AFM Basics

Xinyong Chen

Outline

• How AFM works– Scanning– Feedback control– Contact mode and tapping mode

• Force measurements with AFM– How AFM measures forces– Calibrations

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How AFM works

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How AFM works

• Direct mechanical contact between the probe and the sampler surface– Essential difference from traditional

microscopy

• How AFM “feels” the surface topography?– Optical level detection

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Photodiode

Laser

Scanner

Cantilever + Sharp probe

Photodiode

Laser

Scanner

Cantilever + Sharp probe

Photodiode

Laser

Scanner

Cantilever + Sharp probe

Photodiode

Laser

Scanner

Photodiode

Laser

Scanner

Photodiode

Laser

Scanner

Cantilever + Sharp probe

Photodiode

Laser

Scanner

Cantilever + Sharp probe

Photodiode

Laser

Scanner

Cantilever + Sharp probe

Photodiode

Laser

Scanner

Cantilever + Sharp probe

Photodiode

Laser

Scanner

Cantilever + Sharp probe

Photodiode

Laser

Scanner

Cantilever + Sharp probe

Photodiode

Laser

Scanner

Cantilever + Sharp probe

During scanning, the sample surface maylift the cantilever up, resulting in correspondingmove up of the optical reflection spot on thephotodiode. However, this single photodiodecouldn’t detect small position change of the spot.(Click for the next)

Let’s split the photodiode into two – the “top” and the “bottom”.Assume that the optical reflection spot originally locates in theexactly middle of this split photodiode, resulting in the exactlysame voltage output from the two photodiodes. So, the differencebetween the “top” (T) and the “bottom” (B) is zero.(Click for the next)

Optical level detection

Photodiode

Laser

Scanner

Cantilever + Sharp probe

VoltageDifference

BetweenTop & BottomPhotodiodes

Photodiode

Laser

Scanner

Cantilever + Sharp probe

Top-Bottom Signal (V)or Deflection (nm)or Force (nN)

Quad photodiode to detectBoth vertical and horizontalMovements of the light spot.

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With this “split photodiode”, any slight vertical movement ofthe reflection spot position is detected by checking thedifference between the “top” and the “bottom” photodiodedutputs (the “T-B signal”).(Click for the next)

• Direct mechanical contact between the probe and the sampler surface– Essential difference from traditional

microscopy

• How AFM “feels” the surface topography?– Optical level detection

• Constant-height scan versus Constant-force scan

How AFM works

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Constant-height scan

www.ntmdt.comClick on graph to play animation (internet connection required)

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Constant-height scan

• Advantages:– Simple structure (no feedback control)– Fast response

• Disadvantages:– Limited vertical range (cantilever bending and

detector dynamic range)– Varied force

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Constant-force scan

www.ntmdt.comClick on graph to play animation (internet connection required)

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Optical level detection in constant-force mode

Photodiode

Laser

Z scanner

Cantilever + Sharp probe

Photodiode

Laser

Z scanner

Cantilever + Sharp probe

Photodiode

Laser

Z scanner

Cantilever + Sharp probe

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In constant-force mode, whenever thesample surface topography would result inthe cantilever deflection change, the otherend of cantilever would be accordingly adjusted so that the cantilever deflection angle,and hence the contact force, would keepconstant.

Horizontal

Feedback control in constant-force mode

P.I.D. Control

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In constant-force mode, the cantilever’svertical position is adjusted by anelectronic feedback loop, with the T-Bsignal as the input and the verticalscanner voltage as the output.

Vertical

Constant-force scan vs.constant-height scan

Constant-force mode Constant-height mode

www.ntmdt.comClick on graph to play animation (internet connection required)

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Constant-force scan vs.constant-height scan

Constant-force• Advantages:

– Large vertical range– Constant force (can be

optimized to the minimum)

• Disadvantages:– Requires feedback

control– Slow response

Constant-height• Advantages:

– Simple structure (no feedback control)

– Fast response

• Disadvantages:– Limited vertical range

(cantilever bending and detector dynamic range)

– Varied force

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How AFM works

• Direct mechanical contact between the probe and the sampler surface– Essential difference from traditional

microscopy

• How AFM “feels” the surface topography?– Optical level detection

• Constant-height scan and constant-force scan

• Feedback control in constant-force scanClick forthe Next

Sample swept by AFM probes

Self-assembly of octadecyl phosphonic acid (ODPA) on single crystal alumina surface imaged in ethanol with tapping mode. The central 1 m × 1 m area was previously scanned in contact mode with heavy loading force.

1 m

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The constant AFM probe contactwith the sample surface may causedamage of the sample, typically shownas “sweeping”. One of the techniquesto avoid such a problem is the“tapping mode”.

Tapping mode AFM

www.ntmdt.comClick on graph to play animation

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Feedback control in tapping mode

P.I.D. Control

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In tapping mode, the system usesthe same feedback control as thatused in constant-force contact mode.However, it usually uses the cantilever’soscillation amplitude (the “AC” signal)instead of its DC component (the“Deflection”) as the input signal.

