BME 210 Lecture 9 Scanning Probe Microscopy

BME 210 Lecture 9 Scanning Probe Microscopy - 9. Scanning...

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9. Scanning Probe Microscopy
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Scanning Probe Microscopy High-resolution images are generated by scanning objects with a small electrical, mechanical, optical or other probe 1. Scanning Tunneling Microscopy 2. Atomic Force Microscopy 3. Near-Field Scanning Optical Microscopy Also: Ion Conductance Microscopy, Scanning Thermal Microscopy, etc
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Principle of Scanning Tunneling Microscopy (STM) The “ Tunneling current” effect: Since electrons are waves, they are de-localized: there is some probability they penetrate a non- conducting barrier (e.g., air) generating electrical current The tip and the sample must be conductive, the space between - non-conductive The tunneling current decreases exponentially , with increasing the tip-sample distance (≈10-fold per 0.1 nm) current measurements are very sensitive to changes in distance Working tip-sample distance ≈1 nm Applied voltage ≈0 - 4 V; tunneling current ≈10 pA - 10 nA
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STM design • The tip is connected to a piezo- electric XYZ scanner (a tube with 3 piezo-crystals on its sides, each controlled independently via applied voltage) • The tip is moved in 2D raster above the sample surface while tunneling current is measured • If the tip height is kept constant, current magnitude translates to surface elevation • Current measurements can be used in a feedback mechanism to keep constant current by changing the tip height • STM image represents a surface profile of electron density STM was invented by G. Binnig and H. Rohrer in 1981 (1986 Noble Prize shared with E. Ruska)
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STM tip Tungsten or Platinum-Iridium Electrochemically etched Tip diameter – down to a single atom Typically – few nm
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BME 210 Lecture 9 Scanning Probe Microscopy - 9. Scanning...

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