OPTICAL PROPERTIES STABILITY OF CsPbX3 NANOCRYSTALS EMBEDDED IN POROUS GLASS MATRIX

All-inorganic perovskite nanocrystals with chemical formula CsPbX3 (X = Cl, Br, and I) attract much scientific attention since they possess unique optical properties, such as high extinction coefficients and values of emission quantum yield, together with ease of their synthesis and tunability in the chemical composition. However, these nanomaterials are still far from their large-scale applications since they lack stability. Here, it was shown that the use of a nanoporous glass matrix allowed obtaining the samples with blue, green, and red perovskite nanocrystals possessing reproducible optical characteristics which are almost similar to that of their colloidal solution. Such a matrix also prevented the fast degradation of nanocrystals both at the storage in ambient and under UV-light exposure and/or in the conditions of increased humidity.


INTRODUCTION
The development of chemical routes for the synthesis of nanocrystals with crystal lattice of perovskite-type (PNCs) with chemical formula CsPbX3 (X = Cl, Br, and I) [1] resulted in the burst of scientific attention since the PNCs possess unique optical and electronic properties: high extinction coefficients, high photoluminescence (PL) quantum yields reaching 1, and high charge carrier mobility [2,3]. Another advantage of these nanomaterials is the tunability of chemical composition together with the ease of fabrication [2] which is important for PNCs future utilization in different areas of photovoltaics and optoelectronics.
However, these materials are unstable and can be easily decomposed under ultra-violet (UV) light exposure and while storing in increased humidity [4,5]. At the moment there several ways to tackle this problem: (i) direct synthesis in polar solvents [6], (ii) passivation of PNC surface via chemical treatment including the ligand engineering [7,8], and (iii) embedding the PNCs into different inert matrices, such as polymers or solid porous matrices [9][10][11]. In the latter approach, the matrix can be chosen from a wide variety of materials either soft/flexible or hard/solid which can be related to their further application.
Here, we investigate the stability of optical responses of all-inorganic PNCs embedded in nanoporous silicate matrix (NSM) under the UV exposure and increased humidity. The developed idea on PNC protection is of wide interest for their further implementation as active media in solar cells, photodetectors, and light-emitting diodes. https://doi.org/10.37904/nanocon.2019.8782

EXPERIMENTAL
Steady-state spectral measurements of samples were carried out using a UV-3600 spectrophotometer (Shimadzu), a FP-1800 spectrofluorometer (Jasco), and a confocal lasing scanning microscope LSM-710 (Zeiss) equipped with 20× (NA=0.4) objective and a 405 nm laser. For transient photoluminescence measurements a confocal microscope MicroTime 100 (PicoQuant) equipped with 100× (NA=0.95) objective and 405 nm pulsed diode laser implementing time-correlated single photon counting. To estimate the value deviations of each optical parameter the signal from the sample was collected at least in 3 different points.
NSMs were fabricated by the procedure reported in [12]. Before the use, the obtained NSMs were annealed at 100 ˚C during 1h in a vacuum oven to get rid of moisture and oxygen presented inside the pores. Figure 1a it is seen that the pores are of nanometer-size and homogeneously distributed within NSM' volume. The 3D PL image reconstruction of chopped NSM with g-PNCs shown in Figure 1b confirmed the PNCs penetration into NSM pores. The confocal PL images of samples showed that the PNCs formed agglomerates on the NSM surface which were most probably located at the pore entry. This can be seen as bright spots in the PL images (Figure 2). Although the formation of agglomerates on NSM surface is an undesirable process that may affect optical properties of PNCs, however, this helped to close as many pores on the surface as possible which, in turn, resulted in the increased protection from the moisture and oxygen penetration within the porous matrix.

From SEM image of NSM shown in
The absorption spectra showed increased optical density in the 400-650 nm spectral region which can be attributed to the presence of the PNCs in the NSM volume. The PL spectra shown in Figure 3 of PNCs in NSM showed almost unchanged peak positions with the increased full width at half maximum (FWHM). It is worth to mention that the average PL lifetime is almost preserved after the PNCs embedding into the NSMs. This observation suggested that the embedding of PNCs into the NSMs didn't result in the appearance of additional nonradiative channels of charge carriers' recombination. PL parameters of investigated samples are summarized in Table 1.   Figure 4. For b-PNCs embedded into NSM a broadening of the PL band was observed with almost the same PL peak position. The average PL lifetime decreased slightly during the UV-exposure. For g-PNCs in NSM PL band didn't undergo any significant changes. As it was observed for b-PNCs average PL lifetime also decreased slightly with increased exposure time.
Second, the stability of the optical properties of g-PNC embedded into the NSM was probed under the increased humidity conditions. For that, the distilled water was dispersed at the 20 cm distance from the sample. After the water dispersing, the PL spectrum was measured, and this procedure was repeated 3 times.
The changes in emission relative efficiency and PL position with increased humidity are shown in Figure 5. As it can be seen from Figure 5 the NSM as a host matrix for PNCs preserved the optical properties of g-PNCs during the increased humidity. However, there was a critical value of humidity reaching which the PL efficiency decreased almost twice. Next, we examined the optical properties of g-PNCs in NSM after dipping the sample into the distilled water. The PL signal disappeared after such a treatment.

CONCLUSION
The use of a nanoporous glass matrix allowed obtaining the samples with blue, green, and red perovskite NCs possessing reproducible optical characteristics that are almost similar to that of colloidal solution. Such a matrix also may prevent the fast degradation of nanocrystals both at the storage in ambient and under UV-light exposure and/or increased humidity. Thus, the nanoporous inert solid matrix is a perspective candidate for its implementation as a host matrix for perovskite nanocrystals for photovoltaic and optoelectronic devices with improved performance.