Phase

Tapping mode AFM

1 m

Height

PLA/PSA blend on Si imaged in air

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In addition to the normal topographic image, tapping mode AFMcan also provide simultaneously a “phase image” map, whichresults from variation in interactions between the AFM probe andthe various sample surfaces.

How AFM works

• Direct mechanical contact between the probe and the sampler surface– Essential difference from traditional microscopy

• How AFM “feels” the surface topography?– Optical level detection

• Constant-height scan and constant-force scan• Feedback control in constant-force scan• Contact mode and tapping mode

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Dimension AFM

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MultiMode AFM

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AFM Tips

80 – 320 m

20 m

35 m

125 m

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AFM sample preparation

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AFM in liquid environment

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One extraordinary feature of AFM is to work in liquid environment. A key pointfor liquid AFM is a transparent solid (usually glass) surface, which, together withthe solid sample surface, retains the liquid environment whilst maintains stableoptical paths for the laser beams. An optional O-ring can be used to form a sealedliquid cell. Otherwise, the system can also work in an “open cell” fashion.

19

Liquid AFM Images

41 45 48 56 60

70 nm t=0 min 20 2212

Effect of DNase I enzyme on G4-DNA (0.5:1) complex, the complex was immediately adsorbed onto mica and imaged until stable images were obtained, then the DNase I was introduced.

Nucleic Acids Research, 2003, Vol. 31, No. 14 4001-4005

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Outline

• How AFM works– Scanning and feedback control– Contact mode and tapping mode

• Force measurements with AFM– How AFM measures forces– Calibrations

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Force measurements with AFM

Z Displacement

Deflect

ion

A B

C D

(A+B)-(C+D) A+B+C+DDefl=

P.I.D. Control

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When an AFM works in force measurementmode, the feedback loop is temporarily“cut off”. The cantilever deflection (the“T-B signal”) is then recorded while theAFM probe is vertically “ramped”towards/backwards the sample surface.(Click step-by-step to see how this isdone.)

Experimental Force CurvesContact slope to study hardness

Adhesion to study intermolecular interactions

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Force

)/(constant Spring

Deflection

)/(y sensitivit Detector

signal B-TnNnmV nmnNVnm

• The Hooke’s lawF = -kx

• Detector sensitivityS = Inverse of the contact slope

measured on a hard surface (nm/V)

• Spring constant (N/m)– Property of the cantilever and

provided by the manufacturer• Large variation due to difficulty in

cantilever thickness control– Should (and can) be

experimentally measured for accuracy requirement

• Thermal fluctuation• Resonance + geometry• Mass adding + resonance• Standard with known spring

constant• etc.

Calibration of force measurements

T-B

S

igna

lZ Displacement (nm)

(V)

x

Slope = D / Z (V/nm)

x

Z

D

Def

lect

ion

(nm

)F

orce

(nN

)

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Humidity affects the adhesionAFM probe

SalbutamolMeasurement

of particle

-particle interact

ion

Lactose1µm

Force (nN)

0

200

400

600

800

1000

1200

<10% 22% 44% 65%

‘‘Nanoscale’ contactNanoscale’ contact‘‘Macroscale’ contactMacroscale’ contact Click forthe Next

Environmental AFM

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Both temperature andhumidity can be controlledin this environmentalchamber.

Intermolecular interactions

Schematic of the force–extension characteristics of DNA: at 65 pN the molecule is overstretched to about 1.7 times its contour length, at 150 pN the double strand is separated into two single strands, one of which remains attached between tip and surface.

MFP

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MFP is specially designed for force measurementpurpose

Adhesion Force Imaging

Height Adhesion

0102030

0 4 8 12

Hei

gh

t (n

m)

0.0

0.3

0.6

0 4 8 12

Ah

esio

n (

V)

Albumin

Albumin

Polystyrene

Si

PS

pH 7

5 m

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Adhesion and Hardness Imaging

PLMA/PmMl6 blend on Si imaged in waterPLMA: poly (lauryl methacrylate)PmMl6: 2-methacryloyloxyethyl phosphorylcholine-co-lauryl methacrylate (1:6)

1 m

Height Adhesion Stiffness

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Simultaneous Height, Adhesion and Stiffness maps are obtainedwith “Pulsed-Force” AFM technique.

Conclusions

• How AFM works– Constant-height and constant-force scans (contact mode)– Feedback control in constant-force mode– Contact mode and tapping mode

• Force measurements with AFM– Force curves: contact part to measure hardness and adhesion to

measure intermolecular interactions– Calibrations:

• Detector sensitivity (nm/V) = Inverse of contact slope on a hard surface => Convert the measured T-B signal (V) to cantilever deflection (nm)

• Spring constant (N/m) => Convert the cantilever deflection to force (N) [F=-kx]

End