Tailoring hot-exciton emission and lifetimes in semiconducting nanowires via whispering-gallery nanocavity plasmons

Journal name:
Nature Materials
Volume:
10,
Pages:
669–675
Year published:
DOI:
doi:10.1038/nmat3067
Received
Accepted
Published online

The manipulation of radiative properties of light emitters coupledwith surface plasmons is important for engineering new nanoscale optoelectronic devices, including lasers, detectorsand single photon emitters1, 2, 3, 4, 5, 6, 7, 8. However, so far the radiative rates of excited states in semiconductors and molecular systems have been enhanced only moderately, typically by a factor of 10–50, producing emission mostly from thermalizedexcitons2, 6, 9, 10, 11. Here, we show the generation of dominant hot-exciton emission, that is, luminescence from non-thermalized excitons that are enhanced by the highly concentrated electromagnetic fields supported by the resonant whispering-gallery plasmonic nanocavities of CdSSiO2Ag core–shell nanowire devices. By tuning the plasmonic cavity size to match the whispering-gallery resonances, an almost complete transition from thermalized exciton to hot-exciton emission can be achieved, which reflects exceptionally high radiative rate enhancement of >103 and sub-picosecond lifetimes. Core–shell plasmonic nanowires are an ideal test bed for studying and controlling strong plasmon–exciton interaction at the nanoscale and opens new avenues for applications in ultrafast nanophotonic devices.

At a glance

Figures

  1. Core–shell nanowire plasmonic nanocavity.
    Figure 1: Core–shell nanowire plasmonic nanocavity.

    a, Schematic of the CdSAg core–shell structure separated by the SiO2 interlayer, where the CdS nanowire core provides the semiconductor excitons and the Ag nanoshell supports plasmon cavity modes. b, Transmission electron microscope image showing the 5 nm SiO2 layer on the CdS nanowire produced by atomic layer deposition. Red lines indicate the thickness of the SiO2 layer. c, Transmission electron microscope image of the fabricated CdSSiO2Ag core–shell structure. Red lines indicate the 15 nm Ag shell thickness. d, Elemental mapping image showing the highly conformal coating of SiO2 and Ag on the CdS nanowire measured by energy-dispersive X-ray spectroscopy.

  2. Hot-exciton emission from plasmonic nanowires.
    Figure 2: Hot-exciton emission from plasmonic nanowires.

    a, Photoluminescence spectra (at 77 K) of the photonic (blue) and plasmonic (magenta) nanowire (NW) with core CdS diameters of 115 nm and 140 nm respectively. Hot-exciton emission lines, which appear as a progression, are indicated by nLO, where n is the number of participating LO phonons. b, Schematic diagram of the exciton dispersion showing the relaxation and emission processes for the normal exciton (blue arrows) and the hot-exciton (magenta arrows) emission, where EL is the energy of laser excitation. c,d, Calculated field distribution of the magnetic field intensity for the photonic (c) and plasmonic (d) nanowires (d=140 nm) at the 4LO energy of 2.556 eV. The plasmonic nanowire shows the whispering-gallery plasmon cavity mode, which is absent for the photonic nanowire. The white circles indicate the interfaces of the core–shell structure. The cavity mode profile of the electric field intensity is given in Supplementary Fig. S5. e, Time-resolved photoluminescence spectral map from an ensemble of 300–500 plasmonic nanowires (average CdS core diameter, 140±50 nm) at 300 K. f, Time-resolved integrated emission intensity for plasmonic (upper panel) and photonic (lower panel) nanowires. Solid lines indicate dominant exponential decay lifetimes of 7 ps and 1,600 ps, respectively.

  3. Temperature- and polarization-dependent properties of the hot-exciton emission.
    Figure 3: Temperature- and polarization-dependent properties of the hot-exciton emission.

    a, Temperature-dependent quenching of the emission intensity for the hot exciton (3LO peaks) and free A-exciton, which are measured for the plasmonic (d=140 nm) and photonic (d=150 nm) nanowires, respectively. The intensities are normalized to the values obtained at 77 K for each nanowire. b, Temperature-dependent photoluminescence spectra for the plasmonic nanowire, showing the resonance enhancement between the 4LO hot exciton and free B-exciton. c,d, Polarization properties of the normalized emission intensity as a function of the rotation angle, θ, of the linear polarizer for the photonic (c) and plasmonic (d) nanowires, where the rotation angle is defined as the angle from the nanowire longitudinal axis. The nanowire axis is indicated by the purple horizontal line and the c axis is tilted at an angle of ~55° from the nanowire axis for both wires. The excitation laser is circularly polarized and the linear polarizer is rotated in front of the CCD detector.

  4. Size-dependent properties of the whispering-gallery plasmon nanocavity.
    Figure 4: Size-dependent properties of the whispering-gallery plasmon nanocavity.

    a, Relative enhancement of the 4LO hot-exciton transition as a function of CdS nanowire diameter. The calculated electric field intensity per unit area inside the cavity is plotted together with the measured enhancement of 4LO hot-exciton emission, where the electric field density is normalized by the maximum value obtained for d=60 nm. b, Photoluminescence spectra for four representative plasmonic nanowires with different sized whispering-gallery plasmon nanocavities. All the spectra are normalized to their free A-exciton intensity. ce, Calculated field profiles for the resonant cases at the 4LO energy of 2.556 eV, showing the whispering-gallery plasmon cavity modes for CdS d=60 nm(azimuthal mode number, m=2) (c), 100 nm (m=3) (d), and 135 nm (m=4) (e). The white circles refer to the interfaces of the CdSSiO2Ag core–shell structure.

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Affiliations

  1. Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA

    • Chang-Hee Cho,
    • Carlos O. Aspetti,
    • Sung-Wook Nam &
    • Ritesh Agarwal
  2. Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA

    • Michael E. Turk &
    • James M. Kikkawa

Contributions

C-H.C. and R.A. developed the concept and design of the devices. C-H.C. carried out the device fabrication and steady-state optical measurements. C.O.A. performed the numerical simulations. M.E.T. and J.M.K. performed the time-resolved photoluminescence measurements. S-W.N. performed the transmission electron microscope measurement. C-H.C., C.O.A. and R.A. analysed the results and wrote the manuscript.

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The authors declare no competing financial interests.

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