A Talk On

ELECTRON TOMOGRAPHY

Imaging by sections or sectioning, through the use of any kind of penetrating wave

• Tomograph

• Tomogram

• Tomographic reconstruction

Superposition of free tomographic cross sections S1 and S2 compared with the projected image P.

Physical phenomenon Type of tomogram

X-rays CT hyperlink\ct.docx

Radio-frequency waves MRI hyperlink\mri.docx

Muons Muon tomography hyperlink\mt.docx

Magnetic particles Magnetic particle imaging hyperlink\mpi.docx

Gamma rays SPECT Electron-position annihilation PET hyperlink\pet.docx

archaeology Geophysics Oceanography Radiology Biology

Materials science Astrophysics Quantum Information and other sciences…

ELECTRON TOMOGRAPHY

Wave nature of electrons

λ=h/mvλ=h/mv

½ mv½ mv22 =eV =eV

λλ=h/=h/√2meV√2meV

Electron beam hits the sample

Producing electron and photon signals

Collected by detectors and converted to a voltage and amplified.

Applied to grid of CRT

Image is formed

Figures of Merit-higher resolving power~0.1nm

-higher magnification~10,00,000X

-higher depth of field

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.

The two shared half of the 1986 Nobel Prize in physics for developing STM.

Binnig Rohrer

In classical physics e flows are not possible without a direct connection

On an atomic scale a quantum mechanical particle behaves in its wave function.

An electron will “jump” from one surface to the other of lower potential.

How tunnelling works?????

"... I think I can safely say that nobody understands Quantum Mechanics"Richard P. Feynman

L

Basic Principles of STM

Electrons tunnel between the tip and sample, a small current I is generated (10 pA to 1 nA).

d ~ 6 Å Bias voltage:mV – V range

Transmission Probability: T ≈ 16ε(1 – ε)e-2κL

Two Modes of Scanning

Constant Height Mode

Constant Current Mode

Usually, constant current mode is superior.

Instrumental Design: Controlling the Tip

Raster scanning

Precise tip control is achieved with Piezoelectrics

Displacement accurate to ± .05 Å

Raster the tip across the surface

The tip-surface separation is controlled to be constant by keeping the tunneling current at a constant value.

The voltage necessary to keep the tip at a constant separation is used to produce a computer image of the surface.

AdvantagesNo damage to the sample

Vertical resolution superior to SEM

Spectroscopy of individual atoms

Relatively Low Cost

DisadvantagesSamples limited to conductors and semiconductors

Limited Biological Applications

Generally a difficult technique to perform

Figures of Merit

Maximum Field of View: 100 μm

Maximum Lateral Resolution: 1 Å

Maximum Vertical Resolution: .1 Å

An STM representation of the surface of silicon at the atomic level

Copper Surface

Iron on Copper

Iron on Copper

Carbon Monoxide Man: CO on Platinum

Xenon on Nickel

Wikipedia

Google

Scanning Tunneling Microscopy.” National Center for Photovoltaic at the National Renewable Energy Laboratory. http://nrel.gov/measurements/tunnel.html

“The Nobel Prize in Physics 1986.” Nobel e Museum. http://www. nobel.se/physics/laureates/1986/index.html

STM Image Gallery. http://www.almaden.ibm.com/vis/stm/gallery.html