Ultrathin Highly Luminescent Two‐Monolayer Colloidal CdSe Nanoplatelets

Abstract

Surface effects in atomically flat colloidal CdSe nanoplatelets (NLPs) are significantly and increasingly important with their thickness being reduced to subnanometer level, generating strong surface related deep trap photoluminescence emission alongside the bandedge emission. Herein, colloidal synthesis of highly luminescent two-monolayer (2ML) CdSe NPLs and a systematic investigation of carrier dynamics in these NPLs exhibiting broad photoluminescence emission covering the visible region with quantum yields reaching 90% in solution and 85% in a polymer matrix is shown. The astonishingly efficient Stokes-shifted broadband photoluminescence (PL) emission with a lifetime of ≈100 ns and the extremely short PL lifetime of around 0.16 ns at the bandedge signify the participation of radiative midgap surface centers in the recombination process associated with the underpassivated Se sites. Also, a proof-of-concept hybrid LED employing 2ML CdSe NPLs is developed as color converters, which exhibits luminous efficacy reaching 300 lm W opt −1 . The intrinsic absorption of the 2ML CdSe NPLs (≈2.15 × 10 6 cm −1 ) reported in this study is significantly larger than that of CdSe quantum dots (≈2.8 × 10 5 cm −1 ) at their first exciton signifying the presence of giant oscillator strength and hence making them favorable candidates for next-generation light-emitting and light-harvesting applications.

Excitonic Spin-Coherence Lifetimes in CdSe Nanoplatelets Increase Significantly with Core/Shell Morphology

“…Upon continuing growth to a 3ZnS monolayer shell, lifetime decreases to ∼21 ps perhaps influenced as strain builds up with the thicker, small-lattice-spacing shell. Overall, the significant lifetime increase suggests that removal of the proximal organic surface termination or passivation of dangling surface bonds for the core/shell structure serves to increase spin lifetimes in both the CdSe/CdS and CdSe/ZnS systems, consistent with theoretical investigations of Rodina et al , Additionally, this finding suggests that the hole spin relaxation mechanism is not centrally important, as this charge remains contained within the CdSe core for the latter core/shell.…”

“…Notably, the 2CdSe does not fit the otherwise systematic trend, instead exhibiting similar dynamic behavior as 5CdSe, suggesting that other contributing factors in the generation of long-lived spin-polarized species impact the particularly thin 2CdSe NPLs. Thinner NPLs and especially 2CdSe experience appreciable strain in comparison to thicker NPLs that feature more internal bonding, which conceivably could disrupt the otherwise systematic trend. − At the same time, the >3 eV energy gap of 2CdSe may play a role if exciton wave functions begin to permeate the high potential barrier of the organic surface . This thickness dependence for core-only samples conveys lifetimes that are short and comparable to those recently reported for 2D perovskite spin-polarized lifetimes versus inorganic layer thickness .…”

Luminescence Dynamics of Single Self-Assembled Chains of Förster (FRET)-Coupled CdSe Nanoplatelets

“…Delayed emission was then attributed to electron trapping and detrapping. Slow emission from a second red-shifted level was also reported by some authors and attributed to hole trapping or surface trap sites …”

Chirality in Atomically Thin CdSe Nanoplatelets Capped with Thiol-Free Amino Acid Ligands: Circular Dichroism vs. Carboxylate Group Coordination

“…The obtained absorption spectra are shown in Figure 5. For the initial CdSe394OA NPLs, standard exciton transitions from the zones of heavy holes (hh), light holes (lh) and spin-orbital (so) to the conduction band at wavelengths of 394, 366 and 320 nm, respectively, as previously noted in the literature [25,38] were observed. The wide absorption band at 240 nm refers to the transitions at the boundary of the 2D Brillouin zones (L point) [39].…”