AFM: 6 Must Know

Measurements

Dalia Yablon

SurfaceChar LLC

dalia.yablon@surfacechar.com

AFM - Principles of operation

piezo

AFM can

measure many

other useful

surface

properties

10nm lateral resolution

Must know

measurements with AFM

• Topography – 3 dimensional maps

• Mechanical contrast: Mapping with materials

contrast

• Mechanical properties: Nanoscale adhesion and

modulus

• Electrical properties: Mapping with electrical contrast

• Magnetic properties: Mapping with magnetic contrast

• Optical: Chemical information

Topography – 3 dimensional maps

• Most common measurement

• AFM provides true measurement of z (height)

• (in contrast to electron/optical microscopy)

• Enables measurements like roughness analysis

• Topography is the backbone of all AFM imaging measurements

• Feedback is on the z piezo to provide topography

• Any other imaging mechanism contrast (mechanical, electrical, magnetic) are all collected simultaneously with the topography

Materials contrast imaging:

Phase imagingTapping mode!

– Drive base of cantilever with a sinewave generator

– Phase lag between cantilever drive and response

– Response is due to complicated material response including

elastic and viscoelastic properties

– Key method to discriminate materials, though not well understood

driveresponse

f

D amplitude

Differentiating materials via phase imaging

f

Df DfPhase lag image

Solution-cast triblock

copolymer film (equilibrated)

ABA block copolymer, solvent-vapor annealed:

poly (styrene-isobutylene-styrene) or SIBS

(styrene cylinders standing or lying down)

Relatively bright regions: glassy phase

of triblock copolymer → less dissipative

Stiffness, adhesion, viscosity,…can give dissipation

100 nm

From G. Haugstad

Component

#1 and #2

Component

#3

Component

#4

Component

#5

Phase

Topography

Single point mechanical measurements:

Force curve/force spectroscopy

Adhesion

Modulus

Force curves on impact

copolymer

Create force maps…

Height Adhesion

Forcemaps on cells

Adhesion force (nN)

Elastic modulus (kPa)

cell

background

Data from nanosurf

Epithelial cells

Fibroblast cells

Adhesion

Elastic modulus

Elastic modulus

Data from Asylum Research

(online image gallery)

Kelvin probe microscopy

• Maps surface potential

• What can it measure?

– Trapped charge, spontaneous

polarization, work function variation,

potential difference

• Requires cantilevers with electrically

conductive coating

• Two passes

– First pass: topography with typical piezo

actuation/tapping mode

– Second pass: AC electrical bias plus a

DC bias to vibrate the tip

[cantilever driven electrically]

Electrical properties: surface potential

(Kelvin probe force microscopy KPFM)

BN

Graphene

Copper foil

Image courtesy of Asylum Research

Magnetic Properties

(Magnetic force microscopy – MFM)

10um 10um

Topography MFM

Data and image courtesy of ParkAFM

AFM-IR

• Combine AFM + IR

– Apertureless scattering method

– Photothermal method

Courtesy of Neaspec

• Adds chemical information

to the AFM topography

Courtesy of Anasys Instruments

AFM-IR: Nanoscale spatial resolution

for polymer thin film

min

max

Nano-FTIR Absorption

min

max

200 nm

0nm

3.8nm

Topography

0nm

2.3nm

100 nm

1730 cm-1

ON Resonance

1650 cm-1

OFF

Resonance

High-resolution PMMA/PS diblock thin films (period: 50nm)

Data courtesy of Neaspec

Summary

• Topography – 3 dimensional maps

• Mechanical contrast: Phase imaging

• Mechanical properties: Force spectroscopy

• Electrical properties: Kelvin probe microscopy

• Magnetic properties: Magnetic force microscopy

• Optical: AFM-IR

• Nanoscale Resolution

• Environmental flexibility

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