introduction to scanning tunneling microscopy · introduction to scanning tunneling microscopy...
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Introduction to Scanning Tunneling Microscopy
Xiaozhe Zhang
04/24/2014
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Scanning tunneling microscope
• The scanning tunneling microscope (STM) is a type of electron
microscope that shows three-dimensional images of a sample.
• Monitors the electron tunneling current between a probe and
a sample surface
• What is electron tunneling?
– Classical versus quantum mechanical model
– Occurs over very short distances
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Quantum Tunneling
Classically, when an object hits a potential that it
doesn’t have enough energy to pass, it will never go
though that potential wall, it always bounces back.
Classical Wave Function
For Finite Square Well
Potential Where E<V
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Quantum Tunneling
In quantum mechanics when a particle hits a potential
that it doesn’t have enough energy to pass, when inside
the square well, the wave function dies off exponentially.
If the well is short enough, there will be a noticeable
probability of finding the particle on the other side.
Quantum
Wave Function For Finite Square
Well Potential Where E<V
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Quantum tunnelling
Quantum tunnelling through a barrier. The energy of the tunnelled particle is the same but the amplitude is decreased
Quantum tunnelling through a barrier. At the origin (x=0), there is a very high, but narrow potential barrier. A significant tunnelling effect can be seen
An electron wavepacket directed at a potential barrier. Note the dim spot on the right that represents tunnelling electrons.
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Schematic of a Scanning Tunneling Microscope
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Constant
Height Mode
Constant
Current Mode
Two Modes of Scanning
Usually, constant current mode is superior.
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Applications
Due to the remarkable detail and STM can give about the
surface of a material, they are very useful for studying friction,
surface roughness, defects and surface reactions in materials
like catalysts.
STMs are also very important tools in research surrounding
semiconductors and microelectronics.
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Surface of platinum.IBM, Almaden Research Facility
Surface of nickel.IBM, Almaden Research Facility
Surface of copper.IBM, Almaden Research Facility
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Iron on Copper
Quantum Corrals
Imaging the standing wave created by interaction of species
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The wave effect can be reinforced via constructive interference to create standing waves
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Image from an STM
Iron atoms on the surface of Cu(111)
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Advantages and Disadvantages
• No damage to the sample
• Spectroscopy of individual atoms
• Relatively Low Cost
• Samples limited to conductors and semiconductors
• Limited Biological Applications: AFM
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Thank you for your time!