Research ArticleChemistry

Asymmetric photon transport in organic semiconductor nanowires through electrically controlled exciton diffusion

See allHide authors and affiliations

Science Advances  16 Mar 2018:
Vol. 4, no. 3, eaap9861
DOI: 10.1126/sciadv.aap9861
  • Fig. 1 EP propagation in BPEA nanowires for asymmetric photon transport.

    (A) TEM image of a single BPEA wire. Insets: SAED pattern (left) collected from the microarea marked with a white square and the thermodynamically stable molecular stacking in the crystal nanowire (right). The packing arrangement of molecules is along the b axis with the shortest intermolecular distance of 3.37 Å, which improves the migration of excitons and the exciton-photon coupling. (B) Photoluminescence image of the BPEA nanowires under ultraviolet (330 to 380 nm) excitation. (C) Microarea photoluminescence image of a typical wire locally excited from the middle and guided distance-dependent photoluminescence intensities at different wavelengths. The photoluminescence spectra were collected from the left tip of the wire by accurately shifting the excitation laser spot. EX, excited spot; WG, waveguided emission; a.u., arbitrary units. (D) Schematic depiction of the asymmetric light propagation under an electric field. The electrically induced asymmetric EP migration results in a stronger outcoupling along the field compared with that against the field.

  • Fig. 2 Realization of asymmetric photon transport.

    (A) Schematic depiction of the device configuration. The field is applied in-plane with the light propagation to provide an efficient way for asymmetric power coupling to the waveguides. The nanowire was successively excited from one end, and the outcoupled light was collected from the opposite end. When the excitation beam is focused at one end, EPs would propagate forward and backward with respect to the direction of the electric field along the wire axis. (B) Photoluminescence microscopy image of the device and a typical wire for the optical measurements. The white circles mark the outcoupled emissions from the wire tip without and with forward and backward electric fields (1.0 × 106 V/m), respectively, which indicates that the forward propagation of the waveguided emission along the field increases, and the backward propagation decreases in comparison with that in the absence of the field. (C) Corresponding outcoupled emission spectra collected from the tips of the wire (~34 μm) under applied fields of 0 and 1.0 × 106 V/m (forward and backward). (D) Plot of asymmetric ratio versus wavelength, which shows a good photon transport asymmetry over a large wavelength bandwidth (>200 nm).

  • Fig. 3 Principle and modulation of the asymmetric photon transport.

    (A) Schematic of the exciton diffusion in the absence and presence of an external electric field. With respect to the exciton diffusion without the electric field effect, the applied electric field would alter the exciton diffusion ability through the extra interaction potential Vext = −μe·E, resulting in the increase or decrease in the local potential. (B) Schematic depiction of the spatial relationship between BPEA molecular transition dipole moment (blue arrows) and the growth direction (red arrow) of the nanowire. The projection of the transition dipole (light blue arrow) leans at an angle of 45° to the long wire axis. (C) Plots of forward and backward photoluminescence intensity modulations at 610 nm versus α. (D) Iforward/Ibackward versus α at an electric field strength of 1.0 × 106 V/m. The line is fitted with the cos(α − 45°) function. Error bars represent the SD of three representative measurements.

  • Fig. 4 Design and realization of SPDT optical switch based on the asymmetric photon transport.

    (A and B) Configurations of a dual-output device in the absence (A) and presence (B) of an electric field. Insets: Photoluminescence images of a wire with α = 45°. The two tips (marked with white circles) are monitored as the output ports O1 and O2. WG, waveguide. (C) Outcoupled spectra of O1 and O2 under different field strengths ranging from 0 to 1.0 × 106 V/m. Insets: Linear fits of the outcoupling modulation from ports O1 and O2 versus the electric field strength. (D) Schematic illustration of the SPDT optical switches that can be controlled by operating the electric field. (E) Cyclic on/off switching behavior, which is changed by repeatedly altering the direction of electric field. (F) Series of light pulses with response time of 4.2 ns generated by electrical pulses with a rise time of 3.8 ns. (G) Temporal intensity profiles of O1 and O2 with 10-MHz electrical pulses.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/3/eaap9861/DC1

    section S1. Calculation of BPEA excitons

    section S2. Preparation and structural characterization of BPEA nanowires

    section S3. Formation of EPs in the BPEA nanowires

    section S4. Structure of the device

    section S5. Diffusion of the excitons under the applied electric field

    section S6. Electric effect on the passively waveguided light

    section S7. Orientation of BPEA excitons in the molecule-stacked nanostructures

    section S8. Switching speed and switching frequency measurements for the electrically controlled SPDT optical switch

    section S9. Schematic of the experimental measurements

    fig. S1. Molecular structure of BPEA.

