Increasing the quantum yields of InP quantum dots is important for their applications, particularly for use in consumer displays. While several methods exist to improve quantum yield, the addition of inorganic metal halide salts has proven promising. To further investigate this phenomenon, InP quantum dots dispersed in tetrahydrofuran were titrated with ZnCl 2 , ZnBr 2 , and InCl 3 . The optical properties were observed, and the reactions were studied by using quantitative 1 H NMR and thermodynamic measurements from isothermal titration calorimetry. These measurements contradict the previously hypothesized reaction mechanism in which metal halide salts, acting as Z-type ligands, passivate undercoordinated anions on the surface of the quantum dots. This work provides evidence for a newly proposed mechanism wherein the metal halide salts undergo a ligand exchange with indium myristate. Thermodynamic measurements prove key to supporting this new mechanism, particularly in describing the organic ligand interactions on the surface. An Ising model was used to simulate the quantum dot surface and was fit by using thermodynamic and 1 H NMR data. Together, these data and the proposed exchange mechanism provide greater insight into the surface chemistry of quantum dots.

Quantum dot surfaces can have a substantial effect on their physical, chemical, and optoelectronic properties. When the chemistry that occurs at the surface of nanocrystals is studied, critical insights can be gained into the fundamental structural, thermodynamic, and optical properties of quantum dot materials providing a valuable guide for how to best adapt them for desired applications. Colloidal quantum dots are often terminated with organic ligands that consist of a long aliphatic chain and a head group that binds tightly to the nanocrystal surface. While extensive work has been done to understand how ligand head groups influence quantum dot properties, studies to unravel the influence of the organic ligand tail on ligands and surface reaction equilibria are incomplete. To further investigate the driving forces of quantum dot surface modification, a series of ligand exchange reactions with oleic acid were performed on indium phosphide quantum dots, initially terminated with straight-chain carboxylates of variable lengths. The reaction was monitored using isothermal titration calorimetry and 1H NMR to determine the extent of each reaction and its associated thermodynamics. From these measurements, interligand interactions were observed to be dependent on the length of the straight-chain ligand. A modified Ising model was used to investigate the enthalpic and entropic effects contributing to these ligand exchanges and reveal that interligand interactions play a much larger role than previously thought. Additional experimentation with phosphonic acid ligand exchange reveals complexity in the reaction mechanism but further illustrates the significant impact of ligand tail group length on thermodynamics, even in cases where there is a large difference in head group binding energy.

Powder X-ray diffraction is one of the key techniques used to characterize the inorganic structure of colloidal nanocrystals. The comparatively low scattering factor of nuclei of the organic capping ligands and their propensity to be disordered has led investigators to typically consider them effectively invisible to this technique. In this report, we demonstrate that a commonly observed powder X-ray diffraction peak around $$q=1.4{\AA}^{-1}$$ q = 1.4 Å − 1 observed in many small, colloidal quantum dots can be assigned to well-ordered aliphatic ligands bound to and capping the nanocrystals. This conclusion differs from a variety of explanations ascribed by previous sources, the majority of which propose an excess of organic material. Additionally, we demonstrate that the observed ligand peak is a sensitive probe of ligand shell ordering. Changes as a function of ligand length, geometry, and temperature can all be readily observed by X-ray diffraction and manipulated to achieve desired outcomes for the final colloidal system.

Adsorbed surface ligands play an important role in determining the chemical and physical properties of colloidal semiconductor nanocrystals. Most particularly, these ligands influence the optical properties of these nanocrystals. For instance, the luminescence of type II–VI quantum dots has been shown to decrease as metal carboxylates are stripped from the surface. To gain a better understanding of the thermodynamics and equilibria that influence the optical properties of colloidal quantum dots, we studied the adsorption energies of aliphatic cadmium carboxylates to the surfaces of cadmium sulfide quantum dots. Direct calorimetric measurements of the adsorption energies of such ligands have previously proven to be challenging because they are tightly adsorbed to the quantum dot surface. Here, we show that tetrahydrofuran can be used as a coordinating solvent, allowing cadmium oleates to more easily be stripped from the surface, thereby creating a dynamic ligand equilibrium. Taking advantage of this dynamic equilibrium, ligand-deficient cadmium sulfide quantum dots were prepared, and the adsorption energy of adding cadmium oleate to the surface was measured via isothermal titration calorimetry. Quantum chemical calculations were performed to investigate the binding energy of the cadmium complex with tetrahydrofuran and to calculate the adsorption energy of cadmium oleate to the surface of cadmium sulfide in the absence of a coordinating solvent. Additionally, a modified Ising-model-based simulation was used to estimate the enthalpic and entropic contributions of interligand interactions to the reaction thermodynamics, which play a significant role in describing the quantum dot surface. Lastly, the effects of a dynamic ligand shell on the optical properties of quantum dots were studied, suggesting that a static ligand shell provides higher quantum yields.

Increasing access to undergraduate research is critical in efforts to retain students pursuing careers in STEM. Alternatives to traditional research positions, such as course based undergraduate research experiences (CUREs), have played important roles in engaging more undergraduates in research. However, these opportunities are only a subset of possible nontraditional research experiences which can enrich the undergraduate experience. In this study, we critically examined and improved the research-group-led undergraduate research program (GURP) to better meet the needs of undergraduates seeking research. Specifically, we investigated if the program was successful with a greater diversity of data sets, if assessments of learning were consistent across different data sets and teaching environments, and if the program promoted student engagement in research. We have found that this model is scalable, robust, and adaptable to different implementations while producing consistent and positive learning outcomes for students. Especially remarkable are students' increased self-identification as scientists and statistically significant gains in self-perceived competency across multiple domains of knowledge. This program model has shown promising results as a partially and fully online research experience for undergraduates and has benefited program alumni in their research careers. To assist research groups starting similar programs, we have created public data sets and instructional resources. We believe that GURP programs can work in a variety of situations and hope that they can become a tool to increase interest and build communities for young researchers.