Revisiting the Role of Metallic Antennas to Control Light Emission by Lead Salt Nanocrystal Assemblies

Hongyue Wang, Abdelhanin Aassime, Xavier Le Roux, Nick J. Schilder, Jean-Jacques Greffet, and Aloyse Degiron

Phys. Rev. Applied 10, 034042 – Published 20 September 2018

Abstract

Thin films of lead salt nanocrystals (NCs) offer attractive opportunities as active media for near-infrared optoelectronics but suffer from limiting trade-offs between optical and electrical properties. While NCs separated by nanometer-long ligands are good light emitters, NCs capped with shorter molecules provide a high carrier mobility but degrade the photo- and electroluminescence and broaden the narrow emission spectrum. Here we show that this severe quenching and spectral broadening can be averted with an unconventional use of metallic antennas. The resulting NC-antenna hybridization not only provides a strong boost in luminescence, but also makes it possible to remodel the emission spectrum in radical ways, even at wavelengths where the NC assembly does not emit light. These results cannot be explained with the standard theory of single-emitter luminescence assisted by optical antennas. We propose an alternative model based on a statistical description of light emission by an ensemble of emitters and discuss important consequences of our findings for nano-optics and solution-processed optoelectronics.

Received 22 May 2018
Revised 27 July 2018

DOI:https://doi.org/10.1103/PhysRevApplied.10.034042

Physics Subject Headings (PhySH)

Electroluminescence Nanoantennas Optoelectronics Plasmonics

Nanoparticles Quantum dots

Fluorescence

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Hongyue Wang¹, Abdelhanin Aassime¹, Xavier Le Roux¹, Nick J. Schilder², Jean-Jacques Greffet^2,*, and Aloyse Degiron^1,†

¹Centre de Nanosciences et de Nanotechnologies, CNRS, Univ. Paris-Sud, Université Paris-Saclay, C2N–Orsay, 91405 Orsay cedex, France
²Laboratoire Charles Fabry, Institut d’Optique Graduate School, CNRS, Université Paris-Saclay, 91127 Palaiseau Cedex, France

^*jean-jacques.greffet@institutoptique.fr
^†aloyse.degiron@u-psud.fr

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Vol. 10, Iss. 3 — September 2018

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Images

Figure 1
(a) Photoluminescence intensity of a NC film before and after ligand exchange (blue and red curves, respectively). The noisy shoulder between 1345 and 1480 nm is an experimental artifact due to the absorption lines of the atmosphere. (b) Absorbance of a NC film before and after ligand exchange [same color code as panel (a)]. The black curve represents the absorbance of the $Pb S$ nanocrystals in solution. The different traces are offset for clarity. (c) Current vs voltage between two $Au$ pads connected with $Pb S$ NCs. The distance between the two $Au$ pads is 20 µm and the $Pb S$ NCs are drop-cast in the slit between the pads. Blue curve: result before ligand exchange; red curve: result after ligand exchange. (d) Schematic view of the LEDs considered in the remainder of the study.
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Figure 2
(a) Reflectance of a LED operating with nonresonant $Au$ nanodiscs. The continuous trace is the experimental spectrum and the dotted line is the result of finite-element simulations. Inset: SEM view of the nanoparticle array. Scale bar: 500 nm. (b),(c) PL and EL spectra of the structure with $Au$ nanodiscs. The black trace on the PL plot is the spectrum of a reference LED without $Au$ nanodiscs. (d)–(f) Same as panels (a)–(c), except for a LED operating with resonant $Au$ discs and rings. (g) Light-voltage curves of the nonresonant and resonant structures considered in panels (a)–(f) (blue and red traces, respectively). Black curve: reference sample without $Au$ nanostructure. (h) Finite-element simulations of the nonresonant structure with $Au$ nanodiscs. Each plot represents the vertical component of the electric field in a unit cell at a different wavelength (1005, 1185, 1380, and 1565 nm). The structure is illuminated under normal incidence and the field is evaluated in a plane located 7.5 nm above the $Au$ disc (outlined in black). (i) Same as panel (h) for the resonant structure with $Au$ discs and rings. The field amplitude (in V m^-1) is roughly 3 times larger than for the nonresonant $Au$ nanodiscs.
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Figure 3
(a) Red curve: PL spectrum of a sample with resonant discs and rings [same parameters as in Fig. 2]. Gray and black curves: EL spectra of the sample biased at 2.5 and 3.5 V, respectively. Blue curve: EL spectrum at 7 V of a structure fabricated with the same specifications, except that the granularity of the ITO anode has been modified to hamper the injection of the electrons. All spectra are normalized between 0 and 1. (b) Result of the Wien radiance renormalization described in the text. (c) Black curve: same EL spectrum as the black trace in panel (a). Light-green curve: EL spectrum of a structure fabricated with the same specifications, except that the NCs have a smaller radius, shifting their bandgap by 200 nm to the blue. Dark-green curve: Wien radiance renormalization of the light-green trace.
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Figure 4
(a) Reflectance and EL of a LED operating with resonant $Au$ squares. Inset: SEM view of the $Au$ antenna array. Scale bar: 500 nm. (b) Reflectance and EL of a LED operating with resonant $Au$ nanorods. Orange (yellow) traces: signal measured through a polarizer aligned along the vertical (horizontal) axis of the rods. Inset: SEM view of the $Au$ antenna array. Scale bar: 500 nm. It should be noted that all the spectra of this figure are obtained with the same assemblies of ligand-exchanged NCs as those considered in Figs. 2, 3 and 3.
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