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Quantum Dot Research: Spectroscopy

This overview covers the fundamental concepts of quantum dot spectroscopy, focusing on the synthesis of quantum dots and their characterization through band gap energy and size determination. The discussion includes the principles of fluorescence, the effects of nanoscale dimension on band gap energy, and practical applications in materials science, notably in conductors, insulators, and semiconductors. Key experiments utilize absorbance spectroscopy and fluorescence spectra to explore electronic transitions and energy emission, highlighting the importance of quantum dots in modern technologies such as LEDs and fluorescent lights.

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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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