Bottom-up approach for the synthesis of graphene-alumina nanosheet interfaces

We developed a new strategy for the production of graphene-oxide interfaces which opens a promising route to the direct synthesis on a number of graphene/high-κ dielectrics interfaces.

L. Omiciuolo et al., Nat. Comm. 5, 5062 (2014).

The coupling of graphene with 2D nanosheet dielectrics is an important topic in contemporary materials science, due to its potential impact on a number of high-performance nanoelectronic applications. In this respect, Al₂O₃ films play a key role because of their employment as high-κ oxides in transistors and low-power chips. However, the conventional methods so far used for producing graphene-alumina interfaces inevitably lead to a degradation of the electronic properties of graphene, by introducing defects and contaminants, or to the coexistence of monolayers with bi- or multi-layer Gr. It is therefore desirable to develop some alternative, scalable and economically affordable method for the transfer-free production of Gr-dielectric interfaces. Starting from a clean Ni₃Al(111) surface, we here propose a novel approach for the direct synthesis of Gr-alumina interfaces, by growing Gr on the substrate and subsequently oxidizing the metallic surface, in such a way to induce the formation of an alumina nanosheet.
Low Energy Electron Diffraction, X-ray Photoelectron Spectroscopy, Low Energy Electron Microscopy and μ-LEED experiments were performed at the Surface Science Laboratory and at the SuperESCA and Nanospectroscopy beamlines at Elettra, complemented by Angle Resolved Photoelectron Spectroscopy measurements carried out at the ASTRID synchrotron and by DFT calculations (CSIC-Madrid and UCL-London).
A graphene layer was first grown on Ni₃Al(111) by ethylene chemical vapour deposition at 950 K. The first direct evidence of Gr formation on Ni₃Al(111) comes from the C1s core level spectrum, which shows the presence of two narrow components, which were assigned to non equivalent C atoms in a top-fcc configuration (with respect to the substrate adsorption site) by comparison to our DFT calculations. The system was then exposed to a high flux of molecular oxygen at 520 K. Following this procedure, oxygen can underneath the Gr layer and selectively oxidize the Al atoms at the interface. In the C1s spectra wee observe the appearance of a new component at lower BE which we ascribe, from a comparison with literature, to regions of Gr sitting on an oxide substrate. After the carbon layer has been completely decoupled from the substrate, the C1s spectrum shows a single, narrow component centered at 284.2 eV.

On the other hand, following the evolution of the Al2p core level spectra, we observed the growth of a broad peak, centered at about 73.8 eV, in agreement with the typical values for alumina films on Ni₃Al. The estimated oxide layer thickness is 1.5±0.2 nm. ARPES measurements confirmed that, after oxidation, the Gr layer decouples from the metallic substrate and indicated also an oxide-induced p-doping of the Gr layer (~200 meV), in agreement with our DFT calculations.
To assess the quality of the Gr layer after oxidation, we compared the LEED patterns of the following structures: clean Ni₃Al, Gr/Ni₃Al and Gr/Al₂O₃. The transition from a 3-fold to a 6-fold symmetry in the circular line profile is consistent with the formation of a single-Gr layer on top of Ni₃Al. The width of the diffraction spots remains almost unchanged from the Gr/Ni₃Al to the Gr/Al₂O₃ structure, thus suggesting that the long-range structural order is largely preserved upon oxidation. Our diffraction experiments also show that, beside epitaxial Gr, exactly in registry with the substrate, a small amount of rotated regions (± 16°) are present. On the other hand, our LEEM experiments showed that Gr extends continuously across the sample, nicely confirming that it is not etched upon substrate oxidation. The film is composed by rather large tiles, each one extending for several microns.
Besides its simplicity, this method offers the advantage of using low-cost raw materials and, for this reason, it is expected to open new perspective for the production of new generation Gr-based devices.

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Bottom-up approach for the low-cost synthesis of graphene-alumina nanosheet interfaces using bimetallic alloys.
L. Omiciuolo, E. R. Hernández, E. Miniussi, F. Orlando, P. Lacovig, S. Lizzit, T. O. Menteş, A. Locatelli, R. Larciprete, M. Bianchi, S. Ulstrup, P. Hofmann, D. Alfè and A. Baraldi.
Nat. Comm. 5, 5062 (2014).

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