To Battle Surface Traps on CdSe/CdS Core/Shell Nanocrystals: Shell Isolation versus Surface Treatment

Abstract

Electronic traps at the inorganic-organic interface of colloidal quantum dots (QDs) are detrimental to their luminescent properties. Several types of interface traps were identified for single-crystalline CdSe/CdS core/shell QDs, which were all found to be extrinsic to either the core/shell structure or their optical performance. The electron traps-presumably excess or unpassivated Cd surface sites-are shallow ones and could be readily isolated from the electron wave function of the excitons with more than ∼2 monolayers of CdS shell. There were two identifiable deep hole traps within the bandgap of the QDs, i.e., the surface adsorbed H2S and unpassivated surface S sites. The surface adsorbed H2S could be removed by either degassing processes or photochemical decomposition of H2S without damaging the QDs. The unpassivated surface S sites could be removed by surface treatment with cadmium carboxylates. Understanding of the surface traps enabled establishment of new phosphine-free synthetic schemes for either single-precursor or successive-ion-layer-adsorption-and-reaction approach, which yielded CdSe/CdS core/shell QDs with near-unity photoluminescence quantum yield and monoexponential photoluminescence decay dynamics with 2-10 monolayers of CdS shell.

Rational Design and Synthesis of Highly Luminescent Multinary Cu‐In‐Zn‐S Semiconductor Nanocrystals with Tailored Nanostructures

“…Interestingly, further increase of Zn precursor feeding dosage leads to a greater contribution of the fast decay channel (τ 1 = 3.2 ns) and a longer lifetime of the third emissive channel (τ 3 = 309.9 ns) than those of Zn‐0.5 sample, and thus the average lifetime of 212 ns is obtained in the Zn‐1.5 sample. Based on the aforementioned speculation, the partial cation diffusion and ZnS shelling process is competitive during the formation of Cu‐In‐Zn‐S NCs, and plenty of Zn ions accumulate on the surface of the Zn‐1.5 sample to form a soft core/shell structure, which may serve as the charge traps for nonradiative recombination . This is consistent with the fact that the PLQY of Zn‐1.5 sample is very low.…”

“…As a result, the greater contribution of fast decay channel in Zn‐1.5 sample mainly arises from the nonradiative recombination channel. Moreover, excess Zn ions can also passivate the surface to form a soft core/shell structure, which may lead to ligand‐to‐metal charge‐transfer recombination, and thus the third PL decay channel becomes slower …”

Excitation wavelength dependence of the photoluminescence quantum yield and decay behavior of CdSe/CdS quantum dot/quantum rods with different aspect ratios

“…This ΦPL decrease may arise from an increasing number of defect states at the core/shell interface or within the shell with increasing QDQR length, favoring non-radiative deactivation of the excitons, 8 or from the presence of non-emitting material that absorbs within the same wavelength range as the shell, e.g., CdS nucleates formed as side product during shell growth. 46,47 Another possible explanation are trap states hindering exciton diffusion, leading to a decrease of the shellto-core exciton localization efficiency with increasing QDQR length. 48,49 To identify the mechanism(s) leading to the excitation energy loss, and thus, to the observed wavelength-dependent ΦPL, the PL decay times (τPL) of the QDQRs were determined at three different λexc.…”

Single Enzyme Direct Biomineralization of CdSe and CdSe-CdS Core-Shell Quantum Dots

“…The substantial increase in quantum yield is perhaps the strongest evidence for the formation of a core-shell structure. 31,32 The 12% quantum yield reported here for the core-shell structure is substantially higher than that for the CdSe core nanocrystal and higher than we have achieved for CdS nanocrystals utilizing smCSE catalyzed biomineraliza-tion. 12, 13 The single particle XEDS analysis in Figure 6c further confirms the coexistence of Cd, Se, and S within a single particle, indicating sequential mineralization of CdSe and CdS in a single particle rather than the formation of a secondary population of CdS particles.…”

“…This compares well with the 12% quantum yield we measured for the biomineralized 2.74 nm CdSe cores with 0.7 monolayers of CdS coverage. While substantial improvements in the size and shape uniformity and resulting optical properties of high temperature, organic phase, chemically synthesized materials have been achieved over the past 2 decades, with reported quantum yields exceeding 90%, 31 room temperature single enzyme aqueous phase quantum dot biomineralization is in its comparative infancy and will undoubtedly undergo further optimization as the technology matures.…”