Successfully reported this slideshow.
We use your LinkedIn profile and activity data to personalize ads and to show you more relevant ads. You can change your ad preferences anytime.

STM

2,422 views

Published on

edited

Published in: Technology, Business
  • Be the first to comment

  • Be the first to like this

STM

  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>

×