Stm 07.08.13


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Stm 07.08.13

  1. 1. Scanning Tunneling Microscopy 09TT20 CHARACTERIZATION OF TEXTILE POLYMERS Presentation I S.Dhandapani – 11MT62
  2. 2. Introduction  Invented by Binnig and Rohrer at IBM in 1981 (Nobel Prize in Physics in 1986).  Binnig also invented the Atomic Force Microscope(AFM) at Stanford University in 1986.
  3. 3. Introduction  Topographic (real space) images  Spectroscopic (electronic structure, density of states) images
  4. 4. Introduction  Scanning tunneling microscopy is a microscopical technique that allows the investigation of electrically conducting surfaces down to the atomic scale.  Atomic resolution, several orders of magnitude better than the best electron microscope.  Quantum mechanical tunnel-effect of electron.  Material science, physics, semiconductor science, metallurgy, electrochemistry, and molecular biology.
  5. 5. Working Principle of STM  In the scanning tunneling microscope the sample is scanned by a very fine metallic tip.  The tip is mechanically connected to the scanner, an XYZ positioning device realized by means of piezoelectric materials.  The sample is positively or negatively biased so that a small current, the "tunneling current" flows if the tip is in contact to the sample.
  6. 6. Working Principle of STM  A sharp conductive tip is brought to within a few Angstroms of the surface of a conductor (sample).  The surface is applied a bias voltage, Fermi levels shift.  The wave functions of the electrons in the tip overlap those of the sample surface.  Electrons tunnel from one surface to the other of lower potential.  The tunneling system can be described as the model of quantum mechanical electron tunneling between two infinite, parallel, plane metal surfaces
  7. 7. Working Principle of STM  This feeble tunneling current is amplified and measured.  With the help of the tunneling current the feedback electronic keeps the distance between tip and sample constant.  If the tunneling current exceeds its preset value, the distance between tip and sample is decreased, if it falls below this value, the feedback increases the distance.  The tip is scanned line by line above the sample surface following the topography of the sample.
  8. 8. Experimental methods  the sample you want to study  a sharp tip mounted on a piezoelectric crystal tube to be placed in very close proximity to the sample  a mechanism to control the location of the tip in the x-y plane parallel to the sample surface  a feedback loop to control the height of the tip above the sample (the z-axis) Basic Set-up
  9. 9. Tunneling Tips Cut platinum – iridium wires Tungsten wire electrochemically etched Tungsten sharpened with ion milling Best tips have a point a few hundred nm wide In reality is relatively easy to obtain such tips by etching or tearing a thin metal wire. Very small changes in the tip-sample separation induce large changes in the tunneling current.
  10. 10. How to operate?  Raster the tip across the surface, and using the current as a feedback signal.  The tip-surface separation is controlled to be constant by keeping the tunneling current at a constant value.  The voltage necessary to keep the tip at a constant separation is used to produce a computer image of the surface.
  11. 11. Two Modes of Scanning Constant Height Mode Constant Current Mode Usually, constant current mode is superior.
  12. 12. Tunneling Current  The reason for the extreme magnification capabilities of the STM down to the atomic scale can be found in the physical behavior of the tunneling current.  The tunneling current flows across the small gap that separates the tip from the sample,in better approach of quantum mechanics the electrons are "tunneling" across the gap.  The tunneling current I has a very important characteristic: it exhibits an exponentially decay with an increase of the gap d.
  13. 13. I= K*U*e -(k*d) k and K are constants, U is the tunneling bias Tip-sample tunneling contact Exponential behavior of the tunneling current I with distance d
  14. 14. STM Tips  Tunneling current depends on the distance between the STM probe and the sample Surface Tunneling current depends on distance between tip and surface
  15. 15. Tunneling Current • It shows a cross section of a sample surface with two surface atoms being replaced by foreign atoms, for instance adsorbates (black). • While at low bias (red) the tip may follow the "actual" topography, there may also be a bias where no contrast is obtained (green) or a bump is seen above the adsorbates (blue). • This bias dependent imaging is used to create the color images: three individual STM images of the same sample area are obtained at different tunneling bias.
  16. 16. Advantages  No damage to the sample  Vertical resolution superior to SEM  Spectroscopy of individual atoms  Relatively Low Cost Disadvantages  Samples limited to conductors and semiconductors  Limited Biological Applications: AFM  Generally a difficult technique to perform Figures of Merit Maximum Field of View: 100 μm Maximum Lateral Resolution: 1 Å Maximum Vertical Resolution: .1 Å
  17. 17. Interesting Images with STM Copper Surface Xenon on Nickel Single atom lithography
  18. 18. Iron on Copper Quantum Corrals Imaging the standing wave created by interaction of species
  19. 19. Carbon Monoxide Man: CO on Platinum
  20. 20. Graphite is a good example!  STM images of graphite  Structure of graphite  Overlay of structure shows only every other atom is imaged
  21. 21. Thank you
  22. 22. Basic Principles of STM Electrons tunnel between the tip and sample, a small current I is generated (10 pA to 1 nA). I proportional to e-2κd , I decreases by a factor of 10 when d is increased by 1 Å.  d ~ 6 Å Bias voltage:
  23. 23. Instrumental Design: Controlling the Tip Raster scanning Precise tip control is achieved with Piezoelectrics Displacement accurate to ± .05 Å