Department of Chemistry
Indian Institute of Technology Delhi

Unraveling the Surface Chemistry of Colloidal Two-Dimensional Nanostructures

Dr. Shalini Singh
Physics and Chemistry of Nanostructures, Ghent University, 9000 Ghent, Belgium Center for Nano and Biophotonics, Ghent University, 9000 Ghent, Belgium

Date: November 16th 2017 (Thursday)
Time: 4 PM
Venue: Committee Room, Chemistry Department, 6th Floor

The surface-ligand interface of the colloidal semiconductor nanocrystals has achieved a significant research interest owing to their dominant role in tuning and tailoring the physical and chemical properties of functional nanomaterials. The surface of metal chalcogenide quantum dot (QD) constitute of excess under coordinated metal atoms with dangling bonds which often contribute to localized set of electronic states behaving as traps for electrons or holes. These trap states quench the luminescence and hamper the performance of nanocrystal-based devices. However, in ligand passivated QD, the strong covalent bond between surface cation and anionic ligands such as carboxylate, exhibit clean band gap, free of trap states and preserves the QD luminescence. The surface-chemistry studies have been focused primarily on QDs of small facet size having curved morphology and multiple edge- and corner sites with the assumptions that these concepts can be applied to 1D nanowires, 2D nanoplatelets and other nanoscale objects. But, establishing the relationship between the surface structure and optical properties for the entire gamut of nanocrystals with different forms remains a crucial open question in the field as the increasing anisotropicity of multidimensional nanostructures often results into new surface topology. For example, 2-D CdSe nanoplatelets, have an atomically flat surface with cadmium rich 100 facets as their top and bottom planes that are 7-10 times larger than the edge facets, which takes the facet specific ligand coordination chemistry to a whole new level. 2-D platelets are a very fascinating system as they exhibit extremely narrow yet tunable photoluminescence bands, faster carrier recombination and low-threshold lasing properties. Also, their absorption and emission spectra are highly sensitive to changes in surface ligation. Despite of the growing interest in the optical properties of nanoplatelets, the synthesis of 2-D colloidal nanoplatelets is limited by difficulties in synthesis reproducibility, low colloidal stability and unwanted stacking of platelets in bundles. The tendency of nanoplate to aggregate easily in solution quench their luminescence to a certain extent and limits their processibility in device fabrication. This hampering instability of nanoplatelets in solution and the question of link between surface and optical properties definitely calls for a thorough investigation of the organic-inorganic interface of nanoplatelet surface. Herein we report a detailed study of the surface-chemistry of 2-D nanoplatelets taking archetypal CdSe nanoplatelets as the model system. We have improvised the synthesis strategy ideally used for platelet synthesis and obtained colloidally stable and aggregation free suspensions of nanoplatelets. These nanoplatelets have very high quantum yield despite of the presence of a very large surface to volume ratio. Via solution 1H NMR, we show surface layer of the CdSe nanoplatelets are capped with z-type cadmium carboxylate ligands. These z-type complexes are labile and could be easily dissociated from the surface by introduction of Lewis bases. The displacement of cadmium carboxylate has an irreversible yet linear correlation with the PLQY quenching of the nanoplatelets. Detailed density functional theory (DFT) calculations were performed to observe the relationship between electronic structure of 2-D platelets and the ligand coverage on the surface of platelets. This systematic study of the organic-inorganic interface of the nanoplatelets surface provides better insight on the effect of ligand coverage on optical properties and the perturbation on the electronic structure of the 2-D materials caused by removal of ligands.

All are cordially invited to attend.
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