Transcript Quantum Dots: Confinement and Applications

Quantum Dots: Confinement
and Applications
John Sinclair
Solid State II
Dr. Dagotto
Spring 2009
Outline
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Confinement
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What do we mean?
Small dot or Quantum
Dot?
Experimental Evidence
Applications
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Lasers
Biology
Recent History and Motivation
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Advances in imaging
techniques all us to
image things at the
angstrom level
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Scanning Tunneling
Electron Microscopes
Atomic Force Microscopy
Scanning Transmission
Electron Microscopes
AFM Image InAs
SEM Image of graphene
Quantum Confinement
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3-D
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2-D or Quantum Wells
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The carriers act as free carriers
in a plane
First observed in
semiconductor systems
1-D or Quantum Wires
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All carriers act as free carriers
in all three directions
The carriers are free to move
down the direction of the wire
0-D or Quantum Dots
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Systems in which carriers are
confined in all directions (no
free carriers)
Confinement Continued
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So what if a material is
confined in one
direction?
As the material
becomes confined its
Density of States
changes
In the confined
direction you can think
of the carriers as
particles in boxes
What is the relevant length scale?
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Optical Excitations
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Optical excitations should require the band gap
In semiconductors excitations exist just below the
band gap
The Exciton
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These excitations are bound hole electron pairs
Below the band gap due to binding energy
Hydrogen like quasi particle
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Hydrogen like energy states
Effective Bohr Diameter
Exciton Bohr Diameter
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Material Dependent Parameter
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The same size dot of different materials may not both be
quantum dots
The Bohr Diameter determines the type of
confinement
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3-10 time Bohr Diameter: Weak Confinement
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ΔE ~ 1/M*
M* effective mass of exciton
Smaller than 3 Bohr Diameter: Strong Confinement
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ΔE ~ 1/μ*
μ* effective mass of hole and electron
Exciton Bohr Diameter
Experimental Observation of
Confinement
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Just imaging a small dot is not enough to say
it is confined
Optical data allows insight into confinement
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Optical Absorption
Raman Vibration Spectroscopy
Photoluminescence Spectroscopy
Optical Absorption
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Optical Absorption is a
technique that allows one to
directly probe the band gap
The band gap edge of a
material should be blue
shifted if the material is
confined
Bukowski et al. present the
optical absorption of Ge
quantum dots in a SiO2
matrix.
As the dot decreases in size
there is a systematic shift of
the band gap edge toward
shorter wavelengths
The Blue Shift
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The amount of Blue
Shift is a material
dependent property
It is largest for Ge, but
Why?
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The amount of blue shift
scales with the concavity
of the band gap
Particularly the portion of
the band that is important
as confinement sets in
and the DOS changes
Band Gap Comparison
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Band gap comparison
of Ge and CdTe
Must greater concavity
of Ge translates to
larger blue shift
Raman Vibrational Spectroscopy
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Raman vibrational
spectroscopy probes the
vibrational modes of a
sample using a laser
As the nanocrystal
becomes more confined the
peak will broaden and
shrink
Here we see a peak shift
toward the laser line
Various Ge dots of different
sizes on an Alumina film
Direction of Raman Shift
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Here we see the same
broadening and shrinking of
the Raman Peak
We see a peak shift away
from the laser line
No systematic shift of the
Raman line
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Shifts toward the laser line
are due to confinement
Shifts away from the line
are due to lattice tension
due to film miss-match
Ge dots in a SiO2 matrix
Photoluminescence Spectroscopy
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Photoluminescence
spectroscopy is a
technique to probe the
quantum levels of
quantum dots
Here we see dots of
various size in a
quantum well
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(a) is quantum well
spectrum
(d) is smallest particles
80 nm
Promise from Photoluminescence
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Photoluminescence
spectrum of a 3-layer stack
of InP quantum dots
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Very narrow absorption
should allow for production
of great lasers
At present QD lasers only
out perform other solid state
lasers at low temperatures
(below room temperature)
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Problems arise due to high
threshold currents at high
temperature
Some QD lasers do not
even lase at room
temperature
A Brief Look at Biological Applications
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Attaching ligand molecules and receptors to surface of
quantum dots can create new functional form of joined dots
Patterned substrates can cause QDs to form intricate
patterns
QDs can be used as cellular structure tags with attachment of
appropriate ligands
References
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Tracie J. Bukowski, Critical Reviews in Solid State and Materials.
Sciences (2002)
D. L. Huaker, G. Park and D. G. Deppe, Applied Physics. Journal
(1998)
S. Hoogland, V. Sukhovatkin, Optics Express. (2006)
Teresa Pellegrino, Stefan Kudera and W. J. Parak. small (2005)
N. N. Ledentsov, et al., Quantum dot heterostructures:
fabrication, properties, lasers. Semiconductors (1998)
http://www.condmat.physics.manchester.ac.uk/
http://www.essential-research.com/Quantum