Environmental scanning electron microscopy

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
p contrast
• more gas molecules cause
more scattering
p resolution
best image quality
~4 Torr

Temperature working range in ESEM
Cooling stage
Heating stage 1
Heating stage 2
-30 0 30 60 1000 1500
temperature [°C]

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
soot with solved inclusions
undried or even living biogenic
material (e.g. dermal tissue)
mites

RH = 90%
Activation of unsoluble particles
at high relative humidities
soot
RH = 99%
RH = 100%
increasing
relative humidity
water

Deliquescence and efflorescence
drop formation
recrystallization
RH = 80% 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
85
Na 2
SO 4
DRH [%] at 25°C, this study
80
75
70
65
NH 4
NO 3
NaCl
(NH 4
) 2
SO 4
60
60 65 70 75 80 85
DRH [%] at 25°C, references*
*Ebert et al., 2002

Deliquescence und efflorescence of sodium chloride
RH inc., ESEM, D P
> 100 nm, T = 5° RH dec., Jout. et al., D P
= 100 nm T = 23°C
RH dec., ESEM, D P
> 100 nm, T = 5° RH inc., Hämeri et al., D P
= 50 nm, T = 25°C
RH inc., Jout. et al., D P
= 100 nm, T = 23°C RH dec., Hämeri et al., D P
= 50 nm, T = 25°C
2.5
NaCl
growth factor
2.0
1.5
crystallisation
ESEM
drop
formation
crystallisation
Joutsensaari et al.
1.0
20 30 40 50 60 70 80 90 100
relative humitidy [%]

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
SE detector
pole shoe
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
• 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.
• Hygroscopic behaviour (e.g. deliquescnece, efflorescence, activation
ice forming prperties..) of particles can be studied in situ.
• No charging effects ⇒ no coating of non-conductive samples.
• 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).