The Synthesis of Core/Shell Quantum Dots - Semiconductor Quantum Dots: Organometallic and Inorganic Synthesis - page 130

In a related example, MnS/ZnS was also prepared. In this example,
Mn(CO 2 (CH 2 ) 16 CH 3 ) 2 was dissolved in octadecene (ODE) and heated to 270 C
with octadecylamine (ODA), into which was injected an excess of sulfur in
hot ODE. This was then immediately cooled to obtain small clusters of
MnS. 67 The reaction mixture was then heated to 250 C, followed by
addition of Zn(COOC 17 H 35 ) 2 , forming an initial overcoating of ZnS, which was
then repeated to obtain a thicker shell. The deposition of the initial
shell resulted in a di

d n 1 y 4 n g | 3

ectively forming
a MnS/ZnS : Mn/ZnS heterostructure. The core particles did not appear to
emit, and exhibited an excitonic feature below 300 nm. Addition of the shell
resulted in an absorption feature at ca. 315 nm due to the ZnS, with orange
emission at ca. 600 nm, the intensity of which could be controlled by the ratio
of Zn : Mn. The orange emission, with a maximum quantum yield of ca. 35%
at 6 monolayers, consistent with manganese in a ZnS host, was again attrib-
uted to the 4 T 1 /

usion layer of ZnS : Mn, thus e

6 A 1 transition in Mn 2+ . Slight emission was also observed at
ca. 500 nm, from the trap states of ZnS.
Another interesting report describes the use of a Zn 1 x Mn x S shell, up to 6
monolayers thick, to passivate CdSe, which introduced the paramagnetic
species into the shell material. 68 CdSe particles were prepared by a typical
green synthesis as described in Chapter 1. The shell was then deposited onto
puri

ed core particles using Et 2 Zn and Me 2 Mn as precursors, which was
introduced into the reaction vessel containing the core particles, TOPO and
HDA at 170 C, while H 2 S was simultaneously introduced, and the resulting
reaction was maintained for at 170 C for 2 hours. The material was then
phase-transferred to water using an amphiphilic polymer described in
Chapter 6. The



.

ed to remove surface manganese
species, and electron spin resonance (ESR) con

nal particles were puri

 

rmed that manganese was
incorporated into the ZnS shell. The amount of manganese deposited in the
shell was dependent on the amount of precursor used and the shell thick-
ness, and was controlled between 2 Mn 2+ ions per particle (for 1.5 mono-
layers of shell) and 52 ions per particles in the thicker-shelled species. The
optical properties of the CdSe/Zn 1 x Mn x S closely resembled typical CdSe/ZnS
particles, with quantum yields of 30



-

60%, although the emission quantum
yield was found to reduce when large amounts of manganese were incor-
porated into the shell, attributed to the manganese accumulating at the
CdSe/ZnS interface. No evidence was found for Mn 2+ emission. The particles
were successfully used in optical cell imaging and in simple magnetic
resonance imaging (MRI) experiments.

Core/Shell Alloys
Alloyed CdTe x Se 1 x /CdS core/shell particles have been speci cally designed

for use in biological imaging, where the emission wavelength could be tuned
between 600 and 850 nm, ideal for imaging applications. 69 Upon precursor
addition, it was observed that the tellurium precursor (TOPTe) reacted faster
than the selenium precursor (TOPSe), thus e



ecting the nal composition of

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