Scanning Tunneling Microscopy/Spectroscopy and X-ray Absorption Spectroscopy Studies of Co Adatoms and Nanoislands
on Highly Oriented Pyrolytic Graphite
P. K. 1. Wong*, L. Leonardus, M. P. de long, M. H. Siekman, H. 1. W. Zandvliet and W. G. van der Wiel
MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands
Ferromagnetic metal-organic systems lie at the heart of organic spintronics, which emphasizes active usage and manipulation of carrier spins in carbon-based materials [1,2]. Such systems hold strong promise as spin transport materials because of their potentially weak hyperfine and spin-orbit interactions, thus resulting in long spin lifetimes. In contrast to the commonly adopted lateral field-effect transistor geometry [3], Karpan et al. have put forward a stimulating idea to combine epitaxial Co and/or Ni with multilayer-graphene/graphite, at which interfaces a perfect spin filtering due to the k-vector conservation can be possible [4]. When successful, one could expect a new class of spintronic devices similar to magnetic tunnel junctions, but with a carbon-based molecular interlayer and thus a lower intrinsic resistance area product, which is beneficial for e.g. current-driven magnetization reversal at low drive voltages.
As a very first step to realize this experimentally, we have used a variable-temperature ultra-high vacuum (UHV ) scanning tunneling microscope (STM) to characterize the initial growth of Co deposited in situ bye-beam evaporation in an interconnected custom-made sample preparation chamber on freshly-cleaved highly oriented pyrolytic graphite (HOPG). For the STM imaging, mechanically-cut Pt-Ir tips were used. Statistics of the acquired images revealed that the Co coverages in our study were below 5% for which the Co growth was found to follow the Volmer-Weber mode and to remain at the nucleation-dominated regime, thus allowing us to image the isolated adatoms and nanoislands (mean size � 4 nm) of Co. In the atomic resolution STM image shown in Fig.1, we observed individual Co adatoms occupying the j)-sites of the graphite surface lattice, and nucleation of nanoclusters by filling the hole-sites, thus suggesting that the j)-sites which have higher density of state at and near the Fermi level are stronger adsorption sites for Co. Larger nanoislands were found to be dome-like with a typical height-to-width aspect ratio of �0.15.
It is reported that oxygen contaminants and defects on the graphite surface as well as minute residual gases even in the UHV condition have a profound influence to the intrinsic properties of the nanostructured Co [5]. Accordingly, element-specific X-ray absorption (XAS) spectroscopy acquired in the total electron yield mode at 77 K has been utilized to show that the adsorbed Co islands on HOPG were metallic, as evidenced by the lack of multiplet structure at the Co L2,3
edges in the measured spectra. While the local electronic properties of the Co nanoislands on HOPG were studied by scanning tunneling spectroscopy. All tunneling spectroscopy in this study was carried out by simultaneously collecting current-voltage spectra with the image containing a 16 x 16 pixel grid. The spectra taken on the Co nanoislands again revealed the metallic nature of the magnetic nanostructures. More detailed results will be given in the full manuscript.
978-1-4244-6644-3/10/$26.00 ©2010 IEEE
This work was supported by the European Community's Seventh Framwork Programme (FP712007-2013) under grant agreement 228424 for the MINOTOR project.
Fig.l. Room temperature atomic resolution STM image of Co adatoms adsorbed on HOPG, taken at I = 1.04 nA and
V = 2.19 m V. The Co adatoms occupy preferentially at the fJ-site of graphite lattice.
References
[1] W.J.M. Naber et al., J. Phys. D: Appl. Phys. 40, R205 (2007) [2] VA. Dediu et al., Nature Mater. 8, 707 (2009)
[3] N. Tombros et al., Nature 448, 571 (2007)
[4] VM. Karpan et al., Phys. Rev. Lett. 99, 176602 (2007) [5] T. Kondo et al., J. Phys. Chern. C 112, 15607 (2008)
