The Synthesis of Core/Shell Quantum Dots - Semiconductor Quantum Dots: Organometallic and Inorganic Synthesis - page 149

a quantum yield of ca. 10%. 140 The shell was synthesised using zinc stearate
and dodecanethiol (DDT) with an injection temperature of 130 C and a
growth temperature of 240 C. Replacing DDT with TOPSe resulted in a ZnSe
shell, giving InAs/ZnSe particles with quantum yields of up to 15%. The InAs/
ZnSe particles actually increased in brightness over 30 days in ambient
conditions.
Multiple shells can also be grown on an InAs core, introducing a bu

d n 1 y 4 n g | 3

er
layer to minimise the interfacial strain of a mismatched shell which ulti-
mately leads to a higher quantum yield. 141 A core/shell/shell structure of InAs/
CdSe/ZnSe has been reported, prepared using the SILAR technique. The InAs
cores (slightly larger than most InAs cores at 3.8 nm diameter) were prepared
as outlined in Chapter 2, using TOP as a capping agent. Following redis-
persion of the particles in toluene with ODE and ODA, Cd(CO 2 (CH 2 ) 7 (CH

]
CH)(CH 2 ) 7 CH 3 ) 2 and Se/ODE were added sequentially at 15 minute intervals
at 260 C, followed by Me 2 Zn in toluene/ODE and the selenium precursor to
deposit the rst cadmium layer, the emission

increased notably from 1%, and continued to increase as further shells were
added up to a maximum quantum yield of 50%. A

nal shell. A  er addition of the

 

 er the growth of the CdSe

shell, a red shi in emission was observed from ca. 1050 nm to 1150 nm due

to the small potential barrier, although this returned to the original position
a



er the deposition of the ZnSe shell. Notably, the optical spectra appeared
largely unchanged a



nal deposition, suggesting the particles
retained their monodispersed character. This impressive increase in emis-
sion quantum yield could not be achieved by simply depositing CdSe or ZnSe
shells alone, and a combination of the two was found to be essential. The
emission could be tuned by altering the core size, with 1.9 nm cores giving
emission with a 70% quantum yield at 885 nm, while the larger core of 6.3
nm diameter gave emission at 1425 nm, although the

 er the



.

nal quantum yields



were only 2.5%.
Multiple-shell particles based on III V materials have also been prepared

using alloyed cores of InAs x P 1 x , with stoichiometries from InAs 0.33 P 0.66 to
InAs 0.82 P 0.18 , designed for applications in biological imaging. 142 The cores
were prepared as outlined in Chapter 2 by the simultaneous addition of
As(SiMe 3 ) 3 and P(SiMe 3 ) 3 into an ODE solution of In(CO 2 (CH 2 ) 7 (CH

-

]
CH)(CH 2 ) 7 CH 3 ) 3 at 270 C, followed by growth for 1 hour, giving graded
structures with a phosphorus-rich core due to the faster decomposition of the
phosphine. An InP shell was then deposited by cooling the reaction mixture
to 140 C, followed by addition of In(CH 3 CO 2 ) 3 and P(SiMe 3 ) 3 with growth at
180 C for 1 hour. A further ZnSe shell could be added by increasing the
reaction temperature to 200 C, followed by the dropwise addition of Me 2 Zn
and TOPSe over a further hour, a

er which the reaction was cooled to room
temperature and precipitated with ethanol. Emission from the cores could be
varied from 614 nm (pure InP), through 652 nm (InAs 0.33 P 0.66 ), 699 nm
(InAs 0.66 P 0.33 ), 738 nm (InAs 0.82 P 0.18 ), to 755 nm (pure InAs) with quantum
yields of about 2%. Addition of an InP shell to InAs 0.82 P 0.18 resulted in red
shi



 s in the emission pro le of up to over 60 nm, due to exciton leakage into

