Recent Developments in the Synthesis of Metal-Tipped Semiconductor Nanorods

Semiconductor nanocrystals (NCs), commonly known as quantum dots (QDs), have received great attention over the last two decades due to their, unique size and shapedependent optoelectronic properties, as well as their flexible surface chemistry. While efforts to produce colloidal NCs date back to the pioneering work of Rossetti et al at Bell Labs1 and Ekimov et al at the Vavilov State Optical Institute2 in the early 1980's, the ability to obtain monodisperse spherical and highly crystalline NCs remained largely elusive until the introduction of the hot injection method by Murray et al. in 1993.3 In this method, organometallic precursors of the semiconductor material are rapidly injected into an organic solvent at an elevated temperature under inert conditions. This results in a rapid cooling of the reaction mixture, effectively separating the nucleation and growth phases of the intended semiconductor NCs. Despite the addition of organic surface-capping groups, assynthesized core NCs typically suffer from poor surface passivation and possess surface trap states. These surface trap states result in fast non-radiative relaxation pathways for photogenerated charge carriers, thus leading to reduced fluorescence quantum yields (QYs) typically on the order of ~10-20%. In order to improve the fluorescence efficiency as well as the photostability of semiconductor NCs, growth of an inorganic shell (typically a wider bandgap semiconductor) is generally adopted. One of the earliest and most widely used techniques for the overcoating of semiconductor NCs even today was introduced by Hines et al. in 1996,4 where precursors of the semiconductor shell material are added dropwise to a relatively dilute solution of NC cores at temperatures sufficiently low to prevent homogeneous nucleation of the precursors or Ostwald ripening of the NC cores. Growth of the semiconductor shell can lead to effective surface passivation of the core NC, leading to near unity QYs in the case of CdSe/CdS core-shell NCs,5 although QYs in the range of 5070% are more common. The synthetic development of various II-VI, IV-VI and III-V colloidal semiconductor NCs have been reported to date,3,6,7 and have led to intense research efforts in the study of their fundamental optoelectronic properties as well as their use in applications in areas as diverse as light-emitting diodes (LEDs),8 solar cells,9 and biological imaging.10