Strong Electron-Phonon Coupling in 2D Silver Phenyl Chalcogenolates Revealed by Ultrafast Impulsive Vibrational Spectroscopy

Metal organo chalcogenolates (MOCs) are a new class of 2D semiconductors that combine many of the most advantageous properties of existing 2D material families, such as transition metal dichalcogenides (TMDs) and 2D layered perovskites. Silver phenyl selenolate (AgSePh) is the prototypical example, comprised of covalently-bonded 2D sheets that exhibit a thickness-independent direct bandgap, narrow bandwidth blue emission, 2D in-plane exciton anisotropy, and other unique optoelectronic properties. Additionally, crystalline AgSePh has a straightforward synthesis method and remains stable for weeks under ambient conditions. Owing to these advantages, MOCs have the potential for use in next-generation light emission and detection devices, among other possible applications.<br/><br/>While promising, much of the underlying physics which gives AgSePh its unique characteristics has not been fully explored. In this study, we employ resonant impulsive vibrational spectroscopy (IVS), an ultrafast pump-probe technique experimentally similar to transient absorption spectroscopy, to investigate the degree of electron-phonon coupling in AgSePh. Using this time-domain Raman method at cryogenic temperatures, we can easily resolve multiple coherent vibrational signatures, suggesting strong electron-phonon coupling in this material. At 5K, these impulsively stimulated vibrations exhibit lifetimes approaching or exceeding 10 picoseconds before scattering or dampening processes occur. Further, some vibrational modes can even be resolved in room temperature IVS data.<br/><br/>A comparison of IVS results to DFT and MD simulations allows for assignment of the structural distortions associated with the dominant electronically-coupled vibrational modes in this system. When combined with low frequency Raman and temperature-dependent photoluminescence data, assignment of individual modes as coupled to either the ground or excited electronic state can be performed.<br/><br/>These results present a convincing picture of large electronic-vibrational interactions in AgSePh and MOCs more generally. This motivates future exploration of other possible unique properties in this material, such as self-trapped exciton formation, which can be found in other 2D materials with strong electron-phonon coupling (e.g. 2D perovskites). A better fundamental understanding of the underlying structural and electronic properties of MOCs can also help to inform future development of alternative compositions or more effective synthetic methods to further expand the capabilities of this new material family.