Optical Microscope; • Scanning Electron Microscope (SEM ...

Lecture 3 Brief Overview of Tra

• Optical Microscope;
• Scanning Electron Micr
• Transmission Electron
• Comparison with scannin

aditional Microscopes

roscope (SEM);
Microscope (TEM);
ng probe microscope (SPM)

General philosophy

Human beings use two kinds
objectives:
Seeing --- through eyes (light
↔ electronic microscope (high
wavelength);
Touching --- through hands (

Converting wavelength (nm) to electron
example, a 500 nm light corresponds to

of means to gauge

t) ↔ optical microscope
her resolution due to short

(probe) ↔ SPM.

energy (eV): 1240.7/λ(nm), for
1240.7/500 = 2.48 eV

Principles of an optical micr

roscope

Development of microscope takes

Robert Hooke’s "Micr

us back to 400 years ago

rographia" (1665)

A hair

r knot

Room

m dusts

Red blo

ood cell

Flu v

virus

Organic nanobelts w

with strong emission

Advantages and Disadvantages

Advantages:
• Direct imaging with no need of sampl

microscopy for real color imaging.
• Fast, and adaptable to all kinds of sam

and to solid sample systems, in any s
• Easy to be integrated with digital cam

analysis.
Disadvantages:
• Low resolution, usually down to only

nanometers, mainly due to the light d

s of Optical Microscope

le pre-treatment, the only
mple systems, from gas, to liquid,
shapes or geometries.
mera systems for data storage and

sub-micron or a few hundreds of
diffraction limit.

Electronic Microscope for highe

• Resolution limit of optical microscopes is d
resolution can be estimated as wavelengt
lens, usually ~ 1.0): for white light, averag
resolution is thus a few hundreds nm.

• Decreasing the wavelength is the way to i
would deal with UV light.

• Electron wave is a unique medium that ca
the electrons into high energy beam (via h
created is far shorter than white light. For
from a 20 kV gun, the wavelength is only 1
corresponding to a resolution limit of λ/2 =
image a species as small as 0.3 Å. Most

er resolution

due to the light diffraction; roughly optical
th λ/2NA (NA is the numerical aperture of
ge wavelength is around 500 nm, the best

improve the resolution, though nobody

an be used in imaging. By accelerating
high voltage), the wavelength thus
example, for an electron beam produced
1240.7/20,000 (eV) = 0.06 nm = 0.6 Å,
= 0.3 Å --- theoretically, it can be used to
atoms are in size of 2-3 Å.

History of Electr

• The first electromagnetic lens was develope

• According to Dennis Gabor, the physicist Leó
build an electron microscope, for which he h

• The German physicist Ernst Ruska and the e
prototype electron microscope in 1931, capa
apparatus was the first demonstration of the
later, in 1933, Ruska built an electron micros
with an optical (light) microscope. Moreover,
Siemens-Schuckertwerke, obtained the pate

• In 1932, Ernst Lubcke of Siemens & Halske
electron microscope, applying concepts desc
Five years later (1937), the firm financed the
and employed Helmut Ruska (Ernst’s brothe
especially with biological specimens. Also in
scanning electron microscope. The first prac
1938, at the University of Toronto, by Eli Fra
Hillier, and Albert Prebus; and Siemens prod
electron microscope (TEM) in 1939. Althoug
capable of two million-power magnification, a
upon Ruska’s prototype.

ronic Microscope

ed in 1926 by Hans Busch.

ó Szilárd tried in 1928 to convince Busch to
had filed a patent.

electrical engineer Max Knoll constructed the
able of four-hundred-power magnification; the
e principles of electron microscopy. Two years
scope that exceeded the resolution attainable
, Reinhold Rudenberg, the scientific director of
ent for the electron microscope in May 1931.

built and obtained images from a prototype
cribed in the Rudenberg patent applications.
e work of Ernst Ruska and Bodo von Borries,
er) to develop applications for the microscope,
1937, Manfred von Ardenne pioneered the
ctical electron microscope was constructed in
anklin Burton and students Cecil Hall, James
duced the first commercial transmission
gh contemporary electron microscopes are
as scientific instruments, they remain based

http://en.wikipedia.org/wiki/Electron_microscope

History of Electr

The Nobel Prize in P

"for his fun
optics, and
electron mi

Ernst Ruska
Born: 25 December 1906, Heidelberg
Died: 27 May 1988, West Berlin, Ger
Affiliation at the time of the award:
Gesellschaft, Berlin, Federal Republic

ronic Microscope

Physics 1986

ndamental work in electron
d for the design of the first
icroscope"

g, Germany
rmany
: Fritz-Haber-Institut der Max-Planck-
c of Germany

What is SEM?

