Chemistry of InP Nanocrystal Syntheses

Chemically synthesized InP nanocrystals (NCs) are drawing a large interest as a potentially less toxic alternative to CdSe-based nanocrystals. With a bulk band gap of 1.35 eV and an exciton Bohr radius of similar to 10 nm the emission wavelength of InP NCs can in principle be tuned throughout the whole visible and near-infrared range by changing their size. Furthermore, a few works reported fluorescence quantum yields exceeding 70% after overcoating the core NCs with appropriate shell materials. Therefore, InP NCs are very promising for use in lighting and display applications. On the other hand, a number of challenges remain to be addressed in order to progress from isolated research results to robust and reproducible synthesis methods for high quality InP NCs. First of all, the size distribution of the as-synthesized NCs needs to be reduced, which directly translates into more narrow emission line widths. Next, reliable protocols are required for achieving a given emission wavelength at high reaction yield and for further improving the emission efficiency and chemical and photostability. Advances in these directions have been hampered for a long time by the specific properties of InP, such as the rather covalent nature of binding implying harsh synthesis conditions, high sensitivity toward oxidation, and limited choice of phosphorus precursors. However, in recent years a much better understanding of the precursor conversion kinetics and reaction mechanisms has been achieved, giving this field new impulse. In this review we provide a comprehensive overview from initial synthetic approaches to the most recent developments. First, we highlight the fundamental differences in the syntheses of InP-based NCs with respect to established II-VI and IV-VI semiconductor NCs comparing their nucleation and growth stages. Next, we inspect in detail the influence of the nature of the phosphorus and indium precursors used and of reaction additives, such as zinc carboxylates or alkylamines, on the properties of the NCs. Finally, core/shell systems and doped InP NCs are discussed, and perspectives in this field are given.