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

Oxidation-assisted graphene heteroepitaxy on copper foil

We propose an innovative, easy-to-implement approach to synthesize large-area singlecrystalline graphene sheets by chemical vapor deposition on copper foil. This method doubly takes advantage of residual oxygen present in the gas phase. First, by slightly oxidizing the copper surface, we induce grain boundary pinning in copper and, in consequence, the freezing of the thermal recrystallization process. Subsequent reduction of copper under hydrogen suddenly unlocks the delayed reconstruction, favoring the growth of centimeter-sized copper (111) grains through the mechanism of abnormal grain growth. Second, the oxidation of the copper surface also drastically reduces the nucleation density of graphene. This oxidation/reduction sequence leads to the synthesis of aligned millimeter-sized monolayer graphene domains in epitaxial registry with copper (111). The as-grown graphene flakes are demonstrated to be both single-crystalline and of high quality.Comment: Main text (18 pages, 6 figures) + supplementary information (26 pages, 15 figures

Quantifying the local mechanical properties of twisted double bilayer graphene

Nanomechanical measurements of minimally twisted van der Waals materials remained elusive despite their fundamental importance for device realisation. Here, we use Ultrasonic Force Microscopy (UFM) to locally quantify the variation of out-of-plane Young's modulus in minimally twisted double bilayer graphene (TDBG). We reveal a softening of the Young's modulus by 7% and 17% along single and double domain walls, respectively. Our experimental results are confirmed by force-field relaxation models. This study highlights the strong tunability of nanomechanical properties in engineered twisted materials, and paves the way for future applications of designer 2D nanomechanical systems

Coherent and ballistic transport in InGaAs and Bi mesoscopic devices

In clean' confined conductors (the so-called mesoscopic systems), the electronic phase and momentum can be preserved over very long distances compared to the system dimensions. This gives rise to peculiar transport properties, bearing signatures of electron interferences, ballistic electron trajectories, electron-electron interactions, regular-chaotic electron dynamics and (in some cases) spin-orbit coupling. Examples of such effects are the Universal Conductance Fluctuations (UCFs) and the Weak Localization observed in the low-temperature magnetoconductance of many confined electronic systems. Of central importance, the electronic phase coherence time and the spin-orbit coupling time determine the amplitude of these quantum effects. In the first part of this thesis, we use UCFs to extract these characteristic timescales in open ballistic quantum dots (QDs) fabricated from InGaAs heterostructures. We observe an intrinsic saturation of the coherence time at low temperature in the InGaAs QDs. The origin of this phenomenon has been intensely debated during the last decade. Based on our observations and previous experimental data in QDs, we propose an explanation: the dwell time becomes the limiting factor for electron interferences in QDs at low temperature. Then, we report on magnetoconductance measurements in a bismuth ballistic nano-cavity. The cavity is found to be zero-dimensional for phase coherent processes at low temperature. We evidence an anomalous reduction of the phase coherence time in the cavity with respect to data obtained in thin Bi films, while the spin-orbit coupling time is similar in both systems. Finally, we examine the current-voltage characteristics of asymmetric InGaAs nano-junctions in the nonlinear regime. We observe a new tunable rectification effect, whose amplitude and sign are governed by the conductances of the junctions' channels. We show that the effect is ballistic and exhibits new features with respect to predictions of available models.(FSA 3)--UCL, 200

Synthesis of Ru, Ni and Fe supported graphene nanoplatelets catalysts for hydrogenation of glucose into sorbitol

Non-covalent method of graphene nanoplatelets’ functionalization using the pyrene-tagged Ru, Ni and Fe precursors developed in this work offers an easy way to prepare bimetallic catalysts with different metal ratio and different compositions as Ni-Ru, Fe-Ru and Ni-Fe. First, four different metallic precursors were synthetized: Ni (0), Ni (II), Fe and Ru. Then, 16 different catalysts were prepared and fully characterized by ICP, XPS, XRD and TEM. All catalysts were then tested in the catalytic reaction of the hydrogenation of glucose into sorbitol. The ruthenium supported catalyst with 2.23 wt% shows a good conversion (66%) of glucose and high sorbitol selectivity (74%) at 160 °C under 30 bar H2 pressure during 2 h. The non-noble metallic catalysts Ni and Fe show very low conversion and selectivity. However, the combination of these two metals shows good results: 58% of glucose conversion and 40% of sorbitol selectivity obtained with 4.12 wt% in both metals

Imager les interférences quantiques dans les semi-conducteurs

Une nouvelle microscopie à sonde locale a été développée pour étudier les dispositifs mésoscopiques semiconducteurs dont les électrons de conduction sont enfouis à plusieurs dizaines de nanomètres sous la surface. Cette microscopie mesure les variations de conductance pendant qu’une pointe nanométrique à laquelle on applique une tension est déplacée au-dessus du dispositif pour modifier localement le potentiel électrostatique comme une grille locale. Cet article présente les effets observés expérimentalement sur des anneaux quantiques, ainsi que des simulations théoriques mettant en évidence la correspondance entre les images de conductance et la densité d’états locale