Page 122 - Read Online

Page 4 of 19 Lee et al. Microstructures 2023;3:2023021 https://dx.doi.org/10.20517/microstructures.2023.08

tunability and uniformity, make these materials good candidates as host scaffolds for QDs.

An interesting topic in the field of synthesizing QDs in zeolites is the use of cadmium sulphide (CdS) QDs
in zeolite Y. While this concept was established and researched in the 1980s, it continues to provide new
insights for the next generation of researchers. CdS quantum dots are one of the most studied II-IV binary
semiconductor materials due to their narrow band gap of 2.42 eV . However, the performance of the
[43]
material highly depends on its size and three-dimensional structure. Thus, much research effort has been
dedicated to avoiding aggregation, size control and stabilisation of QD structure. Herron et al. introduced
the idea of cadmium sulphide quantum dots (CdS QDs) in zeolite Y to improve their stability and
optoelectronic properties . In this work, CdS semiconductor clusters were synthesized using an aqueous
[44]
solution process which involves cadmium ion exchange followed by hydrogen sulphide (H S) gas flowing to
2
create CdS cluster within the zeolite pores. It was reported that during synthesis, QDs are formed in sodalite
cages (5 Å) instead of supercage pores (13 Å) of zeolite Y structure through a percolative process
[45]
[Figure 1A] . However, a recent study showed that CdS QDs exist in supercages rather than sodalite cages.
The author found that the migrated Cd ions in sodalite cage during drying process diffused back to
2+
supercage in later reaction process with H S to form QDs, showing the highly dynamic nature of the QD
2
even in their condensed phase. The isolated CdS nanoclusters form interconnection through supercage
window in [CdS] unit, as shown in Figure 1B . This example demonstrates the templated growth
[46]
4
methodology to form QDs within the micropores, and it also clearly shows that the QDs in the template are
still preserving their dynamic properties.

Another important guest material is lead sulphide (PbS) QD which has a narrow band gap and size-
dependent optical properties due to its large exciton Bohr radius which makes it a good candidate for third-
order nonlinear optical (3NLO) applications [47-49] . High 3NLO responses are enabled by smaller-size QDs
and increased material density in the same matrix volume. However, the increased population of QDs in a
small volume introduces aggregations and reduces the efficacy of the composite. Kim et al. demonstrated a
significant increase in the third-order nonlinear optical (3NLO) activity of PbS quantum dots (QDs) by
embedding them within the nanopores of zeolite Y . This work also highlighted the interplay between the
[50]
+
host and guest materials. By replacing of H cation in the zeolite Y matrix with cations of different sizes,
such as NH , Li , Na , K and Rb , the stability and 3NLO activity of the composite were noticeably
+
+
+
+
+
4
enhanced . Considering it is difficult to tailor the chemical functionality or structure properties of zeolites
[50]
based on guest materials, like other evolving porous materials, metal-organic frameworks (MOFs) or
covalent organic frameworks (COFs), systematic replacement of zeolite’s counter cations to guest material is
one of the viable options to control pore volume, framework donor strength, cation acceptor strength, and
electric field strength. This work shows a promising new approach to improve 3NLO activity within the
zeolite matrix and the potential guest QDs.
In recent years, CsPbX (X = Cl, Br, or I) perovskites QDs are receiving great attention as PVs or LEDs as
3
they have a narrow emission band and high PLQY [51,52] . They can be easily processed in liquid form, making
them particularly suitable for templated growth within zeolites . Kim et al. used zeolite X as a host matrix
[53]
for Na Cs PbBr QDs, benefiting from its hydrophilic nature and relatively high aluminium content, which
4
6
4
resulted in an increased number of extra framework cations (Na ) that effectively enhanced the stability of
+
[54]
the quantum dots when compared to zeolite Y . This work was initially designed to create CsPbBr QDs
3
within zeolite X based on the previous research that encapsulates CsPbX QDs within zeolite Y matrix.
3
+
+
However, the author found out that the Cs cations interact with counter cations (Na ), forming new types
of QDs, Na Cs PbBr . Those QDs are placed within the supercage of zeolite structure and interconnected
6
4
4
through the window, as shown in Figure 2. The composite showed a much narrower light emission band