Abstract
The Rh/Graphite system was studied by means of STM, UPS and XPS, after
metal deposition on the basal plane of graphite in UHV at room temperature.
The STM images show clearly that Rh grows as three dimensional particles
on the graphite substrate even at coverages far below a monolayer (Volmer Weber
growth mode). Under the experimental conditions employed in this study
no chemical interaction between adsorbate and substrate, e.g., the formation
of carbides or intercalates, could be found. Rhodium atoms exhibit very
high mobility, resulting in a high rate of defect nucleation and step decoration.
Beginning at a coverage of approximately 1.65 monolayers, fractal growth
of Rh islands is observed. The islands are partially built up by spherical
Rh segments. The spherical shape of these segments is consistent with the
observation that Rh had no tendency of wetting the graphite surface and
with its high cohesive energy.
Information on the electronic structure of the Rh clusters was obtained
by analysis of the shape and the binding energy Eb of the photoelectron
lines as a function of the coverage of the metal. We were able to demonstrate
that the binding energy of the core level 3d electrons increases by 0.3±
0.1 eV. This increase is accompanied by a FWHM increase of about 1.5 eV
for the smallest amount of the deposited metal (up to 0.2 ML). We interpret
the core level shift as a consequence of rehybridisation effects (initial
state effect) on one hand and the change in core hole screening as a function
of cluster dimension on the other (final state effect). In UPS we detected
three different Rh signals at 0.75, 3.2 and 4.4 eV below the Fermi level.
By comparison with UV photoelectron spectra of the Rh(111), Rh(100) and
Rh(110) single crystal surfaces it was concluded that the islands and the
round segments are predominantly close packed. The valence electron signals
can be identified with the Rh d band (0.75 eV), an s-d hybrid band (4.4
eV). The emission at 3.2 eV was interpreted as rhodium atoms with essentially
the electronic structure of isolated atoms and not of bulk Rh. In the coverage
region of 0.2 0.4 monolayers the FWHM of the d band is stagnating before
increasing with increasing cluster size. In the same coverage region, the
d band intensity begins to grow, accompanied with the development of the
Fermi-edge. From these data it was calculated that the insulator/metal transition
occurs at cluster sizes of 70-100 Rh atoms.
The investigation of the Rh/Re(0001) system with STM, AES, LEED, XPS
and TDS shows that rhodium grows for the first two monolayers in a layer by layer
fashion (pseudo Frank van-der-Merwe growth mode), followed by a transition
to a three dimensional (or Stranski-Krastanov) growth mode. In LEED experiments,
no ordered layer could be proved neither at T = 300 K nor at T = 900 K.
At low coverages, the homogenous nucleation process dominates, in line
with a the low mobility of the metal atoms resulting from a strong metal surface
interaction. As an indicator for the strength of the adsorbate/substrate
interaction the desorption behaviour of CO was investigated as a function
of rhodium cluster size. The TPD spectra for the highest Rh coverage were
consistent with those of the CO/Rh(111) system but with a reduced desorption
temperature of 113 K or 32.5 kJ/mol (Redhead analysis). The strong interaction
with the substrate and the charge transfer from rhodium to the surface
weakens the 2p*-backbonding and therefore also
the CO-Rh bond. The picture of closed-packed rhodium islands is supported
by the fact that the recombination signal decreased with increasing rhodium
concentration on the surface. This is reminiscent of the CO-desorption
behaviour on the Rh(111) and Rh(100) surfaces. As a consequence of the
packing density the misfit should become greater, which could be the reason
for the beginning Stranski Krastanov growth mode after completion of the
first two layers. At T > 900 K, volume alloying
starts by thermally induced rhodium diffusion into the rhenium bulk. This
was proven by several temper experiments in a temperature region far below
the desorption temperature of rhodium (1849 K for the monolayer and 1822
K for the multilayer). |
