1. Scanning Probe Microscopy
visualization of
motor coordinates =
± surface coordinates = ±
surface topography
Basic idea
recording of
motor positions
x,y
feedback between
measured interaction
and motor drives
motors moving
the tip up and down
plus laterally
Scanning proceedure
- tip at position x,y is
moved towards the
surface (z-direction)
- interaction is measured
- when prefixed value of
interaction is reached, tip
is stopped and position
x,y,z recorded
-tip is retracted and moved
laterally to next position
(± Dx and/or Dy) z
device measuring tip
surface interaction (force,
tunneling current)
sharp tip
z position at which the
prefixed interaction value
is reached
surface
atoms
Visualization of the x,y,z
positions at which a certain
tunneling current between
the tip and a graphite
surface is reached
(Bristol SPM Group, 2000)

2. Scanning Tunnelling Microscopy
The interaction used in scanning tunnelling microscopy is the current “tunnelling” across the vacuum between a conductive tip and a conductive surface under a certain potential..
tunneling
current
tip
path of tip d
90%
99% of current
Tip - sample geometry in a STM.
The distance between tip and surface
is usually a few Ångstroems.
Two scanning modes are possible:
top: constant height mode
bottom: constant current mode

3. Atomic Force Microscopy (AFM) I
The interaction used in scanning tunnelling microscopy is the force acting between the tip and the sample
Photodetector
Force
Solid State
Laser Diode
Cantilever and Tip A B
intermittant
contact
Amplifier
contact
distance
Sample
non- contact
Register and
control unit
Forces (Van der Waals, repuslive forces)
acting between tip and surface as function
of distance.
AFM set-up. Interaction of the tip with
the surface lead to deviations of the laser
spot in the photodetector. The deviations are
proportional to the surface roughness.

4. Atomic Force Microscopy (AFM) I
Surface analysis
Atomic Force Microscopy (AFM) I
water drop
cantilever
tip
Cantilever with
Si3N4 tip
There are different operating modes
for an AFM. In tapping mode (top) the cantilever oscillates. The interaction with the surface changes the oscillation frequency,which is recorded by the photodetector. In contact mode the tip is in contact with the surface.
Commercial AFM microscope

5. SPM example I AFM images of a calcite surface after 20 and 45 min of
contact with a maleic acid solution
PTCL, University
of Oxford

6. SPM examples II
STM image of a galena (100) surface taken at +200mV sample bias and 1.8 nA tunnel current under oil. Interpretation ? (Eggleston and Hochella,
1992)

7. SPM examples III old position
STM image of a {001} hematite surfaces in contact with a Cr containing solution
The two bright dots on the left image correspond to single Cr atoms. The right image
has been taken 30 seconds later. One of the two Cr atoms has moved by approximately
a lattice distance. Two new Cr atoms entered the field of view (top middle) (courtesy C.
Eggleston)

8. Wulff construction I
Minimization of the total free surface energy determines the equilibrium shape of a crystal. The following sum has to be minimized
The Wulff construction is a geometrical tool to obtain the equilibrium shape of a crytal:
From the origin draw radius vectors ri perpendicular to all possible faces. The length of the vector is proportional to the surface energy of the face. (polar plot (n)).
Draw lines (planes) pi perpendicular through the tip of the radius vectors
The convex envelope of these perpendicular planes describes the crystal shape in equilibrium.
Consequence: The distance of a surface plane from the center of mass of the crystal is proportional to the surface energy of the plane. pi ri

9. Wulff construction II Two examples of the Wulff construction Gamma 3030 60 90
120
210
240
270
300
330
Gamma
Gamma
0.5 1
1.5
Gamma
Two examples of the Wulff construction

10. Wulff construction III
111
100
Anisotropy of surface tensions: The ratio of (hkl) vs. (111) is plotted as function of orientation for different temperature. The distance from the center of the crystal on the SEM image to the the crystal face were taken as surface tension proxies.
SEM image of a lead crystal at 200°C.