ATOMIC FORCE MICROSCOPY
Introduction and theoretical background
Jenny Malmstrom
AFM invented by Binning and co-workers in 1986.
Belongs to the Scanning Probe Microscopy family
Binning et al., Physics Review Letters 1986
AFM invented by Binning and co-workers in 1986.
Belongs to the Scanning Probe Microscopy family
AFM, atomic force microscopy [1]
Contact AFM
Non-contact AFM
Dynamic contact AFM
Tapping AFM
BEEM, ballistic electron emission microscopy[2]
CFM, chemical force microscopy
C-AFM, conductive atomic force microscopy[3]
ECSTM electrochemical scanning tunneling microscope[4]
EFM, electrostatic force microscopy[5]
FluidFM, Fluidic force microscopy[6][7][8][9]
FMM, force modulation microscopy[10]
FOSPM, feature-oriented scanning probe microscopy[11]
KPFM, kelvin probe force microscopy[12]
MFM, magnetic force microscopy[13]
MRFM, magnetic resonance force microscopy[14]
NSOM, near-field scanning optical microscopy (or SNOM, scanning
near-field optical microscopy)[15]
(Wikipedia 2015)
PFM, Piezoresponse Force Microscopy[16]
PSTM, photon scanning tunneling microscopy[17]
PTMS, photothermal microspectroscopy/microscopy
SCM, scanning capacitance microscopy[18]
SECM, scanning electrochemical microscopy
SGM, scanning gate microscopy[19]
SHPM, scanning Hall probe microscopy[20]
SICM, scanning ion-conductance microscopy[21]
SPSM spin polarized scanning tunneling microscopy[22]
SSM, scanning SQUID microscopy
SSRM, scanning spreading resistance microscopy[23]
SThM, scanning thermal microscopy[24]
STM, scanning tunneling microscopy[25]
STP, scanning tunneling potentiometry[26]
SVM, scanning voltage microscopy[27]
SXSTM, synchrotron x-ray scanning tunneling microscopy[28]
SSET Scanning Single-Electron Transistor Microscopy [29]
Binning et al., Physics Review Letters 1986
PRINCIPLE
Physical probe that raster scans a specimen
Key elements:
1. Probe
2. Detector & Feedback
3. Piezo actuators
http://ssd.phys.strath.ac.uk/index.php/Scanning_tunnelling_luminescence
Scanning Probe Microscopy
Scanning Tunneling Microscopy (STM)
Atomic Force Microscopy (AFM)
Operate by using a small tip (the probe) to scan very closely across a surface,
detecting forces present between the surface and the tip.
Atomic scale resolution possible
Can be operated in air and liquid
Slow
STM
Allows:
To see (1981)
To manipulate (1988)
• Signal origin is quantum tunneling effect
• 0.1 nm lateral resolution and 0.01 nm depth
resolution
•Can be used to manipulate individual atoms, trigger
chemical reactions, or reversibly produce ions by
removing or adding individual electrons from atoms or
molecules.
Image of reconstruction on
a clean Au(100) surface.
• Very small scan ranges
http://en.wikipedia.org/
STM image of self-assembled
supramolecular chains of the organic
semiconductor Quinacridone on Graphite.
Scanning tunneling microscopy
no limitation to crystalline structures
ultimate resolution in ultra-high vacuum
also other environments possible such as air and liquids
but by far more difficult
limited to metal surface
problem of sample preparation
AFM
Atomic Force Microscopy (AFM)
Signal origin from short-range forces between the tip and the
sample: van der Waals, capillary, electrostatic
1 nm lateral resolution and 0.1 nm depth resolution
Contact mode
Tapping mode
AFM COMPONENTS
Figures from Wikipedia
Cantilevers
Length
thickness
Cantilever dimensions determine how easy to bend it is.
Image quality depends on tip size and shape
Tip
trace
contact point
Slide courtesy of Duncan Sutherland
Influence of tip sharpness
Tip
trace
Tip
trace
Slide courtesy of Duncan Sutherland
Contact Mode AFM
Laser
Detector
Z
Cantilever
Tip
Simple
X,Y
Not too affected by humidity
Operation in liquid
Feedback: Deflection of cantilever
Damage to soft samples
Slide courtesy of Duncan Sutherland
Tapping Mode AFM
Detector
Cantilever oscillates can be driven
at a resonant frequency ~10-500 KHz
Piezoelectric material
drives oscillations
10-100nm
The surface acts to damp the resonance
Feedback: Oscillation amplitude
Slide courtesy of Duncan Sutherland
Repulsive regime
Attractive regime
Phase imaging in tapping mode
Delay related to tip surface interaction
Phase shift
Two regions on the surface with different
Tip- surface interactions
Slide courtesy of Duncan Sutherland
Phase only really means something in the repulsive mode
(more contributions in the attractive mode)
Many of the more advanced imaging modes has to be run in the repulsive mode
Not always possible
MFP-3D Origin AFM
Imaging in contact and tapping mode. Force curves in single or multiple points.
Image large area with the Origin AFM (80um scan size), 15-20 um z-piezo
Can do future liquid imaging (if we buy a ‘skirt’)
Cypher ES
Image relatively fast to a very high resolution of flat samples (e.g. nanofiber on mica)
Image samples with features up to 5 um in height
Flow inert gas into the imaging chamber
Control the temperature of the sample stage (0-150 deg)
Image by tapping mode in liquid (using blue drive)
Applications available that needs a bit of learning and that will not work for all
samples:
AMFM
EAFM
SKPM
Things we can modify the system to do in the future:
Using perfusion cell to exchange solutions
Electrochemistry (ports for electrodes available)
AM-FM Viscoelastic Mapping Mode
Quantitatively maps both the storage modulus (elastic response) and loss modulus or loss
tangent (viscous response) with nanoscale resolution based on tapping mode, so it’s gentle
and high resolution.
Works in both gas and liquid environments
Using both ground tone resonance and first overtone. Several feedbacks (A, A1, f).
Need to calibrate tip on known sample.
EFM: Electrical AFM
High resolution technique. Has to be done in air.
Need conductive probe.
First tap along one raster line. Then do a non-contact trace (fly) with applied electric
field and monitor forces on cantilever. “Nap” mode
Can “see” hidden conductive elements under the surface of the material (carbon
nanotubes in polymer)
SKPM: Scanning Kelvin Probe Microscopy
Monitors surface potential. Less resolution than EFM. Uses a conductive probe and
applied a bias to the tip. Detects potential difference between tip and sample. “Napmode”
SKPM is QUANTITATIVE
PFM: Piezo responsive force microscopy
Tune the tip in contact with the surface – which gives the resonance of the ‘combined
system’. This can be used to investigate internal dipoles in the system and can also be
used as lithography technique if the dipoles are rewritable.
New developments:
Improved optics – smaller laser spot
Smaller cantilevers – faster scanning
Fast scanning cantilevers are 10–20x shorter than conventional cantilevers
The Cypher laser spot is perfectly sized for even the smallest conventional
cantilevers.
blueDrive
Photothermal excitation
SUMMARY
AFM is a member of the scanning probe family.
It uses a sharp tip on the end of a flexible cantilever
to ‘feel’ the sample surface.
Things AFM cannot do:
Image very rough samples
Image something inside another material
Image something that cannot be deposited on a solid material
Things AFM is particularly suitable for:
Imaging hard nanostructures
Imaging soft nanostructures (proteinfibers, polymers)
Imaging single molecules on a flat substrate
Imaging adherent cells