ACS Nano, 2012, 6 (11), pp 9551–9558.
Rosanna Larciprete , Soren Ulstrup, Paolo Lacovig, Matteo Dalmiglio, Marco Bianchi,Federico Mazzola, Liv Hornekær, Fabrizio Orlando , Alessandro Baraldi, Philip Hofmann, Silvano Lizzit.
CNR-Institute for Complex Systems, Via Fosso del Cavaliere 100, 00133 Roma, Italy
Department of Physics and Astronomy, Interdisciplinary Nanoscience Center, Aarhus University, 8000 Aarhus C, Denmark.
Sincrotrone Trieste, S.S. 14 Km 163.5, 34149 Trieste, Italy.
Physics Department and Center of Excellence for Nanostructured Materials, University of Trieste, Via Valerio 2, 34127 Trieste, Italy.
IOM-CNR Laboratorio TASC, Area Science Park, S.S.14 Km 163.5, 34149 Trieste, Italy.
Abstract
Using photoemission spectroscopy techniques, we show that oxygen intercalation is achieved on an extended layer of epitaxial graphene on Ir(111), which results in the “lifting” of the graphene layer and in its decoupling from the metal substrate. The oxygen adsorption below graphene proceeds as on clean Ir(111), giving only a slightly higher oxygen coverage. Upon lifting, the C 1s signal shows a downshift in binding energy, due to the charge transfer to graphene from the oxygen-covered metal surface. Moreover, the characteristic spectral signatures of the graphene–substrate interaction in the valence band are removed, and the spectrum of strongly hole-doped, quasi free-standing graphene with a single Dirac cone around the K̅ point is observed. The oxygen can be deintercalated by annealing, and this process takes place at around T = 600 K, in a rather abrupt way. A small amount of carbon atoms is lost, implying that graphene has been etched. After deintercalation graphene restores its interaction with the Ir(111) substrate. Additional intercalation/deintercalation cycles readily occur at lower oxygen doses and temperatures, consistently with an increasingly defective lattice. Our findings demonstrate that oxygen intercalation is an efficient method for fully decoupling an extended layer of graphene from a metal substrate, such as Ir(111). They pave the way for the fundamental research on graphene, where extended, ordered layers of free-standing graphene are important and, due to the stability of the intercalated system in a wide temperature range, also for the advancement of next-generation graphene-based electronics.
Copyright © 2012 American Chemical Society
Additional Information
The mass production of graphene based electronic devices requires the synthesis of high quality, i.e. with low defect concentration, and large area graphene layers. Among the different methods that can be used for graphene preparation, epitaxial growth on transition metal surfaces is one of the most promising techniques to achieve graphene layers of the desired quality. The interaction with the substrate, however, is the major drawback of epitaxial graphene, resulting, even for weakly interacting systems like graphene on Ir(111), in additional features in the electronic structure (replica bands and minigaps) close to the Fermi level. A possible solution is the decoupling of graphene by intercalation of metals, silicon, fluorine or hydrogen in order to restore the characteristic linear band dispersion and the peculiar electronic properties of this material.
Oxygen intercalation appears as a viable route to decouple graphene-metal interfaces, but so far intercalation has been demonstrated only for incomplete graphene layers or islands. Using Synchrotron Radiation X-ray Photoemission Spectroscopy (SR-XPS) (at the SuperESCA beamline of the Elettra synchrotron radiation facility in Italy) and Angular Resolved Photoemission Spectroscopy (ARPES) (at the SGM 3 beamline of the ASTRID synchrotron radiation facility in Denmark), we showed that oxygen intercalation can be achieved on an extended layer of epitaxial graphene on Ir(111), which results in the “lifting” of the graphene layer and in its decoupling from the metal substrate. Oxygen intercalation is obtained by exposing the sample to oxygen pressure in the 10-3 mbar range while keeping the temperature at 520 K. After this treatment the characteristic spectral signatures of the graphene-substrate interaction in the valence band are removed, and the Dirac cone of strongly hole-doped, quasi free-standing graphene with a linear π-band dispersion is observed. The thermal stability of the “lifted” graphene was also tested: temperature programmed fast-XPS measurements point out that abrupt oxygen deintercalation with a slight carbon etching occurs around 600 K. After deintercalation, graphene restores its interaction with the Ir(111) substrate [1].
We made one step further by exploiting the tendency of oxygen and of many adsorbates to intercalate under a graphene layer to promote the chemical synthesis of materials between graphene and its metal substrate. In particular we demonstrated that intercalation of silicon atoms below graphene grown on Ru(0001) and subsequent oxygen exposure at 640 K results in the in situ synthesis of a SiO2 dielectric layer which electrically insulates graphene from its metal substrate [2,3]. The procedure described can be applied in principle to other dielectric materials of high interest for graphene-based nanotechnology applications, opening novel design options for device fabrication.
References
[1] K. Tsakmakidis, “Research Highlights – Graphene levitation”, Nature Materials 12, 3 (2013)
[2] S. Lizzit, R. Larciprete, P. Lacovig, M. Dalmiglio, F. Orlando, A. Baraldi, L. Gammelgaard, L. Barreto, M. Bianchi, E. Perkins, and P. Hofmann, “Transfer-Free Electrical Insulation of Epitaxial Graphene from its Metal Substrate”, Nano Letters 12, 4503-4507 (2012).
[3] F. Pulizzi,”Research Highlights – Graphene: Silica in between”, Nature Nanotech. 7, 613 (2012).
