Quantum Dots

1
Quantum Dots a peep in to Synthesis Routes
Saurabh Madaan Graduate student, Materials
Science and Engineering, University of
Pennsylvania
2
Layout

• Brief introduction
• Synthesis routes an overview

3
First Vision of Quantum Dot device
Arakawa, Sakaki gt Efroz, Brus gtBawendi
Alivisatos
4
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

5
Nanocrystals, Artificial Atoms

• Blue shift tunable spectra
• High quantum efficiency
• Good candidates for biological tagging, sensing
  applications

6
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)

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Lithography/ Etching
8
Lithography/ Electric Field

• Quantum well gt quantum wire gt quantum dot by
  etching
• Confinement growth direction qwell lateral
  directions electrostatic potential

9
Lithography Route Limitations

• Edge effects
• Defects due to reactive ion etching
• Less control over size
• Low quantum efficiency
• Slow, less density, and prone to contamination

10
MBE Self-assembled NCs

• Initial stage InAs (7 mismatch) grows
  layer-by-layer 2D mechanism.
• Strained layer wetting layer
• When amount of InAs exceeds critical coverage
  (misfit gt 1.8 ), 3D islands are formed
• Stranski-Krastanow 3D growth

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MBE Vertical Coupling in S-K growth
PHYSICAL REVIEW B 54 (12) 8743-8750 SEP 15 1996
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MBE Self-assembled NCs 2 modes
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MBE Self-assembled NCs 2 modes
S-K Grown ALE Grown
GaAs substrateltInAs monolayerslt island-like self-organization of InAs qdots. 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.
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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!

15
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
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Solution-phase Route (continued)

• high-T supersaturation
• or
• 2. low-T supersaturation
• When rate of injection lt 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).
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Colloidal Route Compounds
Compound Source Precursor 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 gt flocculate gt drying
3. Size-selective precipitation solvent/non-solve
nt pairs eg. Pyridine/hexane
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Further Treatments
More steric hinderance?
Layer of high band-gap SC, higher quantum
efficiency
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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 injectionltfeedstock
  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
• Inhomogeneity in injection of precursors
• Mixing of reactants
• Temperature gradients in flask

20
Mass-limited Growth in Templates
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Finally
Colors from the Bawendi Lab _at_ MIT http//www.yout
ube.com/watch?vMLJJkztIWfg