Quantum Dots – a peep in to Synthesis Routes
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Transcript of Quantum Dots – a peep in to Synthesis Routes
Quantum Dots – a peep in to Synthesis
RoutesSaurabh MadaanGraduate student,Materials Science and Engineering,University of Pennsylvania
• Brief introduction• Synthesis routes – an overview
Layout
Arakawa, Sakaki… > Efroz, Brus >Bawendi & Alivisatos…
First Vision of Quantum Dot device
• 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
Quantum Dots – an Introduction
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
1. Quantum well > quantum wire > quantum dot : by etching
2. Confinement: growth direction – qwell; lateral directions – electrostatic potential
Lithography/ Electric Field
1. Edge effects2. Defects due to reactive ion etching3. Less control over size4. Low quantum efficiency5. Slow, less density, and prone to
contamination
Lithography Route – Limitations
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 ALE GrownGaAs substrate<InAs monolayers< island-like self-organization of InAs
qdots.
1. InAs and GaAs monolayers alternately
grown. Self-organization 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.
- 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!
MBE Self-assembled NCs: Features
Monodisperse NCs – Colloidal Route
Murray, Kagan, Bawendi
•La Mer and Dinegar – discrete nucleation followed by slow growth
• uniform size distribution, determined by time of growth
• Ostwald Ripening in some systems
Solution-phase Route (continued)
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).
1. high-T supersaturation
or
2. low-T supersaturation
When rate of: injection < consumption, no new nuclei form
Colloidal Route – Compounds
Compound Source Precursor Coordinating Solvent
Semiconductor NCs
Metal-alkyls (group II)
R3PE or TMS2E (E = group VI)
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 more nucleation, small
size
• Surfactant chemistry provide capping layer. So, more binding,
more steric effect, small size
• Reagent addition 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)
Possible problems: 1. Inhomogeneity in injection of precursors
2. Mixing of reactants3. Temperature gradients in flask
Mass-limited Growth in Templates
Colors from the Bawendi Lab @ MIT
http://www.youtube.com/watch?v=MLJJkztIWfg
Finally…