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  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… > Efroz, Brus >Bawendi & 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)

  7. Lithography/ Etching

  8. Lithography/ Electric Field • Quantum well > quantum wire > 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 > 1.8% ), 3D islands are formed • Stranski-Krastanow 3D growth

  11. MBE: Vertical Coupling in S-K growth PHYSICAL REVIEW B 54 (12): 8743-8750 SEP 15 1996

  12. MBE Self-assembled NCs: 2 modes

  13. MBE Self-assembled NCs: 2 modes

  14. 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

  16. 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).

  17. 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

  18. Further Treatments More steric hinderance? Layer of high band-gap SC, higher quantum efficiency

  19. 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

  20. Mass-limited Growth in Templates

  21. Finally… Colors from the Bawendi Lab @ MIT http://www.youtube.com/watch?v=MLJJkztIWfg