Lisbon, Portugal
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Leitao D.C.,INESC MN and in | Ventura J.,University of Porto | Sousa C.T.,University of Porto | Teixeira J.M.,University of Porto | And 6 more authors.
Nanotechnology | Year: 2012

The introduction of voids in a magnetic thin-film alters the stray field distribution and enables the tailoring of the corresponding physical properties. Here we present a detailed study on thin magnetic nanohole arrays (NhAs) grown on top of hexagonally-ordered anodic aluminum oxide (AAO) substrates. We address the effect of AAO topography on the corresponding electrical and magneto-transport properties. Optimization of the AAO topography led to NhAs with improved resistance and magnetoresistance responses, while retaining their most important feature of enhanced coercivity. This opens new pathways for the growth of more complex structures on AAO substrates, a crucial aspect for their technological viability. © 2012 IOP Publishing Ltd.


Leitao D.C.,INESC MN and in | Gameiro L.,INESC MN and in | Gameiro L.,University of Lisbon | Silva A.V.,INESC MN and in | And 4 more authors.
IEEE Transactions on Magnetics | Year: 2012

This work compares the performance of spin valve sensors comprising magnetic flux concentrators (MFCs) composed of Co 93Zr 3Nb 4 (CZN) or (Co 70Fe 30) 80B 20-based synthetic-antiferromagnet (SAF) multilayer stacks. In addition, the influence of a tapered MFC tip is also studied. When compared to CZN films, SAFs have the disadvantage of lower magnetic susceptibility (\chi =196 for SAF vs. \chi =753 for CZN), which affects negatively the field gain in gap (10 for SAF vs. 43 for CZN). However, from the overall noise spectrum, one can conclude that the magnetic field detections for sensors incorporating CoFeB/Ru multilayers as MFCs are close to the ones obtained with CZN, being mainly determined by the sensor intrinsic properties instead. For low frequencies, field detection levels at 10 Hz improved from ∼61 nT/Hz 0.5 for single spin valve sensors down to ∼1.8 nT/Hz 0.5 when CZN concentrators with a steep-profile are used. © 1965-2012 IEEE.


Sousa C.T.,University of Porto | Leitao D.C.,INESC MN and in | Leitao D.C.,University of Lisbon | Proenca M.P.,University of Porto | And 3 more authors.
Applied Physics Reviews | Year: 2014

Due to its manufacturing and size tailoring ease, porous anodic alumina (PAA) templates are an elegant physical-chemical nanopatterning approach and an emergent alternative to more sophisticated and expensive methods currently used in nanofabrication. In this review, we will describe the ground work on the fabrication methods of PAA membranes and PAA-based nanostructures. We will present the specificities of the electrochemical growth processes of multifunctional nanomaterials with diversified shapes (e.g., nanowires and nanotubes), and the fabrication techniques used to grow ordered nanohole arrays. We will then focus on the fabrication, properties and applications of magnetic nanostructures grown on PAA and illustrate their dependence on internal (diameter, interpore distance, length, composition) and external (temperature and applied magnetic field intensity and direction) parameters. Finally, the most outstanding experimental findings on PAA-grown nanostructures and their trends for technological applications (sensors, energy harvesting, metamaterials, and biotechnology) will be addressed. © 2014 AIP Publishing LLC.


Pinto R.M.,INESC MN and in | Pinto R.M.,University of Lisbon
Journal of Physical Chemistry C | Year: 2014

The primary kinetic processes leading to photocurrent generation in rubrene/7,7,8,8-tetracyanoquinodimethane (rubene/TCNQ) heterojunctions are investigated using a combination of quantum-chemical methods, Marcus nonadiabatic electron-transfer theory, and Onsager-Braun model for charge separation. Charge-transfer (CT), -recombination (CR), and -separation (CS) rates are obtained for heterodimers representative of two device models: single-crystal planar bilayer, in which crystal orientation is preserved and rubrene's fused π-system is sterically hindered, and bulk-heterojunctions (BHJs), where donor and acceptor molecules approach cofacially with the π-system fully exposed. Results point to low geminate pair recombination due to higher donor-acceptor separation in crystalline bilayers, while maintaining ultrafast CT (∼109 s-1). Moreover, HOMO-LUMO coupling is an order of magnitude higher in cofacial orientation, leveraging CR in BHJs for which kCR ∼ 106 s-1 and kCT ∼ 109 s-1. This work provides a molecular perspective rationale for the high photoresponse reported for rubrene/TCNQ single-crystal bilayer interfaces. © 2014 American Chemical Society.


