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2013
Nandy, S, Gonçalves G, Pinto JV, Busani T, Figueiredo V, Pereira LÍ, {Paiva Martins} RF, Fortunato E.  2013.  {Current transport mechanism at metal-semiconductor nanoscale interfaces based on ultrahigh density arrays of p-type NiO nano-pillars.}. Nanoscale. 5:11699–709., Number 23 AbstractWebsite

The present work focuses on a qualitative analysis of localised I-V characteristics based on the nanostructure morphology of highly dense arrays of p-type NiO nano-pillars (NiO-NPs). Vertically aligned NiO-NPs have been grown on different substrates by using a glancing angle deposition (GLAD) technique. The preferred orientation of as grown NiO-NPs was controlled by the deposition pressure. The NiO-NPs displayed a polar surface with a microscopic dipole moment along the (111) plane (Tasker's type III). Consequently, the crystal plane dependent surface electron accumulation layer and the lattice disorder at the grain boundary interface showed a non-uniform current distribution throughout the sample surface, demonstrated by a conducting AFM technique (c-AFM). The variation in I-V for different points in a single current distribution grain (CD-grain) has been attributed to the variation of Schottky barrier height (SBH) at the metal-semiconductor (M-S) interface. Furthermore, we observed that the strain produced during the NiO-NPs growth can modulate the SBH. Inbound strain acts as an external field to influence the local electric field at the M-S interface causing a variation in SBH with the NPs orientation. This paper shows that vertical arrays of NiO-NPs are potential candidates for nanoscale devices because they have a great impact on the local current transport mechanism due to its nanostructure morphology.

2014
Branquinho, R, Salgueiro D, Santos L??dia, Barquinha P, Pereira L??s, Martins R, Fortunato E.  2014.  {Aqueous combustion synthesis of aluminum oxide thin films and application as gate dielectric in GZTO solution-based TFTs}. ACS Applied Materials and Interfaces. 6:19592–19599., Number 22 Abstract

Solution processing has been recently considered as an option when trying to reduce the costs associated with deposition under vacuum. In this context, most of the research efforts have been centered in the development of the semiconductors processes nevertheless the development of the most suitable dielectrics for oxide based transistors is as relevant as the semiconductor layer itself. In this work we explore the solution combustion synthesis and report on a completely new and green route for the preparation of amorphous aluminum oxide thin films; introducing water as solvent. Optimized dielectric layers were obtained for a water based precursor solution with 0.1 M concentration and demonstrated high capacitance, 625 nF cm(-2) at 10 kHz, and a permittivity of 7.1. These thin films were successfully applied as gate dielectric in solution processed gallium-zinc-tin oxide (GZTO) thin film transistors (TFTs) yielding good electrical performance such as subthreshold slope of about 0.3 V dec(-1) and mobility above 1.3 cm(2) V(-1) s(-1).

Zanarini, S, Garino N, Nair JR, Francia C, Wojcik PJ, Pereira L, Fortunato E, Martins R, Bodoardo S, Penazzi N.  2014.  {Contrast Enhancement in Polymeric Electrochromic Devices Encompassing Room Temperature Ionic Liquids}. International Journal of Electrochemical Science. 9:1650–1662., Number 4 Abstract
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Nunes, D, Pimentel A, Barquinha P, Carvalho PA, Fortunato E, Martins R.  2014.  {Cu2O polyhedral nanowires produced by microwave irradiation}. JOURNAL OF MATERIALS CHEMISTRY C. 2:6097–6103., Number 30 Abstract
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Pereira, S, Gonçalves A, Correia N, Pinto J, Pereira LÍ, Martins R, Fortunato E.  2014.  {Electrochromic behavior of NiO thin films deposited by e-beam evaporation at room temperature}. Solar Energy Materials and Solar Cells. 120, Part:109–115. AbstractWebsite

In this work we report the role of thickness on electrochromic behavior of nickel oxide (NiO) films deposited by e-beam evaporation at room temperature on ITO-coated glass. The structure and morphology of films with thicknesses between 100 and 500 nm were analyzed and then correlated with electrochemical response and transmittance modulation when immersed in 0.5 M LiClO4–PC electrolyte. The NiO exhibits an anodic coloration, reaching for the thickest film a transmittance modulation of 66{%} between colored and bleached state, at 630 nm, with a color efficiency of 55 cm2 C−1. Very fast switch between states was obtained, where coloration and bleaching times are 3.6 s cm−2 and 1.4 s cm−2, respectively.

