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Less frequently, X-ray diffraction (XRD) is used, also employing dried samples 23, 26. Aside from relying on the availability of the core particles, core/shell intermixing or other unexpected deviations of the core/shell nanostructure like inhomogeneities of the shell cannot be detected. To obtain statistically significant results with this single particle technique, several hundred particles must be evaluated which is a general problem of TEM investigations 24, 25. The size difference between both nanoobjects is then taken as a measure for shell thickness. Ideally, first the as-synthesized core is analyzed and then the whole core/shell nanostructure after deposition of the shell(s). Very small inorganic NP like QD with sizes below 10 nm are mainly analyzed with TEM, measuring dried samples on a substrate in vacuum 23. Size and shape of core and core–shell nanostructures and insights into the particle architecture are commonly obtained with analytical techniques like (high resolution) transmission electron microscopy ((HR-)TEM), scattering techniques like small and wide angle X-ray scattering (SAXS and WAXS), and wavelength dispersive X-ray spectroscopy (WDX), as well as dynamic light scattering (DLS) and small-angle neutron scattering (SANS) 22. Also the chemical nature and number of stabilizing ligands per NP is relevant as they determine the colloidal stability, processability, e.g., the ease of incorporation into matrices like polymers for device fabrication, and (bio)functionalization 20, 21. Relevant features include size, shape, and size distribution of the core and core–shell nanostructure, as well as the thickness and chemical composition of the inorganic surface passivation shell(s) 18, 19.
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The crucial importance of passivation shell thickness control for QD with high Φ PL 15, 16, 17 calls for reliable methods to assess particle architecture. for bioimaging and single particle tracking studies 14, 15. These design concepts can also provide non- or barely blinking and very photostable QD, e.g. This has meanwhile led to few reports of QD with Φ PL close to unity for the best studied class of II/VI QD, either exploiting so-called “giant” CdSe/CdS core/shell quantum dots (g-QD) 11, 12, or a post-synthetic treatment with e.g. For optimum performance, i.e., a very high photoluminescence quantum yield ( Φ PL) close to unity and an excellent photostability, the non-radiative recombination of charge carriers commonly associated with trap states must be suppressed 5, 6, 7, typically with the aid of tight inorganic surface passivation shells of sufficient thickness 8, 9, 10. 1) with their size tunable optical properties and narrow emission bands with applications in display technology, solid state lighting, and solar energy conversion 3, 4.
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Examples present nanometer-sized semiconductor quantum dots (QD) (see Fig. Surface chemistry is an increasingly relevant field in nanoparticle (NP)-related research and particularly relevant for many applications of nanomaterials 1, 2.
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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. 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. The size of this spectroscopically assessed QD, is in the range of the information depth of usual laboratory XPS. 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. 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. Usually, to determine shell thickness, core and core/shell particle are measured individually requiring the availability of both nanoobjects. Controlling thickness and tightness of surface passivation shells is crucial for many applications of core–shell nanoparticles (NP).