The surface chemistry of a colloidal nanoparticle is intrinsic to both its structure and function. It is therefore necessary to characterize the surfaces of colloidal materials to rationally underpin any synthetic, catalytic, or transformative mechanisms they enable. Here we characterize the surface properties of colloidal InP clusters and quantum dots by examining the binding of traditional stabilizing ligands including carboxylates, phosphonates, and thiolates. By using the In37P20X51 (X = carboxylate) cluster species as an ideally monodisperse and well-defined starting scaffold, we quantify surface-exchange equilibria. Using quantitative 1H and 31P NMR spectroscopy, we show that 1:1 metathesis-type binding models are insufficient to fully describe the surface dynamics. In particular, for the case of the highly reversible carboxylate ligand exchange, a more detailed isotherm approach using a two-site, competitive model is necessary. This model is used to deconvolute L- and X-type binding modalities. We additionally quantify the reversible and irreversible ligand-exchange reactions observed in the thiolate and phosphonate  systems.

CONSPECTUS: Clusters are unique molecular species that can be viewed as a bridge between phases of matter and thus between disciplines of chemistry. The structural and compositional complexity observed in cluster chemistry serves as an inspiration to the material science community and motivates our search for new phases of matter. Moreover, the formation of kinetically persistent cluster molecules as intermediates in the nucleation of crystals makes these materials of great interest for determining and controlling mechanisms of crystal growth. Our lab developed a keen interest in clusters insofar as they relate to the nucleation of nanoscale semiconductors and the modeling of postsynthetic reaction chemistry of colloidal materials. In particular, our discovery of a structurally unique In 37 P 20 X 51 (X = carboxylate) cluster en route to InP quantum dots has catalyzed our interest in all aspects of cluster conversion, including the use of clusters as precursors to larger nanoscale colloids and as platforms for examining postsynthetic reaction chemistry. This Account is presented in four parts. First, we introduce cluster chemistry in a historical context with a focus on main group, metallic, and semiconductor clusters. We put forward the concept of rational, mechanism-driven design of colloidal semiconductor nanocrystals as the primary motivation for the studies we have undertaken. Second, we describe the role of clusters as intermediates both in the synthesis of well-known material phases and in the discovery of unprecedented nanomaterial structures. The primary distinction between these two approaches is one of kinetics; in the case of well-known phases, we are often operating under high-temperature thermolysis conditions, whereas for materials discovery, we are discovering strategies to template the growth of kinetic phases as dictated by the starting cluster structure. Third, we describe reactions of clusters as model systems for their larger nanomaterial progeny with a primary focus on cation exchange. In the case of InP, cation exchange in larger nanostructures has been challenging due to the covalent nature of the crystal lattice. However, in the higher energy, strained cluster intermediates, cation exchange can be accomplished even at room temperature. This opens opportunities for accessing doped and alloyed nanomaterials using postsynthetically modified clusters as single-source precursors. Finally, we present surface chemistry of clusters as the gateway to subsequent chemistry and reactivity, and as an integral component of cluster structure and stability. Taken as a whole, we hope to make a compelling case for using clusters as a platform for mechanistic investigation and materials discovery.

We have synthesized InP nanocrystals of an unprecedented crystal phase at low temperature (35-100 °C) by templated growth of InP magic-sized clusters. With the addition of stoichiometric equivalents of P(SiMe ) to the starting cluster, we demonstrate nanocrystal growth mediated through a partial dissolution and recrystallization pathway. This growth process was monitored using a combination of in situ UV/Vis and P NMR spectroscopy, revealing the intermediacy of smaller cluster species of higher symmetry. The nanocrystals that result from this templated growth exhibit a crystal structure that is neither zincblende nor wurtzite, and instead is derived from the original cluster. This structure is best described as a 3D polytwistane phase as deduced from a combination of X-ray diffraction, Raman, and solid-state NMR spectroscopy methods.

We have synthesized InP nanocrystals of an unprecedented crystal phase at low temperature (35-100 8 8C) by templated growth of InP magic-sizedc lusters.W itht he addition of stoichiometric equivalents of P(SiMe 3 ) 3 to the starting cluster,w ed emonstrate nanocrystal growth mediated through ap artial dissolution and recrystallization pathway. This growth process was monitored using ac ombination of in situ UV/Vis and 31 PNMR spectroscopy, revealing the intermediacy of smaller cluster species of higher symmetry. The nanocrystals that result from this templated growth exhibit ac rystal structure that is neither zincblende nor wurtzite,a nd instead is derived from the original cluster.This structure is best described as a3 Dp olytwistane phase as deduced from ac ombination of X-ray diffraction, Raman, and solid-state NMR spectroscopym ethods.