Zubizarreta, C, Berasategui EG, Bayon R, Galindo ER, Barros R, Gaspar D, Nunes D, Calmeiro T, Martins R, Fortunato E, Barriga J.  2014.  {Evaluation of the optoelectronic properties and corrosion behavior of Al2O3-doped ZnO films prepared by dc pulsed magnetron sputtering}. JOURNAL OF PHYSICS D-APPLIED PHYSICS. 47, Number 48 Abstract
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Bernacka-Wojcik, I, Ribeiro S, Wojcik PJ, Alves PU, Busani T, Fortunato E, Baptista PV, Covas JA, Águas H, Hilliou L, Martins R.  2014.  {Experimental optimization of a passive planar rhombic micromixer with obstacles for effective mixing in a short channel length}. RSC Advances. 4:56013–56025., Number 99 AbstractWebsite

This paper presents the performance of a passive planar rhombic micromixer with diamond-shaped obstacles and a rectangular contraction between the rhombi. The device was experimentally optimized using water for high mixing efficiency and a low pressure drop over a wide range of Reynolds numbers (Re = 0.1–117.6) by varying geometrical parameters such as the number of rhombi, the distance between obstacles and the contraction width. Due to the large amount of data generated, statistical methods were used to facilitate and improve the results of the analysis. The results revealed a rank of factors influencing mixing efficiency: Reynolds number {\textgreater} number of rhombi {\textgreater} contraction width {\textgreater} inter-obstacles distance. The pressure drop measured after three rhombi depends mainly on Re and inter-obstacle distance. The resulting optimum geometry for the low Re regime has a contraction width of 101 $μ$m and inter-obstacles distance of 93 $μ$m, while for the high Re regime a contraction width of 400 $μ$m and inter-obstacle distance of 121 $μ$m are more appropriate. These mixers enabled 80{%} mixing efficiency creating a pressure drop of 6.0 Pa at Re = 0.1 and 5.1 × 104 Pa at Re = 117.6, with a mixer length of 2.5 mm. To the authors' knowledge, the developed mixer is one of the shortest planar passive micromixers reported to date.

Liu, A, Liu GX, Zhu HH, Xu F, Fortunato E, Martins R, Shan FK.  2014.  {Fully Solution-Processed Low-Voltage Aqueous In2O3 Thin-Film Transistors Using an Ultrathin ZrOx Dielectric}. ACS Applied Materials {&} Interfaces. 6:17364–17369., Number 20 AbstractWebsite

We reported here ?aqueous-route? fabrication of In2O3 thin-film transistors (TFTs) using an ultrathin solution-processed ZrOx dielectric thin film. The formation and properties of In2O3 thin films under various annealing temperatures were intensively examined by thermogravimetric analysis, Fourier transform infrared spectroscopy, and atomic force microscopy. The solution-processed ZrOx thin film followed by sequential UV/ozone treatment and low-temperature thermal-annealing processes showed an amorphous structure, a low leakage-current density (?1 ? 10?9 A/cm2 at 2 MV/cm), and a high breakdown electric field (?7.2 MV/cm). On the basis of its implementation as the gate insulator, the In2O3 TFTs based on ZrOx annealed at 250 °C exhibit an on/off current ratio larger than 107, a field-effect mobility of 23.6 cm2/V·s, a subthreshold swing of 90 mV/decade, a threshold voltage of 0.13 V, and high stability. These promising properties were obtained at a low operating voltage of 1.5 V. These results suggest that ?aqueous-route? In2O3 TFTs based on a solution-processed ZrOx dielectric could potentially be used for low-cost, low-temperature-processing, high-performance, and flexible devices.

Liu, GX, Liu A, Shan FK, Meng Y, Shin BC, Fortunato E, Martins R.  2014.  {High-performance fully amorphous bilayer metal-oxide thin film transistors using ultra-thin solution-processed ZrOx dielectric}. Applied Physics Letters. 105:113509., Number 11 AbstractWebsite

In this study, we report high-performance amorphous In2O3/InZnO bilayer metal-oxide (BMO) thin-film transistor (TFT) using an ultra-thin solution-processed amorphous ZrOx dielectric. A thin layer of In2O3 offers a higher carrier concentration, thereby maximizing the charge accumulation and yielding high carrier mobility. A thick amorphous layer of InZnO controls the charge conductance resulting in low off-state current and suitable threshold voltage. As a consequence, the BMO TFT showed higher filed-effect mobility (37.9 cm2/V s) than single-layer InZnO TFT (7.6 cm2/V s). Apart from that we obtain an on/off current ratio of 109, a subthreshold swing voltage of 120 mV/decade, and a voltage shift ≤ 0.4 V under positive bias stress for 2.5 h, for a gate voltage of 3 V and drain voltage of 1 V. These data demonstrate that the BMO TFT has great potential for a broad range of applications as switching low-power transistors.

