The Synthesis of Core/Shell Quantum Dots - Semiconductor Quantum Dots: Organometallic and Inorganic Synthesis - page 142

The deposition of an extra shell has also been made on type II QD core/
shell structures. 119 As the charge carriers are not necessarily con ned to the

core in type II materials, extra passivation with a wide-bandgap material
further protects the structure, preventing charge carriers from reaching
surface defects and trapping sites. In the case of CdSe/ZnTe, the energy levels
highlighted the charge carrier distribution was spread across the particle,
with holes residing in the shell. If holes migrated to surface defects, emission
would be quenched to some degree. Particles of CdSe/ZnTe (5 nm in diam-
eter) synthesised as described earlier using metal oxides as precursors have
been prepared, isolated, and then dispersed in TOPO and ODA. Deposition of
a 0.4 nm thick ZnS shell was achieved by thermolysis of Zn(CO 2 (CH 2 ) 16 CH 3 ) 2
and TBPS at 190 C for up to 1.5 hours, giving a core/shell/shell material with
the electronic structure as shown in Figure 5.7. The wide bandgap of ZnS
dictated the electrons and holes were restricted to the CdSe/ZnTe structure.
The particles were isolated as normal, and could be phase-transferred to
water using dihydrolipoic acid (DHLA, see Chapter 6). Upon addition of
the ZnS shell, a slight red shi



d n 1 y 4 n g | 3

was observed as the exciton leaked into the
ZnS layer. This ruled out formation of an alloy, which would have resulted
in a slight blue shi



. The emission quantum yield for the CdSe/ZnTe
particles was estimated at 0.004%, which increased to 0.12% upon addition



.

Figure 5.7 Diagram depicting the energy levels for CdSe/ZnTe/ZnS core/shell/shell
particles. Reprinted with permission from C.-T. Cheng, C.-Y. Chen,
C.-W. Lai, W.-H. Liu, S.-C. Pu, P.-T. Chou, Y.-H. Chou and H.-T. Chiu,
J. Mater. Chem. , 2005, 15, 3409. Copyright 2005 The Royal Society of
Chemistry.