In situ spectroscopic characterization of graphene growth on LMCat
Anastasios Manikas, Christos Tsakonas, Marinos Dimitropoulos, Costas Galiotis
Composites and Nanomaterials, Institute of Chemical Engineering Sciences, Foundations for Research and Technology-Hellas, Greece
Graphene is a perfect 2D crystal of covalently bonded carbon atoms and forms the basis of all graphitic structures. Graphene is the best conductor of heat we know, the thinnest and the stiffest material ever made, it conducts electricity much better than silicon, is >200 times stronger than steel and has unique optical properties. These superior characteristics can be exploited in many areas of research; new possibilities are being recognized all the time as the science of graphene and other two-dimensional materials progresses. Nevertheless, graphene cannot have significant impact, until efficient production techniques will develop to harvest their unique properties in global applications and devices. Mass production of high-quality graphene remains a challenge. Chemical Vapor Deposition (CVD) is the most well-known method of graphene growth based on the decomposition of hydrocarbon molecules and their nucleation on a catalytic surface at high temperatures. The fabrication process is rather complex, as it involves multiple steps such as hydrocarbon decomposition, carbon adsorption and subsequently diffusion on the catalytic substrate the generation of the nucleation point and finally the growing. In-situ monitoring of such a complex procedure is of paramount importance for the control of graphene growth and the understanding of growing kinetics.
In the framework of LMCat project we aim to study the catalytic properties and the formation of graphene on LMCats. Raman spectroscopy as a chemically-sensitive technique is capable of detecting molecules and chemical bonds can play a crucial role for this study. Among other optical techniques, Raman spectroscopy has been used extensively for studying nanomaterials in general and graphene (on solid substrates) in particular. It is capable to detect traces of the precursor adsorbates, intermediate reaction species, and characteristics of final graphene products on the LMCat surface, including number of layers, stacking type, defect density, and presence of doping or contamination. However, performing in situ Raman spectroscopy at high temperatures (T > 1000 K) needs special considerations, otherwise the weak Raman signal could be easily dominated by the intense thermal radiation of the molten surface. In our case a close to UV laser line, at 405 nm, for reducing the black body radiation effect in the Raman-Stokes spectral region was used. Raman spectra from graphene were acquired on liquid Cu during growth and verified the existence of graphene even at primary stages. This result is of paramount importance since it is the first time that a chemically sensitive techniques like Raman spectroscopy was implemented for the in-situ monitoring of graphene growth. Furthermore, a novel metrology system based on reflectance spectroscopy for the in-situ monitoring of surface changes during graphene growth by taking advantage of reflectance variations was developed. The production of graphene sheets is taking place in a CVD reactor, by a mixture of Methane (CH4), as the carbon source, Hydrogen (H2) and Argon (Ar) on copper (Cu) foils at high temperatures (~ 1270 K). Simultaneously, reflectance fluctuations on the surface of copper are monitored and analyzed. The results indicated that the growth rate of graphene can be estimated from the measured differential reflectance. Changes to the growth rate were recorded for different growth conditions confirming that reflectance spectroscopy can be successfully adopted for in-situ characterization of graphene growth. Finally, ex-situ Raman spectroscopic analysis is carried out in order to identify the presence and the quality of graphene films after the growth.