Araujo, A, Caro C, Mendes MJ, Nunes D, Fortunato E, Franco R, Aguas H, Martins R.  2014.  {Highly efficient nanoplasmonic SERS on cardboard packaging substrates}. NANOTECHNOLOGY. 25, Number 41 Abstract
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Veigas, B, Branquinho R, Pinto JV, Wojcik PJ, Martins R, Fortunato E, Baptista PV.  2014.  {Ion sensing (EIS) real-time quantitative monitorization of isothermal DNA amplification}. Biosensors and Bioelectronics. 52:50–55. AbstractWebsite

Field-effect-based devices are becoming a basic structural element in a new generation of microbiosensors. Reliable molecular characterization of DNA and/or RNA is of paramount importance for disease diagnostics and to follow up alterations in gene expression profiles. The use of such devices for point-of-need diagnostics has been hindered by the need of standard or real-time PCR amplification procedures. The present work focuses on the development of a tantalum pentoxide (Ta2O5) based sensor for the real-time label free detection of DNA amplification via loop mediated isothermal amplification (LAMP) allowing for quantitative analysis of the cMYC proto-oncogene. The strategy based on the field effect sensor was tested within a range of 1×108–1011 copies of target DNA, and a linear relationship between the log copy number of the initial template DNA and threshold time was observed allowing for a semi-quantitative analysis of DNA template. The concept offers many of the advantages of isothermal quantitative real-time DNA amplification in a label free approach and may pave the way to point-of-care quantitative molecular analysis focused on ease of use and low cost.

Gaspar, D, Fernandes SN, de Oliveira AG, Fernandes JG, Grey P, Pontes RV, Pereira L, Martins R, Godinho MH, Fortunato E.  2014.  {Nanocrystalline cellulose applied simultaneously as the gate dielectric and the substrate in flexible field effect transistors}. Nanotechnology. 25:94008., Number 9 AbstractWebsite

Cotton-based nanocrystalline cellulose (NCC), also known as nanopaper, one of the major sources of renewable materials, is a promising substrate and component for producing low cost fully recyclable flexible paper electronic devices and systems due to its properties (lightweight, stiffness, non-toxicity, transparency, low thermal expansion, gas impermeability and improved mechanical properties). Here, we have demonstrated for the first time a thin transparent nanopaper-based field effect transistor (FET) where NCC is simultaneously used as the substrate and as the gate dielectric layer in an ‘interstrate' structure, since the device is built on both sides of the NCC films; while the active channel layer is based on oxide amorphous semiconductors, the gate electrode is based on a transparent conductive oxide. Such hybrid FETs present excellent operating characteristics such as high channel saturation mobility ({\textgreater}7 cm 2 V −1 s −1 ), drain–source current on/off modulation ratio higher than 10 5 , enhancement n-type operation and subthreshold gate voltage swing of 2.11 V/decade. The NCC film FET characteristics have been measured in air ambient conditions and present good stability, after two weeks of being processed, without any type of encapsulation or passivation layer. The results obtained are comparable to ones produced for conventional cellulose paper, marking this out as a promising approach for attaining high-performance disposable electronics such as paper displays, smart labels, smart packaging, RFID (radio-frequency identification) and point-of-care systems for self-analysis in bioscience applications, among others.

Pimentel, A, Nunes D, Duarte P, Rodrigues J, Costa FM, Monteiro T, Martins R, Fortunato E.  2014.  {Synthesis of Long ZnO Nanorods under Microwave Irradiation or Conventional Heating}. The Journal of Physical Chemistry C. 118:14629–14639., Number 26 AbstractWebsite

The present work reports the synthesis of zinc oxide (ZnO) nanostructures produced either under microwave irradiation using low cost domestic microwave equipment or by conventional heating, both under hydrothermal conditions. X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, room/low temperature photoluminescence, and Raman spectroscopy have been used to investigate the structure, morphology, and optical properties of the produced ZnO nanorods. Identical structures with aspect ratio up to 13 have been achieved for both synthesis routes displaying similar final properties. The hexagonal wurtzite structure has been identified, and a red-orange emission has been detected in the presence of UV irradiation for all the conditions studied. Thermal stability of the as-prepared nanostructures has been evaluated through thermogravimetric measurements revealing an increase of superficial defects. The as-prepared ZnO nanorods were tested as UV sensors on paper substrate, which led to fast response (30 s) and rapid recovery (100 s) times, as well as sensitivity up to 10 indicating that these materials may have a high potential in low cost, disposable UV photodetector applications.

