XAFS and TEM-EELS studies on the electronic state and crystallographic structure of graphene on nickel catalyst
Transition metal-catalyzed chemical vapor deposition is a leading method for large-scale graphene synthesis. Transition metals, especially nickel and cobalt, have a high catalytic ability in the decomposition of hydrocarbon and hydrogen, so they are promising metals for graphene synthesis. In this study, we analyzed the electronic state and crystallographic structure of graphene on Ni catalyst by observing the X-ray absorption fine structure (XAFS) and transmission electron microscope electron energy loss spectroscopy (TEM-EELS) in order to understand the origin of graphene growth from the metal catalyst.
Graphene synthesis was performed with radial line slot plane antenna plasma chemical vapor deposition (RLSA-CVD), which has advantages towards the low-charge damage and the low growth temperature process because of the high-density radical generation in the process field. The carbon sources were ethylene (C2H4) and hydrogen (H2), and the substrates were Ni (30 nm) / TiN (15 nm) / SiO2 / Si (100) wafers. The substrate temperature was 350 - 470°C during the graphene synthesis.
The Raman spectra and the TEM images of graphene showed that multi-layer graphene was formed on the Ni catalyst. The electron diffractions from the graphene on the Ni catalyst indicated that graphene (0001) was formed on the fcc-Ni preferred orientated (111) face. The TEM-EELS spectra of the graphene and Ni catalyst represented the featured peaks of the π* excitation in graphene (285 eV), L3-edge Ni (855 eV) and L2-edge Ni (872 eV), respectively caused by the excitation of the 2p3/2 and 2p1/2 electrons in Ni atoms. The X-ray absorption near edge structure (XANES) spectra of the K-edge Ni (8331.7 eV) corresponded with that of pure metallic nickel. The nearest neighbors’ distances and the crystallographic disorder in the Ni catalyst were characterized by the Fourier transformed spectra of the extended X-ray absorption fine structure (EXAFS). The EXAFS sharp peak at r = 0.21 nm was well assigned to the nearest neighbor Ni-Ni bond.
In conclusion, the XAFS and TEM-EELS analyses revealed that transition metal catalysis has the need to structure and sustain a well defined crystal for successful graphene synthesis, especially at very low-temperatures.
This work was performed as part of the “Ultra Low Voltage Device Project” supported by the New Energy and Industrial Technology Development Organization (NEDO) and the Ministry of Economy, Trade and Industry (METI) of Japan.