Synthesis, Structural, Linear, and Nonlinear Optical Studies of Inorganic–Organic Hybrid Semiconductors (R–C₆H₄CHCH₃NH₃)₂PbI₄, (R = CH₃, Cl)

Abstract

Synthesis, crystal structure, and optical properties of two-dimensional (2D) layered structurally slightly different inorganic–organic (IO) hybrid semiconductors (R–C6H4C2H4NH3)2PbI4 (R = CH3, Cl) are presented. They are naturally self-assembled systems where two (RNH3)+ moieties are sandwiched between two infinitely extended 2D layers of the [PbI6]4– octahedral network and treated as natural IO multiple quantum wells. While the former compound crystallizes into an orthorhombic system in the Cmc21 space group, the latter crystallizes into a monoclinic system in the space group P21/c. As a thin film, they are well-oriented along the (l00) direction. Both single crystals and thin films show strong room-temperature Mott type exciton features that are highly sensitive to the self-assembly and crystal packing. Linear (one-photon) and nonlinear (two-photon) optical probing of single crystals for exciton photoluminescence imaging and spectral spatial mapping provide deep insight into the layered re-arrangement and structural crumpling due to organic conformation. The strongly confined excitons, within the lowest band gap of inorganic, show distinctly different one- and two-photon excited photoluminescence peaks: free excitons from perfectly aligned 2D self-assembly and energy down-shifted excitons originated from the locally crumpled layered arrangement. Their structural aspects are successfully presented with proper correlation that emphasize various differences in physical and optical properties associated between these novel IO hybrids.

Structure-Dependent (Non)Linear Optical Excitons in Primary Cyclic Ammonium (CnH2n–1NH2;n= 3–8)-Based Inorganic–Organic Hybrid Semiconductor Series

“…These show the typical 2D layered arrangement ((R–NH 3 ) 2 PbI 4 ). However, as the cyclic ring size increases ( n > 6), the structure deviates from the expected 2D self-assembly. ,,,− …”

“…The optical features of organometal halide-based IO hybrids play a crucial role in attaining designed optical properties for various optoelectronic applications. ,,− The 2D layered IO hybrids in the form of (R–NH 3 ) 2 PbI 4 show strong room-temperature Mott-type exciton features in the green spectral region. The room-temperature excitons having enhanced binding energies (∼200–300 meV) are due to quantum confinement effects of the quantum-sized PbI 6 network and dielectric contrast between inorganic (quantum well) and organic (quantum barriers) moieties. ,,,, Extensive work on optical features for a variety of organic moieties (based on the size and shape), self-assembly processes, thicknesses, and environmental effects is reported in our previous studies. ,, The deviation from the 2D self-assembled layered arrangement produces red-shifted absorption and emission features. ,, …”

“…The powder XRD patterns derived from the reported crystallographic data of ref are plotted in Figure b for comparison only. Following the (R–NH 3 ) 2 PbI 4 structure, the crystal packing of cyclic IO hybrid compounds from CPPI ( n = 3) to CHXPI ( n = 6) consist of a corner-shared PbI 6 infinitely extended network separated by organic moieties, with a slight deviation in their Pb–I–Pb network angles. ,,, Figure a,b exemplifies the schematic representation of n = 3–6 compounds’ corner-shared 2D layered PbI 6 arrangement (side view) and their irregular quadrilateral corner-shared PbI 6 octahedra (top view). Two organic moieties are bonded to the Pb–I network by hydrogen bonding between organic ammonium cations and the terminal iodide atoms of Pb–I networks.…”

“…The aforementioned results clearly demonstrate the coexistence of high-energy excitons (PL HE ) from nearly perfect top surfaces and the IR-probed red-shifted low-energy excitons (PL LE ) from the deep layers. Here, we demonstrate the PL imaging and PL spatial maps of IO hybrid single-crystal flakes using a modified confocal microscope connected with (i) 400 nm (CW) for 1PA-PL and (ii) 800 nm (120 fs, 84 MHz) for 2PA-PL as excitation sources. ,, The PL spatial mapping at peak maxima and PL line scan wavelength mapping for CHXPI ( n = 6) single-crystal bits are shown in Figure for both 1PA-PL and 2PA-PL.…”

Nonlinear Optical Properties of Emerging Nano‐ and Microcrystalline Materials

“…Reproduced with permission. [100] Copyright 2019, American Chemical Society. b) Luminescence spectra (black plots) for crystal obtained under excitation with 400 nm CW laser (top panel) and 800 nm fs laser (lower panel).…”