Synthesis, surface chemistry, and self-assembly

Historically, one of the most accomplished nanoscale building blocks is the quantum dot, also known as a semiconductor nanocrystal (NC). Since the early 1980s, the chemistry of colloidal nanostructures has been almost exclusively concentrated on colloidal semiconductors such as CdS and TiO2. Those early efforts rose from the oil crisis in the late 1970s, and semiconductor NCs with enhanced surface chemistry were considered highly important for efficient harvesting of solar energy via photoelectrochemistry [1] and later using dye-sensitized solar cells [2]. The interest in quantum dots was further triggered by the discovery of quantum-size effects in the optical spectra of nanometersized semiconductors, by the teams of A. Ekimov and A. Efros in the USSR [3, 4], and by L. Brus at Bell Laboratories in the USA [5]. The first observations were collected from optical measurements on the semiconductor dots dispersed in glass matrices [3] or aqueous semiconductor sols [5]. Until the mid-1990s, a major challenge was to master the production of quantum dots in the form of uniform, size-tunable and isolable NCs, with adjustable physical and chemical properties. This goal was largely accomplished only in 1993 when the hot-injection technique was introduced by Murray, Norris, and Bawendi, enabling synthesis of highlymonodisperse Cd chalcogenide NCs [6]. Surfactant-assisted nucleation and growth in organic solvents, initiated by hot-injection [6] or by heating [7] the reaction mixture became standard for growing monodisperse colloidal NCs of metals, metal oxides, and other classes of inorganic compounds. Even water was demonstrated as a suitable synthesis medium for certain compounds, such as thiol-capped II–VI semiconductor NCs [8] with bright and stable photoluminescence. Themultitude ofmethods for producing nanosize particles and crystals is truly impressive (Figure 1.1). The top-down approaches for the formation of engineered nanomaterials include fragmentation and structuring of macroscopic solids, either mechanically (e.g., ball-milling) or chemically (lithography, exfoliation, etching, etc.). The bottom-up assembly starts with molecules, atoms, and ions and proceeds via gas-phase or liquidphase chemical reactions, aggregation, and crystallization. Liquid-phase synthesis in aqueous or non-aqueous solvents turned out to be particularly convenient and successful.