Lecture 11 - QDs - MSE 803: N ANOMATERIALS AND N...

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Unformatted text preview: MSE 803: N ANOMATERIALS AND N ANOTECHNOLOGY Professor Xudong Wang 213 MSE Building Spring 2010 Class 11: Quantum Dots Quantum Dots Definitions Atoms and molecules -- single to coordinated groups of atoms Clusters >100 atoms: term describes a seed that has the same stoichiometry and crystal structure as bulk material, but not the electronic & optical properties of the bulk material Cluster to Crystallite transition: ~ 700 2,000 atoms (2.8 4.5 nm) Crystallites: Quantum Dots: 10,000 atoms - size < 6 nm, Still less than 20 lattice spacing & crystal essentially not large enough for intrinsic bulk band gap to form Quantum semiconductors and metals Basic Physics of Semiconductors Consider combination of impurity doping + quantum dots to form a new class of luminescent material. Absorption & Emission in Bulk Semiconductors Transparent Absorption and Exciton Formation Electron seeks lowest energy state. Recombination and Photon Emission g E hc wavelength Absorption (a.u.) wavelength Intensity CdS Energy Gap moves to shorter Wavelengths (increased energy) Also Quantization ZnSe & CdS Spectra vs. Size of Particles Optical density (absorption) : From bulk to 2.8 nm dots: Band gap energy increases corresponding wavelength shift 440 nm to 350 nm ZnSe Size Effects in Semiconductor Quantum Dots Exciton Bohr Radius Effective Mass ) 1 1 ( h e HB B m m r r Exciton Energy Parameter ) 1 )( ( 6 . 13 2 h e h e y m m m m eV R y R g E = 2.8eV o m = mass of electron in vac. e m = 0.17 o m HB r = 0.0529nm h m = 0.6 o m = 8.1 B r = 3.23nm = 0.0275eV Gaponenko 1997 y B y B g r R R r R R r E E ) ( 786 . 1 ) ( 2 2 Effective Mass Approximation Breakdown of EMA Quantum Confinement Model Quantum Confinement Modeling...
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This note was uploaded on 03/23/2010 for the course MATERIAL S 803 taught by Professor Samuel during the Spring '10 term at Wisconsin.

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Lecture 11 - QDs - MSE 803: N ANOMATERIALS AND N...

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