Chloride-Induced Thickness Control in CdSe Nanoplatelets

Abstract

Current colloidal synthesis methods for CdSe nanoplatelets (NPLs) routinely yield samples that emit, in discrete steps, from 460 to 550 nm. A significant challenge lies with obtaining thicker NPLs, to further widen the emission range. This is at present typically achieved via colloidal atomic layer deposition onto CdSe cores, or by synthesizing NPL core/shell structures. Here, we demonstrate a novel reaction scheme, where we start from 4.5 monolayer (ML) NPLs and increase the thickness in a two-step reaction that switches from 2D to 3D growth. The key feature is the enhancement of the growth rate of basal facets by the addition of CdCl, resulting in a series of nearly monodisperse CdSe NPLs with thicknesses between 5.5 and 8.5 ML. Optical characterization yielded emission peaks from 554 nm up to 625 nm with a line width (fwhm) of 9-13 nm, making them one of the narrowest colloidal nanocrystal emitters currently available in this spectral range. The NPLs maintained a short emission lifetime of 5-11 ns. Finally, due to the increased red shift of the NPL band edge photoluminescence excitation spectra revealed several high-energy peaks. Calculation of the NPL band structure allowed us to identify these excited-state transitions, and spectral shifts are consistent with a significant mixing of light and split-off hole states. Clearly, chloride ions can add a new degree of freedom to the growth of 2D colloidal nanocrystals, yielding new insights into both the NPL synthesis as well as their optoelectronic properties.

Record High External Quantum Efficiency of 19.2% Achieved in Light‐Emitting Diodes of Colloidal Quantum Wells Enabled by Hot‐Injection Shell Growth

“…Semiconductor colloidal quantum wells (CQWs), also commonly nicknamed as nanoplatelets, have emerged as a new, highly promising family of optoelectronic materials over the last decade . Thanks to their strong quantum confinement solely in the vertical direction, 2D CQWs possess many unique thickness‐dependent optical characteristics including ultranarrow emission with suppressed inhomogeneous broadening, giant oscillator strengths, extraordinarily large linear and nonlinear absorption cross‐sections, and molar extinction coefficients . All these superior properties render CQWs to offer great potential for optoelectronic applications including solar energy harvesting, lasing, and light‐emitting diodes (LEDs) …”

“…[1][2][3][4][5][6][7][8] Thanks to their strong quantum confinement solely in the vertical direction, 2D CQWs possess many unique thickness-dependent optical characteristics including ultranarrow emission with suppressed inhomogeneous broadening, giant oscillator strengths, extraordinarily large linear and nonlinear absorption cross-sections, and molar extinction coefficients. [9][10][11][12][13][14] All these superior properties render CQWs to offer great potential for optoelectronic applications including solar energy harvesting, lasing, and light-emitting diodes (LEDs). [15][16][17][18] CQW-LEDs are highly promising for display and lighting applications, owing to their high color purity with ultranarrow full-width at half-maximum (FWHM), easy solution-processed fabrication procedures, and good compatibility with flexible electronics.…”

A comparative study demonstrates strong size tunability of carrier–phonon coupling in CdSe-based 2D and 0D nanocrystals

“…First predictions have suggested strong tunability of the emission spectra and decay times for platelets and their heterostructures. [24][25][26][27] Applications of these nanoparticles for efficient field effect devices 28 or strong electro-absorption response 22,29,30 have been demonstrated.…”

Brightly Luminescent Core/Shell Nanoplatelets with Continuously Tunable Optical Properties

“…One major limitation of CdSe‐based NPLs is the discreteness of their optical properties for thicknesses from two to five MLs. Here we note that during preparation of this paper, Moreels and co‐workers reported the first successful synthesis of CdSe NPLs with thickness exceeding 5 MLs, i.e., 6, 7, and 8 ML thick plates, achieved by switching the growth from 2D to 3D in the presence of cadmium chloride . Notwithstanding this achievement, the resulting optical properties, in particular the PLQYs, especially of red‐emitting samples, call for further improvement.…”