Quantum Dots – a peep in to Synthesis Routes Saurabh Madaan Graduate 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 effects 2.Defects due to reactive ion etching 3.Less control over size 4.Low quantum efficiency 5.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): SEP

MBE Self-assembled NCs: 2 modes

S-K GrownALE Grown GaAs 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 CompoundSource PrecursorCoordinating Solvent Semiconductor NCs Metal-alkyls (group II) R 3 PE or TMS 2 E (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 ( angstrom range possible) Possible problems: 1.Inhomogeneity in injection of precursors 2.Mixing of reactants 3.Temperature gradients in flask

Mass-limited Growth in Templates

Colors from the Bawendi MIT Finally…