Přednášející: Amadeo L. Vázquez de Parga (Dep. Física de la Materia Condensada, Universidad Autónoma de Madrid Instituto Madrileno de Estudios Avanzados en Nanociencia, Madrid, Spain)
Místo: Cukrovarnicka, knihovna, budova A, 1.p.
Jazyk: anglicky
Pořadatelé:
Oddělení tenkých vrstev a nanostruktur
Ultra perfect graphene monolayers, islands and ribbons can be epitaxially grown on different
single crystal metal surfaces under Ultra High Vacuum conditions. These graphene layers
are spontaneously nanostructured in a periodic array of ripples by the Moiré patterns caused
by the difference in lattice parameter with the different substrates.
We characterize its perfection at the atomic scale by means of Scanning Tunneling
Microscopy (STM) and determine its electronic structure in the real space by local tunnelling
spectroscopy (STS). In-situ STM imaging of graphene monolayers on Ru(0001) reveals
periodic corrugations with 12x12 periodicity. The apparent corrugation depends strongly on
the bias voltage and can even be inverted above +2.6 V, revealing that, in addition to the
geometric corrugation, a much stronger electronic corrugation exists. Moiré patterns have
been observed with STM on different systems and their interpretation, in some cases, is not
straightforward. The main reason is that in STM images the geometric corrugation and the
electronic structure are entangled [1, 2]. Graphene grown on Ru(0001) presents periodic
variations in the electronic structure induced by the chemical interaction between the carbon
atoms and the ruthenium ones [3]. By means of STS we observe inhomogeneities in the
charge distribution along the moiré unit cell [4].
The inhomogeneities in the local surface potential landscape can be explored with
nanometer resolution measuring the Field Emission Resonances (FERs). The STM is
operated in constant current mode and the expected energy position for the FERs is given by
the expression founded by Gundlach some time ago [5]. With that expression it is possible to
determine experimentally the local surface potential along the moiré unit cell. For graphene
on Ru(0001) we have found a difference of the order of 0.25 eV in the work function values
depending on the position on the moiré pattern. The energy position of the first FER presents
strong spatial variations due to the hybridization with a Ru(0001) surface resonance [6].
Finally I will discuss briefly the deposition of electron acceptor molecules on this surface.
[1] B. Borca, S. Barja et al., New J. Phys. 12, 093018 (2010)
[2] D. Stradi, S. Barja et al., Phys. Rev. Lett. 106, 186102 (2011)
[3] A.B. Preobrajenski et al., Phys. Rev. B 78, 073401 (2008)
[4] A.L. Vázquez de Parga, F. Calleja at al., Phys. Rev. Lett. 100, 106802 (2008)
[5] K.H. Gundlach, Solid State Electron. 9, 949 (1966)
[6] B. Borca, S. Barja et al, Phys. Rev. Lett. 105, 036804 (2010)
Copyright © 2008-2010, Fyzikální ústav AV ČR, v. v. i.
