Surface Analysis
•
surfanalmap

Scanning Probe Microscopy ( STM / AFM )
•All of the techniques are based upon scanning a probe (typically called the tip in STM , since it
literally is a sharp metallic tip) just above a surface whilst monitoring some interaction between
the probe and the surface.
scat7_6a
STM - is the tunnelling current between a metallic tip and a conducting substrate which are in very
close proximity but not actually in physical contact.
AFM - is the van der Waals force between the tip and the surface; this may be either the short
range repulsive force (in contact-mode) or the longer range attractive force (in non-contact mode).

Let us look at the region where the tip approaches the surface in greater detail ....
•The position of the tip with respect to the surface must be very accurately controlled (to within
about 0.1 Å) by moving either the surface or the tip.
•The tip must be very sharp - ideally terminating in just a single atom at its closest point of
approach to the surface.
•
scat7_6b

Zoom x106
•... the end of the tip will almost invariably show a certain amount of structure, with a variety
of crystal facets exposed ...
scat7_6c

Zoom x108
•… and if we now go down to the atomic scale ....
scat7_6d
... there is a reasonable probability of ending up with
a truly atomic tip.

If the tip is biased with respect to the surface by the application of a voltage between them then
electrons can tunnel between the two, provided the separation of the tip and surface is
sufficiently small - this gives rise to a tunnelling current.
The direction of current flow is determined by the polarity of the bias.
If the sample is biased -ve with respect to the tip, then electrons will flow from the surface to
the tip as shown on the left, whilst if the sample is biased +ve with respect to the tip, then
electrons will flow from the tip to the surface as shown on the right.
Bias

In this model, the probability of tunnelling is exponentially-dependent upon the distance of
separation between the tip and surface : the tunnelling current is therefore a very sensitive probe
of this separation.
Model of the tunnelling
scat7_6g
In this model, the probability of tunnelling is exponentially-dependent upon the distance of
separation between the tip and surface : the tunnelling current is therefore a very sensitive probe
of this separation.

Imaging of the surface
1.In constant height mode (in which the tunnelling current is monitored as the tip is scanned
parallel to the surface)
2.In constant current mode (in which the tunnelling current is maintained constant as the tip is
scanned across the surface)
scat7_6h
If the tip is scanned at what is nominally a constant height above the surface, then there is
actually a periodic variation in the separation distance between the tip and surface atoms. At one
point the tip will be directly above a surface atom and the tunnelling current will be large whilst
at other points the tip will be above hollow sites on the surface and the tunnelling current will
be much smaller.

Imaging of the surface
In practice, however, the normal way of imaging the surface is to maintain the tunnelling current
constant whilst the tip is scanned across the surface. This is achieved by adjusting the tip's
height above the surface so that the tunnelling current does not vary with the lateral tip
position. In this mode the tip will move slightly upwards as it passes over a surface atom, and
conversely, slightly in towards the surface as it passes over a hollow.
scat7_6i
The image is then formed by plotting the tip height (strictly, the voltage applied to the z-piezo)
v's the lateral tip position.

Scanning Tunnelling Microscopy
ScanningTunnelingMicroscope_schematic[1]


Now we paint the whole surface uniformly gray and switch on the lights.
We can use several lights at different positions and with different colors.
Instead of painting the surface just gray we can use a color palette and paint it according to
height.
Or we choose the color according to another surface property, let's say curvature.
STM How-to Pictures 5-8 STM How-to Pictures 1-4
A tip is scanned over a surface at a distance of a few atomic diameters in a point-by-point and
line-by-line fashion. At each point the tunneling current between the tip and the surface is
measured. The tunneling current decreases exponentially with increasing distance and thus, through
the use of a feedback loop, the vertical position of the tip can be adjusted to a constant distance
from the surface.
The amount of these adjustments is recorded and defines a grid of values which can be displayed as
a grayscale image.
Instead of assigning the values to a color we can also use them to deform the grid in the direction
perpendicular to the surface.
Now we can bring back the grayscale and paint each square according to an average of the four
defining grid points.

xenon na Ni
Title : The Beginning
Media : Xenon on Nickel (110)

Surface state electrons on Cu(111) were confined to closed structures (corrals) defined by barriers
built from Fe adatoms. The barriers were assembled by individually positioning Fe adatoms using the
tip of a low temperature scanning tunneling microscope (STM). A circular corral of radius 71.3
Angstrom was constructed in this way out of 48 Fe atoms.
ring[1]

Title : Quantum Corral
Media : Iron on Copper (111)
kvantová ohrada

makecirc[1]
Title : The Making of the Circular Corral
Media : Iron on Copper (111)

stm4
Title : Untitled
Media : Cesium & Iodine on Copper (111)
Here we see the artists' first use of color mapping. Color is assigned to the surface not only by
lights but by local curvature of the surface. This helps to delineate the shape of the object
(which, for the curious, is a molecule that they assembled from 8 cesium and 8 iodine atoms).
[Hopkinson, Lutz & Eigler]

stm5
Title : Carbon Monoxide Man
Media : Carbon Monoxide on Platinum (111)

STM Image, 35 nm x 35 nm, of single substitutional Cr impurities (small bumps) in the Fe(001)
surface.
STM image, 35 nm x 35 nm, of single substitutional Cr impurities (small bumps) in the Fe(001)
surface

STM image 7 nm x 7 nm of a single zig-zag chain of Cs atoms (red) on the GaAs(110) surface (blue)
STM image 7 nm x 7 nm of a single zig-zag chain of Cs atoms (red) on the GaAs(110) surface (blue)