scanning tunneling microscope & atomic force microscopy
TRANSCRIPT
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Gerd BinnigJuly 20, 1947
German IBM Zurich
scanning tunneling microscope (1981)
Nobel Prize in Physics (1986) shared withErnst Ruska (born 1906 TEM 1933)
http://en.wikipedia.org/wiki/File:Gerd_Binnig_sw.jpg -
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Invention of the STM, 1981
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SPM Timeline
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Multimode SPM
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PRINCIPLE OF STM The STM is based on the concept of quantum
tunneling. When a conducting tip is brought very near to the
surface to be examined, a bias (voltage difference)applied between the two can allow electrons to
tunnel through the vacuum between them. The resulting tunneling currentis a function of tipposition, applied voltage, and the local density ofstates (LDOS) of the sample
Information is acquired by monitoring the current as
the tip's position scans across the surface, and isusually displayed in image form.
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http://upload.wikimedia.org/wikipedia/commons/f/f9/ScanningTunnelingMicroscope_schematic.png -
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STM STM can be a challenging technique, as it
requires extremely clean and stable surfaces,sharp tips, excellent vibration control, andsophisticated electronics.
Resolution is considered to be 0.1 nm lateralresolution and 0.01 nm depth resolution
Individual atoms within materials are imagedand manipulated.
STM can be used in ultra high vacuum, air,water, and various other liquid or gas ambients,and
at temperatures ranging from near zero kelvinsto a few hundred degrees Celsius
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STM - PROCEDURE First, a voltage bias is applied and the tip is brought
close to the sample by some coarse sample-to-tipcontrol, which is turned off when the tip and sampleare sufficiently close.
At close range, fine control of the tip in all threedimensions when near the sample is typically
piezoelectric, maintaining tip-sample separation Wtypically in the 4-7 range, which is the equilibriumposition between attractive (3
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STM - PROCEDURE
If the tip is moved across the sample in the x-y plane, thechanges in surface height and density of states causechanges in current. These changes are mapped inimages.
This change in current with respect to position can bemeasured itself, or the height, z, of the tip correspondingto a constant current can be measured.
These two modes are called constant height mode andconstant current mode, respectively.
In constant current mode, feedback electronics adjustthe height by a voltage to the piezoelectric height controlmechanism[
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Atomic force microscopy The precursor to the AFM, the scanning tunneling
microscope The AFM consists of a cantilever with a sharp tip
(probe) at its end that is used to scan the specimensurface. The cantilever is typically silicon or siliconnitride with a tip radius of curvature on the order ofnanometers. When the tip is brought into proximity ofa sample surface, forces between the tip and thesample lead to a deflection of the cantilever
Typically, the deflection is measured using a laser
spot reflected from the top surface of the cantileverinto an array of photodiodes
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Force
Surface Atoms
Tip Atoms
Photo detector
Laser Beam
Tip
Cantilever
Line Scan
Surface
Working Principle of AFM
www.shef.ac.uk/~htsl/afm.htm
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Mode of Operation Force of Interaction
Contact mode Strong (repulsive) - constant force or constant distance
Non-contact mode Weak (attractive) - vibrating probe
Intermittent contact mode Strong (repulsive) - vibrating probe
Lateral force mode Frictional forces exert a torque on the scanning cantilever
Magnetic force Magnetic field of the surface is imaged
Thermal scanning Distribution of thermal conductivity is imaged
Different Modes of AFM
http://www.mobot.org/jwcross/spm/
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Repulsive
Attractive
Contact Region
Non-ContactRegion
DistanceF
or
ce
Contact and Non- Contact Modes of AFM
www.shef.ac.uk/~htsl/afm.htm
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Three common types of AFM tip
Normal tip
(3 m tall)
Supertip Ultralever
(3 m tall)
Courtesy: Jean-Paul Revel, Caltech & Park Scientific Instruments
http://stm2.nrl.navy.mil/how-afm/how-afm.html
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Scanning Modes of AFM
Constant Height Modes Constant Force Modes
Force Force
www.shef.ac.uk/~htsl/afm.htm
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Comparison of AFM & other Microscopy Techniques
AFM TEM SEM
Max resolution Atomic Atomic 1s nm 100s nm
Imaging Environment Air, fluid, Vacuum, Special gas vacuum vacuum Air, Fluid
In-situ Yes No No Yes
In fluid Yes No No Yes
Sample preparation Easy Difficult Easy Easy
OM
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STM systems
Measure the quantum tunnelling current between a wire or metal-coated
silicon tip and the object surface. An electronic feedback system maintains a
constant current by positioning the tip to exactly contact the surface.
AFMsystemsDetect the z-displacement of the cantilever by the reflection of a laser beam
focused on the top surface of the cantilever. The feedback from this sensor
maintains the probe at a constant force
AFM Vs STM
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VVv
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Nano Fabrication
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Applications of AFM