Synthèse de graphène par CVD catalytique sur cuivre et nickel

Cette thèse présente la synthèse de graphène par CVD catalytique sur feuille de cuivre, wafers de silicium revêtus de nickel et sur mousse de nickel. Les dépôts ont été réalisé à partir de méthane et d'éthylène. Pour l'ensemble de ces substrats, les études faites ont permis de mieux appréhender les mécanismes de croissance et de déterminer les paramètres opératoires optimaux. Des tests applicatifs ont été effectué pour utiliser le graphène synthétisé comme électrode d'OLED et de batterie Li-ion

Graphene in silicon photovoltaic cells

Graphene is an allotrope of carbon. Its structure is one-atom-thick planar sheets of carbon atoms that are densely packed in a honeycomb crystal lattice [1]. The richness of optical and electronic properties of graphene attracts enormous interest. Its true potential seems to be in photonics and optoelectronics, where the combination of its unique optical and electronic properties can be fully exploited. The optical absorption of graphene layers is proportional to the number of layers, each absorbing A=1-T=πα=2.3% over the visible spectrum [2].The rise of graphene in photonics and optoelectronics is shown by several recent results, ranging from solar cells and light emitting devices, to touch screens, photodetectors and ultrafast lasers. Current photovoltaic (PV) technology is dominated by Si cells, with an energy conversion coefficient up to 25% [3]. Such an inorganic PV consists in a current transparent conductor (TC) replacing one of the electrodes of a PIN photodiode. The standard material used so far for these electrodes is indium-tinoxide, or ITO. But indium is expensive and relatively rare, so the search has been on for a suitable replacement. A possible substitute made from inexpensive and ubiquitous carbon is graphene. Being only constituted of carbon, it will become cheap and easily recyclable. But at the moment, the major difficulty consists in its fabrication and/or transfer. Our project consists in synthetizing graphene by CVD (Chemical Vapor Deposition) on Cu and in transferring the obtained layer on silicon PV cells, and then in testing their energy conversion efficiency

High quality graphene synthesized by atmospheric pressure CVD on copper foil

Graphene was synthesized at 1000°C by Atmospheric Pressure Chemical Vapor Deposition on copper foil from methane diluted in argon and hydrogen. The influence of the main synthesis parameters was studied on 2x2 cm2 foils in order to obtain continuous monolayer graphene without crystalline defect. The uniformity, crystal quality and number of layers of graphene were analyzed by Raman spectroscopy and Scanning Electronic Microscopy. First, an increase of the annealing pre-treatment duration induced an increase of the average size of copper grains, leading to larger graphene flakes of higher crystallinity presenting a lower number of layers. Similar evolutions of graphene characteristics were observed when decreasing the methane concentration to 20 ppm, whereas an increase of run duration led to a loss of graphene quality and to a higher number of graphene layers, confirming that graphene formation is not self-limiting on copper. An optimum hydrogen/methane ratio was found, quite different from other results of the literature, probably due to differences in the copper pre-treatment step. Finally, an optimized three steps process was developed to form monolayer continuous graphene of high quality, successfully transposed to 7x7 cm2 substrates after a reactor scale-up

Graphene synthesis on coper and nickel by catalitic cvd

Cette thèse présente la synthèse de graphène par CVD catalytique sur feuille de cuivre, wafers de silicium revêtus de nickel et sur mousse de nickel. Les dépôts ont été réalisé à partir de méthane et d'éthylène. Pour l'ensemble de ces substrats, les études faites ont permis de mieux appréhender les mécanismes de croissance et de déterminer les paramètres opératoires optimaux. Des tests applicatifs ont été effectué pour utiliser le graphène synthétisé comme électrode d'OLED et de batterie Li-ion.This study concerns graphene synthesis by catalytic CVD (Chemical Vapor Deposition) on copper foils, silicon wafers coated with nickel and nickel foam. Deposits have been synthesized from methane and ethylene. For the whole substrates studied, the results obtained have allowed to better understand the mechanisms of nucleation/growth of graphene and to determine the optimal operating parameters. Some applicative tests have been performed in the fields of OLED and Li-ion battery

Three Dimensional Graphene Synthesis on Nickel Foam by Chemical Vapor Deposition from Ethylene

3D multi-layers graphene networks were synthesized on nickel foam from ethylene between 700 and 1000°C by chemical vapor deposition. Large nickel foam substrates were used allowing the accurate measurement of graphene masses. The weight of graphene increased with run duration and when decreasing temperature. Graphene was also present inside the hollow branches of the foam. We demonstrated that the weights of graphene formed largely exceed the masses corresponding to carbon solubility into nickel. Indeed weight percentages of graphene as high as 15% were obtained, corresponding to graphene layers of 500 nm to 1 micron thick. This means that graphene formation could not be due only to carbon dissolution into nickel and then precipitation during the cooling step. Another mechanism probably co-exists, involving continuous graphene formation in presence of ethylene either by segregation from the dissolved carbon into nickel or by surface CVD growth

Graphene synthesis on copper from ethylene by Catalytic Chemical Vapor Deposition

