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COLOR-IMAGES WITH THE SCANNING TUNNELING MICROSCOPE



A Scanning Tunneling Micoscope (STM) in principle shows the topography of the
sample being scanned, down to atomic dimensions. But this description is  only a
good appoximation, in reality the electronic structure of the sample plays an
important role in the imaging process. This fact can be used to obtain STM-
images in color, with these images not only being very aesthetical, but with the
color variations representing varying electronic structures of the sample
surface.


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STM: The Basics


Color-Images: Technique





Sample:
Surface of a 10 monolayer thick island of Tb on W(110). The green region is the
clean Tb(0001) surface, while the blue-violet region is an adsorbate induced
reconstruction of the Tb(0001) surface. The adsorbates, possibly CO, result from
impurities from the evaporant, since the degassing (cleaning) process of the Tb
evaporation-source had not been finished as this sample was prepared.
Image-size:
120 nm x 120 nm
Image: topography
Color-information:
three constant current topographical images at:
red = 0.3 V
green = 1 V
blue = 1.8 V








Sample: Crystallites or "islands" of GdFe2, prepared by simultanously
evaporating Gd and Fe onto the W(110) substrate and annealing at 430°C. The left
island is 2.8 nm thick. Among the islands, the substrate is covered with a
monolayer of GdFe2. The GdFe2 monolayer exhibits a   slightly different color
than the GdFe2 of the islands, what means, that the electronic structures are
different. The reason is the monolayer being stressed by the underlying W(110)
substrate, with a continous stress release to the thicker islands.
Image-size::
75 nm x 75 nm
Image:
10% topography+ 90% deviated topography at 0.1 V
Color-information:
three constant current topographical images at:
red = 0.1 V
green = 1 V
blue = 2.1 V







Sample: About 4 monolayers of Tb on W(110), annealed at 330°C. At these
temper-parameters, the islands mainly grow along the substrate steps. Among the
islands, the substrate is covered with a monolayer of Tb. The blue regions are
hydrogen being absorbed on top of the surface. Hydrogen is always present in
small amounts in the ultra-high vacuum chamber.
Image-size::
140 nm x 140 nm
Image:
15% topography+ 85% deviated topography at -0.3 V Color-information:
current imaging tunneling spectroscopy, obtained after the  topography, dI/dU
images at:
red = 0.1 V
green = 1 V
blue = 2.1 V







Sample::
About 0.3 Monolayer of Tb, less than a closed layer, have been evaporated on top
of the W(110) single crystal. The Tb atoms arrange in different, particularely
chain-like superstructures. The varying inter-chain distance alone leads to
different electronic structures of the Tb atoms and therefore different colors
of the superstructures in these images. 
Image-size::
125 nm x 125 nm
Image:
topography
Color-information:
three constant current topographical images at:
red = 0.3 V
green = 0.8 V
blue = 1.2 V







Sample::
Island of Tb on W(110). Again an insufficient degassing process of the evaporant
Tb leads to the colorful, but otherwise unwanted adsorbed contamination at the
rim of the island.

Image-size::
450 nm x 450 nm
Image: 10% topography+ 90% deviated topography at 0.3 V Color-information:
three constant current topographical images at:
red = 0.3 V
green = 1 V
blue = 1.8 V







Sample::
Tb terraces on W(110); during the evaporation process the substrate temperature
was lowered from 350 to 300 °C, resulting in the formation of such step pyramid
like structures, with each step being only one single atomic layer (0.28 nm)
high. Beside hydrogen adsorbtion sites (blue) the terraces exhibit two kind of
regions of slightly different color, yellowish and greenish. The difference is
due to stacking faults that are present in the Tb(0001) surface.
Image-size::
200 nm x 150 nm
Image:
10% topography+ 90% deviated topography at -0.3 V Color-information:
current imaging tunneling spectroscopy, obtained after the  topography, dI/dU
images at:
red = 0.1 V
green = 1 V
blue = 2.1 V







Sample::
Gd- Islands on W(110). In the blue regions hydrogen has been adsorbed on top of
th Gd surface. The many small second monlayer islands seem to be more sensitive
to hydrogen adsorbtion than the thicker ones, since hydrogen has adsorbed
preferentially on them.
The reason that the color-information is dominated by the hydrogen signal in
this image lies in the very high tunneling bias of the three color channels.
Image-size::
150 nm x 150 nm Image: 10% topography+ 90% deviated topography at 0.3 V
Color-information:
current imaging tunneling spectroscopy, obtained after the  topography, dI/dU
images at:
red = 1.2 V
green = 2.0 V
blue = 2.9 V







Sample::
Gd and Fe have been evaporated in relation of 1:1 on top of the W(110) single
crystal, resulting in in the formation of a GdFe2 alloy, visible as the
olive-green regions. The surplus of Gd results in the formation of the typical
Gd on W(110) superstructure, visible as the blue regions.
Image-size::
140 nm x 140 nm
Image: topography
Color-information:
three constant current topographical images at:
red = 0.3 V
green = 0.8 V
blue = 1.2 V














Most of the technical terms are explained in a glossary.

Acknowledgement:
All measurements shown here were obtained during my thesis in the

Scanning Probe Methods Group
at the
Institute of Applied Physics, University of Hamburg