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Francesco La Porta

Formation of graphene on liquid copper: In situ synchrotron X-ray study

Published on 18 March 2021
Thesis presented March 18, 2021

After its easy synthesis by exfoliation in 2004, graphene astonished the scientific community. Its peculiar physical properties e.g. of electrical charge transfer, mechanical strength and flexibility, and transparency are the key ingredients for a revolution of modern technology.
One of the main issues that hinder the entry of this material into the market is the lack of a cheap method that guarantees a high-quality product on a large scale. The graphene properties can be easily altered by defects, grain boundaries, and thickness variations. One of the most promising methods of synthesis is chemical vapor deposition (CVD), which consists of the dissociation of a gaseous precursor on a transition metal catalyst. The carbon adatoms then diffuse on the metal and aggregate into a graphene crystal structure. Once all the surface is covered by one single layer of graphene, the gas cannot reach the catalytic site anymore, and the reaction stops. The drawback of CVD is that the morphology of the metal surface influences the quality of the graphene produced. The graphene, as a matter of fact, nucleates on grain boundaries and defects of the metal surface. Many nucleation points result in a polycrystalline material that has many grain boundaries. The ideal flat uniform defect-free surface morphology is very hard to achieve on solid metal.
The use of a liquid substrate instead of a solid one can overcome these issues. The liquid naturally has a uniform and atomically flat surface. The firsts papers on graphene on a liquid metal show great quality and high reproducibility of the reaction. Another interesting phenomenon that occurs on the surface of liquid metal is that the flakes can move and rotate, and they tend to aggregate and auto-align. The state of the art of CVD growth of graphene on liquid copper is discussed in Chapter 1.
All previously published experiments of CVD growth of graphene on liquid metal follow a similar approach: the metal is melted, the graphene is grown, and lastly, the sample is resolidified before characterization. This approach, however, alters the surface significantly. Furthermore, in this way, information on the dynamic of the growth is lost entirely. The work described in this thesis is situated in this scenario and has the main objective to fill this gap. A reactor, was designed for the in-situ characterization with the simultaneous combination of x-ray scattering techniques and optical microscopy. An introduction of the techniques is done in Chapter 2, while the reactor and the instrumentation are described in Chapter 3. The metal chosen for the catalyst is liquid copper.
The optical microscope revealed itself to be an essential tool to understand the dynamics of the flake movement on the surface of the liquid and to have feedback on the status of the growth. This is described in Chapter 4. A graphene flake larger than 2 mm was produced. It was observed that the flakes of graphene could self-align, and, with the help of the collaborators of the project, a mechanism was proposed.
The synchrotron measurements done were x-ray reflectivity (XRR) and grazing incident x-ray diffraction (GIXD). The analysis of the data for the XRR was complexified by the convex shape of the surface. With a convex surface, in fact, the reflected beam spreads, and the incident angle depends partially on the curvature of the sample. In chapter 5, the effects of a bent surface are described, and a novel method for analyzing the data is presented. In Chapter 6, the results of x-ray scattering techniques are exposed. With the GIXD, the lattice parameter of graphene was measured for the first time on liquid copper. The XRR measurements proved that the distance between the graphene and the liquid copper atom was 1.40 Å and that the roughness of the graphene and the liquid were similar, at 1.24 Å.

graphene, liquid metals, synchrotron, x-ray reflectivity, x-ray diffraction, optical microscope