(Ph 2 Pr) 4 ]
2þ and [Cd 10 Se 4 (SePh) 12 (PPr 3 ) 4 ] exhibit only trapped emission [85]. The
clusters can be viewed as having ‘‘imperfect’’ surfaces, which results in the generation of surface trap states. The energy of the trapped emission is independent of
the size of the cluster, an effect that is also observed in colloidal samples. This indicates that the nature of the emitting state is similar for all species, also giving
strength to the argument that surface states are responsible for this type of emission. The effect of temperature on deep trap emission is such that the intensity
of the emission decreases as the temperature increases. This can be explained
by non-radiative recombination processes dominating at higher temperatures [86].
The non-radiative relaxation in the CdSe nanoclusters has been attributed to the
involvement of a multiphonon relaxation mechanism mediated by low-frequency
vibrations of the surface phenylselenolate ligands [85]. The net result is such that,
at ambient temperatures, the fast non-radiative decay processes compete with the
slower deep-trap radiative recombination from these states, and no emission is
observed from these clusters. At lower temperatures, the non-radiative decay processes slow down and the quantum yield of the emission increases dramatically.
The quantum yields of these emissions at low temperatures range from 10 to 40%,
with lifetimes of 0.8–10 ms. The effect of the chemical make-up of the surfaces of
XII–XVI clusters on emission properties is also demonstrated for the adamantoid
clusters [Cd 10 E 4 (EAr) 12 L 4 ]
nÀ (Ar ¼ aryl; L ¼ PR 3 , n ¼ 0; L ¼ halide, n ¼ 4) [87,
88].
The phenyl groups bonded to the selenium centers on the surfaces of these
Cd 10 Se, Cd 17 Se and Cd 32 Se clusters are not ideal for passivation of the inorganic
core i.e. the elimination of deep-trap surface states. Efficient passivation of nanoparticles is often achieved via the growth of a wide band-gap XII–XVI surface onto
a quantum dot seed of another XII–XVI material with a smaller band gap energy.
In these ‘‘core–shell’’ particles, the band-edge emission intensities are enhanced
markedly relative to those of particles of the core material only [75]. The nanocluster [(N,N
0 -tmeda) 5 Zn 5 Cd 11 Se 13 (SePh) 6 (OC 4 H 8 ) 2 ] 25 (Figure 13.21) consists of
a Cd 11 Se 13 core in which 10 of the Se
2À ligands are bonded to the five (N,N
0 -
tmeda)Zn sites, located at the cluster surface. Cluster 25 can thus be viewed as the
‘‘molecular limit’’ of core–shell CdSe/ZnSe particles [89]. Consistent with this
view is the demonstrated band-edge luminescence observed at room temperature
for 25. It is unclear at present however whether the increased emission quantum
yield of 25 versus the CdSe clusters described above is a consequence of efficient
surface passivation by ZnSe, or whether it is the absence of phenyl moieties on
half of the cluster surface that limits the non-radiative decay efficiencies.
13.5
Ternary MMOE
In addition to the cage-like cluster molecules [Mo 8 Cu 12 S 28 E 4 ] (E ¼ O, S) that have
been synthesized from the reaction of tetrachalcogenomolybdates with Cu(I) salts
[90, 91], alternate synthetic methods have recently led to the isolation of ternary
13 Large Semiconductor Molecules
444
2þ and [Cd 10 Se 4 (SePh) 12 (PPr 3 ) 4 ] exhibit only trapped emission [85]. The
clusters can be viewed as having ‘‘imperfect’’ surfaces, which results in the generation of surface trap states. The energy of the trapped emission is independent of
the size of the cluster, an effect that is also observed in colloidal samples. This indicates that the nature of the emitting state is similar for all species, also giving
strength to the argument that surface states are responsible for this type of emission. The effect of temperature on deep trap emission is such that the intensity
of the emission decreases as the temperature increases. This can be explained
by non-radiative recombination processes dominating at higher temperatures [86].
The non-radiative relaxation in the CdSe nanoclusters has been attributed to the
involvement of a multiphonon relaxation mechanism mediated by low-frequency
vibrations of the surface phenylselenolate ligands [85]. The net result is such that,
at ambient temperatures, the fast non-radiative decay processes compete with the
slower deep-trap radiative recombination from these states, and no emission is
observed from these clusters. At lower temperatures, the non-radiative decay processes slow down and the quantum yield of the emission increases dramatically.
The quantum yields of these emissions at low temperatures range from 10 to 40%,
with lifetimes of 0.8–10 ms. The effect of the chemical make-up of the surfaces of
XII–XVI clusters on emission properties is also demonstrated for the adamantoid
clusters [Cd 10 E 4 (EAr) 12 L 4 ]
nÀ (Ar ¼ aryl; L ¼ PR 3 , n ¼ 0; L ¼ halide, n ¼ 4) [87,
88].
The phenyl groups bonded to the selenium centers on the surfaces of these
Cd 10 Se, Cd 17 Se and Cd 32 Se clusters are not ideal for passivation of the inorganic
core i.e. the elimination of deep-trap surface states. Efficient passivation of nanoparticles is often achieved via the growth of a wide band-gap XII–XVI surface onto
a quantum dot seed of another XII–XVI material with a smaller band gap energy.
In these ‘‘core–shell’’ particles, the band-edge emission intensities are enhanced
markedly relative to those of particles of the core material only [75]. The nanocluster [(N,N
0 -tmeda) 5 Zn 5 Cd 11 Se 13 (SePh) 6 (OC 4 H 8 ) 2 ] 25 (Figure 13.21) consists of
a Cd 11 Se 13 core in which 10 of the Se
2À ligands are bonded to the five (N,N
0 -
tmeda)Zn sites, located at the cluster surface. Cluster 25 can thus be viewed as the
‘‘molecular limit’’ of core–shell CdSe/ZnSe particles [89]. Consistent with this
view is the demonstrated band-edge luminescence observed at room temperature
for 25. It is unclear at present however whether the increased emission quantum
yield of 25 versus the CdSe clusters described above is a consequence of efficient
surface passivation by ZnSe, or whether it is the absence of phenyl moieties on
half of the cluster surface that limits the non-radiative decay efficiencies.
13.5
Ternary MMOE
In addition to the cage-like cluster molecules [Mo 8 Cu 12 S 28 E 4 ] (E ¼ O, S) that have
been synthesized from the reaction of tetrachalcogenomolybdates with Cu(I) salts
[90, 91], alternate synthetic methods have recently led to the isolation of ternary
13 Large Semiconductor Molecules
444