisted flow deposition (LAFD) is a recently developed growth method, based on a vapour-solid mechanism, which proved to be a powerful approach in the production of ZnO micro/nanostructures with different morphologies as well as high crystallinity and optical quality. In the present work we report the use of the LAFD technique to grow functional ZnO nanostructures (nanoparticles and tetrapods) working as nano templates to improve the dye-sensitized solar cells (DSSCs) efficiency. The structural and morphological characterization of the as-grown ZnO crystals were performed by X-ray diffraction and electron microscopy, respectively, and the optical quality was assessed by photoluminescence spectroscopy. DSSCs were produced using a combination of these nanostructures, which were subsequently sensitized with N719 dye. An efficiency of ∼3{%} was achieved under simulated AM 1.5 illumination conditions for a dye loading time of 1 h.

Nunes, D, Calmeiro TRR, Nandy S, Pinto JVV, Pimentel A, Barquinha P, Carvalho PAA, Walmsley JCC, Fortunato E, Martins R.  2016.  {Charging effects and surface potential variations of Cu-based nanowires}, nov. Thin Solid Films. 601:45–53. AbstractWebsite

The present work reports charging effects and surface potential variations in pure copper, cuprous oxide and cupric oxide nanowires observed by electrostatic force microscopy (EFM) and Kelvin probe force microscopy (KPFM). The copper nanowires were produced by wet synthesis, oxidation into cuprous oxide nanowires was achieved through microwave irradiation and cupric oxide nanowires were obtained via furnace annealing in atmospheric conditions. Structural characterization of the nanowires was carried out by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and energy dispersive X-ray spectroscopy. During the EFM experiments the electrostatic field of the positive probe charged negatively the Cu-based nanowires, which in turn polarized the SiO2 dielectric substrate. Both the probe/nanowire capacitance as well as the substrate polarization increased with the applied bias. Cu2O and CuO nanowires behaved distinctively during the EFM measurements in accordance with their band gap energies. The work functions (WF) of the Cu-based nanowires, obtained by KPFM measurements, yielded WFCuO {\textgreater} WFCu {\textgreater} WFCu2O.

