Dr. Bill Miller
Sacramento City College

Scanning Electron Microscopy (SEM) Images
Ant’s Head
Alan Hicklin, Spectral Imaging Facility, UCD

SEM Images
Ant’s Stinger
Alan Hicklin, Spectral Imaging Facility, UCD

SEM Images
Murine mast cells on a Self-Assembled Monolayer on Gold
Jie-Ren Li, Spectral Imaging Facility, UCD

SEM Images
Scanning Electron Micrograph of the Surface of a Kidney Stone
Kempf, E. K.

SEM Images
Single Gold Nanoparticle on Glass Tip
Nanonics Co.

Hitachi S-4100 T SEM

Hitachi S-4100 T SEM

Optical Microscopy eyepiece objective lens sample condenser lens light collector light source

Optical Microscopy light source light collector condenser lens sample objective lens eyepiece

Optical Microscopy vs. SEM
Iowa State SEM page

Optical Microscopy vs. SEM
Light Source is replaced by an electron source: Why electrons?
1. Visible light has a wavelength of ___________ nm = ___________ m.
 
2. Electrons have a wavelength that depends on their ___________. h mv h = Planck’s constant = 6.626 x 10 –34 J•s m = mass of an electron = 9.1 x 10 –28 g = 9.1x 10 –31 kg v = velocity of an electron = 20% of the speed of light
= 6.0 x 10 7 m/s
3.

= 1.2 x 10 –11 m = 0.012 nm !!

Optical Microscopy vs. SEM
How is the electron beam created?
Method 1: Heating a tungsten filament
A. Tungsten has a high melting point and a low vapor pressure.
T m
= 3695 K
B. Apply a voltage/current to the
Tungsten to heat it up (similar to an incandescent light bulb).
C. At >2500 K, tungsten will emit electrons (and light and heat).
D. Typical operating T=2800 K.

Optical Microscopy vs. SEM
How is the electron beam created?
Method 2: Cold Field Emission Gun
(Cold FEG)
A. An extraction voltage is applied to a sharp Tungsten tip.
B. Operating temperature is 300 K.
C. Electrons are preferentially extracted from the very tip of a metal towards another positively charged metal.
D. Requires “flashing” of tip.

Optical Microscopy vs. SEM
How is the electron beam created?
Method 2: Cold Field Emission Gun
(Cold FEG)
A. An extraction voltage is applied to a sharp Tungsten tip.
B. Operating temperature is 300 K.
C. Electrons are preferentially extracted from the very tip of a metal towards another positively charged metal.
D. Requires “flashing” of tip.

Comparison of Electron Guns

Optical Microscopy vs. SEM
How are the electrons accelerated?
With an accelerating voltage!
The anode is a positively charged plate that attracts the electrons from the tip. Those that miss the anode continue past it at a higher speed.
Accelerating voltages range from 500 v to 30 kV.
The higher the voltage, the faster the electrons!

Optical Microscopy vs. SEM
How are the electrons focused?
Using a “magnetic lens”: wire,
By passing a current through a a magnetic field is created.
This magnetic field “bends” the path of an electron.

Optical Microscopy vs. SEM
How is the image created?

Optical Microscopy vs. SEM
How is the image created?
Optical Microscopy:
Lenses expand and focus image
“Detector” records entire image at once

Optical Microscopy vs. SEM
How is the image created?
Magnetic lenses focus the electron beam to a point.
Scanning coils “raster” this point across the surface.
As the electrons “smash” into the surface, there are three types of “interactions”
1. Secondary electrons are ejected and ...
2. … X-rays are emitted.
3. Backscattered electrons are ejected.
Each type of radiation tells us something about the sample.

SEM Sample Preparation
SEM is conducted under high vacuum of 1 x 10 –5 mbar or less.
For Cold FEG, use < 1 x 10 –10 mbar
1.013 bar = 1 atm = 760 torr
This is so that the electrons don’t hit gas particles.
At these pressures all water immediately evaporates, so samples must be dry.
Biological samples must be freeze dried to keep their shape and structures.

SEM Sample Preparation
The emission current in a tungsten filament SEM is typically 200 µA, which is roughly 10 13 electrons per second.
SEM samples are typically coated in gold to conduct these electrons away from the surface of the sample so that the sample does not become charged.
For cold FEG, the amount of current going into the sample is 20 times less
(and, of course, it’s more complicated than this) but: little to no charging of surface = no gold coating necessary!

