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Anomalous Above-Gap Photoexcitations and Optical Signatures of Localized Charge Puddles in Monolayer Molybdenum Disulfide
journal contribution
posted on 2017-01-24, 00:00 authored by Nicholas J. Borys, Edward S. Barnard, Shiyuan Gao, Kaiyuan Yao, Wei Bao, Alexander Buyanin, Yingjie Zhang, Sefaattin Tongay, Changhyun Ko, Joonki Suh, Alexander Weber-Bargioni, Junqiao Wu, Li Yang, P. James SchuckBroadband
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.
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Keywords
Narrow excitation resonancesband gapdevice architecturesdesigner optoelectronicsfree-carrier excitationscarrier densitymonolayer MoS 2broadband photoexcitation spectrumband gap excitonsphotoluminescence excitation spectroscopyAnomalous Above-Gap Photoexcitationsstate manifoldsubstrate inhomogeneityexciton binding energyhigher-energy excitonslight harvesting technologiessemiconductorfeatures outlinesas-grown materialsmany-body phenomenafirst-principles calculationselectromagnetic spectrumSuch variationsbroadband optoelectronic applicationsLocalized Charge Puddlesstates exhibitcharge puddling phenomenaBrillouin zoneuse substrate patterningband gapsMonolayer Molybdenum Disulfide Broadband optoelectronicscarrier density fluctuationsOptical Signatures
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