Effect of Ligand Exchange on the Stability and Optical Properties of CdSe Quantum Dots
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Effect of Ligand Exchange on the Stability and Optical Properties of CdSe Quantum Dots Jacqueline T. Siy, Lindsay Leone, and Michael H. Bartl Department of Chemistry, University of Utah, 315 South 1400 East, Rm 2020, Salt Lake City, UT, 84112 Corresponding author: M.H.B. ([email protected]) ABSTRACT A new post-synthesis heat/ligand-exchange treatment on CdSe nanocrystal quantum dots that allows fine-tuning the quantum dots size without losing important properties (e.g. photoluminescence) is presented. The careful control of parameters such as the reaction temperature, the quantum dot to alkyl-amine ligand mole ratio and the solvent volume is shown to provide tunable size reduction of the nanocrystals. This size reduction phenomenon provides new insights into the surface chemistry and stability of nanocrystals and can serve as a platform for designing, controlling and processing nanomaterials with tailored properties.
INTRODUCTION Semiconductor nanocrystal quantum dots (QDs) are crystals composed of II-VI, III-V, IV-VI group materials. Recent advances have shown that II-VI semiconductors, in particular CdSe, have a wide range of potential applications such as in biological labeling[1-3], as alternatives to semiconductor polymers in light emitting diodes[4], micro-lasers[5,6], and solar cells[7]. The extensive scope of applications of QDs has prompted intensive study towards optimizing colloidal nanocrystal synthesis and utilizing their size-dependent unique properties. However, despite this wealth of emerging applications, both the stability and growth mechanism of these nanomaterials are still not fully understood. In this work, a unique behavior of CdSe QDs is presented wherein the nanocrystals undergo a tunable reduction of the nanoparticle size over a large range accompanied by a change in their optical properties when subjected to a post-synthesis heat-treatment/ligand-exchange procedure. Previous attempts at selective size reduction of semiconductor and metal nanocrystals generally resulted in a decrease or even elimination of photoluminescence properties and/or limited controllability of the final nanocrystal size and shape[8,9] Therefore, the post-synthesis size engineering method that allows for precise fine-tuning of QD properties while retaining their excellent luminescent efficiencies presented here promise to provide a number of interesting applications.
EXPERIMENTAL Chemicals Cadmium acetate dihydrate (Aldrich, 98%), cadmium oxide (Aldrich, 99.99+%), octadecylamine (ODA, Aldrich, 97%), 1-octadecene (ODE, Aldrich, 90%), selenium (Aldrich, 99.99%), stearic acid (SA, Alfa Aesar, 99%), tributylphosphine (Aldrich, 97%), trioctylphosphine (Aldrich, 90%), and trioctylphosphine oxide (Aldrich, 99%). Synthesis of CdSe QDs CdSe QDs were synthesized utilizing the modified “hot injection” method by Peng and co-workers[10] wherein the precursors were Cd(CH3COO)2·2H2O and elemental Se. SA and TOPO were the Cd precursor ligands, and TOP was the Se precursor ligand used. The nanocrystals were purifie
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