THE SCHOOL OF MATERIALS SCIENCE AND ENGINEERING

GEORGIA INSTITUTE OF TECHNOLOGY

Under the provisions of the regulations for the degree

DOCTOR OF PHILOSOPHY

on Wednesday, December 6, 2017

1:00 PM
in MoSE 1201A

will be held the

DISSERTATION DEFENSE

for

Yanjie He

"Nanostructured materials with well-controlled dimensions, compositions and architectures via precise molecular design of non-linear copolymers"

Committee Members:

Dr. Zhiqun Lin, Advisor, MSE

Dr. Vladimir V. Tsukruk, MSE

Dr. Dong Qin, MSE

Dr. Meisha Shofner, MSE

Dr. Yulin Deng, ChBE

Abstract:

Nanostructured materials with well-controlled dimensions, chemical compositions and complex architectures prepared by conventional colloidal bottom-up approaches offer superior ways to  investigate self-assembly behaviors as well as diverse applications in the last two decades. Despite the impressive advancement of conventional synthetic colloidal approaches, there is still a lack of general and facile synthetic approach that can fully decouple the parameters, such as dimension, chemical composition, morphology, architecture and surface chemistry in one-step synthetic route. In this thesis, we developed a versatile and robust polymer-templated nanoreactors approach for synthesizing well-controlled nanostructured materials with controllable size, shape, morphology, functionality and surface chemistry. We employed judiciously designed 21-armed star-like triblock copolymers as nanoreactors to direct the growth of various functional nanocrystals in a controlled manner owing to the selective interaction between metal precursors with star-like triblock copolymers. This nanoreactors strategy enabled us to answer the in-depth fundamental questions with regarding to the quantum chemistry, fabricate ultra-stable light emitting nanocrystals and improve the performance efficiency of the state-of-the-art perovskite solar cells. Specifically, the major achievements can be summarized as follows:

First, polystyrene capped semiconducting hollow lead chalcogenides (e.g., PbTe and PbS) nanocrystals was crafted with tunable core dimension and shell thickness via the utilization of well-defined star-like PS-b-PAA-b-PS as nanoreactors where metal precursors can be selectively partitioned into the space populated by PAA chains due to the strong coordination ability of PAA to PbTe precursors and the nanocrystals growth can be initiated at high temperature. Our nanoreactors strategy firstly demonstrated that semiconducting hollow lead chalcogenide nanocrystals which cannot be prepared via conventional hollow nanocrystals synthetic approaches can be fabricated with not only precise and readily dimension control but also tunable surface chemistry. The surface capping ligands can be tuned from insulating polystyrene to semiconducting conjugated PEDOT, thereby generating organic-inorganic hybrids consisting of semiconducting PbTe hollow nanocrystals as nano-fillers and the conducting PEDOT as matrix, which is promising for application in high performance organic-inorganic hybrid thermoelectrics. In addition, semiconducting hollow lead chalcogenides nanocrystals exhibited blue-shift of NIR absorption in comparison to solid lead chalcogenides counterparts, which provides another freedom of control when it comes to the design of organic-inorganic hybrids.

Second, we designed two series of amphiphilic 21-armed P4VP-b-PtBA-b-PEO and P4VP-b-PtBA-b-PS via ATRP in conjunction with click reaction and utilized these judiciously designed star-like macromolecules as nanoreactors to direct the growth of lead halide nanocrystals followed by in-situ conversion of these lead halide nanocrystals into the corresponding organo-lead halide perovskite nanocrystals. Encapsulation of perovskite nanocrystals with silica shell was achieved via in-situ hydrolysis of alkoxide silica precursor in the space occupied by PAA chains, leading to the formation of polymer-capped perovskite /silica core/shell nanocrystals with tunable core size as well as shell thickness. The resulting core-shell-1/shell-2 morphology consisting of perovskite nanocrystals as core, silica coating as shell-1 and the polymer ligands as shell-2 exhibited excellent stability while maintaining supreme solution processability, which is a promising candidate for the next generation of LEDs.

Finally, we extended this nanoreactors strategy to craft plasmonic-semiconducting (Au-CdS) core/shell nanocrystals with large lattice mismatch between core and shell materials via utilizing star-like amphiphilic 21-armed P4VP-b-PtBA-b-PEO as nanoreactors and demonstrated core shell nanocrystals’ application in perovskite solar cells. Star-like amphiphilic 21-armed P4VP-b-PtBA-b-PEO nanoreactors circumvented synthetic limitation imposed by epitaxial growth of core shell nanocrystals through independently tuning the size of Au diameter owing to the strong coordination ability of pyridine groups in P4VP blocks and selective partitioning of CdS precursor by PAA chains. In comparison to conventionally prepared Au-CdS capped by oleylamine and oleic acid, the resulting PEO- capped Au/CdS core shell nanocrystals simultaneously achieved the optimum length of PEO to facilitate the transportation of carriers and the chemical compatibility with organo-lead iodide active layer in perovskite solar cells, thereby leading to the descent photovoltaic conversion efficiency improvement.

Thus the bottom-up crafting of nanostructured materials with complicated architectures, tunable dimensions, controlled chemical compositions and well-defined surface chemistry promises new opportunities to tailor-make next generation of nanostructured materials for diverse applications in lithium ion battery, photovoltaic devices, thermoelectrics, catalysis, electronics, nanotechnology, and biotechnology.