What can electrons do

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What can electrons do?

• Jian-Guo Zheng
• Materials Science Engineering
• EPIC/NUANCE Center
• Northwestern University
• 2220 Campus Drive, 1156 Cook Hall
• Evanston, IL 60208-3108, USA
• Phone [phone], Fax [phone]
• E-mail j-zheng3_at_northwestern.edu

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Goals

• This project is to educate students about
  important roles of electron microscopy in
  nanoscience and nanotechnology. It will cover
  some properties of electrons, principles of
  electron microscopy, applications of electron
  microscopy in characterizing nano-scale
  materials, and the development of electron
  microscopy to meet challenges in nanotechnology.
• Electron microscopy normally refers to scanning
  electron microscopy (SEM) and transmission
  electron microscopy (TEM), including tens of
  special techniques. Nano-structured materials
  includes 0 to 3 dimensional nanometer scale
  structures such as nano-particles, nanowires,
  interfaces and multilayers. Electron microscopy
  not only let students see a real nano-word, but
  also provides other varieties of information such
  as composition, crystal structure, and electronic
  structure.
• The project will lead students to the nano-world.
  The knowledge they gain from this project will
  greatly benefit them in their career and future
  life.

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What can electrons do?
outline

• I. Introduction
• II. Electron microscopy TEM and SEM
• III. Seeing with electrons imaging
• IV. More than images
• V. Challenges and opportunities

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I. Introduction
Key modern technologies, for example,
television, computer, flash memory for digital
camera results from the knowledge we have gained
from electrons. It is hardly to think what our
daily life looks like without electrons.
Electrons not only have huge impacts on our
daily life, but also provide us with a key to
explore nano-world which attracted tremendous
attention in recent years. Electrons will play a
key role for us to gain knowledge from the
nano-world and develop new science and technology
(now called nanoscience and nanotechnology).

• I.1 Electrons part of an atom
• I.2 Particle and wave
• I.3 Resolution and wavelength
• I.4 Interaction between electrons and specimen
• I.5 Why electrons?

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I.1 Electrons part of an atom

• The electron is a lightweight fundametal
  subatomic particle that carries a
• negative electric charge (-1.602  10-19 C).
  Its mass is 9.109  10-31 kg, or 1/1836 amu or
  0.510 MeV/c2
• and spin ½.

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The first few Hydrogen Atom electron orbitals
Color-coded probability density (cross-sections)
http//en.wikipedia.org/wiki/Electron
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Some history about electrons

• 1891, G. Johnstone Stoney, coined Electron to
  denote the unit of charge found in experiments
  that passed electric current through chemicals.
• 1894, the word electron derived from the term
  electric force by William Gilbert. Its origin is
  the Greek ??e?t??? (elektron), meaning amber.
• 1897, J.J. Thomson, discovered cathode rays are
  negative charged particles (which he called
  "corpuscles") with very small mass-to-charge
  ratio (over one thousand times smaller than that
  of a charged hydrogen atom). He went further,
  saying that these corpuscles are constituents of
  the atom and these corpuscles are the only
  constituents of the atom.
• 1897 Irish physicist George Francis FitzGerald,
  suggested that Thomson's corpuscles were really
  "free electrons".
• 1911, Ernest Rutherford, suggested that the atom
  might resemble a tiny solar system, with a
  massive, positively charged center circled by
  only a few electrons

JJ Thomson one of his experimental apparatus
http//www.aip.org/history/electron/jjhome.htm
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I.2 Duality Particle and Wave

• Newtonian Mechanics Macro-world
• At the macroscopic scale we are used to two broad
  types of phenomena
• waves and particles.
• Briefly, particles are localized phenomena which
  transport both mass and energy as they move,
  while waves are de-localized phenomena (that is
  they are spread-out in space) which carry energy
  but no mass as they move. Physical objects that
  one can touch are particle-like phenomena (e.g.
  cricket balls), while ripples on a lake (for
  example) are waves (note that there is no net
  transport of water hence no net transport of
  mass).
• Quantum Mechanics very small objects
• In Quantum Mechanics this neat distinction
  between waves and particles is blurred. Entities
  which we would normally think of as particles
  (e.g. electrons) can behave like waves in certain
  situations, while entities which we would
  normally think of as waves (e.g. electromagnetic
  radiation light) can behave like particles.
  (DUALITY)
• Electrons can create wave-like diffraction
  patterns upon passing through narrow slits, just
  like water waves do as they pass through the
  entrance to a harbor.
• Conversely, the photoelectric effect (i.e. the
  absorption of light by electrons in solids) can
  only be explained if the light has a particulate
  nature (leading to the concept of photons).

http//newton.ex.ac.uk/research/qsystems/people/je
nkins/mbody/mbody2.html
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Louis deBroglie hypothesis

• Anything (particles) which had a momentum (p) had
  an intrinsic wavelength (?) inversely
  proportional to it. That is,
• the deBroglie wavelength ? h/p

U ? 10kV 0.0123 nm 100kV 0.00137 nm 200 kV
0.00251 nm 400 kV 0.00164 nm
X-ray Cu ka ?0.154 nm
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I.3 Resolution and wavelength

• The Rayleigh criterion defines the resolution (R)
  of light microscope as
• Rd/20.61?/nsina
• where ? is the wavelength of the radiation, n is
  the refractive index of the view medium and a is
  the semi-angle of collection of the magnifying
  lens.

• 280 nm near ultraviolet wavelength
• 380-420 nm wavelength of violet light
• 420-440 nm wavelength of indigo light
• 440-500 nm wavelength of blue light
• 500-520 nm wavelength of cyan light
• 520-565 nm wavelength of green light
• 565-590 nm wavelength of yellow light
• 590-625 nm wavelength of orange light
• 625-740 nm wavelength of red light

Due to the limitations of the values a, ?, and n,
the resolution limit of a light microscope using
visible light is about 200 nm
The shortest wavelength of visible light is blue
(? 450nm), a for the best lens is about 70
(sina 0.94), and the typical high resolution
lenses are oil immersion lenses (n 1.56) R187
nm
X-ray Cu ka ?0.154 nm
Electron 200 kV, 0.00251 nm
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I.4 Interaction between electrons and specimen
Signals Elastic BSE (EE0) Bragg diffractions
Inelastic Phonon loss (?EltmeV) Plasmon
loss(?Elt50eV) Cathodoluminescence (CL) SEs
(Ese1-10eV) Continuum X-ray (white) Ionization
(Auger, X-ray, core loss)
specimen
Courtesy of Prof. Vinayak P Dravid
The interaction generates a lot of signals which
can be used in scanning electron microscopy (SEM)
and transmission electron microscopy (TEM)
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I.5 Why electrons?
Short Wavelength Ultra-high resolution (down
to single atom) Dynamic interaction with
specimen Multitude of analytical
signals Negative Charge Controllable
trajectory (electro-magnetic lens)