Quantitative size-dependent structure and strain determination of CdSe nanoparticles using atomic pair distribution function analysis

Abstract

The size-dependent structure of CdSe nanoparticles, with diameters ranging from 2 to 4 nm, has been studied using the atomic pair distribution function (PDF) method. The core structure of the measured CdSe nanoparticles can be described in terms of the wurtzite atomic structure with extensive stacking faults. The density of faults in the nanoparticles ∼ 50% . The diameter of the core region was extracted directly from the PDF data and is in good agreement with the diameter obtained from standard characterization methods suggesting that there is little surface amorphous region. A compressive strain was measured in the Cd-Se bond length that increases with decreasing particle size being 0.5% with respect to bulk CdSe for the 2 nm diameter particles. This study demonstrates the size-dependent quantitative structural information that can be obtained even from very small nanoparticles using the PDF approach.

Nanocrystals: Breaking limitations of data analysis

“…Despite of a poor fit, this work shows big differences in the structure of the examined nano-CdSe samples relative to non-nano crystals. The specific structural parameters refined in [17] are not much reliable and they are in contradiction to our knowledge about the CdSe structure. For instance, the (c 0 =a) ratio corresponding to the refined values of the lattice parameters changes from 0.8154 for microcrystalline sample to 0.8084 for 2.2 nm grains; for cubic and hexagonal modifications of CdSe it is about 0.8165.…”

“…Here, instead of presenting our own calculations, we recall and comment on an example of refinement of the structural parameters of CdSe nanocrystals obtained from PDF analysis and performed by Masadeh et al [17]; in our opinion, it is a typical example of misinterpretation of experimental diffraction data collected for nanocrystals. Using PDFFIT program [14] the refinement have been done for CdSe nanocrystals with diameters of 2.2, 3.2, and 3.7 nm.…”

Total scattering atomic pair distribution function: new methodology for nanostructure determination

“…The PDF refinement results indicated that the CdSe_QD structures can be described as intermediate between two structures (mixed structure model), wurtzite and zinc-blende. [7] The PDF method also has been applied to estimate the stacking fault density presents in such small QDs. [7,40] Yang et al have described a 'rapid approximate approach for extracting the stacking fault density by fitting wurtzite-zinc-blende two-phase models to the low-r region of the PDF data'.…”

“…The poor fitting of the mixedphase model to the ultrasmall CdSe QDs indicates that the structure of ultrasmall QDs is different from larger QDs. Recently, Masadeh et al [7] presented a detailed size-dependent nanostructural study, where they reported structural information from PDF analysis on (2À4 nm) CdSe nanoparticles. Yang et al employed the introduced PDF-based methodology to investigate the atomic structure of the CdSe ultrasmall nanoparticles.…”

“…[22,23] Recent developments in both data collection [24À27] and modelling [28À32] make this a potentially powerful tool in quantitative structure determination of nanoparticles, as has been successfully demonstrated in some previous studies. [6,7,33,34] On the other hand, as small nanoparticles show a nonuniform structure, additional extensions to the modelling of nanoparticle structure may be needed to accommodate core/shell structure model. [35,36] This was noted recently on in a study by Palosz et al [36] where they reported the limitations and the perspectives of diffraction studies of the atomic structure of nanoparticles.…”