Combining HR-TEM and XPS to elucidate the core–shell structure of ultrabright CdSe/CdS semiconductor quantum dots

Abstract

Controlling thickness and tightness of surface passivation shells is crucial for many applications of core–shell nanoparticles (NP). Usually, to determine shell thickness, core and core/shell particle are measured individually requiring the availability of both nanoobjects. This is often not fulfilled for functional nanomaterials such as many photoluminescent semiconductor quantum dots (QD) used for bioimaging, solid state lighting, and display technologies as the core does not show the application-relevant functionality like a high photoluminescence (PL) quantum yield, calling for a whole nanoobject approach. By combining high-resolution transmission electron microscopy (HR-TEM) and X-ray photoelectron spectroscopy (XPS), a novel whole nanoobject approach is developed representatively for an ultrabright oleic acid-stabilized, thick shell CdSe/CdS QD with a PL quantum yield close to unity. The size of this spectroscopically assessed QD, is in the range of the information depth of usual laboratory XPS. Information on particle size and monodispersity were validated with dynamic light scattering (DLS) and small angle X-ray scattering (SAXS) and compared to data derived from optical measurements. In addition to demonstrating the potential of this novel whole nanoobject approach for determining architectures of small nanoparticles, the presented results also highlight challenges faced by different sizing and structural analysis methods and method-inherent uncertainties.

Chemical in‐depth analysis of (Ca/Sr)F2 core–shell like nanoparticles by X‐ray photoelectron spectroscopy with tunable excitation energy

“…Consequently, intermixing of calcium and strontium containing phases occurs. This is in accordance with previous results from19 F NMR spectroscopy, also revealing CaF 2 -rich domains, SrF 2 -rich domains, and an intermixing zone 13. However, NMR spectroscopy cannot distinguish which domain is the shell and which is the core.…”

“…13 The core-shell particle morphology formation was deduced indirectly from an analysis of the nanoparticles' luminescence properties. Furthermore, 19 F nuclear magnetic resonance (NMR) spectroscopy reveals different signals from Ca-rich and Sr-rich domains probably located at the surface and center of the particle and an intermixing domain presumably located between them.…”

“…18 The importance of the method for this field is underlined by the number of articles and book chapters that have been published in recent years. [19][20][21][22][23][24][25][26][27][28] These publications not only report great advances but also point out the numerous challenges that must be considered in order to produce meaningful results with an XPS analysis of nanoparticles and nanoparticle coatings. SR-XPS has been successfully applied in the past to investigate the internal heterostructure of nanoparticles.…”

“…XPS is a very powerful tool for the characterization of nanoparticles and nanoparticle coatings due to its outstanding surface sensitivity combined with an element specific sensitivity down to 0.1 at% 18 . The importance of the method for this field is underlined by the number of articles and book chapters that have been published in recent years 19–28 . These publications not only report great advances but also point out the numerous challenges that must be considered in order to produce meaningful results with an XPS analysis of nanoparticles and nanoparticle coatings.…”

Composition-tuned MAPbBr3 nanoparticles with addition of Cs+ cations for improved photoluminescence

“…X-ray photoelectron spectroscopy (XPS) provided an independent verication of composition, and while XPS is surface sensitive, photoelectrons can escape from the centre of quantum dots/nanoparticles and the extracted compositions are representative of the perovskite chemistry. 32,33 XPS examined the binding energies of the Cs 4d (74-78 eV) and Br 3d (64-71 eV) emissions (Fig. 1a and S3 †), and as the intensity of Cs peaks reduces monotonically with increasing Br content, the integrated doublet peak areas were corrected for sensitivity to obtain atomic ratios.…”