Alfredo R. Vaz

Self-Assembled and One-Step Synthesis of Interconnected 3D Network of Fe3O4/Reduced Graphene Oxide Nanosheets Hybrid for High-Performance Supercapacitor Electrode

In the present work, we have synthesized three-dimensional (3D) reduced graphene oxide nanosheets (rGO NSs) containing iron oxide nanoparticles (Fe3O4 NPs) hybrids (3D Fe3O4/rGO) by one-pot microwave approach. Structural and morphological studies reveal that the as-synthesized Fe3O4/rGO hybrids were composed of faceted Fe3O4 NPs induced into the interconnected network of rGO NSs. The morphologies and structures of the 3D hybrids have been characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy and X-ray photoelectron spectrometer (XPS). The electrochemical studies were analyzed by cyclic voltammetry, galvanostatic charge/discharge measurements, and electrochemical impedance spectroscopy, which demonstrate superior electrochemical performance as supercapacitors electrode application. The specific capacitances of 3D hybrid materials was 455 F g-1 at the scan rate of 8 mV s-1, which is superior to that of bare Fe3O4 NPs. Additionally, the 3D hybrid shows good cycling stability with a retention ratio of 91.4 after starting from ∼190 cycles up to 9600 cycles. These attractive results suggest that this 3D Fe3O4/rGO hybrid shows better performance as an electrode material for high-performance supercapacitors.

Towards practical liquid and gas sensing with photonic crystal fibres: side access to the fibre microstructure and single-mode liquid-core fibre

Photonic crystal fibres (PCFs) have important applications in sensing the optical properties of fluids. To this end, the material should be inserted into the fibre holes in order to interact with the propagating field. When dealing with liquids, it is particularly interesting to exclusively insert the sample into the core of a hollow-core PCF, which then guides light through the liquid via total internal reflection. Nevertheless, there is still a series of issues to be addressed before fluid sensing with PCFs becomes practical. The work described here proposes and demonstrates possible solutions for two of these issues: (a) how to insert the sample through a lateral access to the fibre longitudinal holes so that the fibre tips are free for optical handling and accessing and (b) in the case of a liquid-core PCF, how to reduce the number of propagating modes.

Microwave-assisted synthesis and deposition of a thin ZnO layer on microwave-exfoliated graphene: optical and electrochemical evaluations

A rapid and facile microwave-assisted method has been developed for the deposition of a zinc oxide layer (ZnOL) in situ on partially microwave exfoliated graphene (MpEG). The formation of the ZnO layer on microwave partially exfoliated graphene (ZnOL@MpEG) hybrid only requires zinc nitrate and the MpEG sheets are reacted under a low level of microwave irradiation (700 W) for 5 min. The deposited thin ZnOL on the MpEG sheets is uniformly well-dispersed and covers the whole MpEG sheets. The as-prepared ZnOL@MpEG hybrids demonstrate enhanced electrochemical properties as supercapacitors and also show quenching phenomena for photoluminescence. The fluorescence quenching observed for the ZnOL@MpEG hybrids compared to ZnO, indicates photoinduced electron transfer from ZnO to the MpEG layers, which shows recombination of hole and electron charge carriers. The electrochemical measurements exhibit that the ZnOL@MpEG hybrids have a large integral area of the cyclic voltammetry loop, indicating that such hybrids are promising for application in supercapacitors. The material displayed a high specific capacitance of 347 F g−1 at a current density of 5.0 A g−1. A mechanism for the formation of the ZnOL@MpEG hybrids via the microwave method has also been proposed.

Fabrication of interdigitated micro-supercapacitor devices by direct laser writing onto ultra-thin, flexible and free-standing graphite oxide films

In this work we present graphene-based in-plane flexible interdigitated micro-supercapacitor devices fabricated through direct laser writing onto ultra-thin graphite oxide (GO) films. The fabrication route is simple, fast, additive-free, mask-free and cost effective. This involves direct micro-writing of reduced graphene oxide (rGO) by a pulsed UV laser on a very small area (1.14 cm2). The fabricated micro-supercapacitor contains nineteen pairs of rGO electrodes separated by the unreduced portion of the GO film. The single laser patterned rGO electrode presents low resistivity, while the unpatterned portion is non-conducting. Under the optimized laser parameters the 2.2 μm ultra-thin GO films were completely and uniformly reduced. The electrochemical measurements showed that the micro-supercapacitor, packed in a glass cavity, and in the presence of a liquid electrolyte have a capacitance nearly 288% higher (288.7 mF cm−3) compared to the as-fabricated device (0.36 mF cm−3). The as-fabricated micro-supercapacitor without electrolyte also shows some capacitance due to the presence of free ions in the unreduced portion of GO which plays a crucial role. Furthermore, the cycling stability of the as-fabricated micro-supercapacitor is robust, with not much performance degradation for more that 5000 cycles.

Low contact resistivity and strain in suspended multilayer graphene

Method to prepare suspended multilayer graphene (MLG) flakes and to form highly conductive (contact resistivity of ∼0.1 kΩ μm2) and tight mechanical connection between MLG and metal electrodes is described. MLG flakes prepared from natural graphite were precisely deposited over tungsten electrodes using dielectrophoresis, followed by high-temperature thermal annealing in high-vacuum. Considerable strain induced in the suspended part of flakes was revealed by Raman imaging.

