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Atomic Force Microscopy (AFM)Atomic Force Microscopy (AFM)

Arvind Raman, Associate ProfessorMechanical Engineering

Birck Nanotechnology Center NASA 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 byF. Bassani, J. Leidl, and P. Wyder, Elsevier Science Ltd., 2004.§ D. Sarid, Scanning Force Microscopy with Applications to Electric, Magnetic

and 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 voltageand the density of states in the tip and the sample, and

§ For most metals, Φ˜ 4eV, so that κ t=1Å-1

§ Most current carried by “front atom”blunt tips , so atomic resolution possibleeven with relatively blunt tips

§ Only electrically conductive samples, restricting its principal use to metalsand semi-conductors

The starting point- STMThe starting point- STM

Φ2

0( ) t z

t I z I e κ −=2 / t mκ = Φ h

F. Giessibl ’ s

Rev. 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 aSabbatical in IBM Almaden they collaborated with Cal Quate (Stanford) toproduce 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





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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 forceUp

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

S i gn a l

d e t e c t or

Piezoelectric

scanner

SFM 3 dimensional image of a

tumor cell HeLa (37x37µm2 )

D i gi t al S i gn al

P r o c

e s s or

Courtesy- J. Gomez- Herrero, UAM, Spain

z dither piezo

xy

z



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OutlineOutline





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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 forcesensed at the tip

§ k=3EI/L3 is called the bending stiffness of thecantilever

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 spectroscopyForce 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 nearsnap-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





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Dynamic AFMDynamic AFM§ Cantilever driven near resonance§ Non-contact AFM, Tapping mode AFM, Amplitude

Modulated AFM, Frequency Modulated AFM are alldynamic 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 isdecreased§ From this it is possible to

reproduce the Force gradientsbetween the tip and the sample

§ Even non-conservativeinteractions can be resolved§ Offers many advantages over

static-force distance curvebased force spectroscopy

§ Quantitative information is hardto come by because the forcesare nonlinear



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OutlineOutline





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s

s

x

x

y

First tip contacts surface with some setpoint normalforce 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 resonancefrequency and the amplitude of oscillation is kept constant whilethe tip intermittently enters the repulsive regime

Surfaceinteraction

Tapping ModeTapping Mode

Ph I i



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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



C b t b ti (CNT)C b t b ti (CNT)



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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 trenchon Si using conventional silicon probe(left) and a MWCNT probe (right)

1µ

Raman et al, Ult ramicroscopy (2003) , Nanotechnology (2003, 2005)

CNT ti t i dCNT ti t i d



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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

Exc i ta t ion f requency (kHz)

T i p a m p l i t u d e ( a . u . )

CNT tips – tapping modeCNT 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, Ult ramicroscopy (2003) , Nanotechnology (2003)

Z gap decrease

Buckling signature



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Displacement towards sample (nm)

A B

C

D

E

FG

H

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 (200

Static force-distance curvesStatic force-distance curves

Sh t CNT ti t t dSh t CNT ti t t d



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Shorter CNT tips- noncontact modeShorter CNT tips- noncontact mode

§ Divot artifacts associated with switching between attractive (noncontact) andrepulsive (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 using47 kHz microcantilever on wildand mutant bacteriorhodopsinmembrane§2nd bending mode freq ~7*1st

0 100 200 300 400 500

-60

-40

-20

0

20

40

a)

B2/H7

H1

P S D ( d B )

Frequency (kHz)0 2 4 6 8 10

-60

-40

-20

0

20

40

b)

H3H2B2/H7

H1

P S D

( d B )

Frequency (kHz)0 2 4 6 8 10 12

-60

-40

-20

0

20

40

c)

H3H2B2/H7

H1

P S D

( d B )

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)

A li ti t l l dh i ti tiA li ti t l l dh i ti ti



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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 oflocal attractive forces of soft biomolecules

§ Can be extended to electrostatic force microscopy orcapacitance microscopy for dopant profiling

“Probing Van der Waals forces at the nanoscale using higher harmonic dynamic force microscopy Crittenden, Raman, Reifenberger (in press PRB)

Vi id h i t di d i AFMVi id h i t di d i AFM



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Virus capsid mechanics studied using AFMVirus capsid mechanics studied using AFM

BacteriophageP22 (in buffer)

A computer modelof the prohead

structure of the P22and HK97 virus capsids. (T. Ferrin, UCSF Computer Graphics Lab)

L o c a l

G l o b a l

b u c k l i n g

Experimental force-indentation

F i ti F MiF i ti F Mi



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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 J

§ Torsional vibrations due to atomic andmolecular friction

§

Lateral forces are specific§ Applications to nanotribology, probebased 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

Application to local adhesion estimationApplication to local adhesion estimation



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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 oflocal attractive forces of soft biomolecules

§ Can be extended to electrostatic force microscopy orcapacitance 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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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

afm-a. raman

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