A Tale of Tails: Thermodynamics of CdSe Nanocrystal Surface Ligand Exchange

Abstract

The surface ligands of semiconductor nanocrystals (NCs) are central for determining their properties and for their flexible implementation in diverse applications. Thus far, the thermodynamic characteristics of ligand exchange reactions were attained by indirect methods. Isothermal titration calorimetry is utilized to directly and independently measure both the equilibrium constant and the reaction enthalpy of a model ligand exchange reaction from oleate-capped CdSe NCs to a series of alkylthiols. Increased reaction exothermicity for longer chains, accompanied by a decrease in reaction entropy with an overall enthalpy−entropy compensation behavior is observed, explained by the lengthdependent interchain interactions and the organization of the bound ligands on the NCs' surface. An increase in the spontaneity of the reaction with decreasing NC size is also revealed, due to their enhanced surface reactivity. This work provides a fundamental understanding of the physicochemical properties of the NC surface with implications for NC surface ligand design.

The role of organic ligand shell structures in colloidal nanocrystal synthesis

“…Thermodynamics of the ligand shell. As for micelles, the thermodynamics of organic ligand shells of colloidal nanocrystals are best investigated through isothermal titration calorimetry, often through a ligand-exchange reaction 17,27,28,31 . By titrating a reactant into a nanocrystal solution, the heat of that reaction can be measured.…”

“…From these measurements, the ligand-exchange thermodynamics can be understood. Reactions involving quantum dots were performed in isothermal titration calorimeters, such as ligand exchanges with different head groups, for example, metal halides 17 , alkylthiols 27 and alkylphosphonates 28,31 , as well as other reactions such as nanocrystal syntheses 78 , cation exchanges 85 and tail-group exchanges 31 . Although these measurements help with understanding the reactions on the surfaces of ligands, they also have the ability to probe the collective effects of ligands on nanocrystal surfaces 17,31 .…”

“…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.…”

Ligand Conversion in Nanocrystal Synthesis: The Oxidation of Alkylamines to Fatty Acids by Nitrate

“…[9][10][11] In these syntheses, long-chain aliphatic ligands are used to dissolve the precursors and control nucleation and growth. 12,13 After the syntheses, some ligands remain bound to the NC surface providing colloidal stability and determining solubility, 14,15 reactivity, [16][17][18][19][20] and electronic structure. [21][22][23][24][25] Therefore, unveiling the structure and binding motif of surface ligands is fundamental to understand NC properties, 26 design ligand exchange strategies, 20,[27][28][29][30] and envision potential applications.…”

“…12,13 After the syntheses, some ligands remain bound to the NC surface providing colloidal stability and determining solubility, 14,15 reactivity, [16][17][18][19][20] and electronic structure. [21][22][23][24][25] Therefore, unveiling the structure and binding motif of surface ligands is fundamental to understand NC properties, 26 design ligand exchange strategies, 20,[27][28][29][30] and envision potential applications. 31,32 In reactions where several organic ligands can bind to the NCs, the surface chemistry is typically studied in detail by nuclear magnetic resonance (NMR) spectroscopy, e.g., for CdSe, 28,33 PbS, [34][35][36] and InP 29,37 NCs.…”