180 likes | 322 Views
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.
E N D
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
Small Molecules and Bulk Materials From J. Chem. Ed. Vol 84 pp709-710
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
Gap Energy • Electronic transition in molecule is broad due to multiple vibrational states in addition to condensed phase considerations • Need the gap energy
Quantum Dot Size • From cutoff wavelength, calculate the nanoscale material band gap (Egnano) • All constant values given in handout
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
Fluorescence Spectrum Application: Fluorescent Lights
Metals • Sea of electrons model • MO model • Electrons and holes
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