Deuermeier, J, Wardenga HF, Morasch J, Siol S, Nandy S, Calmeiro T, Martins R, Klein A, Fortunato E.  2016.  {Highly conductive grain boundaries in copper oxide thin films}, jun. JOURNAL OF APPLIED PHYSICS. 119, Number 23 Abstract
Bahubalindruni, PG, Kiazadeh A, Sacchetti A, Martins J, Rovisco A, Tavares VG, Martins R, Fortunato E, Barquinha P.  2016.  {Influence of Channel Length Scaling on InGaZnO TFTs Characteristics: Unity Current-Gain Cutoff Frequency, Intrinsic Voltage-Gain, and On-Resistance}, jun. JOURNAL OF DISPLAY TECHNOLOGY. 12:515–518., Number 6 Abstract
Cramer, T, Sacchetti A, Lobato MT, Barquinha P, Fischer V, Benwadih M, Bablet J, Fortunato E, Martins R, Fraboni B.  2016.  {Radiation-Tolerant Flexible Large-Area Electronics Based on Oxide Semiconductors}, jul. ADVANCED ELECTRONIC MATERIALS. 2, Number 7 Abstract
Lyubchyk, A, Vicente A, Soule B, Alves PU, Mateus T, Mendes MJ, Águas H, Fortunato E, Martins R.  2016.  {Mapping the Electrical Properties of ZnO-Based Transparent Conductive Oxides Grown at Room Temperature and Improved by Controlled Postdeposition Annealing}, jan. Advanced Electronic Materials. 2:n/a–n/a., Number 1 AbstractWebsite
Santos, L, Silveira CM, Elangovan E, Neto JP, Nunes D, Pereira LÍ, Martins R, Viegas J, Moura JJG, Todorovic S, Almeida GM, Fortunato E.  2016.  {Synthesis of WO3 nanoparticles for biosensing applications}, feb. Sensors and Actuators B: Chemical. 223:186–194. AbstractWebsite
Kiazadeh, A, Gomes HL, Barquinha P, Martins J, Rovisco A, Pinto JV, Martins R, Fortunato E.  2016.  {Improving positive and negative bias illumination stress stability in parylene passivated IGZO transistors}. APPLIED PHYSICS LETTERS. 109, Number 5 Abstract
Bahubalindrun, P, Tavares V, Barquinha P, de Oliveira PG, Martins R, Fortunato E.  2016.  {InGaZnO TFT behavioral model for IC design}. Analog Integrated Circuits and Signal Processing. 87:73–80., Number 1 AbstractWebsite
Bahubalindruni, P, Tavares V, Borme J, Barquinha P, Oliveira P, Fortunato E, Martins R.  2016.  {InGaZnO Thin Film Transistor Based Four-Quadrant High-Gain Analog Multiplier on Glass}. IEEE Electron Device Letters. :1–1. AbstractWebsite
Pimentel, A, Ferreira S, Nunes D, Calmeiro T, Martins R, Fortunato E.  2016.  {Microwave Synthesized ZnO Nanorod Arrays for UV Sensors: A Seed Layer Annealing Temperature Study}. Materials. 9:299., Number 4 AbstractWebsite
Goswami, S, Nandy S, Calmeiro TR, Igreja R, Martins R, Fortunato E.  2016.  {Stress Induced Mechano-electrical Writing-Reading of Polymer Film Powered by Contact Electrification Mechanism}. Scientific Reports. 6:19514. AbstractWebsite
Deuermeier, J, Bayer TJM, Yanagi H, Kiazadeh A, Martins R, Klein A, Fortunato E.  2016.  {Substrate reactivity as the origin of Fermi level pinning at the Cu2O/ALD-Al2O3 interface}. MATERIALS RESEARCH EXPRESS. 3, Number 4 Abstract
Besleaga, C, Stan GE, Pintilie I, Barquinha P, Fortunato E, Martins R.  2016.  {Transparent field-effect transistors based on AlN-gate dielectric and IGZO-channel semiconductor}. Applied Surface Science. 379:270–276. AbstractWebsite

The degradation of thin-film transistors (TFTs) caused by the self-heating effect constitutes a problem to be solved for the next generation of displays. Aluminum nitride (AlN) is a viable alternative for gate dielectric of TFTs due to its good thermal conductivity, matching coefficient of thermal expansion to indium–gallium–zinc-oxide, and excellent stability at high temperatures. Here, AlN thin films of different thicknesses were fabricated by a low temperature reactive radio-frequency magnetron sputtering process, using a low cost, metallic Al target. Their electrical properties have been thoroughly assessed. Furthermore, the 200 nm and 500 nm thick AlN layers have been integrated as gate-dielectric in transparent TFTs with indium–gallium–zinc-oxide as channel semiconductor. Our study emphasizes the potential of AlN thin films for transparent electronics, whilst the functionality of the fabricated field-effect transistors is explored and discussed.

Barquinha, P, Pereira S, Pereira LÍ, Wojcik P, Grey P, Martins R, Fortunato E.  2015.  {Flexible and Transparent WO 3 Transistor with Electrical and Optical Modulation}, may. Advanced Electronic Materials. 1:n/a–n/a., Number 5 AbstractWebsite
Rodrigues, J, Mata D, Pimentel A, Nunes D, Martins R, Fortunato E, Neves AJ, Monteiro T, Costa FM.  2015.  {One-step synthesis of ZnO decorated CNT buckypaper composites and their optical and electrical properties}, may. Materials Science and Engineering: B. 195:38–44. AbstractWebsite
Kiazadeh, A, Salgueiro D, Branquinho R, Pinto J, Gomes HL, Barquinha P, Martins R, Fortunato E.  2015.  {Operational stability of solution based zinc tin oxide/SiO2 thin film transistors under gate bias stress}, jun. APL Materials. 3:062804., Number 6 AbstractWebsite

In this study, we report solution-processed amorphous zinc tin oxide transistors exhibiting high operational stability under positive gate bias stress, translated by a recoverable threshold voltage shift of about 20{%} of total applied stress voltage. Under vacuum condition, the threshold voltage shift saturates showing that the gate-bias stress is limited by trap exhaustion or balance between trap filling and emptying mechanism. In ambient atmosphere, the threshold voltage shift no longer saturates, stability is degraded and the recovering process is impeded. We suggest that the trapping time during the stress and detrapping time in recovering are affected by oxygen adsorption/desorption processes. The time constants extracted from stretched exponential fitting curves are ≈106 s and 105 s in vacuum and air, respectively.

