1. Semiconductor Nanocrystals
Aka Quantum Dots
Rachel Wooten
February 28, 2006
Fat Tuesday!
Transmission Electron Microscope image of CdSe nanocrystal

2. Outline What are semiconductor nanocrystals? How do we make them?
How do they work?
Applications: Why do we care?

3. Semiconductor Nanocrystals
As the name implies, Semiconductor nanocrystals are tiny (generally fewer than 100 Angstroms in diameter) crystals of a semiconductor material.

4. How to make them Vapor deposition Ion implantation Sol-gel methods
Micelle methods
Organometallic synthesis

5. Why they work - Bohr exciton radius
Roughly follows particle in a box model
In bulk CdSe, absorption of a photon creates an electron hole pair
Electron and hole maintain a characteristic distance known as the bulk Bohr exciton radius. This value depends on the material properties.
For CdSe, this radius ~ 56 Å
Time scale ~ microseconds

6. Bohr exciton radius
When electron gets “kicked up” into the band gap, it physically separates from the hole it leaves behind.
However, they don’t get any farther away from each other than the Bohr exciton radius due to Coulomb attraction
Bohr radius is larger for semiconductors with higher dielectric constants (Coulomb field is hampered more by larger dielectric constant, so electron and hole “feel” each other less.)

7. Quantum confinement
When nanocrystal has a diameter of less than 112 Å, electron and hole cannot achieve their desired separation.

8. Particles in a box Quantum Confinement
Electron and hole become “particles in a box” because they cannot escape the outer walls of the crystal.
The outer wall presents an infinite potential for practical purposes.

9. Tunable infinte spherical well
Qualitatively same behavior as particle in an infinite square well
As the crystal gets smaller, the energy of the first excited state increases; as is gets larger, the energy decreases.
Small 1.7nm << >>Large 7.0nm

10. The Brus Model Three assumptions 1. The nanocrystal is spherical.
2. The interior of the nanocrystal is a uniform medium; there are no point charges or occupied spaces other than the excited electron and hole
3. The potential energy outside the nanocrystal is infinite; thus the electron and hole are always found within the nanocrystal

11. An imperfect model
Unfortunately, model only works well for larger nanocrystals
The smaller nanocrystals have transition energies much higher than predicted in Brus’s model.
* There are better models, but they are much more complicated

12. Applications Photovoltaics Single-electron transistors
Fluorescent tags for biological imaging
Surface paints and coatings
Tunable Light Emitting Diodes (LEDs)
The new lightbulb?

13. Photovoltaics
Sunlight incident on semiconductor produces electron-hole pairs: move electrons laterally using a bias voltage.
Relatively cheap, easy to produce
However, currently very inefficient due to electron-hole recombination losses.

14. Single-elelctron transistors
Because nanocrystals are tiny semiconductors, could be used in microscopic circuits
Might be made by vapor deposition to lay them out regularly on a substrate (e.g. a circuit)

15. Fluorescent tags for biological imaging
Attach bioactive molecule to single color nanocrystals and apply light to system.
Nanocrystals fluoresce at their specific  making microscopic chemical processes visible.
Simultaneous monitoring of multiple processes due to narrow, size dependent spectra.
Fluorescent tags for biological imaging

16. Qdots beat organic molecules.
Resistance to photodegradation, improved brightness, nontoxicity
Size-dependent, narrow emission spectra allows simultaneous observation of multiple processes using different colors.
The absorption spectra are continuous above band gap, so one light source excites different sized nanocrystals.
(Extra energy absorbed into thermal motion!)

17. Surface Paints and Coatings
Nanocrystals produced by organometallic synthesis have tri-octane chains attached to exterior cadmiums
Dissolve in oil or plastics (or any non-polar liquid)
Could be used as a fluorescent paint, especially for raves. Again, excellent color tuning.

18. Tunable LEDs
Dots absorb high energy light and reemit light at a lower frequency that can be controlled by crystal size.
They can also emit light when electron-hole pair produced by electrical stimulation
Currently not producing true quantum dot LEDs, and coatings are cheaper, but true LEDs may yet be produced.

19. White light LEDs?
Recently discovered that very tiny crystals (1.7nm-7nm) emit broad-spectrum white light rather than the expected narrow band very blue light. Huh?
Thought to be due in part to high surface to volume ratio.
Large crystals have many areas that see similar local potentials. In small crystals, all areas are different.
+, smallest ones have high energy of containment

20. HERE IT IS !!!!!
Three extra features seen. Maybe one peak is from surface effects, one from interior effects?
The other? Needs some study