Photochemical Sciences Ph.D. Dissertations

Title

Aggregative Growth of Colloidal Semiconducting Nanocrystals for Nanoshell Quantum Dots and Quantum Dot Molecules

Date of Award

2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Photochemical Sciences

First Advisor

Mikhail Zamkov (Advisor)

Second Advisor

Alexander Tarnovsky (Committee Member)

Third Advisor

Pavel Anzenbacher (Committee Member)

Fourth Advisor

Michael Arrigo (Other)

Abstract

With continuing progress in the chemical synthesis of colloidal semiconductor nanocrystals (NC), one property that remains elusive to the rational design is the ensemble photoluminescence (PL) line width. Given the growing demand for NC-based light-emitting materials, substantial research effort has been dedicated to this issue. Here, we demonstrate a postsynthetic strategy that allows reducing emission line widths of CdSe and CdS NCs to near single-particle levels while enhancing the PL quantum yield. The key idea behind the synthetic approach lies in employing a nonclassical coalescence growth mechanism, which leads to size focusing irrespective of the initial sample morphology. Numerical simulations accurately predict the observed particle size evolution, confirming the ability of coalescence growth to promote size focusing of semiconductor colloids. Ultimately, we expect that the demonstrated coalescence growth strategy could enable a rational control of nanocrystal size distributions and corresponding spectral line widths in many types of semiconductor NCs. The optoelectronic properties of colloidal semiconductor nanocrystals (NCs) can be manipulated by changing their geometric shape. The precise synthetic control over particle morphologies, however, has remained elusive. Conventional growth techniques rely on the kinetic assembly of atomic units, where supersaturation and precipitation processes can lead to a broad distribution of particle shapes. In this paper, we demonstrate that replacing atomic precursors with small-size nanocrystals as building blocks for larger colloids offers an easier, more predictive control over nanoparticle shape evolution. The reported growth strategy is illustrated via shape-selective syntheses of CdSe and CdS NC cubes, spheres, rods, as well as iv unprecedented “donut” and ring-like structures. Different particle morphologies were obtained through a thermodynamically driven growth, using a distinct combination of coordinating compounds that minimize the surface free energy. The demonstrated aggregative growth is explained using a thermodynamic model for interacting viscous colloids. Auger decay of multiple excitons represents a significant obstacle to photonic applications of semiconductor quantum dots (QDs). This nonradiative process is particularly detrimental to the performance of QD-based electroluminescent and lasing devices. Here, we demonstrate that semiconductor quantum shells with an “inverted” QD geometry inhibit Auger recombination, allowing substantial improvements to their multiexciton characteristics. By promoting a spatial separation between multiple excitons, the quantum shell geometry leads to ultralong biexciton lifetimes (>10 ns) and a large biexciton quantum yield. Furthermore, the architecture of quantum shells induces an exciton–exciton repulsion, which splits exciton and biexciton optical transitions, giving rise to an Auger-inactive single-exciton gain mode. In this regime, quantum shells exhibit the longest optical gain lifetime reported for colloidal QDs to date (>6 ns), which makes this geometry an attractive candidate for the development of optically and electrically pumped gain media.

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