Pereira, L, Gaspar D, Guerin D, a Delattre, Fortunato E, Martins R.  2014.  {The influence of fibril composition and dimension on the performance of paper gated oxide transistors}. Nanotechnology. 25:94007., Number 9 AbstractWebsite

Paper electronics is a topic of great interest due the possibility of having low-cost, disposable and recyclable electronic devices. The final goal is to make paper itself an active part of such devices. In this work we present new approaches in the selection of tailored paper, aiming to use it simultaneously as substrate and dielectric in oxide based paper field effect transistors (FETs). From the work performed, it was observed that the gate leakage current in paper FETs can be reduced using a dense microfiber/nanofiber cellulose paper as the dielectric. Also, the stability of these devices against changes in relative humidity is improved. On other hand, if the pH of the microfiber/nanofiber cellulose pulp is modified by the addition of HCl, the saturation mobility of the devices increases up to 16 cm 2 V −1 s −1 , with an I ON / I OFF ratio close to 10 5 .

Santos, L, Neto JP, Crespo A, Nunes D, Costa N, Fonseca IM, Barquinha P, Pereira LÍ, Silva J, Martins R, Fortunato E.  2014.  {WO3 Nanoparticle-Based Conformable pH Sensor}. ACS Applied Materials {&} Interfaces. 6:12226–12234., Number 15 AbstractWebsite

pH is a vital physiological parameter that can be used for disease diagnosis and treatment as well as in monitoring other biological processes. Metal/metal oxide based pH sensors have several advantages regarding their reliability, miniaturization, and cost-effectiveness, which are critical characteristics for in vivo applications. In this work, WO3 nanoparticles were electrodeposited on flexible substrates over metal electrodes with a sensing area of 1 mm2. These sensors show a sensitivity of ?56.7 ± 1.3 mV/pH, in a wide pH range of 9 to 5. A proof of concept is also demonstrated using a flexible reference electrode in solid electrolyte with a curved surface. A good balance between the performance parameters (sensitivity), the production costs, and simplicity of the sensors was accomplished, as required for wearable biomedical devices.

2015
Ullaha, S, Matteis DF, Branquinho R, Fortunato E, Martins R, Davoli I.  2015.  {A combination of solution synthesis solution combustion synthesis for highly conducting and transparent Aluminum Zinc Oxide thin films}. 2015 IEEE 15th International Conference on Nanotechnology (IEEE-NANO). :144–147. Abstract

Aluminum Zinc Oxide has been extensively investigated as a cheap alternative to transparent conducting tin oxide films for electronic and optoelectronic applications. Thin films of Aluminum Zinc Oxide have been developed successfully through a combination of solution combustion synthesis and solution synthesis. Zn(NO3)3·6H2O as metal source was dissolved in 2-methoxyethanol as solvent through combustion synthesis with Urea as fuel while dopant source of AlCl3·6H2O was mixed separately in solvent to avoid aluminum oxide formation in the films. Precursor solutions were obtained mixing Zn {&} Al separate solutions in 9:1, 8:2, and 7:3 ratios respectively with oxide, fuel and dopant concentrations of 0.5, 0.25, 0.1, and 0.05 M. The film stacks have been prepared through spin-coating with heating at 400°C for 10 minutes after each deposition to remove residuals and evaporate solvents. Thermal annealing in oven at 600°C for 1 hour followed by rapid thermal annealing at 500°C {&} 600°C first in vacuum and then in N2-5{%}H2 environment respectively for 10 minutes each reduced the resistivity of film stacks. Film stack with 10 layers for an average thickness of 0.5$μ$m gave the best Hall Effect resistivity of 3.2 × 10-2 $Ømega$-cm in the case of 0.5M solution with Zn:Al mixing ratio of 9:1 for RTA annealings at 600°C with an average total transparency of 80 {%} in the wavelength range of 400-1200 nm. The results show a clear trend that increasing the amount of ingredients resistivity could further be decreased.

