Periodic Reporting for period 2 - Ne2DeM (Creating the new generation of 2D light emitters)
Reporting period: 2021-08-01 to 2023-01-31
Semiconductor nanocrystals (NCs) have size tuneable optical features. Among them, nanoplatelets (NPLs) are 2D NCs with especially well controlled optical features. As a result, these are the semiconductor nanoparticles with the narrowest luminescence signal, which make them very appealing for light emission application and in particular large gamut displays. In the Ne2DeM project, the NPLs will be used as a platform for the design of a new generation of light emitters which applications span from display lightning, to infrared sensing or 3D imaging displays.
The development of a new tool box based on NPLs will be the main ambition of the Ne2DeM project. More specifically I intend to develop three new functionalities to NPLs dedicated to light emission. First, I aim to synthesize particles with dual green and red narrow emissions. This goal relies on the design of a new highly complex 2D heterostructure that will be used as light down converter for white light production with an easier integration than the current technology or materials. The second objective will relate to expanding the wavelength range in the infrared where light can be emitted from NPLs while preserving narrow and fast luminescence. Such short-wave infrared source will be suited in applications such as biolabeling or active imaging. Finally, I intend to develop NPL with circularly polarized emission. To do so, I will develop enantioselective synthesis of 2D NPLs and control the folding of these lasts. This provides high potential for 3D imaging display.
The targeted materials are highly promising to bring back Europe into the market of NC for light emission, while it remains currently uninvolved at the industrial scale. Beyond the development of a new generation of NCs, Ne2DeM also aims to develop new original method for in-situ investigation of the NC growth and to investigate the development of materials electronic structure to fasten their integration into devices.
The development of a new tool box based on NPLs will be the main ambition of the Ne2DeM project. More specifically I intend to develop three new functionalities to NPLs dedicated to light emission. First, I aim to synthesize particles with dual green and red narrow emissions. This goal relies on the design of a new highly complex 2D heterostructure that will be used as light down converter for white light production with an easier integration than the current technology or materials. The second objective will relate to expanding the wavelength range in the infrared where light can be emitted from NPLs while preserving narrow and fast luminescence. Such short-wave infrared source will be suited in applications such as biolabeling or active imaging. Finally, I intend to develop NPL with circularly polarized emission. To do so, I will develop enantioselective synthesis of 2D NPLs and control the folding of these lasts. This provides high potential for 3D imaging display.
The targeted materials are highly promising to bring back Europe into the market of NC for light emission, while it remains currently uninvolved at the industrial scale. Beyond the development of a new generation of NCs, Ne2DeM also aims to develop new original method for in-situ investigation of the NC growth and to investigate the development of materials electronic structure to fasten their integration into devices.
During the firorkpackagest 30 months of the project, 4 students and I have dedicated our research to the three work-packages (WPs). Besides, we have written a Chemical review on the II-VI semiconductor nanoplatelets ( NPLs).
WP 1: We have demonstrated that native carboxylate ligands of semicondcutor NPLs can be replaced by halides followed by a partial dissolution of cadmium chalcogenide NPLs at the edges. The released monomers then recrystallize on the wide top and bottom facets, leading to an increase in NPL thickness. This dissolution/recrystallization method is used to increase NPL thickness to 9 MLs while using 3 ML NPLs as the starting material. We also demonstrate that this method is not limited to CdSe and can be extended to CdS and CdTe to grow thick NPLs. When a metal halide precursor is introduced with a chalcogenide precursor on the NPLs, CdSe/CdSe, CdTe/CdTe, CdSe/CdTe core/shell homo- and heterostructures are achieved. Finally, when an incomplete layer is grown, NPLs with steps are synthesized. These stress-free homostructures are comparable to type I heterostructures, leading to recombination of the exciton in the thicker area of the NPLs. Following the growth of core/crown and core/shell NPLs, it affords a new degree of freedom for the growth of structured NPLs with designed band engineering. This work represents one of the objectives of the first WP. The main objective was also to show dual emission coming from one single nanoparticle. We have shown the design of CdSe/CdTe/CdSe core/crown/crown heterostructures enables to localize the exciton either in the external crown, leading to a green emission or at the interface between CdSe/CdTe leading to a red emission. The ratio between these two emission can be tuned through the lateral dimensions of the NPLs or the excitation power. Besides, we have demonstrated that these NPLs could be inserted in LEDs.
WP 2: We have worked on the comparison between HgTe NPLs and low-energy-absorption HgTe nanocrystals and probed the optical transmission of HgTe nanoparticles over the 0.26–1.8 eV range, from 0 to 300 K temperatures and under simultaneous pressure, up to 4 GPa. While the pressure dependence of NPLs follows the one observed for bulk and nanocrystals, the temperature dependence dramatically differs for NPLs. The modeling of the electronic energy dispersion using up to 14-band k.p formalism suggests that the second conduction band and higher bands of HgTe play a vital role in describing and explaining the HgTe nanoparticle spectroscopies. In the objective to get narrow NIR emitters, we have shown that cation exchange with mercury could be performed on alloy CdSeTe NPLs. The choices of solvent, temperature, precursors and ligands have their own importance. So, we have demonstrated the synthesis of HgSeTe NPLs with tunable optical properties depending on the composition of the alloy and that emit in the NIR range. We are now working and the control of cation exchange on 2D particles with the surface chemistry. When ligands bring a stress in the direction of the targeted material lattice parameter, the 2D shapes of the NPLs is preserved. We are showing that the kinetic of cation exchange is a key parameter to preserve the 2D shape of particles. We are also working on further cation exchange to synthesize NPLs with confinement in 1D with optical properties in the NIR range. These two objectives were in this WP.
