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The Powerful Diversity of the AFM
Probe
Stefan B. Kaemmer, Bruker Nano Surfaces Division, Santa Barbara, CA 93117
stefan.kaemmer@bruker-nano.com
March 21, 2011

Introduction
The tip allows us to measure a quantity. This quantity is based
on what interaction the selected tip is sensitive to.
• Surface topography
• Molecular forces
(Pulling, molecular recognition)
• Nanomechanical information
(PeakForce QNM)
• Electrical information (CAFM,
SCM, SPoM,…)
• Optical information
(Raman, IR, Fluorescence)
• Thermal information
(SThM, nTA)
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What do we cover in this talk?
In order to get quality information one needs the right
tools:
• A high performance AFM,
• The correct tip
600 nm height image of Lambda DNA
adsorbed onto a mica surface.
(TappingMode TM in fluid.)
500 nm phase image of E. coli S-layer
membranes exhibiting the
characteristic 14nm lattice periodicity.
(TappingMode TM in fluid.)
45 mm
180 mm image of Human
endothelial cells captured at 2k x
2k pixel resolution. (Contact
mode in fluid)
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Cantilever and beam bounce setup
Amplification: B = 3s/l but ultimate sensitivity is independent of l and
proportional to 1/s
l: length of cantilever
S: distance between detector and cantilever
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Where to find information about tips?
www.BrukerAFMprobes.com
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Probe selection
Imaging
Environment
AFM Mode
Sample Type Probe Family/Model Liquid Air Tapping Contact Force
Curves
Biomolecules (nucleic
acids, proteins, lipids,
carbohydrates, etc)
Silicon OTESPA - X X - -
RTESP - X X - -
TESP - X X - -
Biolever X - - - X
Etched Silicon Cantilever
5-10 nm
Biomolecules (nucleic
acids, proteins, lipids,
carbohydrates, etc)
Silicon
Nitride
SNL X - X X X
MSNL X - X X X
NP-STT X - X X -
Cells Silicon Biolever X - - X X
Cells
Silicon
Nitride
DNP X - X X X
MLCT X - X X X
Tissues Silicon TESP - X X - -
Silicon Nitride Cantilever
10 - 40 nm
Tissues
Silicon
Nitride
DNP X - X X X
MLCT X - X X X
SNL X - X X X
MSNL X - X X X
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Example: Force measurements
AFM is used for force measurements in pN (10 -12 N) range
Approach
Retract
z
• Hooke‘s law shows us that the force measured is directly
proportional to the cantilever spring constant
• So the solution is easy: Just make a super soft cantilever
and have a go. Or not?
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Example: Force measurements
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Example: Force measurements
J Hutter, J Bechhoefer, Rev. Sci. Instrum. 64 (1993)
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Example: Force measurements
Example: OBL lever ―Biolever‖ (from BrukerAFMprobes.com)
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Tip functionalization
Measure the specific interaction
between a molecule attached to the tip apex
(A) and another one attached to a support
(from atomically flat support to living cells) in
(most of the time) a liquid environment.
A
A
A
A
A
A
A A
A
A
A
A
A A A A A A A A
B B B B B B B B B B B
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Molecular Recognition Mapping
Malaria-Infected Erythrocytes
AFM Probe functionalized
with endothelial surface
receptor CD36.
• Malaria-infected RBCs (IE‘s) show
different shape and appearance of
knob-like surface structures.
• ‗Knobs‘ believed involved in adherence
to endothelial cells
Imaging of IE‘s with CD36 probe showed
adhesion sites mapped to ‗knob-like‘
structures.
Li et al. WCB 2010.
