Atomic Force Microscopy
Robyn Snow
1
https://www.pinterest.com/lictorn/gary-larson-the-far-side
2
Brief History of Microscopes
• 1590’s
– Zacharias Jansen and father
• Several lenses in a tube
– Enlarged objects only ~9X
http://www.history-of-the-microscope.org/history-of-the-microscope-who-invented-the-microscope.php
3
Brief History of Microscopes
• Anton van Leeuwenhoek (1632-1723)
– 1st to make a “real” microscope
– 1st to see and describe bacteria, yeast plants…
– Developed superior lenses
• 270X magnifying power
• Robert Hooke (1635-1703)
– Verified van Leeuwenhoek’s work
– Published Micrographia, 1665
• Observed pores in cork, called them “cells”
– Hooke’s Law 
4
IMAGE: http://www.nlm.nih.gov/exhibition/hooke/hookesbooks.html
Brief History of Microscopes
• Nobel Prize in Physics, 1986
– Ernst Ruska
• German physicist
– Fundamental work in electron optics
– Designed 1st electron microscope
– Gerd Binnig , Heinrich Rohrer
• IBM Zurich Research Lab
– Design of scanning tunneling
microscope
5
http://ernst.ruska.de/daten_e/library/documents/999.nobellecture/lecture.html
General Overview of AFM
• Surface analysis technique
– Surface topography
– Elasticity
– Friction
– Magnetic forces
– Electrostatic forces
6
General Operation
– Small probe is scanned across surface
– Data from interaction with surface is stored for
each point
– Image is displayed as an intensity map, I(x,y) ; I =
parameter sensed by the probe
• For AFM, the height of the surface is sensed
• LFM, friction
• MFM, magnetic fields
7
Change of the surface properties along the scan line
Interaction force between probe and sample
Deflection of cantilever
Changes of the laser signal to the photosensitive
position detector
Electric signal
Signal processing to generate image
8
Atomic Force Microscopy
• Advantages:
–
–
–
–
Minimal to no surface prep
Non-destructive imaging
Atmospheric conditions
Sample not required to be conductive
• Polymers, ceramics, glass
• Metals
• Biological samples
• Obtain image at or near atomic resolution
9
Surface Preparation
-Depends on sample, generally:
1. Clean substrate
2. Sample must be adhered to the surface of
substrate
– Mica, glass, gold
– HOPG (highly ordered pyrolitic graphite)
3. Rigidly mount sample to the stage
– No vibration
• If contamination layer present, use contact
mode or UHV
10
Resolution
• Lateral resolution ~30nm
• Vertical resolution ~0.1nm
• Limited to scan areas of 100μm
•
•
•
•
•
Vertical Resolution:
Laser intensity noise
Photodiodes noise
Thermal noise of
cantilever
Vertical scanner resolution
<1Å(0.1nm)
Noise: electrical,
mechanical, acoustic
Lateral Resolution
• Tip sharpness / shape
• Scanner resolution in X,Y
• Pixelization
– EX: 50μmX 50μm image
– Samples/line @512
– Pixel size=0.098 μm
=>cannot resolve features
smaller than 98nm with
50μm scan size
11
Gold Nanoparticles
http://www.afmworkshop.com/products-main/imagegallery.html#!gold_nanoparticles_100_nm_pm
http://www.afmworkshop.com/productsmain/image-gallery.html#!nanotriangle_pm
12
Diagram of Instrument
Probe
13
Probe
• Probe: Si or Si3N4
• Only part that contacts sample
– Like the “eye” of the instrument
– > shape is critical! -> Resolution depends on it
14
Probe Tip
• Conical Probe
• more preferable
• higher resolution image
• Pyramidal Probe
• can see distortion of image
• High Radius of Curvature
Artifact seen in image
15
Fat-tip Effect: apparent width
measured by large tip
𝑥2
= 𝑅𝑡𝑖𝑝 + 𝑅𝑠𝑎𝑚𝑝𝑙𝑒
2
− 𝑅𝑡𝑖𝑝 − 𝑅𝑠𝑎𝑚𝑝𝑙𝑒
𝑥 = 2 𝑅𝑡𝑖𝑝 𝑅𝑠𝑎𝑚𝑝𝑙𝑒
𝑤 = 2𝑥 = 4 𝑅𝑡𝑖𝑝 𝑅𝑠𝑎𝑚𝑝𝑙𝑒
2
Rtip
x
w
RSample
•When Rtip ~ ¼ Rsample, measured width = 2Rsample
•Normal tip size, ~ 20 nm or larger.
16
SEM images of probes
Si3N4 Tip
Atomic Force Microscopy: Theory, Practice , Applications
Paul E. West, Ph.D.