WP 3: NPLs are not only extremely thin but their thickness present a subatomic roughness all along the particles with lateral dimensions extended up to few hundreds nm. The crystal structure of the NPLs in ad-equation with the wide facets orientation induces the assembly of organic ligands over thousands of nm2. And, in order to release the surface stress brought by these ligands, NPLs fold as helices. We are working on the control of the NPLs helices radii through the ligands described as an anchoring group and an aliphatic chain of a given length. A mechanical model accounting for the misfit strain between the inorganic core and the surface ligands enables to predict the helices radii. This work represents one of the objectives of the third WP. We are now dedicated our work to the optical properties of these folding NPLs. Besides, we are studying the effect of NPLs shape and thickness with various chiral ligands on the circular dichroism properties of these particles.
These results lead to 7 main publications.
WP 1: We have demonstrated that native carboxylate ligands of semicondcutor NPLs can be replaced by halides followed by a partial dissolution of cadmium chalcogenide NPLs at the edges. The released monomers then recrystallize on the wide top and bottom facets, leading to an increase in NPL thickness. This dissolution/recrystallization method is used to increase NPL thickness to 9 MLs while using 3 ML NPLs as the starting material. We also demonstrate that this method is not limited to CdSe and can be extended to CdS and CdTe to grow thick NPLs. When a metal halide precursor is introduced with a chalcogenide precursor on the NPLs, CdSe/CdSe, CdTe/CdTe, CdSe/CdTe core/shell homo- and heterostructures are achieved. Finally, when an incomplete layer is grown, NPLs with steps are synthesized. These stress-free homostructures are comparable to type I heterostructures, leading to recombination of the exciton in the thicker area of the NPLs. Following the growth of core/crown and core/shell NPLs, it affords a new degree of freedom for the growth of structured NPLs with designed band engineering. This work represents one of the objectives of the first WP. The main objective was also to show dual emission coming from one single nanoparticle. We have shown the design of CdSe/CdTe/CdSe core/crown/crown heterostructures enables to localize the exciton either in the external crown, leading to a green emission or at the interface between CdSe/CdTe leading to a red emission. The ratio between these two emission can be tuned through the lateral dimensions of the NPLs or the excitation power. Besides, we have demonstrated that these NPLs could be inserted in LEDs.
WP 2: We have worked on the comparison between HgTe NPLs and low-energy-absorption HgTe nanocrystals and probed the optical transmission of HgTe nanoparticles over the 0.26–1.8 eV range, from 0 to 300 K temperatures and under simultaneous pressure, up to 4 GPa. While the pressure dependence of NPLs follows the one observed for bulk and nanocrystals, the temperature dependence dramatically differs for NPLs. The modeling of the electronic energy dispersion using up to 14-band k.p formalism suggests that the second conduction band and higher bands of HgTe play a vital role in describing and explaining the HgTe nanoparticle spectroscopies. In the objective to get narrow NIR emitters, we have shown that cation exchange with mercury could be performed on alloy CdSeTe NPLs. The choices of solvent, temperature, precursors and ligands have their own importance. So, we have demonstrated the synthesis of HgSeTe NPLs with tunable optical properties depending on the composition of the alloy and that emit in the NIR range. We are now working and the control of cation exchange on 2D particles with the surface chemistry. When ligands bring a stress in the direction of the targeted material lattice parameter, the 2D shapes of the NPLs is preserved. We are showing that the kinetic of cation exchange is a key parameter to preserve the 2D shape of particles. We are also working on further cation exchange to synthesize NPLs with confinement in 1D with optical properties in the NIR range. These two objectives were in this WP.
WP 3: NPLs are not only extremely thin but their thickness present a subatomic roughness all along the particles with lateral dimensions extended up to few hundreds nm. The crystal structure of the NPLs in ad-equation with the wide facets orientation induces the assembly of organic ligands over thousands of nm2. And, in order to release the surface stress brought by these ligands, NPLs fold as helices. We are working on the control of the NPLs helices radii through the ligands described as an anchoring group and an aliphatic chain of a given length. A mechanical model accounting for the misfit strain between the inorganic core and the surface ligands enables to predict the helices radii. This work represents one of the objectives of the third WP. We are now dedicated our work to the optical properties of these folding NPLs. Besides, we are studying the effect of NPLs shape and thickness with various chiral ligands on the circular dichroism properties of these particles.
These results lead to 7 main publications.
My teams and I are working on the three different workpackages in order to synthesis NPLs with dual emission, NPLs with narrow optical properties in the near IR range and NPLs with circularly polarized optical properties.
We have recently shown that even in homostructured NPLs, we could specifically localized exciton in thicker areas of the NPLs. Joining these results with the right choice of semicondcutors, we will be able to synthesis NPLs with dual narrow emission. Our second objective is to expand optical properties of NPLs to the infrared while preserving narrow and fast luminescence. Different strategies will be explored but after having shown that CdSe NPLs could be grown in the thickness, we plan to expand this strategy with mercury chalcogenides NPLs. Such short-wave infrared source will be suited for active imaging. Finally, We will synthesize NPLs with circularly polarized optical properties while carefully choosing precursors to induce enantioselective growth of nanoparticles.
We have recently shown that even in homostructured NPLs, we could specifically localized exciton in thicker areas of the NPLs. Joining these results with the right choice of semicondcutors, we will be able to synthesis NPLs with dual narrow emission. Our second objective is to expand optical properties of NPLs to the infrared while preserving narrow and fast luminescence. Different strategies will be explored but after having shown that CdSe NPLs could be grown in the thickness, we plan to expand this strategy with mercury chalcogenides NPLs. Such short-wave infrared source will be suited for active imaging. Finally, We will synthesize NPLs with circularly polarized optical properties while carefully choosing precursors to induce enantioselective growth of nanoparticles.