Scanning Probe Microscopy

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Scanning Probe Microscopy Scanning Tunneling
Microscope STM Atomic Force Microscope
AFM Nearfield Scanning Optical Microscope NSOM
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HEINRICH ROHRER
GERD BINNIG
IBM spelled in Xenon atoms
Shared the 1986 Nobel prize in Physics for their
invention of the scanning tunneling microscope
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HEINRICH ROHRER
GERD BINNIG
IBM spelled in Xenon atoms
Shared the 1986 Nobel prize in Physics for their
invention of the scanning tunneling microscope
Ernst Ruska was the other winner
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Electron Tunneling
In scanning tunnneling microscopy a small bias
voltage V is applied so that due to the electric
field the tunneling of electrons results in a
tunneling current I. The height of the barrier
can roughly be approximated by the average
workfunction of sample and tip.
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When the tip of the STM probe is sufficiently
close to the surface of the specimen ( 1nm) a
tunneling current can become established
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Ideally a STM probe tip is very pointed (1-2
atoms at the end) and has a relatively low work
function. Etched tungsten crystals are ideal and
are nearly identical to field emitters.
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The tunneling current is exponentially
proportional to the distance and thus via a
feedback loop the tip can be maintained at a
constant distance from the surface by maintaining
a constant tunneling current.
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If the tunneling current is kept constant the Z
position of the tip must be moved up and down.
If this movement is recorded then the topography
of the specimen can be inferred.
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Alternatively if the Z position of the tip is
kept constant the tunneling current will change
as it moves across the surface. If the changes
in current are recorded the then the topography
of the specimen can be inferred.
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The probe is scanned over the surface in a raster
pattern similar to that of a SEM or Confocal.
Each coordinate (X,Y, Z) is recorded by a
computer.
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The ability to precisely position the probe of an
STM is made possible by an XYZ Piezo-Scanner
which coupled to a feedback regulator keeps track
of the tunneling current and precisely positions
the tip accordingly.
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Crystals which acquire a charge when compressed,
twisted or distorted are said to be
piezoelectric. Piezoelectric ceramic materials
have found use in producing motions on the order
of nanometers in the control of STMs and other
devices.
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The Piezoelectric Effect
Forces applied to a segment of material lead to
the appearance of electrical charge on the
surfaces of the segment. The specific
distribution of electric charges in the unit cell
of a crystal is the source of this phenomenon.
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Surface of Platinum
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Iron corrals on Cu
Positioning of atoms for a mass data storage
system
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ATOMIC FORCE MICROSCOPE
HOW DOES IT WORK?
The atomic force microscope (AFM), uses a sharp
tip attached to the end of a cantilever rasters
across an area while a laser and photodiode are
used to monitor the tip force on the surface. A
feedback loop between the photodiode and the
piezo crystal maintains a constant force during
contact mode imaging and constant amplitude
during intermittent contact mode imaging.
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As with the STM the probe tip of an AFM must be
very small but because there is no need to
establish a tunneling current one can use a
variety of materials, not just those with a low
workfunction.
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Similar to a phonograph needle the probe is
actually in contact with the specimen and is
physically moved up and down due to the repulsion
of van der Waals forces
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The AFM records the position of the probe by
bouncing a laser off the back surface of the
probe and recording how the light is deflected
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By using a four quadrant detector the relative
amount of laser light hitting each quadrant can
be used to determine how the tip has been
deflected as it moves over the surface of the
specimen
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AFM of Chromosome
Since an AFM relies on contact rather than
current many nonconductive materials can be
examined
AFM derived models of nuclear pore complex
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Since the contact of the tip with the specimen
can cause physical damage to the specimen many
AFMs employ a tapping mode in which the probe
vibrates up and down as the sample is moved.
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ATOMIC FORCE MICROSCOPE
WHAT CAN WE LEARN?
Topography
Mechanical Properties
Elasticity
Friction
AFM Image and manipulation of an Adenovirus.
Binding
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A recent development uses an AFM to write with
biomolecules such as DNA sequences. This will
allow for the creation of micro DNA chips which
can be used a wide variety of applications
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There are now a number of systems that combine an
AFM with a conventional inverted microscope so
that light and surface information can be
collected from the same samples
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Nearfield Scanning Optical Microscopy (NSOM) In
NSOM a subwavelength (20 - 200 nm) aperture is
placed in close proximity to the surface to be
imaged (of the order of 10 nm). Light passing
through the aperture remains collimated for a
distance of the order of one aperture
diameter. If the aperture is maintained in the
near-field and scanned over a sample surface, an
image can be reconstructed point by point with
spatial resolution limited by the aperture
diameter rather than the wavelength of light.
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A key
technological advance was the development of the
tapered optical fiber probe by Betzig and
Trautman et. al. in 1991. Tapered optical fiber
tips are fabricated from single mode optical
fiber using a commercially available micro
pipette puller with a focused carbon dioxide
laser as the heat source. The aperture is formed
by coating the tapered fiber with a high
reflectivity metal (Al or Ag) via standard
thermal evaporation.
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As with STM the probe is rastered by the movement
of a Piezo-electric device
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AFM vs. STM vs. NSOM
In all three resolution is largely dependent on
probe size and the ability to control
scanning. STM requires a conductive specimen,
AFM and NSOM do not and both of these can be used
in air, vacuum, or in liquids. AFM physically
contact the specimen but STM and NSOM do not.
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A schematic of an atomic force microscope is
shown in the diagram above. The sample is mounted
on a piezo ceramic which can be moved extremely
accurately in the x, y and z directions. The
sample is then rastered in the x and y directions
under a sharp tip. This tip is mounted at the
free end of a cantilever (as shown) onto which a
laser beam is focussed. The beam is reflected
from the back of the cantilever to a set of four
photosensitive diodes. These act to detect any
deflection of the laser beam arising from the
cantilever moving as the sample is rastered. A
feedback loop then acts to move the piezo in the
z direction taking the laser beam back to its
original position. In this way the sample is
scanned with a constant force and the resulting z
piezo motion produce a How the AFM
works. topographical map of the region scanned
with a vertical resolution much smaller than 1A
in favorable cases. The AFM can be used in a
variety of environments, in air,in UHV or under
liquids.
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