  A complex InAs/CdSe/ZnSe Core/Shell1/Shell2 (CSS) structure is synthesized,
where the intermediate CdSe buffer layer decreases strain between the InAs core
and the ZnSe outer shell. This structure leads to significantly improved
fluorescence quantum yield as compared to previously prepared core/shell
structures and enables growth of much thicker shells. The shell growth is done
using a layer-by-layer method in which the shell cation and anion precursors
are added sequentially allowing for excellent control and a good size
distribution is maintained throughout the entire growth process. The CSS
structure is characterized using transmission electron microscopy, as well as
by X-ray diffraction and X-ray-photoelectron spectroscopy which provide
evidence for shell growth. The quantum yield for CSS with small InAs cores
reaches over 70% - exceptional photoluminescence intensity for III-V
semiconductor nanocrystals. In larger InAs cores there is a systematic decrease
in the quantum yield, with a yield of ~40% for intermediate size cores down to
a few percent in large cores. The CSS structures also exhibit very good
photostability, vastly improved over those of organically coated cores, and
transformation into water environment via ligand exchange is performed without
significant decrease of the quantum yield. These new InAs/CdSe/ZnSe CSS
nanocrystals are therefore promising near-IR chromophores for biological
fluorescence tagging and optoelectronic devices.