Graphene is a promising material thanks to its physical properties and presents many potential applications for example as transparent electrodes in the field of OLED, solar cells or sensitive flat displays. However its production at low cost and large scale with controlled characteristics remains elusive. This is the reason why its synthesis has still to be improved in order to control its crystallinity and its number of layers over large areas. Catalytic CVD (Chemical Vapor Deposition) appears to be the most promising commercially viable process, since it allows forming cm2 scale areas of good quality graphene. However, most high quality CVD graphene is at present grown at temperatures close to 1,035°C on copper catalytic substrates from methane. This temperature is very close to the melting point of Cu (~1,085°C), and then creates intense Cu evaporation and then condensation fluxes upon cooling, which can affect the reproducibility of graphene synthesis and also decrease the samples quality and the reactor lifetime [1]. Some attempts have been made to decrease the synthesis temperature of graphene using alternative precursors like toluene, benzene, ethylene or acetylene [1-3]. Ethylene seems to be a good candidate to replace methane since it is cheap and easy to handle, and presents a higher reactivity than methane [1-2]. High quality graphene has already been obtained using ethylene on Cu foils at 850°C [1-2], but only at low total pressure (max. 100 Pa) and without a complete analysis of the key synthesis parameters influence. In the present study, the influence of the main deposition conditions on the graphene crystalline quality and number of layers has been analyzed using ethylene diluted into hydrogen and argon on copper foils (25 um thick, 99,999% Alfa Aesar) of 2x2 cm2. The operating temperature was varied between 700 and 850°C, the hydrogen on ethylene inlet molar ratio between 1.5 and 14 and the total pressure between 3 and 700 Torr, as detailed in Table 1. The ethylene partial pressure was maintained at 30 mTorr for all experiments conducted at the total pressure of 3 Torr and was equal to 7 Torr at 700 Torr of total pressure

Development of a multi-steps CVD process to produce bi-layers graphene for anode of Organic Light Emitting Diodes

Graphene is one of the most interesting candidates for the next generation of transparent conductive electrodes (TCEs) for electrical devices, because of its unique electronic structure. Furthermore, the optical transparency of graphene films surpasses that of conventional TCEs such as indium tin oxide (ITO) [1]. However, graphene anode for Organic Light Emitting Diodes (OLEDs) still presents several problems owing to its low work function and high sheet resistance [1], which may be related to a poor control of graphene quality. Chemical vapor deposition (CVD) on copper from methane seems to be the most efficient approach to form high quality transferable graphene for opto-electronic applications, due to the potential for commercially viable production at large scale. However, CVD processes need to be optimized for obtaining selective single or bilayers growth, as well as highly crystalline, full coverage, large area domains [2]. Indeed, CVD graphene films are typically composed of relatively small polycrystalline flakes. A high density of grain boundaries degrades the properties of graphene [2]. Thus, it is desirable to prepare large single-crystal graphene to minimize the impact of defects existing at grain boundaries. The most recent studies of the literature show that this objective can be met by using very low concentration of methane (<100 ppm), but in these conditions, it is difficult to obtain a full coverage of the substrate [3]. The most efficient way to obtain a continuous high quality monolayer of graphene seems to use a multi-steps process, first involving a very low methane concentration in order to form strictly monolayer graphene flakes with low nucleation density. Then, methane concentration is progressively increased, to counterbalance the decrease of the active catalytic copper surface [3]. In the present study, a two-step process then a three-steps one have been developed only differing by the third step, in order to produce graphene for OLED application. Methane diluted into hydrogen and argon was used on copper foils (25 mm thick, 99,999% Alfa Aesar) of 2x2 cm2. The operating temperature was fixed at 1,000°C and the total pressure was of 700 Torr. The hydrogen on methane inlet molar ratio was fixed to 800 for steps 1 and 2. The CH4 concentration was of 10 ppm for step 1 and 40 ppm for step 2, and their duration was of 60 min for each one. For step 3, only 5 min long, the CH4 concentration was of 9,000 ppm and the H2/CH4 ratio of 10. Optical microscope and Raman spectroscopy measurements (confocal Raman microscope Labram – Horiba Yvon Jobin) were carried out to investigate the quality and extend of graphene sheets

High quality graphene synthesized by atmospheric pressure CVD on copper foil

International audienceGraphene was synthesized at 1000°C by Atmospheric Pressure Chemical Vapor Deposition on copper foil from methane diluted in argon and hydrogen. The influence of the main synthesis parameters was studied on 2x2 cm2 foils in order to obtain continuous monolayer graphene without crystalline defect. The uniformity, crystal quality and number of layers of graphene were analyzed by Raman spectroscopy and Scanning Electronic Microscopy. First, an increase of the annealing pre-treatment duration induced an increase of the average size of copper grains, leading to larger graphene flakes of higher crystallinity presenting a lower number of layers. Similar evolutions of graphene characteristics were observed when decreasing the methane concentration to 20 ppm, whereas an increase of run duration led to a loss of graphene quality and to a higher number of graphene layers, confirming that graphene formation is not self-limiting on copper. An optimum hydrogen/methane ratio was found, quite different from other results of the literature, probably due to differences in the copper pre-treatment step. Finally, an optimized three steps process was developed to form monolayer continuous graphene of high quality, successfully transposed to 7x7 cm2 substrates after a reactor scale-up