MIT Libraries homeMIT Libraries logoDSpace@MIT

MIT
View Item 
  • DSpace@MIT Home
  • MIT Libraries
  • MIT Theses
  • Theses - Dept. of Chemical Engineering
  • Chemical Engineering - Ph.D. / Sc.D.
  • View Item
  • DSpace@MIT Home
  • MIT Libraries
  • MIT Theses
  • Theses - Dept. of Chemical Engineering
  • Chemical Engineering - Ph.D. / Sc.D.
  • View Item
JavaScript is disabled for your browser. Some features of this site may not work without it.

Lead sulfide nanocrystal ligand structure and its influence on superlattice self-assembly

Author(s)
Winslow, Samuel W.(Samuel Walter)
Thumbnail
Download1193321484-MIT.pdf (31.02Mb)
Other Contributors
Massachusetts Institute of Technology. Department of Chemical Engineering.
Advisor
James W. Swan and William A. Tisdale.
Terms of use
MIT 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. http://dspace.mit.edu/handle/1721.1/7582
Metadata
Show full item record
Abstract
Colloidal 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.
 
I 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.
 
In 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.
 
This 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.
 
Description
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemical Engineering, May, 2019
 
Cataloged from the official PDF of thesis.
 
Includes bibliographical references (pages 217-235).
 
Date issued
2019
URI
https://hdl.handle.net/1721.1/127577
Department
Massachusetts Institute of Technology. Department of Chemical Engineering
Publisher
Massachusetts Institute of Technology
Keywords
Chemical Engineering.

Collections
  • Chemical Engineering - Ph.D. / Sc.D.
  • Chemical Engineering - Ph.D. / Sc.D.

Browse

All of DSpaceCommunities & CollectionsBy Issue DateAuthorsTitlesSubjectsThis CollectionBy Issue DateAuthorsTitlesSubjects

My Account

Login

Statistics

OA StatisticsStatistics by CountryStatistics by Department
MIT Libraries homeMIT Libraries logo

Find us on

Twitter Facebook Instagram YouTube RSS

MIT Libraries navigation

SearchHours & locationsBorrow & requestResearch supportAbout us
PrivacyPermissionsAccessibility
MIT
Massachusetts Institute of Technology
Content created by the MIT Libraries, CC BY-NC unless otherwise noted. Notify us about copyright concerns.