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Atomic Force Microscopy (AFM)Atomic Force Microscopy (AFM)
Arvind Raman, Associate ProfessorMechanical Engineering
Birck Nanotechnology CenterNASA Institute of Nanoelectronics and Computation (I
NAC)
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Further readingFurther reading
§ J. Gomez-Herrero, and R. Reifenberger, “Scanning Probe Microscopy”, to appear in Encyclopaedia of Condensed Matter Physics, edited by F. Bassani, J. Leidl, and P. Wyder, Elsevier Science Ltd., 2004.
§ D. Sarid, Scanning Force Microscopy with Applications to Electric, Magneticand Atomic Forces, Revised Edition, Oxford University Press, 1994.
§ U. Dürig, “Interaction sensing in dynamic force microscopy”, New Journal of Physics, Vol. 2, pp. 5.1-5.12, 2000.
§ F. Giessibl, “Advances in atomic force microscopy”, Reviews of Modern Physics, Vol. 75, pp. 949-983, 2003.
§ R. García, R. Pérez, “Dynamic atomic force microscopy methods”, Surface Science Reports, Vol. 47, pp. 197-301, 2002.
§ B. Cappella, G. Dietler, “Force-distance curves by atomic force microscopy”Surface Science Reports, Vol. 34, pp. 1-104, 1999.
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OutlineOutline
§ History of Atomic Force Microscopy (AFM)§ Instrumentation§ Static force-distance curves and force
spectroscopy§ Dynamic AFM and force gradient spectroscopy§ Imaging§ Applications and emerging areas
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§ Binnig, Gerber, Rohrer, Wiebel (1982)§ Binnig and Rohrer awarded Nobel Prize in Physics in 1986 for STM§ If |Vt| is small compared to workfunction , and tunneling current is given
by where z is the gap I0 is a function of the applied voltage and the density of states in the tip and the sample, and
§ For most metals, Φ˜ 4eV, so that κt=1Å-1
§ Most current carried by “front atom”blunt tips , so atomic resolution possible even with relatively blunt tips
§ Only electrically conductive samples, restricting its principal use to metals and semi-conductors
The starting point- STMThe starting point- STM
Φ2
0( ) t ztI z I e κ−=
2 /t mκ = Φ h
F. Giessibl’sRev. Mod. Phys.
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The AFMThe AFMG. Binnig, C. F. Quate and Ch. Gerber, PRL 56, 930 (1986)
§ Binnig invented the AFM in 1986, and while Binnig and Gerber were on a Sabbatical in IBM Almaden they collaborated with Cal Quate (Stanford) to produce the first working prototype in 1986
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G. Binnig, C. F. Quate and Ch. Gerber, PRL 56, 930 (1986)
Early AFM ImagesEarly AFM Images
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OutlineOutline
§ History of Atomic Force Microscopy (AFM)§ Instrumentation§ Static force-distance curves and force
spectroscopy§ Dynamic AFM and force gradient spectroscopy§ Imaging§ Applications and emerging areas
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The microcantilever – the force sensorThe microcantilever – the force sensor
www.olympus.co.jp
www.nanosensors.com
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Detecting deflectionDetecting deflection
§STM tip
§Capacitance/laser interferometry
§Beam deflection
Courtesy- J. Gomez, UAM, Spain
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Photodiode
Laser
The beam deflection methodThe beam deflection method
a) Normal force
Up
Down
b)Lateral Force
Right
leftA+B= UP
C+D=DOWN
A+C= LEFT
B+D=Right
Courtesy- J. Gomez-Herrero, UAM, Spain
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AFM Block DiagramAFM Block Diagram
Personal Computer
SPM Signals
HV Amplifiers and signal conditioning
SPM tip
Signaldetector
Piezoelectricscanner
SFM 3 dimensional image of a tumor cell HeLa (37x37µm2 )
Digital SignalProcessor
Courtesy- J. Gomez-Herrero, UAM, Spain
z dither piezo
xy
z
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OutlineOutline
§ History of Atomic Force Microscopy (AFM)§ Instrumentation§ Static force-distance curves and force
spectroscopy§ Dynamic AFM and force gradient spectroscopy§ Imaging§ Applications and emerging areas
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§ Long-range electrostatic and magnetic forces (upto 100 nm)
§ Capillary forces (few nm)§ Van der Waals forces (few nm) that are
fundamentally quantum mechanical (electrodynamic) in nature
§ Casimir forces§ Short-range chemical forces (fraction of
