Light and Quantum dots

• The same light into a semiconductor nanoparticle will produce different colors radiating out depending on particle size because size controls ΔE .
E2 ∆E
E2 ∆E
allowed energies for electrons
E1
*This phenomenon of energy levels and forbidden energies is part of what is
called “Quantum Mechanics”
Copyright April 2009 The Pennsylvania State University
Copyright April 2009 The Pennsylvania State University
Photo-electric effect: UV light shining onto a charged metal caused it to loose its charge, while shining visible light on the metal had no effect
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ቤተ መጻሕፍቲ ባይዱ
How is that light produced? Atomic Excitation
Atom in natural state
Excited (heated) atom:
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Each jump corresponds to a different energy = frequency (color) of emitted light
Einstein: light behaves as a bunch of particles (photons). Each photon is like a “bundle of energy”. UV photons carry more energy than visible-light ones: no matter how much visible light you shine on the metal, none of the photons has enough energy to kick out an electron
Copyright April 2009 The Pennsylvania State University
Details
The emitted light from a semiconductor nanoparticle is called fluorescence. Nanoparticles can fluoresce very strongly and, the color of the fluorescence can be adjusted by adjusting the size of the nanoparticles. Semiconductor nanoparticles with this strong fluorescence property are called “quantum dots”, because of the connection of the light color to ΔE and the connection of ΔE to the allowed energies predicted by quantum mechanics.
/en/simulation/photoelectric
Copyright April 2009 The Pennsylvania State University
The Electromagnetic Spectrum
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Producing light of a particular color: The atomic spectrum
allowed energies for electrons E2 ∆E
allowed energies for electrons
E1
Copyright April 2009 The Pennsylvania State University
allowed energies for electrons
allowed energies for electrons
E1
Same light in Light out depends on nanoparticle size
“Relative Size of a Qdot® Nanocrystal.” Invitrogen Corporation. 2009.
Copyright April 2009 The Pennsylvania State University
The Product: Quantum dots These are available commercially and used in, for example, medical research for their very strong fluorescence color. The color they give off when excited can be changed simply by buying different sized dots which changes ΔE and thereby the fluorescing color.
“Relative Size of a Qdot® Nanocrystal.” Invitrogen Corporation. 2009.
This picture of vials containing actual quantum dots was captured after the samples were placed in front of a UV hand lamp which excited the electrons from below E1 to above E2.
Copyright April 2009 The Pennsylvania State University
• Electrons in semiconductor nanoparticles can only have allowed energies* and they have forbidden energy ranges ΔE just like the electrons in atoms. (fact) • The size of semiconductor nanoparticles determine the energy difference ΔE between the bands of allowed energies: ΔE gets bigger as the nanoparticle size gets smaller. (why? See Heisenberg principle)
Elements heated by a flame glow their characteristic color. The color of each element is revealed to consist of a pattern of distinct frequencies: its atomic spectrum
Light
17th century: Newton suggested that light should be regarded as a “stream of particles” Early 19th century: Young shows light interference with double-slit experiment -> it’s a wave! (Newton was wrong?) End of 19th century: Experiments cannot be explained with wave theory
• Incoming light with energy equal to or bigger than ΔE can excite an electron in the nanoparticle from the “ground state” band of energies below the energy E1 to the more energetic, “excited state” band of energies above energy E2. • The excited electron will eventually relax back down to the “ground state” band below E1 and, in a semiconductor nanoparticle, release its extra energy ΔE as emitted light.
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/en/simulation/discharge-lamps
Copyright April 2009 The Pennsylvania State University
Quantum Dots: Semiconductor nanoparticles