Halide-Assisted Synthesis of Cadmium Chalcogenide Nanoplatelets

Abstract

Atomically flat colloidal semiconductor CdSe nanoplatelets (NPLs) with precisely controlled thickness possess a range of unique optoelectronic properties. Here, we study the growth of CdSe, CdTe, and CdS NPLs with the aim of synthesizing thicker NPLs in order to extend their optical activity further into the lower energy/larger wavelength range. We employ cadmium halides, which lead to faster reaction kinetics as confirmed by control experiments with cadmium hydroxide as a Cd-precursor. Addition of halides in all cases led to the formation of thicker NPL species, as compared with the corresponding syntheses without these additives. Analysis of a recent theoretical model of the platelet growth mechanism confirms an earlier suggestion that reducing both the reaction enthalpy and the surface energy of CdSe, by replacing acetate ligands with chloride ions, should indeed lead to thicker NPLs as observed. We noticed a formation of Cd0-metal nanoparticles in the first stage of the synthesis by preparing the Cd-precursor, which is another key finding of our work. We assume that these particles can serve as an active cadmium source facilitating the growth of the NPLs. The resulting 6 ML CdSe NPLs exhibited bright photoluminescence with quantum yield of up to 50%, exceptionally narrow spectrum centered at 582 nm with full width at half-maximum of approx. 11 nm, and small Stokes shift of 2 nm. Moreover, we demonstrated the synthesis of heterostructured core/shell CdSe/CdS NPLs based on 6 ML CdSe platelets, which also exhibited bright fluorescence. This work shows the possibility to overcome energetic barrier limiting the size (thickness) control by using appropriate promoters of the growth of CdSe, CdTe, and CdS 2D structures.

CdS crown growth on CdSe nanoplatelets: core shape matters

“…We attribute this difference to the growth of a CdS crown, which is in part slightly thicker than 4 MLs, as this will result in additional light absorption at higher wavelengths than 405 nm. 21,43 The original emission wavelength, however, will remain nearly unchanged in this case, as the thickness of the CdSe core does not change and the conduction band offset between CdSe and CdS is still comparably small, so that the exciton binding energy cannot be overcome. 44 It needs to be noted, that under the applied reaction conditions the formation of a complete CdS layer on the top and bottom facets of the core NPLs is generally unlikely, as it requires higher reaction temperatures and/or an excess of the chalcogenide precursor.…”

“…1(a)]. Therefore, we applied two already published protocols 21,43 as well as a new one inspired by the procedure for CdTe (CdSe) growth on CdSe NPLs. 32 In the procedure introduced by Tessier et al, 21 a pre-prepared mixture of oleic acid, cadmium acetate and sulphur in ODE is injected at a xed rate into a solution of the CdSe core NPLs in ODE at the reaction temperature.…”

“…32 In the procedure introduced by Tessier et al, 21 a pre-prepared mixture of oleic acid, cadmium acetate and sulphur in ODE is injected at a xed rate into a solution of the CdSe core NPLs in ODE at the reaction temperature. Meerbach et al, 43 in contrast, added a S/ ODE solution at a slow injection rate whereas cadmium acetate and oleic acid were present in the reaction ask. Our new protocol is similar to the latter one, with the difference, that TOPS/ODE instead of S/ODE was applied as the sulphur precursor.…”

“…In addition to the described protocol, two literature procedures were applied for the CdS crown growth. 21,43 The reaction parameters of all three protocols are compared in Table 1.…”

Dielectric Screening Modulates Semicondcutor Nanoplatelet Excitons

“…[0][1][2][3][4] Another appealing aspect of these NPLs is the atomic precision over their growth anisotropy: II-VI cadmium chalcogenide NPLs can be synthesized with precisely 2-11 monolayer (ML) thicknesses, and show increasing control over their 2D lateral extent. [5][6][7][8][9] Such structures can also template other II-VI NPLs, including core-shell and core-crown Cd/Zn S/Se/Te heterostructures and mercury chalcogenide infrared emitters, which demonstrate comparable synthetic control. [10][11][12] With increased tunability of the NPL thickness, colloidal 2D materials warrant further exploration of the extent of synthetic modulation over their photophysical properties and device applicability.…”