Elevated Temperature Photophysical Properties and Morphological Stability of CdSe and CdSe/CdS Nanoplatelets

Abstract

Elevated temperature optoelectronic performance of semiconductor nanomaterials remains an important issue for applications. Here we examine 2D CdSe nanoplatelets (NPs) and CdS/CdSe/CdS shell/core/shell sandwich NPs at temperatures ranging from 300 to 700 K using static and transient spectroscopies as well as in situ transmission electron microscopy. NPs exhibit reversible changes in PL intensity, spectral position, and emission line width with temperature elevation up to ∼500 K, losing a factor of ∼8 to 10 in PL intensity at 400 K relative to ambient. Temperature elevation above ∼500 K yields thickness-dependent, irreversible degradation in optical properties. Electron microscopy relates stability of the core-only NP morphology up to 555 and 600 K for the four and five monolayer NPs, respectively, followed by sintering and evaporation at still higher temperatures. Reversible PL loss, based on differences in decay dynamics between time-resolved photoluminescence and transient absorption, results primarily from hole trapping in both NPs and sandwich NPs.

Highly Stable, Near‐Unity Efficiency Atomically Flat Semiconductor Nanocrystals of CdSe/ZnS Hetero‐Nanoplatelets Enabled by ZnS‐Shell Hot‐Injection Growth

“…From TEM images in Figure c, the vertical thickness of the CdSe/ZnS core/HI thin‐shell sample is measured as 3.04 ± 0.3 nm. Using thick‐shell (or additional shell coating) is well known to improve the stability, as reported in many earlier works, due to the effective protection from the surrounding environment . Here CdSe/ZnS core/HI thick‐shell sample was synthesized with some modifications and using extra shell precursor in the HI thin‐shell recipe presented in the Experimental Section.…”

“…An effective method for tackling the stability issue is the growth of a semiconductor layer around the core NPLs. This approach generally helps to reduce the surface nonradiative recombination sites by passivating surface traps, leading to increased quantum yield and decreased emission blinking at a single particle level . Among the shell structures used for CdSe‐core NPLs, the most notable ones demonstrated thus far include CdS and ZnS, which results in quasi‐type‐II and ‐type‐I band alignment structures, respectively.…”

“…Among the shell structures used for CdSe‐core NPLs, the most notable ones demonstrated thus far include CdS and ZnS, which results in quasi‐type‐II and ‐type‐I band alignment structures, respectively. In the past few years, research on the core/shell NPLs has concentrated mostly on the colloidal synthesis of CdSe/CdS core/shell NPLs owing to the smaller lattice mismatch between these materials and the resulting quasi‐type II nature of electronic structure enabling highly tunable excitonic features . For example, Rossinelli et al have recently reported the uniform growth CdS shell on CdSe NPLs at higher temperatures resulting in narrower emission linewidth (≈20 nm) with suppressed blinking and moderately high photoluminescence quantum yield (QY) of 55–60% .…”

“…In addition, we observe that the thermal stability is greatly enhanced by using the HI‐shell growth approach beyond 500 K. By systematically conducting thermal tests under ambient air conditions which induce additional degradation, we observe that the thick‐shell CdSe/ZnS core/shell NPLs completely recover their initial PL intensity (100% PL recovery) during a heating cycle from 300 to 400 K while exhibiting 76% PL recovery for the range of 300–525 K. This HI thick‐shell sample also exhibits the best performance in UV stability tests and preserves their higher QY even after multiple cleaning steps. Previously, the thermal stability of different NPLs was reported by Rowland et al for CdSe/CdS core/shell NPLs under vacuum . In this previous study, as a benchmark, the heating cycle up to 450 K yields fully reversible PL intensity under vacuum for CdSe/CdS core/shell NPLs with six monolayers of CdS shell that was produced using the c‐ALD method; however, the PL intensity was found to drop to 60% at the end of heating cycle up to 500 K. These results show that the CdSe/ZnS hetero‐nanoplatelets synthesized herein exhibit superior thermal stability compared to their predecessors.…”

Low‐Threshold Amplified Spontaneous Emission and Lasing from Thick‐Shell CdSe/CdS Core/Shell Nanoplatelets Enabled by High‐Temperature Growth

“…The properties of colloidal core‐only NPL materials could be further improved in core/shell or core/crow heterostructures . Among these, CdSe/CdS core/shell NPLs have been increasingly attractive due to their excellent shell‐thickness‐dependent optical performance . CdSe/CdS NPLs are synthesized with higher quantum yields (QYs) and stability as compared to CdSe core‐only NPLs, because of the effective passivation of surface trap states by the epitaxial growth of a wider bandgap shells .…”