3.052 Nanomechanics of

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3.052 Nanomechanics of Materials and
Biomaterials
LECTURE 5 EXPERIMENTAL ASPECTS OF
HIGH-RESOLUTION FORCE SPECTROSCOPY II
Prof. Christine Ortiz DMSE, RM 13-4022 Phone
[phone] Email cortiz_at_mit.edu WWW
http//web.mit.edu/cortiz/www
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A Typical High-Resolution Force Spectroscopy
Technique General Components
I. high-resolution force transducer
computer controls system performs data
acquisition, display, and analysis
d
sample
III. high-resolution displacement control
z
d transducer displacement or deflection z
displacement of sample normal to sample surface
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REVIEW LECTURE 2 Experimental Aspects of
High-Resolution Force Spectroscopy I The
High-Resolution Force Transducer
How do we measure such small forces (i.e. nN or
pN) ? High Resolution Force Sensor or Transducer
that is 1) soft and 2) small
microfabricated cantilever beams and probe tips
deflect in response to an applied force (e.g.
types, dimensions, attachments, material
properties, cantilever beam theory) a force
transducer or sensor can be represented by a
linear elastic, Hookean spring Fkd
ddisplacement at end of cantilever (m) ? we
measure in force spectroscopy experiment Fexterna
l force applied to cantilever (N) ? we calculate
from d kcantilever spring constant 3EI/L3
(N/m) ? we know independently EYoungs (elastic)
modulus of cantilever material (Pa) Imoment of
inertia of cross-sectional area (m4) Lcantilever
length (m) force transducer sensitivity
k?keff force detection limits thermal noise
limitation (model force transducer as a free,
1-D harmonic oscillator) ltFm2gt1/2 ?(k BTk
) ? ltFm 2gt1/2?k

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Cantilever Beam Theory
d
F
0
L
x
d(max)
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Example of a Force Transducer The Cantilever
Beam
(NRL http//stm2.nrl.navy.mil/how-afm/how-afm.h
tml)
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Fundamental Limit of Force Detection
cantilever
d
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Stiffness Requirements for a Force Transducer
Force Sensitivity
FTFFs dTdds
FT,dT
k
Fkd
?
ks
Fsksds
sample surface
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Displacement Detection Optical Lever (Beam)
Deflection Technique
Lateral Force Microscopy (LFM)
VAC-VBD
mirror
B
A
VA B-VCD
C
D
Normal Force Microscopy (NFM)
4-quadrant position sensitive photodiode
laser beam
cantilever
sample
probe tip
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Displacement Detection Optical Lever (Beam)
Deflection Technique
ZERO FORCE
mirror
4-quadrant position sensitive photodiode
laser beam
d0
cantilever
probe tip
REPULSIVE FORCE
ATTRACTIVE FORCE
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Displacement Control How can we move something
one nanometer at a time?
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Poling of Piezoelectric Materials
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Advantages and Disadvantages of Piezoelectic
Materials
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Displacement Control Piezoelectric Tube Scanners
(Digital Instruments JV PZT scanner)
DDD
d
D
-Z
LDL

Z
L
voltage applied
Y
-X
X
electrodes
connecting wires
polarization
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Conversion of z-Displacement Data, z to
Tip-Sample Separation Distance, D
IN-CONTACT REPULSIVE FORCE
IN-CONTACT ZERO FORCE
OUT-OF-CONTACT ATTRACTIVE FORCE
d
z
d
sample
sample
z
piezo
D
piezo
sample
piezo
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Atomic Force Microscope (AFM) General
Components and Their Functions (Binnig, G.
Quate, C. F. Gerber, Ch. Phys. Rev. Lett. 1986,
56 (9), 930-933)
A
B
mirror
laser diode

C
D
cantilever
position sensitive photodetector
d
?10-15
sensor output ? d ? F
sample
probe tip
piezoelectric scanner
computer
z
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Surface Forces Apparatus (Israelachvili, J.N.,
et al. J. Chem. Soc. Faraday Trans. 1978, 74,
975.)
(http//squid.ucsb.edu/sfalab/mark-III.html)
New surface forces apparatus (SFA Mk III) for
measuring the forces between two molecularly
smooth surfaces. Mk III employs four distance
controls instead of three as in Mk II. The four
controls are micrometer, differential micrometer
,different spring and piezoelectric tube. The
mica surfaces are glued to cylindrical support
disks of radius R and positioned in a crossed
cylinder geometry. The lower surface is mounted
on a variable-stiffness double-cantilever
force-measuring spring within the lower chamber
and is connected to the upper (control) chamber
via a Teflon bellows.
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Optical Tweezers (Ashkin, et al. Phys. Rev.
Lett.1985, 54, 1245.)
(http//www.embl-heidelberg.de/CellBiophys/LocalP
robes)
(http//atomsun.harvard.edu/tweezer/2j.jpg)
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Biomembrane Surface Probe
(R. MERKEL, P. NASSOY, A. LEUNG, K.
RITCHIE E. EVANS Nature 397, 50 - 53 (1999))
Vertical Assembly- The epi-illuminated microscope
images the nanoscale positional changes of the
probe microsphere. Light from arc clamp D is
made monochromatic though filter F1 and linearly
polarized through polarizer P1. The light
travels to objective E to reflect from the sample
container and probe microsphere is recollected by
the objective. An analyzer polarizer P2 enhances
image contrast before imaging by camera C and
digitization and analysis by computer A.
Simultaneously computer A using feedback from the
analyzed image controls the high voltage power
supply B that drives piezo element F and hence
controls the probe assembly position above the
sample.
force transducer
pressurized glass pipet
microsphere probe
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Typical Force Versus Distance Curve on a Stiff
Substrate
RAW DATA
CONVERTED DATA
substrate compression
no interaction
repulsive regime
jump-to-contact
Photodiode Sensor Output, s (V)
Force, F (nN)
0
adhesion
kc
attractive regime
0
0
Tip-Sample Separation Distance, D (nm)
z-Piezo Deflection, z (nm)