Atomic Force Microscopy
imaging and beyond
Arif Mumtaz
Magnetism and Magnetic Materials Group
Department of Physics, QAU
Coworkers:
Prof. Dr. S.K.Hasanain
M. Tariq
Khan Alam
Imaging and beyond
Scanning Probe Microscopy
Imaging Modes
Beyond Imaging
The first AFM
Surface imagistics and topography
The most widespread techniques for surface imaging and morphological
characterization
AFM schematically
Photodiode
Laserdiode
Mirror
AFM-Probe mounted on spring
Spring deflection detection
Sample - Probe displacement
Sample
Feedback Mechanism
Feedback
XYZ Piezo-Scanner
AFM Imaging
Idea : Stay at the same separation by keeping tip-sample
Interactions constant while scanning the sample
z-Position controlled by feedback
xy-Position is scanned
Possible feedback parameters :
Spring deflection : Contact Mode
For vibrating tips : Amplitude of vibration
Contact Mode
forces between probe and
surface
•
Van der Waals force: always present, attractive, outer
electrons, long distance
•
contact force: repulsion, chemical, core electrons
•
capillary force: attractive, water layer!
•
electrostatic and magnetic force
•
friction force
•
forces in liquids
strong repulsive forces in
contact
long range forces:
•
•
•
in air: 10-100 nN
in liquids: 1-100 pN
in UHV: 0.1-10 nN
These long range forces must be
compensated through short
range repulsive forces: risk of
damage for delicate samples!
reduce by lever bending:
Fshort range = Flong range - k zlever
Methods Using Vibrating Tips
Feedback parameter : Amplitude
Advantages :
No permanent tip-sample contact
No shear forces
Non-contact imaging possible
Tapping Mode, Intermittent Contact Mode
And Non-Contact Mode are the
most successful methods for pure imaging
Tapping Mode
Phase imaging
Example of phase imaging
AFM image of PZT-Nickel Ferrite composite
500 nm
Other SPM Techniques
•
•
•
•
MFM – Magnetic Force Microscopy
EFM – Electric Force Microscopy
SCM – Scanning Capacitance Microscopy
Many more…
Magnetic force Microscope
• Special probes are used for MFM. These are
magnetically sensitized by sputter coating
with a ferromagnetic material.
• The cantilever is oscillated near its resonant
frequency (around 100 kHz).
• The tip is oscillated 10’s to 100’s of nm above
the surface
• Gradients in the magnetic forces on the tip
shift the resonant frequency of the cantilever .
• Monitoring this shift, or related changes in
oscillation amplitude or phase, produces a
magnetic force image.
• Many applications for data storage
technology
Magnetic force Microscope
MFM Probe
MFM of Hard Drive
HEIGHT
MFM IMAGE
AFM and MFM image of
PZT-Nickel Ferrite composite
AFM Beyond imaging
• Force Spectroscopy
Example: Phase segregated binary polymer blend
• AFM Electrostatic Nanolithography
Phase segregation in polymers
Segregation at nano scale under certain
conditions
Interest: antireflection coatings, polymer
brushes, nanopatterning and template
formation
Model System:
Polystyrene (PS)--Poly(methylmethacrylate) (PMMA)
Question: identification of structure and the
matrix
Phase segregation in polymers
Height image
Phase image
PS/PMMA blend thin film vacuum annealed at 210oC for 1 hour
2umX2mu image of 90PS/10PMMA
5×5 µm2 images of 80PS/20PMMA
50×50µm2 image of 40PS/60PMMA
50×50µm2 image of 60PS/40PMMA
• Local probing of surface structure and
mechanical properties (elastic modulus,
frictional and adhesive forces, shear
stress, etc.) with a submicron resolution
became possible after the introduction of
atomic force microscopy
Force distance measurements
The approach curve is roughly divided in thee parts
Zero line
No interaction between tip-sample
Snap in
Attractive force > stiffness of tip
Contact line
sample is further pushed against
the tip after contact
Approach
Withdrawal
Hertz Model
The Young modulus determination
from Force-Distance curve
Force vs penetration depth: 80PS/20PMMA
matrix
structure
70
70
Data: Data1_B
Model: Hook's Law
Equation: y = k*x
Weighting:
y
No weighting
60
50
40
2.03145
Chi^2/DoF
= 41.96869
R^2
= 0.81618
k
±0.0327
5.20459
±0.22265
40
Force (nN)
Force (nN)
50
Chi^2/DoF
= 5.72889
R^2
= 0.96686
k
Data: Data1_B
Model: Hook's Law
Equation: y = k*x
Weighting:
y
No weighting
60
30
20
30
20
10
10
0
0
-10
-10
-30
-20
-10
0
10
20
30
Pentration Depth (nm)
(a)
Young Modulus = 2.6 GPa
Bulk Young Modulus PS = 2.8 GPa
-30
-20
-10
0
10
20
30
Pentration Depth (nm)
(b)
Young Modulus = 3.8 GPa
Bulk Young Modulus PMMA = 4.06 GPa
Heating Stage
Electrostatic Nanolithography
Electric field applied
across polymer film
between AFM tip and
conductive back-plane
AFM Tip
Dielectric
liquid
T=Tg
Electric field 108-1010 Vm-1
results in electronic breakdown
through polymer film
T > Tg
Localized Joule heating
occurs in high field
region from current flow
Polymer film
Conductive film
Nanolithography
Conclussions
Considerations for AFM imaging
- a smooth sample surface is required
- Particles must
- be strongly attached to the substrate.
- AFM a probe or a tool?
Thank You
•
•
•
Surface segregation of PMMA can be explained mainly in terms of
two factors, conformational entropy and translational entropy of PS
and PMMA components
Note an amorphous polymer chain in the bulk takes random coil
conformation, its conformation entropy increases with increase in
molecular weight
polymeric chains existing at film surface are compressed along the
direction perpendicular to the film surface. Thus the conformational
entropy of a chain at the film surface is fairly smaller than that in bulk
because conformations break at film surface. Therefore there is
conformational entropy penalty at the surface and this
conformational entropic penalty decreases with decrease in
molecular weight