nm) § Contact forces § Electrostastic double-layer forces§ Solvation forces§ Nonconservative forces (Dürig (2003))
Tip-sample gap
Tip-sample interaction force
Attractive
Repulsive
Nanosensors Gmbh
Tip-sample interaction forces in AFMTip-sample interaction forces in AFM
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The microcantilever – the force sensorThe microcantilever – the force sensor
§ From elementary beam theory, if E=Young’s modulus, I=bh3/12 then
§ δ=w(L)=F L3/(3EI), and θ=dw(L)/dx=FL2/(2EI)
§ Deflection and slope linearly proportional to force sensed at the tip§ k=3EI/L3 is called the bending stiffness of the
cantilever
www.olympus.co.jp
F
L
b h δθ
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d
Force-displacement curvesForce-displacement curves
Z
δkδ
F(d)
F(d)=kδ
d
F(d)
Z
k
kδ
Z
δ
WAdhesion=blue shaded area above
1
WCantilever=shaded area above
13 3
Inaccessible region
Snap-in
2
2’2
2’
4’
4
Pull-off
4
4’
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Force spectroscopy Force spectroscopy
§ Three distinct regions§ If k is known then from the static-force distance curve, F(d)
can be calculated for all d except for inaccesible range near snap-in§ It can be shown that WCantilever is related to the WAdhesion
§ Slope in III is good measure of repulsive forces (local elasticity)
kδ
Z
IIIIII
Animation courtesy J. Gomez-Herrero, UAM, Spain
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OutlineOutline
§ History of Atomic Force Microscopy (AFM)§ Instrumentation§ Static force-distance curves and force
spectroscopy§ Dynamic AFM and force gradient spectroscopy§ Imaging§ Applications and emerging areas
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Dynamic AFMDynamic AFM§ Cantilever driven near resonance§ Non-contact AFM, Tapping mode AFM, Amplitude
Modulated AFM, Frequency Modulated AFM are all dynamic AFM§ The cantilever's resonant frequency, phase and
amplitude are affected by short-scale force gradients
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Dynamic AFM & force gradient spectroscopyDynamic AFM & force gradient spectroscopy§ Variation of amplitude,
resonance frequency, and phase measured as Z is decreased § From this it is possible to
reproduce the Force gradients between the tip and the sample§ Even non-conservative
interactions can be resolved§ Offers many advantages over
static-force distance curve based force spectroscopy§ Quantitative information is hard
to come by because the forces are nonlinear
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OutlineOutline
§ History of Atomic Force Microscopy (AFM)§ Instrumentation§ Static force-distance curves and force
spectroscopy§ Dynamic AFM and force gradient spectroscopy§ Imaging§ Applications and emerging areas
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ss
x
xy
First tip contacts surface with some setpoint normal force which is kept constant during the scan
Contact Mode ImagingContact Mode Imaging
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In Tapping mode the tip is oscillated at the resonance frequency and the amplitude of oscillation is kept constant while the tip intermittently enters the repulsive regime
Surface interaction
Tapping ModeTapping Mode
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Phase ImagingPhase Imaging
AFM height (left) and phase (right) images of poly(methylmethacrylate)
(Digital Instruments, Inc.)
n Regular tapping mode implemented but signal phase monitoredn Phase contrasts are related to differences in local dissipation
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OutlineOutline
§ History of Atomic Force Microscopy (AFM)§ Instrumentation§ Static force-distance curves and force
spectroscopy§ Dynamic AFM and force gradient spectroscopy§ Imaging§ Applications and emerging areas
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Carbon nanotube tips (CNT)Carbon nanotube tips (CNT)
§ Provide high resolution§ Show little evidence of wear§ Promising technology for critical dimension metrology of
semiconductors, and nanobiological investigations§ Buckling, friction and stiction of CNT become important
Dynamic AFM images of a 100 nm trench on Si using conventional silicon probe (left) and a MWCNT probe (right)
1µ
Raman et al, Ultramicroscopy (2003), Nanotechnology (2003, 2005)
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71.5 7 2 72.5 7 3 73.50
2 0
4 0
6 0
8 0
1 0 0
1 2 0
Exci ta t ion f requency (kHz)
Tip
am
plitu
de (
a.u.
