Surface Engineering of Quantum Dots with Designer Ligands

Semiconductor nanocrystal particles (quantum dots, QDs) exhibit unique electronic and optical properties, which depend on their size and composition, and can be tuned by a controlled variation of the particle dimensions. Due to this tunability, applications of QDs in optoelectronics, sensing, biolabeling and biodiagnostics have attracted great interest. For these applications it is essential to consider the particular structure of the nanoparticles, as well as the surrounding, usually organic, shells. In addition to the size and composition dependence, the optical and electronic properties of QDs are modulated by the chemical composition, attachment, and molecular conformation of the organic (ligand) shell. Control of the ligand-shell attachment to QD surfaces, and understanding of its influence on the physicochemical, optical, and electronic properties of isolated QDs, and QDs incorporated in particle ensemble of molecularly controlled matrices are essential to successfully achieve most of the promising applications. For example, chemical derivatization of the ligands improves the emission characteristics of the QDs, can render the QDs water soluble and biocompatible, enables one to incorporate the QDs into composite materials, promotes their assembly into hierarchical structures, makes coupling of the QDs to surfaces possible, and finally, opens the possibility to introduce biorecognition functions onto their surface. Modulation of the photophysical properties of QDs through molecular engineering of the ligand shell unfolds new avenues in chemical sensing based on energy- or electron-transfer processes. This chapter briefly reviews the progress and current status of chemical functionalization of the surface of QDs.