Transcript Quantum Dots – a peep in to Synthesis Routes Saurabh Madaan Graduate student, Materials Science and Engineering, University of Pennsylvania.

Quantum Dots – a peep in to
Synthesis Routes
Saurabh Madaan
Graduate student,
Materials Science and Engineering,
University of Pennsylvania
Layout
• Brief introduction
• Synthesis routes – an overview
First Vision of Quantum Dot device
Arakawa, Sakaki… > Efroz, Brus >Bawendi & Alivisatos…
Quantum Dots – an Introduction
• Confined 3-D structures – bohr-exciton radius is less than
material dimensions (5.6 nm for CdSe)
• Unique electronic, optical properties ~ particle in a box
Nanocrystals, Artificial Atoms
• Blue shift; tunable
spectra
• High quantum
efficiency
• Good candidates
for biological
tagging, sensing
applications
Synthesis Routes
TOP-DOWN
• Lithography (Wet-chemical etching, E-field)
BOTTOM-UP
• Epitaxy (self assembly or patterned; S-K or ALE)
• Colloidal chemistry routes
• Templating (focused ion beam, holographic lithography,
direct writing)
Lithography/ Etching
Lithography/ Electric Field
1.
2.
Quantum well > quantum wire > quantum dot : by etching
Confinement: growth direction – qwell; lateral directions – electrostatic
potential
Lithography Route – Limitations
1.
2.
3.
4.
5.
Edge effects
Defects due to reactive ion etching
Less control over size
Low quantum efficiency
Slow, less density, and prone to contamination
MBE – Self-assembled NCs
1. Initial stage – InAs (7% mismatch)
grows layer-by-layer 2D mechanism.
2. Strained layer – wetting layer
3. When amount of InAs exceeds critical
coverage (misfit > 1.8% ), 3D islands
are formed
Stranski-Krastanow 3D growth
MBE: Vertical Coupling in S-K growth
PHYSICAL REVIEW B 54 (12): 8743-8750 SEP 15 1996
MBE Self-assembled NCs: 2 modes
MBE Self-assembled NCs: 2 modes
S-K Grown
GaAs substrate<InAs monolayers<
island-like self-organization of
InAs qdots.
ALE Grown
1.
InAs and GaAs monolayers
alternately grown. Selforganization of high In
composition dots surrounding
low In region.
Thin wetting layer covers the substrate.
No wetting layer.
Additional barrier layer needed to
embed dots in high band-gap
material.
Dot formation takes place in low In
content InGaAs layer, which
serves as barrier layer.
MBE Self-assembled NCs: Features
- No edge effects, perfect Xtal structure
- Qdot lasers, single photon generation, detection
- Annealing leads to blue shift
• Undesired fluctuations in size and density – broadened
spectra
• Random distribution on lateral surface area – lack of
positioning control
• Cost!
Monodisperse NCs – Colloidal Route
•La Mer and Dinegar –
discrete nucleation
followed by slow
growth
• uniform size
distribution, determined
by time of growth
• Ostwald Ripening in
some systems
Murray, Kagan, Bawendi
Solution-phase Route (continued)
1. high-T supersaturation
or
2. low-T supersaturation
When rate of: injection < consumption, no
new nuclei form
Fig: a) synthesize NCs by high T solution-phase
route, b) narrow size dist by size selective ppt, c)
deposit NC dispersions that self-assemble, d) form
ordered NC assemblies (superlattices).
Colloidal Route – Compounds
Compound
Semiconductor
NCs
Source Precursor
Metal-alkyls
(group II)
R3PE or TMS2E
(E = group VI)
Coordinating Solvent
alkylphosphines
1. Nucleation and Growth:
2. Isolation and purification:
anyhdrous methanol > flocculate > drying
3. Size-selective precipitation:
solvent/non-solvent pairs
eg. Pyridine/hexane
Further Treatments
More steric hinderance?
Layer of high band-gap SC, higher quantum efficiency
Colloidal Route – Controlling size
• Time
growth, Ostwald ripening
• Temperature
growth, O. r.
• Reagent/Stabilizer concentration
• Surfactant chemistry
• Reagent addition
more nucleation, small size
provide capping layer. So, more binding, more steric effect, small size
rate of injection<feedstock addition… “focus” the size-distribution
• When desired size is reached (absorption spectra), further
growth is arrested by cooling (15-115 angstrom range possible)
1.
Possible problems:
Inhomogeneity in injection of precursors
2. Mixing of reactants
3. Temperature gradients in flask
Mass-limited Growth in Templates
Finally…
Colors from the Bawendi Lab @ MIT
http://www.youtube.com/watch?v=MLJJkztIWfg