Download Semiconductor Nanocrystals

Semiconductor
Nanocrystals
Aka Quantum Dots
Rachel Wooten
February 28, 2006
Fat Tuesday!
Transmission Electron Microscope image of CdSe nanocrystal
Outline
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What are semiconductor nanocrystals?
How do we make them?
How do they work?
Applications: Why do we care?
Semiconductor Nanocrystals
As the name implies,
Semiconductor
nanocrystals are tiny
(generally fewer than
100 Angstroms in
diameter) crystals of a
semiconductor
material.
How to make them
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Vapor deposition
Ion implantation
Sol-gel methods
Micelle methods
Organometallic synthesis
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
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.)
Quantum confinement
• When nanocrystal has a diameter of less than 112 Å,
electron and hole cannot achieve their desired separation.
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.
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
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
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
Applications
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Photovoltaics
Single-electron transistors
Fluorescent tags for biological imaging
Surface paints and coatings
Tunable Light Emitting Diodes (LEDs)
The new lightbulb?
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.
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)
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.
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!)
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.
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.
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
HERE
IT
IS
!!!!!
• Three extra features seen. Maybe one peak is
from surface effects, one from interior effects?
• The other? Needs some study