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RelativeResourceManager(21) - Scanning probe microscopy...

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Scanning probe microscopy Chem 420 November 19, 2008 Professor Ryan C. Bailey
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Principles of scanning probe microscopy (SPM) z All SPM instruments monitor interactions between small probe tips and a specimen z STM measures the tunneling current between a metallic tip and a conducting substrate z AFM measures the van der Waals force between tip and sample (also other forces)
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Development of Scanning Probe Microscopy z Binnig and Rohrer z Nobel Prize in Physics in 1986 for their design of the scanning tunneling microscope (STM). z Other tip-sample interactions including direct physical contact
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Design requirements z Accurate control of the tip-surface distance to within 0.01nm by moving the tip or sample z Tip must be very sharp-ideally only one atom is at the closest tip-surface distance z Accurate measurement of chemical and physical interactions
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High precision position control piezo scanners Principle of piezo element. The applied voltage makes the element longer or shorter The combination of three piezo elements makes it possible to move the STM tip in the X-, Y-, and Z-directions. In most modern scanning probe microscopes, one uses a tube geometry.
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Scanning Tunneling Microscopy z Conductive tip rastered across conductive sample z Constant current (height mode): z tip-sample distance held constant by changing tunneling current z Constant height (current mode): z preset current value maintained by feedback control of z piezo
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Zooming in on tip
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Zooming in to the atomic level z At higher magnification, the tip is rough z At the atomic scale, there are only a few atoms at the outermost edge
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Current flows between the conducting sample and tip Direction of current flow depends on polarity of the bias. Shown here, the current flows from surface to tip
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In a metal, the energy levels of the electrons are filled up to a particular energy, known as the ‘Fermi energy’ E F . In order for an electron to leave the metal, it needs an additional amount of energy Φ , the so-called ‘work function’ When the specimen and
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RelativeResourceManager(21) - Scanning probe microscopy...

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