As first DEOM aimed to fabricate layer-confined and large size low-dimensional metal halide perovskite quantum well (QW) to use as building blocks for vertically stacked visible optoelectronic metamaterials. We design a liquid surface assembly method similar to Langmuir film formation, where organic-inorganic hybrid perovskite solutions are spread on a liquid substrate and there the crystallization is confined at the liquid-air interface. Through engineering precursor and solvent chemistry, we have achieved large scale growth of over 50 μm lateral sized perovskite sheets, with thickness under 10 nm (5 layers) and high purity in phase composition and optical bandgap.
A low surface energy spreading of the solution was achieved by using acetonitrile as a solvent and phenyl-perfluorinated organic spacer molecules (pentafluorophenylethylammonium, 5FPEA+) in constructing n=1 low dimensional perovskites. The subphase should have a large density as well as surface energy such that the spread solutions can cover the exposed air-liquid interface. In the meantime, its compatibility with the solvents and solutes in perovskite solution needs to be considered to ensure proper crystallization of the 2D perovskite. After several trials, o-xylene and chlorobenzene were found to be the best subphase candidates. All the n=1 sheets obtained have a strong PL emission peak at 510 nm.
Furthermore it was developed an approach to fabricating 2D superlattices of PbS QD in a Langmuir-Schaefer setup with external compression which showed unprecedented area coverage and homogeneity. We have shown that the degree of compression before triggering the ligand exchange is a crucial parameter for obtaining highly ordered, crack-free superlattices. Careful structural characterisation revealed that due to the compression, the ligand-exchanged samples retained the hexagonal geometry thus maximizing the packing of the structure and the number of nearest-neighbours. The control of the lateral compression, and of the structural properties of the superlattices, give rise to significant differences in their transport properties. Ion gel-gated field effect transistors displayed average electron mobility of 5±2 cm2V-1s-1 for the lower compression, increasing to 19±8 cm2V-1s-1 for the highly compressed ones thanks to the better coverage, lack of cracks, better packing and increased translational ordering. This demonstrates the effectiveness of our method in achieving high carrier mobility in superlattices with millimetre square coverage which is unprecedented in literature. Translating this approach to three-dimensional SL as well might have important implications for the technological applications of such metamaterials in optoelectronics.
Another important activity in DEOM has been the fabrication of QDs photodetectors, the activity is very important to prepare not only the right devices structure in view of the use of 3D QD superlattices but also to screen ligands for proper QDs passivation. For the first time, we demonstrated that the MAPbI3 ligand can achieve complete phase-transfer ligand exchange for large-size QDs with excitonic peaks from 1300 nm to 1700 nm. The resulting inks have a prolonged shelf time of at least 10 weeks thanks to the comprehensive passivation of [100] facet. Transport measurements in field-effect transistors showed that our strategy improves carrier mobility and extends the stability for all the QD sizes. The high quality of the inks was further proven by the fabrication by blade coating of high-performance SWIR PbS QDs photodiodes. The resulting device achieved a 76% of EQE at the exciton peak at 1300 nm and 1.8 × 1012 Jones of specific detectivity. This performances are commensurate with the state-of-the-art literature reports of short wavelenght infrared (SWIR) QDs photodetectors fabricated by spin-coating deposition. This is an extremely important demonstration as it reveals the scalable (and not wasteful) fabrication of SWIR PbS QDs photodetectors without performance loss.