Pinto R.M.,INESC MN and IN | Pinto R.M.,University of Lisbon | Macoas E.M.S.,University of Lisbon | Neves A.I.S.,INESC MN and IN | And 5 more authors.
Journal of the American Chemical Society | Year: 2015

Exciton diffusion is at the heart of most organic optoelectronic devices' operation, and it is currently the most limiting factor to their achieving high efficiency. It is deeply related to molecular organization, as it depends on intermolecular distances and orbital overlap. However, there is no clear guideline for how to improve exciton diffusion with regard to molecular design and structure. Here, we use single-crystal charge-transfer interfaces to probe favorable exciton diffusion. Photoresponse measurements on interfaces between perylenediimides and rubrene show a higher photocurrent yield (+50%) and extended spectral coverage (+100 nm) when there is increased dimensionality of the percolation network and stronger orbital overlap. This is achieved by very short interstack distances in different directional axes, which favors exciton diffusion by a Dexter mechanism. Even if the core of the molecule shows strong deviation from planarity, the similar electrical resistance of the different systems, planar and nonplanar, shows that electronic transport is not compromised. These results highlight the impact of molecular organization in device performance and the necessity of optimizing it to take full advantage of the materials' properties. © 2015 American Chemical Society.


PubMed | INESC MN and IN and University of Lisbon
Type: Journal Article | Journal: Journal of the American Chemical Society | Year: 2015

Exciton diffusion is at the heart of most organic optoelectronic devices operation, and it is currently the most limiting factor to their achieving high efficiency. It is deeply related to molecular organization, as it depends on intermolecular distances and orbital overlap. However, there is no clear guideline for how to improve exciton diffusion with regard to molecular design and structure. Here, we use single-crystal charge-transfer interfaces to probe favorable exciton diffusion. Photoresponse measurements on interfaces between perylenediimides and rubrene show a higher photocurrent yield (+50%) and extended spectral coverage (+100 nm) when there is increased dimensionality of the percolation network and stronger orbital overlap. This is achieved by very short interstack distances in different directional axes, which favors exciton diffusion by a Dexter mechanism. Even if the core of the molecule shows strong deviation from planarity, the similar electrical resistance of the different systems, planar and nonplanar, shows that electronic transport is not compromised. These results highlight the impact of molecular organization in device performance and the necessity of optimizing it to take full advantage of the materials properties.


PubMed | INESC MN and IN and University of Lisbon
Type: Journal Article | Journal: ACS applied materials & interfaces | Year: 2015

The efficiency of organic photodetectors and optoelectronic devices is strongly limited by exciton diffusion, in particular for acceptor materials. Although mechanisms for exciton diffusion are well established, their correlation to molecular organization in real systems has received far less attention. In this report, organic single-crystals interfaces were probed with wavelength-dependent photocurrent spectroscopy and their crystal structure resolved using X-ray diffraction. All systems present a dynamic photoresponse, faster than 500 ms, up to 650 nm. A relationship between molecular organization and favorable exciton diffusion in substituted butyl-perylenediimides (PDIB) is established. This is demonstrated by a set of PDIBs with different intra- and interstack distances and short contacts and their impact on photoresponse. Given the short packing distances between PDIs cores along the same stacking direction (3.4-3.7 ), and across parallel stacks (2.5 ), singlet exciton in these PDIBs can follow both Frster and Dexter exciton diffusion, with the Dexter-type mechanism assuming special relevance for interstack exciton diffusion. Yet, the response is maximized in substituted PDIBs, where a 2D percolation network is formed through strong interstack contacts, allowing for PDIBs primary excitons to reach with great efficiency the splitting interface with crystalline rubrene. The importance of short contacts and molecular distances, which is often overlooked as a parameter to consider and optimize when choosing materials for excitonic devices, is emphasized.


