期刊
ACS NANO
卷 15, 期 12, 页码 19165-19174出版社
AMER CHEMICAL SOC
DOI: 10.1021/acsnano.1c08286
关键词
inter- and intravalley excitons; electron-phonon interaction; electron-electron interaction; upconversion; photoluminescence; WSe2 monolayer
类别
资金
- Polish National Science Center (NCN) Grant Beethoven 2 [2016/23/G/ST3/04114]
- DFG Beethoven 2 Grant [DE 2206/2-1]
- Polish NCN Grant Maestro [2014/14/A/ST3/00654]
- RFBR-DFG [19-52-12038]
- DFG [TRR 160]
- Mercur Foundation [Pe-2019-0022]
- Ministry of Science and Technology, Taiwan [MOST 1102112-M-011-002]
Semiconducting monolayers of transition-metal dichalcogenides are important platforms to study various excitonic interactions, with the brightening of dark excitons and trions being crucial in semiconductor physics. A mechanism is reported here in which a dark intervalley exciton can convert light into a bright intravalley exciton under specific conditions, highlighting the importance of dark excitons in the optics of two-dimensional materials.
Semiconducting monolayers of transition-metal dichalcogenides are outstanding platforms to study both electronic and phononic interactions as well as intra- and intervalley excitons and trions. These excitonic complexes are optically either active (bright) or inactive (dark) due to selection rules from spin or momentum conservation. Exploring ways of brightening dark excitons and trions has strongly been pursued in semiconductor physics. Here, we report on a mechanism in which a dark intervalley exciton upconverts light into a bright intravalley exciton in hBN-encapsulated WSe2 monolayers. Excitation spectra of upconverted photoluminescence reveals resonances at energies 34.5 and 46.0 meV below the neutral exciton in the nominal WSe2 transparency range. The required energy gains are theoretically explained by cooling of resident electrons or by exciton scattering with Lambda- or K-valley phonons. Accordingly, an elevated temperature and a moderate concentration of resident electrons are necessary for observing the upconversion resonances. The interaction process observed between the inter- and intravalley excitons elucidates the importance of dark excitons for the optics of two-dimensional materials.
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