Diamond Coated tip
17
Conical Tip and Cantilever
Silicon
Cantilever Length (µm): 225
Res. Frequency (kHz): 28
Spring Constant (N/m): 0.1
Tip Radius ~8nm
http://store.nanoscience.com/store/pc/viewPrd.asp?idproduct=2556&idcategory=4
18
Cantilever and Probe
• Spring system/Force Sensor
• Bends in presence of attractive/repulsive
forces
• Cantilever deflection converted to force using:
– Hooke’s Law!
𝐹 = 𝑘𝑠
s : deflection
k : spring constant
19
Hooke’s Law
Fe : Restorative force due to spring
F : force due to sample
s : springs displacement from equilibrium
IMAGE FROM : http://www.physics.usyd.edu.au/teach_res/jp/waves/hwaves1001.htm
20
Ideal Spring System
• Max deflection for given force
– Spring as soft as possible (small k)
• Minimize interference due to vibrations of building
(~100Hz)
– Stiff spring with high natural frequency(𝑓0 )
Natural Frequency of spring(𝑓0 ):
1
𝑓0 =
2𝜋
𝑘
𝑚0
1
2
To achieve high frequency and max deflection:
↓k and ↓m0
k = spring constant
m0 = mass of spring
21
Spring Constant, k
• For a rectangular cantilever:
𝐸𝑤𝑡 3
𝑘=
4𝑙 3
𝑤
𝑡
• E = Young’s modulus
• Measure of stiffness
• For Si, E = 1.3*1011N/m2
𝑙
• Typical values for k : 0.001 to 100 N/m
• ~100microns in length , a few microns thick
22
Piezoelectric Scanner
• Piezoelectric material
– Typically (PZT) lead zirconate titanate (Pb(ZrTi)O3)
• Piezoelectric effect
– Expand or contract in presence of potential difference
– Develop potential in response to mechanical pressure
• Allows for ability to precisely manipulate
movement of sample or probe
23
Piezoelectricity
• Polarization in one direction occurs due to
applied electric field
– Change in length ∝ field strength
• ~0.1nm per applied Volt
http://www.ytca.com/lead_free_piezoelectric_ceramics
24
25
Light Lever Sensor
• Force Sensor
– Detects changes in height/ deflection based on
angle of reflected light
26
Photodetector
A
B
D
C
• Photosensitive elements (photodiodes)
• Photocurrent is produced upon
illumination for each quadrant
• The ratio between the photocurrent
from each quadrant determines the
relative position of the laser beam
𝐴+𝐷 − 𝐵+𝐶
X−Position =
𝐴+𝐵+𝐶+𝐷
𝐴+𝐵 − 𝐷+𝐶
Y−Position =
𝐴+𝐵+𝐶+𝐷
27
Probe-Surface Interaction
• Interaction between
probe tip and
surface =>Atomic
Forces
– Creates potential
energy(PE)
Attractive forces- takes minimal energy to
bring atoms closer together at distance
Equilibrium : distance when potential
energy minimized
• + PE : repulsive
forces , atoms very
close
• - PE : attractive
forces, van der waals, Repulsive forces- very small r, takes a lot of
atoms further away energy to bring atoms closer together>repulsive forces dominant
28
Lennard-Jones Potential (LJP)
• Empirical Model, describes potential energy (V)
of interaction between outermost atom of tip
and surface atoms
𝑉 𝑟 =4∈
𝜎
𝑟
12
𝜎
−
𝑟
6
∈ : well depth- measure of strength of attraction
𝜎 : distance at which V is zero (equilibrium)
r : distance of separation
29
n
30
http://chemwiki.ucdavis.edu/@api/deki/files/8914/Figure_B.jpg
Van der Waals Forces
𝑉 𝑟 =4∈
𝜎
𝑟
12
𝜎
−
𝑟
6
𝜎
𝑟
𝜎
𝑟
12
6
31
Modes of Imaging
Repulsive
Tapping Mode
Attractive
Non-Contact Mode
Contact Mode
http://virtual.itg.uiuc.edu/training/AFM_tutorial/
32
Contact Mode
• Tip is in very close contact with surface
<0.5nm -> Repulsive forces
– Cantilever bends
𝐹 = 𝑘𝑠
s : displacement of cantilever ->height/Force
measure
– Force varies dramatically based on distance
between tip and sample
• Two types of contact mode:
1. Constant Height
2. Constant Force
33
Constant Height Contact Mode
• Maintain constant height of sampler
– Variations of deflection of lever are recorded as
topography
• Deflection ↑ as height of sample ↑
• Deflection ↓ as height of sample ↓
• Advantages
– Higher scanning speeds
– High resolution
34
Fig. 1 STM and AFM imaging of pentacene on Cu(111).
