The role of organic ligand shell structures in colloidal nanocrystal synthesis

Abstract

olloidal nanocrystals bridge the gap between molecules and bulk materials, and enable researchers to probe fundamentals of nanomaterials, surfaces and size-dependent physics 1,2 . Beyond this, they have found a variety of applications from quantum dot emitters in displays 3,4 to photovoltaics 5,6 . Their colloidal nature allows them to be dispersed into solvents for optical studies, which include single-particle measurements 7,8 and assemblies of these materials can be created to investigate collective effects and their use as artificial atoms 9,10 .Fundamentally, most colloidal nanocrystals are composites that consist of an inorganic core and an organic ligand shell. This organic ligand shell enables colloidal stability in solvents 11,12 , prevents inorganic cores from fusing 10,13 , passivates undercoordinated atoms that lead to exciton traps in quantum dots [14][15][16][17] , acts as soft matter in the assembly of artificial solids 9,10,18,19 and provides a platform for the modification of inorganic cores 15,17,20 . Beyond contributing to colloidal nanocrystals properties, they are essential in synthesis.Organic ligands mediate growth by binding to growing nanocrystal surfaces as a surfactant. The group that binds to the nanocrystal surface is referred to as the head group, and the rest of the capping ligand is referred to as the tail group 1,19,21 . Typically, in hydrophobic environments, when growth is terminated by the exhaustion of precursors or reaction quenching, these ligands form an organic shell around each nanocrystal, in which the head groups face into the nanocrystal and the organic tail groups face outward. The nature of the ligands dictates the inorganic nanocrystal surface structure 11,19,21,22 , and has been designed to promote surfaces that minimize electronic trap states in quantum dots 15,17,23,24 . A variety of head groups are used to terminate colloidal nanocrystal surfaces, such as carboxylates, amines, phosphonates, thiols and halides, and often a mixture of capping groups is used [15][16][17]20,[25][26][27][28][29][30][31][32] . The amount, type and impurities of the ligands used in nanocrystal synthesis have implications for the nanocrystal size, shape, crystal structure and stability, as well as for their optical and electronic properties 33,34 .Head groups play a critical role in the control of colloidal nanocrystal growth, and different nanocrystal shapes can be formed by manipulating head group binding. Using combinations of

Calibrate Ligand-ligand Interaction on Nanocrystals via the Dynamic Volume of Chain Segments

“…The measurement of dynamic volume by NMR. The segmental dynamics of ligand can be assessed with a different approach based on the motional averaging of 1 H- 13 C dipolar coupling on CH2 units. This is normally achieved by the dipolar chemical shift correlation (DIPSHIFT) NMR experiment which can provide more quantitative measures of the motional amplitude 57 .…”

“…This is normally achieved by the dipolar chemical shift correlation (DIPSHIFT) NMR experiment which can provide more quantitative measures of the motional amplitude 57 . The DIPSHIFT relies on the 13 C chemical shift to resolve different segments and provides evolution curves resulting from 1 H-C dipolar coupling for each segment. The larger curvature of the curve indicates the stronger residual dipolar interaction and the smaller motional amplitude.…”

“…The confinement of electronic states in the semiconductor core endows them with tailorable optoelectronic properties 9 and photocatalytic capabilities 6 . While the surface layer of organic ligands plays a crucial role to stabilize their core structures 10 , to control their shapes and morphology [11][12][13] , and to modulate the surface chemistry 14,15 .…”

“…Many studies are focused on the head groups of the organic ligands as their coordination affects the formation of surface facets [16][17][18] , determines the binding strength of ligands 10,19,20 , and controls the abundance of surface defect sites [21][22][23][24] . Differently, the non-coordinating tails utilize weak interactions and entropic effects to offer tremendous power in controlling the colloidal stability and assembly 13 .…”

“…The amplitude and characteristics of these segmental motions could be captured by various NMR parameters such as relaxation, anisotropic lineshapes, or by multidimensional exchange spectroscopy 51,52 . For instance, the analyses on quadrupolar patterns of 2 H NMR and 1 H- 13 C dipolar interaction offer quantitative measures of motional rate and amplitude of chain segments 53 .…”