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Title: Magnetic brightening and control of dark excitons in monolayer WSe2

Abstract

Monolayer transition metal dichalcogenide crystals, as direct-gap materials with strong light-matter interactions, have attracted much recent attention. Because of their spin-polarized valence bands and a predicted spin splitting at the conduction band edges, the lowest-lying excitons in WX 2 (X = S, Se) are expected to be spin-forbidden and optically dark. To date, however, there has been no direct experimental probe of these dark excitons. Here, we show how an in-plane magnetic field can brighten the dark excitons in monolayer WSe 2 and permit their properties to be observed experimentally. Precise energy levels for both the neutral and charged dark excitons are obtained and compared with ab initio calculations using the GW-BSE approach. As a result of their spin configuration, the brightened dark excitons exhibit much-increased emission and valley lifetimes. These studies directly probe the excitonic spin manifold and reveal the fine spin-splitting at the conduction band edges.

Authors:
 [1];  [2];  [3];  [4];  [5];  [3];  [6];  [5];  [4];  [6];  [2];  [7]
  1. Columbia Univ., New York, NY (United States); Stanford Univ., Stanford, CA (United States); SLAC National Accelerator Lab., Menlo Park, CA (United States)
  2. Univ. of California, Berkeley, CA (United States); Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
  3. National High Magnetic Field Lab., Tallahassee, FL (United States); Florida State Univ., Tallahassee, FL (United States)
  4. The Pennsylvania State Univ., University Park, PA (United States)
  5. Columbia Univ., New York, NY (United States)
  6. National High Magnetic Field Lab., Tallahassee, FL (United States)
  7. Stanford Univ., Stanford, CA (United States); SLAC National Accelerator Lab., Menlo Park, CA (United States)
Publication Date:
Research Org.:
SLAC National Accelerator Lab., Menlo Park, CA (United States); Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES); National Science Foundation (NSF); Betty and Gordon Moore Foundation
OSTI Identifier:
1367175
Alternate Identifier(s):
OSTI ID: 1458504
Grant/Contract Number:  
AC02-76SF00515; GBMF4545; FA9550-14-1-0040; AC02-05CH11231; DMR-1508412; DMR-1420634; DMR-1157490
Resource Type:
Accepted Manuscript
Journal Name:
Nature Nanotechnology
Additional Journal Information:
Journal Volume: 12; Journal Issue: 9; Journal ID: ISSN 1748-3387
Publisher:
Nature Publishing Group
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE

Citation Formats

Zhang, Xiao -Xiao, Cao, Ting, Lu, Zhengguang, Lin, Yu -Chuan, Zhang, Fan, Wang, Ying, Li, Zhiqiang, Hone, James C., Robinson, Joshua A., Smirnov, Dmitry, Louie, Steven G., and Heinz, Tony F. Magnetic brightening and control of dark excitons in monolayer WSe2. United States: N. p., 2017. Web. doi:10.1038/NNANO.2017.105.
Zhang, Xiao -Xiao, Cao, Ting, Lu, Zhengguang, Lin, Yu -Chuan, Zhang, Fan, Wang, Ying, Li, Zhiqiang, Hone, James C., Robinson, Joshua A., Smirnov, Dmitry, Louie, Steven G., & Heinz, Tony F. Magnetic brightening and control of dark excitons in monolayer WSe2. United States. doi:https://doi.org/10.1038/NNANO.2017.105
Zhang, Xiao -Xiao, Cao, Ting, Lu, Zhengguang, Lin, Yu -Chuan, Zhang, Fan, Wang, Ying, Li, Zhiqiang, Hone, James C., Robinson, Joshua A., Smirnov, Dmitry, Louie, Steven G., and Heinz, Tony F. Mon . "Magnetic brightening and control of dark excitons in monolayer WSe2". United States. doi:https://doi.org/10.1038/NNANO.2017.105. https://www.osti.gov/servlets/purl/1367175.
@article{osti_1367175,
title = {Magnetic brightening and control of dark excitons in monolayer WSe2},
author = {Zhang, Xiao -Xiao and Cao, Ting and Lu, Zhengguang and Lin, Yu -Chuan and Zhang, Fan and Wang, Ying and Li, Zhiqiang and Hone, James C. and Robinson, Joshua A. and Smirnov, Dmitry and Louie, Steven G. and Heinz, Tony F.},
abstractNote = {Monolayer transition metal dichalcogenide crystals, as direct-gap materials with strong light-matter interactions, have attracted much recent attention. Because of their spin-polarized valence bands and a predicted spin splitting at the conduction band edges, the lowest-lying excitons in WX 2 (X = S, Se) are expected to be spin-forbidden and optically dark. To date, however, there has been no direct experimental probe of these dark excitons. Here, we show how an in-plane magnetic field can brighten the dark excitons in monolayer WSe 2 and permit their properties to be observed experimentally. Precise energy levels for both the neutral and charged dark excitons are obtained and compared with ab initio calculations using the GW-BSE approach. As a result of their spin configuration, the brightened dark excitons exhibit much-increased emission and valley lifetimes. These studies directly probe the excitonic spin manifold and reveal the fine spin-splitting at the conduction band edges.},
doi = {10.1038/NNANO.2017.105},
journal = {Nature Nanotechnology},
number = 9,
volume = 12,
place = {United States},
year = {2017},
month = {6}
}

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Figures / Tables:

Fig. 1 Fig. 1: Conduction band structure of monolayer WSe2 and magnetic brightening of dark excitons. a, False color plot of the measured emission spectrum for monolayer WSe2 at a temperature of 30 K as a function of the strength of the applied B||. The displayed energy range includes emission from themore » neutral A exciton (X0) and the associated trion state (XT). Emission features from the dark exciton (XD) and dark trion (XDT) grow with increasing B||. b, For monolayer WSe2, electrons in the lower conduction band (CB2) have spin opposite that in the upper valence band (VB), rendering the lowest transition optically dark. Only transitions from the upper CB (at energy ℎ𝜐0 ) are allowed. The spin-split CB bands, CB1 and CB2, can be described as the result of an effective out-of-plane magnetic field Bint acting on the electron magnetic moment. c, Under an external in-plane magnetic field B||, the total effective field Beff = Bint + B|| is tilted away from the surface normal, resulting in tilted spin polarization of the CB electrons. Optical transitions at energy ℎ𝜐D from an exciton formed mainly from the lower CB and corresponding to the dark exciton, then become weakly allowed.« less

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