Nandy, S, Thapa R, Kumar M, Som T, Bundaleski N, Teodoro OMND, Martins R, Fortunato E.  2015.  {Efficient Field Emission from Vertically Aligned Cu2O1-delta(111) Nanostructure Influenced by Oxygen Vacancy}. ADVANCED FUNCTIONAL MATERIALS. 25:947–956., Number 6 Abstract
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Fernandes, M, Leones R, Costa AMS, Silva MM, Pereira S, Mano JF, Fortunato E, Rego R, {de Zea Bermudez} V.  2015.  {Electrochromic devices incorporating biohybrid electrolytes doped with a lithium salt, an ionic liquid or a mixture of both}. Electrochimica Acta. 161:226–235.: Elsevier Ltd AbstractWebsite
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Kololuoma, T, Leppäniemi J, Majumdar H, Branquinho R, Herbei-Valcu E, Musat V, Martins R, Fortunato E, Alastalo A.  2015.  {Gravure printed sol–gel derived AlOOH hybrid nanocomposite thin films for printed electronics}. J. Mater. Chem. C. 3:1776–1786., Number 8 AbstractWebsite
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Vicente, A, Águas H, Mateus T, Araújo A, Lyubchyk A, Siitonen S, Fortunato E, Martins R.  2015.  {Solar cells for self-sustainable intelligent packaging}. J. Mater. Chem. A. 3:13226–13236., Number 25 AbstractWebsite
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Wojcik, PJ, Santos L, Pereira L, Martins R, Fortunato E.  2015.  {Tailoring nanoscale properties of tungsten oxide for inkjet printed electrochromic devices}. Nanoscale. 7:1696–1708., Number 5: Royal Society of Chemistry AbstractWebsite

This paper focuses on the engineering procedures governing the synthesis of tungsten oxide nanocrystals and the formulation of printable dispersions for electrochromic applications. By that means, we aim to stress the relevancy of a proper design strategy that results in improved physicochemical properties of nanoparticle loaded inks. In the present study inkjet printable nanostructured tungsten oxide particles were successfully synthesized via hydrothermal processes using pure or acidified aqueous sol-gel precursors. Based on the proposed scheme, the structure and morphology of the nanoparticles were tailored to ensure the desired printability and electrochromic performance. The developed nanomaterials with specified structures effectively improved the electrochemical response of printed films, resulting in 2.5 times higher optical modulation and 2 times faster coloration time when compared with pure amorphous films.

Branquinho, R, Salgueiro D, Santa A, Kiazadeh A, Barquinha P, Pereira L, Martins R, Fortunato E.  2015.  {Towards environmental friendly solution-based ZTO/AlOx TFTs}. SEMICONDUCTOR SCIENCE AND TECHNOLOGY. 30, Number 2, SI Abstract
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Nunes, D, Santos L, Duarte P, Pimentel A, Pinto JV, Barquinha P, Carvalho PA, Fortunato E, Martins R.  2015.  {Room temperature synthesis of Cu₂O nanospheres: optical properties and thermal behavior.}, feb. Microscopy and microanalysis : the official journal of Microscopy Society of America, Microbeam Analysis Society, Microscopical Society of Canada. 21:108–19., Number 1 AbstractWebsite

The present work reports a simple and easy wet chemistry synthesis of cuprous oxide (Cu2O) nanospheres at room temperature without surfactants and using different precursors. Structural characterization was carried out by X-ray diffraction, transmission electron microscopy, and scanning electron microscopy coupled with focused ion beam and energy-dispersive X-ray spectroscopy. The optical band gaps were determined from diffuse reflectance spectroscopy. The photoluminescence behavior of the as-synthesized nanospheres showed significant differences depending on the precursors used. The Cu2O nanospheres were constituted by aggregates of nanocrystals, in which an on/off emission behavior of each individual nanocrystal was identified during transmission electron microscopy observations. The thermal behavior of the Cu2O nanospheres was investigated with in situ X-ray diffraction and differential scanning calorimetry experiments. Remarkable structural differences were observed for the nanospheres annealed in air, which turned into hollow spherical structures surrounded by outsized nanocrystals.

Santos, L, Wojcik P, Pinto JV, Elangovan E, Viegas J, Pereira LÍ, Martins R, Fortunato E.  2015.  {Structure and Morphologic Influence of WO 3 Nanoparticles on the Electrochromic Performance of Dual-Phase a -WO 3 /WO 3 Inkjet Printed Films}, feb. Advanced Electronic Materials. 1:n/a–n/a., Number 1-2 AbstractWebsite
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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.