Bernacka-Wojcik, I, Aguas H, Carlos FF, Lopes P, Wojcik PJ, Costa MN, Veigas B, Igreja R, Fortunato E, Baptista PV, Martins R.  2015.  {Single Nucleotide Polymorphism Detection Using Gold Nanoprobes and Bio-Microfluidic Platform With Embedded Micro lenses}, jun. BIOTECHNOLOGY AND BIOENGINEERING. 112:1210–1219., Number 6 Abstract
G-Berasategui, E, Zubizarreta C, Bayón R, Barriga J, Barros R, Martins R, Fortunato E.  2015.  {Study of the optical, electrical and corrosion resistance properties of AZO layers deposited by DC pulsed magnetron sputtering}, jun. Surface and Coatings Technology. 271:141–147. AbstractWebsite

Aluminium-doped zinc oxide (AZO) is a common material used as a front contact layer on chalcopyrite CuInGaSe2 (CIGS)-based thin-film solar cells since it combines optimum optical and electrical properties with low cost and abundant elemental availability. Low-resistivity and high-transmission front contacts are required to develop high-performance CIGS solar cells. However, the durability of the cells is highly influenced by the corrosion resistance behaviour of the AZO layers. In this work, an exhaustive study of the aluminium-doped zinc oxide layers (AZO) deposited by pulsed DC magnetron sputtering (MS) has been performed. The optical, electrical and electrochemical corrosion resistance properties of the AZO layers have been evaluated as a function of the deposition pressure. The results show that adjusting the deposition pressure could develop AZO layers with very high electrochemical corrosion resistance in chlorinated aqueous media combined with optimum electrical and optical properties. Layers grown at 3×10−3mbar pressure present very high corrosion resistance values (in the order of 106 {\$}Ømega{\$}) and very high electrochemical stability, indicating no tendency for electrochemical corrosion degradation. Besides, these layers are highly transparent with an average transmittance in the visible range above 90{%} and with a low resistivity of 6.8×10−4 {\$}Ømega{\$}cm for a 1000nm films thickness, making them optimum candidate front contact for high-performance and high durability CIGS solar cells.

Aguas, H, Mateus T, Vicente A, Gaspar D, Mendes MJ, Schmidt WA, Pereira L, Fortunato E, Martins R.  2015.  {Thin Film Silicon Photovoltaic Cells on Paper for Flexible Indoor Applications}, jun. ADVANCED FUNCTIONAL MATERIALS. 25:3592–3598., Number 23 Abstract
Veigas, B, Fortunato E, Baptista PV.  2015.  {Field Effect Sensors for Nucleic Acid Detection: Recent Advances and Future Perspectives}, jan. Sensors. 15:10380–10398., Number 5: Multidisciplinary Digital Publishing Institute AbstractWebsite

In the last decade the use of field-effect-based devices has become a basic structural element in a new generation of biosensors that allow label-free DNA analysis. In particular, ion sensitive field effect transistors (FET) are the basis for the development of radical new approaches for the specific detection and characterization of DNA due to FETs' greater signal-to-noise ratio, fast measurement capabilities, and possibility to be included in portable instrumentation. Reliable molecular characterization of DNA and/or RNA is vital for disease diagnostics and to follow up alterations in gene expression profiles. FET biosensors may become a relevant tool for molecular diagnostics and at point-of-care. The development of these devices and strategies should be carefully designed, as biomolecular recognition and detection events must occur within the Debye length. This limitation is sometimes considered to be fundamental for FET devices and considerable efforts have been made to develop better architectures. Herein we review the use of field effect sensors for nucleic acid detection strategies—from production and functionalization to integration in molecular diagnostics platforms, with special focus on those that have made their way into the diagnostics lab.

