Linear and nonlinear optical probing of various excitons in 2D inorganic-organic hybrid structures

Abstract

Nonlinear optical properties, such as two-(or multi-) photon absorption (2PA), are of special interest for technologically important applications in fast optical switching, in vivo imaging and so on. Highly intense infrared ultrashort pulses probe deep into samples and reveal several underlying structural perturbations (inter-layer distortions, intra-layer crumpling) and also provide information about new excited states and their relaxation. naturally self-assembled inorganic-organic multiple quantum wells (io-MQWs) show utility from room-temperature exciton emission features (binding energies ~200-250 meV). These Mott type excitons are highly sensitive to the self-assembly process, inorganic network distortions, thickness and inter-layer distortions of these soft two-dimensional (2D) and weak van der Waal layered hybrids. We demonstrate strong room-temperature nonlinear excitation intensity dependent two-photon absorption induced exciton photoluminescence (2PA-PL) from these IO-MQWs, excited by infrared femtosecond laser pulses. Strongly confined excitons show distinctly different one-and two-photon excited photoluminescence energies: from free-excitons (2.41 eV) coupled to the perfectly aligned MQWs and from energy down-shifted excitons (2.33 eV) that originate from the locally crumpled layered architecture. High intensity femtosecond induced pL from one-photon absorption (1PA-PL) suggests saturation of absorption and exciton-exciton annihilation, with typical reduction in PL radiative relaxation times from 270 ps to 190 ps upon increasing excitation intensities. From a wide range of IR excitation tuning, the origin of 2PA-PL excitation is suggested to arise from exciton dark states which extend below the bandgap. Observed two-photon absorption coefficients (β ~75 cm/ GW) and two-photon excitation cross-sections (η 2 σ 2 ~ 110GM), further support the evidence for 2PA excitation origin. Both 1PA-and 2PA-PL spatial mappings over large areas of single crystal platelets demonstrate the coexistence of both free and deep-level crumpled excitons with some traces of defectinduced trap state emission. We conclude that the two-photon absorption induced pL is highly sensitive to the self-assembly process of few to many mono layers, the crystal packing and deep level defects. This study paves a way to tailor the nonlinear properties of many 2D material classes. Our results thus open new avenues for exploring fundamental phenomena and novel optoelectronic applications using layered inorganic-organic and other metal organic frameworks. High intensity ultrashort laser pulses interact with nanomaterials to produce fascinating parametric and nonparametric nonlinear optical phenomena such as two-photon (or multi-photon) absorption 1 , harmonic generation 2 , coherent anti-Stokes Raman scattering (CARS) 3 , excited state absorption and ultrafast charge carrier dynamics 4,5. Studies of two-(or multi-) photon absorption in layered materials (such as graphene oxide, MoS 2 type transition-metal dichalcogenides) 6-11 have given new in...

Differences and Similarities of Photocatalysis and Electrocatalysis in Two-Dimensional Nanomaterials: Strategies, Traps, Applications and Challenges

“…The bottom-up wet chemical synthesis approaches are easier to realize for large-scale production and provides a more controllable synthesis of 2D nanomaterials. These include surfactant-assisted synthesis [298], surfactant self-assembly [299], chemical vapor deposition [300][301][302][303], template-assisted synthesis [304], inorganic-organic lamellar [305] and solvothermal synthesis [306,307]. The following discussion will discuss typical methods for 2D nanomaterials synthesized using bottom-up methods.…”

Nonlinear Optical Properties of Emerging Nano‐ and Microcrystalline Materials

“…Indeed, it is a known matter that highly ordered media such as thin films and single crystals support so-called Mott-Wannier excitons whose properties are strongly dependent on the details of crystal packing such as the presence of defects, shifts and deformations in layers' stacking; these phenomena are referred to as structural crumpling. [99] An interesting analysis of disparate one-photon and two-photon luminescence properties of layered perovskites and impact of structural crumpling on them has been presented by Prakash and collaborators for 2D perovskites of formula (R-C 6 H 4 C 2 H 4 NH 3 ) 2 PbI 4 (R = CH 3 , Cl), referred to as MEPI and CEPI, respectively. [99] By using linear (400 nm) and nonlinear (800 nm) excitations they registered luminescence spectra as well as constructed maps of exciton photoluminescence, allowing for visual recognition of areas where exciton of given kind is present.…”

“…[99] An interesting analysis of disparate one-photon and two-photon luminescence properties of layered perovskites and impact of structural crumpling on them has been presented by Prakash and collaborators for 2D perovskites of formula (R-C 6 H 4 C 2 H 4 NH 3 ) 2 PbI 4 (R = CH 3 , Cl), referred to as MEPI and CEPI, respectively. [99] By using linear (400 nm) and nonlinear (800 nm) excitations they registered luminescence spectra as well as constructed maps of exciton photoluminescence, allowing for visual recognition of areas where exciton of given kind is present. Since excitation-dependent properties of MEPI and CEPI are essentially similar, only data for the latter will be discussed.…”

Charge currents and induced magnetic fields in a bounded two-dimensional hydrogen atom

“…Since an exciton is a hydrogen-like bound state of an electron and a hole, the motivation for this study is based on the practical applications to the systems of excitons in the heterostructures. The subject of excitons in 2D heterostructures and their interactions with the electric, magnetic and laser fields has been well studied and plays an important role in the fabrication of novel photonic and optoelectronic devices [37][38][39][40][41][42][43]. A detailed investigation of the quantum confined excitons in 2D materials can be found in [44] and references therein.…”