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