Nakano M.,University of Geneva | Alves H.,INESC MN and in | Molinari A.S.,Technical University of Delft | Ono S.,University of Geneva | And 3 more authors.
Applied Physics Letters | Year: 2010

We investigated transport properties of organic heterointerfaces formed by single-crystals of two organic donor-acceptor molecules, tetramethyltetraselenafulvalene and 7,7,8,8-tetracyanoquinodimethane (TCNQ). Whereas the individual crystals have unmeasurably high resistance, the interface exhibits a resistivity of few tens of megohm with a temperature dependence characteristic of a small gap semiconductor. We analyze the transport properties based on a simple band diagram that naturally accounts for our observations in terms of charge transfer between two crystals. Together with the recently discovered tetrathiafulvalene-TCNQ interfaces, these results indicate that single-crystal organic heterostructures create functional electronic systems with properties relevant to both fundamental and applied fields. © 2010 American Institute of Physics.


Apolinario A.,University of Porto | Sousa C.T.,University of Porto | Ventura J.,University of Porto | Costa J.D.,University of Porto | And 6 more authors.
Journal of Materials Chemistry A | Year: 2014

Highly ordered TiO2 nanotubes (NTs) were synthesized by electrochemical anodization of Ti foils. We investigated the effect of the Ti surface roughness (applying different pre-treatments prior to the anodization) on the length, growth rate and degree of self-organization of the obtained NT arrays. The mechanisms related to the TiO2 NT formation and growth were correlated not only with the corresponding anodization curves but also with their appropriate derivatives (1st order) and suitable integrated and/or obtained parameters, to reveal the onset and end of different electrochemical regimes. This enables an in-depth interpretation (and physical-chemical insight), for different levels of surface roughness and topographic features. We found that pre-treatments lead to an extremely small Ti surface roughness, offer an enhanced NT length and also provide a significant improvement in the template organization quality (highly ordered hexagonal NT arrays over larger areas), due to the optimized surface topography. We present a new statistical approach for evaluating highly ordered hexagonal NT array areas. Large domains with ideally arranged nanotube structures represented by a hexagonal closely packed array were obtained (6.61 μm2), close to the smallest grain diameter of the Ti foil and three times larger than those so far reported in the literature. The use of optimized pre-treatments then allowed avoiding a second anodization step, ultimately leading to highly hexagonal self-ordered samples with large organized domains at reduced time and cost. © 2014 the Partner Organisations.


PubMed | Singulus Technologies AG, INESC MN and IN and AGH University of Science and Technology
Type: Journal Article | Journal: Sensors (Basel, Switzerland) | Year: 2016

As single tunneling magnetoresistive (TMR) sensor performance in modern high-speed applications is limited by breakdown voltage and saturation of the sensitivity, for higher voltage applications (i.e., compatible to 1.8 V, 3.3 V or 5 V standards) practically only a series connection can be applied. Thus, in this study we focused on sensitivity, 3 dB bandwidth and sensitivity-bandwidth product (SBP) dependence on the DC bias voltage in single and series-connected TMR sensors. We show that, below breakdown voltage, the strong bias influence on sensitivity and the 3 dB frequency of a single sensor results in higher SBP than in a series connection. However, the sensitivity saturation limits the single sensor SBP which, under 1 V, reaches the same level of 2000 MHzV/T as in a series connection. Above the single sensor breakdown voltage, linear sensitivity dependence on the bias and the constant 3 dB bandwidth of the series connection enable increasing its SBP up to nearly 10,000 MHzV/T under 5 V. Thus, although by tuning bias voltage it is possible to control the sensitivity-bandwidth product, the choice between the single TMR sensor and the series connection is crucial for the optimal performance in the high frequency range.

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