title: Anomalous Above-Gap Photoexcitations and Optical Signatures of Localized Charge Puddles in Monolayer Molybdenum Disulfide.
abstract: Broadband optoelectronics such as artificial light harvesting technologies necessitate efficient and, ideally, tunable coupling of excited states over a wide range of energies. In monolayer MoS2, a prototypical two-dimensional layered semiconductor, the excited state manifold spans the visible electromagnetic spectrum and is comprised of an interconnected network of excitonic and free-carrier excitations. Here, photoluminescence excitation spectroscopy is used to reveal the energetic and spatial dependence of broadband excited state coupling to the ground-state luminescent excitons of monolayer MoS2. Photoexcitation of the direct band gap excitons is found to strengthen with increasing energy, demonstrating that interexcitonic coupling across the Brillouin zone is more efficient than previously reported, and thus bolstering the import and appeal of these materials for broadband optoelectronic applications. Narrow excitation resonances that are superimposed on the broadband photoexcitation spectrum are identified and coincide with the energetic positions of the higher-energy excitons and the electronic band gap as predicted by first-principles calculations. Identification of such features outlines a facile route to measure the optical and electronic band gaps and thus the exciton binding energy in the more sophisticated device architectures that are necessary for untangling the rich many-body phenomena and complex photophysics of these layered semiconductors. In as-grown materials, the excited states exhibit microscopic spatial variations that are characteristic of local carrier density fluctuations, similar to charge puddling phenomena in graphene. Such variations likely arise from substrate inhomogeneity and demonstrate the possibility to use substrate patterning to tune local carrier density and dynamically control excited states for designer optoelectronics.
authors.forename: Nicholas J
authors.identifier: 0000-0001-5434-1191
authors.initials: NJ
authors.lastname: Borys
authors.forename: Edward S
authors.initials: ES
authors.lastname: Barnard
authors.affiliation: Department of Physics, Washington University in St. Louis , St. Louis, Missouri 63130, United States.
authors.forename: Shiyuan
authors.initials: S
authors.lastname: Gao
authors.forename: Kaiyuan
authors.initials: K
authors.lastname: Yao
authors.affiliation: Department of Materials Science and Engineering, University of California Berkeley , Berkeley, California 94720, United States.
authors.forename: Wei
authors.initials: W
authors.lastname: Bao
authors.forename: Alexander
authors.initials: A
authors.lastname: Buyanin
authors.forename: Yingjie
authors.initials: Y
authors.lastname: Zhang
authors.affiliation: Department of Materials Science and Engineering, University of California Berkeley , Berkeley, California 94720, United States.|Department of Materials Science and Engineering, Arizona State University , Tempe, Arizona 85287, United States.
authors.forename: Sefaattin
authors.initials: S
authors.lastname: Tongay
authors.affiliation: Department of Materials Science and Engineering, University of California Berkeley , Berkeley, California 94720, United States.
authors.forename: Changhyun
authors.initials: C
authors.lastname: Ko
authors.affiliation: Department of Materials Science and Engineering, University of California Berkeley , Berkeley, California 94720, United States.
authors.forename: Joonki
authors.initials: J
authors.lastname: Suh
authors.forename: Alexander
authors.initials: A
authors.lastname: Weber-Bargioni
authors.affiliation: Department of Materials Science and Engineering, University of California Berkeley , Berkeley, California 94720, United States.
authors.forename: Junqiao
authors.initials: J
authors.lastname: Wu
authors.affiliation: Department of Physics, Washington University in St. Louis , St. Louis, Missouri 63130, United States.
authors.forename: Li
authors.initials: L
authors.lastname: Yang
authors.forename: P James
authors.initials: PJ
authors.lastname: Schuck
keywords: broadband optical properties; charge puddles; exciton Stokes shift; localized carrier density; monolayer molybdenum disulfide; transition metal dichalcogenides