1. Encased Cantilevers for Gentle Atomic force
Microscopy and low noise Force Spectroscopy In
Liquids
Keep Cantilever Dry Interferometric Detection
Reduce Force Noise Reduce Position Sensing Noise
Dominik Ziegler PhD
Molecular Foundry
Lawrence Berkeley National Lab

2. Imaging Soft Samples
Deformation Limits Resolution
AFM measurements require force between the tip and sample.
This force can cause deformation and damage.

3. Damping of Cantilevers in viscous Media
4k BTk
Fn = = 4k BTb
ω0Qi
Force noise: “Smallest force one can
measure in a given bandwidth”
Viscous damping defines force resolution.
ω0≈150 kHz Fn≈20fN/√Hz
Q≈200
ω0≈50 kHz Fn≈200fN/√Hz
Q≈4
In water AFM cantilevers are about 10
100 µm long, 30 µm wide times less sensitive than in air!
Copyrighted Material dziegler@lbl.gov

4. Damping of Cantilevers in viscous Media
4k BTk
Fn = = 4k BTb
ω0Qi
b = 6πRη
Size of Viscosity
object
ω0≈150 kHz Fn≈20fN/√Hz
Q≈200
ω0≈50 kHz Fn≈200fN/√Hz
Q≈4
In water AFM cantilevers are about 10
100 µm long, 30 µm wide times less sensitive than in air!
Copyrighted Material dziegler@lbl.gov

5. Damping of Cantilevers in viscous Media
4k BTk
Fn = = 4k BTb
ω0Qi
b = 6πRη
Size of Viscosity
object
Force Noise
Fn=23fN/√Hz
Requires special
2um instrument
2 µm
Olympus BioleverFAST
Detection limits size
Viani et. al. JAP v86 p2258 (1999)
Copyrighted Material dziegler@lbl.gov

6. Damping of Cantilevers in viscous Media
4k BTk
Fn = = 4k BTb
ω0Qi
b = 6πRη
Size of Viscosity
object
Keeping the cantilever dry (reduce viscosity).
Copyrighted Material dziegler@lbl.gov

7. Let`s go Scuba Diving!

8. Fabrication of Encased Cantilevers
SiO2 / Parylene
Length of levers can be tuned
short is ideal for high speed imaging
long and soft for contact mode force spectroscopy
Copyrighted Material dziegler@lbl.gov

9. Fabrication of Encased Cantilevers
SiO2 / Parylene
TEM shows that tip radius <10nm is maintained
Copyrighted Material dziegler@lbl.gov

10. Performance in Water
High Q factor and high resonance frequency while submerged in water
Hydrophobic encasement prevents water from entering (>2 days)
Copyrighted Material dziegler@lbl.gov

11. Gentle Imaging OF Lipid BiLayers
Measured height of supported DPPC bilayer on mica
(softest possible tapping mode).
5.34nm
2000
1500
Number of events
1000
500
0
0 2 4 6 8 10 12
Topography [nm]
Height Histograms
“Hydra Levers” by AppNano 12x12 µm2
Cantilever Length: 200 µm
6.32nm
Res. Frequency: 17 kHz
Number of events
3000
2500
2000
1500
1000
500
0
0 5 10 15 20
Topography [nm]
Height Histograms
Encased silicon lever (Mikromasch Thicker measured height
NSC36 A) etched back 110 µm, 12x12 µm2  Less deformation by tip
stiffness of 0.95N/m and frequency
85kHz.
Copyrighted Material dziegler@lbl.gov

12. Contact Mode - Force Spectroscopy
Regular cantilever in liquid
(Silicon, 350 µm long, 1µm thick, 35 µm wide)
Same cantilever with encasement
Encasement also lowers noise in DC
measurements.
worm like chain model
Protein Pulling (unfolding) experiments
Simulation for worm like chain model
At bandwidth of 2kHz noise of the
cantilevers dominates the measurements
Copyrighted Material dziegler@lbl.gov

13. Cantilever with integrated Interferometer
Fabry-Pérot Etalon
(Reflectance Mode)
Destructive interference for gap
size g = n λ/2
Finesse:
Copyrighted Material dziegler@lbl.gov

14. Fabrication Defects Reduce Effective Finesse
White Light Spectroscopy
Finesse ~ 1
Non uniform gap size along
lever!
Finesse ~3
Copyrighted Material dziegler@lbl.gov

15. Optical Setup (Home-Built AFM)
Finesse ~3
Copyrighted Material dziegler@lbl.gov

16. Optical Setup (Home-Built AFM)
k=45N/m
f0=320kHz
70nm
Network of Carbon Nanotubes
Optical Beam Deflection Interferometric Cantilevers,
(Measured before deposition of gold layer on encasement) Home-built AFM
Multimode Bruker ~6fm/ √ Hz! (@ 10.6mW,~770nm)
~25fm/ √ Hz (@ ~1mW,680nm)
Copyrighted Material dziegler@lbl.gov

17. Summary
• Encased cantilevers perform up to •First prototypes of interferometric
10x better in liquid than regular cantilevers outperform position noise in
cantilevers with similar dimensions state-of-the-art commercial AFMs!
Force Noise of 12fN/ √ Hz
Position Noise of 6fm/ √ Hz
• Work in any commercial instruments achieved
with optical beam deflection
•Successfully used for imaging
• Tunable length of cantilever for
different applications
Copyrighted Material dziegler@lbl.gov

18. Acknowledgements
Paul Ashby PhD
See his talk tomorrow 2:00 PM, UU5.02. Breaking the Raster Scan Paradigm.
Interferometric Cantilevers:
Adrian Nievergelt Mechanical Engineering, ETH Zurich, Switzerland
Preparation of Lipid Bilayers:
David Chmielewski LBNL, Berkeley
CNT Samples provided by
Aminy Ostfeld, LBNL, Berkeley
ALL MEMBERS OF NANOFABRICATION AND NANOIMAGING GROUPS
dziegler@lbl.gov
#DE-AC02-05CH11231

19. Acknowledgements
Want to become a beta tester of
encased cantilevers?
Contact us, and become a user of
the Molecular Foundry.
dziegler@lbl.gov www.foundry.lbl.gov