Z-contrast scanning transmission electron microscopy on self-assembled CdSe quantum dots in ZnSe and (Zn,Mn)Se matrices

摘要

Z-contrast imaging in the scanning transmission electron microscope (STEM) was employed to study single and multi-sheet arrangements of self-assembled CdSe quantum dots (QDs) in ZnSe and (Zn_(0.9)Mn_(0.1))Se matrices. The negligible influence of strain field on Z-contrast images allow for an analysis of the shapes and sizes of a large number of QDs. Higher levels of self-ordering could be proven by such analyses in a sample for which a fractional monolayer of MnSe was deposited prior to the deposition of a single CdSe sheet, that subsequently self-assembled into three dimensional (3D) quantum dots. In the multi-sheet structure, we observed the co-existence of a variety of differently shaped, sized, and self-ordered Cd rich agglomerates. Several of these agglomerates showed different types of compositional modulation on the atomic scale. The advantages of using atomic resolution Z-contrast imaging in STEM for the elucidation of these modulations are demonstrated. Results of the standardless qualitative elemental mapping technique of Z-contrast STEM are in agreement with the results of semi-quantitative energy dispersive X-ray spectroscopy (EDS).