The Synthesis of Core/Shell Quantum Dots - Semiconductor Quantum Dots: Organometallic and Inorganic Synthesis - page 139

energy levels were aligned such that carrier localisation was in the shell (type I
behaviour — it is worth noting that this is still considered type I even though

the con nement is in the shell, not the core as found in CdSe/ZnS particles).

This highlighted that the charge carriers can be restricted to certain parts of
the nanoparticle and hence the recombination tuned by simply altering the
shell thickness. ZnTe-based core/shell materials have been reported with CdS,
CdSe and CdTe shells, prepared by a one-pot reaction. 105 In this case, the ZnTe
cores were prepared using TOPTe and Me 2 Zn in TOP as precursors, which
were then injected into an ODE solution of ODA at 280 C, followed by cooling
to 240 C in readiness for shell growth. Shell precursors (CdO in ODE and
oleic acid, TOPSe, TOPTe or ODE/S) were then injected sequentially using the
SILAR technique to controllably grow the required number of monolayers.
The resulting type II structures exhibited tuneable emission, in the case of
ZnTe/CdSe, from ca. 550 nm (0.5 monolayers of CdSe on a 4.5 nm ZnTe core)
to ca. 900 nm (4.5 monolayers of CdSe on a 7.6 nm ZnTe core) with emission
quantum yields of up to 20%. When OAm was used as a ligand during shell
growth, the spherical ZnTe/CdSe particles eventually grew into a pyramidal
structure and ultimately into tetrapods. 106 The ZnTe/CdSe tetrapods exhibited
a feature in the absorption spectra at 650 nm, consistent with the presence of
CdSe, but showed no evidence of emission.
Type II structures with a ZnTe/ZnSe structure have also been prepared. 107
In this case, the core particles were made by the thermolysis of Et 2 Zn and
TOPTe at 270 C in ODE, using HDA as a capping agent. Isolation and
puri



d n 1 y 4 n g | 3

cation were carried out before the core particles were redispersed in
fresh HDA/ODE, and Et 2 Zn and TOPSe were added dropwise over 6 hours at
200



.

250 C. A further ZnS shell could be added without puri

cation, by
slowly adding Et 2 Zn and S(SiMe 3 ) 2 at 220 C over 2 hours followed by 1 hour
stirring. The bare ZnTe particles were not luminescent, but were emissive
between 500 nm and 580 nm when the ZnSe shell was deposited. The
emission, tuneable by the shell thickness, had a maximum quantum yield
of 6%, which could be improved to 12% with the deposition of a further
ZnS shell.
Related materials (ZnSe/CdSe) have been prepared using metal alkyl-based
routes as described in Chapter 1. The ZnSe cores were prepared by the
thermolysis of Et 2 Zn and TOPSe in long-chain amines at ca. 300 C. Shell
growth was achieved by addition of a required amount of Me 2 Cd and TOPSe
to give the required shell thickness. An annealing stage of up to 2 days was
required to achieve quantum yields of up to 80%. Emission could be tuned
between 430 and 600 nm, with FWHM of 20

-



- 40 nm. These particles were

then used in light ampli cation studies. 108

Core/shell ZnSe/CdSe particles have also been prepared using CdO as
a precursor and ODE as a solvent for the shell deposition, using 2.8 nm ZnSe
cores, giving materials with similar optical properties to those described
above. 109 In this case, the di



raction patterns of the ZnSe particles suggested
a cubic core, which changed to a hexagonal pattern with the deposition of
6 monolayers of CdSe shell. The emission and absorption spectra provided