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…