Nonlocal laser annealing to improve thermal contacts between multi-layer graphene and metals

The accuracy of thermal conductivity measurements by the micro-Raman technique for suspended multi-layer graphene flakes has been shown to depend critically on the quality of the thermal contacts between the flakes and the metal electrodes used as the heat sink. The quality of the contacts can be improved by nonlocal laser annealing at increased power. The improvement of the thermal contacts to initially rough metal electrodes is attributed to local melting of the metal surface under laser heating, and increased area of real metal-graphene contact. Improvement of the thermal contacts between multi-layer graphene and a silicon oxide surface was also observed, with more efficient heat transfer from graphene as compared with the graphene-metal case.

Catalyst-free synthesis of a three-dimensional nanoworm-like gallium oxide–graphene nanosheet hybrid structure with enhanced optical properties

We here report the synthesis and growth of catalyst-free three-dimensional β-gallium oxide nanoworm-like nanostructures on graphene nanosheets (3D β-Ga2O3@GNSs) using a solid mixture of graphite oxide and gallium acetylacetonate by the microwave (MW)-assisted method for the first time. The MW-assisted synthesis of the 3D β-Ga2O3@GNSs hybrids contains 1D semiconducting β-Ga2O3 nanoworms (NWs) and 2D highly conducting graphene nanosheets (GNSs) materials. The β-Ga2O3 NWs have an average diameter of 200 nm and lengths of up to ∼1 μm grown on the GNSs. These 3D β-Ga2O3@GNSs hybrids have been synthesized in a very short time with scalable amounts. The controlling parameters such as MW irradiation time and power were found to greatly influence the structural morphology of the as-synthesized 3D β-Ga2O3@GNSs hybrid. This method for the synthesis of 3D β-Ga2O3@GNSs hybrids is imperative due to it allowing excellent control over experimental parameters, being low cost and having better reproducibility. Also, the catalyst-free MW-assisted method is a much more rapid and thus higher throughput alternative for effective and scalable growth over the conventional heating method. The crystallinity, structure, morphology, and optical analysis of the 3D β-Ga2O3@GNSs hybrids are carried out utilizing several techniques. The formation of the 3D β-Ga2O3@GNSs hybrids shows a band gap variation from 4.94 to 4.48 eV associated with the structural evolution. A suitable growth mechanism has been suggested for the formation of these 3D β-Ga2O3@GNSs hybrids.

Platinum thin films deposited on silicon oxide by focused ion beam: characterization and application

Focused ion beam system was used for deposition of platinum (Pt) thin films on thermally oxidized silicon (Si). Various test patterns (squares and lines) were deposited for electrical characterization of the films, using 2- and 4-terminal measurements. Tests with parallel Pt lines were also carried out, and considerable leakage was detected for the interline distances in the sub-micron range. We investigated two ways to decrease the leakage current: inducing surfaces roughness and using an oxygen plasma after patterns deposition. A method of dielectrophoresis with an AC electric field was applied to align and deposit metallic multi-wall carbon nanotubes (CNT) between pre-fabricated metal, gold, and palladium electrodes with a micron-scale separation. Further, using focused electron and ion beam-deposited Pt contacts in two different configurations (“Pt-on-CNT” and “CNT-on-Pt”), 4-terminal measurements have been performed to evaluate intrinsic nanotube resistances.

Burning Graphene Layer-by-Layer

Graphene, in single layer or multi-layer forms, holds great promise for future electronics and high-temperature applications. Resistance to oxidation, an important property for high-temperature applications, has not yet been extensively investigated. Controlled thinning of multi-layer graphene (MLG), e.g., by plasma or laser processing is another challenge, since the existing methods produce non-uniform thinning or introduce undesirable defects in the basal plane. We report here that heating to extremely high temperatures (exceeding 2000 K) and controllable layer-by-layer burning (thinning) can be achieved by low-power laser processing of suspended high-quality MLG in air in “cold-wall” reactor configuration. In contrast, localized laser heating of supported samples results in non-uniform graphene burning at much higher rates. Fully atomistic molecular dynamics simulations were also performed to reveal details of oxidation mechanisms leading to uniform layer-by-layer graphene gasification. The extraordinary resistance of MLG to oxidation paves the way to novel high-temperature applications as continuum light source or scaffolding material.

Electrical Characterization of Platinum Thin Films Deposited by Focused Ion Beam

Dual beam FIB (focused ion beam)/SEM (scanning elelctron microscope) systems are commonly used for imaging, selective etch and deposition of materials like platinum. The paper presents the results of electrical characterization of platinum thin films deposited by focused ion beam. For measurements, two types of test structures were fabricated: (i) 150x150 :m and 20x20 :m squares with thickness of 5, 10, 30 and 100 nm, and (ii) 30 :m long resistors with variable cross - section (50 nm x 50nm to 1:m x 1:m). The Pt film resistivity has been measured by a four points probe method, to give the value of ~10 x 10-4 A.cm.