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Quantum Dot Research: Spectroscopy

Quantum Dot Research: Spectroscopy. S117 Fall 2011. Overview. Post-Lab Discussion of Synthesis Pre-lab: Quantum Dot Spectroscopy Band gap theory review Determination of band gap energy Determination of quantum dot size Fluorescence Application to materials.

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Quantum Dot Research: Spectroscopy

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  1. Quantum Dot Research:Spectroscopy S117 Fall 2011

  2. Overview • Post-Lab • Discussion of Synthesis • Pre-lab: Quantum Dot Spectroscopy • Band gap theory review • Determination of band gap energy • Determination of quantum dot size • Fluorescence • Application to materials

  3. Small Molecules and Bulk Materials From J. Chem. Ed. Vol 84 pp709-710

  4. Particle in a Box! • Tune the energy of band gap through size of quantum dot • Measure band gap energy—use theory to determine size • How do we measure? Absorbance spectroscopy

  5. Gap Energy • Electronic transition in molecule is broad due to multiple vibrational states in addition to condensed phase considerations • Need the gap energy

  6. Wavelength Cutoff

  7. Quantum Dot Size • From cutoff wavelength, calculate the nanoscale material band gap (Egnano) • All constant values given in handout

  8. Fluorescence • Photons absorbed to promote electron into various vibrational modes in electronic excited state • Partial relaxation of electron through loss of vibrational energy (heat-during collisions) to ground vibrational state of excited state • Emission of photon in return to ground state—lower in energy

  9. Fluorescence Spectrum Application: Fluorescent Lights

  10. Stokes shift

  11. Application to Materials

  12. Conductors, Insulators, and Semi-conductors

  13. Metals • Sea of electrons model • MO model • Electrons and holes

  14. Carbon: Conductor or Insulator?

  15. Semiconductor • More difficult, but still possible, to transfer valence electrons into the conduction band • Free electrons and holes have energy difference • Recombination releases energy in form of light • Not very efficient

  16. Doping: More efficient Semiconductors

  17. Two Distinct Materials

  18. Light Emitting Diodes

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