Quantum Dots: Confinement and Applications
John SinclairSolid State IIDr. DagottoSpring 2009
Outline
Confinement What do we mean? Small dot or Quantum
Dot? Experimental Evidence
Applications Lasers Biology
Recent History and Motivation Advances in imaging
techniques all us to image things at the angstrom level Scanning Tunneling
Electron Microscopes Atomic Force Microscopy Scanning Transmission
Electron Microscopes
AFM Image InAs
SEM Image of graphene
Quantum Confinement
3-D All carriers act as free carriers
in all three directions 2-D or Quantum Wells
The carriers act as free carriers in a plane
First observed in semiconductor systems
1-D or Quantum Wires The carriers are free to move
down the direction of the wire 0-D or Quantum Dots
Systems in which carriers are confined in all directions (no free carriers)
Confinement Continued
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? Optical Excitations
Optical excitations should require the band gap In semiconductors excitations exist just below the
band gap The Exciton
These excitations are bound hole electron pairs Below the band gap due to binding energy Hydrogen like quasi particle
Hydrogen like energy states Effective Bohr Diameter
Exciton Bohr Diameter
Material Dependent Parameter The same size dot of different materials may not both be
quantum dots The Bohr Diameter determines the type of
confinement 3-10 time Bohr Diameter: Weak Confinement
ΔE ~ 1/M* M* effective mass of exciton
Smaller than 3 Bohr Diameter: Strong Confinement ΔE ~ 1/μ* μ* effective mass of hole and electron
Exciton Bohr Diameter
Experimental Observation of Confinement Just imaging a small dot is not enough to say
it is confined Optical data allows insight into confinement
Optical Absorption Raman Vibration Spectroscopy Photoluminescence Spectroscopy
Optical Absorption
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
The amount of Blue Shift is a material dependent property
It is largest for Ge, but Why? 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
Band gap comparison of Ge and CdTe
Must greater concavity of Ge translates to larger blue shift
Raman Vibrational Spectroscopy 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
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 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 Photoluminescence
spectroscopy is a technique to probe the quantum levels of quantum dots
Here we see dots of various size in a quantum well (a) is quantum well
spectrum (d) is smallest particles
80 nm
Promise from Photoluminescence Photoluminescence
spectrum of a 3-layer stack of InP quantum dots 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) 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 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
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
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