become overshadowed by surface (SR) transitions. In cluster 24a (R ¼ Ph) a l max
of 358 nm is observed, whereas in 24b (R ¼ CH 2 CH(OH)CH 3 ), a blue shift of the
first excitation is observed with l max ¼ 325 nm. As the Cd 32 S 50 cores are identical
in the two, this inconsistency must be attributed to the different surface ligands
on the clusters, and the mixing of intraligand transitions with the ‘‘band gap’’
absorption [80]. As CdS has a relatively wide band gap (2.4 eV in the bulk), when
the particle size is reduced to that of molecular clusters, the transitions of the first
excited state become overshadowed by surface (SR) transitions.
CdSe, on the other hand, has a smaller band gap energy (1.7 eV) than CdS, and
thus surface ligand transitions are less likely to mask the ‘‘band gap’’ excitation
in the absorption spectra of nanoclusters of this material. Recent synthetic success
using RSeSiMe 3 /E(SiMe 3 ) 2 reagents has permitted an investigation of the optical
properties of a series of CdSe nanoclusters of different sizes that are isostructural
with the CdS complexes 22–24. The position of the first excitation is shifted to
lower energy with increasing cluster size for the adamantoid/barrelanoid clusters
[Cd 10 Se 4 (SePh) 12 (PPr 3 ) 4 ] (3.88 eV) [81], [Cd 17 Se 4 (SePh) 24 (PPh 2 Pr) 4 ]
2þ (3.42 eV)
[82], and [Cd 32 Se 14 (SePh) 36 (PPh 3 ) 4 ] (3.32 eV) [83]. The Cd 32 Se 50 cluster is formed
by dissolving the smaller precursor [Cd 10 Se 4 (SePh) 12 (PPh 3 ) 4 ] in tetrahydrofuran,
similar to the methods used for the preparation of [Cd 32 S 14 (SR) 36 ÁL 4 ] 24a from
[Cd 10 S 4 (SPh) 12 ] [79] and [Hg 32 Se 14 (SePh) 36 ] from [Hg 10 Se 4 (SePh) 12 (PPh 2
n Pr) 4 ]
[84]. The @1.5 nm size of these monodisperse M 32 nanoclusters overlaps with
those reported for small XII–XVI colloids [78]. Surprisingly, the absorption profiles
of the CdSe clusters are relatively broad, and if inhomogeneous broadening is a
result of size distribution, then particles of a uniform size should negate this effect.
The source of this broadening is unclear, however, it is suggested that it may be
due to the inhomogeneous distribution of surface coverage by the phenyl ligands
[85].
Because of their small size, the number of surface atoms in nanoclusters is a
large fraction of the total. As a result, the optical properties of these particles are
largely affected by the nature of the surface. It has already been demonstrated
above how surface ligand transitions can play a role in the overshadowing of ‘‘band
gap’’ absorptions and in the inhomogeneous broadening of absorption spectra. The
largest effect of surface states, however, can be observed in the emission spectra
using photoluminescence (PL) spectroscopy. The photoluminescence of XII–XVI
nanoparticles occurs as one of two types; band edge PL and deep trap PL [86].
Band edge emission occurs due to the recombination of trapped charge carriers
from shallow trap states within the particle. This type of emission results in a
slight red shift with respect to the absorption maximum, due to the relaxation of
the trapped charge carrier into lower lying energy states prior to recombination,
and it usually has a narrow line width. Deep trap emission, which is formally forbidden, occurs via the recombination of trapped localized carriers from deep and
mid-gap trap states, which are usually hole traps at the surface of the cluster (or
colloid). Deep trap emission is characterized by a broad line width, shifted significantly to the red of the excitonic absorption [86].
