Metal-Induced 3D to 2D Phase Transformation: Formation of Ruddlesden-Popper Superlattice Nanoplatelets

Mondal, Sudipta (2022) Metal-Induced 3D to 2D Phase Transformation: Formation of Ruddlesden-Popper Superlattice Nanoplatelets. Masters thesis, Indian Institute of Science Education and Research Kolkata.

Text (MS dissertation of Sudipta Mondal (17MS018)) 17MS018_Thesis_file.pdf - Submitted Version Restricted to Repository staff only Download (10MB)

Abstract

Lead halide perovskites have been dominating the world as a promising class of semiconductor materials for a long time now and gradually focus has shifted from three-dimensional (3D) perovskites to two-dimensional (2D) perovskites owing to their unique structural as well as optical properties and enhanced stability. 2D perovskites having RP (Ruddlesden-Popper), DJ (Dion-Jacobson) and ACI (Alternating Cation in Interlayer) phases are mainly prepared as bulk materials using acid precipitation method for application in photovoltaics or optoelectronics. Synthesis of 2D perovskites on a nanoscale with control over the number of layers poses a formidable challenge. Recently, research work on two-dimensional perovskite nanoplatelets, having thickness of only a few unit cells, exhibiting strong quantum confinement effect, dielectric effect, tunable optical properties and high photoluminescence quantum yield (PLQY) have seen a boom along with their various optoelectronic applications. Here in, we have for the first time, shown the formation of colloidal 2D RP-phase nanoplatelets (NPLs) upon tin substitution beyond a critical concentration at the B-site of cubic α-phase FAPbI3 nanocrystals (NCs) using hot-injection method in Schlenk line. Phase transitions have been studied in perovskites previously but change in phase from 3D to 2D via metal substitution in hybrid organic-inorganic perovskite NCs has not been reported till date. We have systematically studied the effect of tin substitution at the lead site by synthesising a range of systems with increasing Sn/Pb precursor ratio as well as the role of tin in this phase transition. From 30% precursor solution Sn-wt %, the phase begins to change and 2D RP-phase nanoplatelets form having the general formulae L₂[FASnxPb₁-xI₃]n-₁SnxPb₁-xI₄ where L represents the ligands present in between the inorganic layers, n represents the number of inorganic layers and x represents the solution Sn-fraction in the precursor. For x = 0.35, a mixture of n = 3 and n = 4, 2D RP-phase NPLs form. From this x value, the formation of 2D RP-phase NPLs is accompanied by the creation of a superlattice structure via the self-assembly of the nanoplatelets. For x ≥ 0.5, n = 2, 2D RP-phase nanoplatelets dominantly form. The characteristic periodic XRD pattern of the systems having multiple satellite peaks which arise from the superlattice reflections were analysed to reveal important structural insights. These 2D RP superlattices are made of alternating layers of organic ligands and inorganic nanoplatelets arranged in an ordered fashion over a large extent and this results in a multiple quantum well superlattice structure having varying quantum well width, which has been evidenced by the presence of multiple closely spaced peaks in low temperature photoluminescence spectra along with the characteristic XRD pattern. Moreover, we have used trioctylphosphine as a surface passivating agent to minimise surface defects and non-radiative recombination pathways to increase PLQY to impressive levels for 2D perovskite nanoplatelets, reaching a highest value of 98.4 % in case of x = 0.3, which consists of a mixture of n = 4 and higher n value phase, 2D RP NPLs.

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