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dc.contributor.advisorJames W. Swan and William A. Tisdale.en_US
dc.contributor.authorWinslow, Samuel W.(Samuel Walter)en_US
dc.contributor.otherMassachusetts Institute of Technology. Department of Chemical Engineering.en_US
dc.date.accessioned2020-09-15T22:04:37Z
dc.date.available2020-09-15T22:04:37Z
dc.date.copyright2020en_US
dc.date.issued2019en_US
dc.identifier.urihttps://hdl.handle.net/1721.1/127577
dc.descriptionThesis: Ph. D., Massachusetts Institute of Technology, Department of Chemical Engineering, May, 2019en_US
dc.descriptionCataloged from the official PDF of thesis.en_US
dc.descriptionIncludes bibliographical references (pages 217-235).en_US
dc.description.abstractColloidal semiconductor nanocrystals (NCs) or "quantum dots" are used in next-generation optoelectronic devices such as photovoltaics, displays, photodetectors, and thermoelectrics. For deployment in these architectures, NCs are cast out into the solid state. Because the NC ensembles are monodisperse, they readily self-assemble into an ordered superlattice (SL). Commonly for PbS NCs, body-centered cubic (BCC), body-centered tetragonal (BCT), and face-centered cubic (FCC) phases are observed with varying degrees of NC orientation relative to adjacent SL sites. Predictive control over the organization of NCs into SLs with long-range order remains a challenge. In this Thesis, oleate-capped PbS NCs are used as a convenient, prototypical system to establish a predictive framework for NC SL formation with respect to newly identified and existing tuning parameters.en_US
dc.description.abstractI first identify and fully characterize unbound/free ligand as an important, controllable parameter to continuously adjust SL symmetry with theoretically single-molecule resolution. Increasing either the bound or unbound ligand populations shifts the SL uniaxially from the BCC to FCC phase. A high free ligand fraction has implications for the ease of formation of oriented SLs via spin-casting. Next, I measure a universal distortion of SL symmetry when cooling from room to cryogenic temperatures in which the SL contracts along one axis while expanding along the other two, ultimately shifting towards the BCC symmetry. Both hysteresis and non-monotonic, surprising trends in unit cell volume are observed and rationalized. The distortion is delineated by thermal markers of the surface-capping ligands and is generalizable to other material systems. I establish small-angle neutron scattering (SANS) as a valuable experimental tool for complete characterization of NC surfaces.en_US
dc.description.abstractIn order to fit SANS data, I develop a model inspired by the NC structure sampled from molecular dynamics (MD) simulations and introduce a Markov chain Monte Carlo (MCMC) algorithm for efficient parameter inference and uncertainty estimation. I quantify an epitaxial monolayer of PbCl₂ on the surface of PbS NCs synthesized from a large excess of PbCl₂ instead of from PbO. This elucidation reconciles sizing curves from the literature and explains the suitability of a specific NC synthesis for different applications. Finally, I extend the SANS method to measure the structure factor of semi-dilute PbS NC dispersions and liken the interactions to that of a square well fluid. The data-fitting yields a repulsive core size larger than the physical NC core diameter which stems from a densely-packed ligand layer near the NC surface. I also measure a weak attractive strength ~1 k[subscript B]T.en_US
dc.description.abstractThis novel understanding of ligand-mediated NC interactions is extended to parameterize patchy particle simulations which predict a complete PbS NC SL phase diagram consistent with all previous tuning strategies. This Thesis provides a complete description of the predictive framework for self-assembled SLs and develops new computational tools which may be applied to other material systems.en_US
dc.description.statementofresponsibilityby Samuel W. Winslow.en_US
dc.format.extent235 pagesen_US
dc.language.isoengen_US
dc.publisherMassachusetts Institute of Technologyen_US
dc.rightsMIT theses may be protected by copyright. Please reuse MIT thesis content according to the MIT Libraries Permissions Policy, which is available through the URL provided.en_US
dc.rights.urihttp://dspace.mit.edu/handle/1721.1/7582en_US
dc.subjectChemical Engineering.en_US
dc.titleLead sulfide nanocrystal ligand structure and its influence on superlattice self-assemblyen_US
dc.typeThesisen_US
dc.description.degreePh. D.en_US
dc.contributor.departmentMassachusetts Institute of Technology. Department of Chemical Engineeringen_US
dc.identifier.oclc1193321484en_US
dc.description.collectionPh.D. Massachusetts Institute of Technology, Department of Chemical Engineeringen_US
dspace.imported2020-09-15T22:04:36Zen_US
mit.thesis.degreeDoctoralen_US
mit.thesis.departmentChemEngen_US


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