The ultrafast onset of exciton formation in 2D semiconductors

Trovatello, Chiara; Katsch, Florian; Borys, Nicholas J.; Selig, Malte; Yao, Kaiyuan; Borrego-Varillas, Rocio; Scotognella, Francesco; Kriegel, Ilka; Yan, Aiming; Zettl, Alex; Schuck, P. James; Knorr, Andreas; Cerullo, Giulio; Conte, Stefano Dal

doi:10.1038/s41467-020-18835-5

The ultrafast onset of exciton formation in 2D semiconductors

Journal Article · Mon Oct 19 00:00:00 EDT 2020 · Nature Communications

DOI:https://doi.org/10.1038/s41467-020-18835-5· OSTI ID:1764538

Trovatello, Chiara ^[1]; ^[2]; ^[3]; Selig, Malte ^[2]; Yao, Kaiyuan ^[4]; Borrego-Varillas, Rocio ^[5]; Scotognella, Francesco ^[1]; Kriegel, Ilka ^[6]; ^[7]; ^[7]; ^[4]; Knorr, Andreas ^[2]; ^[5]; ^[1]

Politecnico di Milano (Italy)
Technische Univ. Berlin (Germany)
Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Molecular Foundry; Montana State Univ., Bozeman, MT (United States)
Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Molecular Foundry; Univ. of California, Berkeley, CA (United States); Columbia Univ., New York, NY (United States)
Politecnico di Milano (Italy); IFN-CNR, Milano (Italy)
Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Molecular Foundry; Inst. Italiano di Tecnologia (IIT), Genova (Italy)
Univ. of California, Berkeley, CA (United States); Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States); Kavli Energy NanoScience Institute, Berkeley, CA (United States)

The equilibrium and non-equilibrium optical properties of single-layer transition metal dichalcogenides (TMDs) are determined by strongly bound excitons. Exciton relaxation dynamics in TMDs have been extensively studied by time-domain optical spectroscopies. However, the formation dynamics of excitons following non-resonant photoexcitation of free electron-hole pairs have been challenging to directly probe because of their inherently fast timescales. Here, we use extremely short optical pulses to non-resonantly excite an electron-hole plasma and show the formation of two-dimensional excitons in single-layer MoS₂ on the timescale of 30 fs via the induced changes to photo-absorption. These formation dynamics are significantly faster than in conventional 2D quantum wells and are attributed to the intense Coulombic interactions present in 2D TMDs. A theoretical model of a coherent polarization that dephases and relaxes to an incoherent exciton population reproduces the experimental dynamics on the sub-100-fs timescale and sheds light into the underlying mechanism of how the lowest-energy excitons, which are the most important for optoelectronic applications, form from higher-energy excitations. Importantly, a phonon-mediated exciton cascade from higher energy states to the ground excitonic state is found to be the rate-limiting process. These results set an ultimate timescale of the exciton formation in TMDs and elucidate the exceptionally fast physical mechanism behind this process.

View Accepted Manuscript (DOE)

Research Organization:: Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States)

Sponsoring Organization:: USDOE Office of Science (SC), Basic Energy Sciences (BES). Materials Sciences & Engineering Division; European Research Council (ERC); National Science Foundation (NSF)

Grant/Contract Number:: AC02-05CH11231; SC0019443

OSTI ID:: 1764538

Journal Information:: Nature Communications, Journal Name: Nature Communications Journal Issue: 1 Vol. 11; ISSN 2041-1723

Publisher:: Nature Publishing GroupCopyright Statement

Country of Publication:: United States

Language:: English

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