Strongly Quantum-Confined Blue-Emitting Excitons in Chemically Configurable Multiquantum Wells

Abstract

Light matter interactions are greatly enhanced in twodimensional (2D) semiconductors because of strong excitonic effects. Many optoelectronic applications would benefit from creating stacks of atomically thin 2D semiconductors separated by insulating barrier layers, forming multiquantum-well structures. However, most 2D transition metal chalcogenide systems require serial stacking to create van der Waals multilayers. Hybrid metal organic chalcogenolates (MOChas) are self-assembling hybrid materials that combine multiquantum-well properties with scalable chemical synthesis and air stability. In this work, we use spatially resolved linear and nonlinear optical spectroscopies over a range of temperatures to study the strongly excitonic optical properties of mithrene, that is, silver benzeneselenolate, and its synthetic isostructures. We experimentally probe s-type bright excitons and p-type excitonic dark states formed in the quantum confined 2D inorganic monolayers of silver selenide with exciton binding energy up to ∼0.4 eV, matching recent theoretical predictions of the material class. We further show that mithrene's highly efficient blue photoluminescence, ultrafast exciton radiative dynamics, as well as flexible tunability of molecular structure and optical properties demonstrate great potential of MOChas for constructing optoelectronic and quantum excitonic devices.

Chemical crystallography by serial femtosecond X-ray diffraction

“…Mithrene and tethrene both have strong light-matter interactions; yellow mithrene has a direct bandgap and emits blue light with a short photoluminescent lifetime (approximately 50 ps); 18 whereas orange tethrene has an indirect bandgap 36 and emits broadly with a longer lifetime (approximately 1 ns) (Fig. 4j, k).…”

“…Our results give a concrete structural explanation for these observations. We have previously demonstrated that excitons in mithrene, delocalized in two dimensions across the argentophilic network of Ag-Ag bonds, give rise to its visible absorption and emission spectra 18,21,37 . The argentophilic interactions of thiorene are marked by linear Ag-Ag chains, which do not support the two-dimensional delocalization of excitons observed in mithrene and tethrene.…”

“…After indexing the sparse serial patterns by a graph theory approach 14 , the resulting datasets can be solved and refined using standard tools for single-crystal diffraction data [15][16][17] . We describe the ab initio structure solutions of mithrene (AgSePh) [18][19][20] , thiorene (AgSPh) and tethrene (AgTePh), of which the latter two were previously unknown structures. In thiorene, we identify a geometric change in the silver-silver bonding network that is linked to its divergent optoelectronic properties 20 .…”

“…Powder diffraction and electron microdiffraction are established methods for determining structures of microcrystalline substances, but each can be limited by some combination of the challenges above. As an example, the structure of the excitonic and blue-emitting mithrene, AgSeC 6 H 5 , was determined by SCXRD [18][19][20] but the remaining and optically divergent silver benzenechalcogenolates (AgEC 6 H 5 , E = S, Te) have not yielded to characterization by Rietveld refinement or micro-electron diffraction 21 . In this work, we present the room-temperature (298 K) crystal structures of all three materials as determined by X-ray free-electron laser (XFEL) small-molecule serial femtosecond crystallography (smSFX).…”

Investigation of Nucleation and Growth at a Liquid–Liquid Interface by Solvent Exchange and Synchrotron Small-Angle X-Ray Scattering

“…Mithrene, silver (I) benzeneselenolate, is a well understood example of this material class. It is a direct gap semiconductor with a large exciton binding energy, that exhibits optical properties similar to transition metal dichalcogenide monolayers like MoS 2 ( Chen et al, 2017 ; Trang et al, 2018 ; Maserati et al, 2020 ; Yao et al, 2020 ). A classic synthetic method for mithrene is nucleation and growth occurring at an oil-water interface, in Figure 1A an image of the toluene-water interface and a schematic in Figure 1B help illustrate this synthetic environment.…”