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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.

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slide1

Quantum Dots – a peep in to Synthesis Routes

Saurabh Madaan

Graduate student,

Materials Science and Engineering,

University of Pennsylvania

slide2

Layout

  • Brief introduction
  • Synthesis routes – an overview
slide3

First Vision of Quantum Dot device

Arakawa, Sakaki… > Efroz, Brus >Bawendi & Alivisatos…

slide4

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
Nanocrystals, Artificial Atoms
  • Blue shift; tunable spectra
  • High quantum efficiency
  • Good candidates for biological tagging, sensing applications
synthesis routes
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 electric field
Lithography/ Electric Field
  • Quantum well > quantum wire > quantum dot : by etching
  • Confinement: growth direction – qwell; lateral directions – electrostatic potential
lithography route limitations
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
mbe self assembled ncs
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 > 1.8% ), 3D islands are formed
  • Stranski-Krastanow 3D growth
mbe vertical coupling in s k growth
MBE: Vertical Coupling in S-K growth

PHYSICAL REVIEW B 54 (12): 8743-8750 SEP 15 1996

mbe self assembled ncs features
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
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
Solution-phase Route (continued)
  • 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
Colloidal Route – Compounds

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
Further Treatments

More steric hinderance?

Layer of high band-gap SC, higher quantum efficiency

colloidal route controlling size
Colloidal Route – Controlling size
  • Time growth, Ostwald ripening
  • Temperature growth, O. r.
  • Reagent/Stabilizer concentration more nucleation, small size
  • Surfactant chemistryprovide capping layer. So, more binding, more steric effect, small size
  • Reagent additionrate 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:
  • Inhomogeneity in injection of precursors
  • Mixing of reactants
  • Temperature gradients in flask
slide22

Finally…

Colors from the Bawendi Lab @ MIT

http://www.youtube.com/watch?v=MLJJkztIWfg