)
CNT tips – tapping mode CNT tips – tapping mode
(SEM images: NASA & Purdue, 2002)
Straight MWCNT
§ CNT attached strongly to Force modulation etched Si probe (Ni evaporation)§ Straight MWCNT, diameter 10 nm, length 7.5µm, Frequency 72.5 kHz§ Repulsive and attractive states do not appear to co-exist for long CNT tips
Raman et al, Ultramicroscopy (2003), Nanotechnology (2003)
Z gap decrease
Buckling signature
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Displacement towards sample (nm)
AB
CD
EF
GH
J
I
A B C D E
J I H G F
§ CNT buckles, slips, and slides§ High adhesion on the CNT sidewalls
Raman et al, Nanotechnology (2003)
Static force-distance curvesStatic force-distance curves
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Shorter CNT tips- noncontact modeShorter CNT tips- noncontact mode
§ Divot artifacts associated with switching between attractive (noncontact) and repulsive (tapping states)
§ Ringing artifacts associated with CNT adhesion and stiction to sidewalls
100 nm Si grating
300 nm Tungsten nanorods
0.4 µ CNT
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Exploiting anharmonic oscillationsExploiting anharmonic oscillations
§ NC vibration spectrum depends on local adhesion properties§ Experiments performed using 47 kHz microcantilever on wild and mutant bacteriorhodopsinmembrane §2nd bending mode freq ~7*1st
0 100 200 300 400 500
-60
-40
-20
0
20
40
a)
B2/H7
H1
PS
D (d
B)
Frequency (kHz)0 2 4 6 8 10
-60
-40
-20
0
20
40
b)
H3H2B2/H7
H1
PS
D (
dB)
Frequency (kHz)0 2 4 6 8 10 12
-60
-40
-20
0
20
40
c)
H3H2B2/H7
H1
PS
D (
dB)
Frequency (kHz)
Thermal vibration Driven in air On mica (50 % setpoint)
“Probing Van der Waals forces at the nanoscale using higher harmonic dynamic force microscopy”, Crittenden, Raman, Reifenberger (in press PRB)
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Application to local adhesion estimationApplication to local adhesion estimation3500 nm x 3500 nm scans
proteins Lipid deposits
Topography Second harmonic image
Seventh harmonic image
§ Clear distinction between lipids and proteins§ Presence of internal resonance critical in the method§ The method shows promise for the measurement of
local attractive forces of soft biomolecules§ Can be extended to electrostatic force microscopy or
capacitance microscopy for dopant profiling“Probing Van der Waals forces at the nanoscale using higher harmonic dynamic force microscopy”Crittenden, Raman, Reifenberger (in press PRB)
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Virus capsid mechanics studied using AFMVirus capsid mechanics studied using AFM
BacteriophageP22 (in buffer)
A computer model of the proheadstructure of the P22 and HK97 virus capsids. (T. Ferrin, UCSF Computer Graphics Lab)
Loca
l
Glo
bal
buck
ling
Experimental force-indentation
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Friction Force MicroscopyFriction Force Microscopy
3.0µm
Friction force image of a self assembled monolayer(Riefenberger Group)
www.chem.nwu.edu/~mkngrp/Dip-pen lithography
Contact mode oxidationlithography
Conley, Raman, Krousgrill, submitted JAP
§ Torsional vibrations due to atomic and molecular friction§ Lateral forces are specific§ Applications to nanotribology, probe
based lithography
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AcknowledgementsAcknowledgements§ Students§ Sebastian Rützel (Darmstadt)§ Mark Strus§ Shuiqing Hu§ Mau Deridder§ Xin Xu§ Bill Conley
§ Postdocs§ Soo-Il Lee (Univ. of Seoul)
§ Collaborators§ Steve Howell (Sandia) § Scott Crittenden (ARL)§ Cattien Nguyen (NASA Ames)§ Ron Reifenberger (Physics, Purdue)§ Amy McGough (Biology, Purdue)
§ Funding Agencies§ NSF Korean Center for Nanomechatronics and Manufacturing§ NASA§ DoE§ Purdue Research Foundation
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Questions & AnswersQuestions & Answers
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Application to local adhesion estimationApplication to local adhesion estimation3500 nm x 3500 nm scans
proteins Lipid deposits
Topography Second harmonic image
Seventh harmonic image
§ Clear distinction between lipids and proteins§ Presence of internal resonance critical in the method§ The method shows promise for the measurement of
local attractive forces of soft biomolecules§ Can be extended to electrostatic force microscopy or
capacitance microscopy for dopant profiling“Probing Van der Waals forces at the nanoscale using higher harmonic dynamic force microscopy”Crittenden, Raman, Reifenberger (in press PRB)
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Phase ImagingPhase Imaging
AFM height (left) and phase (right) images of poly(methylmethacrylate)
(Digital Instruments, Inc.)
n Regular tapping mode implemented but signal phase monitoredn Phase contrasts are related to differences in local dissipation