A: Ball-and-stick model
B: Constant current STM
C,D : Constant height AFM images
Leo Gross et al. Science 2009;325:1110-1114
Published by AAAS
http://www.sciencemag.org/content/325/5944/1110.full
Constant Force Contact Mode
• Maintain constant force between tip and
sample
– Regulate height of sample relative to the tip
• Feedback loop : photodetector and piezoelectric scanner
– Height of sample ↑ , Force ↑, lower height
of sample to maintain constant force
• Slower scan speeds
• Advantage:
– Can simultaneously measure other
characteristics/forces
36
Lateral Force Microscopy
• In constant force mode:
– Scan perpendicular to longitudinal axis of cantilever
– Measures surface friction
– Friction-force map
• Four quadrant
photodiode
detector
– Difference between
left and right
segments ∝ friction
37
Lateral Deflection
• Magnitude depends on:
– Frictional coefficient of the sample
– Topography of sample surface
– Cantilevers lateral spring constant
38
Lateral Force Spring Constant
• Lateral spring constant:
𝐺𝑤𝑡 3
𝑘𝑡 =
3𝑙𝑟 2
G : shear modulus-measure of elasticity
r : length of tip
∆s
𝒍
Force
𝒕
𝒘
39
Lateral Force Calibration
• Calibration of :
– normal and lateral forces, F
– Photodiode sensitivity, S
•
•
•
•
Using reference sample
𝐹𝑁 = 𝑘𝑁 𝑆𝑁 ∆𝑉𝑁 Normal Force
𝐹𝐿 = 𝑘𝐿 𝑆𝐿 ∆𝑉𝐿 = 𝛼∆𝑉𝐿 Lateral Force
𝛼 = lateral calibration factor
– Transforms lateral ΔV ->Friction Force (nN)
40
Human Hair Image : AFM vs LFM
a) AFM image of topography
b) LFM image
http://www.parkafm.com/images/spmmodes/standard/Lateral-Force-Microscopy-(LFM).pdf
41
LFM Image
Topography
LFM image
2μm x 2 μm of Nickel CD stamper matrix
T.Göddenhenrich, S.Müller and C.Heiden, Rev. Sci. Instrum. 65, (1994) 2870
42
Contact Mode: pros and cons
Advantages
• High resolution >50nm
• Fastest
• No problem with
surface pollution
– Can image in air or liquid
Disadvantages
• High contact pressure
– Can damage/ not
analyze soft samples
• Probe and sample
experience lateral
forces
• Lateral resolution
limited by tip sharpness
• Lowers lifetime of tip
43
Non-contact Mode
• Lever(spring) oscillates close to its resonance
frequency from driving piezo
– => Use z-piezo to vibrate the cantilever near its
resonant frequency
• Forces shift this oscillation
tip-sample distance of ~5-10nm
44
Z-drive Piezo for Non-contact AFM
• Informs feedback loop of motion of tip/cantilever:
• Frequency
• Amplitude
• Allows for frequency modulation(FM-AFM) or
amplitude modulation (AM-AFM/tapping mode)
45
Frequency Modulation
• Excitation Amplitude constant
• Tip-sample interaction-> frequency ↓
• Attractive forces
Tip-sample
interaction->
natural
frequency shift
∆f
Natural
frequency, no
interaction
46
Frequency Modulation
• ∆f : info about tip-sample interaction
• Feedback loop : adjusts tip-sample distance to
achieve constant Amplitude
47
Non-contact AFM of C60
STM image
A : model
B to E : AFM
showing Δf at
differing tip heights
F : image used for
measure of bond
length
• measured bond
lengths are Lh =
1.38Å
• Lp = 1.454 Å
http://www.sciencemag.org/content/337/6100/1326.figures-only
48
“h”
“p”
"C60a" by Original uploader was Mstroeck at en.wikipedia Later versions were uploaded by Bryn C at
en.wikipedia. - Originally from en.wikipedia; description page is/was here.. Licensed under CC BY-SA 3.0 via
Wikimedia Commons - http://commons.wikimedia.org/wiki/File:C60a.png#/media/File:C60a.png
49
Amplitude Modulation/ Tapping Mode
• Cantilever excited to resonance frequency
• Tip-sample distance ↓ , amplitude (A)↓
• A reaches set point, below resonance A ->
height is measured
• Feedback loop adjusts height to maintain set A
as sample is scanned
50
Set-point
51
Tapping mode pros and cons
Advantages
• Reduced forces on
surface
– Good for soft materials
• No friction forces
– Can use sharper tips
• Can be in air or liquid
• Improved lateral
resolution
Disadvantages
• Slower than contact
mode
– Up to20 minutes per
scan
• Tip is damaged after
several scans
– ~5nm
52
Tapping mode image
DNA deposited on mica
2.5μm scan size
http://www.veeco.com/pdfs/database_pdfs/B54_Rev_A1_Caliber_300.pdf
53
High Resolution Image of muscovite mica
in water
High-resolution dynamic atomic force microscopy in liquids with different feedback architectures
John Melcher, David Martínez-Martín, Miriam Jaafar, Julio Gómez-Herrero, Arvind Raman
Beilstein J. Nanotechnol. 2013, 4, 153–163.
