Introduction to Atomic Force Microscopy

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Introduction to Atomic Force Microscopy
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AFM Background

• Invented by Binnig, Quate, and Gerber in 1986
• Measures the interaction forces between the tip
  and surface (e.g. repulsive and attractive
  forces).
• Vertical resolution 0.1 nm
• Lateral resolution 2-10 nm
• Most common modes of AFM Contact and tapping
  mode
• Topography and other important information about
  the surface can be constructed into an image
• Forces are measured in AFM and obey Hookes Law
  F k z
• (k cantilever spring constant z vertical
  deflection)

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Basic Operating Principle of AFM
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Basic Operating Principle of AFM
photodiode detector
The beam from a diode laser is deflected off the
back of an AFM cantilever to a quadrant
photodetector. As the AFM tip is raster scanned
across the surface, the up-down and left-right
movement of the tip is monitored by the detector
and converted into images of topography and
friction, respectively.
diode laser
laser light
tip
piezoceramic tube scanner
sample
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• SEM micrographs of
• a square-pyramidal PECVD Si3N4tip
• a square pyramidal etched single crystal silicon
  tip
• (c) a three-sided pyramidal natural diamond
  tip

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Force Distance-Curves in AFM
Stage (i) The tip is far away from the surface
and there is no interaction between tip and the
surface. No cantilever deflection occurs during
stage i. Stage (ii) As the tip approaches the
surface, the tip senses attractive forces causing
the tip to snap into contact with the surface.
Stage (iii) As the tip keeps approaching the
surface, long and short-range repulsive forces
make the cantilever deflect. Stage (iv) When
the tip keeps approaching the surface, the
cantilever bends resulting in a straight line
appeared in cantilever deflection axis. As
deflection reaches to maximum, the tip stops
approaching and retracts from the
surface. Stage (v) The retracting line
appears because of the adhesive forces between
the tip and the surface. Once the tip-surface
separation is large enough to overcome long-range
attractive interaction, the cantilever returns to
zero deflection.
Solid line tip approaching surface Dashed line
tip retracting from surface
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Contact Mode AFM

• Tip remains in contact with the surface during
  scanning
• Operates in the repulsive regime of the force
  curve
• Tip is maintained at a constant force by moving
  the cantilever up and down as it scans
• Topography and friction (i.e. lateral force)
  images are acquired simultaneously during
  scanning
• Disadvantages There exists large lateral forces
  on the sample as the tip is dragged over the
  surface. Problems with contact mode are caused by
  excessive forces applied by the probe to the
  sample.
• Thus, possibility of damaging delicate
  samples.

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Tapping Mode AFM

• Also called intermittent contact mode AFM
• Tip oscillates near its resonance frequency over
  the sample during scanning (gently taps the
  surface).
• Lateral forces are dramatically reduced and is
  therefore useful for working with delicate or
  poorly immobilized specimens on the surface.
• Topography (height) and phase (elastic
  properties) data are acquired simultaneously
  during scanning.
• Phase imaging measures the difference between
  the phase of the driving signal and the phase of
  output signal detected by the photodiode
  detector.

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Tapping Mode AFM
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AFM-based Nanolithography Techniques
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Structures of Alkanethiol and Alkylsilane
Self-assembled Monolayers (SAMs)
Alkanethiol SAM
Organosilane SAM
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Structures of Alkanethiol SAMs on Au(111)
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Nanoshaving Procedure
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Nanoshaving Using AFM
Nanoshaved patterns produced within an
octadecanethiol (ODT) SAM -The topographic image
displays twelve dark squares written into an ODT
matrix in ethanol. -The dark squares correspond
to uncovered areas of Au(111). The areas of
brighter contrast indicate taller features
whereas the dark areas are shallower.
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Nanolithography Techniques
Nanografting
Acc. Chem. Res. 2000, 33, 457. (S. Xu, G.-Y.
Liu)

• Depending on the choice of molecules,
  nanografting can generate patterns that are
  taller or shorter then the SAM matrix

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Nanografting (contd)
(A) Cross-shaped pattern of 11-mercaptoundecanol
(MUD) written into matrix of ODT. (B) The
simultaneously acquired frictional force image
exhibits dark contrast for the MUD areas of the
cross, which are terminated with hydroxyl groups.
The surrounding methyl-terminated matrix areas
of ODT exhibit lighter frictional contrast,
clearly distinguishing the differences in surface
chemistry after nanografting. (C) The line
profile indicates the nanostructure is 0.7 0.3
nm shorter than the matrix SAM, in close
agreement with the theoretical differences in
thickness (ODT 2.1 nm, MUD 1.5 nm).
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Nanolithography Techniques
Biased-induced Lithography
Chem. Rev. 2003, 103, 4367. (C. Gorman) Adv.
Mater. 1999, 11, 55. (R. Maoz, S. R. Cohen, J.
Sagiv)

• Oxidation or replacement of surface molecules
• When a elevated bias voltage is applied, an
  electric field is generated between a conductive
  tip and a conductive/semi-conductive sample
• Surface molecules beneath the tip will become
  oxidized or replaced.
• Operating conditions
• - Performed in air
• - Tip is in direct contact with the
  sample during scanning

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Nanolithography Techniques

Dip-Pen Lithography
Science 1999, 283, 661. (C. Mirkin)

• The AFM tip (pen) is coated with the molecules
  to be written (ink), and a clean substrate serves
  as the paper.
• The molecular ink must have an affinity for both
  the AFM tip and for the substrate. For example,
  n-alkanethiols attach to silicon nitride AFM tips
  through physisorption (physical adsorption) and
  bind to surfaces of certain metals through
  chemisorption.
• Molecules are transported to the surface via
  capillary diffusion through the nanoscopic water
  meniscus formed between the tip and sample in an
  ambient environment.
• The transfer of the ink is dependent on the
  relative humidity, which affects the size of the
  water meniscus formed between the tip and the
  surface.
• Tip is held in contact with the surface for
  certain time intervals.

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Dip-Pen Lithography (contd)
Octadecanethiol and mercaptohexadecanoic acid
nanopatterns written with DPN
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An approach curve or force-distance curve
displays the vertical cantilever bending vs.
lever-sample displacement. This displacement is
measured between the sample and the rigidly held
rear end of the cantilever (as opposed to the
front end with the tip which will bend in
response to interaction forces). (i) The lever
and sample are initially far apart and no forces
act. (ii) As the lever is brought close to the
sample, the tip senses attractive forces which
cause the end of the lever to bend downward, thus
signifying a negative (attractive) force. (iii)
The attractive force gradient exceeds the spring
constant of the lever at this point, and this
instability causes the tip to snap into contact
with the sample. (iv) The lever-sample
displacement can continue to be reduced. Since
this tip is in repulsive contact with the sample,
the front end of the lever is pushed further and
further upward. The force corresponds to the
externally applied load. (v) The motion is
reversed. Adhesion between the tip and sample
maintains the contact although there is now a
negative (tensile) load. (vi) Finally the
tensile load overcomes the adhesion or pull-off
force and the tip snaps out of contact with the
sample.
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