Bottom-up approach for the low-cost synthesis of graphene-alumina nanosheet interfaces using bimetallic alloys
The coupling of graphene (Gr) 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 Gr-alumina interfaces inevitably lead to a degradation of the electronic properties of Gr, 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 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.
LEED (Low Energy Electron Diffraction), HR-XPS (High Resolution X-ray Photoelectron Spectroscopy), LEEM (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 ARPES (Angle Resolved Photoelectron Spectroscopy) measurements carried out at the SGM-III beamline of ASTRID synchrotron and by Density Functional Theory calculations (ICMM-CSIC, Madrid and UCL-London).
Figure 1. (a) (bottom) C1s and Al2p photoemission spectra acquired at different O2 exposure. (top) C1s and Al2p high-resolution spectra acquired after the formation of a thick alumina nanosheet underneath Gr, the fit components are shown superimposed to the experimental data. (b) Low energy electron diffraction patterns of the clean Ni3Al(111), Gr/Ni3Al(111) and Gr/Al2O3 structures. (c) LEEM image of an epitaxial and a rotated Gr flake. The two images, which were obtained using the corresponding first order diffraction beams, map the lateral extent of two distinct Gr single crystals within the same microscopic region of the sample. μ-LEED pattern from the same two single-crystals, as measured by probing an area of diameter 500 nm.
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, presented in Fig. 1a (left). The spectrum 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 intercalate underneath the Gr layer and selectively oxidize the Al atoms at the interface. Fig. 1a (left) shows a set of C1s spectra acquired after different oxygen exposures. We 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 (Fig. 1a, right), 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. We observed a transition from a 3-fold to a 6-fold symmetry which is consistent with the formation of a single-Gr layer on top of Ni3Al. (Fig. 1b). 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 (Fig. 1c) showed that Gr extends continuously across the sample with rather large tiles, each one extending for several microns, nicely confirming that it is not etched upon substrate oxidation. Finally, the epitaxial and rotated flakes exhibits similar characteristics.
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.
This research was conducted by the following research team:
- Luca Omiciuolo, Elisa Miniussi, Fabrizio Orlando and Alessandro Baraldi, University of Trieste, Trieste, Italy
- Eduardo R. Hernández, Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC), Madrid, Spain
- Paolo Lacovig, Silvano Lizzit, Tevfik Onur Menteş and Andrea Locatelli, Elettra-Sincrotrone Trieste, Trieste, Italy
- Rosanna Larciprete, CNR, Institute for Complex Systems, Roma, Italy
- Marco Bianchi, Søren Ulstrup and Philip Hofmann, Aarhus University, Aarhus, Denmark
- Dario Alfè, University College London, London, United Kingdom and CNR-IOM, DEMOCRITOS National Simulation Centre, Trieste, Italy
Contact person:
Alessandro Baraldi:
