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Introduction to Scanning Tunneling Microscopy Xiaozhe Zhang 04/24/2014
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Page 1: Introduction to Scanning Tunneling Microscopy · Introduction to Scanning Tunneling Microscopy Xiaozhe Zhang 04/24/2014. ... –Occurs over very short distances. Quantum Tunneling

Introduction to Scanning Tunneling Microscopy

Xiaozhe Zhang

04/24/2014

Page 2: Introduction to Scanning Tunneling Microscopy · Introduction to Scanning Tunneling Microscopy Xiaozhe Zhang 04/24/2014. ... –Occurs over very short distances. Quantum Tunneling

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

Page 3: Introduction to Scanning Tunneling Microscopy · Introduction to Scanning Tunneling Microscopy Xiaozhe Zhang 04/24/2014. ... –Occurs over very short distances. Quantum Tunneling

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

Page 4: Introduction to Scanning Tunneling Microscopy · Introduction to Scanning Tunneling Microscopy Xiaozhe Zhang 04/24/2014. ... –Occurs over very short distances. Quantum Tunneling

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

Page 5: Introduction to Scanning Tunneling Microscopy · Introduction to Scanning Tunneling Microscopy Xiaozhe Zhang 04/24/2014. ... –Occurs over very short distances. Quantum Tunneling

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.

Page 6: Introduction to Scanning Tunneling Microscopy · Introduction to Scanning Tunneling Microscopy Xiaozhe Zhang 04/24/2014. ... –Occurs over very short distances. Quantum Tunneling

Schematic of a Scanning Tunneling Microscope

Page 7: Introduction to Scanning Tunneling Microscopy · Introduction to Scanning Tunneling Microscopy Xiaozhe Zhang 04/24/2014. ... –Occurs over very short distances. Quantum Tunneling

Constant

Height Mode

Constant

Current Mode

Two Modes of Scanning

Usually, constant current mode is superior.

Page 8: Introduction to Scanning Tunneling Microscopy · Introduction to Scanning Tunneling Microscopy Xiaozhe Zhang 04/24/2014. ... –Occurs over very short distances. Quantum Tunneling

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.

Page 9: Introduction to Scanning Tunneling Microscopy · Introduction to Scanning Tunneling Microscopy Xiaozhe Zhang 04/24/2014. ... –Occurs over very short distances. Quantum Tunneling

Surface of platinum.IBM, Almaden Research Facility

Surface of nickel.IBM, Almaden Research Facility

Surface of copper.IBM, Almaden Research Facility

Page 10: Introduction to Scanning Tunneling Microscopy · Introduction to Scanning Tunneling Microscopy Xiaozhe Zhang 04/24/2014. ... –Occurs over very short distances. Quantum Tunneling

Iron on Copper

Quantum Corrals

Imaging the standing wave created by interaction of species

Page 11: Introduction to Scanning Tunneling Microscopy · Introduction to Scanning Tunneling Microscopy Xiaozhe Zhang 04/24/2014. ... –Occurs over very short distances. Quantum Tunneling

The wave effect can be reinforced via constructive interference to create standing waves

Page 12: Introduction to Scanning Tunneling Microscopy · Introduction to Scanning Tunneling Microscopy Xiaozhe Zhang 04/24/2014. ... –Occurs over very short distances. Quantum Tunneling

Image from an STM

Iron atoms on the surface of Cu(111)

Page 13: Introduction to Scanning Tunneling Microscopy · Introduction to Scanning Tunneling Microscopy Xiaozhe Zhang 04/24/2014. ... –Occurs over very short distances. Quantum Tunneling

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

Page 14: Introduction to Scanning Tunneling Microscopy · Introduction to Scanning Tunneling Microscopy Xiaozhe Zhang 04/24/2014. ... –Occurs over very short distances. Quantum Tunneling

Thank you for your time!


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