Correlated fluorescence blinking in two-dimensional semiconductor heterostructures

Journal name:
Nature
Volume:
541,
Pages:
62–67
Date published:
DOI:
doi:10.1038/nature20601
Received
Accepted
Published online

‘Blinking’, or ‘fluorescence intermittency’, refers to a random switching between ‘ON’ (bright) and ‘OFF’ (dark) states of an emitter; it has been studied widely in zero-dimensional quantum dots1 and molecules2, 3, and scarcely in one-dimensional systems4, 5. A generally accepted mechanism for blinking in quantum dots involves random switching between neutral and charged states6, 7 (or is accompanied by fluctuations in charge-carrier traps8), which substantially alters the dynamics of radiative and non-radiative decay. Here, we uncover a new type of blinking effect in vertically stacked, two-dimensional semiconductor heterostructures9, which consist of two distinct monolayers of transition metal dichalcogenides (TMDs) that are weakly coupled by van der Waals forces. Unlike zero-dimensional or one-dimensional systems, two-dimensional TMD heterostructures show a correlated blinking effect, comprising randomly switching bright, neutral and dark states. Fluorescence cross-correlation spectroscopy analyses show that a bright state occurring in one monolayer will simultaneously lead to a dark state in the other monolayer, owing to an intermittent interlayer carrier-transfer process. Our findings suggest that bilayer van der Waals heterostructures provide unique platforms for the study of charge-transfer dynamics and non-equilibrium-state physics, and could see application as correlated light emitters in quantum technology.

At a glance

Figures

  1. Fluorescence blinking in a loosely contacted WS2/MoSe2 2D bilayer heterostructure.
    Figure 1: Fluorescence blinking in a loosely contacted WS2/MoSe2 2D bilayer heterostructure.

    a, Illustrations of a typical blinking quantum dot (top; with ON/OFF states) and a blinking 2D bilayer heterostructure (bottom; with bright/neutral/dark states). b, Optical image of a trapezoidal WS2/MoSe2 bilayer heterostructure, formed by aligned stacking of a WS2 monolayer and a MoSe2 monolayer. ce, Fluorescence image taken with a colour camera at different times, showing a bright (c), neutral (d) and dark (e) emission state of WS2 at the WS2/MoSe2 junction. The exposure time was 50 ms. f, g, Snapshots of representative fluorescence images at different times, showing emission from WS2 (f) and MoSe2 (g); the exposure time for each frame was 8 ms. h, i, Time traces showing variations in emission intensity for: h, a WS2 monolayer alone (black trace) or within a WS2/MoSe2 bilayer (red trace); and i, a MoSe2 monolayer alone (black trace) or within a WS2/MoSe2 bilayer (red trace), over a time period of 20 s. These traces were calculated from the greyscale of frames from two videos recorded with a high-speed camera, with a 645-nm bandpass filter for WS2 (h) and a 794-nm bandpass filter for MoSe2 (i).

  2. The IICT model of fluorescence blinking in 2D heterostructures.
    Figure 2: The IICT model of fluorescence blinking in 2D heterostructures.

    a, A neutral state. Top, band alignment showing the conduction band minimum (CBM) and valence band maximum (VBM) of a typical bilayer TMD heterostructure, with one component acting as an electron donor (for example, MoSe2 in WS2/MoSe2) and its counterpart as an electron acceptor (for example, WS2 in WS2/MoSe2). Grey arrows indicate the emission of an A-exciton from the electron acceptor (1) and from the electron donor (2). Bottom, diagram showing the junction between the monolayers in the TMD heterostructure. e, electron; h+, hole. b, An electron-dominated carrier-transfer process, which will result in a dark emission state in the electron donor and a bright state in the electron acceptor. c, A hole-dominated carrier-transfer process, which will result in a bright emission state in the electron donor and a dark state in the electron acceptor. d, A bilayer heterostructure with unimpeded carrier-transfer channels (for both electrons and holes). Emission from electron donor and acceptor will be quenched. This process will probably appear when the two monolayers of the bilayer are in close contact, and will allow emission of an interlayer exciton (3). e–g, Transient absorption measurements from a MoSe2/WSe2 heterostructure. e, Optical (top) and fluorescence (bottom) images, showing a near-neutral region (state ‘a’) and a darker region (state ‘d’). f, g, Pump–probe measurements for the near-neutral region (f) and the dark region (g). In the main panels, the normalized differential transmission (ΔT/T(λ,t)) of the broadband 13-fs probe beam is depicted in two-dimensional false-colour maps versus the pump–probe delay time and the wavelength. In both cases, the spectrum of the 60-fs excitation pulse (810 nm to 845 nm) is adjusted to overlap with the absorption of MoSe2 monolayer. The top panels depict the signal dynamics evaluated at fixed wavelengths, indicated on the colour maps with horizontal colour-coded dashed lines. The right panels depict dynamic spectral signatures of the signal evaluated at fixed delay times t and indicated by black vertical dashed lines.