• In scanning electron microscopy (SE
a small probe and is rastered across

• Several interactions with the sample
electrons or photons occur as the ele

• These emitted particles can be collec
yield valuable information about the

• The most immediate result of observ
microscope is that it displays the sha

• The resolution is determined by beam

EM) an electron beam is focused into
s the surface of a specimen.
e that result in the emission of
ectrons penetrate the surface.
cted with the appropriate detector to
material.
vation in the scanning electron
ape of the sample.
m diameter.

Scanning Electron

• Electron microscope follows the same id
electrons instead of light;

• “Lens” here are not the optical materials

Microscope (SEM)

deas of optical microscope, but uses
s (like glass), but electrical field.

Detailed working

diagram of SEM

Basic components of SEM

Electron optics:
1. condenser lens --- focusing the electron bea
2. objective lens --- responsible for the size of
3. electromagnetic coils --- responsible for dri
electron beam.

• Sample and sample holder:
1. size --- centimeters.
2. rotation --- sample can be rotated freely at th
at all directions.
3. conductivity --- required for sample to be m
biological specimens, metal

• Transducers (detectors):
1. scintillation device --- doped glass or plastic
photons when struck
2. semiconductor transducers --- when struck b
generated, thu

M

am to the objective lens.
f electron beam impinging on sample surface
iving the raster scanning by deflecting the

hree dimensions, x, y, z, to achieve imaging
measured. For non-conducting samples, like

llic coating is required.

c target that emits a cascade of visible
k by electrons.
by electrons, electron-hole pairs are
us increasing the conductivity.

Basic signals of SEM

The electrons interact with the atoms at or close
contain information about the sample's surface to
such as electrical conductivity. The types of sign
electrons, back-scattered electrons (BSE), chara
luminescence), specimen current and transmitted

Secondary electrons: they are electrons genera
'secondary' because they are generated by other
radiation can be in the form of ions, electrons, or
exceeding the ionization potential. Secondary ele
SEM with secondary electron imaging or SEI can
sample surface, revealing details less than 1 nm

Back-scattered electrons (BSE): they are beam
sample by elastic scattering. BSE are often used
made from the characteristic X-rays. Because th
related to the atomic number (Z) of the specimen
the distribution of different elements in the sampl

Characteristic X-rays: they are emitted when th
electron from the sample, causing a higher energ
These characteristic X-rays are used to identify t
of elements in the sample.

to sample surface produces signals that
opography, composition, and other properties
nals produced by an SEM include secondary
acteristic X-rays, light (cathodo-
d electrons.

ated as ionization products. They are called
r radiation (the primary radiation). This
r photons with sufficiently high energy, i.e.
ectron detectors are common in all SEMs. A
n produce very high-resolution images of a
m in size.

m electrons that are reflected from the
d in analytical SEM along with the spectra
he intensity of the BSE signal is strongly
n, BSE images can provide information about
le.

he electron beam removes an inner shell
gy electron to fill the shell and release energy.
the composition and measure the abundance

Brief schemes show

wing how SEM works

Brief schemes show

wing how SEM works

Scanning Electron M

Microscope (SEM)

Electron microscope constructed by
Ernst Ruska in 1933

Advantages and Disadvant

Advantages:
• Almost all kinds of samples, conducti

coating needed);
• Based on surface interaction --- no re

sample.
• Imaging at all directions through x-y-z
Disadvantages:
• Low resolution, usually above a few t
• Usually required surface stain-coating

conducting.

tages of SEM

ing and non-conducting (stain
equirement of electron-transparent
z (3D) rotation of sample.
tens of nanometers.
g with metals for electron

Note: Specialized SEM works dir

• Nonconductive specimens tend to charg
and especially in secondary electron im
and other image artifacts. So, for usual
coated with an ultrathin coating of electr
gold.

• However, nonconducting specimens ma
SEM instrumentation such as the "Envir
gun (FEG) SEMs operated at low voltag
(FEG) is capable of producing high prim
size even at low accelerating potentials
(gun) is a tungsten filament cathode.

• At the Physics Department of UU, there
emitter electron source. Feature sizes o
vacuum feature which allows imaging o
samples need no special preparation).

rectly on non-conductive samples

ge when scanned by the electron beam,
maging mode, this causes scanning faults

SEM imaging, the samples must be
rically-conducting material, commonly

ay be imaged uncoated using specialized
ronmental SEM" (ESEM) or field emission
ge, and low vacuum. field emission guns
mary electron brightness and small spot
s. In a regular SEM, the electron source

e is a FEI NanoNova SEM, with a field
of 2nm can be resolved. There is a low
of non-conducting samples (biological

SEM images conducting an

ZnO nano-wires

nd semiconducting materials

Carbon nano-tubes

TEM pr

SEM images of (a) ZnO nanobelts a
through chemical reaction with H2S.

Z. L. W

rinciple Nanobelts

and (b) the ZnS nanobelts converted

Wang, Annual Review of Physical Chemistry, 2004, Vol. 55: 159-196