Solution Properties Of Nanocrystal-ligands Complexes

Size-dependent physical chemistry property and solution processibility are two intrinsic properties of colloidal nanocrystals. However, knowledge on solution properties of colloidal nanocrystals is scarce and inconsistent, and sometimes even controversial. By analyzing the dissolution/precipitation process of typical colloidal nanocrystal-ligands complexes, the paper aimed to investigate the solution properties of colloidal nanocrystals systemically and quantitatively, and provide the support for colloidal nanocrystal synthesis and application in the future.High purity colloidal nanocrystal is the key to understand their solution properties. However, all the purification method so far suffer low efficiency, and only a little progress has been made even though large effort has been put on improving purification methods. According to the structure of nanocrystal-ligands complexes and key impurities, a new scheme based on chloroform-acetonitrile precipitation was identified as an ideal system. By this new scheme, metal precursors, especially for metal fatty acid salts could be efficiently eliminated from colloidal nanocrystal solutions. Results revealed that, the new scheme worked universally for CdSe nanocrystals and also for CdS and oxide (such as Fe3O4) nanocrystals coated with similar ligands. Furthermore, as chloroform and acetonitrile both are aprotic solvents, they were found to be benign to the surface ligands coverage and fluorescence efficiency of colloidal nanocrystals.CdSe nanocrystal-ligands complexes are the most studied nanocrystals systems. Their surface stable ligands are usually fatty acids and long-chain thiols. The size-dependent optical properties, excellent size monodispersity and size-adjustable properties of CdSe nanocrystals established a unique foundation for solution properties of nanocrystal-ligands complexes. Results shows that, similar to organic molecules, solubility is also a valid concept for nanocrystal-ligands complexes. Dfferent from common organic molecules, the solubility of nanocrystal-ligands complexes possess surprisingly strong yet well-defined dependence on temperature, particle size, solvents and the length of hydrocarbon chain ligands. Furthermore, results revealed that, size-and temperature-dependence originated from the surface ligands, including strong ligand inter-digitation between adjacent particles in solid and the ligands internal rotation entropy in solution.The properties of ligands mentioned above— inter-digitation and internal rotation entropy have not been reported in traditional models. According to our experimental results, considering the size difference between nanocrystal-ligands complexes and tranditional colloidal pariticles, we developed a theoretical model focusing on molecular level. In this model, dissolution releases freedom of C-C a bond rotation/bending and intra-particle rotation vibration, whose low frequency characteristics could correspond to enormous ligands conformation entropy. It is the increase of enormous intraparticle entropy, rather than the traditional steric barrier that balanced the huge melting enthalpy. Furthermore, the model confirmed that the huge melting enthalpy in solid of nanocrystal-ligands complexes was not due to the attraction between inorganic cores, but strong ligand inter-digitation between adjacent particles. For common straight chain ligands, as long as the size of the inorganic crystals is< 20 nm, the inter-digitation interaction between ligands could be stronger than core-core attraction.Such strong entropy-enthalpy competition of ligands inspired us to introduce a new kind of ligand—"entropy ligand", which made the ligands inter-digitation between particles become almost negligible, but had little effect on C-C a bond rotation/bending. Results showed that, generic entropic ligands increased solubility of metal, semiconductor and magnetic oxides by 2-5 orders of magnitude, and usually coming to>100mg/ml in common solvents. Meanwhile, compared with common straight chain ligands, the geometric length of entropic ligands decrease greatly, which could reduce the thickness of insulation layer, and offer means to greatly enhance electronic accessibility and performace in nanoelectric and optoelectronic devices.