Atomic Force Microscop (AFM)

1
?? ??? ???
1
2
Atomic Force Microscop (AFM)
3

• History and Definitions in Scanning Probe
  Microscopy (SPM)
• History
• Scanning Tunneling Microscope (STM)
• Developed in 1982 by Binning, Rohrer, Gerber, and
  Weibel at IBM in Zurich, Switzerland.
• Binning and Rohrer won the Nobel Prize in Physics
  for this invention in 1986.
• Atomic Force Microscope (AFM)
• Developed in 1986 by Binning, Quate, and Gerber
  as a collaboration between IBM and Stanford
  University.
• Definitions
• Scanning Probe Microscopy (SPM) Consists of a
  family of microscopy forms where a sharp probe is
  scanned across a surface
• The two primary forms of SPM consist of
• Scanning Tunneling Microscopy (STM)
• Atomic Force Microscopy (AFM) (also called
  Scanning Force Microscopy (SFM))
• There are 3 primary modes of AFM
• Contact Mode AFM
• Non-contact Mode AFM
• TappingMode AFM

4

• The atomic force microscope (AFM) or scanning
  force microscope (SFM) is a very high-resolution
  type of scanning probe microscopy, with
  demonstrated resolution of fractions of a
  nanometer, more than 1000 times better than the
  optical diffraction limit.
• The information is gathered by "feeling" the
  surface with a mechanical probe.
• Piezoelectric elements that facilitate tiny but
  accurate and precise movements on (electronic)
  command enable the very precise scanning.
• The AFM consists of a cantilever with a sharp tip
  (probe) at its end that is used to scan the
  specimen surface. The cantilever is typically
  silicon or silicon nitride with a tip radius of
  curvature on the order of nanometers.
• Typically, the deflection is measured using a
  laser spot reflected from the top surface of the
  cantilever into an array of photodiodes.

5
(No Transcript)
6
(No Transcript)
7
Atomic Force Microscope
in air and in buffer solutions
8
Basic SPM Components
9
Atomic Force Microscope
laser
photodiode
cantilever
z
piezo
cantilever tip
x
y
10

• Contact mode AFM operates by scanning a tip
  attached to the end of a cantilever across the
  sample surface while monitoring the change in
  cantilever deflection with a split photodiode
  detector.
• The tip contacts the surface through the adsorbed
  fluid layer on the sample surface.
• A feedback loop maintains a constant deflection
  between the cantilever and the sample by
  vertically moving the scanner at each (x,y) data
  point to maintain a "setpoint" deflection.
• By maintaining a constant cantilever deflection,
  the force between the tip and the sample remains
  constant.
• The force is calculated from Hooke's Law where
• F Force
• k spring constant
• x cantilever deflection.
• Force constants usually range from 0.01 to 1.0
  N/m
• The distance the scanner moves vertically at each
  (x,y) data point is stored by the computer to
  form the topographic image of the sample surface.
• Operation can take place in ambient and liquid
  environments.

11
Contact Mode AFM
12

• Tapping Mode AFM operates by scanning a tip
  attached to the end of an oscillating cantilever
  across the sample surface.
• The cantilever is oscillated at or near its
  resonance frequency with an amplitude ranging
  typically from 20nm to 100nm. The frequency of
  oscillation can be at or on either side of the
  resonant frequency.
• The tip lightly taps on the sample surface
  during scanning, contacting the surface at the
  bottom of its swing.
• The feedback loop maintains a constant
  oscillation amplitude.
• The vertical position of the scanner at each
  (x,y) data point in order to maintain a constant
  "setpoint amplitude is stored by the computer
  to form the topographic image of the sample
  surface.
• By maintaining a constant oscillation amplitude,
  a constant tip-sample interaction is maintained
  during imaging.
• Operation can take place in ambient and liquid
  environments. In liquid, the oscillation need not
  be at the cantilever resonance.

13
TappingMode AFM
14

• Contact Mode AFM
• Advantages
• High scan speeds
• Contact mode AFM is the only AFM technique which
  can obtain "atomic resolution" images.
• Rough samples with extreme changes in vertical
  topography can sometimes be scanned more easily
  in contact mode.
• Disadvantages
• Lateral (shear) forces can distort features in
  the image.
• The forces normal to the tip-sample interaction
  can be high
• Tapping Mode AFM
• Advantages
• Higher resolution on most samples (1 nm to 5 nm).
• Lower forces and less damage to soft samples
  imaged in air.
• Disadvantages
• Slightly slower scan speed than contact mode AFM.

15
(No Transcript)
16
(No Transcript)
17
(No Transcript)
18
Advantages and disadvantages

• The AFM has several advantages over the scanning
  electron microscope (SEM). Unlike the electron
  microscope which provides a two-dimensional
  projection or a two-dimensional image of a
  sample, the AFM provides a true three-dimensional
  surface profile. Additionally, samples viewed by
  AFM do not require any special treatments (such
  as metal/carbon coatings) that would irreversibly
  change or damage the sample. While an electron
  microscope needs an expensive vacuum environment
  for proper operation, most AFM modes can work
  perfectly well in ambient air or even a liquid
  environment. This makes it possible to study
  biological macromolecules and even living
  organisms. In principle, AFM can provide higher
  resolution than SEM. It has been shown to give
  true atomic resolution in ultra-high vacuum (UHV)
  and, more recently, in liquid environments.

19

• A disadvantage of AFM compared with the scanning
  electron microscope (SEM) is the image size. The
  SEM can image an area on the order of millimetres
  by millimetres with a depth of field on the order
  of millimetres. The AFM can only image a maximum
  height on the order of 10-20 micrometres and a
  maximum scanning area of around 150 by 150
  micrometres.
• Another inconvenience is that an incorrect choice
  of tip for the required resolution can lead to
  image artifacts. Traditionally the AFM could not
  scan images as fast as an SEM, requiring several
  minutes for a typical scan, while a SEM is
  capable of scanning at near real-time

20
Thanks for your attention
20