Marques, AC, Santos L, Costa MN, Dantas JM, Duarte P, Gonçalves A, Martins R, Salgueiro CA, Fortunato E.  2015.  {Office paper platform for bioelectrochromic detection of electrochemically active bacteria using tungsten trioxide nanoprobes.}, jan. Scientific reports. 5:9910. AbstractWebsite

Electrochemically active bacteria (EAB) have the capability to transfer electrons to cell exterior, a feature that is currently explored for important applications in bioremediation and biotechnology fields. However, the number of isolated and characterized EAB species is still very limited regarding their abundance in nature. Colorimetric detection has emerged recently as an attractive mean for fast identification and characterization of analytes based on the use of electrochromic materials. In this work, WO3 nanoparticles were synthesized by microwave assisted hydrothermal synthesis and used to impregnate non-treated regular office paper substrates. This allowed the production of a paper-based colorimetric sensor able to detect EAB in a simple, rapid, reliable, inexpensive and eco-friendly method. The developed platform was then tested with Geobacter sulfurreducens, as a proof of concept. G. sulfurreducens cells were detected at latent phase with an RGB ratio of 1.10 ± 0.04, and a response time of two hours.

Santos, L, Nunes D, Calmeiro T, Branquinho R, Salgueiro D, Barquinha P, Pereira LÍ, Martins R, Fortunato E.  2015.  {Solvothermal synthesis of gallium-indium-zinc-oxide nanoparticles for electrolyte-gated transistors.}, jan. ACS applied materials {&} interfaces. 7:638–46., Number 1 AbstractWebsite

Solution-processed field-effect transistors are strategic building blocks when considering low-cost sustainable flexible electronics. Nevertheless, some challenges (e.g., processing temperature, reliability, reproducibility in large areas, and cost effectiveness) are requirements that must be surpassed in order to achieve high-performance transistors. The present work reports electrolyte-gated transistors using as channel layer gallium-indium-zinc-oxide nanoparticles produced by solvothermal synthesis combined with a solid-state electrolyte based on aqueous dispersions of vinyl acetate stabilized with cellulose derivatives, acrylic acid ester in styrene and lithium perchlorate. The devices fabricated using this approach display a ION/IOFF up to 1 × 10(6), threshold voltage (VTh) of 0.3-1.9 V, and mobility up to 1 cm(2)/(V s), as a function of gallium-indium-zinc-oxide ink formulation and two different annealing temperatures. These results validates the usage of electrolyte-gated transistors as a viable and promising alternative for nanoparticle based semiconductor devices as the electrolyte improves the interface and promotes a more efficient step coverage of the channel layer, reducing the operating voltage when compared with conventional dielectrics gating. Moreover, it is shown that by controlling the applied gate potential, the operation mechanism of the electrolyte-gated transistors can be modified from electric double layer to electrochemical doping.

Pavan, M, Rühle S, Ginsburg A, Keller DA, Barad H-N, Sberna PM, Nunes D, Martins R, Anderson AY, Zaban A, Fortunato E.  2015.  {TiO2/Cu2O all-oxide heterojunction solar cells produced by spray pyrolysis}, jan. Solar Energy Materials and Solar Cells. 132:549–556. AbstractWebsite

Here we present for the first time a TiO2/Cu2O all-oxide heterojunction solar cell entirely produced by spray pyrolysis onto fluorine doped tin oxide (FTO) covered glass substrates, using silver as a back contact. A combinatorial approach was chosen to investigate the impact of the TiO2 window layer and the Cu2O light absorber thicknesses. We observe an open circuit voltage up to 350mV and a short circuit current density which is strongly dependent of the Cu2O thickness, reaching a maximum of {\~{}}0.4mA/cm2. Optical investigation reveals that a thickness of 300nm spray pyrolysis deposited Cu2O is sufficient to absorb most photons with an energy above the symmetry allowed optical transition of 2.5eV, indicating that the low current densities are caused by strong recombination in the absorber that consists of small Cu2O grains.