Research ArticleCONDENSED MATTER PHYSICS

Bimodal exciton-plasmon light sources controlled by local charge carrier injection

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Science Advances  25 May 2018:
Vol. 4, no. 5, eaap8349
DOI: 10.1126/sciadv.aap8349
  • Fig. 1 Overview of STM-induced electroluminescence on C60 thin films.

    (A) Pseudo–three-dimensional topographic STM image of the C60 surface overlaid with the simultaneously obtained electroluminescence photon map. The C60 film exhibits a three-dimensional growth mode on the atomically flat Ag(111) metallic substrate. The top terraces exhibit a homogeneous BKG luminescence (red-colored terraces). In addition, ECs of nanometer extension with different intensities (yellow-colored regions) appear scattered over the surface. Arrows indicate the positions of an EC and a terrace with BKG luminescence. Image size, 150 nm × 150 nm. kcts, kilocounts. (B) Typical optical spectra of the exciton (top), bimodal (middle), and plasmon (bottom) luminescence mechanisms. The data were recorded under conditions optimized for each case and were rescaled to make them comparable (U = −3.0 V and I = 100 pA).

  • Fig. 2 Characterization of luminescence processes on an EC and on a BKG region.

    (A) LDOS map of the C60 CB in the vicinity of an EC on Ag(111), constant height; voltage applied between tip and sample, U = 1.2 V. (B) Luminescence versus current function, P(I), measured on an EC, U = −3.0 V. A sublinear dependence is observed. (C) LDOS map of the VB in the vicinity of an EC, constant height, U = −3.0 V. (D) Constant height photon map of an EC. The molecules with the highest luminescence intensity are marked by white dashed circles to emphasize the similarity with the VB map (C). Color scale, 0 to 16 kcts/s. (E) Schematic energy diagram of STM-induced excitonic luminescence on C60 films. (F) LDOS map of the CB of a C60 film on Au(111) with BKG emission, constant height, U = 1.5 V. (G) P(I) for BKG luminescence showing a linear dependence of intensity on current, U = −3.0 V. (H) LDOS map of the VB of a BKG surface region, constant height, U = −3.1 V. (I) Constant height photon map of a BKG surface region. The molecules with the highest luminescence intensity are marked by white dashed circles to emphasize the similarity with the CB map (F). Color scale, 0 to 23 kcts/s. (J) Schematic energy diagram of STM-induced plasmonic luminescence on C60 films. All STM and photon maps have dimensions of 5 nm × 5 nm.

  • Fig. 3 Bimodal electroluminescence by position-controlled mixing.

    (A) Schematic representation of the experiment presented in (B) to (E). The STM tip approaches an EC laterally at constant current and voltage. (B) Constant current STM topography of the investigated surface area of a film grown on Au(111); four C60 molecules are imaged 1.5 nm × 4 nm, U = −3.3 V, I = 50 pA. (C) Constant current photon map obtained simultaneously with (B). The five positions of the measurements in (D) and (E) are marked and color-coded. (D) Optical spectra obtained on the positions marked in (C). A transition from a broad to a line-dominated spectrum is observed from bottom to top. (E) Luminescence versus current function, P(I), measured at the same positions as indicated in (C). The evolution from linear to sublinear behavior correlates with the appearance of the exciton line at 690 nm in the optical spectra presented in (D). Every P(I) measurement in the series is incrementally offset by multiples of 8 kcts/s for clarity. The dashed lines correspond to fits using the model described in the text. For details, see the Supplementary Materials.

  • Fig. 4 Control of the bimodal electroluminescence by charge injection dynamics.

    (A) Schematic of the experiment presented in (B) and (C). The STM tip approaches the EC perpendicular to the surface at constant bias voltage, thus increasing the tunnel current, I. (B) Optical spectra at a fixed surface position and U = −3 V for the indicated currents. The accumulation time of each spectrum, T, was chosen such that the total injected charge T·I = 0.05 μC is constant. (C) P(I) measurement extended to high currents with three regimes [(i) to (iii)] discussed in the text.

  • Fig. 5 Spatial distribution and dynamic control of plasmon and exciton emission.

    Photon maps (13.7 nm × 9.3 nm, U = −3 V) of a flat C60 terrace on Au(111) containing an EC and a region with BKG luminescence. The constant current maps are stacked as layers on top of each other in a three-dimensional presentation. The current and the corresponding average time between charges are indicated for every layer (scales on the left and right sides, respectively). At injection rates faster than 100 ps, the plasmonic and the excitonic intensities become comparable, and the EC cannot be distinguished from the BKG. All photon maps share the same color scale ranging from 0 to 10 kcts/s.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/5/eaap8349/DC1

    section S1. EC and BKG luminescence on the molecular scale

    section S2. Evaluating Pex(I) in the three-state rate model that includes exciton-charge annihilation

    section S3. Fits of P(I) in Fig. 3

    section S4. Reversibility of the current dependence of the exciton-to-plasmon ratio

    fig. S1. Topography and light intensity channels of EC and BKG luminescence at the molecular scale.

    fig. S2. Scheme of the three-state model.

    fig. S3. Fits of P(I) of the curves presented in Fig. 3.

    fig. S4. Reversibility of the bimodal light source.

  • Supplementary Materials

    This PDF file includes:

    • section S1. EC and BKG luminescence on the molecular scale
    • section S2. Evaluating Pex(I) in the three-state rate model that includes exciton-charge annihilation
    • section S3. Fits of P(I) in Fig. 3
    • section S4. Reversibility of the current dependence of the exciton-to-plasmon ratio
    • fig. S1. Topography and light intensity channels of EC and BKG luminescence at the molecular scale.
    • fig. S2. Scheme of the three-state model.
    • fig. S3. Fits of P(I) of the curves presented in Fig. 3.
    • fig. S4. Reversibility of the bimodal light source.

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