  3. Variation in trion and exciton emission from heterostructures over time.
    Figure 3: Variation in trion and exciton emission from heterostructures over time.

    Emission was monitored from a WS2 monolayer in blinking MoS2/WS2 and WS2/WSe2 bilayer heterostructures. X0, neutral exciton; X+, positively charged trion; X, negatively charged trion. a, Typical emission spectra, with Gaussian fitting, from a blinking MoS2/WS2 heterojunction: top, a brighter state (but less bright, or weaker, than that produced by emission from bare WS2); bottom, a darker state. The left insets are representative fluorescence images. The right insets are illustrations of different carrier-transfer states, showing slight electron transfer (top, thin blue line) and strong electron transfer (bottom, thick blue line). b, Top, time trace of integrated total emission intensity from WS2 in MoS2/WS2 (I(X0 + X+); red curve), and time trace of the intensity ratio of trion/exciton emission (I(X+)/I(X0)) from WS2 in MoS2/WS2 (blue curve). Bottom, time traces of emission energy from neutral exciton emission (E(X0); red curve) and trion emission (E(X+); blue curve). Red and black vertical dashed lines indicate t = 73.2 seconds and t = 76.8 seconds, respectively. c, d, Statistical analyses of state distribution on the basis of the time traces in b, allowing visualization of the relationship between emission intensity and the trion/exciton ratio (c), and of the relationship between emission intensity and emission energy (d). Data from an isolated WS2 region are plotted for reference. e, f, Similar state distribution of WS2 emission from a blinking WS2/WSe2 bilayer heterojunction, the main difference being that X+ emission is replaced with X emission, because WS2 becomes an electron acceptor in WS2/WSe2. All spectra were taken with a 50-ms exposure, with an interval of 150 ms in MoS2/WS2 and of 50 ms in WS2/WSe2. All excitation was carried out with a 532-nm continuous wave laser.

  4. Fluorescence cross-correlation spectroscopy analyses of a blinking WS2/MoSe2 bilayer heterostructure.
    Figure 4: Fluorescence cross-correlation spectroscopy analyses of a blinking WS2/MoSe2 bilayer heterostructure.

    a, Simultaneous recording of spectra for WS2 and MoSe2 emission, using a 150 lines mm−1 grating on an HR Evolution spectrometer. Each spectrum has an exposure time of 5 ms, and the time interval between two acquisitions is 3.7 ms. The inset shows an optical image of the WS2/MoSe2 bilayer heterostructure sample. b, Time-dependent intensity fluctuations of WS2 (red curve) and MoSe2 (blue curve) emission from the spectra shown in a. c, G(τ) calculated from the time-trace data in b, indicating a prominent negative correlation at time zero. d, Set-up for measuring the time-resolved fluorescence correlation. BS, beam splitter; BP, bandpass filter centred at the indicated frequencies; SPAD, single-photon avalanche diode detector; TCSPC, time-correlated single-photon counting. e, Fluorescence lifetime imaging microscopic image of the same WS2/MoSe2 heterostructure. f, Time-decay traces for the monolayer WS2 region, the monolayer MoSe2 region, and the heterostructure region. g, A 40-second period of emission trajectories (Top, intensity versus time; bottom, averaged lifetime versus time) for MoSe2 and WS2 in the heterostructure, measured simultaneously by two SPADs. Binning time is 100 ms. h, G(τ) calculated from the intensity traces shown in g, showing a negative correlation. The data are fitted with , where T1= 378 ms and T2= 88.6 ms. i, ON/neutral/OFF probability analysis of the blinking dynamics. The ON/OFF distributions are fitted with a multi-exponential model, , where τ1= 8.4 ms, τ2= 1.9 ms and τ3= 0.62 ms for the ON times; and τ1= 7.5 ms, τ2= 1.8 ms and τ3= 0.58 ms for the OFF times. The neutral distribution is fitted with a truncated power law, with α = 0.48 and β = 1.6 ms. j, Statistical analysis of the correlated state distribution deduced from the data in g (and Supplementary Fig. 13), suggesting a typical hole-dominant charge transfer (CT) state (at t = 24.8 s) and a typical electron-dominant CT state (at t = 29.5 s).