The PL spectra of the nanoclusters [Cd 32 Se 14 (SePh) 36 (PPh 3 ) 4 ], [Cd 17 Se 4 (SePh) 24 Á
13.4 Group XII-chalogenides and the Quantum Confinement Effect 443
of 358 nm is observed, whereas in 24b (R ¼ CH 2 CH(OH)CH 3 ), a blue shift of the
first excitation is observed with l max ¼ 325 nm. As the Cd 32 S 50 cores are identical
in the two, this inconsistency must be attributed to the different surface ligands
on the clusters, and the mixing of intraligand transitions with the ‘‘band gap’’
absorption [80]. As CdS has a relatively wide band gap (2.4 eV in the bulk), when
the particle size is reduced to that of molecular clusters, the transitions of the first
excited state become overshadowed by surface (SR) transitions.
CdSe, on the other hand, has a smaller band gap energy (1.7 eV) than CdS, and
thus surface ligand transitions are less likely to mask the ‘‘band gap’’ excitation
in the absorption spectra of nanoclusters of this material. Recent synthetic success
using RSeSiMe 3 /E(SiMe 3 ) 2 reagents has permitted an investigation of the optical
properties of a series of CdSe nanoclusters of different sizes that are isostructural
with the CdS complexes 22–24. The position of the first excitation is shifted to
lower energy with increasing cluster size for the adamantoid/barrelanoid clusters
[Cd 10 Se 4 (SePh) 12 (PPr 3 ) 4 ] (3.88 eV) [81], [Cd 17 Se 4 (SePh) 24 (PPh 2 Pr) 4 ]
2þ (3.42 eV)
[82], and [Cd 32 Se 14 (SePh) 36 (PPh 3 ) 4 ] (3.32 eV) [83]. The Cd 32 Se 50 cluster is formed
by dissolving the smaller precursor [Cd 10 Se 4 (SePh) 12 (PPh 3 ) 4 ] in tetrahydrofuran,
similar to the methods used for the preparation of [Cd 32 S 14 (SR) 36 ÁL 4 ] 24a from
[Cd 10 S 4 (SPh) 12 ] [79] and [Hg 32 Se 14 (SePh) 36 ] from [Hg 10 Se 4 (SePh) 12 (PPh 2
n Pr) 4 ]
[84]. The @1.5 nm size of these monodisperse M 32 nanoclusters overlaps with
those reported for small XII–XVI colloids [78]. Surprisingly, the absorption profiles
of the CdSe clusters are relatively broad, and if inhomogeneous broadening is a
result of size distribution, then particles of a uniform size should negate this effect.
The source of this broadening is unclear, however, it is suggested that it may be
due to the inhomogeneous distribution of surface coverage by the phenyl ligands
[85].
Because of their small size, the number of surface atoms in nanoclusters is a
large fraction of the total. As a result, the optical properties of these particles are
largely affected by the nature of the surface. It has already been demonstrated
above how surface ligand transitions can play a role in the overshadowing of ‘‘band
gap’’ absorptions and in the inhomogeneous broadening of absorption spectra. The
largest effect of surface states, however, can be observed in the emission spectra
using photoluminescence (PL) spectroscopy. The photoluminescence of XII–XVI
nanoparticles occurs as one of two types; band edge PL and deep trap PL [86].
Band edge emission occurs due to the recombination of trapped charge carriers
from shallow trap states within the particle. This type of emission results in a
slight red shift with respect to the absorption maximum, due to the relaxation of
the trapped charge carrier into lower lying energy states prior to recombination,
and it usually has a narrow line width. Deep trap emission, which is formally forbidden, occurs via the recombination of trapped localized carriers from deep and
mid-gap trap states, which are usually hole traps at the surface of the cluster (or
colloid). Deep trap emission is characterized by a broad line width, shifted significantly to the red of the excitonic absorption [86].
The PL spectra of the nanoclusters [Cd 32 Se 14 (SePh) 36 (PPh 3 ) 4 ], [Cd 17 Se 4 (SePh) 24 Á
13.4 Group XII-chalogenides and the Quantum Confinement Effect 443