Fe adatoms on graphene/Ru(0001): Adsorption site and local electronic properties

We have investigated the electronic properties and adsorption site of iron adatoms on monolayer and bilayer graphene on Ru(0001) by means of scanning tunneling microscopy/spectroscopy at 5 K. On monolayer graphene we find that Fe adatoms reside in the valleys and the ripples’ edges within the moiré pattern. We correlate the position of Fe atoms on the edges with geometrical and charge effects. If Fe atoms are adsorbed on bilayer graphene the impurities are attracted by defects and strained regions of graphene. Scanning tunneling spectroscopy reveals inhomogeneous electronic properties of Fe adatoms, depending on their adsorption site.

Figure 1

(a) STM image of monolayer (MLg) and bilayer (BLg) graphene after deposition of a small amount of Fe atoms at 12 K ($U$ $=$ 1 V and $I$ $=$ 0.25 nA). (b) STM topography of 2 Fe atoms located on the edges of ripples. On MLg the apparent height of the Fe adatoms is 0.8 Å at $U=+0.3$ V independent of their adsorption site. (c) The atomic resolution of graphene superimposed on the unit cell of the moiré pattern with visible lattice distortion on the corners of triangular ripples ($U$ $=0.6$ V and $I$ $=0.5$ nA). (d) STM topography of MLg covered by Ni atoms. The Ni atoms are adsorbed on the edges of ripples similarly to Fe adatoms ($U$ $=0.2$ V and $I$ $=0.1$ nA). (e) STS spectra taken on Fe atoms adsorbed on the hills and the valleys. Tunneling parameters are $U$ $=1$ V and $I$ $=0.5$ nA.

Figure 2

(a) STM topography revealing Fe 3D clusters on monolayer graphene (MLg) and Fe 2D islands on the Ru(0001) surface. ($U$ $=$ 1 V and $I$ $=$ 0.5 nA. (b) Tunneling spectra taken on the center of small and big Fe clusters residing in the valleys as indicated by color-coded circles in (a). Tunneling parameters are $U$ $=1$ V and $I$ $=0.5$ nA.

Figure 3

(a) Atomically resolved STM image of a border region between MLg and BLg ($U$ $=0.2$ V and $I$ $=0.5$ nA). (b) The tunneling spectra recorded on defect-free BLg and two different defects as indicated by circles in (a). The STS curves are shifted by offsets of 0.3 arb. units with respect to each other. Tunneling parameters are $U$ $=$ 0.5 V and $I$ $=$ 0.5 nA.

Figure 4

(a) STM topography presenting two different Fe atoms on BLg ($U$ $=0.1$ V and $I$ $=0.25$ nA). (b) The same area as shown in (a) after picking up Fe${}_A$. Picking up of the Fe atom on the apex of the tip results in a double tip ($U$ $=0.2$ V and $I$ $=0.25$ nA). (c) Tunneling spectra of Fe atoms siting on small and big defects. Tunneling parameters are $U$ $=1$ V and $I$ $=0.2$ nA).