205 research outputs found
The influence of residual oxidizing impurities on the synthesis of graphene by atmospheric pressure chemical vapor deposition
The growth of graphene on copper by atmospheric pressure chemical vapor
deposition in a system free of pumping equipment is investigated. The emphasis
is put on the necessity of hydrogen presence during graphene synthesis and
cooling. In the absence of hydrogen during the growth step or cooling at slow
rate, weak carbon coverage, consisting mostly of oxidized and amorphous carbon,
is obtained on the copper catalyst. The oxidation originates from the
inevitable occurrence of residual oxidizing impurities in the reactor's
atmosphere. Graphene with appreciable coverage can be grown within the
vacuum-free furnace only upon admitting hydrogen during the growth step. After
formation, it is preserved from the destructive effect of residual oxidizing
contaminants once exposure at high temperature is minimized by fast cooling or
hydrogen flow. Under these conditions, micrometer-sized hexagon-shaped graphene
domains of high structural quality are achieved.Comment: Accepted in Carbo
Additive Photonic Colors in the Brazilian Diamond Weevil, Entimus imperialis
Structurally colored nano-architectures found in living organisms are complex optical materials, giving rise to multi-scale visual effects. In arthropods, these structures often consist of porous biopolymers and form natural photonic crystals. A signature of the structural origin of coloration in insects is iridescence, i.e., color changes with the viewing angle. In the scales located on the elytra of the Brazilian weevil Entimus imperialis (Curculionidae), three-dimensional photonic crystals are observed. On one hand, each of them interacts independently with light, producing a single color which is observed by optical microscopy and ranges from blue to orange. On the other hand, the color perceived by the naked eye is due to multi-length-scale light effects involving different orientations of a single photonic crystal. This disorder in crystal orientations alters the light propagation in such a way that the crystal iridescence is removed. Entimus imperialis is therefore a remarkable example of additive photonic colors produced by a complex multi-scale organic architecture. In order to study this specific natural photonic structure, electron microscopy is used. The structure turns out to be formed of a single type of photonic crystal with different orientations within each scale on the elytra. Our modeling approach takes into account the disorder in the photonic crystals and explains why the structure displays bright colors at the level of individual scales and a non-iridescent green color in the far-field.Structurally colored nano-architectures found in living organisms are complex optical materials, giving rise to multi-scale visual effects. In arthropods, these structures often consist of porous biopolymers and form natural photonic crystals. A signature of the structural origin of coloration in insects is iridescence, i.e., color changes with the viewing angle. In the scales located on the elytra of the Brazilian weevil Entimus imperialis (Curculionidae), three-dimensional photonic crystals are observed. On one hand, each of them interacts independently with light, producing a single color which is observed by optical microscopy and ranges from blue to orange. On the other hand, the color perceived by the naked eye is due to multi-length-scale light effects involving different orientations of a single photonic crystal. This disorder in crystal orientations alters the light propagation in such a way that the crystal iridescence is removed. Entimus imperialis is therefore a remarkable example of additive photonic colors produced by a complex multi-scale organic architecture. In order to study this specific natural photonic structure, electron microscopy is used. The structure turns out to be formed of a single type of photonic crystal with different orientations within each scale on the elytra. Our modeling approach takes into account the disorder in the photonic crystals and explains why the structure displays bright colors at the level of individual scales and a non-iridescent green color in the far-field
Graphene-coated holey metal films: tunable molecular sensing by surface plasmon resonance
We report on the enhancement of surface plasmon resonances in a holey
bidimensional grating of subwavelength size, drilled in a gold thin film coated
by a graphene sheet. The enhancement originates from the coupling between
charge carriers in graphene and gold surface plasmons. The main plasmon
resonance peak is located around 1.5 microns. A lower constraint on the
gold-induced doping concentration of graphene is specified and the interest of
this architecture for molecular sensing is also highlighted.Comment: 5 pages, 4 figures, Final version. Published in Applied Physics
Letter
Synthesis and Characterization of Highly Crystalline Vertically Aligned WSe2 Nanosheets
Here, we report on the synthesis of tungsten diselenide (WSe2) nanosheets using an atmospheric pressure chemical vapor deposition technique via the rapid selenization of thin tungsten films. The morphology and the structure, as well as the optical properties, of the so-produced material have been studied using electron microscopies, X-ray photoelectron spectroscopy, photoluminescence, UV–visible and Raman spectroscopies, and X-ray diffraction. These studies confirmed the high crystallinity, quality, purity, and orientation of the WSe2 nanosheets, in addition to the unexpected presence of mixed phases, instead of only the most thermodynamically stable 2H phase. The synthesized material might be useful for applications such as gas sensing or for hydrogen evolution reaction catalysis
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