metallic procedure to synthesize highly crystalline and nearly monodisperse CdSe
QDs [10]. In contrast to previous samples, these nanocrystals had essentially
defect-free crystal structures and size variations smaller than 5% but the achieved
photoluminescence efficiencies were still relatively low (@10%).
The main cause for such low quantum yields is the imperfect surface structure
of the nanocrystals. Low-energy defect states on the particle surface give rise to
nonradiative relaxation pathways that degrade the optical properties of the QDs. In
1996, Hines and Guyot-Sionnest reported core–shell nanocrystals in which the
CdSe core was coated with a ZnS shell layer which improved the luminescence efficiencies to as high as 50% [27]. The growth of a CdS shell was also found to increase the quantum yield of CdSe nanocrystals [28, 29]. There are several reasons
why the surface-capping layer ZnS or CdS has such a beneficial effect on the luminescence efficiency: (i) the shell has similar lattice parameters to the core (since
the ZnaS and CdaS bond lengths are similar to that of CdaSe); (ii) the shell is
capable of crystallizing in the same crystal structure as CdSe (wurtzite structure in
this case); and (iii) the shell has a wider bandgap (energy difference between the
valence band and the conduction band) than the core. The first two properties work
together to minimize lattice mismatch and strain between the core and shell materials, promoting epitaxial growth of the shell, while the latter property shields the
core from the outside environment and further confines the exciton to the nanocrystal core.
At present, the successful synthetic procedures are all based on a common
theme, with small or incremental variations [10]. Briefly, an appropriate cadmium
precursor is combined with the desired chalcogen precursor (sulfur, selenium or
tellurium) in a hot coordinating solvent at a sufficiently high temperature to promote fast particle nucleation, followed by a lower temperature growth period. The
moderate particle growth rate allows the particle size to be monitored during
growth. Once the desired size has been obtained, growth can be terminated by
quenching the reaction. Cadmium precursors for synthesizing nanocrystals cores
include dimethyl cadmium, cadmium oxide, cadmium carbonate and cadmium
acetate, but shell formation is best accomplished using the organometallic reagents
dimethyl cadmium and dimethyl or diethyl zinc. Chalcogen species can be either
elemental sulfur, selenium or tellurium dissolved in tri-n-octyl phosphine (TOP) or
tri-n-butyl phosphine (TBP) as well as trimethylsilyl reagents of these chalcogens,
with either type being equally acceptable for core or shell synthesis. Tri-n-octyl
phosphine oxide (TOPO) is often used as the main solvent, with the addition of cosolvents such as dodecylamine [30], hexadecylamine [31], and stearic acid [32].
The following procedure describes the synthesis of CdE (E ¼ S, Se or Te) QD
cores whose surface is passivated with TOPO. With the cadmium precursor added
to the reaction vessel along with the TOPO, acting as the high-temperature coordinating solvent as well as surfactant, the solution is heated to @300–350
C. At
this point, an equimolar amount of chalcogen reagent dissolved in TOP (typical
stock solutions are 0.25 M in E) is rapidly injected into the hot solvent. Upon injection, the previously colorless solution rapidly gains color, the sequence of which
depends on the type of core material (from colorless to deep yellow for CdS, from
12.3 Synthesis 409
QDs [10]. In contrast to previous samples, these nanocrystals had essentially
defect-free crystal structures and size variations smaller than 5% but the achieved
photoluminescence efficiencies were still relatively low (@10%).
The main cause for such low quantum yields is the imperfect surface structure
of the nanocrystals. Low-energy defect states on the particle surface give rise to
nonradiative relaxation pathways that degrade the optical properties of the QDs. In
1996, Hines and Guyot-Sionnest reported core–shell nanocrystals in which the
CdSe core was coated with a ZnS shell layer which improved the luminescence efficiencies to as high as 50% [27]. The growth of a CdS shell was also found to increase the quantum yield of CdSe nanocrystals [28, 29]. There are several reasons
why the surface-capping layer ZnS or CdS has such a beneficial effect on the luminescence efficiency: (i) the shell has similar lattice parameters to the core (since
the ZnaS and CdaS bond lengths are similar to that of CdaSe); (ii) the shell is
capable of crystallizing in the same crystal structure as CdSe (wurtzite structure in
this case); and (iii) the shell has a wider bandgap (energy difference between the
valence band and the conduction band) than the core. The first two properties work
together to minimize lattice mismatch and strain between the core and shell materials, promoting epitaxial growth of the shell, while the latter property shields the
core from the outside environment and further confines the exciton to the nanocrystal core.
At present, the successful synthetic procedures are all based on a common
theme, with small or incremental variations [10]. Briefly, an appropriate cadmium
precursor is combined with the desired chalcogen precursor (sulfur, selenium or
tellurium) in a hot coordinating solvent at a sufficiently high temperature to promote fast particle nucleation, followed by a lower temperature growth period. The
moderate particle growth rate allows the particle size to be monitored during
growth. Once the desired size has been obtained, growth can be terminated by
quenching the reaction. Cadmium precursors for synthesizing nanocrystals cores
include dimethyl cadmium, cadmium oxide, cadmium carbonate and cadmium
acetate, but shell formation is best accomplished using the organometallic reagents
dimethyl cadmium and dimethyl or diethyl zinc. Chalcogen species can be either
elemental sulfur, selenium or tellurium dissolved in tri-n-octyl phosphine (TOP) or
tri-n-butyl phosphine (TBP) as well as trimethylsilyl reagents of these chalcogens,
with either type being equally acceptable for core or shell synthesis. Tri-n-octyl
phosphine oxide (TOPO) is often used as the main solvent, with the addition of cosolvents such as dodecylamine [30], hexadecylamine [31], and stearic acid [32].
The following procedure describes the synthesis of CdE (E ¼ S, Se or Te) QD
cores whose surface is passivated with TOPO. With the cadmium precursor added
to the reaction vessel along with the TOPO, acting as the high-temperature coordinating solvent as well as surfactant, the solution is heated to @300–350
C. At
this point, an equimolar amount of chalcogen reagent dissolved in TOP (typical
stock solutions are 0.25 M in E) is rapidly injected into the hot solvent. Upon injection, the previously colorless solution rapidly gains color, the sequence of which
depends on the type of core material (from colorless to deep yellow for CdS, from
12.3 Synthesis 409