The Synthesis of Core/Shell Quantum Dots - Semiconductor Quantum Dots: Organometallic and Inorganic Synthesis - page 116

particles exhibiting excellent stability when compared to the phosphine
oxide analogues, attributable to the linear shape and hence e cient packing

of the molecule. Organic monolayers, however, provide only minimal
protection against oxidation and a more complete passivating agent is
usually required. The overcapping of bare particles prepared by organome-
tallic precursors with a further inorganic phase can be traced back to 1994,
where work by Danek et al. examined capping CdSe particles with ZnSe and
ZnS. 23 - 26 Although most of these process were based on metal

d n 1 y 4 n g | 3

organic
chemical vapour deposition (MOCVD), where bare CdSe particles were
incorporated into a ZnS matrix, one report described the reaction of CdSe
with TOPSe and Et 2 Zn prior to deposition by an electrospray process. This
can be seen as the

-

 rst paper to examine core/shell structures of these

particular materials. 25

5.3 Type I Materials
5.3.1 CdSe/ZnS
Hines and Guyot-Sionnest reported the rst complete solution-based

core/shell semiconductor system (CdSe/ZnS, type I) prepared by organome-
tallic precursors in a single-pot reaction. 27 Typically, the core CdSe particles
were prepared as described in Chapter 1, by the injection of a stock solution
consisting of Me 2 Cd, trioctyl phosphine (TOP) and TOPSe in TOPO at 350 C.
The reagents were then allowed to cool to 300 C, whereupon Me 2 Zn and
S(SiMe 3 ) 2 in TOP were then injected in



.

ve portions over a period of about
a minute, to avoid the nucleation and growth of separate ZnS particles. The
molar ratio of Cd/Se : Zn/S precursors was 1 : 4, consistent with earlier
attempts to grow core/shell particles where the shell thickness was compa-
rable to that of the core diameter. 12 The particles were found to be composed
of ca. 3 nm diameter CdSe cores capped with a ZnS shell (6



3 A thick), as
determined by transmission electron microscopy (TEM) comparisons of
particle size before and a

er addition of the shell precursor. A surfactant
layer on the ZnS shell ensured solubility in non-polar hydrocarbons in
a similar manner to the bare dots.
Dabbousi et al. reported a similar, more detailed preparation 28 using the
same precursors, but notably isolated the core particles and narrowed the
size distribution by selective precipitation before redispersing the particles in
TOPO/TOP. Once the size of the particles had been determined by either
small-angle X-ray spectroscopy or electron microscopy, the amount of
precursor required to form a shell of the desired thickness was then calcu-
lated. The core particle mixture was heated to the required optimum
temperature (140 C for 2.3 nm diameter dots, to 220 C for 5.5 nm diameter
dots) where the ZnS precursors were added dropwise over 10 minutes. A



 er

for several hours at 90 C before isolation. A
similar procedure was used to prepare CdSe/CdS core/shell particles,
replacing Et 2 Zn with Me 2 Cd.

addition, the mixture was le 