Download Scanning Tunneling Microscopy.

An STM representation of the surface of silicon at
the atomic level
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.
Stylus Profiler (1929 –Schmalz)
Topographiner (1971 –Young)
Was operated in field emission!
STM
An Introduction to Quantum Mechanical
Tunneling
L
L
Quantum mechanics allows a small particle, such as an electron,
to overcome a potential barrier larger than its kinetic energy.
Tunneling is possible because of the wave-like properties
of matter.
Transmission Probability: T ≈ 16ε(1 – ε)e-2κL
The Tunneling Phenomenon
Chen, C.J. In Introduction to Scanning Tunneling Microscopy; Oxford University Press: New York, 1993; p 3.
p2x
 U x  E
2m
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 > U(x), but when E<U(x) the area is forbidden.
 ( x)  U x( x)  E( x)
The quantum mechanical description of the same electron is H
 ikx
, where k 
In the classically allowed region (E>U), there are two solutions,  ( x )   ( 0)e
2 m E  U 

These give the same result as the classical case. However, in the classically forbidden region (E<U) the solution is
 ( x )   ( 0)e x , where  
2 m  U  E

 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.
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.
STM tips may (or may not) be complex
Tips
Cut platinum – iridium wires
Tungsten wire electrochemically etched
Tungsten sharpened with ion milling
Best tips have a point a few
hundred nm wide
Vibration Control
Coiled spring suspension with magnetic damping
Stacked metal plates with dampers between them
Basic Principles of STM
d~6Å
Bias voltage:
mV – V range
Electrons tunnel between the tip and sample, a small current I is
generated (10 pA to 1 nA).
I proportional to e-2κd, I decreases by a factor of 10 when d is
increased by 1 Å.
Two Modes of Scanning
Constant
Height Mode
Constant
Current Mode
Usually, constant current mode is superior.
Instrumental Design: Controlling the Tip
Precise tip control is achieved with
Piezoelectrics
Displacement accurate to ± .05 Å
Raster scanning
Interpreting STM Images
“Topography” model good for large scale
images, but not for the atomic level.
Electron charge density model more
accurate for atomic level images.
Hydrogen on Gadolinium
Best model requires complex
quantum mechanical considerations
Scanning Tunneling Spectroscopy
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.
Graphite is a good example!
• STM images of graphite
Structure of graphite
• Overlay of structure shows only every
other atom is imaged
Advantages
Disadvantages
No damage to the sample
Samples limited to conductors
and semiconductors
Vertical resolution superior to
SEM
Spectroscopy of individual
atoms
Relatively Low Cost
Limited Biological
Applications: AFM
Generally a difficult
technique to perform
Figures of Merit
Maximum Vertical
Resolution: .1 Å
Maximum Lateral
Resolution: 1 Å
Maximum Field of View: 100 μm
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
Interesting Images with STM
Xenon on Nickel
Single atom lithography
Copper Surface
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.
Quantum Corrals
Imaging the standing wave created by interaction of species
Iron on Copper
Carbon Monoxide Man: CO on Platinum
Question:
• At low voltages and temperature the tunneling current is given by:
I  exp( 2 Kd )
2m
K

• 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 ħ 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%.
Answer
For
where
if current is kept to 2%,  = 4eV, then
Very sensitive technique!
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.
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.
Sources
Stroscio, Joseph A.; Kaiser, William J. Scanning Tunneling Microscopy. 1993. Academic Pres
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/Archive/STM-under-pressure.html
“Scanning Tunneling Microscopy.” National Center for Photovoltaics at the National
Renewable Energy Laboratory. http://nrel.gov/measurements/tunnel.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/1986/index.html