The Preparation of IV–VI Semiconductor Nanomaterials - Semiconductor Quantum Dots: Organometallic and Inorganic Synthesis - page 79

was assigned as an interband transition for electrons and holes with a 1S
envelope function (termed 1S h 1S e ) and notably the second excitonic feature
which was eventually assigned as 1P h 1P e . 25 In this seminal report, mono-
dispersed quasi-spherical particles of PbSe were prepared by the injection of
Pb(CO 2 (CH 2 ) 7 (CH CH)(CH 2 ) 7 CH 3 ) 2 and TOPSe into diphenylether at 150 C,

a notably lower synthesis temperature than those used for the preparation
of II

]

d n 1 y 4 n g | 7

- VI materials, a consistent feature of the synthesis of IV - VI nano-

materials. The particle size of 3 15 nm diameter was tuned by altering the

reaction temperature, with higher temperatures giving larger particles. The
route was then used to explore the optical properties of PbSe in some depth,
demonstrating the tuneable band edge emission with extremely high
quantum yields of up to 85%, ranging typically from 1.3 to 2.0

-

m

m in size. 26,27

The samples exhibited small Stokes shi s of only 22 meV, assigned to the

simple exciton structure. Larger particles (>8 nm in diameter) were prepared
using the same method with either a higher synthesis temperature (250 C)
or a lower temperature (200 C) route with essentially secondary precursor
additions. 28 Larger particles, which typically favoured cubic morphologies,
displayed emission spectra up to 4



m (Figure 3.2), although the quantum
yield dropped noticeably to 0.5% due to electronic coupling with the capping
agent. This drop in quantum yield as the particle increased in size might

m

.

Figure 3.2 Room-temperature emission spectra of a series of PbSe QDs (dashed line
spectrum obtained at 77 K). Reprinted with permission from J. M.
Pietyga, R. D. Schaller, D. Werder, M. H. Stewart, V. I. Klimov and J. A.
Hollingsworth, J. Am. Chem. Soc. , 2004, 126 , 11752. Copyright 2004
American Chemical Society.