Secondary Electrons y‐axis secondary   electron electron   from   gun nucleus n=1 n=2 n=3 x‐axis
Type   of   scattering:   ______________
Typical   Kinetic   Energy   (KE)   of   a   secondary   electron:    ______________
What   secondary   electrons   tells   us   about   the   sample:   ______________

Secondary Electron Image
Ant’s Head
Alan Hicklin, Spectral Imaging Facility, UCD

X-ray Emission y‐axis secondary   electron electron   from   gun nucleus n=1 n=2 n=3 x‐axis
Typical   Wavelength   of   X ‐ ray:   ______________
What   X ‐ rays   tell   us   about   the   sample:   ______________

SE and X-ray Emission Analysis
3.7
keV/photon   =   6.0
x   10 –16 J/photon
  hc

E
(6.626

10

34
J
 s)(3.0

10

8 m / s)
6.0

10

16
J

3.3

10

10 m
Kidney Stone
Digital Microscopy Facility, Mount   Allison   University

SE and X-ray Emission Analysis
Kidney Stone
Digital Microscopy Facility, Mount   Allison   University

Back Scattered Electrons electron from   gun
y‐axis nucleus n=1 n=2 n=3 x‐axis
Type   of   scattering:   ______________
Typical   Kinetic   Energy   (KE)   of   electron:    ______________
What   backscattered   electrons   tell   us   about   the   sample:   ______________

SE and Back Scattered Electron (BSE) Image
SE   Image
Iron Particle on Carbon
ETH-Zurich
BSE   Image

Our Project
In collaboration with
Professor Gang-yu Liu (Department of Chemistry) and
The Spectral Imaging Facility
At UC Davis
Research Experiment
Accomplish 2 Main Goals:
1. Scientifically: Study Human Hair Samples using Field Emission SEM
A. Field Emission SEM allows imaging of hair samples in their natural state
B. Previous studies have established a general relationship between ethnicity and diameter of hair for gold-coated samples. We hope to quantify this relationship using uncoated hair samples and image analysis software.
C. Further studies potentially looking at pollen samples, clay samples, bat hair samples and many other samples.

Our Project
In collaboration with
Professor Gang-yu Liu (Department of Chemistry) and
The Spectral Imaging Facility At UC Davis
Alan Hicklin, Staff Scientist
Research Experiment
Accomplish 2 Main Goals:
2. Educationally: Involve community college students in all aspects of this project
A. Learning about SEM
B. Participating in this study by donating a hair sample and receiving back an SEM image of your hair including diameter analysis.
C. Participate in this study by analyzing hair samples for their diameter on your computer.
D. Participate in this study by completing a second seminar on the specifics of using the Hitachi S-4100T Field Emission SEM. Then, accompany me to
UC Davis to prepare and analyze your own hair sample.
E. Participate in this study as a $500 paid intern: includes all of the above many times over and helping to coordinate the project.

Preliminary Results

Preliminary Results
75 µm
Miller

Preliminary Results hair not cleaned with soap
Miller damaged hair
Miller

Preliminary Results
58 µm
Trotter
99 µm
Sanchez

Preliminary Results
303 µm cat whisker different scale
55 µm dog fur

Preliminary Results cat whisker close-up

Preliminary Results pollen grain

Conclusions/Summary
Scanning Electron Microscopy
1. The SEM uses a beam of electrons scanned across the surface of a sample.
2. Collecting secondary electrons produces a 3-D image of the surface.
3. Collecting x-rays allows an elemental analysis of the surface.
4. Collecting back scattered electrons allow an elemental mapping of the surface.
Research Project
1. Hair samples have been imaged using a Field Emission SEM without any conductive coating on the sample.
2. These hair images have a variety of thicknesses.
3. There are opportunities for student involvement in this project.

Acknowledgements
Thank you to Gang-yu Liu and Sacramento City College for support.
Thank you to Alan Hicklin and Jie-Ren Li for scientific assistance.
Thank you to Scott Trotter, Eric Sanchez, Ceanne Brunton and Tam Le for assistance in acquiring images of their hairs. http://wserver.scc.losrios.edu/~millerb/sem.htm