Chem 823 presentation


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Chem 823 presentation

  1. 1. Chemical identification of individual surface atoms by Atomic Force Microscopy Sugimoto, Pou, Abe, Jelinek, Peres, Morita & Custance Nature 446, 64-67, 2007 Vivek J P Chem 823 presentation March 2 nd 2007
  2. 2. Outline <ul><li>Introduction </li></ul><ul><li>Background </li></ul><ul><li>Discussions on the work </li></ul><ul><li>Conclusion and comments </li></ul>
  3. 3. Scanning Probe microscopy <ul><li>Describes a series of imaging techniques </li></ul><ul><ul><li>based on tip surface interactions. </li></ul></ul><ul><ul><ul><li>STM Tunneling current </li></ul></ul></ul><ul><ul><ul><li>AFM Atomic forces </li></ul></ul></ul><ul><ul><ul><li>MFM Magnetic force </li></ul></ul></ul><ul><ul><ul><li>EFM Electrostatic forces </li></ul></ul></ul><ul><ul><ul><li>SECM Electrochemical potential or current </li></ul></ul></ul><ul><ul><ul><li>…… .. (and many more) </li></ul></ul></ul><ul><ul><li>STM </li></ul></ul><ul><ul><ul><li>Resolve individual atoms </li></ul></ul></ul><ul><ul><ul><li>measure electrical properties </li></ul></ul></ul><ul><ul><ul><li>Can be used to image conducting samples only </li></ul></ul></ul><ul><ul><li>AFM </li></ul></ul><ul><ul><ul><li>Resolve individual atoms </li></ul></ul></ul><ul><ul><ul><li>Can be used to image conducting and non conducting samples </li></ul></ul></ul>
  4. 4. Atomic Force Microscope Modes of operation : Static Dynamic Amplitude Modulation Frequency modulation Probing the force between sharp tip and sample Sample + holder cantilever probe Laser Photo detector mirror Electronic control system
  5. 5. How does AFM work (Dynamic, frequency modulated) <ul><li>Cantilever vibrates </li></ul><ul><li>Interacts with sample’s surface </li></ul><ul><li>Causing the resonant frequency to change </li></ul><ul><li>Measured as a function of position </li></ul><ul><li>And converted into a 3D image </li></ul>
  6. 6. Chemical identification of individual atoms One of the thorniest problems in surface science. In principle, dynamic force microscopy should make it possible (motivation of the present work) Determining Chemical composition of nanostructures remains a challenge (especially for insulators)
  7. 7. Discussions on the work Chemical identification of individual atoms <ul><li>Dependence of the force on the distance between the tip and the individual atoms on the surface </li></ul><ul><li>(Sn and Pb atoms adsorbed on Si) </li></ul><ul><li>Dynamic force microscopy topographic images of a single-atomic layer of Sn (d) and Pb (e) grown over a Si(111) substrate. </li></ul><ul><li>The idea is to measure maximum attractive force </li></ul>d e ( 4.3 x 4.3) nm 2
  8. 8. Discussions on the work (contd…) <ul><li>Sets of short range force curves </li></ul><ul><li>(a) Sn and Si atoms and (b) Sn and Si atoms, in different sets of experiments </li></ul><ul><li>The results were not reproducible ! </li></ul><ul><li>Exact composition and shape of the tip’s apex on the atomic scale is impossible to establish with certainty. </li></ul>a b
  9. 9. Discussions on the work (contd…) The force curves of Sn/Si and Pb/Si before (a&c) and after (b&d)normalization (Key step )
  10. 10. Discussions on the work (contd… ) Theoretical modeling for the interaction between tip and surface Force curves before normalization Force curves after normalization
  11. 11. Discussions on the work (contd…) (a) Topographic image of a surface alloy composed by Si, Sn and Pb atoms (b) Height distribution of the atoms in (a) (c) and (d): Local chemical composition
  12. 12. Discussions on the work (contd…) <ul><li>Strongest interaction was always between tip and Si </li></ul><ul><li>The ratio remains distinct and reproducible </li></ul><ul><li>Atomistic modeling supports these observations </li></ul><ul><li>The experiment was successful to label atoms on an alloy of Sn, Si and Pb atoms on Si surface </li></ul>
  13. 13. Conclusions Comments: <ul><li>Chemical identification of complex and insulating surfaces yet to be attained. </li></ul><ul><li>In-plane local spatial variation of long range forces may affect, on extending this approach to other systems. </li></ul><ul><li>Possibility of maximum attractive force overlapping. </li></ul><ul><li>A statistical approach to measuring force can provide the local composition and structure of a semiconductor surface at the atomic level. </li></ul><ul><li>May lead to extremely discriminatory chemical analyses. </li></ul>
  14. 14. <ul><ul><li>Thank you </li></ul></ul>
  15. 16. Sample + holder cantilever probe Laser Photo detector mirror Electronic control system