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Mechanical Property Mapping – Live and
dead cells
PeakForce deformation channel
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Probe selection
Imaging
Environment
AFM Mode
Sample Type Probe Family/Model Liquid Air Tapping Contact Force
Curves
Biomolecules (nucleic
acids, proteins, lipids,
carbohydrates, etc)
Silicon OTESPA - X X - -
RTESP - X X - -
TESP - X X - -
Biolever X - - - X
Etched Silicon Cantilever
5-10 nm
Biomolecules (nucleic
acids, proteins, lipids,
carbohydrates, etc)
Silicon
Nitride
SNL X - X X X
MSNL X - X X X
NP-STT X - X X -
Cells Silicon Biolever X - - X X
Cells
Silicon
Nitride
DNP X - X X X
MLCT X - X X X
Tissues Silicon TESP - X X - -
Silicon Nitride Cantilever
10 - 40 nm
Tissues
Silicon
Nitride
DNP X - X X X
MLCT X - X X X
SNL X - X X X
MSNL X - X X X
• Resonance frequency of probes in fluid drops to 1/2 -1/3 of the resonance frequency of the probe in air.
• Resonance frequency in fluid easily identified through use of thermal tune.
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Tapping Mode
Newtons 2 nd law of motion
Effective resonance
frequency
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Tapping Mode
Tapping Mode
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Lateral resolution
• Smallest features that can be resolved
• In optics it is determined by the spot size of the focused beam, in
SPM by tip size and tip-sample distance
Tip size -> What does the sample actually see of the tip? E.g. STM
resolution is restricted to area of tip at which current changes less than a
magnitude (CJ Chen. Intro to STM 1993) In force microscopy we have to
look at the change in tip-sample interaction forces to define the lateral
resolution.
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Why do we actually want to stay close to
the surface?
Lets take the example of electrostatic measurements.
z
r
s
x
q
Grounded tip away from a point
charge q. The force on the tip will
be F=f(x) with F max at x=0
The force is a function of F(x,s,r). One
can show that:
Force
DLat
DLat is:
• proportional to sqrt(r)
• directly proportional to the tip-sample
distance s.
q1
q2
This is why you want to be close to the
surface and not far away like in noncontact
for the highest lateral resolution.
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What tip do we need?
We want to stay close but at the same time want to avoid the jump to
contact.
F Spring
s
F Total = F Spring - F Ext
F Total (s)=0
s
F Ext
s=0
time
So we need a tip with a high enough spring constant to avoid the
instability
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Resonance versus Sub-Resonance Tapping
Cantilever
Response
F ~ s
k
Solution: k 0.1~ 0.4 N/m
F
~
A
Q
s
~ f
Q
Frequency
k
A0
( A0 As ) k
w ~
Q
w10x10 -18 Joule/cycle,
for a tip with R~10 nm
0
Bruker NanoSurfaces Division
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Peak Force Tapping
Trajectory of the tip
1 nN
approaching
withdraw
van der Waals
Peak tapping force
Time
TESP (42 N/m) on Si, MM8
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Resolution & Force Control
C 60 H 122 Height
Stress = 1.27 GPa
1 nN
C 36 H 74 Height
Dia 1 nm
C 18 H 38 Height
80x80 nm
500x500 nm
10pN—1uN
80x80 nm
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True atomic resolution - Gibbsite in water
1 x 1 um 2 topography image of
gibbsite platelets on mica substrate
Tapping mode topography (left) and phase
image (right) of a gibbsite surface in pure water.
Data taken on regular MultiMode-AFM using Fastscan B cantilever. Courtesy
of F. Mugele and D. Ebeling, Univ. of Twente/NL.
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True atomic resolution - Mica in water
Data taken on regular MultiMode-AFM. Courtesy of F. Mugele and D. Ebeling,
Univ. of Twente/NL.
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Conclusion
• Choosing the right cantilever for the job will unlock the full potential of
your AFM
• It is beneficial to make some rough estimates on what can be achieved
with a given cantilever
• Peak Force Tapping achieves extremely high resolution data in air due to
superior force control and the ability to use soft cantilevers
• By working in liquids Tapping Mode using small amplitudes can produce
true atomic resolution data
• High solution imaging: the sensitivity and even noise have been
sufficient for a decade with Multimode IIIa
I would like to thank my colleagues Andrea Slade, Steven Minne, James
Shaw, and Chanmin Su for helpful discussions
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