Hugo Gonçalves

Studying the Wtb vertex structure using recent LHC results

This work was partially supported by Fundacao para a Ciencia e Tecnologia, FCT (Project No. CERN/FP/123619/2011, Grant No. SFRH/BI/52524/2014 and Contract No. IF/00050/2013). The work of M. C. N. Fiolhais was supported by LIP-Laboratorio de Instrumentacao e Fisica Experimental de Particulas, Portugal (Grant No. PestIC/FIS/LA007/2013). The authors would like to thank the support of CRUP (Conselho de Reitores das Universidades Portuguesas) through Accao integrada Ref. E 2/09 and the MAP-Fis Program (the Joint Doctoral Programs of the Universities of Minho, Aveiro and Porto, http://www.map.edu.pt/fis/home). Special thanks go to Juan Antonio Aguilar-Saavedra for all the fruitful discussions and a long-term collaboration.

Enhancing visibility of graphene on arbitrary substrates by microdroplet condensation

In order to take advantage of the enormous potential of graphene for future electronic microcircuits and other applications it is necessary to develop reliable, rapid, and widely applicable methods to visualize graphene-based structures. We report here on a microdroplet condensation technique, which allows for quick visual identification of graphene on a variety of substrates, including some which were previously considered unsuitable for the visualization of carbon layers. The technique should also be applicable to visualize artificially patterned graphene structures which are expected to be key technologically enabling components in electronic microcircuits and other applications.

Enhancement of graphene visibility on transparent substrates by refractive index optimization.

Optical reflection microscopy is one of the main imaging tools to visualize graphene microstructures. Here is reported a novel method that employs refractive index optimization in an optical reflection microscope, which greatly improves the visibility of graphene flakes. To this end, an immersion liquid with a refractive index that is close to that of the glass support is used in-between the microscope lens and the support improving the contrast and resolution of the sample image. Results show that the contrast of single and few layer graphene crystals and structures can be enhanced by a factor of 4 compared to values commonly achieved with transparent substrates using optical reflection microscopy lacking refractive index optimization.

Intense optical second harmonic generation from centrosymmetric nanocrystalline para-nitroaniline

We demonstrate an approach that uses normally centrosymmetric, but highly polarizable organic molecules of para-nitroaniline (p-NA) as very efficient second harmonic generator (SHG) for near infra-red light. The approach is based on an effective manipulation of the nanocrystalline size and their assembly into a highly orientated mesocrystalline structures. The resulting mesocrystalline form of p-NA consists of multiple nanocrystals with common orientation, providing a set of surfaces with highly aligned molecules forming a head-to-tail polar arrangement. The second harmonic generation efficiency from this arrangement of mesocrystalline nanostructures is comparable to that from the conventional non-centrosymmetric organic crystals. The experimental results indicate that a very strong second-order nonlinear response can be obtained from small D-π-A organic molecules with elevated molecular hyperpolarizabilities even if these molecules tend to crystallize in centrosymmetric structures.

Fluorescent phenanthroimidazoles functionalized with heterocyclic spacers: synthesis, optical chemosensory ability and two-photon absorption (TPA) properties

Three series of fluorescent phenanthroimidazoles bearing heterocyclic spacers were synthesized in moderate to excellent yields and evaluated as optical chemosensors for ions as well as two-photon absorbing chromophores. Interaction of compounds 5–7 with anions and cations in acetonitrile and acetonitrile/H2O (95 : 5) showed them to be selective receptors for several anions (AcO−, CN− and F−) and cations (Fe3+, Cu2+ and Pd2+), with compound 7a being the most sensitive receptor for Fe3+ and Cu2+. On the other hand, compounds 5a, 7b and 7c were the most sensitive receptors for AcO−, CN− and F−. The binding stoichiometry between the receptors and the anions and cations was found to be 1 : 2 (ligand to anion/metal cation). The binding process was also followed by 1H NMR titrations. The evaluation of the TPA properties of chosen phenanthroimidazoles 7a–c by the two-photon induced fluorescence method revealed that compound 7c, which contains a bithienyl spacer, exhibited the highest TPA cross-section (σ2) value.

Long range energy transfer in graphene hybrid structures

We thank Nuno Peres and Bruno Amorim for informative discussions. This work was supported by FEDER through the COMPETE Program and by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Project PEST-C/FIS/UI607/2011 and under grant PTDC/FIS/101434/2008. TS has been supported by Spain’s MINECO under Grant No. FIS2014-57432-P and by the Comunidad de Madrid under Grant No. S2013/MIT-3007 MAD2D-CM.

New optical techniques to improve the visibility of graphene on multiple substrates

Optical microscopy as a means to identify graphene is hampered by the low absorptivity of its monolayers and few-layer structures. However for many of the upcoming applications for graphene, it is essential to develop techniques to readily deliver images of graphene based structures. We report on two novel techniques and additionally on a well-known, but modified technique for the identification of graphene. All of the described methods employ standard optical reflection and transmission microsocopy and can be readily adapted in most laboratories. One of the novel techniques is based on the enhancement of the optical contrast by refractive index matching using oil immersion microscopy. The second technique, microdroplet condensation, exploits the hydrophobicity difference between the carbonic sheets and almost any arbitrary substrate. The third technique is a modification of the already well known technique to enhance the visibility contrast of graphene using interferometric effects by employing a Si wafer coated with a dielectric of specific thickness.

The influence of nanocrystal size on optical second harmonic generation by para-nitroanaline embedded in electro-spun polymeric fibers

Poly(methyl methacrylate) electro-spun fibers with embedded nanocrystals of the paradigmatic donor–acceptor nonlinear chromophore para-nitroaniline have been recently demonstrated to be efficient generators of second harmonic light. To understand the influence of the size and local strain experienced by the embedded para-nitroaniline nanocrystals, a Williamson−Hall analysis was carried out on the X-ray diffraction intensity. Both the mean crystal size and strain can be tuned by simple changes in the deposition parameters of flow rate and applied voltage. The observed second harmonic signal is well correlated with the ratio of the fiber diameter to the mean para-nitroaniline crystal size suggesting that surface effects are the main source of the strong nonlinear optical response. Adjusting the electro-spinning deposition parameters when producing polymeric fibers doped with strong nonlinear organic chromophores with high dipole moments has the potential to provide a versatile and efficient method for developing second-order nonlinear optical materials.

Easy process to obtain suspended graphene flakes on TEM grids

Much of the ongoing research on graphene requires free-hanging (suspended) graphene to eliminate any influence from underlying substrates. Several methods have been developed for its preparation but they are either very complex or not completely reliable. Here, we describe a simple method for the transfer of graphene single layers from glass or silicon substrates onto TEM grids. The method uses a carrier film for the transfer process. By optimizing the process yields greater than 60% were achieved. The integrity of the transferred films was confirmed using Raman spectroscopy; successful suspension of both mono- and double-layer graphene sheets was obtained.

A versatile fluorescence lifetime imaging system for scanning large areas with high time and spatial resolution

We present a flexible fluorescence lifetime imaging device which can be employed to scan large sample areas with a spatial resolution adjustable from many micrometers down to sub-micrometers and a temporal resolution of 20 picoseconds. Several different applications of the system will be presented including protein microarrays analysis, the scanning of historical samples, evaluation of solar cell surfaces and nanocrystalline organic crystals embedded in electrospun polymeric nanofibers. Energy transfer processes within semiconductor quantum dot superstructures as well as between dye probes and graphene layers were also investigated.