Even though good emission properties make bulk metal halide perovskites natural candidates for efficient light-emitting devices, it is well known that these materials have a limited photoluminescence (PL) quantum yield, due to their small exciton binding energy and to the presence of mobile ion defects. The efficiency of light-emitting devices based on perovskites may be improved by synthesizing two-dimensional (2D), layered materials, which have a much larger exciton binding energy but are still prone to defect formation. The first two goals of the present project will thus be represented by the investigation of the effects of H irradiation on the optical properties of bulk and layered metal halide perovskites. Over the years, the hydrogenation of semiconductors has proven to be an invaluable experimental tool to tune and improve the properties of semiconductors, and preliminary results indeed suggest that H irradiation of bulk perovskites has beneficial effects on the optical properties of these materials. As it regards layered perovskites, however, it is important to note that recent attempts to hydrogenate semiconductors with a similar structure, such as transition-metal dichalcogenides (TMDs) and hexagonal boron nitride (hBN), wherein 2D crystalline layers are held together by weak van der Waals forces, resulted in the accumulation of H2 molecules underneath the first few crystal layers, leading to the formation of micrometer-sized domes that dramatically alter the morphology and the strain state of the material. A further goal of this project will thus be the verification of whether H irradiation of layered perovskites also leads to the formation of similar domes. Finally, the ultimate goals of the present project will be represented by the fabrication of 2D heterostructures, based on the stacking of layered perovskites on top of TMD and hBN microdomes, and on the investigation of the effects that such curved templates have on the strain state of the system.
The results expected in the framework of this project are of high importance, both from the fundamental point of view and, in perspective, for their potential application to the realization of innovative light-emitting devices. To the best of our knowledge, hydrogen's ability to passivate defects has never been demonstrated in perovskites, so that the achievement of the first two goals of the project -that is, the determination the effects of H irradiation on the optical properties of bulk and layered metal halide perovskites, respectively- could represent very important contributions to the field. It is worth mentioning here that after the initial phase, in which we will focus on bulk inorganic perovskites such as as CsPbCl3 and CsPbBr3, we will shift our attention to hybrid organic/inorganic materials such as MAPbI3 (MA = CH3NH3) and FAPbI3 [FA = CH(NH2)2]. The investigation of the effects of H irradiation on these materials is rendered particularly interesting by the recent discovery of the importance of hydrogen vacancies in the determination of the radiative efficiency of these materials, as reported in [Zhang et al., Nat Mater 20, 971 (2021)]. As far as layered perovskites are concerned, on the other hand, we already noted that the large mismatch between the dielectric constants of the inorganic slabs and of the organic barriers leads to very large exciton binding energies [Blancon et al, Nature Communications 9, 2254 (2018)], solving one of the two main issues plaguing the recombination efficiency of these materials. The eventual confirmation that H irradiation can effectively passivate defects in layered perovskites would then go a long way towards the resolution of the second (and final) issue, and it would thus be hailed as a major milestone in the quest towards the development of perovskites with ultimate radiative efficiency.
Finally, in the last part, we will investigate the effects of strain in layered perovskites, and in heterostructures based on 2D perovskites stacked on top of TMD- and hBN-based microdomes. Apart from its fundamental interest, the understanding of these effects would provide us with a crucially important knob for the tuning (and, hopefully, for the optimization) of the properties of these materials. In heterostructures, more specifically, strain tuning would afford us an unprecedented control of the band alignments, providing us with the ability to directly affect the formation and the properties of interlayer excitons [Chen et al., ACS Nano 14, 10258 (2020)] and the establishment of charge transfer phenomena between the heterostructure's constituents [Karpiska et al., ACS Applied Materials & Interfaces Article ASAP (2021)].