The Coordination Structures Of Surface Ligands And Their Effect On The Physical Properties Of Nanocrystals

The nanocrystal is a kind of nanomaterial with wide application prospect.Generally,nanocrystals are composed of inorganic nucleus between 1-20 nm coated by a layer of organic molecules(ligands).The crystal shape,size,and morphology of inorganic nucleus are important to the properties of nanocrystals(quantum).Meanwhile,the organic ligand plays a crucial role in controlling the growth,solution properties,and stability of colloidal nanocrystals.However,there is no reliable method to identify and quantify the coordination configuration of nanocrystal ligands,and little attention has been paid to the interaction of nanocrystals in solution and solid.In this paper,CdSe and CdTe nanocrystals were used as models,and specific purification techniques were used to obtain nanocrystals with specific properties.The dispersibility,stability,and solubility of nanocrystals on a single particle scale were revealed.And the complex coordination modes of surface ligands and the interactions between ligand layers were investigated.Then,by ligand exchange,mixed ligand nanocrystals with rotational/bending degrees of freedom of C-C ? bonds were obtained.Its excellent solubility brought a new revolution to the application of nanocrystals.Finally,nanocrystals at the molecular level are self-assembled into supramolecular materials with excellent mechanical properties in order to open the door of bionic materials.Carboxylic acid molecules of aliphatic chain are commonly used as nanocrystal ligands.We studied the coordination structure of carboxylate ligands on the surface of CdSe nanocrystals with different morphologies,including nanocrystals,hexahedral nanocrystals,and commonly spherical nanocrystals.Combining NMR and FT-IR experiments and corresponding calculation results,it was found that the carboxylic acid ligands were mainly chelated on the surface of CdSe nanocrystals.By means of NMR chemical displacement,we can not only further distinguish the bridge mode from the tilted coordination,but also accurately quantify the proportion of various modes.The NMR method provides more comprehensive and quantitative information than conventional infrared spectroscopy.On the above basis,we studied the ligand exchange mechanism of long-short carboxylic acid and its effect on the solubility of nanocrystals.We found that the ligand exchange reaction did not change the ligand density,but the freedom degree of the ligand movement increased after the exchange,which reduced the ligand-ligand interaction and greatly improved the solubility of the nanocrystal.For the hexanoic acid and tetradecanoic systems,we found the optimal ratio of exchange reaction.We also used NMR technique to explore the exchange process of carboxylic acid in solution.Three different solution states of the ligand,including bound ligand,physical adsorption ligand,and free ligand,were found.We also studied the binding mode of mercaptopropionic acid(MPA)with bifunctional groups on the surface of CdTe nanocrystal.We found that MPA may form single-layer or multi-layer structure influenced by of purification conditions.The monolayer structure is composed of bound ligands and strongly adsorbed ligands,while the multilayer structure is composed of the bound ligand and unbound ligand with different adsorption capacity.The viscoelastic properties of the two nanogels are obviously different,making it possible to design and prepare high viscoelastic gels with noncovalent bond forces.