Among state-of-the-art nanotechnologies, semiconductor nanocrystals are one that successfully reached a mass market as light sources for displays. Compared to former lanthanide phosphor-based emitters, they offer a narrower spectral emission. This property is critical to improve the quality of the green color and thus to increase the gamut (ie the palette of color) of a display.
To date, nanocrystal-based displays rely on an InGaN LED used to generate blue light that excites two populations of green and red emitting nanocrystals. However, this approach faces two limitations. First, the emission in the green remains broad (around 40 nm) limiting the color gamut. Secondly, two populations presenting two distinct surface chemistries have to be blended into a transparent matrix, which raises formulation challenges.
Thus, researchers from ESPCI brought the idea that, by carefully engineering its structure, a single nanocrystal could manage the full down-conversion process from the blue light of an InGaN LED to the white light required for LCD displays. To reach this goal, they selected 2D nanocrystals (the so-called nanoplatelets: NPLs) that are the narrower emitters in the green among nanocrystals (around 1.2 kT of full width at half maximum). Using this geometry, they synthesized a planar heterostructure that breaks Kasha’s rule which generally states that light emission should only occur through the excited ground state. The design of such particle is complex in several aspects, but a careful engineering of the band alignment between the different semiconductors of the heterostructure can tackle most of those issues. In fact, by managing the localization of the charge carriers in the different parts of the heterostructure, they successfully obtained a particle that combines both green (510 nm) and red (640 nm) emissions, see the emission spectra in Figure 1b, while keeping an advantageous radiative to non-radiative decay path ratio.
Furthermore, they demonstrate that the green-to-red emission ratio can be adjusted by tuning the incident power, see Figure 1b. The total emission gets greener under higher excitation power, see Figure 1a. This enables to broadly tune the emitted color as shown on the chromaticity diagram in Figure 1c.
Figure 1 Power-dependent luminescent properties. a. Image of a test tube containing a dilute solution of CdSe/CdTe/CdSe core/crown/crown NPLs, illuminated by a blue 405 nm laser diode. The bottom of the tube acts as a lens and luminescence at the focal point is green while being red away from it. b. Photoluminescence spectra of CdSe/CdTe/CdSe core/crown/crown NPLs under various incident powers. c. Chromaticity diagram and positions of the PL signal resulting from the CdSe/CdTe/CdSe core/crown/crown NPLs as the incident power is increased.
Finally, the authors demonstrate that these bicolor emission properties are not limited to optical excitation and can be also obtained from electroluminescence, see Figure 2b. To do so the bicolor NPLs have been integrated into a diode stack such as the one depicted in Figure 2a
Figure 2 Electroluminescence from bicolor NPL a. Schematic of LED stack used to generate electroluminescence signals from the bicolor emitting NPLs. b. Electroluminescence spectra of bicolor NPLs. The inset is a picture of 8 pixels from the LED turned on.
Paper is now published in:
Dabard, C., Guilloux, V., Gréboval, C. et al. Double-crowned 2D semiconductor nanoplatelets with bicolor power-tunable emission. Nat Commun 13, 5094 (2022). https://doi.org/10.1038/s41467-022-32713-2