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  1. 1. Scanning Techniques and Fast Fourier Transform RM August 28, 2007 Scanning Tunneling Microscopy
  2. 2. Easy Scan <ul><li>Software enabling researchers to view and clarify sample </li></ul><ul><li>Carefully place sample near the tip </li></ul><ul><li>Allow computer to “approach” the sample </li></ul><ul><li>Scan should appear </li></ul><ul><li>Green light- approach is done and successful </li></ul><ul><li>Red light- the sample crashed into tip </li></ul>
  3. 3. Troubles with the tip? <ul><li>Crashing is a test of patience…and it’s inevitable </li></ul><ul><ul><li>Steps to fix a crash: </li></ul></ul><ul><ul><ul><li>Press “withdraw” and retry the approach </li></ul></ul></ul><ul><ul><ul><li>Change approach step- refine it so tip doesn’t crash </li></ul></ul></ul><ul><ul><ul><li>Change current- decrease it so image can scan from greater distance, increase if tip doesn’t reach close proximity with surface </li></ul></ul></ul><ul><ul><ul><li>Change the tip (note that the tip can sustain many crashes before a new one is needed) </li></ul></ul></ul>
  4. 4. Tip Changing 101 <ul><li>Needed when images are blurry and full of noise </li></ul><ul><li>Procedure </li></ul><ul><ul><li>Using pliers, pull out a portion of wire (1/2 in. or less) from STM bag </li></ul></ul><ul><ul><li>Pull wire diagonally with wire cutters (cutting and tugging simultaneously) </li></ul></ul><ul><ul><li>Carefully place under the gold clip </li></ul></ul>
  5. 5. Settings of Easy Scan <ul><li>X-Y Ranges </li></ul><ul><ul><li>Adjust the tilt of the surface </li></ul></ul><ul><li>Z-Range </li></ul><ul><ul><li>Height of tip above surface (depends on current) </li></ul></ul><ul><ul><ul><li>Lower z-range, greater knowledge of surface heights </li></ul></ul></ul><ul><li>Scan Range </li></ul><ul><ul><li>Side length of the square area being scanned </li></ul></ul><ul><ul><ul><li>Smaller scan range, smaller area scanned, </li></ul></ul></ul><ul><ul><ul><li>“ zoomed in” </li></ul></ul></ul>
  6. 6. Even More Settings… <ul><li>Gap Voltage </li></ul><ul><ul><li>Should be varied depending on material studied </li></ul></ul><ul><li>Set point </li></ul><ul><ul><li>The current </li></ul></ul><ul><ul><ul><li>Generally doesn’t need to be changed, except in tip troubles </li></ul></ul></ul>
  7. 7. Filtering and Resolution Techniques <ul><li>When do you need to filter? </li></ul><ul><ul><li>Noise, scan lines, valleys, spikes, blurry images, etc. </li></ul></ul><ul><li>How do you filter? </li></ul><ul><ul><li>Outlier objects- spikes + particles </li></ul></ul><ul><ul><ul><li>Click on filter outlier objects and select objects to be filtered out </li></ul></ul></ul><ul><ul><ul><li>Changing pixels- increase them for clarity- try not to go beyond 256 x 256 pixels- too much time </li></ul></ul></ul>
  8. 8. Filtering and Resolution Techniques, Part 2 <ul><li>Change “I-gain” and “P-gain” </li></ul><ul><ul><li>Correct errors from the feedback loop that maintains constant current </li></ul></ul><ul><ul><ul><li>Sensitive to changes in form on sample </li></ul></ul></ul><ul><ul><ul><ul><li>High gains (very sensitive) can sometimes provide unusable images </li></ul></ul></ul></ul><ul><ul><li>When satisfied with images, scan 2 images </li></ul></ul><ul><ul><ul><li>Very subjective- how much is too much? </li></ul></ul></ul><ul><ul><ul><li>Scan one up, scan the other down </li></ul></ul></ul><ul><ul><ul><ul><li>This checks drift </li></ul></ul></ul></ul>
  9. 9. Other Analysis Tools <ul><li>Line profiles </li></ul><ul><ul><li>Height compared to distance for a section </li></ul></ul><ul><ul><li>Draw line across image, plot forms </li></ul></ul><ul><ul><ul><li>Measure distance between atoms and their heights </li></ul></ul></ul>
  10. 10. Fourier Transform <ul><li>Form of processing and filtering </li></ul><ul><ul><li>Translates data into sine waves, showing differing frequencies and amplitudes </li></ul></ul><ul><ul><li>Signal = sum of sine waves; FT signal = amplitude of a specific frequency </li></ul></ul><ul><li>Why it works </li></ul><ul><ul><li>Noise isn’t periodic; structures are </li></ul></ul><ul><li>Bright spots </li></ul><ul><ul><li>Appear on image after FFT activated </li></ul></ul><ul><ul><li>Draw box around bright spots and press “filter” </li></ul></ul><ul><ul><li>Clear image will appear </li></ul></ul>
  11. 11. Fourier Transform <ul><li>Types of filters: </li></ul><ul><ul><li>Butterworth </li></ul></ul><ul><ul><ul><li>Least ripples; frequency response is the flattest it can be within a passband </li></ul></ul></ul><ul><li>Other filter types: </li></ul><ul><ul><li>Gaussian, Elliptic, Chebyshev I and II </li></ul></ul>
  12. 12. Other Information... <ul><li>Most samples oxidize when left in air and become contaminated </li></ul><ul><ul><li>Causes scanning difficulties </li></ul></ul><ul><li>Constant Current Imaging </li></ul><ul><ul><li>Constant current maintained </li></ul></ul><ul><ul><ul><li>If two regions have same brightness, equal chance of finding electron there </li></ul></ul></ul><ul><li>St. Joe’s work with the STM </li></ul>