7. Prospects for Rational Control of Nanocrystal Shape Through Successive Ionic Layer Adsorption and Reaction (SILAR) and Related Approaches

This chapter describes the use of colloidal successive ionic layer addition as an additive growth method to form inorganic colloidal nanocrystals with controlled shapes. Rational design of nanocrystal dimensions and layer thicknesses in nanocrystal heterostructures is important to many current and anticipated uses of nanocrystals in optoelectronics, energy conversion, and fluorescence imaging. One approach to shape control is the use of a series of self-limiting surface reactions to build up crystals one atomic layer at a time. This approach is especially applicable to nanocrystals made of binary ionic or polar-covalent crystalline compounds, as are found in colloidal quantum dots (QDs). The intrinsic symmetry present in colloidal nanocrystal nuclei can be suppressed to enforce conformal layer growth or harnessed to promote regioselective growth. We specifically discuss two families of methods that are colloidal analogues of vapor-source atomic layer deposition. In colloidal successive ionic layer adsorption and reaction (colloidal SILAR), reagents are introduced to a nanocrystal solution in metered doses corresponding to the total surface area. In colloidal atomic layer deposition (colloidal ALD), reagents are added in excess, and unreacted reagent is subsequently separated and removed. An extensive literature exists on the use of colloidal SILAR to form nominally isotropic core/shell quantum dots (QDs) with high photoluminescence quantum yield. Colloidal ALD has been introduced more recently. There is increasing interest in applying both methods to the formation of anisotropic nanocrystal heterostructures, both through deposition of conformal layers on anisotropic substrates and through controlled anisotropic growth. We review the historical development of these methods, common precursors, and recent developments in monitoring of reaction progress and mechanisms. We also present contemporary examples of isotropic and anisotropic growth, and prospects for future development in the context of several representative applications including cell membrane voltage measurements and fluorescence anisotropy.