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Surface atom stoichiometry is controlled for Ag2S NCs through ion-pair ligand-assisted surface reactions. Predominantly cation-rich-surfaced Ag2S NCs (Ag-Ag2S), anion-rich-surfaced Ag2S NCs (S-Ag2S), and perfectly stoichiometric Ag2S NCs (st-Ag2S) are prepared. Various combinations of cation and anion pairs are studied for Ag2S NC surface reactions. Depending on the hardness/softness of the ion-pair ligands, the NC surface shows disparate reactions of (i) X-type anion replacing native X- and L-type ligands, (ii) binding as the whole ion-pair ligand, and (iii) one-to-one exchange by the positive cation ligand. Ion-pair-assisted surface chemistry enables the synthesis of perfectly stoichiometric-surfaced Ag2S NCs, which shows that the fluorescence intensity increased by more than ten times, and epitaxial ZnS growth on st-Ag2S was enhanced more than three times when compared with non-stoichiometric NCs.

Magic-sized clusters (MSCs) are often observed as intermediates during the growth of semiconductor quantum dots (QDs) as transiently thermodynamically stable, with diameters typically ranging 1–2 nm. Exploiting MSCs as a new class of precursors for QD syntheses is of great importance. Roles of MSCs in the nucleation and growth of QDs and interactions between MSCs and conventional molecular precursors have attracted intensive attention. During QD synthesis, an MSC can convert to another MSC or an amorphous cluster. It can also be fragmentized into smaller clusters and molecular species such as metal–ligand complexes, which in turn can be used as a molecular precursor for nucleation or growth of QDs. Two different MSCs, F393-InP:Zn MSC and F360-InP:Zn MSC, were used as a QD synthesis precursor. They acted as a precursor cargo supplying the indium-octadecylphosphonate (In-ODPA) complex, which was subsequently consumed for QD growth. The F393-InP:Zn MSC has higher thermal stability than the F360-InP:Zn MSC. As a result, the F393-InP:Zn MSC released In-ODPA complexes at a higher temperature than the F360-InP:Zn MSC, which could hinder ripening and/or continuous nucleation and yielded narrowly size-distributed QDs. The F360-InP:Zn MSC provided In-ODPA complexes at an earlier stage of QD synthesis, and recreation of the MSC could be observed. Low concentrations of F360-InP:Zn MSC produced narrowly distributed QDs due to the In-ODPA complex effect. However, for large amounts, more MSCs yielded smaller and more broadly size-distributed QDs by competitive precursor consumption. For an excessively large amount, MSCs prevailed over QD nucleation and completely blocked QD production. Understanding the roles of MSCs as precursors for QDs will pave the way for sophisticated synthesis design for high-quality QDs.

We propose a new method for an objective measurement of video quality. By analyzing subjective scores of various video sequences, we find that the human visual system is particularly sensitive to degradation around edges. In other words, when edge areas of a video sequence are degraded, evaluators tend to give low quality scores to the video, even though the overall mean squared error is not large. Based on this observation, we propose an objective video quality measurement method based on degradation around edges. In the proposed method, we first apply an edge detection algorithm to videos and locate edge areas. Then, we measure degradation of those edge areas by computing mean squared errors and use it as a video quality metric after some postprocessing. Experiments show that the proposed method significantly outperforms the conventional peak signal-to-noise ratio (PSNR). This method was also independently evaluated by independent laboratory groups in the Video Quality Experts Group (VQEG) Phase 2 test. The method consistently provided good performances. As a result, the method was included in international recommendations for objective video quality measurement.