Anharmonic Exciton‐Phonon Coupling in Metal‐Organic Chalcogenides Hybrid Quantum Wells

Abstract

conditions, and recent improvements in their synthesis approaches allow producing high-quality samples suitable both for spectroscopic investigation and device fabrication. [3][4][5] In particular, the excitonic behavior of [AgSePh] ∞ , a prototypical MOC, has just been investigated by us, [6,7] and two other groups, [8][9][10] highlighting strongly bound anisotropic resonances with blue, direct gap emission and very short lifetime. This short exciton lifetime is peculiar and could be potentially leveraged for ultrafast optical applications. As we recently reported, the [AgSePh] ∞ ultrafast de-excitation appears to be dominated potentially by intrinsic selftrapping of excitons. [6,7] Such self-trapping of carriers often occurs in semiconductors with a large electron-phonon coupling. [11] In the latter case, a proper description of optical excitations needs to consider the dressing of excitons with lattice phonons, which ultimately gives rise to a new quasiparticle, the exciton-polaron, in the strong interaction limit. While the subfield of 2D MHPs already reported several experiments highlighting the important role of phonons and lattice distortions in the optical transitions and carrier dynamics, [12][13][14][15][16][17] the HQWs based on MOCs still lack an understanding of such interplay between excitons and phonons.Here, we study exciton-phonon coupling in the prototypical MOC [AgSePh] ∞ by a combination of time-resolved and time-integrated optical spectroscopies. Impulsive stimulated Raman scattering (ISRS) [18,19] via resonant transient absorption In contrast to inorganic quantum wells, hybrid quantum wells (HQWs) based on metal-organic semiconductors are characterized by relatively soft lattices, in which excitonic states can strongly couple to lattice phonons. Therefore, understanding the lattice's impact on exciton dynamics is essential for harnessing the optoelectronic potential of HQWs. Beyond 2D metal halide perovskites, layered metal-organic chalcogenides (MOCs), which are an airstable, underexplored material class hosting room-temperature excitons, can be exploited as photodetectors, light emitting devices, and ultrafast photoswitches. Here, the role of phonons in the optical transitions of the prototypical MOC [AgSePh] ∞ is elucidated. Impulsive stimulated Raman scattering (ISRS) allows the detection of coherent exciton oscillations driven by Fröhlich interaction with low-energy optical phonons. Steady state absorption and Raman spectroscopies reveal a strong exciton-phonon coupling (Huang-Rhys parameter ≈1.7) and its anharmonicity, manifested as a nontrivial temperaturedependent Stokes shift. The ab initio calculations support these observations, hinting at an anharmonic behavior of the low-energy phonons <200 cm −1 . These results untangle complex exciton-phonon interactions in MOCs, establishing an ideal testbed for room-temperature many-body phenomena.

Resonant Exciton Scattering Reveals Raman Forbidden Phonon Modes in Layered GeS

“…To estimate the strength of this Fröhlich interaction we follow the approach described in ref ( 42 ). We consider the presence of Wannier excitons which is confirmed by Pastorino et al 43 showing that the wave function of the first bright exciton in GeS is highly delocalized and spreads over several atomic layers.…”

Resonant exciton scattering reveals Raman forbidden phonon modes in layered GeS

“…To estimate the strength of this Fröhlich interaction we follow the approach described in Ref. 42. We consider the presence of Wannier excitons which is con rmed by Pastorino et al [43] showing that the wave function of the rst bright exciton in GeS is highly delocalized and spreads over several atomic layers.…”

The Critical Role of Anharmonic Lattice Dynamics for Macroscopic Properties of the Visible Light Absorbing Nitride Semiconductor CuTaN2

“…[1,3] More recent work has highlighted that atomic motions transcending the harmonic phonon picture can heavily influence the optoelectronic properties of many different classes of emerging semiconductors, including organic crystals, [4] halide perovskites [5][6][7][8][9][10][11][12][13][14][15][16] and both metal oxides [17][18][19] and chalcogenides. [20,21] Indeed, updated microscopic understanding is now being generated to elucidate how more complicated atomic motion -captured in the framework of anharmonic lattice dynamics -can trigger interesting consequences for key physical quantities, such as the fundamental bandgap, [5,6,12,14,15] structural phase transitions, [22][23][24] and nature of quasi-particle excitations. [21,25] Ternary nitride semiconductors are fascinating materials and offer a versatile range of desirable properties for sustainable energy conversion, including solar energy harvesting, thermoelectrics, and solid-state lighting.…”

“…[20,21] Indeed, updated microscopic understanding is now being generated to elucidate how more complicated atomic motion -captured in the framework of anharmonic lattice dynamics -can trigger interesting consequences for key physical quantities, such as the fundamental bandgap, [5,6,12,14,15] structural phase transitions, [22][23][24] and nature of quasi-particle excitations. [21,25] Ternary nitride semiconductors are fascinating materials and offer a versatile range of desirable properties for sustainable energy conversion, including solar energy harvesting, thermoelectrics, and solid-state lighting. [26][27][28][29] Compared to intensively studied metal oxide compounds, nitrides offer narrower bandgaps for efficient light absorption, along with increased covalent character that can enhance long-range charge transport.…”

“…[ 20,21 ] Indeed, updated microscopic understanding is now being generated to elucidate how more complicated atomic motion – captured in the framework of anharmonic lattice dynamics – can trigger interesting consequences for key physical quantities, such as the fundamental bandgap, [ 5,6,12,14,15 ] structural phase transitions, [ 22–24 ] and nature of quasi‐particle excitations. [ 21,25 ]…”