Research ArticleCONDENSED MATTER PHYSICS

Quantum plasmonic control of trions in a picocavity with monolayer WS2

See allHide authors and affiliations

Science Advances  11 Oct 2019:
Vol. 5, no. 10, eaau8763
DOI: 10.1126/sciadv.aau8763
  • Fig. 1 Quantum plasmonic generation of trions in a Au-Ag picocavity with monolayer WS2.

    (A) Schematic of TEQPL imaging with monolayer WS2 in a picocavity formed by the Ag tip and the Au substrate. The 532-nm laser beam was focused on the tip apex, and the sample was scanned to obtain the PL spatial maps of neutral excitons (X0) and trions (X). The tunneling-induced X0X transition takes place for the short tip-sample distance. (B) Sketch of the Au-Ag cavity with d > 1 nm tip-sample distance that corresponds to the classical coupling (CC) regime. (C) PL spectra of X0 and X in monolayer WS2 in the CC regime. Blue and red solid lines are Gaussian fitting functions centered at 614 and 625 nm, respectively. a.u., arbitrary units. (D and E) Corresponding sketch and PL spectra of monolayer WS2 in the Au-Ag picocavity in the quantum coupling (QC) regime with a tip-sample distance d < 0.35 nm, where the charge tunneling [blue arrow in (D)] contributes to the formation of trions. The PL intensity of X becomes larger than that of X0 in the QC (E) compared to the CC (C) regime.

  • Fig. 2 Picoscale quantum plasmonic control of neutral excitons (X0) and trions (X) in monolayer WS2 in a Au-Ag cavity.

    (A) AFM image of the triangular monolayer WS2 nanoflake. PL intensity of neutral excitons (X0; blue) and trions (X; red) measured in a spatial location marked by a circle in (A) as a function of the tip-sample distance in the picometer scale (B) and in the whole range (C) shows the PL quenching of both signals at the picoscale distances. (D) However, the ratio IX/(IX0 + IX) shows an increase in the trion relative to the neutral exciton signal at distances shorter than 300 pm.

  • Fig. 3 TEQPL imaging of a complex WS2 nanoflake.

    (A) AFM image shows the height topography with WS2 triangular monolayer periphery and a few-layer triangular central region. Several areas of interest are marked A to F in different parts of the flake. (B) Kelvin probe force microscopy (KPFM) image under a 532-nm laser illumination shows inhomogeneous contact potential difference (CPD) signal at the surface of the sample. G to I mark the top, left, and right corners, respectively. NF neutral exciton, X0 (C), and trion, X (E), TEQPL, and FF X0 (D) and X (F) PL images of the complex WS2 nanoflake in a Au-Ag cavity with tip-sample distances of 0.31 and 10 nm, respectively. Black dashed lines indicate the outlines of the WS2 nanoflake. The imaging step size is 50 nm. (G and H) Line profiles of the AFM, KPFM, and PL signals from the marked white dashed lines (i) and (ii), respectively. Vertical orange and red dashed lines in (G) mark the width of the FF and NF PL profiles, respectively, of trions at the Au-WS2 interface at the edge of the flake. Vertical blue and red dashed lines in (H) mark the positions of the maximum signal intensities of the NF PL signal profiles of neutral excitons and trions, respectively, showing the relative shift of the two signals.

  • Fig. 4 Tip-sample distance dependence of PL of WS2 nanoflake in a Au-Ag cavity.

    (A) Energy diagram of the tip-sample-substrate (Ag-WS2-Au) system with Schottky barrier (SB). Tip-sample distance dependence of the PL signal intensities of neutral excitons (X0) and trions (X) from two spatial locations marked A (B and C) and C (D and E) in Fig. 3A. Three regimes of tip-sample coupling are identified in (B): (i) FF with no tip-sample coupling (d > 10 nm), (ii) NF with classical tip-sample coupling (NF CC) with 0.35 nm < d < 10 nm, and (iii) NF with quantum tip-sample coupling (NF QC) with d < 0.35 nm. Green and blue dashed lines indicate the FF and the short-distance NF X0 PL signals, respectively. Zoomed-in picoscale tip-sample distance dependence of TEQPL signals from spatial locations A (C) and C (E) in the QC regime. The vertical black dashed lines separate the CC and QC regimes at the vdW tip-sample contact distance (0.35 nm).

  • Fig. 5 Subwavelength control of trions in an Au-Ag picocavity with monolayer WS2.

    Zoomed-in picoscale tip-sample distance dependence of TEQPL signals of neutral excitons X0 (blue) and trions X (red) from spatial locations E (A) and D (C) in the QC regime. (B, D) The corresponding peak ratios IX/(IX0 + IX) show the relative number of trions and neutral excitons and reveal the underlying quantum plasmonic mechanisms. Inset shows the tip-sample distance control parameter d and the AFM image of the central part of the complex WS2 nanoflake, with the marked locations of spots D and E separated by 60 nm. The peak ratios show two different types of behavior from the closely spaced locations, which may be switched by varying the tip-sample distance by only a few picometers.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/10/eaau8763/DC1

    Fig. S1. AFM force-distance diagram.

    Fig. S2. Tip-sample distance dependence of the neutral exciton/trion PL.

    Fig. S3. Classical TEPL imaging.

    Fig. S4. Kelvin probe force microscopy.

    Fig. S5. AFM obtained during TEQPL imaging.

    Fig. S6. Repeated distance dependence measurements of spot B.

    Fig. S7. Anticorrelated distributions of neutral excitons and trions under TEQPL.

    Fig. S8. Mixed distributions of neutral excitons and trions.

    Fig. S9. The model and simulation of TEQPL.

    Fig. S10. PL and peak assignment of WS2 on Si/SiO2.

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. AFM force-distance diagram.
    • Fig. S2. Tip-sample distance dependence of the neutral exciton/trion PL.
    • Fig. S3. Classical TEPL imaging.
    • Fig. S4. Kelvin probe force microscopy.
    • Fig. S5. AFM obtained during TEQPL imaging.
    • Fig. S6. Repeated distance dependence measurements of spot B.
    • Fig. S7. Anticorrelated distributions of neutral excitons and trions under TEQPL.
    • Fig. S8. Mixed distributions of neutral excitons and trions.
    • Fig. S9. The model and simulation of TEQPL.
    • Fig. S10. PL and peak assignment of WS2 on Si/SiO2.

    Download PDF

    Files in this Data Supplement:

Stay Connected to Science Advances

Navigate This Article