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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. Source

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. Source

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. Source

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. Source

Vieira S.M.C.,INESC MN and in | Hsieh G.-W.,University of Cambridge | Unalan H.E.,Middle East Technical University | Dag S.,Lawrence Berkeley National Laboratory | And 2 more authors.
Applied Physics Letters | Year: 2011

The ambient stability of thin-film transistors (TFTs) based on zinc oxide (ZnO) nanowires embedded in poly(3,3″-dialkyl-quarterthiophene) was monitored through time dependence of electrical characteristics over a period of 16 months. The hybrid-based TFT showed an initial hole mobility in the linear regime of 4.2× 10-4cm2/V s. After 16 months storage in ambient conditions (exposed to air, moisture, and light) the mobility decreased to 2.3× 10-5 cm2/V s. Comparatively the organic-based TFT lost total carrier mobility after one month storage making the hybrid-based TFTs more suitable for transistor applications when improved stability combined with structural flexibility are required. © 2011 American Institute of Physics. Source

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