Altering the properties of graphene on Cu(111) by alkali halides

Defect free, monocrystalline graphene can be obtained by catalytic growth on
transition metal surfaces like Cu(111). [1] On the one hand this bottom-up approach
provides a clean and controllable route. On the other hand, graphene's
remarkable properties are deminished by the binding to the transition metal
substrate. [2]
Intercalation opens new horizons to tailor the properties of absorbed graphene
as desired. By engineering the graphene-metal interface, either free-standing
or doped graphene can be obtained. [3] In addition, new properties like magnetism
can be introduced by intercalation. [4] Understanding and controlling
the process of intercalation is crucial.
Strain may plays an important role on the intercalation. On coalescence of
graphene and alkali halide islands, grain boundaries were formed. The formation
can be opposed by a compressive intrinsic strains that already existed
in a thin-film prior the coalescence. [5] The Moire is a manifestation of the
intrinsic compressive strain in graphene. [6] Alkali halides provide an ideal
toolbox to test this hypothesis. For NaCl the strain is tensile, while in KBr it
is compressive. In addition higher adatom mobility, induced by heating, can
deepen compressive strains. [5]
The intrinsic properties of KBr and graphene opposed the formation of grain
boundaries, promoting further epitaxial growth. In contrast NaCl should have
favoured the formation of grain boundaries.
The effect of alkali halides on graphene investigated by Dynamic Force Microscopy
(DFM) and Kelvin Probe Force Microscopy (KPFM) will be presented
in chapter 3. Indeed, smooth interfaces were found for KBr co-adsorption.
For NaCl the grain boundaries were grown over the graphene edges.
For the first time the work function of graphene on Cu(111) was measured by
KPFM and in perfect agreement to the value predicted by theory. Both alkali
halides were found to induce changes in work function of graphene. While
only a negligible shift was found for NaCl, graphene's work-function became
comparable to free-standing graphene in presence of KBr.
A simple plate capacitor model for the interface of graphene on copper [7]
was applied to calculate the expected contact potential difference. Very good
agreement between the model and the measured CPD was found in the cases
of graphene on copper and KBr-intercalated graphene.
In cooperation with the group of Prof. Clelia Rhigi it was proven by means of
DFT, that intercalation by KBr was favorable over the absorption of KBr on
copper. The first results also revealed a cooperative effect between the surface
and the islands of KBr and graphene.
These observations were complemented by spectroscopic measurements on the
Moire structures of graphene on Cu(111) and of graphene intercalated by a
monolayer of KBr.