Research ArticleCONDENSED MATTER PHYSICS

Electrically driven photon emission from individual atomic defects in monolayer WS2

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Science Advances  16 Sep 2020:
Vol. 6, no. 38, eabb5988
DOI: 10.1126/sciadv.abb5988
  • Fig. 1 Tunneling electron-induced photon emission.

    Scheme of experimental configuration (A) and electron energy levels (B) involved in electrically driven photon emission from an atomic point defect. An electron from a gold tip (initial state ψi in red) tunnels inelastically into an atomic defect (top sulfur vacancy) of monolayer WSS (final state ψf in purple). The excess of transition energy ħω is released into the emission of one photon, mediated by intermediate plasmon states of the tip cavity. The 2D material is supported on a graphene layer (electric contact) on top of SiC. The tip-sample bias voltage Vbias controls the electron injection energy. An inelastic electron transition is represented by the green downward pointing arrow in (B). The dark and light gray boxes represent filled and empty states, respectively. Photo credit: Ignacio Gaubert

  • Fig. 2 Tunneling bias dependence of photon emission.

    (A) Side view of atomic layers and the two types of WS2 defects that we study here: a top sulfur vacancy (VacS top) and a chromium substituting tungsten (CrW). (B) Energy level diagrams showing the two unoccupied VacS defect states (red) and three unoccupied CrW defect states (orange), lying in the gap between valence and conduction bands (VB and CB). (C and D) STM topography (constant current I = 20 pA and bias V = 1.1 V) of VacS top (C) and CrW (D). (E) We probe the local density of states through dI/dV spectroscopy on VacS (red) and CrW (orange) defects compared with a position far from any defect (gray). The dI/dV spectra of top and bottom VacS are identical (36). (F) Spectrally integrated photon emission induced by inelastic electron tunneling at constant current I = 10 nA. Measurements in (E) and (F) were recorded at the same defect sites and with the same tip.

  • Fig. 3 Photon emission spatial imaging of a single sulfur vacancy.

    (A) Spectrally integrated photon map of a VacS top defect acquired with I = 20 nA constant current and V = 2.9 V bias. This map is in excellent agreement with the STM image of the in-gap defect orbital (see Fig. 2C). (B) Spectrally integrated photon emission across VacS top as a function of tunneling bias. The white dashed line indicates the bias at which the map in (A) was acquired. The defect exhibits strongly localized photon emission at a tunneling bias substantially lower than the onset of substrate emission. (C) STM topography of VacS top simultaneously acquired during the photon map shown in (A). (D) CO tip noncontact atomic force microscopy (nc-AFM) image of a VacS top.

  • Fig. 4 Spectrally resolved photon emission on a single sulfur vacancy.

    (A) Photon emission spectrum on VacS at different tunneling biases. The spectral width and the minimal spectral variation with lateral tip position suggest a plasmon-mediated emission process. (B) 2D representation of the data in (A). Oblique arrows indicate transitions into the two discrete VacS defect states. The oblique dashed line marks the onset of substrate emission corresponding to inelastic tunneling into the WSS conduction band minimum (CBM). (C) Photon counts at ħω = 1.85 eV [cut along the vertical dashed line in (B)]. We identify two emission onsets corresponding to inelastic transitions into the lower and higher VacS defect state, as well as a higher-energy onset assigned to transitions into the WSS CBM.

Supplementary Materials

  • Supplementary Materials

    Electrically driven photon emission from individual atomic defects in monolayer WS2

    Bruno Schuler, Katherine A. Cochrane, Christoph Kastl, Edward S. Barnard, Edward Wong, Nicholas J. Borys, Adam M. Schwartzberg, D. Frank Ogletree, F. Javier García de Abajo, Alexander Weber-Bargioni

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    This PDF file includes:

    • STML theoretical modelling
    • Sample synthesis
    • Supplementary STS measurements
    • Supplementary STML measurements
    • Tip preparation
    • Figs. S1 to S12
    • References

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