The geometry of the clusters must be determined by quantum chemistry methods
that often use group theory and the optic response is described in terms of molecular
transitions whose positions and intensities are predicted by sophisticated calculations of quantum mechanics [20]. As a pioneering work, the absorption spectra
computed by equation-of-motion coupled-cluster singles and doubles
(EOM-CCSD) and similarity transformed equation-of-motion coupled-cluster singles and doubles (STEOM-CCSD) methods with all-electron basis sets, for the most
stable structures of Ag 5–8 , nicely illustrated the molecular-like behavior of nanoclusters leading to an electronic energy quantization and the changes in the leading
features of the patterns as a function of the cluster size [24]. While the optical
properties of such small metal clusters have been largely investigated in the gas
phase, their study in the solid and liquid phase requires some “stabilization” that
prevents them from fragmentation or degradation. The use of solid gas or inorganic
matrices permits to protect gold or silver clusters from photodissociation [25–28].
Organic scaffolds allow the formation and stabilization of metal clusters in solution.
The use of organic scaffolds for fluorescent metal nanoclusters is relatively new,
first reported by the Zheng and Dickson [29] for silver nanoclusters. These organic
scaffolds have tremendous potentials, as the interaction between the ligands and
metal clusters can be adjusted, leading to tunability in their spectroscopic properties. For example, by using DNA oligomers as organic scaffolds and by playing
with the nucleotide sequence of DNA oligomers, it is possible to synthesize silver
nanoclusters that emit from the blue to near-infrared region [30].
Ligands play very important roles in the formation of NCs as protective agents,
which can prevent the metal clusters from aggregation and then keep the
size-dependent fluorescence property. The formation and stabilization of gold or
silver nanoclusters in solution have been accomplished in various ways and with
different scaffolds (Figs. 5.2 and 5.3). The proper choice of parameters for the
reaction, including the temperature, the reducing method, the stabilizers, and the
initial ratio of metal salt and stabilizer, plays a crucial role in the successful synthesis of nanoclusters and to limit the size to few-atom nanoclusters. In addition to
the ultrasmall size, the ligands used for NCs preparation also have impacts on their
fluorescence properties. Wu and Jin [31] demonstrated that for gold NCs, the
surface ligands of NCs not only can be used as capping agent but also largely affect
the fluorescence of NCs through charge transfer from surface ligand to the gold
core. When the surface ligands have strong electron donation capability, the
fluorescence can be enhanced. And the ligands with electron-rich atoms or groups
have been found as a very effective choice for the promising surface ligand of NCs
to enhance the fluorescence.
142
R. Antoine
that often use group theory and the optic response is described in terms of molecular
transitions whose positions and intensities are predicted by sophisticated calculations of quantum mechanics [20]. As a pioneering work, the absorption spectra
computed by equation-of-motion coupled-cluster singles and doubles
(EOM-CCSD) and similarity transformed equation-of-motion coupled-cluster singles and doubles (STEOM-CCSD) methods with all-electron basis sets, for the most
stable structures of Ag 5–8 , nicely illustrated the molecular-like behavior of nanoclusters leading to an electronic energy quantization and the changes in the leading
features of the patterns as a function of the cluster size [24]. While the optical
properties of such small metal clusters have been largely investigated in the gas
phase, their study in the solid and liquid phase requires some “stabilization” that
prevents them from fragmentation or degradation. The use of solid gas or inorganic
matrices permits to protect gold or silver clusters from photodissociation [25–28].
Organic scaffolds allow the formation and stabilization of metal clusters in solution.
The use of organic scaffolds for fluorescent metal nanoclusters is relatively new,
first reported by the Zheng and Dickson [29] for silver nanoclusters. These organic
scaffolds have tremendous potentials, as the interaction between the ligands and
metal clusters can be adjusted, leading to tunability in their spectroscopic properties. For example, by using DNA oligomers as organic scaffolds and by playing
with the nucleotide sequence of DNA oligomers, it is possible to synthesize silver
nanoclusters that emit from the blue to near-infrared region [30].
Ligands play very important roles in the formation of NCs as protective agents,
which can prevent the metal clusters from aggregation and then keep the
size-dependent fluorescence property. The formation and stabilization of gold or
silver nanoclusters in solution have been accomplished in various ways and with
different scaffolds (Figs. 5.2 and 5.3). The proper choice of parameters for the
reaction, including the temperature, the reducing method, the stabilizers, and the
initial ratio of metal salt and stabilizer, plays a crucial role in the successful synthesis of nanoclusters and to limit the size to few-atom nanoclusters. In addition to
the ultrasmall size, the ligands used for NCs preparation also have impacts on their
fluorescence properties. Wu and Jin [31] demonstrated that for gold NCs, the
surface ligands of NCs not only can be used as capping agent but also largely affect
the fluorescence of NCs through charge transfer from surface ligand to the gold
core. When the surface ligands have strong electron donation capability, the
fluorescence can be enhanced. And the ligands with electron-rich atoms or groups
have been found as a very effective choice for the promising surface ligand of NCs
to enhance the fluorescence.
142
R. Antoine