Environmental scanning electron microscopy
a technique that offers unique possibilities for exposure studies
Martin Ebert
and
Stephan Weinbruch
CONTENTS
1. Individual Particle Analysis by SEM
2. Basics of Environmental Scanning Electron Microscopy (ESEM)
3. ESEM applications in exposure studies
Cabability of Scanning Electron Microscopy
-Excellent resolution (~ 1nm)
-Excellent depth of field
-Chemical element identification by EDX
But all samples in the SEM are exposed to high vacuum
⇒ no volatile or nonconductive samples can be investigated
(without sample preparation)
Why do we need high vacuum in a SEM ?
3. Basics of Environmental Scanning Electron Microscopy (ESEM)
Gaseous Secondary Electron Detector (GSE)
• gas molecules are used for
amplification
pÇ Ö contrast Ç
• more gas molecules cause
more scattering
pÇ Ö resolution È
best image quality
~4 Torr
Temperature working range in ESEM
Heating stage 2
Heating stage 1
Cooling stage
- 30
0
30
1000
60
temperature [°C]
1500
Druck u. Temperatur-Arbeitsbereich im ESEM
Druck in Torr
Der mögliche Druck und
Temperatur-Arbeitsbereich
im Esem ermöglicht die
Untersuchung von Wasser
sowohl im flüssigen als
auch im festen Zustand.
4. ESEM applications in exposure studies
4.1. Characterization of volatile and astable aerosol components
At pressures of 1- 10 mbar most of
the volatile components of the
aerosol can be studied in the ESEM
(sulfates, nitrates, organics, liquid
water).
Secondary electron picture
of ammonium nitrate particles
in the ESEM
Characterization of volatile aerosol components
fresh soot in the ESEM
same particle
exposed to high vacuum
Investigation of pollen and spores
a
b
Particles deposited on gelantine covered substrates can directly
be ínvestigated in the ESEM
Investigation of pollen and spores
a
b
c
Spores bursting and degassing under electron bombardement
4.2. Investigation of water containing samples
and interactions with water
mites
soot with solved inclusions
undried or even living biogenic
material (e.g. dermal tissue)
RH = 90%
Activation of unsoluble particles
at high relative humidities
RH = 99%
soot
RH = 100%
increasing
relative humidity
water
Deliquescence and efflorescence
drop formation
RH = 80%
recrystallization
RH = 85%
RH = 60%
Deliquescence and efflorescence of a sodium sulfate particle
DRH at 25°C determined in the ESEM, compared with the
values obtained by other techniques
Na2SO4
DRH [%] at 25°C, this study
85
(NH4)2SO4
80
NaCl
75
70
65
NH4NO3
60
60
65
70
75
80
DRH [%] at 25°C, references*
85
*Ebert et al., 2002
Deliquescence und efflorescence of sodium chloride
RH inc., ESEM,
DP > 100 nm, T = 5°
RH dec., ESEM,
DP > 100 nm, T = 5°
RH inc., Jout. et al., DP = 100 nm, T = 23°C
RH dec., Jout. et al., DP = 100 nm T = 23°C
RH inc., Hämeri et al., DP = 50 nm, T = 25°C
RH dec., Hämeri et al., DP = 50 nm, T = 25°C
2.5
growth factor
NaCl
2.0
crystallisation
ESEM
drop
formation
1.5
crystallisation
Joutsensaari et al.
1.0
20
30
40
50
60
70
relative humitidy [%]
80
90
100
Deliquescence behavior of Ni-containing particles
b) Partial deliquescence
RH 72 %
RH 95 %
4.3. Ice in the ESEM
aerosol particles and ice nucleation in the atmosphere
Homogeneous ice nucleation of a supercooled cloud droplet:
does not start until -38°C !!!
Heterogeneous ice nucleation (induced by special aerosol particles):
was observed already at -5°C !!!
aerosol particles
ice
nuclei
?
CCN
sample chamber of the ESEM with cooling stage
pole shoe
SE detector
sample
peltier
cooling
stage
CCD camera
gaseous SE
detector
water
cooling
Heterogeneous ice nucleation in the ESEM
ice
mica
ice
mica
T = - 5°C
4.4. additional devices:
Micro manipulator
micromanipulator in ESEM
scratching the surface
drop off or pick up
electrical conducting
micro injector (drop off or pick up solution)
CONTEXT
z Additionally
to the capabilities of conventional SEM, ESEM enables
electronmicroscopic analysis at pressures up to 50 Torr (~6700 Pa) and
temperatures between –30 to 1500°C.
Analysis of non conductive, wet, astable and living material becomes
suitable. This enables a more complete picture of the ambient aerosols.
z
Hygroscopic behaviour (e.g. deliquescnece, efflorescence, activation
ice forming prperties..) of particles can be studied in situ.
z
z
No charging effects ⇒ no coating of non-conductive samples.
z
Observation of in-situ processes (SE/BSE/EDX).
The scanning electron microscope can be used as a reaction
chamber, customized for different requirements (cryo-SEM, micromanipulator, microinjector, residual gas analysis).
z