Videos

  1. High speed dynamic fluorescence imaging of the WS2 component in the WS2/MoSe2 heterostructure
    Video 1: High speed dynamic fluorescence imaging of the WS2 component in the WS2/MoSe2 heterostructure
    High speed dynamic fluorescence imaging of the WS2 component in the WS2/MoSe2 heterostructure.
  2. High speed dynamic fluorescence imaging of the MoSe2 component in the WS2/MoSe2 heterostructure
    Video 2: High speed dynamic fluorescence imaging of the MoSe2 component in the WS2/MoSe2 heterostructure
    High speed dynamic fluorescence imaging of the MoSe2 component in the WS2/MoSe2 heterostructure.
  3. Dynamic fluorescence imaging of the WS2 component in a MoS2/WS2 heterostructure acquired with a color CCD camera
    Video 3: Dynamic fluorescence imaging of the WS2 component in a MoS2/WS2 heterostructure acquired with a color CCD camera
    Dynamic fluorescence imaging of the WS2 component in a MoS2/WS2 heterostructure acquired with a color CCD camera.

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Author information

Affiliations

  1. Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371 Singapore

    • Weigao Xu,
    • Weiwei Liu,
    • Weijie Zhao,
    • Xin Lu,
    • Weibo Gao &
    • Qihua Xiong
  2. Department of Physics and Center for Applied Photonics, University of Konstanz, D-78457 Konstanz, Germany

    • Jan F. Schmidt,
    • Timo Raab &
    • Denis V. Seletskiy
  3. Laboratoire Pierre Aigrain, Ecole Normale Supérieure, PSL Research University, CNRS, Université Pierre et Marie Curie, Sorbonne Universités, Université Paris Diderot, Sorbonne Paris-Cité, 24 Rue Lhomond, 75231 Paris Cedex 05, France

    • Carole Diederichs
  4. MajuLab, CNRS–UNS–NUS–NTU International Joint Research Unit, UMI 3654 Singapore

    • Carole Diederichs &
    • Qihua Xiong
  5. NOVITAS, Nanoelectronics Center of Excellence, School of Electrical and Electronic Engineering, Nanyang Technological University, 639798 Singapore

    • Qihua Xiong

Contributions

W.X. and Q.X. designed the research; W.X., W.L. and X.L. prepared the heterostructures and carried out steady-state/transient fluorescence spectroscopy measurements and correlation measurements; J.F.S., W.Z., T.R. and D.V.S. performed transient absorption spectroscopy measurements; W.X., W.L., X.L., W.Z., J.F.S., D.V.S., C.D., W.G. and Q.X. analysed the data; and W.X., W.L. and Q.X. wrote the manuscript. All authors commented on the manuscript.

Competing financial interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to:

Reviewer Information Nature thanks X. Cui and A. Malko for their contribution to the peer review of this work.

Author details

Supplementary information

Video

  1. Video 1: High speed dynamic fluorescence imaging of the WS2 component in the WS2/MoSe2 heterostructure (4.92 MB, Download)
    High speed dynamic fluorescence imaging of the WS2 component in the WS2/MoSe2 heterostructure.
  2. Video 2: High speed dynamic fluorescence imaging of the MoSe2 component in the WS2/MoSe2 heterostructure (8.16 MB, Download)
    High speed dynamic fluorescence imaging of the MoSe2 component in the WS2/MoSe2 heterostructure.
  3. Video 3: Dynamic fluorescence imaging of the WS2 component in a MoS2/WS2 heterostructure acquired with a color CCD camera (1.8 MB, Download)
    Dynamic fluorescence imaging of the WS2 component in a MoS2/WS2 heterostructure acquired with a color CCD camera.

PDF files

  1. Supplementary Information (5.7 MB)

    This file contains Supplementary Text, Supplementary Figures 1-16 and Supplementary References.

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