Quantification of the Photon Absorption, Scattering, and On-Resonance Emission Properties of CdSe/CdS Core/Shell Quantum Dots: Effect of Shell Geometry and Volumes

Abstract

Reliable quantification of the optical properties of fluorescent quantum dots (QDs) is critical for their photochemical, -physical, and -biological applications. Presented herein is the experimental quantification of photon scattering, absorption, and on-resonance-fluorescence (ORF) activities of CdSe/CdS core/shell fluorescent QDs as a function of the shell sizes and geometries. Four spherical QDs (SQDs) with different diameters and four rod-like QDs (RQDs) with different aspect ratios (ARs) have been analyzed using UV–vis, fluorescence, and the recent polarized resonance synchronous spectroscopic (PRS2) methods. All quantum dots are simultaneous absorbers and scatterers in the UV–vis wavelength region, and they all exhibit strong ORF emission in the wavelength regions where the QDs both absorb and emit. The absorption and scattering cross-sections of the CdS shell are linearly and quadratically, respectively, proportional to the shell volume for both the SQDs and RQDs. However, the effects of CdS shell coating on the core optical properties are different between SQDs and RQDs. For RQDs, increasing the CdS shell volume through the length elongation has no effect on either the peak wavelength or intensity of the CdSe core UV–vis absorption and ORF, but it reduces the QD fluorescence depolarization. In contrast, increasing CdS shell volume in the SQDs induces red-shift in the CdSe core peak UV–vis absorption and ORF wavelengths, and increases their peak cross-sections, but it has no effect on the SQD fluorescence depolarization. The RQD ORF cross-sections and quantum yields are significantly higher than their respective counterparts for the SQDs with similar particle sizes (volumes). While these new insights should be significant for the QD design, characterization, and applications, the methodology presented in this work is directly applicable for quantifying the optical activities of optically complex materials where the common UV–vis spectrometry and fluorescence spectroscopy are inadequate.

Quantification of the Optical Properties of Perovskite Nanocrystals Using a Combination of Polarized Resonance Synchronous and Polarized Anti-Stokes, On-Resonance, and Stokes-Shifted Spectroscopy

“…The key limitation with the dark-field spectroscopy is, however, the challenge in separating the materials’ light scattering from their on-resonance fluorescence (ORF), the photon emission at wavelengths identical to the excitation wavelengths. Such an ORF has been observed in a broad range of fluorescent materials including molecular and nanoscale fluorophores such as fluorescent quantum dots. − It occurs invariably in the wavelength region where the fluorophores both absorb and emit. Indeed, even for dark-field spectra obtained with an excitation and detection monochromator, care should always be exercised in interpreting the dark-field spectra obtained for fluorescent samples to avoid mistaking ORF signals as scattered photons.…”

CdS@CdSe Core/Shell Quantum Dots for Highly Improved Self-Powered Photodetection Performance

“…Heterojunction nanostructures formed between different semiconductor materials provide a promising route to rationally tune electronic properties and have shown potential applications in solar cells, − photoelectrochemical cells, ,, field-effect transistors, , etc. A built-in electric field can usually be created at the interfaces of the heterojunction nanostructures, which can increase the separation efficiency of the photogenerated e–h pairs and regulate the carrier transportation in (photo)­electronic devices. − In the past decade, heterostructures based on cadmium sulfide (CdS) and cadmium selenide (CdSe) have been considered as promising materials for versatile applications, such as photoluminescence, − water splitting, photon scattering, photocatalysis, ultrafast photonics, etc. For example, in 2020, Xu et al used different shell sizes and geometries of CdSe/CdS core/shell QDs for experimental quantification of photon scattering, absorption, and on-resonance fluorescence activities.…”

Missing Links between the Structures and Optical Properties of Porphyrin Assemblies

“…This is in contrast to the earlier studies performed with fluorescent quantum dots and plasmonic gold nanoparticles of different sizes. 31,42 The scattering extinction of the quantum dots and gold nanoparticles both increase with increasing scatters' volume. This discrepancy strongly indicates that the interior structure of the ZnTPyP assemblies with different shapes is very critical in defining their optical properties.…”