1 / 37

ECE 802-604: Nanoelectronics

ECE 802-604: Nanoelectronics. Prof. Virginia Ayres Electrical & Computer Engineering Michigan State University ayresv@msu.edu. Lecture 27, 03 Dec 13. Molecular Electronics: Why not polyacetylene? or any conjugated “ene”? Examples of possibilities Actual performance

carolbutler
Download Presentation

ECE 802-604: Nanoelectronics

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. ECE 802-604:Nanoelectronics Prof. Virginia Ayres Electrical & Computer Engineering Michigan State University ayresv@msu.edu

  2. Lecture 27, 03 Dec 13 • Molecular Electronics: • Why not polyacetylene? or any conjugated “ene”? • Examples of possibilities • Actual performance • Electronic (p) structure brief review • Mechanical (s) structure brief review • New: bond alteration structure in polyacetylene • Electronic result of bond alteration structure • Qualitative • Quantitative • Solitons (polarons): Su-Schreiffer-Heeger (SSH) model VM Ayres, ECE802-604, F13

  3. H H H H c c c c c c c c c H H H H H New: Bond alteration polyacetylene: HAA types:no formula changes due to long and short bonds “A” “B” -a +a VM Ayres, ECE802-604, F13

  4. H H H H c c c c c c c c c H H H H H New: Bond alteration polyacetylene: HAB types -a +a “A” “B” “B” VM Ayres, ECE802-604, F13

  5. Two “identical” bond alterations VM Ayres, ECE802-604, F13

  6. Describe as: a perturbation of the original.Two chances of it happening A bit less A bit more A bit less A bit more VM Ayres, ECE802-604, F13

  7. Describe as: a perturbation on the original. Two possibilties more less VM Ayres, ECE802-604, F13

  8. Original: VM Ayres, ECE802-604, F13

  9. Two possibilities: VM Ayres, ECE802-604, F13

  10. VM Ayres, ECE802-604, F13

  11. For HW: do the 2nd nearest neighbor “B” atoms N = 2 in the original model Also ask: Where does HAB come form? VM Ayres, ECE802-604, F13

  12. Now repeat with unequal bond lengths: Now have four possibilities for where Carbon “B” is:: VM Ayres, ECE802-604, F13

  13. t0 = Example: Units of t0 = ? Units of a x0 = ? Units of a = ? VM Ayres, ECE802-604, F13

  14. t0 = Answer: Units of t0 = eV Units of a x0 = eV Units of a = eV/ (distance = Ang) a is a phonon coupling coefficient: Converts the extra bit distance into the impact this perturbation has on the energy levels VM Ayres, ECE802-604, F13

  15. E-k relationship for more realistic polyacetylene with bond alteration: VM Ayres, ECE802-604, F13

  16. E-k relationship for more realistic polyacetylene with bond alteration: Solve for E: This bond alteration realism “opened up a gap” but it seems narrow so what’s the problem with the slow transport? For polyacetylene: VM Ayres, ECE802-604, F13

  17. Polyactylene without bond alterations Polyactylene with bond alterations Egap = 0.4 eV +0.2 eV - 0.2 eV Electrons will want to bond using the lowest energy level possible. Bond alteration configurations “lock”. VM Ayres, ECE802-604, F13

  18. Polyactylene without bond alterations Polyactylene with bond alterations Egap = 0.4 eV +0.2 eV - 0.2 eV Electrons will want to bond using the lowest energy level possible. Bond alteration configurations “lock”. The major problem: VM Ayres, ECE802-604, F13

  19. Polyactylene with bond alterations Minor problem: Egap: Not so narrow: Egap = 0.4 eV +0.2 eV - 0.2 eV VM Ayres, ECE802-604, F13

  20. Lecture 27, 03 Dec 13 • Molecular Electronics: • Why not polyacetylene? or any conjugated “ene”? • Examples of possibilities • Actual performance • Electronic (p) structure brief review • Mechanical (s) structure brief review • New: bond alteration structure • Electronic result of bond alteration structure • Qualitative • Quantitative • Solitons (polarons): Su-Schreiffer-Heeger (SSH) model VM Ayres, ECE802-604, F13

  21. 2 “identical” bond alterationsNomenclature: both are = “fully isomerized”: means: large segments of each chain type can form. VM Ayres, ECE802-604, F13

  22. What about this? Some connection here Can be neutral or charged VM Ayres, ECE802-604, F13

  23. This defect is a soliton. w Defect = “soliton” VM Ayres, ECE802-604, F13

  24. A soliton is a defect site that separates the two “phases” of polyacetylene “W” = the soliton “wall width” VM Ayres, ECE802-604, F13

  25. ES( ) VM Ayres, ECE802-604, F13

  26. ES( ) ES( ) The minimum energy of the soliton ES is ALWAYS within the gap Egap! Egap VM Ayres, ECE802-604, F13

  27. ES( ) ES( ) Another way to say this is that there is a localised electronic state (the soliton) at the center of the gap Egap VM Ayres, ECE802-604, F13

  28. ES( ) Plot of the probability distribution of the localised electronic state (the soliton) at the center of the gap VM Ayres, ECE802-604, F13

  29. ES( ) ES( ) Yet another way to say this is that “the soliton formation energy is less than that needed to create a band excitation”. That means an electron doesn’t go into the conduction band – it goes into the creation of a charged soliton Egap VM Ayres, ECE802-604, F13

  30. PART 01 of problem: A and B structures form VM Ayres, ECE802-604, F13

  31. PART 02 of problem: A and B structures are connected by a defect with its own local energy state in the middle of the bandgap. “the soliton formation energy is less than that needed to create a band excitation”. That means an electron doesn’t go into the conduction band – it goes into the creation of a charged soliton VM Ayres, ECE802-604, F13

  32. Energy of an electron in the soliton region solved using a Green’s function approach VM Ayres, ECE802-604, F13

  33. Corresponding wavefunction for the electron in the soliton region VM Ayres, ECE802-604, F13

  34. 2 n = 0, ± 2, 4, 6,….. (for odd n: f0(n) = 0) l is a “stretching parameter” that scales n/l a = 1.22 Angstroms = the x-spacing between CH groups VM Ayres, ECE802-604, F13

  35. A neutral soliton has an unpaired electron: VM Ayres, ECE802-604, F13

  36. Two different transport situations defeated by soliton: Situation 01 on left: This is in a single polyacetylene chain. A dopant added to polyacetylene chain, say a nitrogen atom N. Soliton becomes charged with one dopant-contributed electron. Charged soliton grabs an off-chain impurity = the parent phosphorous N+ ion at a distance of about 2 angstroms and becomes neutral. Everyone’s happy except the experimenter. Pinning results. Transport tanks. VM Ayres, ECE802-604, F13

  37. Two different transport situations defeated by soliton: Situation 02 on right: This is in a self-assembled monolayer of many aligned polyactylene chains. Experimenter liberates an electron from a neutral soliton using a laser. It’s supposed to go into the conduction band of that polyactylene chain. Actually it goes into charging up another soliton on an adjacent chain at distance of about 4 angstroms. The two solitons, the first + charged and the second - charged lock up. End of transport. The experimenter predicts it will take 20 years to finish his/her Ph.D. and tears hair out VM Ayres, ECE802-604, F13

More Related