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Ceballos F, Cui Q, Bellus MZ, Zhao H. Exciton formation in monolayer transition metal dichalcogenides. NANOSCALE 2016; 8:11681-11688. [PMID: 27219022 DOI: 10.1039/c6nr02516a] [Citation(s) in RCA: 75] [Impact Index Per Article: 9.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/05/2023]
Abstract
Two-dimensional transition metal dichalcogenides provide a unique platform to study excitons in confined structures. Recently, several important aspects of excitons in these materials have been investigated in detail. However, the formation process of excitons from free carriers has yet to be understood. Here we report time-resolved measurements on the exciton formation process in monolayer samples of MoS2, MoSe2, WS2, and WSe2. The free electron-hole pairs, injected by an ultrashort laser pulse, immediately induce a transient absorption signal of a probe pulse tuned to the exciton resonance. The signal quickly drops by about a factor of two within 1 ps and is followed by a slower decay process. In contrast, when excitons are resonantly injected, the fast decay component is absent. Based both on its excitation excess energy and intensity dependence, this fast decay process is attributed to the formation of excitons from the electron-hole pairs. This interpretation is also consistent with a model that shows how free electron-hole pairs can be about twice more effective than excitons in altering the exciton absorption strength. From our measurements and analysis of our results, we determined that the exciton formation times in these monolayers to be shorter than 1 ps.
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Affiliation(s)
- Frank Ceballos
- Department of Physics and Astronomy, The University of Kansas, Lawrence, Kasas 66045, USA.
| | - Qiannan Cui
- Department of Physics and Astronomy, The University of Kansas, Lawrence, Kasas 66045, USA.
| | - Matthew Z Bellus
- Department of Physics and Astronomy, The University of Kansas, Lawrence, Kasas 66045, USA.
| | - Hui Zhao
- Department of Physics and Astronomy, The University of Kansas, Lawrence, Kasas 66045, USA.
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Chernikov A, van der Zande AM, Hill HM, Rigosi AF, Velauthapillai A, Hone J, Heinz TF. Electrical Tuning of Exciton Binding Energies in Monolayer WS_{2}. PHYSICAL REVIEW LETTERS 2015; 115:126802. [PMID: 26431003 DOI: 10.1103/physrevlett.115.126802] [Citation(s) in RCA: 126] [Impact Index Per Article: 14.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/10/2015] [Indexed: 05/09/2023]
Abstract
We demonstrate continuous tuning of the exciton binding energy in monolayer WS_{2} by means of an externally applied voltage in a field-effect transistor device. Using optical spectroscopy, we monitor the ground and excited excitonic states as a function of gate voltage and track the evolution of the quasiparticle band gap. The observed decrease of the exciton binding energy over the range of about 100 meV, accompanied by the renormalization of the quasiparticle band gap, is associated with screening of the Coulomb interaction by the electrically injected free charge carriers at densities up to 8×10^{12} cm^{-2}. Complete ionization of the excitons due to the electrical doping is estimated to occur at a carrier density of several 10^{13} cm^{-2}.
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Affiliation(s)
- Alexey Chernikov
- Departments of Physics and Electrical Engineering, Columbia University, New York, New York 10027, USA
| | - Arend M van der Zande
- Departments of Physics and Electrical Engineering, Columbia University, New York, New York 10027, USA
- Department of Mechanical Engineering, Columbia University, New York, New York 10027, USA
- Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA
| | - Heather M Hill
- Departments of Physics and Electrical Engineering, Columbia University, New York, New York 10027, USA
| | - Albert F Rigosi
- Departments of Physics and Electrical Engineering, Columbia University, New York, New York 10027, USA
| | - Ajanth Velauthapillai
- Departments of Physics and Electrical Engineering, Columbia University, New York, New York 10027, USA
- Department of Physics and Materials Sciences Center, Philipps-Universität Marburg, Marburg 35032, Germany
| | - James Hone
- Department of Mechanical Engineering, Columbia University, New York, New York 10027, USA
| | - Tony F Heinz
- Departments of Physics and Electrical Engineering, Columbia University, New York, New York 10027, USA
- Department of Applied Physics, Stanford University, Stanford, California 94305, USA
- SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA
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Schleife A, Rödl C, Fuchs F, Hannewald K, Bechstedt F. Optical absorption in degenerately doped semiconductors: Mott transition or Mahan excitons? PHYSICAL REVIEW LETTERS 2011; 107:236405. [PMID: 22182110 DOI: 10.1103/physrevlett.107.236405] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/19/2011] [Indexed: 05/31/2023]
Abstract
Electron doping turns semiconductors conductive even when they have wide fundamental band gaps. The degenerate electron gas in the lowest conduction-band states, e.g., of a transparent conducting oxide, drastically modifies the Coulomb interaction between the electrons and, hence, the optical properties close to the absorption edge. We describe these effects by developing an ab initio technique which captures also the Pauli blocking and the Fermi-edge singularity at the optical-absorption onset, that occur in addition to quasiparticle and excitonic effects. We answer the question whether free carriers induce an excitonic Mott transition or trigger the evolution of Wannier-Mott excitons into Mahan excitons. The prototypical n-type zinc oxide is studied as an example.
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Affiliation(s)
- André Schleife
- Institut für Festkörpertheorie und -optik, Friedrich-Schiller-Universität, Max-Wien-Platz 1, 07743 Jena, Germany.
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Glutsch S, Dan NT, Bechstedt F. Hartree contribution to the band-gap renormalization in semiconductor microstructures. PHYSICAL REVIEW. B, CONDENSED MATTER 1995; 52:13776-13779. [PMID: 9980587 DOI: 10.1103/physrevb.52.13776] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
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