glassy substrates. One of the first reports on the optical properties of silver clusters in
silica-based glasses was performed by Borsella and co-workers [49]. In this study
different silver ion concentrations and gases used during the heat treatment of the
samples were tested. A plausible formation mechanism was attributed to the creation
of defects on the glass structure generated by the ion implantation technique, such
defects served as nucleation sites to trigger the formation of luminescent silver
clusters (<8 Ag atoms). In the same report, the use of these materials in optoelectronic applications was suggested. Since this report, different groups started to
investigate the influence of the initial composition of the glass matrix on the optical
properties of the formed silver clusters, for instance, the use of oxyfluoride [50],
lanthanide-containing [51], and metal co-doped glassy substrates [52]. Next to the
use of traditional activation techniques, researchers started to look for more controlled synthesis conditions for the fabrication of luminescent silver clusters in glassy
substrates. Combinatorial approaches in which X-ray irradiation, heat treatment, and
photon-induced activation [53–55] are employed, emerged as alternative activation
procedures to fabricate, in a more controlled fashion, luminescent silver clusters in
glassy substrates with specific functionalities. On the other hand, microporous
materials have been pointed as suitable materials to stabilize sub-nanometer metal
clusters; for instance, zeolites (Fig. 1) have been used as scaffolds for the fabrication
of silver clusters and nanoparticles over the past five decades [56]. Recently, a novel
type of microporous functional materials, metal organic frameworks (MOFs), were
postulated as potential candidates to serve as scaffolds for the confinement of
functional luminescent metal clusters and nanoparticles. This is exemplified in the
work of Falcaro and Furukawa [57], in which a MOF material was doped with
lanthanides ions (Eu
3+ , Tb
3+ ) to generate a red emitting material. On the other hand,
several attempts have been carried to fabricate metal nanoparticles within MOFs
nanocavities. This was recently achieved in a study by Ameloot and collaborators
[58] in which MOFs were used as photoactive matrices for the generation of metallic
silver microstructures. In a different example, the rational design of the ligand prior
to MOF assembly to encapsulate palladium nanoparticles was applied to fabricate
advanced heterogeneous catalysts [59]. It is expected that these types of functional
materials (metal doped MOFs) will emerge as a viable route for the fabrication of
luminescent metal clusters as illustrated by recent reports [60, 61].
2 Luminescent Silver Clusters Confined in Zeolites
2.1 Early Studies on Silver-Exchanged Zeolites
Zeolites are aluminosilicate materials with a molecularly sized cages arrangement
which makes them perfect candidates to accommodate sub-nanometer silver clusters, whereas their high cation exchange capacity facilitates the uptake of silver ion
precursors within their pores and cages. A subsequent reduction step (heat treatment,
Photoactivation X-ray irradiation) converts metal cations into oligoatomic clusters,
82
E. Coutino-Gonzalez et al.
silica-based glasses was performed by Borsella and co-workers [49]. In this study
different silver ion concentrations and gases used during the heat treatment of the
samples were tested. A plausible formation mechanism was attributed to the creation
of defects on the glass structure generated by the ion implantation technique, such
defects served as nucleation sites to trigger the formation of luminescent silver
clusters (<8 Ag atoms). In the same report, the use of these materials in optoelectronic applications was suggested. Since this report, different groups started to
investigate the influence of the initial composition of the glass matrix on the optical
properties of the formed silver clusters, for instance, the use of oxyfluoride [50],
lanthanide-containing [51], and metal co-doped glassy substrates [52]. Next to the
use of traditional activation techniques, researchers started to look for more controlled synthesis conditions for the fabrication of luminescent silver clusters in glassy
substrates. Combinatorial approaches in which X-ray irradiation, heat treatment, and
photon-induced activation [53–55] are employed, emerged as alternative activation
procedures to fabricate, in a more controlled fashion, luminescent silver clusters in
glassy substrates with specific functionalities. On the other hand, microporous
materials have been pointed as suitable materials to stabilize sub-nanometer metal
clusters; for instance, zeolites (Fig. 1) have been used as scaffolds for the fabrication
of silver clusters and nanoparticles over the past five decades [56]. Recently, a novel
type of microporous functional materials, metal organic frameworks (MOFs), were
postulated as potential candidates to serve as scaffolds for the confinement of
functional luminescent metal clusters and nanoparticles. This is exemplified in the
work of Falcaro and Furukawa [57], in which a MOF material was doped with
lanthanides ions (Eu
3+ , Tb
3+ ) to generate a red emitting material. On the other hand,
several attempts have been carried to fabricate metal nanoparticles within MOFs
nanocavities. This was recently achieved in a study by Ameloot and collaborators
[58] in which MOFs were used as photoactive matrices for the generation of metallic
silver microstructures. In a different example, the rational design of the ligand prior
to MOF assembly to encapsulate palladium nanoparticles was applied to fabricate
advanced heterogeneous catalysts [59]. It is expected that these types of functional
materials (metal doped MOFs) will emerge as a viable route for the fabrication of
luminescent metal clusters as illustrated by recent reports [60, 61].
2 Luminescent Silver Clusters Confined in Zeolites
2.1 Early Studies on Silver-Exchanged Zeolites
Zeolites are aluminosilicate materials with a molecularly sized cages arrangement
which makes them perfect candidates to accommodate sub-nanometer silver clusters, whereas their high cation exchange capacity facilitates the uptake of silver ion
precursors within their pores and cages. A subsequent reduction step (heat treatment,
Photoactivation X-ray irradiation) converts metal cations into oligoatomic clusters,
82
E. Coutino-Gonzalez et al.