    fig. S2. Scanning electron microscopy (SEM) image of the BPEA nanowires.

    fig. S3. Absorption and fluorescence (solid) spectra of BPEA powder (black) and BPEA nanowires (red).

    fig. S4. Atomic force microscope (AFM) characterization for a single BPEA nanowire.

    fig. S5. XRD patterns of the BPEA nanowires (black) and a monoclinic powder sample (red).

    fig. S6. TEM image of BPEA nanowire and SAED patterns collected from different areas of a single wire.

    fig. S7. Thermodynamically stable molecular packing in the BPEA nanowire.

    fig. S8. Formation of EPs in the BPEA nanowires.

    fig. S9. Output spectra from the two ends of the nanowire in Fig. 1C when the excitation is located in the middle of the wire.

    fig. S10. SEM image of a typical device.

    fig. S11. Calculated results of the asymmetric distribution of exciton density.

    fig. S12. Electric effect on the passively waveguided light.

    fig. S13. Spatial relationship between the BPEA molecular transition dipole moment (blue arrow) and the [010] growth direction (red arrow) of the BPEA nanowire.

    fig. S14. Polarization angle–dependent photoluminescence measurements.

    fig. S15. Switching speed measurements for the optical SPDT switch.

    fig. S16. Temporal intensity profiles of O1 and O2 ports in the device shown in Fig. 4 obtained by increasing the frequency of the electric signal to ~13 MHz.

    fig. S17. Schematic demonstration of the experimental setup for the steady-state optical measurement.

    fig. S18. Schematic demonstration of the response time and switching frequency measurement.

    References (3649)

  • Supplementary Materials

    This PDF file includes:

    • section S1. Calculation of BPEA excitons
    • section S2. Preparation and structural characterization of BPEA nanowires
    • section S3. Formation of EPs in the BPEA nanowires
    • section S4. Structure of the device
    • section S5. Diffusion of the excitons under the applied electric field
    • section S6. Electric effect on the passively waveguided light
    • section S7. Orientation of BPEA excitons in the molecule-stacked nanostructures
    • section S8. Switching speed and switching frequency measurements for the electrically controlled SPDT optical switch
    • section S9. Schematic of the experimental measurements
    • fig. S1. Molecular structure of BPEA.
    • fig. S2. Scanning electron microscopy (SEM) image of the BPEA nanowires.
    • fig. S3. Absorption and fluorescence (solid) spectra of BPEA powder (black) and BPEA nanowires (red).
    • fig. S4. Atomic force microscope (AFM) characterization for a single BPEA nanowire.
    • fig. S5. XRD patterns of the BPEA nanowires (black) and a monoclinic powder sample (red).
    • fig. S6. TEM image of BPEA nanowire and SAED patterns collected from different areas of a single wire.
    • fig. S7. Thermodynamically stable molecular packing in the BPEA nanowire.
    • fig. S8. Formation of EPs in the BPEA nanowires.
    • fig. S9. Output spectra from the two ends of the nanowire in Fig. 1C when the excitation is located in the middle of the wire.
    • fig. S10. SEM image of a typical device.
    • fig. S11. Calculated results of the asymmetric distribution of exciton density.
    • fig. S12. Electric effect on the passively waveguided light.
    • fig. S13. Spatial relationship between the BPEA molecular transition dipole moment (blue arrow) and the 010 growth direction (red arrow) of the BPEA nanowire.
    • fig. S14. Polarization angle–dependent photoluminescence measurements.
    • fig. S15. Switching speed measurements for the optical SPDT switch.
    • fig. S16. Temporal intensity profiles of O1 and O2 ports in the device shown in Fig. 4 obtained by increasing the frequency of the electric signal to ~13 MHz.
    • fig. S17. Schematic demonstration of the experimental setup for the steady-state optical measurement.
    • fig. S18. Schematic demonstration of the response time and switching frequency measurement.
    • References (36–49)

    Download PDF

    Files in this Data Supplement:

Stay Connected to Science Advances

Navigate This Article