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.

This work reports the synthesis, structural, linear, and nonlinear optical investigations into low-dimensional naturally selfassembled inorganic−organic (IO) hybrid systems based on (C n H 2n−1 NH 3 ) + (n = 3−8) cyclic moieties. The steric effects of cyclic ring sizes convert the IO hybrids from two-dimensional (2D) layered structures to 1D crystal packing. The crystal packing of this class of IO hybrid compounds of cyclic sizes from n = 3 to 6 shows a perfect 2D layered structural arrangement having a crystal structure (R−NH 3 ) 2 PbI 4 . On the contrary, n = 7 hybrid shows a 1D layered structural arrangement, but the adjacent chains are disconnected along the "c"-axis, resulting into (R−NH 3 ) 3 PbI 5 . Moreover, for n = 8 hybrids, the inorganic network structure is infinitely extended along the "a"-axis having (R−NH 3 )PbI 3 1D crystal structure. These structural changes may lead to defect states, which is verified by density functional theory (DFT) calculations. The linear and nonlinear optical probing of room-temperature optical excitons demonstrate the photoluminescence and absorption feature variation from 2D layered crystal packing (n = 3−6) to quasi-1D structures (n = 7, 8). A systematic correlation of one-photon (1PA)-and two-photon (2PA)-excited exciton photoluminescence (PL) features with a cyclic size is discussed and presented. While one-photon absorption-induced photoluminescence (1PA-PL) provides information about strong exciton emission from the top few perfectly aligned layers, twophoton excitation probes the deeper depths. This shows red-shifted PL (2PA-PL) from the structurally distorted crystal packing within the sample and traces of defect-induced emission. The DFT study shows that the I-vacancy defect creates the states at conduction band minimum (CBm), which leads to a sudden reduction in the band gap for n = 7 and 8. The systematic optical probing studies to determine the structural deviations in IO hybrid semiconductors will provide a new platform for advanced photonics and optoelectronic devices.

Real-time monitoring of room-temperature exciton photoluminescence (PL) while irradiated with ultrafast laser excitations (UV and infrared) in long alkyl-chain based (C 12 H 25 NH 3 ) 2 PbI 4 inorganic-organic hybrid semiconductors is presented. These naturally self-assembled 2D hybrid structures show strong room-temperature Mott-type excitons confined within the lowest inorganic bandgap, which are highly sensitive to structural phase flips. Under both one-photon (E 1PA ≥ E g ) and two-photon (2E 2PA ≥ E g ) laser excitations, the exciton PL of unstable phase-II appears initially, and with prolonged laser exposure, the PL peak switches to a new stable blueshifted phase-I peak position. This exciton phase flip demonstrates different laser-induced structural deformations in inorganic quantum wells (PbI 6 extended network) associated with orthorhombic (phase-I) and monoclinic (phase-II) unit cells. One-photon absorption induced PL shows the various time dynamics of laser exposure depending on laser characteristics (continuous wave and ultrashort pulsed lasers), mostly influenced by localized heating, ablation effects, and third-order nonlinear effects such as saturation of linear absorption and exciton-exciton annihilation. However, in two-photon absorption induced PL, the near infrared laser excitation reveals the redshifted crumpled excitons from the deeper depth of the sample, which are induced by multiphoton absorption and avalanche ionization. A series of systematic linear and nonlinear steady-state and time-resolved PL studies are presented. A simplified kinetic model further provides an understanding of the real-time evolution of laser-induced excitons and their related phase flips. These laser-induced exciton phase flips and linear and nonlinear optical probing open a new avenue for novel functional properties and nonlinear absorption-based optoelectronic devices.

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