The Preparation of II–VI Semiconductor Nanomaterials - Semiconductor Quantum Dots: Organometallic and Inorganic Synthesis - page 17

unstable in the reaction mixture, so was added with the selenium reagents.
This resulted in more reproducible reactions; however, the sharp pro le of

the absorption spectra was lost to some degree. ZnSe particles grown with
just fatty acids had quantum yields of below 10%, whereas amine passivated
particles had quantum yields of up to 50% with emission as narrow as 14 nm
(full width at half the maximum, FWHM). Emission could be tuned to
between ca. 370 nm to ca. 440 nm, making the particles ideal blue emitters.
ZnS nanoparticles displayed a di

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d n 1 y 4 n g | 1

erent chemistry, with fatty acid passivated
particles being approximately 10 times brighter than amine/acid passivated
ones, although both types of ZnS displayed signi cant deep trap emission.

This route also utilised similar injection/growth temperatures, when
convention normally dictates a high injection temperature, followed by a low-
temperature growth step to separate nucleation and growth. This step was
necessary due to the delicate balance between nucleation and growth in zinc
chalcogenide particles.
Reiss et al. have reported an almost identical method, 110 which produced
capped ZnSe nanoparticles with a hexagonal crystalline core as opposed to
the cubic analogues reported by Li. The similarity between hexagonal and
cubic di

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d n 4 .

raction patterns coupled with the broadened re ections make

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assignment di cult, although it is possible that particles reported by Li may

exhibit a slight re

ection from the 103 plane, making them more hexagonal
in nature. Reports on similar experiments described by Chen et al. reported
ZnSe prepared by the reaction between zinc oxide and lauric acid (giving the
carboxylate salt), HDA and TOPSe resulting in hexagonal crystalline particles
approximately 4 nm in diameter. 111 Surprisingly, a method of preparing ZnSe
particles using TOPO has been reported, where ZnCl 2 in glycerol was mixed
with HDA and TOPO before a selenium solution of hydrazine was added at
200 C, followed by growth at 160 C. The particles, which were clearly cubic
in nature, were up to 17 nm in diameter and displayed broad band edge
emission at ca. 400 nm. 112 Selenourea has also been used as a precursor for
ZnSe QDs, using Zn(O 2 CCH 3 ) 2 as a precursor, with TOPO and octadecyl-
amine (ODA) as capping agents. 113 In this case, by varying the ratio between
ODA and TOPO, the crystallinity could be tuned between zinc blende and
wurtzite-structured materials and di

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ering particle geometries.
Quasi-spherical and rod-shaped ZnS QDs have also been prepared by the
injection of Et 2 Zn into sulfur/HDA-containing ODE, followed by growth at
300 C. 114 Low injection temperatures resulted in quasi-spherical particles of
intermediate cubic/hexagonal crystallinity, but above 150 C the formation of
rods predominated. The use of a large excess of sulfur also drove the
formation of rods. To increase the yield of the rods, the isolated particles
were annealed in OAm at 60 C for 1 day, resulting in the oriented attachment
of the particles yielding anisotropic particles with a cubic crystalline core.
Again, the use of TOPO did not result in rod formation, producing a random
distribution of particle shapes and sizes. The optical properties of the rods
and spherical particles showed excitonic absorption features at ca. 315 nm,
with near band edge emission, not trap emission as the group previously