Blue : Oxygen
Green : Silicon
Unit Cell: 5.199Å
9.7Å
http://www.beilstein-journals.org/bjnano/single/articleFullText.htm?publicId=2190-4286-4-15
54
Force Modulation Microscopy(FMM)
• Tip in contact with the sample
• z feedback loop maintains a constant
cantilever deflection
• A periodic vertical oscillation signal is applied
to either the tip or the sample.
• The amplitude varies according to the elastic
properties of the sample.
• the system generates a force modulation
image --- a map of the sample's elastic
properties
55
FMM
56
FMM vs AFM
Carbon fiber/polymer Composite Collected Simultaneously (5μm)
FMM
AFM
FMM gives more detailed information about the composition and distribution
of the two components --- soft polymer (dark area) and hard carbon fiber.
57
2-phase block copolymer
AFM
FMM
The softer, more compliant component of the polymer maps in black.
900nm scans. Veeco.
58
Magnetic Force Microscopy (MFM)
• Tip is coated with a ferromagnetic film(Ni, Fe,
Co)
• Scanned in non contact mode
• Provides high resolution image of magnetic
patterns
• Strength of local magnetic interaction
determines the vertical motion of the tip
• Detectable magnetic field ~0.1 gauss (10
microteslas)
59
Magnetic Force
• Described as:
𝐹 = 𝜇0 𝑚𝛻 𝐵
𝜇0 : magnetic permeability of free space (4π 10-7
WbA-1m-1)
𝑚: magnetic moment of the tip
𝐵:strength of magnetic field from sample
60
MFM Limitations
• Type of tip and magnetic coating affect image
• Interaction of magnetic field of tip and sample
can alter each others field
• Highly dependent on scan height
• Inner and surface magnetic charges not able
to be deconstructed
61
MFM Image
magnetic disk; 40 µm x 40 µm
http://www.afmworkshop.com/products-main/image-gallery.html#!
62
MFM Image
3.2GB hard drive
30GB hard drive
http://commons.wikimedia.org/wiki/File:MFM_AFM_JANUSZ_REBIS_INFOCENTRE_PL_HDD_MAGNETIC_MEMORY_EVOLUTION.png#/media/
63
File:MFM_AFM_JANUSZ_REBIS_INFOCENTRE_PL_HDD_MAGNETIC_MEMORY_EVOLUTION.png
Electric Force Microscopy(EFM)
• Measures electric field gradient distribution
on sample surface
• Tip : electrically conducting coating
• applied Voltage between tip and sample
• Deflection of cantilever 𝛼 charge density of
sample
• Use photo-diode detector
64
EFM
65
Applications of EFM
• characterizing surface electrical properties
• Interfacial charge transport and separation for
organic/electrode devices (conducting
polymer, organic semiconductors, etc.)
• detecting defects of an integrated circuit
(silicon surface)
• measuring the distribution of a particular
material on a composite surface.
66
Bio Applications
•
•
•
•
•
•
Imaging
Ligand-receptor binding sites
Antibody-antigen binding sites
Proteins-folding/unfolding
Structural analysis – SMRFM
*need special surface preparation for bio samples
– Absorb sample onto a supported cationic bilayer
(mica) surface and imaged with AFM in aqueous
buffers
67
DNA on multiple mica layers
2 μm X 2 μm
http://www.afmworkshop.com/products-main/image-gallery.html#!AFM_scan_DNA_mica_pm
68
Red Blood Cells, 30μm X 30μm
http://www.afmworkshop.com/products-main/image-gallery.html#!07_img_blood_cells_01_big
69
Single-Molecule Recognition Force
Microscopy (SMRFM)
• Couple a “ligand” molecule to the tip
– Thin PEG chain
• Ligand recognizes complementary receptor
site in sample
– Causes deflection of cantilever / change in
oscillation frequency => maps recognition sites
70
SMRFM
1. NH2 on tip reacts with NHS ester of PEG linker
2. Protein attached to free end of PEG (amine-aldehyde linkage)
http://www.jku.at/biophysics/content/e54633/e54706/e54710/#fig171
How cell membranes respond to their
environment
Light harvesting Complex
Reaction centers
• Membrane organization in
photosynthetic bacteria
• –Rsp. Photometricum
• (exposed to strong
light)
AFM Image
Simon Scheuring , Thomas Boudier , James N. Sturgis
http://www.innovations-report.com/html/reports/life-sciences/report-47236.html
72
Questions
• THANK YOU!! 
73