Scanning Tunneling

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Scanning Tunneling Microscopy (STM)
An STM representation of the surface of silicon
at the atomic level
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What is Scanning Tunneling Microscopy?
Allows for the imaging of the surfaces of metals
and semiconductors at the atomic level.
Developed by Gerd Binnig and Heinrich Rohrer at
the IBM Zurich Research Laboratory in 1982.
Binnig
Rohrer
The two shared half of the 1986 Nobel Prize in
physics for developing STM.
STM has fathered a host of new atomic probe
techniques Atomic Force Microscopy, Scanning
Tunneling Spectroscopy, Magnetic Force
Microscopy, Scanning Acoustic Microscopy, etc.
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Stylus Profiler (1929 Schmalz)
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Topographiner (1971 Young)
Was operated in field emission!
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STM
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An Introduction to Quantum Mechanical Tunneling
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The Tunneling Phenomenon
Chen, C.J. In Introduction to Scanning Tunneling
Microscopy Oxford University Press New York,
1993 p 3.
In classical mechanics, the energy of an electron
moving in a potential U(x) can be shown by
The electron has nonzero momentum when E gt U(x),
but when EltU(x) the area is forbidden.
The quantum mechanical description of the same
electron is
In the classically allowed region (EgtU), there
are two solutions,
These give the same result as the classical case.
However, in the classically forbidden region
(EltU) the solution is
k is a decay constant, so the solution dictates
that the wave function decays in the x
direction, and the probability of finding an
electron in the barrier is non-zero.
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Tunneling Energy Diagram
Behm, R.J. Hosler, W. In Chemistry and Physics
of Surfaces VI Vanselow, R., Howe, R., Eds.
Springer Berlin, 1986 p 361.
This diagram shows the bias dependence on
tunneling. Ev is the vacuum level, or the
reference energy level. EF is the Fermi level,
which is the highest occupied level in a metal.
fs is the work function of the sample. The work
function is defined as the amount of energy
needed to remove an electron from the bulk to
the vacuum level. The work function of the tip
is labeled as ft. If the sample bias is
positive, the Fermi level of the sample is less
than that of the tip, so electrons flow towards
the sample. When the sample bias is negative,
the Fermi level of the sample is at a
higher level than that of the tip, so the
electrons travel from the tip to the sample.
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STM tips may (or may not) be complex
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Basic Principles of STM
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Two Modes of Scanning
Constant Height Mode
Constant Current Mode
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Instrumental Design Controlling the Tip
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Interpreting STM Images
Scanning Tunneling Spectroscopy
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Since you are measuring the electronic states,
images of the same surface can vary!
First images were of the Si (111)
reconstruction The images vary depending on the
electronic state of the material/tip.
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Graphite is a good example!

• STM images of graphite

Structure of graphite

• Overlay of structure shows only every other atom
  is imaged

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Applications of STM
Surface Structure Compare to bulk structure
Stuff Physicists Do Semiconductor surface
structure, Nanotechnology, Superconductors, etc.
Metal-catalyzed reactions
Spectroscopy of single atoms
Limited biological applications Atomic Force
Microscopy
Future Developments Improve understanding of
how electronic structure affects tunneling
current, continue to develop STM offshoots
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Interesting Images with STM
Xenon on Nickel Single atom lithography
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Catalytic Processes

• Tunneling current can be used to dissociate
  single O2 Molecules on Pt(111) surfaces.
• After dissociation O atoms are 1-3 lattice
  sites apart.
• Stipe et al, PRL 78 (1997) 4410.

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Quantum Corrals
Imaging the standing wave created by interaction
of species
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Carbon Monoxide Man CO on Platinum
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Question

• At low voltages and temperature the tunneling
  current is given by
• where d is the distance between the tip and
  sample, K is the decay constant, m is the mass of
  an electron, ? is the barrier height and h is
  planks constant. Assume the local barrier height
  is about 4eV. Show the current sensitivity to
  distance between the tip and sample if the
  current is kept within 2.

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Answer
For
where
if current is kept to 2, ? 4eV, then
Very sensitive technique!
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Question

• Bias-dependent STM images can probe the occupied
  and unoccupied states. Here are the STM images of
  GaAs(110)-2x1surface. Images were obtained by
  applying (a) 1.9V (b) -1.9V to the sample wtih
  respect to the tip. The rectangles in the images
  indicate the corresponding position. And it was
  suggested that the filled states are localized on
  the As atoms, while the empty states are
  localized on the Ga atoms. Draw the GaAs(110)-2x1
  surface. and gives a little explanation as well.

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Answer

• When the sample is biased positive, electrons
  from occupied states of the tip tunnel to the
  unoccupied states of the sample, so image (a)
  (see question) represents the Ga states, while
  image (b) (see question) represents As states.
  The position of surface atoms are schemiatically
  shown in picture (c), where small dots indicate
  As atoms and large dots represent Ga atoms.

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Sources
Stroscio, Joseph A. Kaiser, William J. Scanning
Tunneling Microscopy. 1993. Academic Press,
Inc. San Diego.
Golovchenko, JA. Science. 232, p. 48 53.
Pool, Robert. Science. 247, p. 634 636.
Hansma, PK Elings, VB Marti, O Bracker, CE.
Science. 14 October 1988, p. 209 215.
STM Image Gallery. IBM Corporation 1995.
http//www. almaden.ibm.com/vis/stm/gallery .html
A Practical Guide to Scanning Probe Microscopy.
Veeco Metrology Group. http//www.
topometrix.com/spmguide/contents.htm
Preuss, Paul. A Close Look Exploring the
Mystery of the Surface. Science Beat. April
12, 1999. http//www. lbl.gov/Science-Articles/A
rchive/STM-under-pressure.html
Scanning Tunneling Microscopy. National Center
for Photovoltaics at the National Renewable
Energy Laboratory. http//nrel.gov/measurements/t
unnel.html
Scanning Tunneling Microscopy. http//www.
physnet.uni-hamburg. de/home/vms/ pascal/stm.htm
The Nobel Prize in Physics 1986. Nobel e
Museum. http//www. nobel.se/ physics/laureates/1
986/index.html