This text presents a method for the synthesis of In 37 P 20 (O 2 C 14 H 27) 51 clusters and their conversion to indium phosphide quantum dots. The In 37 P 20 (O 2 CR) 51 clusters have been observed as intermediates in the synthesis of InP quantum dots from molecular precursors (In(O 2 CR) 3 , HO 2 CR, and P(SiMe 3) 3) and may be isolated as a pure reagent for subsequent study and use as a single-source precursor. These clusters readily convert to crystalline and relatively monodisperse samples of quasi-spherical InP quantum dots when subjected to thermolysis conditions in the absence of additional precursors above 200 °C. The optical properties, morphology, and structure of both the clusters and quantum dots are confirmed using UV-Vis spectroscopy, photoluminescence spectroscopy, transmission electron microscopy, and powder X-ray diffraction. The molecular symmetry of the clusters is additionally confirmed by solution-phase 31 P NMR spectroscopy. This protocol demonstrates the preparation and isolation of atomically-precise InP clusters, and their reliable and scalable conversion to InP QDs.

Using nanoscale building blocks to construct hierarchical materials is a radical new branch point in materials discovery that promises new structures and emergent functionality. Understanding the design principles that govern nanoparticle assembly is critical to moving this field forward. By exploiting mixed ligand environments to target patchy nanoparticle surfaces, we have demonstrated a novel method of colloidal quantum dot (QD) assembly that gives rise to 2D structures. The equilibration of solutions of spherical and quasispherical QDs, including CdS, CdSe, and InP, with 2,2′-bipyridine-5,5′-diacrylic acid resulted in the preferential formation of 2D assemblies over the course of days as determined by transmission electron microscopy analysis. Small-angle X-ray scattering confirms the existence of the QD assemblies in solution. The dependence of the assembly on linker properties (length and rigidity), linker concentration, and total concentration was investigated, together with the data point to a mechanism involving ligand redistribution to create a patchy surface that maximizes the steric repulsion of neighboring QDs. By operating in an underexchanged regime, the arising patchiness results in enthalpically preferred directions of cross-linking that can be accessed by thermal equilibration.

Solution‐phase syntheses of semiconductor nanoparticles provide a versatile and scalable approach to obtain nanomaterials with desirable structure and function. The most common mechanism invoked to explain the nucleation and growth of colloidal semiconductor nanocrystals is the LaMer model, an extension of classical nucleation theory. This model is widely used to conceptualize the conversion of molecular precursors into nanoparticles, and is characterized by three stages: monomer buildup, nucleation, and particle growth. Each of these stages represents a fundamental aspect of nanoparticle synthesis, and each has been widely studied over the past several decades. One of the core successes of this model has been the prediction of the “burst nucleation” method, which has enabled the synthesis of monodisperse nanoparticles in many material systems. Increasingly, however, it has become apparent that many chemical systems do not follow classical nucleation theory, but rather grow via nonclassical cluster‐mediated or aggregative growth processes. Regardless of the nucleation mechanism, highly monodisperse semiconductor nanoparticles can be obtained through kinetic size focusing or digestive ripening. Kinetic size focusing occurs during the nanoparticle growth phase when monomer concentration is high and smaller particles grow faster than their larger counterparts, enabling the smaller particles to “catch up” in size to give a more monodisperse ensemble. Digestive ripening, on the other hand, is a postsynthetic strategy where additional ligand is provided to the system to etch larger particles and redistribute monomer to smaller particles to get a more uniform sample. Beyond the synthesis of spherical colloidal nanoparticles, procedures can be modified to achieve a range of anisotropic structures including 1D nanorods and 2D nanosheets. Seeded and templated growth strategies can lend additional synthetic flexibility to more easily access a range of complex architectures. Continued innovation in synthetic methods and mechanistic understanding will pave the way for technological translation and lasting impact across applications from biological imaging to catalysis and many other next‐generation technologies.

No abstract