Double-crowned 2D semiconductor nanoplatelets with bicolor power-tunable emission

nature communications Article https://doi.org/10.1038/s41467-022-32713-2 Double-crowned 2D semiconductor nanoplatelets with bicolor power-tunable emission Received: 21 April 2022 Check for updates 1234567890():,; 1234567890():,; Accepted: 10 August 2022 Corentin Dabard1, Victor Guilloux2, Charlie Gréboval2, Hong Po1, Lina Makke1, Ningyuan Fu 1, Xiang Zhen Xu1, Mathieu G. Silly 3, Gilles Patriarche 4, Emmanuel Lhuillier 2, Thierry Barisien2, Juan I. Climente5, Benjamin T. Diroll6 & Sandrine Ithurria1 Nanocrystals (NCs) are now established building blocks for optoelectronics and their use as down converters for large gamut displays has been their first mass market. NC integration relies on a combination of green and red NCs into a blend, which rises post-growth formulation issues. A careful engineering of the NCs may enable dual emissions from a single NC population which violates Kasha’s rule, which stipulates that emission should occur at the band edge. Thus, in addition to an attentive control of band alignment to obtain green and red signals, non-radiative decay paths also have to be carefully slowed down to enable emission away from the ground state. Here, we demonstrate that core/ crown/crown 2D nanoplatelets (NPLs), made of CdSe/CdTe/CdSe, can combine a large volume and a type-II band alignment enabling simultaneously red and narrow green emissions. Moreover, we demonstrate that the ratio of the two emissions can be tuned by the incident power, which results in a saturation of the red emission due to non-radiative Auger recombination that affects this emission much stronger than the green one. Finally, we also show that dualcolor, power tunable, emission can be obtained through an electrical excitation. Semiconductor nanocrystals (NCs) are nanoparticles with size-tunable optical features thanks to quantum confinement. Beyond the ease to induce a spectral shift, NCs also offer a narrow photoluminescence (PL) signal resulting from low ensemble polydispersity. This property is of utmost interest for the design of down converters for displays. Currently, a quantum dot display relies on a blue light-emitting diode (LED) based on InGaN quantum wells used to excite two populations of NCs emitting in the green and the red. The combination of these three colors is used to generate white light that is later filtered through a liquid crystal filter to generate red, green, and blue pixels. The human eye is most sensitive to green light. For this reason, in this spectral range, a limited change of the spectral linewidth drastically affects the color gamut (i.e., the color palette), while for blue and red the linewidth appears less critical. Thus, designing NCs with narrow green emission is essential to achieve high-quality displays. Green emissions can easily be obtained from III-V (using InP) and II-VI (using CdSe) semiconductors. However, using spherical particles, the PL linewidth of CdSe remains above 25 nm and even higher using 1 Laboratoire de Physique et d’Etude des Matériaux, ESPCI-Paris, PSL Research University, Sorbonne Université Univ Paris 06, CNRS UMR 8213, 10 rue Vauquelin, 75005 Paris, France. 2Sorbonne Université, CNRS, Institut des NanoSciences de Paris, INSP, F-75005 Paris, France. 3Synchrotron SOLEIL, L’Orme des Merisiers, Départementale 128, 91190 Saint-Aubin, France. 4Centre de Nanosciences et de Nanotechnologies, CNRS, Université Paris-Saclay, C2N, Palaiseau 2110, France. 5Departament de Quimica Fisica i Analitica, Universitat Jaume I, E-12080 Castello de la Plana, Spain. 6Center for Nanoscale Materials, e-mail: Argonne National Laboratory, Lemont, IL 60439, US. Nature Communications | (2022)13:5094 1 Article https://doi.org/10.1038/s41467-022-32713-2 InP. CdSe 2D nanoplatelets1–3 (NPLs), thanks to a specific growth mechanism4–6, offer the narrowest PL linewidth among NCs. The 2D growth enables atomic control of the thickness, its only confined direction. As a result, the PL linewidth is limited by homogeneous broadening7. When they are grown with a thickness of 4.5 monolayers (MLs) (i,e., four planes of Se sandwiched by five planes of cadmium), their PL emission presents a maximum at ~510 nm at room temperature, close to the optimal green wavelength for displays, see Fig. 1d. As stated before, the current NC display technology relies on the blend of two populations of NCs embedded into a transparent matrix whose role is to protect the NCs from oxidation, and also to extract heat generated by sub unity PL quantum yield. To maintain long-term stability, this matrix is typically covalently bound to the NC surface. Integration of two NC populations (i.e., one green and one red) makes this chemical coupling more complex. It is thus of utmost interest to design a single NC that (i) combines both emissions, while (ii) presenting a narrow green PL linewidth. In the past, several works have reported bicolor emission from NCs8–12, however, none of these previous works combine the two properties simultaneously. These two constrains require the design of a system that does not obey Kasha’s rule which states that the emission should occur through the lowest excited state due to fast thermalization of the hot carriers. To do so, the non-radiative decay paths and the carrier cooling need to be drastically slowed down. NPLs, with their large volume, appear as promising candidates13,14. NPLs offer a nice playground for quantum engineering of excitonic transitions through heterostructure design, especially with CdSe core/crown geometry12,13 where a second semiconductor is laterally extended around the CdSe core without modification of the thickness. The initial confinement of the NPL is maintained and can thus be used to generate a narrow green emission. Few reports mention bicolor emission from NPLs. Dufour et al. designed a CdSe/CdSeTe core/crown heterostructure presenting two distinct emissions9, however the two orange and red emissions were spectrally overlapping. Khan et al. designed a core/barrier/crown NPL15 as a photon up converter with both green and red emissions. However, the red light was always largely prevailing over the green emission. This latter work suggests that in addition to the combination of a red and a narrow green emission, the two emissions need to be relatively close in magnitude and possibly tunable. Here, we design a single NPL emitter combining the narrow green emission together with a red emission, with tunable relative magnitude of the two peaks. We demonstrate that the introduction of a small CdTe belt in a large CdSe NPL forming a CdSe core/ CdTe crown/CdSe crown heterostructure fulfills all the targeted objectives. We start by discussing the geometry of such NPLs and present the optical transitions involved. We then investigate, experimentally and theoretically, the power-dependence of the spectrum. Finally, we demonstrate that bicolor, power-tunable electroluminescence can also be obtained from this core/crown/crown NPL speci (...truncated)